Studies of the adrenal medulla have provided several scientific "firsts." George Oliver and Edward Albert Schäfer were the first to extract a hormone from an endocrine organ when they showed that a preparation from the adrenal medulla had a profound effect on blood pressure. John Jacob Abel was the first to isolate a hormone when he isolated epinephrine from the adrenal medulla. Jokichi Takamine and Thomas Bell Aldrich, independently, were the first to crystallize a hormone, an important step on the way to chemical identification. Ernst Joseph Friedmann was the first to characterize a hormone by revealing its chemical formula. Hermann Blaschko was the first to outline the biosynthetic pathway of a hormone. Blaschko and Arnold Welch, and Nils-Åke Hillarp, Sten Lagerstedt and Bodil Nilson, were the first to isolate secretary vesicles, the chromaffin vesicle of adrenal medullary cells. This non-exhaustive list of studies with the adrenal medulla, and its principal hormone epinephrine, indicates that this organ has played an important and interesting role in the history of our understanding of the endocrine system and, as we shall see, the nervous system as well. But before we discuss these relatively recent breakthroughs, the events leading up to our current understanding of the adrenal medulla will be examined.

Since in the human, as in all mammals, the adrenal medulla is completely enclosed by the adrenal cortex, the early history of the medulla is necessarily linked to the discovery of the adrenal glands. In fact, it was not recognized until relatively recently that two functionally distinct organs coexisted within the adrenal gland. By some accounts, the first description of the adrenal glands is found in the Bible, specifically Leviticus 3:4. This passage reads (in the King James version, and some key words are different in other versions), "And the two kidneys, and the fat that is on them, which is by the flanks......". The question of whether or not the fat described in the passage refers to the adrenal glands apparently comes down to technicalities of translation, since the word for fat and the description of the relationship of this fat to the kidney are questionable. However, at the most generous, this cannot be considered a detailed description of the adrenal glands.

Another controversy surrounds the accuracy of the descriptions of the adrenal glands by Claudius Galen (c. 130-201). From the German translation of Galen's seven books by Simon in 190696, it is apparent that Galen encountered the adrenal glands in his numerous dissections of animals. However, only the left gland is described in his writings. While the gland is described as "loose flesh," he clearly describes the left adrenal vein connected to the left renal vein. Like many anatomists in the centuries to follow, Galen assumed that this "loose flesh" represented accessory renal tissue. Some historians have not recognized the description of Galen as applying specifically to the adrenal glands, but his accurate description of the left adrenal vein leads one to the conclusion that Galen was the first to publish a description of the mammalian adrenal gland.

The first anatomist to give a detailed description of the human adrenal glands, including accurate illustrations, was Bartholomeus Eustachius34. Eustachius (1520- 1574) was professor at the Collegio della Sapienza at Rome. A significant part of his career was spent preparing detailed copper plates of the human anatomy. The

forty-seventh (and last) of these splendid plates was completed in 1552. Interestingly, these plates were placed in the Papal Library where they remained for over a century. Rolleston89 reported that Pope Clement XI gave them to his medical attendant, G.M. Lancisi, who published them with his own text in 1714. Eustachius published a description of the kidneys in 1563 and referred to the adrenal glands as Glandulae Renibus Incumbentes, a name that implied an accessory renal role. His description, here translated by Lenard65, still holds true. "Even if many will consider sufficient what we have said about the surface of the kidneys, somebody could well object that I have neglected something and I consider it indicated to say something of the glands, diligently overlooked [diligenter praetermissis] by other anatomists. Both kidneys are capped on the extremity towards the cava by a gland. Both are connected with a fold of the peritoneum in such a way that one, if he is not very attentive, does really overlook them, as if they were not present. Their shape resembles that of the kidneys...sometimes one is bigger, sometimes another...early anatomists and those who write ample treatises on this art in our days failed to detect them. They, pretending to be exact, stand so obstinately for their own and their master's errors that often they seem to be fighters rather than searchers of anatomical truth."

It is easy to see that the politics of science flourished in Eustachius' day, perhaps more openly than today. This bitter trend was to continue throughout most of the history of the adrenal medulla. Lenard65 referred to this era as that of the "fighting anatomists" [anatomici contentiosi]. The next blow was delivered by Archangelo Piccolomini who attacked Eustachius in 1586. An excerpt from his Anatomicae prelectiones (from65): "Sometimes one may see two or more glands lying on the kidneys but we do not think they deserve special attention, because they are not to be found in every case and they have no flesh or parenchyma of their own ... so that they might be considered renal excrescences. 'They form part of the renal parenchyma. Why do they exist in a few? The same way supra-abundance of material creates a sixth finger, they originate from a seminal surplus and that is the way they come out of the kidneys."

Some time later, in 1640, André DuLaurens, physician of Henry IV of France, also doubted the existence of the adrenal glands31. He wrote: "Eustachius claims to find a gland above the kidneys. Sometimes we saw that too; often, however, we stated that there was no such gland."

Despite these, and other, detractors, history shows that Eustachius was the first to describe the adrenal glands accurately. Certainly part of the problem of why this was not recognized by his contemporaries was due to the suppression of his beautifully detailed copper plates. We will probably never know exactly why this occurred, but one can speculate that there were negative feelings, held by at least one influential person under the Pope's authority, about the publication of illustrations of human dissections. It apparently was not taboo for Eustachius to publish a written description in 1563. Although the date of 1563 is often given as the earliest description of the adrenal glands, the true date should be set as prior to 1552 when Eustachius completed his plates.

As described by Goldzieher40, other anatomists of the period, including Caspar Bauhin in 1588 and Archangelus Piccolhomineus in 1596, commented on the existence of the adrenal glands. In 1611, another description of the adrenal glands was put forward, which was to have a profound effect on the contemporary concept of the adrenal medulla. The Danish anatomist Caspar Bartholin (1585-1629), a student of Bauhin, described the adrenal glands as hollow organs, filled with "black bile." He gave them the name capsulae atrabiliarae. In his plates the spherical, hollow nature of the glands is emphasized. Apparently, Bartholin's concept was based on the erroneous description by Johann Schenk von Grafenburg (1530-1598) published in 160090 of a central cavity in the gland, corroborated and supplemented by Bauhin's "discovery," in 1605, of a black fluid in the cavity. This concept, and the same plates, were passed on by Caspar's son, Thomas. Thomas Bartholin (1616-1680)7, the second of six sons, was destined to become more famous than his father. And it was Thomas' son, Caspar the Younger, who described the greater vestibular glands of the female perineum. An interesting history of the Bartholins, who were well known as men of letters in addition to being famous anatomists, is provided by Sir Humphry Davy Rolleston89.

