Chronic treatment of male tammar wallabies with deslorelin implants during pouch life: effects on development, puberty, and reproduction in adulthood

The present study evaluated the effects of chronic GnRH agonist (deslorelin) treatment on sexual maturation in the male tammar wallaby. Slow-release deslorelin or placebo implants were administered to male pouch young (n = 10/group) when they were between 180 and 200 days old, to determine if disruption of the pituitary-testicular axis during development altered the timing of sexual maturation or had long-term effects on adult reproductive function. Deslorelin treatment caused retardation of testicular growth and reduced the serum FSH and testosterone concentrations between 12 and 24 mo of age. Maturation of the hypothalamic-pituitary-testicular axis was also delayed in treated animals at 13 and 19 mo of age. Despite these alterations in the pattern and timing of neuroendocrine development, sexual maturation was not permanently blocked in these animals and deslorelin-treated animals reached sexual maturity at the same age as treated animals, as evidenced by a fully functional pituitary-testicular axis and proven fertility at 25 mo of age. The ability of the treated animals to reach puberty at the same time as control animals, despite delayed maturation of the hypothalamic-pituitary-testicular axis, suggests that puberty in the male tammar wallaby is additionally regulated by other, gonadotropin-independent factors.     

Onset of Spermatogenesis in Corriedale Ram Lambs under Extensive Rearing Conditions in Uruguay

The present study was undertaken to estimate the time for the attainment of spermatogenesis in spring-born Corriedale ram lambs under conditions of extensive grazing systems in Uruguay. Clinical (live weight, scrotal circumference, penile/preputial separation), morphologi-cal(light and electron microscopy) and endocrinological (testosterone levels) parameters were examined. Two experiments in 2 consecutive years were carried out. In Exp. I, 40 ram lambs were clinically examined, weighed, and blood-sampled at 2 week-intervals between 78 and 216 days of age. Sixteeen were castrated in 3 selected periods: 132-162 (n: 2), 145–175 (n: 6) and 186–216 days (n: 8). In Exp. II, 40 ram lambs appertaining to the next generation of the same flock were examined as above at 180–210 days of age, and castrated for morphological studies. The time for attainment of complete spermatogenesis correlated significantly with most corporal parameters (i.e. scrotal circumference (r = 0.52); testicular weight (r = 0.61), epididymal weight (r = 0.60), penile/preputial separation (r = 0.75). The age of castrated ram lambs correlated with the degree of spermatogenesis (r = 0.83), although no significant correlations were found with live weight or with levels of circulating testosterone. The histology showed major variations in the degree of development of the seminiferous epithelium. Cells undergoing degeneration were a common finding through the initial stages of spermatogenesis, coincident to the presence of sperm abnormalities and foreign cells in semen. By day 180 and onwards, both histology and seminal picture normalized. It is concluded that, under these rearing conditions, the onset of puberty (expressed as morphologically established spermatogenesis) in Corriedale ram lambs is attained at 180-216 days of age when they reach 23 cm of scrotal circumference and 191 g of testis weight. The finding of a high correlation between these parameters (r: 0.93) confirms the usefulness of the measurement of scrotal circumference during clinical examination of ram lambs in this breed.     

Early puberty in short-haired Guinea pigs kept in laboratory animal facilities

Lab animals, such as Guinea pigs (Cavia porcellus), are crucial for scientific development, as they play an important role in the development and quality control chain of vaccines and drugs distributed by the Brazilian public health system. Investigating their biological and physiological parameters is fundamental to raise and keep these animals, so the handling of the facilities that hold them can be updated whenever new information comes up, with the well-being of the animals and alignment with the 3 Rs in mind. In the search for understanding reproductive aspects of Guinea pigs, the present study had the main goal of studying puberty by means of estrous cycle analysis in short-haired Guinea pigs. Guinea pigs have a vaginal occlusive membrane that covers the vaginal orifice. Its rupture takes place gradually and naturally, moments before labor and during estrus. The present study followed 42 females as for the presentation of the vaginal occlusive membrane. Once the membranes ruptured spontaneously, a swab was collected to study vaginal cytology. Membrane rupture was observed in 39 females; six females showed membrane rupture with less than 21 days of age (17 to 21 days). Twenty-three females were characterized as being in estrus due to cytology showing a prevalence of anucleated superficial cells. One of these females was younger than 21 days old. The opening of the vaginal occlusive membrane took place most frequently in intervals between 17 and 18 days, and the membrane remained open between one and three consecutive days. It was possible to follow three cycles of membrane opening on six females. The present study showed the need to adapt handling guidelines for C. porcellus kept in research animal facilities. The early age of puberty imposes the need of separate the female daughters from their fathers at 16 days old.     

