The fine structure of the zona glomerulosa and the zona fasciculata of the adrenal cortex of the opossum

The normal fine structure of the zona glomerulosa and the zona fasciculata of the opossum is described, with emphasis on the structure of the smooth surfaced endoplasmic reticulum (SER) and mitochondria. The cells of the zona glomerulosa are characterized by the presence of numerous rod-shaped mitochondria with shelf-like cristae, small amounts of tubular SER and moderate amounts of rough surfaced endoplasmic reticulum. The cells of the zona fasciculata possess large quantities of SER, spherical mitochondria with tubular cristae and a few profiles of rough surfaced reticulum. A Golgi apparatus is present in cells of both zones as are coated vesicles, coated invaginations of the plasmalemma, lysosomes and lipid droplets. It is concluded that the tubular SER seen in electron micrographs corresponds closely to the form of this membrane system found in the living cell. The functional significance of the associations observed among lipid droplets, mitochondria and SER are discussed.     

ULTRASTRUCTURAL ZONATION OF THE HUMAN ADRENAL CORTEX

Electron microscopic observations of sections from adrenal cortex obtained from a twenty-four-year-old male with hypertension were made. In the zona glomerulosa the cells showed a large nuclear-cytoplasmic ratio. The mitochondria had a dark matrix and plate-like inner structure. The agranular endoplasmic reticulum revealed a close association with lipid granules. The Golgi apparatus was prominent and frequently accompanied by centrioles. Free ribosomes were abundant. The zona fasciculata can be subdivided into two layers. In the outer layer the cells are characterized by numerous large lipid granules. Two kinds of mitochondria are discernible; one with a pale matrix and tubulo-vesicular or vesicular inner structure and the other with a dark matrix and tubulo-vesicular or lamelllform cristae. In the inner zona fasciculata and reticularis there are two types of cells besides the dark cells. The type 1 cells have a compact cytoplasm, elliptic or irregularly shaped mitochondria with a dark matrix and tubulo-vesicular or vesicular internal structure. The type 2 cells are large and ovoid, having a clear transparent cytoplasm. The mitochondria are numerous having a clear matrix and almost exclusively vesicular inner elements. Abundance in agranular endoplasmic reticulum, decreased lipid granules, and cytoplasmic incisions are common in both the inner fasciculata and reticularis. In addition the zona reticularis is characterized by increased number of dark cells, numerous pigment bodies, and the appearance of huge mitochondria.
On the basis of these observation were discussed the significance of the organelles in steroid biosynthesis, mode of hormone secretion, and cyto-genesls in the adrenal cortex.     

Fat accumulation in tha adrenal gland: effect of oestradiol benzoate on the zona reticularis of the Mongolian gerbil adrenal gland.

Oestrogen promotes accumulation of abundant lipid droplets in the zona reticularis of the Mongolian gerbil. Lipid was observed in zona reticularis parenchymal cells but droplets were especially numerous in macrophages. Enlargement of lipid droplets was facilitated by an envelopment with lysosomal membranes. Focal areas or entire parenchymal cells may then undergo degeneration and necrosis and be incorporated into large macrophages recognizable by distinct areas of lipid-containing material as well as by small mitochondria with short cristae. Oestrogen may well act directly on zona reticularis cells inasmuch as nafoxidine, an antioestrogen, prevents these changes. Zona fasciculata cells were identical to those in controls and an intermediate region between the zona fasciculata and zona reticularis persists, although concentric whorls of rough endoplasmic reticulum seen in controls were replaced by disorganized, randomly dispersed cisternae of rough endoplasmic reticulum. Oestrogen does not elevate cortisol in the plasma.     

Changes in blood cortisol and aldosterone levels and ultrastructure of the adrenal cortex during hemorrhagic shock

