Estrogen synthesis in fetal sheep brain: effect of maternal treatment with an aromatase inhibitor

The aim of the present study was to determine whether the fetal lamb brain has the capacity to aromatize androgens to estrogens during the critical period for sexual differentiation. We also determined whether administration of the aromatase-inhibitor 1,4,6-androstatriene-3,17-dione (ATD) could cross the placenta and inhibit aromatase activity (AA) in fetal brain. Eight pregnant ewes were utilized. On Day 50 of pregnancy, four ewes were given ATD-filled Silastic implants, and the other four ewes received sham surgeries. The fetuses were surgically delivered 2 wk later (Day 64 of gestation). High levels of AA (0.8-1.4 pmol/h/mg protein) were present in the hypothalamus and amygdala. Lower levels (0.02-0.1 pmol/h/mg protein) were measured in brain stem regions, cortex, and olfactory bulbs. The Michaelis-Menten dissociation constant (K(m)) for aromatase in the fetal sheep brain was 3-4 nM. No significant sex differences in AA were observed in brain. Treatment with ATD produced significant inhibition of AA in most brain areas but did not significantly alter serum profiles of the major sex steroids in maternal and fetal serum. Concentrations of testosterone in serum from the umbilical artery and vein were significantly greater in male than in female fetuses. No other sex differences in serum steroids were observed. These data demonstrate that high levels of AA are found in the fetal sheep hypothalamus and amygdala during the critical period for sexual differentiation. They also demonstrate that AA can be inhibited in the fetal lamb brain by treating the mother with ATD, without harming fetal development.     

Effects of exogenous steroids on androgen receptors in fetal guinea pig brain

We treated pregnant guinea pigs on Day 50 of gestation with 10 mg testosterone propionate (TP), obtaining fetuses 2, 4, 8, or 18 h later as well as after 5 days of treatment. In a second group of pregnant guinea pigs, dihydrotestosterone propionate (DHTP), estradiol benzoate (E2B), progesterone (P), or cortisol was given 2 h before obtaining fetuses. Although TP treatment elevated fetal serum T (p less than 0.05), brain cytosolic androgen receptor (ARc) content was unchanged in fetuses of either sex. In female fetuses, nuclear androgen receptors (ARn) increased 10-fold in medial-basal hypothalamus (MBH) and preoptic area (POA) at 2 and 4 h (respectively) after treatment, while fetal male ARn content was unchanged. Maternal injection of other steroids (E2B, P, or cortisol, but not DHTP) significantly increased these hormones in the fetus 2 h later (p less than 0.05). Only androgens affected fetal androgen receptor (AR) content. While TP increased ARn in female MBH, DHTP decreased ARc in fetal anterior pituitary of both sexes. In this latter case, a metabolite of DHT may mediate the effects. We conclude that T crosses the guinea pig placenta and activates ARn in POA and MBH of female fetuses; male ARn appear to be maximally occupied by endogenous T. Steroids of other classes do not induce AR responses in fetal guinea pig brain. These AR changes may represent an initial cellular mechanism in brain sexual differentiation.     

Aromatase activity in adult guinea pig brain is androgen dependent

Androgen metabolism in target tissues constitutes an important step for understanding hormone action. The in situ aromatization of androgen represents one of these metabolic events. We characterized aromatase activity (AA) in a microsomal preparation of brain tissue from adult guinea pigs since earlier reports questioned its presence in neural tissues of this species. Analyses revealed an apparent substrate affinity (approximately 17 nM) that was equivalent in adult males and females. However, adult male brains contained greater quantities of AA than female brains. Specifically, AA in the preoptic area (POA: p less than 0.05) and the medial basal hypothalamus (MBH; p less than 0.01) was greater in males than in females. AA was concentrated in the limbic system and hypothalamus (amygdala greater than POA greater than septum greater than MBH), whereas low levels were consistently measured in cortical tissue. In vitro estrogen formation was significantly lower in POA (p less than 0.05) and MBH (p less than 0.01) after castration. After dihydrotestosterone treatment, AA returned to levels equal to or greater than those observed in intact males. These data indicate that AA does exist in the guinea pig brain and is modulated by androgens through the androgen receptor. The presence of high levels of aromatase activity may suggest a role for locally formed estrogens in brain function in this species.     