Puberty in the male owl monkey (Aotus trivirgatus griseimembra): A study of physical and hormonal development

In the male owl monkey (Aotus trivirgatus),the onset of puberty occurs at between 211 and 337 days (median, 313) as indicated by longitudinal measurements of plasma testosterone in six subjects. Larger rises in plasma testosterone were observed between 300 and 400 days. Pubertal increases in body weight and testicular volume are not pronounced in this species. Young animals first entered the weight range for adult males (800–1080 g) when they were between 370 and 520 days of age (median, 472). Marked growth of the subcaudal scentmarking gland occurred during puberty. The first signs of development of this gland (stiffening and discoloration of overlying hairs) were noted between 282 and 370 days (median 316), and it had attained an adult structure by 336–442 days (median, 397). These changes were androgen dependent as indicated by the fact that treatment of a prepubertal male with testosterone stimulated the subcaudal gland to develop prematurely. Hormonal and physical changes during puberty were the same whether males remained with their parents or were caged alone. There was no indication that puberty was retarded in males which had remained in their natal groups.     

A study of the age factor in experimental silicosis in rats.

In order to study the progress of pulmonary silicosis in rats of different ages, intratracheal injections of (50 mg/150 g body weight) quartz dust of particle size less than 5 mu were given as a single dose and studies were made over a period of 180 days. The pulmonary macrophage reaction and phagocytosis in the younger age group of rats was different from that in the older animals at 30 days postinoculation. The formation of silicotic nodules was delayed in the younger animals. They consisted of thick reticulin fibers and some collagen fibers; in the older group of rats large silicotic nodules with dense collagination developed towards the termination of the experiment (180 days). The present results indicate a possible direct relationship between age and the development of experimental pulmonary silicosis in rats.     

Proestrous hormonal changes preceding the onset of ovulatory failure in lightly androgenized female rats

Proestrous hormonal profiles were characterized in lightly androgenized female rats prior to the onset of the delayed anovulatory syndrome (DAS). In these females, ovulatory failure and persistent vaginal estrus (PVE) occur at a very early age. Female Sprague-Dawley rats were injected with 10 micrograms testosterone propionate (TP) on postnatal Day 5. Control rats were untreated. All animals were weaned at 21 days of age, and following the onset of puberty, estrous cyclicity was monitored by vaginal smear. Rats showing regular 4-day cycles were used. Between 50-70 days of age, intra-atrial cannulae were implanted on a morning of proestrus (0700-0900 h) and blood was sampled at 2-h intervals from 1000 to 2000 h. Additional samples were taken at 0.5-h intervals from 1600 to 1800 h. Plasma was assayed for luteinizing hormone (LH), follicle-stimulating hormone (FSH) and progesterone (P) by radioimmunoassay (RIA). All animals were monitored for the onset of PVE or other alterations in estrous cyclicity. Females treated neonatally with TP that subsequently showed PVE by 150 days of age (PRE DAS) displayed a reduced peak amplitude (P less than 0.01) and delay in onset (1600 vs. 1400 h) of LH but not FSH secretion, when compared to controls. Females treated neonatally with TP that did not enter PVE by 150 days of age (No DAS) also showed a delayed rise in LH when compared to controls. However, the amplitude of LH secretion was not different from controls or PRE DAS females.(ABSTRACT TRUNCATED AT 250 WORDS)     

Alterations in gonadotropin secretion and ovarian function in prepubertal gilts by elevated environmental temperature

The effect of chronic exposure to elevated environmental temperature on gonadotropin secretion and ovarian function was studied in prepubertal gilts. Gilts were maintained under control (15.6 degrees C) or elevated temperature (33.3 degrees C) conditions from 150 to 180 days of age. Endocrine and ovarian responses to bilateral (BLO), unilateral (ULO), and sham ovariectomy were evaluated between 175 and 180 days of age. During the 96-h sampling period after BLO, plasma concentrations of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) were suppressed in heat-stressed females. Similarly, elevated temperatures abolished the transient rise in FSH and subsequent follicular growth normally associated with ULO. In contrast, environmental treatment had no effect on the secretion of FSH and LH after sham ovariectomy, yet the number of small follicles was lower in gilts exposed to elevated temperatures than in females maintained under control conditions. These results indicate that a chronic exposure to elevated environmental temperature during pubertal development diminished the ability of the hypothalamo-hypophyseal axis to secrete FSH and LH, which had physiological consequences on follicular growth. When provided an appropriate stimulus (ULO), an acute period of FSH secretion and subsequent development of follicles failed to occur in females exposed to elevated temperatures. Consequently, we propose that delayed puberty in gilts during periods of elevated environmental temperatures is due, in part, to a diminished capacity for gonadotropin secretion.     

