Development and cytodifferentiation of C cell complexes in dog fetal thyroids

Summary The development of C-cell complexes was investigated in dog fetuses by an immunoperoxidase method with three specific antisera: anti-calcitonin, anti-C-thyroglobulin (C-Tg), and anti-19S thyroglobulin. Ultimobranchial bodies joined with the thyroid anlage and then dispersed into the parenchyma to form large C cell groups. Sparse reaction products of C-Tg initially appeared in C cells with small amounts of cytoplasm. Later at about day 39 of gestation, when the immunoreactivity of calcitonin and 19S thyroglobulin appeared weakly in C cells and follicular cells, C-cell complexes were identified as large cell masses containing numerous undifferentiated cells without no immunoreactivity for any of the antisera. As development proceeded, the undifferentiated cells developed progressively the morphology of C cells. In addition, the undifferentiated cells developed 19S thyroglobulin immunoreactivity, that is, within some of the complexes small clusters of cells filled with material immunoreactive for 19S thyroglobulin. They were not organized into follicles during the fetal period, and were very slow in development. Depending on the degree of development of the undifferentiated cells, several features of the complexes were noted. The present study indicates that not only C cells but also follicular thyroid cells appear to be derived from the ultimobranchial bodies.     

Development of immunoreactive somatostatin in C-cell complexes in the thyroid gland of the dog

Summary In connection with our previous finding that an intense immunoreaction to somatostatin transiently appears in thyroid C cells of the dog during early fetal periods, the present study investigated C-cell complexes in thyroid glands from early fetuses to adults in an attempt to clarify whether the transient appearance of immunoreactivity to somatostatin is dependent on the degree of differentiation of C cells. C-cell complexes retain their fetal characteristics; even in the complexes of postnatal dogs, there are numerous undifferentiated cells, immature C cells and primitive follicular cells, which are not yet organized into follicles. Neither the degree of differentiation of C cells nor that of other constituent elements of the complexes affected the developmental pattern of somatostatin immunoreactivity in C cells. The C cells located in complexes displayed the same pattern of developmental changes in immunoreactivity to somatostatin as the cells in thyroid parenchyma. In the C-cell complexes of early fetal dogs a very intense immunoreactivity for somatostatin was observed; almost all calcitonin-positive cells were also somatostatin positive. The immunoreactivity to somatostatin progressively decreased with age. In the postnatal complexes the number of somatostatin-positive cells was very small compared with that of calcitoninpositive cells.     

Effects of ethanol on the ultrastructure of the hamster thyroid C-cell

The morphology of the thyroid C-cells in golden hamsters after short- and long-term treatment with ethanol was studied. Immunohistochemistry was applied to examine the distribution of the C-cells in the thyroid gland. In the short-term experimental animals, the Golgi complexes and the granular endoplasmic reticulum were well developed and the number of the secretory granules was decreased as compared with those of the control animals. These findings suggest that the cellular activity of the thyroid C-cell is stimulated after short-term treatment with ethanol. The morphology of the thyroid C-cells of the long-term experimental animals was similar to that of the controls. It is conceivable that long-term treatment with ethanol does not affect the function of the C-cell.     

Specific paracrystalline structures of rough endoplasmic reticulum in the follicular (stellate) cells of the dog adenohypophysis

Summary The fine structure of follicular cells of the adenohypophysis was examined in fetal, neonatal, and adult beagle dogs. Prior to birth, undifferentiated follicular cells are common. At birth mature cells that form follicles are routinely encountered. The fine structural appearance of follicular cells is unchanged between birth and adulthood. Follicular cells of puppies and adults are, however, distinguished by the presence of unusual complexes within distended cisternae of the rough endoplasmic reticulum. These complexes vary greatly in morphology, some appear as a maze of interconnecting tubules while others show a highly organized paracrystalline configuration. The presence of these paracrystalloid structures in follicular cells supports the view that they represent a distinct pituitary cell type.Supported by NIH Grants AM19743 and NS12969The authors wish to thank John Patrikes and Helen Mantulin for their expert technical assistance     