So it is clear that by the early 17th century, the presence of the adrenal glands in the human abdomen had been established, although two centuries were to pass before the medulla was unequivocally recognized. We will return to that controversy later.

Whereas the anatomists of this early period knew the adrenal glands were present (only the first descriptions are mentioned above), there was great uncertainty as to their function. This was to remain the case until the time of Thomas Addison (1793-1860). As late as 1827, John Redman Coxe (1773-1863) commented, "Notwithstanding, however, the long period that has elapsed since their discovery, their use in the animal oeconomy still remains a mystery, almost as many offices having been attributed to them, as there have been writers on the subject."25 Many of the early workers, probably including Galen and certainly including Eustachius, theorized that the adrenal glands functioned as accessory kidneys. Eustachius' term glandulae renibus incumbentes implies this. A similar view was hold by Julius Casserius (1545-1616), one of William Harvey's teachers at Padua, who called them renes succenturiati. In an interesting account given by Shumacker94, Antonius Molinetti published a more sophisticated version of the "renal" function of the adrenal glands in 1675. By this time it was noticed that the adrenal glands are relatively larger, with respect to the kidneys, in the fetus than the adult (now known to be due to an enlargement of the cortex). It was theorized by many that the adrenals had a more important function in fetal life than in adulthood. Molinetti proposed that the adrenals functioned only as diverticulae of the blood, diverting most of the arterial blood away from the kidneys and thereby preventing the secretion of urine in the fetus. He reasoned that this would provide some means of avoiding urinary excretion by the fetus, since this was considered to be disastrous. Molinetti felt that most of the arterial blood was diverted from the kidneys, only as much reaching them as they needed for their metabolism, the remainder being shunted back to the heart. From here it passed through the umbilical vein and placenta into the mother's circulation. Upon reaching the maternal kidneys, waste material was eliminated. This explained why women have to urinate more during pregnancy! While this theory was virtually ignored for a century and a half, curiously it was resurrected by Thomas Mitchell in 1813, Charles Bell in 1829, and enthusiastically by Coxe in 1827. In supporting Molinetti's view with persuasive reasoning (but devoid of experimental proof), Coxe ended his case by saying if he is "...correct in the positions thus advanced, I think all the names by which these organs have been hitherto distinguished, should be abolished, and that of diverticula urinae should be substituted in their place."25

Although a renal function for the adrenal glands was an early and long-lasting concept, there were many other functions proposed between the times of Eustachius and Addison. Shumacker94 gives a most thorough account of these. Adrianus Spigelius (1578-1625), who called them capsulae renales, thought that the adrenal glands merely served to occupy the space between the kidneys and the diaphragm, and to aid in supporting the stomach. This view was upheld by Nathaniel Highmore (1613-1685) who additionally suggested that they might serve to absorb humid exudates (sweat?) from the large vessels nearby. Naturally such a simple theory did not satisfy many. Thomas Bartholin replied to Spigelius by writing: "1. Nature makes, nor does nothing in vain or inconsiderately, much less does she appoint a noble animated Part, only to fill a space, which she might have filled by making the kidney a little bigger. 2. These props would have been too weak by reason of their smallness. Nor should this use belong to the Stomach alone, but to other neighboring Parts." 7

Jean Riolan, both the Elder (I538-1605) and his son the Younger (1577-1657), put forth the thesis that the adrenal glands served to sustain the group of nerves above the kidney. According to Shumacker94, the Riolans felt that the chief need for such a support for the nerve plexus was to prevent it from weighing too heavily upon neighboring veins. Molinetti refuted this view by stating that the adrenals lay above the plexus and that the plexus itself was so firmly adherent to the vertebra that it could be freed from it only with a scalpel. To Riolan the Younger is also attributed the theory that the adrenals served no other purpose than to generate perirenal fat in the fetus.

Thomas Wharton (1610-1673) also associated the nearby nerve plexus with the function of the adrenal glands. He referred to the adrenals as glandulae ad plexum. As pointed out by Shumacker94, Wharton was impressed with the large size of the plexus and with the large nerve supply to such a small organ. He suggested that the adrenals received some substance from the nerves (which he assumed to be useless to the nerves); the adrenals absorbed this substance and passed it into the veins where it had some useful purpose. Although there is no direct evidence that Wharton supported his hypothesis with experimentation, to him goes the credit for being the first to associate the adrenal glands with a function of the nervous system. Indeed his description, published in 1656, of the adrenals taking a substance from nerves and transferring it to veins preceded the neuroendocrine concept of the adrenal medulla that we have only appreciated in the 20th century!

Wharton's best-known student, Francis Glisson (1597-1677), supported his younger teacher. In 1657 he described a vessel coming from the body of the adrenal gland that emptied into the neighboring vein. He also described a valve in the vein which prevented return of blood to the adrenal. Glisson thought that the adrenal glands extracted a liquid from the nerves and secreted it into the veins. As pointed out by Shumacker94, it would have been ridiculous to assume that the adrenals should separate a substance from the arterial blood and then secrete it directly back into the venous circulation. Glisson reasoned that it must come from the nerves since they were the only other structures communicating with the glands. Just what this liquid was he did not know.

One of the most widely debated of the early theories of the function of the adrenal glands was that of Caspar Bartholin, later defended by Thomas. As mentioned above, the Bartholins described the adrenal glands as having a cavity filled with a brownish fluid. They held, and staunchly defended, the belief that the adrenals absorbed atrabiliary (a word meaning black bile, but referring to melancholy) juice from the blood exiting the liver and spleen; from the adrenals the juice was carried through the blood to the kidneys for excretion. This explained, among other things, the dark color of the urine in some disease states.

Thomas Petruccius (1648-1711) published a book on the adrenal glands in 1675 and supported the Bartholins' view. As discussed by Shumacker94, Petruccius is said to have discovered valves in the capsular veins placed in a manner contrary to those of other veins and in such a way that they favored the passage of atrabiliary juice to the kidneys. According to Fulton37, this rare book is also remarkable for its inclusion of good bibliographical references to the descriptions in question, a rather unusual thing for a 17th century medical writer to include. However, Petruccius offered another function for the adrenal glands. He pointed out that since "...being a work of God Almighty, they have to contribute to the beauty and utility of the microcosmos we call man. Like everything, they praise God. When do they praise him? When they are viewed with understanding eyes ......" It is no mere coincidence that Petruccius' work was dedicated to a Cardinal.