Changes of secretory activity and structure of the adrenal cortex were observed in dogs subjected to hemorrhagic shock. Blood cortisol and aldosterone levels were rapidly increased after hemorrhage. The cortisol level reached a maximum value about 5 times that of normal 1 h after hemorrhage, while aldosterone levels attained maximum levels showing a 10-fold increase over normal, considerably later (at the end of the impending stage) than the cortisol. Cortisol secretion was quickly depleted at the end of critical stage and terminal stage of normovolemic shock, while the aldosterone secretion remained relatively high. Morphologically, sticking of leukocytes to the sinusoidal wall and accumulation of SER around lipid droplets in the fasciculata and reticularis cells became conspicuous 1 h after hemorrhage. At the end of the impending stage, infiltration of leukocytes occurred in the zona fasciculata and reticularis. Irregular masses of fibrin strands appeared in the sinusoidal space and at large gaps of the sinusoidal wall. Degenerative changes such as aggregation of SER, dilatation of endoplasmic reticulum and edema were observed in the cells of zona fasciculata and reticularis, especially of inner half of the former. At the end of the critical stage, small necrotic foci and hemorrhage with inflammatory infiltration were scattered in the inner half of the zona fasciculata. Degeneration of the cells of zona fasciculata and reticularis became more severe in this period, while the cells of zona glomerulosa remained unchanged. At the terminal stage of normovolemic shock after reinfusion, degeneration and necrosis were remarkable in the cells of zona fasciculata and reticularis, especially of the inner half of the zona fasciculata. Moreover, destruction of cordal structure, hemorrhage and inflammatory infiltration were distinct in the zona fasciculata and reticularis, while the zona glomerulosa was relatively well preserved.     

Electron microscopic radioautographic studies on DNA synthesis and the ultrastructure of the adrenal gland in aging mice

The changes in DNA synthesis of adrenal gland cells in aging mice from prenatal day 19 to postnatal 2 years were observed by electron microscopic radioautography. The percentages of labeled cells (labeling indices) in the zona glomerulosa, zona fasciculata and zona reticularis of the cortex and the medulla after³H-thymidine injection were at their maxima during the perinatal stage and then gradually decreased with age. The ultrastructural features of the cells at the late embryonic and early postnatal stages appeared undeveloped. Well-developed cell organelles, such as smooth surfaced endoplasmic reticulum, mitochondria with tubular or vesicular cristae and lipid droplets, were more frequently observed in the cytoplasm of the unlabeled cells than in the labeled cells in the three zones of the cortex. This paper was presented at the 28th Annual Meeting of the Clinical Electron Microscopy Society of Japan, Osaka, October 17–19, 1996.     

The fine structure of testicular interstitial cells in mice

The testicular interstitial cells of mice contain an abundant agranular endoplasmic reticulum occurring as a network of interconnected tubules. Unusual features of the reticulum are occasional extensive whorls of concentric membranes and the occurrence of bundles of parallel double-walled tubules. The mitochondria have tubular cristae and are occasionally very large. In contrast to the interstitial cells of other species that have been described, the mouse interstitial cells show a segregation of the cytoplasm into areas of dense agranular reticulum and other areas where the agranular reticulum is relatively sparse. The latter areas contain scattered cistenae of granular reticulum, many free ribosomes in clusters, mitochondria, lipid droplets and small granules. The mitochondria and lipid droplets are often encircled by cisternae of the reticulum. Biochemical evidence from other laboratories, taken with the present results, indicates that the membranes of the endoplasmic reticulum in mouse interstitial cells are the site of the enzymes that mediate the synthesis of testosterone from progesterone. There is also an indication that cholesterol biosynthesis is associated with the agranular reticulum. The membranes of the agranular reticulum may also serve as a reservoir for the storage of cholesterol.     

Structure and dynamics of adrenal mitochondria following stimulation with corticotropin releasing hormone

The mitochondria of rat adrenals were investigated qualitatively and quantitatively in different functional states of the adrenal cortex. Following stimulation of the animals with corticotropin releasing hormone (CRH), the corticosterone serum levels reached a maximum 1 hour after stimulation with CRH. The amount of inner mitochondrial membrane within the zona fasciculata increased showing a biphasic time course, with a first maximum 2 hours and a second maximum 8 hours after stimulation. In contrast, a significant rise of mitochondrial volume occurred only 24 hours after CRH stimulation. Therefore, the dense vesicularization of mitochondrial cristae may constitute an early process to enhance the steroidogenic capacity of these cells. Within cells of the transition zone between zona glomerulosa and zona fasciculata, we could depict a special type of mitochondria with characteristic crescent-like cristae only seen after stimulation with CRH. This type of mitochondria may represent an intermediate form between mitochondria of zona glomerulosa and zona fasciculata underlining the impressive transformational capacity of adrenocortical mitochondria. After hypophysectomy, zona fasciculata cells contained mitochondria with tubular inner membranes, representing a hypofunctional state. In contrast, the hypofunctional state after hypophysectomy and the hyperfunctional state after stimulation of the adrenal cortex via CRH injection did not appear to correlate with the morphology of mitochondria from the zona reticularis and adrenal medulla.© Willey-Liss, Inc.     

Structure and dynamics of adrenal mitochondria following stimulation with corticotropin releasing hormone.