Reactivity of monoclonal islet cell antibodies on normal hyperplastic and neoplastic thyroid C-cells

Summary Thyroid C-cell reactivity to 15 monoclonal antibodies raised against a series of pancreatic islet cells (H[human]ISL, B[bovine]ISL and R[rat]ISL) was evaluated using an indirect immunoperoxidase technique on frozen thyroid sections. Of the monoclonal anti-islet cell antibodies, five reacted specifically with bovine C-cells or human hyperplastic and neoplastic C-cells but not with follicular cells. Two monoclonal antibodies of the bovine series showed strong immunoreactivity with C-cells and only a weakly positive immunostaining of follicular cells. Five monoclonal antibodies reacted with both thyroid C-cells and follicular cells, whereas 3 monoclonal anti-islet cell antibodies did not stain any cell type of the thyroid. In human medullary carcinomas, calcitonin- and somatostatin-producing neoplastic cells were immunoreactive with the same monoclonal antibodies as were normal human C-cells. The protein bands identified by the monoclonal antibodies in human medullary carcinomas had the same molecular weight as those from pancreatic islet extracts. Our study demonstrates the presence of similar differentiation antigens on thyroid C-cells and pancreatic islet cells; this further illustrates common modes of differentiation and specialisation of these embryologically different members of the dispersed neuroendocrine system. The crossreactivity of seven of the monoclonal antibodies investigated with follicular epithelium of the thyroid suggests the existence of common antigenic determinants in different endocrine organs and may partly explain the multiple organ autoimmune response found in patients with polyendocrine diseases.     

Influence of development on the number of calcitonin-containing cells in the mouse thyroid.

Calcitonin-containing cells in serial, 6-micrometer sections of the thyroid glands of Swiss Webster mice, at 1 day, 2 weeks, 4 weeks and 8 weeks of age, were demonstrated by an immunoperoxidase method, using antiserum to human calcitonin. C-cell nuclei were counted in every sixth section of both left and right lobes. The average number of C-cells counted in the thyroid glands of 8-week-old animals was 18-fold, 5.5-fold and 2.5-fold greater than the number observed in 1-day, 2-week and 4-week-old animals, respectively. C-cell concentration was found to be greatest in 4-week-old mice. Mitoses of C-cells were observed in animals which were 1 day, 2 weeks and four weeks old. No mitotic figures were seen in 8-week-old animals. A few C-cells were seen in close association with neurons. The volume of the thyroid glands of 8-week-old animals was about 14-, 4- and 3-fold greater than the volume in the 1-day-old, 2-week-old and 4-week-old mice, respectively. These changes in the C-cell population during development provide a model for the study of C-cell proliferation and storage of calcitonin.     

Anomalous occurrence of immunoreactive calcitonin cells in the thymus of the rat

Summary In a study of the effect of pinealectomy on thyroid C-cell number, 8 animals out of 66 were found to have thymic tissue in close association with the thyroid. Cells containing immunoreactive calcitonin were found in all of the thyroids but in only one of the 8 pieces of thymus. These cells found in a piece of thymic tissue associated with the right thyroid lobe were located immediately under the capsule and did not form or associate with follicles. Unlike the other animals the rat with thymic calcitonin cells had an unequal distribution of C-cells between the left and right thyroid lobes, but the total number of thyroidal C-cells was the same as that of the other rats. Since the thymus proper was not examined in these 66 animals, ten additional rats were taken for such a study. Thyroid-associated thymic tissue was found in three of these, but none of these thymi showed any immunoreactive cells.Financial support by the Deutsche Forschungsgemeinschaft (grant Vol 35/7) is gratefully acknowledged     

Effect of fetal age and method of recovery on isolation of preantral follicles from bovine ovaries