The Bartholins were also supported, although not enthusiastically, by Johann Vesling (Veslingius) (1598- 1649). According to Lenard65, Vesling was an immensely popular anatomist, based in Padua. His well- known treatise, published in 1653, suggested that the adrenal glands "probably favour the evacuation of the serous liquid and store the black bile which acts as a ferment, promoting the secretion of liquid from the blood." However, he also said of the adrenals, "What their use is, the human mind has not yet conjectured."

There are many detractors of the Bartholins' theory, most of them arguing on anatomical grounds. They included Molinetti and Highmore. Another writer of the period, Dominicus de Marchettis (1626-1688), pointed out the obvious fact that "melancholy humor" was especially abundant in adults, whereas the adrenal glands are very small in adults and large in fetuses. lsbrand de Diemerbroeck (1609-1674) wrote in favor of Vesling (and by association, the Bartholins) but pointed out a weakness of the theory. According to Shumacker94, de Diemerbroeck was among those who could not demonstrate an appropriate passage from the adrenals to the kidneys. He did conjecture that the adrenals may extract black juice, needed by the venous blood, from the arterial blood, thereby preventing the juice from going to parts where it is not needed.

Theodorus Kerckringius (1640-1693) thought that the function of the adrenal glands was to secrete a juice to color and animate the blood and produce a fermenta- tion in the heart. As related by Shumacker94, this fermentation served in some manner to excite the heart. Could Kerckringius have discovered adrenaline? Of course not, but his theory strikes tantalizingly close to the mark.

Shumacker94 and Coxe25 mention several other writers who associated adrenal function with some essential alteration of the blood. One such group felt that its action was particularly concerned with changes in the blood of the renal circulation. Samuel Collins (1618-1710) suggested that the adrenals extracted a fermentative liquor from nerves which in turn disposed the blood to give up its waste products to the kidneys. Franciscus Sylvius (1614-1672) thought that the adrenals separated some liquor from the blood which was mixed with the blood returning from the kidneys after the secretion of urine, serving to dilute it and prevent coagulation. Hermannus Boerhaave (1668-1738) also taught that the adrenals corrected the fluidity of the blood coming from the kidneys, presumably because it was made more viscous by the extraction of urine. Antonius Deidier (?- 1746) held a similar view, maintaining that the humor from the adrenal cavity was emptied into the renal vein in order to give back to the blood a substance ("serosité") and lymph that it had lost while traversing the kidney. Daniel Tauvry (1669-1701) said that the adrenal glands strained from the blood a liquor which became oily after storage. The adrenals would "after that, throw it into the Veins; where the Blood, that was stripp'd of its Fluidity by the Secretion of the Serous Parts, recovers its former State, and its Parts that hang'd loosely together are compacted by this Liquor."

The central argument for these various theories involving a direct effect of the adrenal glands on the blood was the observation that the glands were hollow and contained a fluid. it is fair to say that this description, made popular by the Bartholins, was largely accepted as fact during the 17th and 18th centuries. It is obvious from the foregoing descriptions that the Bartholins alone were not responsible for this misconception. Scores of anatomists supported the view that the adrenals were hollow. Sorkin101 suggested that the fault should be placed on the humoral conception of medicine and the conviction it carried in an era in which inductive reasoning prevailed.

One of the few dissenters from the concept of hollow adrenals during this time was Jean Riolan (the Younger). As cited by Lenard65, Riolan stated in 1655: "I have never seen a cavity in them. Should they have a cavity, no pea would find a place in it." This view was essentially ignored, even though anatomists were having a difficult time finding the duct that they reasoned should be draining the adrenal cavity. It was Riolan who named the glands capsulae suprarenales, a term equivalent to the modern one (Nomina Anatomica (Fifth edition, 1983): glandula suprarenalis]. It is most unfortunate that Riolan is best remembered as William Harvey's chief opponent. While he was wrong in opposing the emerging view of the circulation, his anatomical concept of the adrenal glands was correct. Neither of his views found acceptance during his lifetime.

It is apparent that peas were (and still are) a popular indication of volume. In direct contradiction of Riolan, Gottfried Welch wrote in his thesis in 1691 that the adrenal gland contains a cavity which is able to hold not only eight, but even twelve peas. This cavity served as a reservoir for lymph. According to Lenard65, Welch apparently had the courage (if that's the right word) to taste the black liquid in the medulla and described it as "acid and astringent." We will return later to the "discovery" of a solid adrenal medulla.

Another category of theories of adrenal physiology involves a role in sexual function. In 1719, Antonius Maria Valsalva (1666-1723) announced the presence of an excretory duct from the adrenal communicating with the left epididymis, although Shumacker94 points out that credit for this "discovery" belongs to Marcus Aurelius Severinus (1580-1656). Valsalva also described a duct between the left adrenal and ovary. It is now apparent that he had located a left gonadal vein. Valsalva supported his view of an adrenal-gonadal relationship not only with anatomical and philosophical arguments, including Biblical references to the close association between the kidneys and genitals, but in what may have been a significant departure from the style of the day, he performed experiments to prove his point. During his career, Valsalva used several animals, including guinea pigs, birds, tortoises, and rats, but in his most famous experiment he removed one testis and the contralateral kidney (and presumably the adrenal) from a dog106. During the period of a few days before the dog died (from sepsis, no doubt), the dog did not "even fawn upon a bitch in heat." (A direct translation would read, "a bitch bitching," which has different meaning in our day.) This therefore "proved" a role for the adrenals in regulating the libido.

Although he was aware of the fact that there is no anatomical connection between the adrenal glands and the gonads, Johanne Friedrich Meckel (1714-1774) also associated the adrenals with sexual function. He published in 1806 that the basis for this relationship was "...their simultaneous, considerable development in several orders of the mammalia...".76. He also cited abnormalities of the adrenal glands in cases associated with sexual abnormalities (castration, syphilis, etc.). It is of interest to note that the direct relationship between the adrenals and gonads, as envisioned by these early workers, had no basis in fact, although today we do recognize important interactions between the gonads and the adrenal cortex.