The mitochondria of rat adrenals were investigated qualitatively and quantitatively in different functional states of the adrenal cortex. Following stimulation of the animals with corticotropin releasing hormone (CRH), the corticosterone serum levels reached a maximum 1 hour after stimulation with CRH. The amount of inner mitochondrial membrane within the zona fasciculata increased showing a biphasic time course, with a first maximum 2 hours and a second maximum 8 hours after stimulation. In contrast, a significant rise of mitochondrial volume occurred only 24 hours after CRH stimulation. Therefore, the dense vesicularization of mitochondrial cristae may constitute an early process to enhance the steroidogenic capacity of these cells. Within cells of the transition zone between zona glomerulosa and zona fasciculata, we could depict a special type of mitochondria with characteristic crescent-like cristae only seen after stimulation with CRH. This type of mitochondria may represent an intermediate form between mitochondria of zona glomerulosa and zona fasciculata underlining the impressive transformational capacity of adrenocortical mitochondria. After hypophysectomy, zona fasciculata cells contained mitochondria with tubular inner membranes, representing a hypofunctional state. In contrast, the hypofunctional state after hypophysectomy and the hyperfunctional state after stimulation of the adrenal cortex via CRH injection did not appear to correlate with the morphology of mitochondria from the zona reticularis and adrenal medulla.     

Ultrastructural types of cell in adrenal cortical adenoma with primary aldosteronism.

By means of electron microscopy, three types of cell were identified in the adrenal cortical adenomas obtained by surgery from three patients with primary aldosteronism. They were zona glomerulosa-type cells, compact cells and clear cells. The former two types of cell were quite similar to those found in the zona glomerulosa and the zona reticularis respectively. The clear cell with large lipid vacuoles possessed mitochondria similar to those found in the outer zona fasciculata. The ultrastructure of the vacuoles was rather complex and their relationship to those in the zona fasciculata cell was not clear. Glycogen particles in the beta-form, which were present in the zona glomerulosa and reticularis cell but not in the zona fasciculata cell, were observed within the cytoplasm of this tumour cell. The clear cells of the tumours were therefore considered to possess features of both the zona glomerulosa and the zona fasciculata cell.     

A correlated thin-section and freeze-fracture analysis of guinea pig adrenocortical cells

Comparison of the fine structural features of guinea pig adrenocortical cells as seen in thin sections with those revealed by freeze-fracture confirms the structural appearance of steroid-secreting cells as interpreted from thin sections and reveals significant new features of the membranous organelles. Smooth-surfaced endoplasmic reticulum appears as a network of tubules, interwoven or in parallel, and as cisternae, fenestrated and non-fenestrated. These elements are tightly packed in the deeper cortical cells, excluding other organelles from their domain. Tubules and fenestrated cisternae possess randomly distributed intramembranous particles on their PF faces, while closely packed non-fenestrated cisternae possess aggregates of particles interspersed with aparticulate regions on their PF faces. These differences in particle distribution suggest functional specialization among the various forms of reticulum. Mitochondria appear as elongated structures of varying shape. Freezefracture reveals that all their cristae have circular origins from the inner membrane. Sinuous tubules, which appear as tubules in section, and straight tubules, which appear as lamellae in section, arise from single sites. Flattened sac-like cristae may have multiple circular origins. Definite contact points seen between inner and outer membranes may facilitate passage of molecules, including steroids, into the mitochondrial compartments. Lysosomes and peroxisomes, which are easily identified in thin sections with the aid of cytochemistry, are difficult to identify with certainty by freezefracture. Single membrane-bound granules of slightly smaller diameter than mitochondria may represent lysosomes. Smaller granules interconnected with the tubular reticulum, as well as dilated regions of this organelle, may represent peroxisomes. Plasma membranes show no indication of tight junctions but do have abundant gap junctions which show a zonal differentiation: small gap junctions throughout the cortex, medium-sized regularly shaped gap junctions in zona fasciculata externa, and large irregular gap junctions in zona fasciculata interna and zona reticularis. The large junctions cover planar areas as well as surfaces of projections of one cell into another. Such junctions may allow passage of ions as well as of low-molecular-weight substances between the cells, facilitating or even amplifying the response to trophic hormone stimulation.     