The objective of this study was to compare enzymatic and mechanical methods, at distinct fetal ages, on isolation of different developmental stages of preantral follicles from bovine ovaries. Fetal ovaries were obtained from pregnant cattle at 150 to 270 d of gestation, and 135,521 preantral follicles at different stages of development were studied. The dissociation of ovaries with a mechanical procedure resulted in an average of 938.16 prenatral follicles. In contrast, 3,715.56 follicles were obtained when enzymatic digestion was used (P = 0.0001). Histological evaluation confirmed follicular stages and demonstrated that both mechanical and mechanical-enzymatic procedure did not affect the cellular integrity of the follicles. Granulosa cell-oocyte complexes surrounded by a basal membrane, were considered preantral follicles in this study. The ratio of different stages of isolated preantral follicles was significantly (P = 0.0001) correlated to fetal age. The earliest fetal age at which tertiary follicles were identified was at 210 d of gestation. The results confirm previous observation that follicular development and atresia are initiated during fetal development. These data provide information on methodologies to isolate intact bovine preantral follicles for investigating the control and regulation of follicular development and the growth of preantral follicles in vitro.     

Culture of differentiated and undifferentiated type II cells from fetal rat lung

We have developed a relatively simple and reproducible method for the isolation and culture of both differentiated and undifferentiated type II cells from fetal rat lung. The technique involves an initial period of explant culture in serum and hormone free medium, followed by enzymatic dissociation of the explants, differential adhesion to remove fibroblasts, incubation of the cell pellet to promote aggregation of the type II cells and monolayer culture of the type II cells. The type II cells form clusters which are surrounded by scattered fibroblasts. When the technique was performed with three differential adhesion steps, cultures contained 86.0 +/- 1.4% type II cells. To obtain a higher degree of purity and greater yield, two differential adhesions followed by gentle trypsinization of the cultures which selectively removes the isolated fibroblasts was performed. This resulted in cultures with 89.4 +/- 1.7% type II cells. The differentiated fetal type II cell cultures were prepared from 19-day fetal rat lungs which were initially maintained in explant culture for 48 h. These differentiated cells demonstrated the characteristic morphologic features of type II cells including lamellar bodies and microvilli. Undifferentiated fetal cells were prepared in a similar manner from 18-day fetal rat lung maintained in explant culture for 24 h. These cells did not contain intracellular osmiophilic granules; the appearance of these granules could, however, be induced by hormones. For this reason they are considered to be pre-type II cells. The viability of the cultured cells was 97%. Both the differentiated and undifferentiated fetal type II cells specifically bound the Maclura pomifera lectin, a type II cell surface marker. The phospholipid profile of the fetal cells was similar to that of adult rat type II cells; the differentiated fetal cells, however, synthesized less phosphatidylcholine than the adult cells did, but more than the undifferentiated fetal cells. The differentiated fetal cells secreted phosphatidylcholine at a basal rate of 0.6% +/- 0.1% during a 90-min incubation. There was dose-dependent stimulation of phosphatidylcholine secretion after exposure to terbutaline. Maximum stimulation (76%) was observed at a concentration of 10 microM. This culture system provides a valuable model for studies of the maturation of the undifferentiated fetal type II cell and surfactant metabolism and secretion in the differentiated fetal type II cell.     

Thyrotropin-releasing hormone receptor expression in thyroid follicular cells: a new paracrine role of C-cells?