Steroid control of sexual behavior and brain aromatase in the dove: effects of nonaromatizable androgens, methyltrienolone (R1881), and 5 alpha-dihydrotestosterone

This study examines the effects of nonaromatizable androgens, methyltrienolone (R1881) and 5 alpha-dihydrotestosterone (DHT) on aggressive courtship and vocal behavior in the male ring dove. Since androgens may influence behavior by increasing the formation of estrogen in the brain, the effects of R1881 and DHT on brain aromatase activity were also studied using an in vitro microassay. Under conditions in which testosterone induced aggressive courtship patterns, the nonaromatizable androgens were ineffective. But DHT and R1881 induced vocal behavior with equal efficiency, indicating that androgens can influence mechanisms of vocal behavior without conversion to estrogens. The behavioral effectiveness of both hormones was reduced (approximately 50%) when the period between castration and treatment was doubled. Testosterone propionate increased formation of E2 from 3H-testosterone in both the preoptic (POA) and anterior hypothalamic areas. Neither of the nonaromatizable androgens affected POA aromatase activity. The results suggest that only the aromatizable androgen, testosterone, which is also required specifically for male courtship, increases preoptic formation of estrogen.     

Brain aromatase in control versus castrated Norway Brown, Sprague-Dawley and Wistar adult rats.

Brain aromatase cytochrome P450 converts androgens to estrogens that play a critical role in the development of sexually dimorphic neural structures, the modulation of neuroendocrine function(s), and the regulation of sexual behavior. We characterized the influence of surgical castration on brain aromatase in Norway Brown and Wistar adult rats and compared their responses to Sprague-Dawley rats that were surgically or biochemically castrated (with flutamide, a known androgen receptor blocker). Aromata enzyme activity was measured by the tritiated water release assay in the medial basal hypothalmus/preoptic area (MBH/POA) and amygdala brain regions. The present results demonstrate that independent of the rat strain examined, MBH/POA aromatase is regulated by androgens (in Sprague-Dawley, Norway Brown and Wistar males). However, intact Wistar animals displayed significantly higher MBH/POA aromatase levels compared to Sprague-Dawley control values. Conversely, in the amygdala region, there was an apparent lack of androgen hormone action upon aromatase enzyme activity in some of the rat strains tested. The importance of brain aromatase regulating estrogen biosynthesis and influencing brain development and function is covered.     

Sex differences and hormone influences on tyrosine hydroxylase immunoreactive cells in the leopard frog

We examined sex differences in tyrosine hydroxylase immunoreactive (TH-ir) cell populations in the preoptic area (POA), suprachiasmatic nucleus (SCN), posterior tuberculum (TP), and caudal hypothalamus (Hy) in the leopard frog (Rana pipiens), in addition to the effects of natural variation in sex steroid hormones on these same populations in both sexes. All four of these populations have been shown to be dopaminergic. Gonadal sex, androgens, and estrogen all influenced TH-ir cell numbers, but in a complicated pattern of interactions. After factoring out the effects of sex steroids by multiple regression, TH-ir cell numbers in all four areas differed between the sexes, with males having a greater number of TH-ir cells. The influence of androgens and estrogen differed by region and sex of the animals. Androgens were the main influence on TH-ir cell numbers in the POA and SCN. Plasma androgen concentrations were positively correlated with TH-ir cell numbers in both areas in males. In females, androgen concentration was negatively correlated with TH-ir cell numbers in the POA; there was no significant relationship in the SCN in females. In the more caudal populations, estrogen (E2) levels were positively correlated with TH-ir cell numbers in the TP of both males and females. In the caudal hypothalamus, E2 levels were positively correlated with TH-ir cell numbers in females, but there was no significant correlation in males. The results indicate that gonadal sex imposes a baseline sex difference in the four TH-ir (dopamine) populations, resulting in a higher number of such cells in males. Individual and sex-linked differences in gonadal steroid hormones lead to variation around this baseline condition, with androgens having a greater influence on rostral populations and estrogen on caudal populations. Last, an individual's gonadal sex determines the effect that androgens and estrogen have on each population.     