Fine structure of adrenal adenomata causing Cushing's syndrome

Summary Adenomata taken from nine patients with Cushing's syndrome were observed by electron microscopy. Agranular endoplasmic reticulum was prominently developed in all cases, occasionally fine dotty granules were observed in the tubules of agranular endoplasmic reticulum.Mitochondria showed a wide variety of changes in size, shape and internal structure, and somtimes contained electron dense droplets in their matrix. Mitochondria resembling those in the normal zona fasciculata intermingled with pathologically altered ones, suggesting that the adenomata which caused Cushing's syndrome were derived from the zona fasciculata.Fibrous structures were seen in the cytoplasm in one case. It is proposed that all of the changes in cellular organelles of the adenoma cells are correlated with increased secretory activity.     

Ultrastructural analysis of the adrenal cortex in rats after bilateral subdiaphragmatic vagotomy

Electron-microscopic investigation of the zona fasciculata and zona reticularis of the adrenal cortex in rats showed that 7 days after bilateral subdiaphragmatic vagotomy the perinuclear space of the adrenocorticocytes and endothelial cells is widened, the tubules of the smooth cytoplasmic reticulum are dilated, the mitochondria edematous, their cristae reduced, and the number and size of the lipid droplets diminished. After 45 days some mitochondria were starting to undergo myelinization, lipid droplets were aggregating, and electron-translucent vacuoles appeared in them. Vagotomy depresses the function of the adrenocorticocytes of the zona fasciculata and zona reticularis of the rat adrenal cortex.Department of Histology and Embryology, Faculty of Internal Medicine, N. I. Pirogov Second Moscow Medical Institute. (Presented by Academician of the Academy of Medical Sciences of the USSR A. P. Avtsyn.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 83, No. 4, pp. 492–494, April, 1977.     

Ultrastructure of the corpus luteum of the white-tailed deer during pregnancy

The fine structure of corpora lutea from 14 white-tailed deer was studied from early through near-term pregnancy. The corpora contained both thecal and granulosal lutein cells. The small, elongate and cylindrical thecal lutein cells contained many lipid droplets, juxtanuclear Golgi elements, abundant agranular endoplasmic reticulum, and other cytoplasmic organelles. These cells were observed throughout pregnancy. Some of the thecal lutein cells became modified during early to midpregnancy. The modified thecal lutein cells possessed many lipid droplets, several lysosomes, packets of PAS-positive glycogen granules, and numerous small membranous whorls of agranular endoplasmic reticulum. The granulosal lutein cells had highly folded and/or ruffled plasma membranes, abundant tortuous tubular and cisternal agranular endoplasmic reticulum, numerous rod-shaped to round mitochondria with tubular and lamellar cristae, Golgi elements, and a few packets of granular endoplasmic reticulum. Many granulosal lutein cells became modified during mid- to near-term pregnancy by the addition of numerous small membrane-bound osmiophilic droplets and variable numbers of large nonmembrane-bound lipid droplets. The granulosal and thecal lutein cells appeared more active in steroid biosynthesis than the modified granulosal and thecal lutein cells.     

Degeneration of the rat and canine adrenal cortex caused by alpha- (1,4-dioxido-3-methylquinoxalin-2-yl) -N-methylnitrone (DMNM)

The antibacterial drug alpha- (1,4-dioxido-3-methylquinoxalin-2-yl)-N-methylnitrone (DMNM) given at a dose of 22.5 mg/kg/bid to 4 dogs for 14 days caused diminished adrenal cortical reserves as determined by decreased plasma corticol (3 dogs) and lower aldosterone levels (4 dogs) following the intravenous infusion of ACTH. A dose of 100 mg/kg/day of DMNM administered to rats for 31 or 35 days resulted in significant decreases in blood glucose. Histologically, the adrenal glands of both species treated with DMNM for a maximum period of 21 days (dogs) and 35 days (rats) had widespread granular and vacuolar degeneration of the cortex. The degeneration, as demonstrated in treated rats, began in the zona reticularis and inner regions of the zona fasciculata and eventually involved the entire cortex including the zona glomerulosa. As a result of treatment, significant ultrastructural alterations within cells of the rat and canine adrenal cortex consisted of degeneration of the mitochondria and an increase in the numbers and lipolysis of lipid droplets. The ultrastructure of the zona reticularis and fasciculata was most severely affected.     

The adrenal cortex in spontaneously hypertensive rats. A quantitative ultrastructural study.