Thyrotropin-releasing hormone (TRH) synthesized in the hypothalamus has the capability of inducing the release of thyroid-stimulating hormone (TSH) from the anterior pituitary, which in turn stimulates the production of thyroid hormones in the thyroid gland. Immunoreactivity for TRH and TRH-like peptides has been found in some tissues outside the nervous system, including thyroid. It has been demonstrated that thyroid C-cells express authentic TRH, affecting thyroid hormone secretion by follicular cells. Therefore, C-cells could have a paracrine role in thyroid homeostasis. If this hypothesis is true, follicular cells should express TRH receptors (TRH-Rs) for the paracrine modulation carried out by C-cells. In order to elucidate whether or not C-cell TRH production could act over follicular cells modulating thyroid function, we studied TRH-Rs expression in PC C13 follicular cells from rat thyroid, by means of immunofluorescence technique and RT-PCR analysis. We also investigated the possibility that C-cells present TRH-Rs for the autocrine control of its own TRH production. Our results showed consistent expression for both receptors, TRH-R1 and TRH-R2, in 6-23 C-cells, and only for TRH-R2 in PC C13 follicular cells. Our data provide new evidence for a novel intrathyroidal regulatory pathway of thyroid hormone secretion via paracrine/autocrine TRH signaling.     

Developmental regulation of phospholipid secretion by fetal type II pneumocytes

Surfactant sufficiency is dependent upon adequate synthesis and secretion of surfactant by the type II alveolar epithelium. Our laboratory has previously shown that basal secretion of surfactant phospholipid by differentiated fetal type II cells is lower than the basal secretion by adult cells. The purposes of this study were to determine if undifferentiated fetal type II cells can secrete phosphatidylcholine, to determine if terbutaline, a β-adrenergic agonist, stimulates secretion of surfactant phospholipids by undifferentiated fetal cells and to examine the effects of differentiation on secretion of surfactant phospholipids by fetal cells. Constitutive (basal) secretion of phosphatidylcholine increased linearly as a function of time in both undifferentiated and differentiated cells, but the rate of secretion was greater in differentiated cells than the rate of secretion in undifferentiated cells. Terbutaline caused a concentration-dependent increase in secretion in both undifferentiated and differentiated cells. Maximal effective concentration and EC₅₀ were similar for undifferentiated (10⁻⁶ M, 0.2 μM) and differentiated (10⁻⁵ M, 0.3 μM) cells. The relative stimulation of secretion above control values was greater for undifferentiated cells. The kinetics of terbutaline stimulation varied significantly with cellular differentiation. Terbutaline resulted in 230% stimulation of secretion in undifferentiated cells at 30 min followed by a decline in the response to terbutaline at 60 to 120 min. In contrast, terbutaline stimulated secretion by differentiated cells showed a sustained linear increase from 0 to 120 min. This regulation of stimulated secretion is not present in undifferentiated cells. We conclude that undifferentiated type II cells are capable of the secretion of phosphatidylcholine and that terbutaline stimulates secretion by undifferentiated cells. Furthermore, basal secretion increases as a function of differentiation of type II cells and the regulation of stimulated secretion seen in differentiated cells is not developed in undifferentiated cells. The developmental regulation of the secretion of surfactant is complex and probably involves both excitatory as well as inhibitory mechanisms which develop at different stages of differentiation of the type II cell.     

Migration of melanoblasts into the developing murine hair follicle is accompanied by transient c-Kit expression.

Disruption of the c-Kit/stem cell factor (SCF) signaling pathway interferes with the survival, migration, and differentiation of melanocytes during generation of the hair follicle pigmentary unit. We examined c-Kit, SCF, and S100 (a marker for precursor melanocytic cells) expression, as well as melanoblast/melanocyte ultrastructure, in perinatal C57BL/6 mouse skin. Before the onset of hair bulb melanogenesis (i.e., stages 0-4 of hair follicle morphogenesis), strong c-Kit immunoreactivity (IR) was seen in selected non-melanogenic cells in the developing hair placode and hair plug. Many of these cells were S100-IR and were ultrastructurally identified as melanoblasts with migratory appearance. During the subsequent stages (5 and 6), increasingly dendritic c-Kit-IR cells successively invaded the hair bulb, while S100-IR gradually disappeared from these cells. Towards the completion of hair follicle morphogenesis (stages 7 and 8), several distinct follicular melanocytic cell populations could be defined and consisted broadly of (a) undifferentiated, non-pigmented c-Kit-negative melanoblasts in the outer root sheath and bulge and (b) highly differentiated melanocytes adjacent to the hair follicle dermal papilla above Auber's line. Widespread epithelial SCF-IR was seen throughout hair follicle morphogenesis. These findings suggest that melanoblasts express c-Kit as a prerequisite for migration into the SCF-supplying hair follicle epithelium. In addition, differentiated c-Kit-IR melanocytes target the bulb, while non-c-Kit-IR melanoblasts invade the outer root sheath and bulge in fully developed hair follicles.     