Postnatal development of vagina, clitoris and urethral glands of the golden hamster

We have made a histological study of the postnatal development of the clitoris, preputial glands, urethral glands and vagina of the golden hamster. The 'phallic groove' of the clitoris is closed at 10 days of life, then the urethra has a cuboidal stratified, a stratified squamous and a stratified keratinized epithelium. The preputial glands are composed of branched saccular glands. These glands develop, with few changes during their maturation period. Formation of the urethral glands begins at 5 days and the alveoli are fully developed at puberty. The hamster vagina has two origins; the upper part is Müllerian, the caudal part is sinusal. The wall of the Müllerian vagina has a cylindrical epithelium at birth, which becomes 'double epithelium' at puberty and thereafter changes cyclically in connection with the estrous cycle. The sinusal vagina is solid at birth, its lumen being formed in the first 10 days of life and its wall having a cuboidal stratified epithelium. At 15 days it becomes a stratified keratinized epithelium, which will later line the vaginal pouch. At the 5th day, an ectodermic invagination (stratified keratinized epithelium) is observed in the zone of the future introitus. At the time of vaginal opening this zone forms the distal segment.     

The Development of Lateral Root Primordia in Vicia faba L. and Their Response to Colchicine

Lateral root primordia in i are first initiated 2–3 daysfollowing the onset of germination, after which they take 5.17–6.35days to complete their development and emerge as lateral roots.Variation in the amount of time elapsing between primordiuminitiation and emergence as a lateral is probably a reflectionof the cell number attained by any one primordium at the timeof emergence. The number of primordia produced per cm of primaryroot growth (5.35–6.65) was not affected by variationin the rate of root elongation, although the number of primordiaproduced each day increased with increase in the rate of rootgrowth. In colchicine-treated roots, the amount of time between primordiuminitiation in the C-tumour and the subsequent emergence of alateral (5.43–6.43 days) was similar to the value obtainedin control roots. Primordia which were present at the time ofcolchicine treatment responded to treatment in a number of differentways, depending on the stage of development reached. Primordiain the first 2.66 days of their development die following treatment;those between 2.66 and 3.69 days old have their developmentinhibited but stay alive; primordia which have been developingfor 3.69–4.91 days following initiation grow out as straightlaterals, while those between 4.91 and 5.77 days old form C-tumoursand emerge as inhibited laterals.     

Metabolic cues for puberty onset in free grazing Holstein heifers naturally infected with nematodes

Leptin is a new plausible candidate for the molecular link between nutritional status and the reproductive axis. In previous studies we described that continuous natural nematode infections in heifers retarded growth and delayed the onset of puberty, and that the insulin-like growth factor I (IGF-I) was involved. In the present study we monitored the leptin levels during development in heifers naturally parasitized versus those chronically treated with ivermectin and we investigated whether growth hormone (GH) accounted for the differences in IGF-I previously noted. Insulin levels were also measured. Prolactin hormone was recorded as an indicator of immune system activation. We found a direct correlation between leptin and body weight during development and a prepubertal surge of the hormone 2 weeks before the first progesterone peak that indicates the onset of puberty. This suggests that leptin may act as a signal for this event. Insulin did not vary during growth and prepuberty. On the other hand, GH as not responsible for diminished IGF-I levels in parasitized animals as levels were similar in both groups. The GH levels were high at birth and then diminished rapidly and remained constant during development and puberty. The last hormone studied, prolactin, followed seasonal changes of sunlight duration and presented sporadic bursts in infected animals. These were related to high nematode infection and are probably involved in the immune response of the host.     