The adrenal cortex of spontaneously hypertensive rats (SHR) has been examined by quantitative morphologic techniques for electron microscopy. The volume and surface area of smooth endoplasmic reticulum and the volume of Golgi apparatuses in zona glomerulosa cells of SHR was significantly greater than those of Wistar-Kyoto strain (W/KY) normotensive controls; the volume of lipid droplets and nucleus was significantly less in SHR than in W/KY animals. A stimulation of the zona glomerulosa in SHR may well be attributable to the elevation in systolic blood pressure. A distinct lipid-free subglomerulosa was observed in the adrenal gland of W/KY rats; the cell volume was similar to that of the zona glomerulosa although the cells showed a significantly greater volume of mitochondria and surface area of mitochondrial membranes and greater volume of smooth endoplasmic reticulum and lysosomes. In the zona fasciculata, cell volume, volumes and surface area of mitochondria and smooth endoplasmic reticulum, and volume of lipid droplets were significantly lower in SHR than in W/KY rats. The volume of the Golgi apparatus was greater in SHR than in W/KY rats. Glycogen particles were observed in focal areas of some zona fasciculata cells. The adrenal cortex of another strain of normotensive Wistar rat (W/CFN) was compared with that of the W/KY and SHR. Although the relative adrenal weights of SHR and W/KY animals were identical, the weight of that in W/CFN was significantly smaller. The volume of the zona glomerulosa of SHR was significantly greater than that of W/KY although the volume of the zona glomerulosa in W/CFN was significantly greater than the other two groups. The volume of nucleus and lipid droplets of zona glomerulosa in W/KY was significantly greater than that in the S/CFN; the volume of the cell, mitochondria, smooth endoplasmic reticulum, lipid droplets, and lysosomes, and the surface area of smooth endoplasmic reticulum and mitochondrial membranes of W/KY animals was significantly greater than those of W/CFN animals. It is concluded that the W/CFN rat is not an appropriate control for spontaneously hypertensive rats.     

Crystalloid configurations in the adrenal cortex of the Siamese tree shrew (Tupaia glis)

Crystalloids of what appear to be smooth reticulum have been observed in the zona fasciculata and zona reticularis in both the stressed and nonstressed adrenal gland of the Siamese tree shrew (Tupaia glis). No crystalloids are observed in the zona glomerulosa. Similar crystalloids have been described in other steroid-secreting organs, including the antebrachial organ of the lemur (Lemur catta), the parotoid gland of Bufo alvarius and in sebaceous gland cells of the Galagos and Macaques. Moreover, the crystalloids in the present investigation resemble the paracrystalline arrays of smooth reticulum present in the adrenal cortex of the fetal guinea pig. The crystalloids show much variation in degree of organization, sometimes appearing as wavy tubules parallel with one another or as fused tubules having a “donut” configuration. In addition, the crystalloids are nearly identical to configurations which have been described in mitochondrial cristae of the protozoan, Pelomyxa carolinensis.     

Stereological analysis of the guinea pig adrenal: Effects of dexamethasone and ACTH treatment with emphasis on the inner cortex

This paper describes the morphological responses of adult male guinea pig adrenals to dexamethasone (DEX) and adrenocorticotrophic hormone (ACTH), in both qualitative and quantitative terms. Most organelles and inclusions are affected, but their responses often vary in the different cell types examined: zona fasciculata externa and interna, and zona reticularis. Following DEX the volume of lipid droplets increases in cells of zona fasciculata externa but decreases in zona reticularis; smooth endoplasmic reticulum decreases in fasciculata externa but increases in reticularis. Following ACTH, exactly the opposite occurs. This strongly suggests differing functions for these subcellular entities in each cell type, particularly for the smooth reticulum, as well as for the cells themselves. The volume of the Golgi complex markedly decreases following DEX in all cells but increases only in zona fasciculata interna and zona reticularis following ACTH. These deeper cortical cells are known to secrete at least one sulfated steroid, dehydroepiandrosterone sulfate, and these changes in the Golgi complex strengthen the suggestion that the Golgi plays a role in sulfation of steroids. Mitochondrial volume and number decrease in all cells following DEX, supporting their role in steroidogenesis. Further decreases in their volume, accompanied by increases in their number following ACTH, may be related to a proliferation of mitochondria in response to ACTH. Changes in peroxisome volume and number, following DEX and ACTH, suggest a possible role for these organelles in steroid cell metabolism. Lysosomes decrease in volume in all cells following ACTH. This does not support the recently suggested concept that they play a role in steroid secretion.     