Merkel cell distribution in human hair follicles of the fetal and adult scalp

The distribution of Merkel cells in fetal and adult terminal hair follicles of human scalp was studied immunohistochemically using cytokeratin (CK) 20 as a specific Merkel cell marker. In hair follicles of adult scalp, abundant Merkel cells were found enriched in two belt-like clusters, one in the deep infundibulum and one in the isthmus region. No Merkel cells were found in the deep follicular portions including the bulb, or in the dermis. In early fetal hair follicles (bulbous peg stage), Merkel cells were only detected in the basal layer of the developing infundibulum but not in deeper follicular areas. In later stages, Merkel cells were also present in the isthmus and bulge. No Merkel cells were seen in the dermis around developing hair follicles. Nerve growth factor receptor was not only present in nerves but was found to be widely distributed within fetal skin. In adult skin, this receptor was localized to the basal cell layers of the outer root sheath of the bulb and the suprabulbar area, but was not detectable in the areas containing Merkel cells. The present study localizing Merkel cells within the permanent hair follicle structures close to their possible stem cells suggests that they have paracrine functions.     

Morphological and histochemical changes of ultimobranchial follicles of the rat thyroid in the course of postnatal life.

Morphological and histochemical changes of ultimobranchial follicles of thyroid have been investigated in rats from newborn to 18 months of age. The first well-delimited ultimobranchial follicles, though with no lumen, were detected in the thyroid gland of 10-day-old rats. At 30 days of age, follicles possessing regular lumina were present in the thyroid. With age, the follicles gradually increased in volume assuming extreme dimensions in adult age. The follicles displayed varying shapes from simple cysts to bizarre forms. From the age of 50 days the cells of the follicular wall are separated from the cell debris contained in the lumen. The latter gave a PAS positive reaction. The cells of the ultimobranchial follicles did not exhibit argyrophilia and metachromasia showing that they differ considerably from the C-cells likewise of ultimobranchial origin, which are known to give marked argyrophilic and metachromatic reactions.     

Quantitative changes in the frequency and distribution of the C-cell population in the rat thyroid gland with age

Summary The development of calcitonin cells (C-cells) was investigated in rat thyroid glands from birth to 120 days, using an immunoperoxidase technique and a point-counting method. The proportion of C-cells to follicular cells was 4.5% on the day of birth and increased progressively to 10.4% by 120 days. The highest density of C-cells was noted in the mid-region of the lobes along a longitudinal axis. The caudal and cephalic regions of the lobes contained smaller numbers of C-cells. The C-cells tended to be more numerous in the posterior aspects of the lobes. Although the numbers of C-cells in 120-day-old animals were markedly increased as compared to animals at the time of birth, the cell distributions within the glands were similar at all ages.     

Local inhibitor of the crayfish telson-flexor motor giant neurons: morphology and physiology