Coxe24, Shumacker94, and Lenard65 all point out some other miscellaneous functions that were ascribed to the adrenal glands by early investigators. Jean Baptiste Senac (1693-1770) supposed the adrenals secreted the fetal meconium. Jean van Helmont (1577- 1644) thought they secreted a lithotriptic juice that prevented the formation of renal calculi. Joseph Lieutaud (1703-1780) is said to have claimed that the adrenals secreted an acrid liquor that prevented concretions in the vena cava. (If it does serve this purpose, it works particularly well in most of us!) Giovanni Morgagni (1682-1771) conjectured that the adrenals drained lymph from the intestines during fetal life since the cisterna chyli does not fill during this period. In an article published in 1846, John Goodsir (1814-1867) theorized that the adrenal glands, thymus, and thyroid have a related embryology and may function similarly to "...elaborate the matter which has already been absorbed by the other parts, and is now circulating in the vessels of the more perfect individual." One can see the germ of modern endocrinology in this concept. Georg Heim (1803-?) suggested that the adrenal glands might serve to divert blood away from the lungs. Evidence for this theory included the observation that the adrenals were large in the fetus when such a diversion took place. In 1789, Cassan made the observation that adrenal glands were larger in Negroes than in Europeans and offered the possibility that the adrenals played a role in pigmenting the skin. And about this same time, Johannes Christophorus Heino Schmidt stated that a secretion was formed in the adrenal glands which was poured into the blood and in turn served to help the action of the heart91. It is tempting to speculate, although without basis, that this concept presaged the positive inotropic effect of epinephrine on cardiac muscle.

To conclude the overview of early concepts of adrenal function, it is instructive to note a literary competition that was offered by the Academy of Sciences of Bordeaux in 1716. The question the essayists were to answer was, "What is the function of the adrenal glands?" Although many essays were submitted, not one was considered worthy of the prize.

As was mentioned above, the thought of the day was strongly influenced by the concept that the adrenal glands were hollow. Riolan was an unheard dissenting voice. In 1797, Matthew Baille (1761-1832) referred to the soft state, which might be almost fluid, of the center of the "renal capsules" and commented: "This is probably what is meant by authors when they say that they have found in the cavity of the renal capsules a fluid like ink. Their description may be regarded as being a little exaggerated." Although earlier descriptions were given, the credit probably goes to Georges Cuvier (1769-1832) for establishing the fact in 1805 that the adrenal glands are solid structures. He appeared to recognize that the center of the gland was morphologically distinct from the outer portion, although the terms for the medulla and cortex were not introduced until 1836. Meckel pointed out in 1832 that the apparent cavity was in all probability due to postmortem softening of the medulla, a point made in modern terms by A. David Smith and Hans Winkler in their definitive characterization of the lysosomes of adrenal medullary cells in 196698. Meckel differentiated the more solid yellow substance, cortex, from the inner, softer, reddish-brown medulla. Particularly strong evidence was presented in 1836 by N. Nagel, who showed by means of injected preparations that the large central vein had been mistaken for a central cavity80. In 1839, Pierre Rayer (1793-1867) explained that the formation of the cavity was due to extravasation of blood from the rupture of veins. This may have been a contributing factor, but in any event it had been established by this time that the adrenal gland is a solid structure and that any cavity observed within was a postmortem artifact.

In his 1836 article Nagel used, apparently for the first time, separate terms for the two portions of the adrenal gland. In his opening sentence, he refers to the human adrenal gland as being composed of a cortical (Rinden) and medullary (Mark) substance80. This terminology persists to the present.

It was during the early part of the 19th century that scientists were able to have a closer look at structure. This involved the use of an improved compound microscope, fitted with the latest in technology, achromatic lenses. As history was to repeat itself a century and a half later when the electron microscope was introduced, the earnest application of the light microscope to biological studies had to await the development of suitable preparative techniques. One of the first to apply this new technology to the adrenal gland was Alexander Ecker (1816-1887). Among other observations, he noted in 1846 that the adrenal medulla is remarkable for its brightness32.

However, the credit for the first complete description of the microscopic anatomy of the adrenal gland goes to (Rudolph) Albert von Kölliker (1817-1905). Describing fine structure in a manner remarkable for his time, Kölliker presented a written account in 1852 that has only been slightly improved since60. His drawings are also remarkable. But perhaps most impressive of all was his analysis of adrenal function. He stated clearly: "I consider the cortical and medullary substances as physiologically distinct." This is a point that was apparently missed by his contemporary, Thomas Addison. As for the different functions of the adrenal cortex and medulla, Kölliker went on to say: "The former may, provisionally, be placed with the so-called 'blood-vascular glands,' and a relation to secretion assigned to it; whilst the latter, on account of its extremely abundant supply of nerves, must be regarded as an apparatus appertaining to the nervous system, in which the cellular elements and the nervous plexus either exert the same reciprocal action as they do in the grey nerve-substance, or stand in a relation as yet wholly unascertained, towards each other." 61 In so stating, Kölliker was affirming Wharton's earlier (1656) idea that the adrenals are related functionally to the nervous system. As those in the field well appreciate, we are still trying to unravel the details of this relationship!

The year 1856 ushered in an era that has not yet ended, the application of histochemistry to the adrenal medulla. These early studies provided definitive proof that the medulla was a distinct and special part of the adrenal gland. Gabriel Constant Colin (1825-1896) noted that the surface of the adrenal medulla became blue when treated with sulphate of iron21. In a more widely cited study, (Edme Félix) Alfred Vulpian (1826-1887) showed that ferric chloride colored the adrenal medulla green111. This reaction was also noted by Rudolf Virchow (1821-1902) in 1857108. Perhaps more importantly, Vulpian also noted that blood in the adrenal veins gave the same reaction, while other organs and the blood from these organs did not. He concluded that the material giving the reaction was related to the function of the adrenal medulla.

The most famous histochemical reaction associated with the adrenal medulla, the chromaffin reaction, named for a relatively specific reaction to chromate salts, was apparently first observed by Bertholdus Werner as a brownish deposit after fixation in chromic acid or dichromate salts113. Similar observations were made by Gregor Joesten in 186457 and (Friedrich Gustav) Jacob Henle (1809-1885) in 186547. Henle was the first to give cytologic details of the color reaction. Following these articles, there was a significant increase in interest in the adrenal medulla. The new techniques of microscopy were coming into general use and there was something peculiar about the way the adrenal medulla reacted with certain metallic solutions, particularly with chromium. However, it was not until the end of the century that the term "chromaffin" was introduced to describe this reaction. The term "chromaffin reaction" was applied to cells that reacted in this manner, the adrenal medulla being the prime example. The "chromaffin cell" and apparently the name of the reaction as well, were so named by Alfred Kohn (1867-1959) by 190259. (A brief story of how Kohn survived in the concentration camp at Theresienstadt is provided by Blaschko10.)

These and related studies in the last half of the 19th century clearly established that the adrenal medulla is histologically and functionally separate from the cortex. What is now known as "chromaffin tissue" was coming into its own. Interestingly, there have been several other terms introduced with which to label this tissue. These proposed terms include "chromophile,"102, "pheochrome,"86 and "fuscogenic"8. While these and other terms have merit, the term "chromaffin" has stuck and is used universally to describe cells of the adrenal medulla and related cells that exhibit the chromaffin reaction.