Regulation of acyl coenzyme A reductase by a heat-stable cytosolic protein during preputial gland development

A low-molecular-weight protein located in the cytosol of mouse preputial glands has been shown to stimulate the activity of a microsomal acyl coenzyme A (CoA) reductase in the gland. This cytoplasmic protein was stable to heating and lyophilization, but was destroyed by trypsin digestion. It was able to bind palmitoyl-CoA and gel elution behavior indicated it had a molecular weight of 10,000–12,000. The level of this stimulatory cytosolic protein and the activity of acyl-CoA reductase were shown to correlate with differentiation of the preputial gland during development of puberty in male mice; the acyl-CoA reductase activity first appeared at 4 weeks of age and increased dramatically up to 6 weeks of age. By 8 weeks, when sexual maturity was attained, the reductase activity decreased to that level found in mature male mice. The cytosol from the preputial glands of the youngest mice (3 weeks) contained sufficient heat-stable acyl-CoA binding protein to stimulate acyl-CoA reduction; however, the 3-week-old preputial gland microsomes had little or no acyl-CoA reductase activity. As the animal matured, the stimulatory capacity in the heat-treated cytosol increased, reaching a maximum at 6 weeks; by 8 weeks, the stimulatory capacity of the soluble fraction had decreased to that found in mature male mouse. Results of this study suggest that the concentration of acyl-CoA, cytoplasmic acyl-CoA binding protein, and acyl-CoA reductase activity regulate the level of fatty alcohols in vivo and that the reductase activity and binding protein have similar patterns of development during puberty.     

Preweaning over- and underfeeding alters onset of puberty in the rat without affecting kisspeptin

The perinatal nutritional environment can permanently influence body weight, potentially leading to changes in puberty onset and reproductive function. We hypothesized that perinatal under- or overfeeding would alter puberty onset and influence concentrations of a neuropeptide crucial for successful puberty, kisspeptin. We manipulated Wistar rat litter sizes to derive small (SL), control (CL), and large (LL) litters containing 4, 12, and 20 rat pups respectively. This manipulation results in an overweight phenotype in SL rats and a lean phenotype in LL that persists throughout life. To investigate whether successful puberty onset is affected by neonatal under- or overfeeding, we examined indices of growth and development, including the onset of puberty, as well as the central expression of Kiss1 mRNA in these pups. Male LL rats reached puberty later than those from CL. These males also had reduced plasma testosterone and elevated 17beta-estradiol concentrations at puberty. The age at puberty onset was not affected in SL males despite accelerated growth. In females, puberty onset was not significantly delayed by having a lean phenotype, and steroid hormones were not affected. The age at onset was, however, younger in the SL females. Kiss1 mRNA in the hypothalamus was not affected by neonatal nutrition either at puberty or 7 days later. Our findings show early life underfeeding in males and overfeeding in females significantly affects puberty onset, altering steroid hormone concentrations in males, but this is not related to changes in hypothalamic kisspeptin.     

Influence of the presence of a mature female on puberty attainment in gilts

At 90 days of age, 40 Large White gilts were assigned to one of two treatments. At 155 days, a mature female which was left intact (Treatment I) or ovariectomized (Treatment O) was placed in each pen of five experimental gilts. From 180 days, estrus was checked daily with the back pressure test, and the occurrence of ovulation was detected by measuring the concentration of plasma progesterone at weekly intervals. From 240 days, a mature boar was introduced, for 5 minutes daily, into each pen during estrus detection. Gilts were slaughtered within 12 days after ovulation or at 270 days of age if they were not cyclic earlier. The percentage of gilts reaching puberty before 225 days of age was significantly higher in Treatment I (7 19 ) than in Treatment O (0 19 ) even though the average age at puberty was similar (I, 231 +/- 24 days; O, 243 +/- 12 days; mean +/- SD). Age at puberty and the number of days between mature female introduction and puberty differred significantly between the pens of gilts in Treatment O but not in Treatment I. Ovarian weights, ovulation rate and percentage of gilts with silent estrus were similar in the two treatments. Thus, the occurrence of pubertal estrus may be influenced by contact with an older, cyclic female or with other contemporary females raised in the same pen.     

Mutagenesis of a single AT basepair in mice transgenic for PhiX174 am3, cs70. II. Brain.