Negative feedback regulation of gonadotropin secretion by androgens in fetal rhesus macaques

Previously we described sex differences in circulating gonadotropin concentrations (greater in females) in fetal rhesus macaques, and demonstrated that these sex differences relate, at least in part, to the negative feedback actions of testicular secretions. A fully functional gonadal-hypothalamic-pituitary feedback relationship is present as early as Day 100 of gestation in fetal males because castration at this time results in a dramatic increase (greater than 10-fold) in fetal luteinizing hormone (LH) concentrations. Although short-term (6-h) treatment of fetuses with testosterone (T) 3 wk after gonadectomy (GX) does not lower LH levels in males, it is completely effective in females. These data suggest that either T is not the primary testicular factor responsible for feedback suppression of LH in fetal males, or the hypothalamic-pituitary axis becomes insensitive to T after GX. To determine if immediate treatment with T after GX is effective in maintaining LH levels, we gonadectomized five fetal rhesus males on Days 98-104 of gestation and immediately implanted crystalline-T-containing intraabdominal Silastic capsules. An additional five fetuses were treated with the nonaromatizable androgen dihydrotestosterone (DHT). Umbilical arterial samples for hormone analysis were obtained prior to GX and again approximately 3 wk later. Serum from control males (n = 11) castrated in utero on Day 100 of gestation contained significantly greater concentrations of LH and follicle-stimulating hormone (FSH) 3 wk after the operation than before GX. Five sham-operated male fetuses did not have elevated levels of either LH or FSH in their serum on Day 120 of gestation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Is androgen-dependent aromatase activity sexually differentiated in the rat and dove preoptic area?

Aromatase activity is higher in the male than in the female anterior hypothalamic-preoptic area (POA) in both the avian and the rodent adult brain. This sex difference is abolished after castration of the male and restored by androgen treatment. Gonadectomy has no effect on POA aromatase in the female. The aim of this study was to find out whether sex dimorphism in adult POA aromatase is only due to a sex difference in circulating gonadal hormones or dependent upon sexual differentiation of the brain. Aromatase activity was measured in vitro in microdissected POA samples using a sensitive radiometric assay. We examined the effects of gonadectomy and testosterone treatment on enzyme activity in adult rats and doves of both sexes. We also studied the effects of neonatal gonadectomy and hormone substitution in male and female rats. The results suggest that levels of POA aromatase in the adult depend primarily on gonadal activity, but that mechanisms involved in the regulation of aromatase activity and enzyme induction may be sex-specific and could result from sexual differentiation of the brain in early life. Further work will be required to determine the developmental stage when this occurs and the exact mechanism(s) responsible for increased sensitivity of the adult male POA to the inductive effect of testosterone.     

Is androgen-dependent aromatase activity sexually differentiated in the rat and dove preoptic area?

Aromatase activity is higher in the male than in the female anterior hypothalamic-preoptic area (POA) in both the avian and the rodent adult brain. This sex difference is abolished after castration of the male and restored by androgen treatment. Gonadectomy has no effect on POA aromatase in the female. The aim of this study was to find out whether sex dimorphism in adult POA aromatase is only due to a sex difference in circulating gonadal hormones or dependent upon sexual differentiation of the brain. Aromatase activity was measured in vitro in microdissected POA samples using a sensitive radiometric assay. We examined the effects of gonadectomy and testosterone treatment on enzyme activity in adult rats and doves of both sexes. We also studied the effects of neonatal gonadectomy and hormone substitution in male and female rats. The results suggest that levels of POA aromatase in the adult depend primarily on gonadal activity, but that mechanisms involved in the regulation of aromatase and enzyme induction may be sex-specific and could result from sexual differentiation of the brain in early life. Further work will be required to determine the developmental stage when this occurs and the exact mechanism(s) responsible for increased sensitivity of the adult male POA to the inductive effect of testosterone.     

Quantitative Analysis of Long-Form Aromatase mRNA in the Male and Female Rat Brain

In vitro studies show that estrogens acutely modulate synaptic function in both sexes. These acute effects may be mediated in vivo by estrogens synthesized within the brain, which could fluctuate more rapidly than circulating estrogens. For this to be the case, brain regions that respond acutely to estrogens should be capable of synthesizing them. To investigate this question, we used quantitative real-time PCR to measure expression of mRNA for the estrogen-synthesizing enzyme, aromatase, in different brain regions of male and female rats. Importantly, because brain aromatase exists in two forms, a long form with aromatase activity and a short form with unknown function, we targeted a sequence found exclusively in long-form aromatase. With this approach, we found highest expression of aromatase mRNA in the amygdala followed closely by the bed nucleus of the stria terminalis (BNST) and preoptic area (POA); we found moderate levels of aromatase mRNA in the dorsal hippocampus and cingulate cortex; and aromatase mRNA was detectable in brainstem and cerebellum, but levels were very low. In the amygdala, gonadal/hormonal status regulated aromatase expression in both sexes; in the BNST and POA, castration of males down-regulated aromatase, whereas there was no effect of estradiol in ovariectomized females. In the dorsal hippocampus and cingulate cortex, there were no differences in aromatase levels between males and females or effects of gonadal/hormonal status. These findings demonstrate that long-form aromatase is expressed in brain regions that respond acutely to estrogens, such as the dorsal hippocampus, and that gonadal/hormonal regulation of aromatase differs among different brain regions.     