While this discussion of the histochemistry of the adrenal medulla has only covered the earliest studies (for more current and much more comprehensive information, see reviews by Rex Coupland24 and David Hopwood54), a particularly interesting article was published in 1918. William Cramer (1873-1930) showed, but not for the first time, that osmic acid had a specific effect on the adrenal medulla. He noted that osmic acid vapor demonstrated "adrenaline granules" as "giving the appearances of fine coal dust scattered over the medulla"26. He was probably the first to give clear evidence that an increase in secretary activity is a result of the passage of these osmiophilic granules into the blood vessels. It has been shown, and continues to be shown, that this observation describes the essential event in the secretion of catecholamines from the adrenal medulla. Only the tools used to show it have changed.

During the middle of the 19th century, embryologists began to examine the development of the adrenal glands. In 1831, Friedrich Arnold (1803-1890) stated that the adrenals developed from the Wolffian (mesonephric) bodies by means of a fissure, and that there was a structural resemblance between the two2. In a rather remarkable series of articles, beginning in 1847, Robert Remak (1815-1865) indicated that the adrenal medulla originated in association with the sympathetic ganglia88. During his brief and extremely productive career, Henry Gray (1827-1861) also investigated the embryology of the adrenal glands. In a paper on "the ductless glands of the chick" published in 1852, he reported that the adrenals arose from "two grayish white masses, which lie one on each side of the aorta, between this tube and the upper and inner extremity of the Wolffian bodies, and are perfectly separate"42.

Kölliker showed that the adrenal cortex is formed first and is subsequently invaded by sympathetic neural elements. Although there were several important studies on comparative embryo- logy of the adrenal glands (see Rolleston89, for review), these early observations were scarcely improved upon until the already-classic studies of chimeras by Nicole Marthe Le Douarin (1930- ) and her colleagues in the 1970s (for example, see64). For a review of more recent work, see18.

The stage was now set for the breakthrough that would give us an understanding of what the adrenal medulla actually does. By the end of the 19th century th basic anatomy, histology, histochemistry, and embryology of the adrenal medulla were understood. It was appreciated both on histological and embryological grounds that the adrenal medulla was somehow associated with the nervous system, specifically the sympathetic nervous system. The classic story of this breakthrough puts George Oliver and Edward Albert Schäfer at center stage. Perhaps this is where they belong but, as we shall see, there are some challengers. As for Oliver (1841-1915) and Schäfer (1850-1935), Sir Henry Dale (1875-1968) gives a charming account of the circumstances surround- ing their discovery27. Oliver, a physician of Harrogate, employed his winter leisure in experiments on his family, using equipment he had designed for clinical testing. In one such experiment he was applying an instrument for measuring the thickness of the radial artery, and having given his young son (whom Dale suggested should receive a special memorial!) an injection of an extract of the adrenal gland prepared from material supplied by the local butcher, Oliver thought that he detected a contraction (others suggest it was an expansion) of the radial artery. When Oliver went to London to tell Professor Schäfer what he thought he had observed, he found Schäfer engaged in an experiment in which the blood pressure of a dog was being recorded. Schäfer was, not unnaturally, incredulous about Oliver's claim and was very impatient at the interruption. But Oliver was in no hurry, and urged only that a dose of his adrenal extract, which he produced from his pocket, should be injected into the dog when Schäfer's own experiment was over. And so, just to convince Oliver that it was all nonsense, Schäfer injected the extract into a vein, and then stood in amazement as they watched the mercury climbing in the arterial manometer until the recording float almost lifted out of the distal limb. Their publication in 189484 resulting from their subsequent experiments is heralded as the first demonstration of a hormonal effect. Many historians regard this study of the adrenal medulla as a milestone in endocrinology.

As pointed out by Medvei77, another scientific team made the same discovery independently of Oliver and Schäfer. Napoleon Cybulski (1854-?) and Ladislaus Szymonowicz performed experiments quite similar to Oliver and Schäfer's in Cracow in December 1894, just one year after the first experiment in London103.

However, the first indirect demonstration of the role of the adrenal medulla as an endocrine organ was in 1892. Joh. Carl Jacobj (1867-?) showed in 1892 that electrical stimulation of the greater splanchnic nerve in the dog decreased the amplitude of contraction of the animal's intestines56. Whereas this article appears not to have been appreciated at the time, hindsight shows us that this was actually a more sophisticated demonstration of adrenal medullary function than the classic study of Oliver and Schäfer.

Even before Jacobj's publication, there was another clue to the function of the adrenal medulla. In 1886, Felix Fränkel reported the first case of a tumor of the adrenal medulla35, a type of tumor that has become known as pheochromocytoma. The patient in this case was an 18 year-old girl who had died suddenly of collapse. Her clinical history and autopsy findings pointed to a severe hypertensive crisis. This, combined with the discovery of an adrenal medullary tumor, presented what appears to be the first evidence, seer-, only in retrospect, of the relationship between the adrenal medulla and blood pressure.

There are some accounts of the history of pathologies of the adrenal glands (for example, Sorkin101 and Rolleston89) and pheochromocytoma in particular (Manger and Gifford74, Sherwin93, Medvei77, and Welbourn112). Just a few highlights of this history will be given here. After Fränkel's report, Paul Manasse (1866-?) reported another patient with pheochromocytoma in 189371. In 1896, Manasse was the first to demonstrate that chromium salts reacted with pheochromocytomas73. In the ensuing years, there was a heated controversy over the nature of this reaction, both with tumors and normal adrenal medullary tissue. Heinrich Poll (?-1939) objected to the word "chromaffin" to describe this reaction because of the uncertain nature of the dark color produced. (Blaschko10 gives an interesting account of Poll's life and death.) Poll introduced the term "pheochrome," and in 1912, Ludwig Pick (1868-1935) incorporated this into the neoplastic counterpart of the adrenal medulla. The term "pheochromocytoma" has been used ever since.

In the early part of this century the clinical picture of pheochromocytoma was emerging. It wasn't until 1926, however, that surgical intervention was attempted. The first case of a successful removal of a pheochromocytoma was performed by César Roux (1857-1934) (cited by Manger and Gifford74 and Welbourn112; this report was not published in the traditional sense, but is apparently only recorded in a thesis at the University of Lausanne). About 7 months later, Charles Horace Mayo (1865-1939) also removed a pheochromocytoma75. The patient in this historic case was Mother Joachim. A fascinating account of Mother Joachim, and her recovery after this surgery, is provided by van Heerden107.

