Paper - The early embryological development of the fetal and permanent adrenal cortex in man (1940)
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Uotila UU. The early embryological development of the fetal and permanent adrenal cortex in man. (1940) Amer. J Anat. 6:259–390.
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The Early Embryological Development of the Fetal and Permanent Adrenal Cortex in Man
Unto U. Uotila
Department Of Anatomy, Harvard Medical School, Boston, Massachusetts
Fellow of the Rockefeller Foundation.
Four Plates (Eleven figures)
This paper presents observations on the development of the human adrenal cortex from its inception until the embryo has grown to a length of 20 mm. Special attention has been given to the genesis of the permanent (true) cortex and its relationship to the fetal cortex.
It is well known that during embryonic, fetal, and neonatal life the cortex of the human adrena.l consists of cells of two histologically distinct types. Keene a11d Hewer (’27) refer to the tissue made up of one of these cell types, which degenerates soon after birth, as the ‘fetal’ cortex, and to the other group of cells, which persists in the adult adrenal, as the ‘true’ cortex.
In this paper the term ‘permanent cortex’ has been substituted for Keene and Hewer’s term ‘true cortex.’ The terms ‘fetal’ and ‘permanent’ cortex are preferable in the light of the development and subsequent fate of the two types of cells. The zonal terminology customarily applied to descriptions of the adult adrenal cannot be applied to embryonic and early fetal adrenals, since the definitive zoning, as seen iii the adult, does not develop until late fetal or early postnatal life.
The development of the human fetal cortex is fairly well known, though descriptions of its origin vary to some extent according to different methods of fixation, or to variation in measurement of the embryo’s length arising from different degrees of embryonic ﬂexure. Proliferating cell nests in contact with the coelomic epithelium were found at the site of the future adrenal gland in 6-mm. human embryos by Soulié (’03), at 6.5 mm. by Zuckerkandl (’12), at 7 mm. by Kolmo (’25), and at 8 mm. by Politzer (’36). On the basis of Soulié’s observations, Poll (’05) estimated that the adrenal primordium is formed at the beginning of the fourth week. According to most authors, the cell mass proliferates and loses its contact with the coelomic epithelium by the time the embryo has grown to a length of 8 to 8.5 mm. After this, differentiation and proliferation continue, the glands acquire a connective tissue capsule, and capillaries appear among the masses of fetal cortical cells. These cells are large, eosinophilic, and homogeneous, in contrast to the cells of the permanent cortex, which are small, basophilic, vacuolated, and possess a darkly—staini11g nucleus (Elliott and Armour, ’11; Hammar, ’25; Hett, ’25; Kohno, ’25; Keene and Hewer, ’27).
Descriptions of the early appearance of the permanent cortex are also variable. Kohno (’25) found the progenitors of these cells in the subcapsular region of an embryo of 14 mm., Hammar (’25) at 15-16 mm., Hett (’25) at 19.6 mm., and Soulié (’03) at 24 mm. By the 30-mm. stage this cell type is recognized as a regular feature of the adrenal gland by most authors. It is generally assumed that the cells of the permanent cortex develop from those of the fetal cortex.
In contrast to this point of view, Keene and Hewer (’27), from study of a small series of human embryos, suggest that the permanent cortex does not arise from the fetal cortex, but has its own separate origin. In a 12-mm. embryo they describe an ‘epithelial cap’ applied to the ventral surface of the fetal cortical mass. This epithelial cap is apparently derived from coelomic epithelium, a11d Keene and Hewer suggest that cells from this cap may encircle the entire fetal cortex from the ve11tral surface, by pushing their way around the gland between the connective tissue capsule and the fetal cortex. They regard this ‘epithelial cap’ as the probable primordium of the ‘true,’ or permanent cortex. However, the number of embryos at the proper stage for a critical study of this process (11-15 mm.) was so limited (onc embryo at 12 mm., described in detail, an unrecorded number at 15 and 16 mm., descriptions of which are not given) that, as Keene and Hewer admit, no final conclusions on the development of the permanent cortex could be made.