The motor circuits that control telson flexion in the crayfish (Procambarus clarkii) include a curiously arranged sub-circuit: a premotor 'command' neuron excites a motor neuron via a trisynaptic pathway, but also inhibits (and prevents firing of) the motor neuron via a shorter latency pathway (Kramer et al. 1981 a). The premotor and motor neurons in this circuit have been previously identified (Kramer et al. 1981 a; Dumont and Wine 1985a, b; see Fig. 1). We have now identified a local interneuron that inhibits the motor neurons. The cell we studied is called the 'C' cell because of its distinctive structure (Figs. 2, 3). A single pair of bilaterally homologous C-cells was found in the last (6th) abdominal ganglion. The C-cells are invariably dye coupled to one another following injections of lucifer yellow into either one of them, and are frequently dye coupled to smaller axons in the 2nd, 3rd, and 6th nerves. In addition, some of the extensive branches of the C-cell extend out into the 6th nerve, where they are in close proximity to the axons of the motor neurons they inhibit (Fig. 3). Two kinds of evidence established that the C-cell directly inhibits the motor neurons. First, when simultaneous recordings were made from the C-cell and the motor neurons, spikes in the C-cell, no matter how evoked, were invariably followed, within 1.5 ms, by depolarizing IPSPs in the motor neuron (Fig. 6). Second, when the C-cell was hyperpolarized so that it could not fire, that same IPSP in the motor neuron was abolished (Fig. 6). The inhibitory pathway to the motor neurons must be fired at short latency in order to prevent firing caused by the trisynaptic excitatory input (Fig. 1). The C-cells were fired at short latency (less than 3 ms) by impulses in either of the escape command cells (Fig. 4), and at even shorter latency by impulses in the Segmental Giant of the 6th ganglion (SG6) (Fig. 5). It has been established elsewhere that the SGs are a major output pathway of the escape command cells; our results suggest that they may be the pathway for command-evoked firing of the C-cell. The C-cells are also excited by two descending, non-giant, flexion premotor neurons, called I2 and I3 (Fig. 5). The EPSPs from a single I2 or I3 impulse were subthreshold, but temporal and spatial summation of EPSPs from the non-giant pathway sometimes fired the C-cells.(ABSTRACT TRUNCATED AT 400 WORDS)     

Immunocytochemical analysis of the expression of gap junction protein connexin 43 in the rat ovary

The present immunocytochemical study examines in the rat ovary the pattern of expression of connexin 43 (Cx43), a subunit of gap junctions. Using a well-characterized specific antiserum against rat Cx43, immunoreactivity was not detected in the fetal ovary, i.e., prior to follicular formation. However, in the ovary of 20-day-old, 35-day-old, and adult rats, strong Cx43-immunore-activity was associated with the cell borders of the follicular epithelium/granulosa cells of all developmental stages (primordial follicles, preantral and antral secondary follicles). In general, immunoreactivity of the granulosa cells of large antral follicles appeared more intense than the one of smaller follicles. Staining was also seen in oocytes (cytoplasmic staining). Theca cells of large antral follicles, but not of small follicles were immunoreactive. Immunoreactive interstitial cells were not seen in ovaries of 20- and 35-day-old animals, but staining in these cells was present in adult rats. In large follicles with signs of atresia, granulosa cells lacked Cx43-immunoreactivity, whereas Cx43-immunoreactivity in their theca interna strikingly increased. Corpora lutea in the cyclic adult rats were heterogeneously stained, with either no detectable immunoreactivity, staining of cell borders of most luteal cells, or with conspicuous staining of only a few cells. In the pregnant animals on gestation days (GD) 12, 14, and 17, all luteal cells stained strongly for Cx43 at the cell surface. Shortly before delivery (GD 21), however, the staining pattern vanished and only few, presumably luteal cells remained immunoreactive. In Western blots (using homogenates of whole ovaries), the Cx43 antiserum recognized a major band of approximate Mr 43 × 10³, together with minor bands, which may reflect the presence of several differently phosphorylated Cx43 forms. This is indicated by treatment with alkaline phosphatase, which reduced the banding pattern to one single band. In summary, the gap junction molecule Cx43 is abundantly expressed in all endocrine compartments of the rat ovary. The staining pattern obtained in the present study indicates that Cx43 and presumably gap-junctional communication are associated with follicular development, atresia, and the development of the interstitial gland, as well as with the development and regression of the corpus luteum. The heterogeneous staining within the ovary furthermore hints to a contribution of the local intraovarian factors in the regulation of Cx43 expression. © 1995 Wiley-Liss, Inc.     