Maurice Charles Pincoffs (1886-1960) is credited with first recognizing a pheochromocytoma preoperatively85. In the same year, 1929, Coleman Berley Rabin (1900- ) noted that a pheochromocytoma contained a pressor agent in excess of that occurring in the normal adrenal medulla and suggested that this might account for the clinical manifestations87. Although several significant features of pheochromocytoma have been published in the meantime (see, for example74,112 for details), the essential characteristics of the principal adrenal medullary tumor had been recognized by this time.
Alongside these studies of the clinical picture of adrenal medullary pathology, physiologists were carrying out the classic studies on the function of the adrenal medulla. Independently, George P. Dreyer (1866-1931) in 189830 and M. Lewandowsky in 189967 noted the correspondence between the effects of stimulation of postganglionic sympathetic nerves and the effects of administered epinephrine. This was confirmed by John N. Langley (1852-1925)63, whose student, Thomas Renton Elliot (1877-1961), suggested in 1904 that the sympathetic nerves produced their effects by liberating epinephrine33. This suggestion is considered by many to be the beginning of our concept of neurochemical transmission.

Although there were many other great scientists pursuing the problem of adrenal medullary function in the first part of this century, the name that stands out is Walter Bradford Cannon (1871-1945). It is largely through the efforts of Cannon and a host of collaborators (see the career-end review by Cannon13) that we have a basic understanding of how the adrenal medulla contributes to the body's efforts to maintain homeostasis during stress. In fact, the still-used term "stress syndrome" was coined by one of Cannon's students, Hans Selye (1907-1982). However, Cannon and his contemporaries encountered phenomena that could not be explained by just the elaboration of epinephrine from the adrenal medulla. The discovery of norepinephrine was not made until after Cannon's death. Furthermore, as pointed out by Davenport28, Cannon used a commercial preparation of natural epinephrine that contained norepinephrine.

Also during the early part of this century, there was a battle raging in the chemical laboratories. Davenport28 tells the intricate tale of the struggle for priority as the discoverer of epinephrine, a struggle particularly between John Jacob Abel (1857-1938) and Jokichi Takamine (1854-1922), but also involving Thomas Bell Aldrich (1861-?). The story is rife with intrigue and discovery, accusations and counter-accusations. A suggestion of how complicated the picture gets is offered by the fact that the Merck Index 178 lists 38 different terms for what is generally called epinephrine. This includes such interesting names as Takamine and Supracapsulin. Incidentally, the term used in the present account is epinephrine, as listed in the United States Pharmacopeia. The widely used term, adrenaline, is listed in the British Pharmacopoeia.

Ernst Joseph Friedmann (1877-1956) published the chemical formula of epinephrine in 190636. An account of Friedmann's interesting life is given by Mitchell79. One of the first schemes of the biosynthetic pathway for epinephrine was proposed by Walter L. Halle, also in 190645. It is interesting to note that Halle's scheme differs very little from that proposed by Hermann Karl Felix Blaschko (1900- ) in 19399. Blaschko's pathway was proven to be correct in 1957 by McChesney Goodall (1916- ) and Norman Kirshner (1923- )41. An account of how the enzymes of the pathway (tyrosine hydroxylase, dopa decarboxylase, dopamine ß-hydroxylase, and phenylethanolamine N-methyl transferase) were discovered is given by Hingerty and O'Boyle52.

The discovery of norepinephrine (noradrenaline), the biosynthetic precursor of epinephrine and a bioactive compound in its own right, as a secretary product of the adrenal medulla was a particularly important break-through. To be sure, there were many scientists involved in the elucidation of the role of norepinephrine in the sympathetic nervous system, but the most prominent of these was Ulf Svante von Euler (1905-1983). His contributions to the field of pharmacology were recognized with the Nobel Prize in 1970. As an example, in 1949 von Euler was among the first to demonstrate that norepinephrine is a constituent of the adrenal medulla110. This report was followed in 5 weeks by an independent report by Marcel Goldenberg and colleagues who reached the same conclusion39. However, the first report of norepinephrine in the adrenal medulla was made some months earlier by Peter Holtz (1902-1970)53. It is of interest to note that these reports came just a few years after Cannon's death. Cannon would have loved to have heard this news.

The next big breakthrough came just a few years later. Under the light microscope the cytoplasm of adrenal chromaffin cells appears granular due to the presence of numerous organelles, often referred to as chromaffin granules. Chromaffin granules were apparently first noticed by Manasse in 189472. Numerous microscopists made note of the granular cytoplasm in subsequent years, but the definitive study of the adrenal medulla at the fight microscopic level was published in 1941 by Henry Stanley Bennett (1910-1997)8. With remarkable resolution, he demonstrated chromaffin granules that appeared to be ripe for the plucking. And plucked they were. In 1953, two independent laboratories reported the isolation of chromaffin granules from the bovine adrenal medulla. Blaschko and Arnold D. Welch (1908- )12 and Nils-Åke Hillarp (1916-1965) and his colleagues50 submitted papers exactly one month apart. Both groups showed that centrifugation of homogenates of the adrenal medulla led to the sedimentation of the major part of the hormones of this organ, the catecholamines. This was the first time a secretary organelle was isolated. Subsequently, the concept of the chromaffin granule as a distinct hormone-containing component of the cytoplasm also paved the way for the discovery of hormone- and neurotransmitter- containing organelles in other organs. As pointed out by Winkler and Carmichael118, it is not an overstatement to say that the first-discovered secretary organelle remains the best characterized and has often served as a "model" organelle. (For further discussion, including 600 references to more recent work, see118).

Shortly after the isolation of chromaffin granules using biochemical techniques, the electron microscope was used in this area. The first relevant application of the electron microscope was by Hillarp and his colleagues49 who examined smears of crushed medullary cells in 1954. The first ultrastructural study of the adrenal medulla was published by Jeffrey Darcy Lever(1923- ) in 195566. In 1956, Fritiof Stig Sjöstrand (1912- ) and Rudolph Wetzstein (1916- )97 were the first to use the term "chromaffin granule" to describe the catecholamine- containing organelle of the adrenal chromaffin cell. Although this organelle has been clearly shown to be a membrane-bound cavity, a structure properly referred to as a vesicle, Sjöstrand still feels the term granule is preferable (personal communication). The terms "chromaffin granule" and "chromaffin vesicle" are used interchangeably in the literature.