Most cell divisions in the mouse brain have ceased by 14 days after birth. Therefore, spontaneous mutations that occur in brain cells can be assumed to be fixed by replication during brain development. Spontaneous and ethylnitrosourea (ENU)-induced reverse mutations at a single AT base pair were measured in brain tissue by using mice transgenic for PhiX174 am3, cs70. The line (am54) has 50 PhiX genomes per haploid genome integrated in a tandem array and is maintained by random breeding on a C57BL/6 background. For mutagenesis studies, homozygous am54 males were mated to non-transgenic C57BL6/J females. Four-day old offspring from this cross were treated with 50 mg/kg ENU and were euthanized at 68-80 days of age. The ENU-treated animals had a significantly higher frequency of am3 revertants in brain than did concurrent controls. In a second experiment, hemizygous male offspring (85 to 94 days old) were treated with 150 mg/kg ENU and euthanized at various post-injection intervals (3, 10 and 110 days). The revertant frequencies 3 and 10 days after treatment were not significantly different from control values. At the 110 days post-injection interval, however, the average revertant frequency in the treated group was significantly lower than controls. In a second study animals were euthanized 3, 10 and 74 days after treatment. Two groups (3 and 74 days post-injection) also showed a significant decrease in the revertant frequency as compared to controls. Additional sets of adult animals were treated with 50 and 150 mg/kg ENU and were euthanized 195 to 201 days after treatment. The average revertant frequency of the animals that were treated with 50 or 150 mg/kg ENU was not significantly different from the control value. Thus, although an increase in mutant frequency is detected in the PhiX174 system when neonatal mice are treated with ENU, treatment of mature mice with ENU did not result in an increase in the mutant frequency. Moreover, under certain conditions, ENU-produced a significantly lower mutant frequency than was observed in the control animals. This decrease in the revertant frequency among the treated animals was likely due to selective killing of cells with a higher spontaneous revertant frequency than cells with lower frequency.     

Influence of season of birth on growth and reproductive development of Brahman bulls

Seasonal effects on reproduction are more dramatic in Bos indicus than Bos taurus cattle. This experiment evaluated reproductive development of fall- (n=7) versus spring- (n = 10) born Brahman bulls to determine if season of birth affects reproductive development. Measurements of growth and reproductive development began after weaning and continued at bi-weekly intervals until each bull reached sexual maturity. Different stages of sexual development were classified according to characteristics of the ejaculate and included first sperm in the ejaculate, puberty (> 50 x 10(6) sperm/ejaculate), and sexual maturity (two ejaculates with > 500 = 10(6) sperm/ejaculate). Average daily increases in all measured traits were similar in fall- and spring-born bulls and there were no differences in age, body weight, scrotal circumference, or paired testis volume between groups at first sperm or puberty. However, fall-born bulls were older (P < 0.05) than spring-born bulls at sexual maturity (553 days versus 481 days, respectively) as the interval between puberty and sexual maturity was longer (P < 0.05) in fall- than in spring-born bulls (82 days versus 54 days, respectively). The prolonged interval between puberty and sexual maturity in fall-born calves coincided with a short photoperiod (winter) whereas the short interval between puberty and sexual maturity in spring-born calves coincided with a long photoperiod (summer). In conclusion, season of birth affected sexual development; photoperiod might be involved in regulating testicular function immediately after puberty in Brahman bulls.     

A role for aromatizable androgens in female rat puberty

The function that aromatizable androgens may have in female puberty is unclear. The present experiments were undertaken to examine, using a quantitative approach, the role that physiological levels of these androgens may play in determining the timing of vaginal opening and first ovulation in female rats. Serum androstenedione (delta 4) levels increased markedly between Postnatal Days 4 and 8, remained elevated through Day 16, and declined thereafter to remain at about 100 pg/ml throughout juvenile development (Days 20-32). Serum testosterone (T) also increased, though less prominently after Postnatal Day 4. Maximal values were found at Day 12 (about 150 pg/ml); thereafter, T levels decreased to intermediate values (about 100 pg/ml), which were maintained during juvenile days. Serum dehydroepiandrosterone (DHA) remained undetectable throughout prepubertal development. At puberty, serum delta 4 increased 2.5-fold, but only at the time of the preovulatory luteinizing hormone (LH) surge. In contrast, T levels increased significantly 2-fold on the early proestrous-2 phase of puberty, 3.5-fold on the morning of first proestrus, and 9-fold at the time of the LH surge. Serum DHA remained undetectable. Implantation of Silastic capsules containing T at 2 or 6 mg/ml oil into juvenile 28-day-old rats resulted in serum T levels similar to those found on early proestrous 2 (about 150-180 pg/ml) and at 1300 h of first proestrus (ca. 300-400 pg/ml), respectively. Both treatments induced precocious vaginal opening, but failed to advance first ovulation. About 50% of the T-implanted rats had ambiguous estrous-type vaginal cytology preceding the day of first diestrus, and failed to show corpora lutea at this time.(ABSTRACT TRUNCATED AT 250 WORDS)     