Structural and ultrastructural differentiation of the thyroid gland during embryogenesis in the grass snake Natrix natrix L. (Lepidosauria, Serpentes)

The differentiation of the thyroid primordium of reptilian species is poorly understood. The present study reports on structural and ultrastructural studies of the developing thyroid gland in embryos of the grass snake Natrix natrix L. At the time of oviposition, the thyroid primordium occupied its final position in the embryos. Throughout developmental stages I-IV, the undifferentiated thyroid primordium contained cellular cords, and the plasma membranes of adjacent cells formed junctional complexes. Subsequently, the first follicular lumens started to form. The follicular lumens were of intracellular origin, as in other vertebrate species, but the mechanism of their formation is as yet unclear. At developmental stages V-VI, the thyroid anlage was composed of small follicles with lumens and cellular cords. Cells of the thyroid primordium divided, and follicles were filled with a granular substance. At developmental stage VI, the cells surrounding the follicular lumen were polarized, the apical cytoplasm contained dark granules and the Golgi complex and the rough endoplasmic reticulum (RER) developed gradually. Resorption of the colloid began at developmental stage VIII. At the end of this stage, the embryonic thyroid gland was surrounded by a definitive capsule. During developmental stages IX-X, the follicular cells contained granules and vesicles of different sizes and electron densities and a well-developed Golgi apparatus and RER. At developmental stage XI, most follicles were outlined by squamous epithelial cells and presented wide lumens filled with a light colloid. The Golgi complex and RER showed changes in their morphology indicating a decrease in the activity of the thyroid gland. At developmental stage XII, the activity of the embryonic thyroid gradually increased, and at the time of hatching, it exhibited the features of a fully active gland.     

Developmental regulation of baboon fetal ovarian maturation by estrogen

Ovarian function in adult human and nonhuman primates is dependent on events that take place during fetal development, including the envelopment of oocytes by granulosa (i.e., folliculogenesis). However, our understanding of fetal ovarian folliculogenesis is incomplete. During baboon pregnancy, placental production and secretion of estradiol into the fetus increases with advancing gestation, and the fetal ovary expresses estrogen receptors alpha and beta in mesenchymal-epithelial cells (i.e., pregranulosa) as early as midgestation. Therefore, the current study determined whether estrogen regulates fetal ovarian follicular development. Pregnant baboons were untreated or treated with the aromatase inhibitor CGS 20267, or with CGS 20267 plus estradiol benzoate administered s.c. to the mother on Days 100-164 (term = Day 184). On Day 165, baboon fetuses were delivered by cesarean section and the number of total follicles and interfollicular nests consisting of oocytes and mesenchymal-epithelial cells in areas (0.33 mm(2)) of the outer and inner cortices of each fetal ovary were quantified using image analysis. Maternal and umbilical serum estradiol levels were decreased by >95% with CGS 20267. Treatment with CGS 20267 and estrogen restored maternal estradiol to normal and fetal estradiol to 30% of normal. Although fetal ovarian weight was unaltered, the mean number of follicles +/- SEM/0.33 mm(2) in the inner (59.0 +/- 1.7) and outer (95.3 +/- 2.4) cortical regions of fetal ovaries in untreated animals was 35%-50% lower (P < 0.01) in estrogen-depleted baboons (25.9 +/- 1.4, inner cortex; 62.5 +/- 2.7, outer cortex) and was restored to normal by treatment with CGS 20267 and estrogen. In contrast, the number of interfollicular nests was 2-fold greater (P < 0.01) in fetal ovaries of estrogen-suppressed animals, a change that was prevented by treatment with estrogen. In summary, fetal ovarian follicular development was significantly altered in baboons in which estrogen was depleted during the second half of gestation and restored to normal by estradiol. We propose that estrogen plays an integral role in regulating, and perhaps programming, primate fetal ovarian development.