Another important contributor to our understanding of the morphology of the adrenal chromaffin cell has been Rex Ernest Coupland (1924- ). His 1965 book on the natural history of the chromaffin cell24 has remained a classic although, alas, it has been out of print for several years. Since these first electron microscopic studies, the ultrastructure of the adrenal medulla has been described in more than 50 animals (for references, see16). The adrenal medulla has also been studied with both the high-voltage (1000 KV)17 and the scanning electron microscopes19.

Up until the work of the mid-1950's, the centuries of work on the adrenal glands and the adrenal medulla had been directed toward the anatomy and physiology of the entities themselves. From the 50's onward, the emphasis shifted from a study of adrenal medullary function per se to an outlook where the adrenal chromaffin cell was regarded as a model neuron in general, a model sympathetic neuron in particular. This view is held to the present day, and is sure to direct research on the adrenal medulla in the future.

There are several examples of how the adrenal medulla has served as a neurobiological model. One is the phenomenon of the uptake of compounds into cytoplasmic vesicles, whether they be chromaffin vesicles, synaptic vesicles, or another organelle such as the lysosome. It was noticed in 1962 that chromaffin granules can take up epinephrine from a dilute solution against a concentration gradient provided that the medium contains adenosine triphosphate (ATP) and magnesium ions14,58, that is, the uptake of epinephrine into chromaffin granules is an active process. It was shown a short time later by Peter Banks(1937- ) that a magnesium-activated ATPase is associated with the chromaffin vesicle, suggesting the uptake of catecholamines is linked to the hydrolysis of ATP by this enzyme5. A decade was to pass before this link was elucidated. George Karoly Radda (1936- ), David Lars Njus (1948- ) and their colleagues82, and independently, Robert Gahagen Johnson (1952- ) and Antonio Scarpa (1942- ) demonstrated this to be a coupled transport system. More specifically, the ATPase actively drives protons into the chromaffin vesicle and the proton gradient (creating pH and electrical gradients across the vesicle membrane) in turn drives catecholamine uptake (see Njus and Radda82 for details). This was the first demonstration of a chemiosmotic mechanism operating in any mammalian organelle other than the mitochondrion. The usefulness of the adrenal medulla in the discovery of this important mechanism (it has since been shown to operate in several other systems) led Njus et al.81 to make the following whimsical proposal about "the legacy of the adrenal medulla": "Often the unrelenting pressure of evolution creates features so exaggerated they seem intended more for the edification of biologists than for the survival of the organism. The adrenal medulla is a good example." While this may not be literally true, it certainly makes good press!

Another example of the value of the adrenal medulla in neurobiology has been studies on the composition of the chromaffin vesicle since this serves as a model neurotransmitter-containing vesicle. The pioneering work of Hillarp in 1959 demonstrated that proteins and nucleotides are contained within the chromaffin vesicle48,51. In 1966, Karen Blaauw Helle (1934- ) published the first attempt to characterize the soluble proteins of chromaffin vesicles in some detail, and also described the first preparation of antibodies against the soluble protein of chromaffin vesicles. These antibodies were used for the first demonstration of exocytosis by Banks and Helle in 19656. Within a year, Blaschko's group had given the name "chromogranin" to a protein within chromaffin vesicles and had shown that it was secreted from the adrenal medulla11, although the name "chromogranin" was first suggested by Helle (personal communication). Anthony David Smith (1938- ) and Hans Christoph Winkler (1939- ), working in Blaschko's laboratory, and, independently, Kirshner, purified chromogranin A (as it is now called)99,100. Impressive progress has been made in the last 20 years in our understanding of the composition of the chromaffin vesicle, but important questions remain about the structure of the vesicle and the "cocktail" that it secretes (see Winkler et al.118 for review).

Studies on the proteins of the chromaffin vesicle have more recently been extended to enkephalins. Since the discovery of enkephalins in the pig brain in 1975, there has been an intense interest in these and related peptides. Enkephalin-like reactivity was first demonstrated in the adrenal medulla by Tomas G.M. Hökfelt (1940- ) and his colleagues92. Within a year, studies in the laboratories of Osvaldo Humberto Viveros (1937-), Sidney Udenfriend (1918- ), and Erminio Costa (1924- ) demonstrated that enkephalins and/or opioid peptides were localized in the adrenal medulla, specifically within chromaffin vesicles23,68,109. Work in these and other laboratories showed that the adrenal medulla is one of the richest sources of enkephalins. Therefore the adrenal medulla has been used extensively in studies on opioid peptides. These include the virtually simultaneous publication of the sequence of the preproenkephalin molecule by three independent groups (Noda et al.83, Gubler et al.44, Comb et al.22). All three groups used the adrenal medulla as the source for material. The adrenal medulla continues to be used extensively for studies on peptide synthesis and processing.

The utilization of adrenal chromaffin cells in neurobiology has been increased by the use of isolated cells. The first successful move in this direction was the discovery by Arthur Steven Tischler (1946- ) and Lloyd Asher Greene (1944- ) of a culturable form of the rat pheochromocytoma cell. This cell line (referred to as PCI2 cells) has proven to be very useful and popular (see43 for references). A recent technique that has helped make the adrenal medulla even more valuable for studying peptides, receptors, and many other aspects of neurobiology was the development of methods for isolating and culturing adrenal chromaffin cells. Among the many people who pioneered these techniques are Jack Carlton Brooks (1941- ), Allan Stanford Schneider (1940- ), Robert Louis Perlman (1938- ) and Bruce Grayson Livett (1943- ) (see Livett69 for details and references). Jack Brooks is known to his friends as "Smilin' Jack"

and he's always a favorite of the ladies! Adrenal chromaffin cells are now being cultured around the world for use in an extensive variety of studies.

Just one more example of current studies on chromaffin cells. In the 1960's, William Wilton Douglas (1922- ) and his colleagues determined that calcium ions are the only ion required for the secretion of catecholamines from the adrenal medulla, although the dependence on calcium had been demonstrated earlier (by Houssay and Molinelli55). Douglas compared this to excitation-contraction coupling in muscle and coined the term "stimulus-secretion coupling" in 196829. The precise role of calcium in this process was not understood then, and it is not understood now (for review, see Baker and Knight4). Measurements of calcium levels in chromaffin cells, using numerous techniques, have been made by many investigators. Many intracellular proteins and other compounds have been implicated; but we still do not know how a rise in cellular calcium levels leads to the release of the chromaffin vesicle contents at the cell surface (a process known as exocytosis). When the answer comes, it will probably be with studies using isolated chromaffin cells.