It is evident consequently that there are still a number of gaps in our knowledge of the development of the permanent cortex. The present paper is based upon a study of a more complete series of human embryos than has been heretofore available. An examination of many embryos up to 20 mm. in length, covering every stage of development after the first appearance of the adrenal primordium, makes it possible to round out in greater detail the main features of early adrenal development. Later stages, after 20 mm., have been well covered by a number of investigators (Hett, ’25; Kohno, ’25; Keene and Hewer, ’27).
Material and Methods
Serial sections of seventy«nine embryos ranging from 4.0 mm. to 20.0 mm. were studied. Forty-three of these were from the Harvard Embryological Collection in the Department of Anatomy, Harvard Medical School, and thirty-six were from the Carnegie Laboratory of Embryology in Baltimore, Maryland. In addition, through the kindness of Dr. George L. Streeter, it was possible to study three embryos (8.5 mm., 12.8 mm., and 22 mm.) of the rhesus monkey (Macaca mulatta). This monkey material proved to be very valuable in clarifying the origin of the permanent cortex.
Most of the embryos were fixed in formol or in Bouin’s ﬂuid, though a few were fixed in sublimate solution. The stains most commonly used were alum cochineal and orange G, borax carmine and Lyons blue, and hematoxylin and eosin.
A. The formation. of the fetal cortex
The human adrenal primordium develops at a site medial to the urogenital ridge and lateral to the mesogastrium, iii the trough of the adrenal groove. On tl1e medial side of the adrenal groove is the adrenal ridge, which plays no part in the development of the adrenal glands.
In an embryo of 4 to 6 mm. the coelomic epithelium at the site of the future adrenal primordium is cuboidal or columnar, and displays frequent mitotic figures. As the embryo grows to a length of 6.5 to 7.0 mm., the coelomic epithelium becomes more columnar, and begins to show evidence of forming several layers (fig. 1).
Figure 2 represents the earliest adrenal primordium found in this series. It is from an 8.0—mm. embryo (C.E.C. No. Template:CS2065) corresponding in development to embryo 24 (fig. X1) in Keibel-Elze’s N01-mentafeln and estimated by them to be about 21 days old. The adrenal primordium is less differentiated in a 7.2-mm. embryo (H.E.C. Template:HEC2745) which resembles no. 25 (fig. XII in the Normentafeln), with an apparent age of about 25 days. No adrenal primordium can be seen in embryos corresponding to figure X or to stages between figures X—XI in Keibel-Elze’s tables. Hence it is evident that the adrenal primordium first forms at the end of the third or at the beginning of the fourth week.
By this time the primordial cells have nuclei similar in size to those of the mesothelium, but larger than those of the adjacent mesenchymal cells. The primordium is in direct contact with the mesothelium (fig. 2). As the gland grows (embryos of 8 and 9 mm.), the caudal part differentiates more rapidly than the cranial portion of the gland. As the caudal half enlarges, it gradually loses its contact with the mesothelium and becomes embedded in the neighboring mesenchymal tissue. The cranial portion becomes similarly separated from the coelomic mesothelium at a slightly older stage. An embryo of 9.4 mm. (H.E.C. No. 1005, corresponding to embryo 41, fig. XVII of Keibel—Elze, with an age estimated at 30-38 days) shows the cranial portion of the gland still to be in contact with the mesothelium, while in another embryo of 9.4 mm. (H.E.C. No. 529) a thin layer of mesenchymal cells definitely intervenes between the glandular epithelium and the mesothelium, so that separation of the gland has been completed.
These observations, illustrated by figures 1 and 2, indicate that the primordium of the human fetal cortex forms by the proliferation and differentiation of mesothelial cells and by the migration of these into the underlying mesenchymal tissue. It should be pointed out that the formation of distinct cell buds has not been observed. Once the primordium has formed, it grows rapidly, displaying intense mitotic activity and reaching a considerable size in embryos of 8 to 9 mm. The cells are large, with prominent nuclei (fig. 3). Capilliform blood vessels first appear in the adrenal in human embryos of 9 to 9.5 mm.