Placental Aromatase Is Deficient in Placental Ischemia and Preeclampsia

IntroductionPreeclampsia is a maternal hypertensive disorder with uncertain etiology and a leading cause of maternal and fetal mortality worldwide, causing nearly 40% of premature births delivered before 35 weeks of gestation. The first stage of preeclampsia is characterized by reduction of utero-placental blood flow which is reflected in high blood pressure and proteinuria during the second half of pregnancy. In human placenta androgens derived from the maternal and fetal adrenal glands are converted into estrogens by the enzymatic action of placental aromatase. This implies that alterations in placental steroidogenesis and, subsequently, in the functionality or bioavailability of placental aromatase may be mechanistically involved in the pathophysiology of PE.MethodsSerum samples were collected at 32–36 weeks of gestation and placenta biopsies were collected at time of delivery from PE patients (n = 16) and pregnant controls (n = 32). The effect of oxygen tension on placental cells was assessed by incubation JEG–3 cells under 1% and 8% O₂ for different time periods, Timed-mated, pregnant New Zealand white rabbits (n = 6) were used to establish an in vivo model of placental ischemia (achieved by ligature of uteroplacental vessels). Aromatase content and estrogens and androgens concentrations were measured.ResultsThe protein and mRNA content of placental aromatase significantly diminished in placentae obtained from preeclamptic patients compared to controls. Similarly, the circulating concentrations of 17-β-estradiol/testosterone and estrone/androstenedione were reduced in preeclamptic patients vs. controls. These data are consistent with a concomitant decrease in aromatase activity. Aromatase content was reduced in response to low oxygen tension in the choriocarcinoma JEG–3 cell line and in rabbit placentae in response to partial ligation of uterine spiral arteries, suggesting that reduced placental aromatase activity in preeclamptic patients may be associated with chronic placental ischemia and hypoxia later in gestation.ConclusionsPlacental aromatase expression and functionality are diminished in pregnancies complicated by preeclampsia in comparison with healthy pregnant controls.     

Low aromatase activity and gene expression in human fetal testes

Because of previous indications that estradiol (E2) plays a role in the regulation of testicular testosterone (T) production in some species, the production of E2 and aromatase gene expression in human fetal testes were investigated. Testicular minces from 14 fetuses (fetal age 15-23 weeks) were incubated with and without 200 ng/ml highly purified hCG, and the production of E2 and T was measured by RIA. Basal T production was high at 15-18 weeks of gestation and decreased thereafter. Estradiol production was low in all testes. Aromatase mRNA (P-450 arom messenger ribonucleic acid) was not detectable in fetal testicular tissues when studied by Northern and dot blot techniques. Placenta and fetal liver expressed aromatase mRNA, but fetal ovary contained only miniscule amounts. HCG significantly stimulated the production of both T and E2 in the testes of older fetuses (19-23 weeks), but the testicular E2 production of the youngest fetuses (15-18 weeks) did not increase significantly after hCG stimulation. These results indicate that aromatase activity and gene expression are very low in human fetal testes. These findings suggest that E2 may not play a major role in testicular T production in the human fetus.     

Aromatase: Neuromodulator in the control of behavior

Estrogens are required for both the organization of the brain in early development and adult behavior. Two approaches have been used in our laboratory to study the behavioral role of brain aromatase. First, brain metabolism of testosterone (T) has been related to behavior in the same individual using a well established neuroendocrine model, the ring dove, in which estradiol-17β (E₂) has specific effects on brain mechanisms of male behavior. Aromatase in preoptic area (POA) (a) has a high activity (V_max) and strong substrate binding affinity (K_m < 5 nM), (b) is regulated by both androgens and estrogens, and the type of regulation differs according to brain area, (c) is influenced by products of an endogenous inactivating pathway, 5β-reduction; 5β-dihydrotestosterone and other 5β-reduced metabolites appear to be non-genomic regulators of the brain aromatase. Preoptic aromatase activity is also influenced by photoperiod and socio-sexual stimuli. The codistribution of regulated aromatase activity and estrogen receptor cells is found to be T-dependent. Our second approach has been to relate the aromatase system to developmental sex differences in brain structure and behavior of the Mongolian gerbil. Neonatal gerbil aromatase is relatively active in the POA, but has a weaker T substrate-binding affinity (K_m = 30 nM) than the dove. T acting via its metabolite, E₂, masculinizes the sexually dimorphic area of the hypothalamus; the differentiating effect is asymmetric. We suggest that the regulation of the brain aromatase system may be lateralized during steroid-sensitive periods of development.     