Two unrelated observations have led to the answer of an old question about the adrenal gland. The first of these observations was made by Richard Jay Wurtman (1936- ) and Julius Axelrod (1912- ) (who won the Nobel Prize along with von Euler in 1970). In 1965, they found that adrenal cortical steroids enhance the conversion of norepinephrine to epinephrine119. Within the past year, it has also been shown that cortical steroids have a direct effect on the peptides (LaGamma and Adler62), including enkephalins (Yoburn et al.120), chromogranins and other proteins (Sietzen et al.95).

The second observation, made independently by Klaus Unsicker (1942- ), Jose-Maria Trifaró (1936- ), their colleagues and others, was that when adrenal chromaffin cells are grown in culture they extend neurite-like processes104,105. This growth is inhibited by steroids in the culture medium. Work in several laboratories has shown that the phenotypic expression of adrenal chromaffin cells is influenced by glucocorticoids (for example, Anderson and Axel1).

With these and related studies in a number of laboratories, a longstanding question has satisfactorily been answered (although additional answers are certain to surface in the future). The question had been: Why are the adrenal glands composed of two organs, the medulla and cortex, that are distinct in their morphology, physiology, embryology, etc.? The two current answers are: first, the cortical hormones influence the activity of the enzyme that converts norepinephrine to epinephrine (phenylethanolamine N-methyltransferase), creating the situation whereby the medulla secretes the generally more active catecholamine into the bloodstream. In a related fashion, the cortex appears to influence the peptide and protein content of the adrenal medulla. Second, cortical hormones also influence the shape of the adrenal chromaffin cells, somehow preventing them from extending processes, as do other postganglionic sympathetic neurons.

For many of us working with the adrenal medulla, the adrenal chromaffin cell is considered as a model neuron and, from the hints offered above, with good reason (for a more general review, see20). More recently, the adrenal chromaffin cell has been considered as a substitute neuron in the human brain! In 1982, parts of the adrenal medulla were transplanted autologously into the brain of a patient with severe Parkinson's disease (Backlund et al.3). It has been shown more recently that, in selected patients, this may be the first effective therapy for this common disease (Madrazo et al.70).

1552 Eustachius completes plates illustrating adrenal glands.
1611 Bartholin describes hollow adrenal glands.
1655 Riolan asserts adrenal glands not hollow.
1656 Wharton associates adrenal glands with nerve plexus.
1805 Cuvier establishes that adrenal glands are solid.
1831 Arnold studies embryology of adrenal glands.
1836 Nagel names adrenal medulla.
1852 Kölliker describes microscopic anatomy of adrenal medulla.
1856 Histochemical studies of adrenal medulla begun.
1886 Fränkel describes tumor of adrenal medulla.
1892 Jacobj shows adrenal medulla is an endocrine organ.
1894 Oliver and Schäfer describe pressor effect of adrenal medulla.
1896 Cybulski and Szymonowicz also describe pressor effect.
1901 Epinephrine discovered independently by Abel and Takamine.
1902 Kohn names chromaffin cells.
1906 Friedmann publishes chemical formula of epinephrine.
1926 First surgical removal of pheochromocytoma (Roux and Mayo).
1939 Blaschko determines biosynthetic pathway of epinephrine.
1948 Holtz detects norepinephrine in adrenal gland.
1953 Chromaffin vesicles isolated.
1955 Lever describes ultrastructure of adrenal medulla.
1968 Douglas coins terms "stimulus-secretion coupling."
1979 Enkephalin measured in adrenal medulla.
1982 Adrenal medulla transplanted into human brain.

Above is an outline of some of the developments that relate to the history of the adrenal medulla. It is of interest to note that the adrenal glands have been recognized for just over four hundred years. For almost half of that period, the glands were thought to be hollow! Since this is an article on history, it is not pertinent to discuss all of the recent and current studies on the adrenal medulla. Although many have been mentioned above, there are other areas of current activity. These include investigations on: mechanisms of exocytosis; the structure and function of catecholamine-synthesizing enzymes; the variety of receptors on chromaffin cells; the regulation of the adrenal medulla by the central nervous system; the role of the adrenal medulla in the metabolism of the organism; and the diagnosis and clinical treatment of tumors of the adrenal medulla (mainly pheochromocytomas) (see15 for references).

There are many more of our contemporaries who have made and are making important contributions to the history of the adrenal medulla. It is impossible to list them all here. By way of illustration, it is documented that over 2,500 scientists authored or co-authored articles relating to the adrenal medulla in the years 1983-1985 alone!15 As large and as talented as this field is, one worker stands out for having made consistently important and accurate discoveries, in addition to calling the attention of the scientific community to the value of the adrenal chromaffin cell as a neurobiological model. This acknowledged leader in the field is Hans Winkler. Winkler has been involved in most of the significant work relating to the adrenal medulla for the last 30 years. Furthermore, he has shown a knack for analyzing the literature and trends, often pointing the way for others. For example, commentaries he has published in Neuroscience on the composition of the chromaffin vesicle in 1976114 and the biogenesis of the chromaffin vesicle in 1977115 are already regarded as classics. His 1976 article has been cited more frequently than any other article in the journal Neuroscience116. In his 1977 article in this journal, he made several predictions regarding the origin and fate of chromaffin vesicles; essentially they have all been shown to be true. Winkler and his colleagues continue to be major contributors to our understanding of the composition and life cycle of the chromaffin vesicle, as well as the biological significance of the adrenal chromaffin cell117. There can be no doubt that Hans Winkler has done more than anyone in promoting interest in the adrenal medulla! And when he drills you with his famous, questioning stare you won't forget it...

It is tempting, but impossible, to tell the future history of the adrenal medulla. While some have expressed the feeling that we are nearing the end of important work with this system, that the relatively simple adrenal chromaffin cell will soon yield all of its secrets, I disagree. As with all good science, as the important questions of today (and only a few general ones have been alluded to in this article) are answered, these discoveries will themselves pose new questions. The adrenal medulla will continue to be a useful model for study. The history of the adrenal medulla will continue to be an important part of the history of neuroscience.

The author thanks Miss Ruth Mann who, as History of Medicine Librarian at Mayo Clinic, urged the author to pursue this project. I also thank Mrs. Nancy Rucker- Johnson, History of Medicine Librarian, who has tracked down numerous articles and bits of information. Also, Mr. Robert Spinner, one of my students, has been invaluable in translating from Latin and Greek. Finally, I thank my colleagues who have supplied information and/or have reviewed the manuscript in its formative stages, including Stephen Briimijoin, Jack Brooks, Frank Mann, David Njus, Steve Sommer, Susan Stoddard, Jon van Heerden, Richard Welbourn, and Hans Winkler.

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