After the complete separation of the gland from the mesothelium, differentiation of the fetal cortex continues in embryos of 10 to 12 mm. (figs. 4 and 5). The cells are large, with conspicuous nuclei, and with cytoplasm staining more darkly than that of the neighboring undifferentiated mesenchymal cells. Capilliform blood vessels invade the gland in greater numbers. Sympathetic elements can be seen on the mediodorsal border of the gland in embryos of 11 to 12 mm. (fig. 4), but at this stage they have not yet actually penetrated the primordium of the cortex. A capsule is barely distinguishable up to the 13-mm. stage and remains rather poorly developed until the permanent cortex has differentiated.
B. Formation of the permanent cortex
Though the fetal cortex has completely separated from the coelomic epithelium by the 10—mm. stage, the activity of the coelomic mesothelium does not cease at this time. It continues to undergo mitosis, giving rise to a second proliferation of cells which exhibit characteristics different from those of the fetal cortex and which are destined to form the permanent. adrenal cortex. By the time that the second type of cell arises from the lining of the coelome, the cells of the coelomic epithelium and their nuclei have become distinctly smaller. Hence the cells which give rise to the permanent cortex are smaller and possess less prominent nuclei than those which formed the fetal cortex. By their darker staining cytoplasm and nuclei, these cells can also be distinguished from undifferentiated mesenchymal cells in the neighborhood of the developing adrenal. The initial aggregation of mesothelial cells, destined to become permanent cortical cells, forms on the ve11tral, ventromedial, and ventrolateral surface of the mass of fetal cortex (figs. 4 a11d 5) and corresponds to the group of cells designated by Keene a11d Hewer (’27) as the ‘epithelial cap.’ Since the glandular capsule at this period is but poorly developed, and is practically absent in the region of initial contact between the cap of mesothelial cells and the fetal cortex, the newly formed cells can attach themselves to and penetrate the fetal cortex. In embryos of 12 to 13 mm. the karyokinetic activity in the central portion of the fetal cortical mass seems to have diminished, persisting mainly at the periphery of the fetal cortex. In the mesothelial cells, wl1icl1 are differentiating into permanent cortex, mitotic activity appears 011 the contrary to be intense. This distribution of mitotic figures affords additional evidence that the cells forming the permanent cortex arise independently on the outside and invade the periphery of the fetal cortex. The earliest embryo with definite permanent cortex (a 14-mm. embryo, H. E. C. No. 2156) showed this tissue only along the ventral surface of the gland (fig. 6).
In embryos from 12 to 14 mm. on, a capsule begins to clifferentiate more definitely around the main portion of the fetal cortex. It assumes the appearance of a loose condensation of ﬂattened cells over the surface of the fetal cortex. As the mesothelial derivatives, which become the permanent cortical cells, continue to proliferate and to spread more widely over the fetal cortex, they appear in scattered places to infiltrate or to invade this capsular zone. This infiltration leads to an intermingling of the mesothelial and capsular elements, so that the capsule becomes disorganized and the two types of cells can no longer be clearly told apart. In such areas numerous mitotic figures are present, and beneath such proliferating foci an indefinite transitional zone, or intermediate layer, appears between the capsule and the fetal cortex, composed of cells which cannot be definitely identified either as capsular or permanent cortical cells (fig. 8). Nevertheless, numerous mitotic figures are also seen in this transitional zone, indicating that new cortical tissue is being formed.
It is possible to identify mesothelial cells immediately outside the capsule and in certain places even to trace them into the capsule. In such areas it becomes impossible subsequently to distinguish individually between mesothelial and capsular elements. Consequently one cannot rule out definitely the possibility that mesenchymal cells of the capsule, or cells originating in the vicinity of the capsule, may not also contribute to the formation of the permanent cortex. In all events it seems probable that the permanent cortex arises in the main from the coelomic mesothelium, but that it may receive an addendum of cells from the mesenchymal elements which are associated with the formation of the capsule. It is quite clear, nevertheless, that the cells of the permanent cortex which form initially as a cap on the ventral aspect of the gland, in that region where there is no capsule, are derived directly and probably exclusively from the cells lining the coelome.