Circulating insulin-like growth factor-I and -II concentrations are not dependent on pituitary influences in the midgestation fetal sheep

Studies were performed to investigate the possible role of pituitary factors on the regulation of circulating concentrations of insulin-like growth factor-I and -II in the midgestation sheep fetus. Four fetuses were decapitated at 59-64 days of gestation and fetal serum obtained at sacrifice at 90-102 days of gestation. Insulin-like growth factor-I and -II concentrations were similar in these samples to those from 6 control fetuses (83-102 days). A further 4 fetuses were studied following electrolytic destruction of the median eminence of the hypothalamus at 108-110 days of gestation. Four sham operated controls were also studied. Circulating growth hormone concentrations were markedly reduced (P less than 0.01) by destruction of the median eminence. However neither insulin-like growth factor-I nor -II levels differed from those of sham operated fetuses. We conclude that, in the midgestation fetal sheep, growth hormone is not essential for the maintenance of circulating concentrations of insulin-like growth factor-I or -II.     

Regulation of rabbit fetal glycogen: effect of in utero fetal decapitation on the metabolism of glycogen in fetal heart, lung, and liver

To understand the control mechanisms involved in the regulation of fetal glycogen, we have studied the effect of in utero fetal decapitations on glycogen metabolism in rabbit fetal heart, lung, and liver. In utero fetal decapitations were performed between days 18 and 21 of gestation. Two to four fetuses on one side of the horn were decapitated. Fetuses were delivered between days 23 and 26 or between days 28 and 30 of gestation. Fetal heart, lungs, and liver were analyzed for DNA, protein, glycogen, glycogen synthase (I and D forms), glycogen phosphorylase (a and b forms), phosphofructokinase, pyruvate kinase, and lactic dehydrogenase. In fetal heart and lung, no difference was observed in any of the above measurements in the intact and decapitated fetuses. In contrast, fetal liver does not appear to develop the glycogen system as indicated by the very low levels of glycogen (0.02 mg/mg DNA) in decapitated fetuses as compared with intact fetuses (0.4 mg/mg DNA). Similarly the levels of glycogen synthase and phosphorylase were two to three times lower in livers from decapitated fetuses as compared with the livers from intact fetuses. The three enzymes phosphofructokinase, pyruvate kinase, and lactic dehydrogenase were not affected by fetal decapitation in all three tissues. These results indicate that the fetal hypothalamic-pituitary-adrenal (thyroid) axis is not required at least after day 18 of gestation for the normal accumulation and subsequent utilization of glycogen in fetal heart and lungs, while it is an absolute requirement for the development of the fetal liver glycogen system.(ABSTRACT TRUNCATED AT 250 WORDS)     

The role of the fetal gonads and placenta in steroid production, maintenance of pregnancy and parturition in the mare.

The effects of fetal gonadectomy on steroid production and the maintenance of pregnancy in the mare were studied. Removal of the fetal gonads resulted in an immediate fall in maternal plasma concentrations of conjugated and unconjugated oestrogens whereas progestagen levels remained unchanged. Hormone profiles in mares carrying sham-operated fetuses remained similar to those in unoperated control mares. Plasma levels of 13,14-dihydro-15-oxo-PGF-2 alpha (PGFM) were much lower, and uterine contractions weaker, during labour in mares carrying gonadectomized foals than in control mares. Pregnancy was maintained until parturition at term in the mares carrying gonadectomized fetuses. However, 3 of the 4 gonadectomized foals were dysmature and died during or soon after birth. The biosynthetic pathways involved in the production of oestrogens by the feto-placental unit and the possible role of oestrogens in fetal development in the pregnant mare are discussed.     