Congenital 17 alpha-hydroxylase deficiency: a clinicopathologic study

The histopathologic features of the adrenal glands in three cases of congenital 17 alpha-hydroxylase deficiency are described in relation to clinical and endocrine findings. Diffuse or nodular adrenocortical hyperplasia, particularly in the zonae fasciculata and reticularis, was observed in all cases examined. The hyperplastic adrenal cortices were composed of cells with morphologic features of hypercorticism and hyperstimulation. Myelolipomatous lesions were detected in two cases. These morphologic findings were consistent with excessive adrenocorticotropic hormone secretion in this disorder. In all the cases examined, the plasma aldosterone concentration was within normal limits, and plasma renin activity was suppressed prior to dexamethasone treatment. Morphologically, however, hyperplasia of the cells with abundant mitochondria and smooth endoplasmic reticulum seemed to involve the zona glomerulosa. Nonencapsulated nests of hypertrophied cortical cells in periadrenal tissue were remarkable in one case. From these morphologic findings, we postulated hyperfunction of the zona glomerulosa as well as involvement of corticosteroids from the zona glomerulosa in the pathophysiology of this disorder.     

Ultrastructural morphometry of normal adrenal cortex and of hyperplastic adrenals in gushing’s disease

Surgical specimens of 4 normal adrenal glands and of 5 hyperplastic ones in Cushing’s disease were studied ultrastructurally. The ultrastructure of the three zones of each adrenal gland was morphometrically and statistically analyzed. Comparing the statistical data of the three zones of the normal gland, the development of smooth endoplasmic reticuium and rough endoplasmic reticuium showed an increase from the outer zona glomerulosa to the inner zona fasciculata and reticularis. Also, the mitochondria were more numerous in the inner zona reticularis than in the outer zones. In Cushing’s disease, the smooth endoplasmic reticuium, the rough endoplasmic reticuium, and the mitochondria were developed to a significantly higher degree than in normal human adrenal glands. The distribution of the cell organelles in the zona fasciculata and zona reticularis is almost alike in normal adrenal glands and in the adrenal cortex in Cushing’s disease, except that the smooth endoplasmic reticuium in the zona reticularis was less extensive than in the zona fasciculata. The volume percentages of lipid vacuoies in the hyperplastic zona fasciculata in Cushing’s disease was strongly and significantly decreased in comparison to normal adrenal glands. Our ultrastructural findings and the statistical data were in accordance with the results from animal experiments with adrenocorticotropic hormone (ACTH) stimulation and confirmed former qualitative ultrastructural findings concerning human adrenal gland changes in ACTH-dependent Cushing’s disease.     

Spontaneous or experimentally induced formation of a special zone in the adrenal cortex of the adult brush-tailed possum (trichosurus vulpecula)

The cytology and ultrastructure of the hypertrophied special zone, which is formed spontaneously in the adrenal cortex of adult female brush-tailed possums (Trichosurus vulpecula), was compared to the adrenocortical tissue in adult males in which the special zone, normally absent, was induced following castration alone or by additional treatment with folliclestimulating hormone (FSH). The special zone in females was situated between the zona fasciculata and the zona reticularis, the latter being a rudimentary zone in this species. Special zone tissue extended as a broad band parallel to and on one side of the adrenal medulla. In the luteal phase of the reproductive cycle, the special zone cells showed ultrastructural features commonly associated with steroidogenic tissues, with many mitochondria and compact masses of smooth endoplasmic reticulum. Cytoplasmic lipid inclusions were rarely observed. In lactating females, however, the special zone cells exhibited cytological and ultrastructural features suggestive of a transformation in their morphology broadly divided into two types of cells: (1) cells at the periphery of the special zone (closest to the zona fasciculata) showed variable quantities of lipid inclusions, mitochondria with dispersed cristae, and segregation of the smooth endoplasmic reticulum into compact masses; (2) cells within the more central regions showed an increasing abundance of lipid inclusions which in many cells became the dominant feature of the cytoplasm. These special zone cells contained very little smooth endoplasmic reticulum and their mitochondria contained few cristae together with amorphous granular material within the matrix. In castrated males, special zone tissue developed between the zona fasciculata and the zona reticularis, appearing initially as focal islands of cells (8 months postcastration) and later (11 months postcastration) expanding into a single zone, probably via the proliferation and differentiation of adjacent cells of the zona fasciculata and longitudinal growth of the special zone. Similar focal aggregations of special zone cells were induced after 14 days of FSH treatment given to 2-month castrated males. In all castrated and FSH-treated castrated males, the ultrastructure of special zone cells was similar to that of special zone cells in luteal-phase female possums. The findings suggest that the formation and cellular composition of the special zone is associated with changes in the pituitary-gonadal axis and that FSH plays a primary role in the differentiation of this tissue.