Study of several excellently stained, serially sectioned monkey embryos adds some interesting additional information to that derived from the human. In an 8.5—mm. monkey embryo no cortical primordium was found. In a 12.8-mm. monkey embryo, fetal cortex was observed very similar to that seen in 10- to 11—mm. human embryos. A 22-mm. monkey embryo, exceptionally well preserved and stained (with sections alternately stained with hematoxylin and eosin, and with azan), shows the beginning of the formation of the permanent cortex (figs. 10 and 11). In this specimen the capsule is a fairly diffuse layer exhibiting numerous mitotic figures (fig. 11). Under the capsule is a narrow intermediate layer of small cells, with small nuclei which are rounder than those of the capsular cells. The intermediate layer is much better defined in the monkey than in the human embryos of our series. This intermediate layer also displays numerous mitotic figures, and merges with masses of subjacent basophilic cells which have definitely differentiated into permanent cortex. The latter can be seen to advantage in figure 10, as colonies of darkly staining cells located at the periphery of the fetal cortex. In figure 11 the details of these zones are clearly shown. These observations on monkey material substantiate the conclusion that the permanent cortex is not formed by the fetal cortex, but is derived from cells which migrate under or through the capsule.
Mitotic activity in the capsule of human embryos diminishes after the 15- to 16-mm. stage, whereas in the subjacent permanent cortex karyokinesis continues. The permanent cortex therefore grows in size and becomes a. constant feature of embryos of 15 to 16 mm. in length and upward.
Cytology of fetal and permanent cortex. The fetal cortical cells are large from the time of their first differentiation (figs. 3, 5 and 9), and possess granular cytoplasm which later (after 12 mm.) becomes acidophilic. In contrast, the cells of the permanent cortex are smaller, with basophilic cytoplasm which, after the 18-mm. stage, frequently becomes vacuolated and spongy (figs. 9, 10 and 11). Moreover, they possess small darkly-staining nuclei. Another type of cell, of Very large size, with an extremely large, round nucleus has not infrequently been found in both fetal and permanent cortex. Its nature could not be determined.
C. Blood vessels and cortical structure
Capilliform blood vessels first appear in the adrenal gland in embryos of 9 to 9.5 mm. (fig. 3), where they are few and scattered and small in size. They increase in number and diameter as the embryo grows through the 10- and 12—mm. stages (figs. 4 and 5), and by 14 mm. small sinusoidal vessels also can be seen in the fetal cortex (fig. 6). By the 16- to 18-mm. stages capilliform blood vessels are present in the peripheral portion of the gland, while large sinusoidal vessels have developed in the center (fig. 7). A central Vein is present and the core of the fetal cortex has been transformed into a reticular structure by the developing plexus of sinusoidal Vessels. It may be seen by further reference to figures 7 and 9 that the cells of the peripheral part of the gland are assuming a primitive fascicular arrangement, and that this fascicular layer is formed principally of acidophilic fetal cortical cells. At the extreme surface of the gland is a layer of permanent cortical cells without any regular arrangement. The stage at which this primitive zoning of cortical tissue commences varies somewhat, but it usually begins in embryos of 14 to 15 mm. and continues through the 20—mm. stage.
The sympathetic elements. Cords of sympathetic nerve cells, formed by small spindle-shaped, darkly-staining cells, can be seen on the dorsomedial aspect of the fetal cortex in embryos of 11 to 12 mm. (6 weeks). Invasion of the gland by the sympathetic elements has not yet started in the majority of embryos of this size. Invasion becomes more definite and constant in embryos of 13 to 14 mm. (6-7 weeks), and continues through the largest embryos in this series (20 mm.), penetration of the capsule taking place on the medial side of the gland. In sagittal sections it appears as though the ‘migrating’ sympathetic elements, approaching from a dorsocranial quarter, occasionally separated or isolated small pieces of tissue from the caudal pole of the cortex which become accessory adrenal nodules.
The present observations are essentially in agreement with the majority of investigators who derive the primordial human fetal cortex from proliferating coelomic mesothelium in the adrenal groove.