The effects of hydrocortisone on lung structure in fetal lambs

The effect of cortisol infusion on fetal lung development was studied in lambs. Changes were compared with those of control groups of saline-infused fetuses of the same age (day 132) and normal late gestation fetuses (142 +/- 4.6 days). Cortisol was infused into five fetal lambs at 129 days of gestation at a rate of 17.0 mg/day. Four fetuses were delivered by hysterotomy at the onset of labour-like uterine activity (58 +/- 3 h). In cortisol-infused fetuses the concentration of cortisol in fetal plasma and tracheal fluid rose to levels similar to those in normal fetuses during the last week of gestation. Progesterone concentration in maternal plasma declined at about 48 h after the start of treatment. Cortisol-infused lambs showed increases in fixed lung volume, specific lung volume, absolute volume of both parenchyma and non-parenchyma and the proportion of the parenchyma which was potential airspace and a decrease in the proportion of parenchyma. For cortisol-infused lambs Type II cell size and the abundance of lamellar bodies, and the volume fraction of cell occupied by the nucleus were similar to the 142 day group, whereas Golgi apparatus and RER were closer to age matched saline-infused (day 132) controls. Glycogen content was midway between the two control groups. We conclude that infusion of cortisol for about 60 h at physiological levels, beginning at 0.85 of gestation, accelerates many, but not all aspects of pulmonary parenchymal maturation, expressed in terms either of morphogenesis of the gas exchange area or differentiation of Type II alveolar cells.     

Sex-related differences in blood and gonad levels of testosterone in silver fox fetuses

The mass of silver fox fetuses of both sexes, their gonads, and adrenals, and the levels of testosterone in blood serum and in gonads and adrenals were determined from day 31 of gestation and every five days thereafter until its termination. Marked sex-related differences were revealed: the blood and gonad levels of testosterone in male fetuses were much higher than those in female fetuses. The fetal adrenals contained significantly less testosterone than the gonads. No sex-related differences in the content of testosterone in the fetal adrenals were found. No differences were found in the body and adrenal mass in female and male fetuses at all the developmental stages studied, while the mass of ovaries exceeded that of testes from day 45 of gestation. The data obtained suggest sex dimorphism in the production of testosterone by gonads in silver foxes appears after day 35 and appears to correspond to the period of morphological differentiation of gonads.     

Circulating LH, FSH, prolactin, testosterone, delta 4-androstenedione, dehydroepiandrosterone sulfate and cortisol levels in the fetus in late gestation and in newborn male and female lambs

Gonadotropins, prolactin (PRL), testosterone (T), delta 4-androstenedione, dehydroepiandrosterone sulfate and cortisol (F) levels were determined from 14 days before birth to term in 3 female and 3 male ovine fetuses with a chronically implanted venous catheter, and in the same animals from birth to 72 h of age. In both sexes, plasma gonadotropins and androgens were low throughout the period of study while plasma F increased with gestational age. After birth, plasma gonadotropins and PRL tended to increase progressively with time while PRL concentrations were significantly higher in female than in male lambs. F and T concentrations decreased significantly within the first 12 and 6 h of postnatal life. Higher T values were again observed at 36 h in male lambs. These data indicate that the fetal hypothalamic-pituitary-gonadal axis is relatively quiescent in the last 14 days of gestation but is activated within the first 72 h after birth.     

Dihydrotestosterone blocks fetal lung fibroblast-pneumonocyte factor at a pretranslational level

Fibroblast pneumonocyte factor (FPF) synthesis by fetal rat lung fibroblasts is augmented during gestation in the presence of cortisol. The control and cortisol-augmented levels of FPF production, as determined by FPF ability to stimulate saturated phosphotidylcholine synthesis by lung epithelial Type II cells, is delayed during development in fibroblasts derived from male fetuses as compared to those derived from female fetuses. The mechanism by which this delay occurs has been addressed. Pregnant rats treated in vivo with dihydrotestosterone (DHT) showed decreased FPF activity from control or cortisol-treated fibroblasts derived from 20-day-old male or female fetuses. In vitro translated proteins of size-fractionated lung RNA from 19-day-old fibroblasts that were pretreated with DHT in vitro showed decreased FPF activity compared to nontreated samples. This decreased FPF activity was present even if the DHT-pretreated cells were stimulated with cortisol prior to RNA preparation. Using a mouse model of testicular feminization that contains no receptors for androgens showed no change in the cortisol augmented FPF activity when the fibroblasts were pretreated with DHT. These data taken together suggest that the delayed FPF production of male-derived lung fibroblasts is a physiologic process which requires androgen receptors, and the mechanism by which androgens inhibit FPF production appears to affect events occurring mainly at a pretranslational level.