On the contrary, most previous workers have assumed that the permanent cortical cells are formed from the fetal cortex. The observations reported here, in keeping with those of Keene and Hewer (’27), show that this is not the case, but that entirely new and distinctive mesothelial elements give rise to the permanent cortex after the fetal cortex has become differentiated. These new mesothelial elements, proliferating from the coelomic wall, come in contact with the ventral surface of the fetal cortex where there is no investing capsule. From this site the mesothelial cells spread over the surface of the fetal cortex, penetrating the adrenal capsule where the latter has begun to difi‘:'erentiate, and intermingling with its cells. Permanent cortex differentiates gradually on the free surface of the fetal cortex, as well as beneath the capsule where the latter exists. The permanent cortex is apparently derived directly from the proliferating mesothelium along the ventral border of the gland a11d, elsewhere, from mesothelial cells which have migrated through the capsule to occupy a subcapsular position. In the latter regions it is possible that the permanent cortex receives cellular components directly from the mesenchyma forming the capsule. The weight of evidence indicates, however, that the permanent cortex is derived principally froni mesothelial cells.
The later development and fate of the fetal and permanent cortices, in human embryos larger than 20 mm., are well described by Hett (’25) and by Keene and Hewer (’27). According to then}, the permanent cortex slowly increases in size throughout intrauterine life, though in a fetus at term it forms scarcely more than a quarter to a third of the cortex. It would hence appear that no great changes occur iii the relative sizes of fetal and permanent cortex between the 20-mm. stage and birth. The degeneration of the fetal cortex starts during the last 10 weeks of intrauterine life (Keene and Hewer, ’27) and is completed by the end of the first year. According to Farber (’39), degeneration is usually complete by the age of 6 or 7 months. Meanwhile the permanent cortex increases in size, replacing the fetal cortex. A zona glomerulosa is seen shortly before term (Hett, ’25), or at the end of the first postnatal month (Peter, ’38) ; the zona fasciculata is defined by the third postnatal week (Keene and Hewer, ’27), and the zona reticularis by 3%; months (Keene and Hewer, ’27) or by the sixth month (Peter, ’38).
Lipins have been reported first to appear in the adrenals in 23-mm. embryos (Hett, ’25) ; at 25 mm. (I-Iammar, ’25); at 50 mm. (Broman, ’11); and at 200 mm. (Keene and Hewer, ’27). Although during fetal life the permanent cortex and to a lesser extent the fetal cortex contain lipins, at term only the permanent cortex is lipin-bearing (Starkel and VVegrzynowski, ’10; Keene and Hewer, ’27).
The function of the permanent cortex in postnatal life is to produce cortical hormone. Nothing definite is known concerning the function of the fetal cortex. The fetal cortex is formed before the permanent cortex; it forms the bulk of the fetal adrenal; it is highly vascular, persists as a well—developed organ throughout intrauterine life, but degenerates soon after birth. This suggests strongly that it serves some important function in the physiology of the embryo and fetus, and is not a mere phylogenetic relic. The fetal cortex would seem to belong to that group of prenatal structures which Streeter (’38) would regard as ‘temporary devices’ with which some ‘particular needs are met.’
It might be borne in mind that the primordium of the fetal cortex develops at the same time (5-7 mm.) _as, and in close proximity to, the primordia of the medullary tubules of the indifferent gonads. These tubules form the male part of the ambisexual gonad. On the other hand, the permanent cortex develops later (11—14 mm.) from smaller mesothelial cells with smaller nuclei, and these cells are probably more differentiated than those of the 5— to 7-mm. embryos. One cannot say whether these features are merely coincidental or have a bearing on the assumed ‘androgenic’ function of the fetal cortex (Grroll— man, ’36).
- The fetal cortex of the human adrenal develops from columnar mesothelial cells in the adrenal groove which proliferate at 20 to 25 days (7-8 mm.), forming a mass of cells which soon separates from the coelomic epithelium (9—9.5 mm.) and becomes differentiated into large, acidophilic cells.
- The permanent cortex forms somewhat later from a further proliferation of mesothelial cells in the same area at the age of 6—6% weeks (11-13 mm.), forming at mass of cells on the ventral surface of the fetal cortex. These cells are smaller than those giving rise to the fetal cortex and have smaller nuclei. They come into direct contact with the ventral portions of the fetal cortex where there is no capsule. Here they continue to multiply and spread along tl1e surface of the gland. In regions where the capsule has begun to develop, these mesothelial elements penetrate the capsule in places, i11tei'111i11gli11;: with the capsular cells so that the two become indistinguishable. Subjacent to the capsule in such areas, islets and subsequently a layer of small basophilic cells collect which become the permanent cortex. These cells are small and basophilic in contrast to the large, acidophilic elements of the fetal cortex. The cells of the permanent and fetal cortex are cytologic-ally distinct from the time of their earliest differentiation.
- Capilliform blood Vessels tirst appear in the fetal cortex at 9 mm., and rapidly increase in complexity and number, forming a network of sinusoidal Vessels in embryos of about 1-1 mm. By the 16- to 18-min. stages capilliform blood vessels are seen in the periphery of the gfland, whereas more centrally large sinusoidal Vessels are observed which join to form a central vein. A primitive zonal arrangement begins to appear in the 16- to 18-mm. stages, consisting of reticular and fascicula r layers formed by the fetal cortex. On the periphery there is 1 layer of permanent cortex which shows no special subdivisions.
- The capsule is very poorly developed until about 13 min., after which it becomes more definite.
- Sympathetic elements appear on the mediodorsal side of the primitive cortex in 11- to 12-mm. embryos, but invasion of the cortex by sympathetic cells does not occur until the 13- to 14-min. stage.
The author wishes to express his gratitude to Dr. George L. Streeter for his kindness in permitting him to examine the human and monkey material in the Embryological Collection of the Carnegie lnstitution. The author is also grateful to Dr. George B. Wisloeki for his kindness and criticisms during the work, and to l)r. H. Stanley Bennett who corrected the English.
BROMAN, T. 1911 Normale und abnorme Entwicklung des Menschen. Bergmann. Wiesbaden.
ELLIOTT, T. R., AND R. G. ARMOUR 1911 The development of the cortex in the human suprarenal gland, and its condition in hemicephaly. J. Path. and Bact., vol. 15, pp. 481-489.
FARBER, S. 1939 Unpublished data.
GROLLMAN, A. 1936 The Adrenals. Williams and Wilkins. Baltimore.
HAMMAR, J. A. 1925 A quelle époque de la vie foetale de l’homme apparaissent les premieres signes d’une activité endocrine. Upsala Lakarefor. Forhand1., N. F., Bd. 30, S. 375-480.
HETT, J. 1925 Ein Beitrag zur Histogenese der menschlichen Nebenniere. Zeit. f. mikr.-anat. Forsch., Bd. 3, S. 179-282.
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1 Transverse section through the adrenal groove of a 6.7—mm. (under 21 days) human embryo (H.E.C. No. 2285). The adrenal groove is situated medially to the. urogenital ridge and the posterior cardinal vein. It lies lateral to the mesogastrium and tl1e adrenal ridge. The aorta can be seen in the mid—line at the top. The columnar mesothelium in the trough of the adrenal groove shows mitotic figures, a11d the resulting cells seem to be pushing their way into the neighboring loose mesenchyma. F01-mol, eoehineal and orange G. X 180.
2 Transverse section through the adrenal primordium of an 8-mm. (about 21 days) human embryo (H.E.C. No. 2065). The mesothelium is still columnar and shows mitotic figures. The mass of cells constituting the adrenal primordium is in Contact with the Inesothelium. The primordial cells are beginning to assume the appearance of fetal cortex being quite large and possessing prominent round or oval nuclei. No blood vessels have yet appeared. The pronephros can be seen on each side, the aorta dorsally. Formol, coehineal and orange G. X 150.
3 Transverse section through the right adrenal primordium of a 9.2—mm. (30-38 (lays) human embryo (H.E.C. No. 2301). At this particular place the fetal cortex has already separated from the mesothelium which still exhibits mitotic figures. Note the large, relatively closely set fetal cortical cells. A few eapilliform blood vessels are now beginning to appear. Pronephros in left corner, aorta to the right. Formol, eoehineal and orange G. X 360.
4 Transverse section through the fetal adrenal of an 11-mm. (about 6 weeks) human embryo (H.E.C. No. 216). The gland is now composed of a we1l-differcntinted mass of fetal cortical cells. The mesothelium lining the coelome has lost its columnar appearance but is still proliferating, and is now giving rise to a new type of smaller cell. These cells—the primordial cells of the permanent cortex— are forming along the ventral surface of the previously differentiated fetal cortex. They differ from the cells of the fetal cortex by virtue of the fact that their nuclei are distinctly smaller. The cortical capilliform blood vessels are increasing in number and size and sympathetic elements are approaching the dorsomedial surface of the gland. The aorta is seen in the middle, the posterior cardinal vein, proncphros and genital ridge on each side. Formol, cochineal and orange G. X 115.
5 Transverse section through the adrenal cortex of a 12-mm. (about 6-6.5 weeks) human embryo (H.E.C. No. 2158). The cells arising from the coclomic rnesotheliuxn and forming the primordium of the permanent cortex are increasing in number and are slowly spreading over the ventral, ventromedial and ventrolateral surface of the central mass of fetal cortex. Note the distinct difference between the primordial cells of the permanent cortex and the fetal cortical cells; the former have smaller nuclei and a more basophilic cytoplasm. The capilliform blood vessels are increasing in number. Formol, cochineal and orange G. X 200.
6 A sagittal section through the adrenal gland of a 14.1-mm. (about 6.5 weeks) human embryo (H.E.C. No. 2156). A zonal permanent cortex has formed along the entire ventral border of the gland (lower border of figure). The permanent cortical cells are smaller and stain more darkly than the fetal cortical cells (upper portion of figure). Sinusoidal capillaries have become numerous. Formol, cochineal and orange G. X 115.
7 Sagittal section through the adrenal gland of a 17—mm. (about 7 weeks) human embryo (C.E.C. No. 576). The acidophilic fetal cortex and basophilie permanent cortex have become differentiated. The peripheral part of the fetal cortex is assuming a fascicular arrangement. The central portion possesses a more reticular structure. Notice the abundant, sinusoidal capillaries within the central portion of the gland. A central vein can also be seen. Formol, cochineal and orange G. X 100.
8 Transverse section from the adrenal gland of a 15—mm. human embryo (H.E.C. No. 2051) showing a portion of the capsule. The figure illustrates the abundance of mitoses at this stage in the capsule and in the subjacent cells. Permanent cortical cells cannot be distinguished in this field. Formol, cochineal and orange G. X 225.
9 Higher magnification of the adrenal (17-mm. embryo) shown in figure 7. In the upper half of the figure are the permanent cortical cells which are small, baophilic and somewhat vacuolated. The lower half of the figure is composed mainly of fetal cortical cells which are much larger and acidophilic. Observe also the beginning fascicular arrangement of the cortex. Formol, hematoxylin and eosin. X 250.
10 Transverse section from the adrenal gland of a 22-mm. monkey embryo (C.E.C. No. 504). In the upper part of the figure, under the capsule, there is a narrow intermediate layer of small cells. Subjacent to this layer are dark, bas0philic nests of permanent cortical cells. The bulk of the gland is formed by lightly staining acidophilic fetal cortex. Bouin, hematoxylin and eosin. X 92.
11 Higher magnification of a portion of figure 10. On the right border of the photograph somewhat ﬂattened, loosely arranged cells are visible constituting an ill-defined capsule. On the left border of the figure a typical subcapsular mass of permanent cortical cells is seen. Between the capsule and the mass of permanent cortical cells a band of intermediate cells is apparent which merges by transitional cell types into both capsule and cortical cells. Mitotic figures are present in the capsule and permanent cortex. In the upper left-hand corner the large nuclei and cell bodies of five fetal cortical cells are distinctly visible. Compare this picture with figure 8. In the latter transitional cells and numerous mitoses are also visible, but no ‘well-defined zone or mass of permanent cortical cells has become di1ferentiated as yet. Bouin, hematoxylin and eosin. X 250.
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