Embryo production by ovum pick up in unstimulated calves before and after puberty

The possibility of producing embryos from oocytes repeatedly collected from unstimulated calves was tested, and results obtained before and after puberty were compared in the same animals. Ovum pick-up (OPU) coupled with in vitro embryo production was used on 2 sets of 7 and 9 calves, aged 7 to 10 m.o. at the start of the experiment. The oocytes were collected twice a week during a 2-m.o. period before puberty and a 1-m.o. period after puberty. Oocytes were fertilized and co-cultured with cumulus cells in modified synthetic oviduct fluid (SOF) up to Day 7 post insemination. Some Day 7 blastocysts were vitrified and transferred to recipient heifers. An average of 3.8 to 6.8 follicles was punctured per OPU session; 1.9 to 3.1 oocytes were collected, of which more than 60% were of good quality. The number of punctured follicles and collected oocytes varied between donors. Blastocyst rates of 19 to 27% were obtained for the 2 sets. Three normal calves were born from the transfer of 20 vitrified embryos. While no significant difference was observed for the first set of calves, a significant decrease in the number of punctured follicles was observed after puberty in the second set. A direct correlation was also obtained between the number of follicles punctured before and after puberty in the same animal. In conclusion, oocytes can be collected by repeated OPU in calves 7 to 10 m.o. old without affecting their growth or the onset of puberty. An average of 5 to 11 (range 0 to 16) blastocysts per donor was produced over 2 month. However, important variations were found between donors. The correlation observed for the number of follicles punctured before and after puberty suggests that this parameter is determined before puberty.     

Body development and puberty in spontaneously hypertensive rats

The body growth and the onset of puberty in spontaneously hypertensive rats (SHR) and in normotensive controls (WKY) have been studied. In female rats the onset of puberty was determined by both the age and the body weight at which the vaginal opening and first estrus appeared, as well as the ability of estradiol and progesterone to induce pituitary LH release. For this purpose females were injected with estradiol benzoate (0.1 mg/kg) and progesterone (1 mg/rat). Control animals received only oil vehicle. In male rats, puberty was assessed by studying the age and body weight at the time of balano-preputial separation. In another experiment, SH and WKY rats were decapitated on day 30 to determine FSH, LH, PRL, GH and testosterone plasma levels in males and FSH and LH in females. The results obtained show: a) A greater body weight, at all the ages studied (every 4 days between days 28 and 92) in SHR animals. b) A delay in vaginal opening and first estrus presentation in SHR females. c) Absence of spontaneous LH peaks in WKY females. d) Advancement in balano-preputial separation in SHR males and e) Higher plasma FSH levels in SHR males than in WKY males, without differences in other hormones.     

Testicular development in Chinese Meishan boars

The developmental process of the testis and age-related changes in the morphology of rete testicular spermatozoa were investigated in Meishan boars at 1 to 364 days of age. Testicular weight and the diameter of seminiferous tubules increased rapidly until 150 to 180 days of age. Leptotene stage spermatocytes, round spermatids and spermatozoa were first found in the section of seminiferous tubules at 30 to 45, 60 and 75 days of age, respectively. However, after 105 to 120 days of age, most rete testicular spermatozoa were morphologically normal. These results indicate that Meishan boars reach puberty as early as 75 days of age, though the testes acquire the ability to produce morphologically normal spermatozoa at about 120 days.     

Puberty and growth rate in thoroughbred fillies

We measured progesterone and estradiol levels from birth to the beginning of adult life in 10 Thoroughbred fillies from the Equilia Stud Farm in Avaré SP, Brasil. The animals were measured and weighed monthly for the determination of body development and of a possible correlation between the rate of weight and height gain and the onset of detectable sex hormone levels. Jugular blood was collected twice a week and stored at -20 degrees C until assay of progesterone by a solid phase RIA with a sensitivity of 0.32 nmol L and of estradiol by liquid phase RIA adapted to low levels (3.67 pmol L ). The fillies were born with high serum levels of both hormones, which fell to undetectable levels by the first week of life. A variation in growth rate was observed, with the highest levels occurring from birth to the 3rd month of life, followed by a reduction until 15 mo of life, when fast growth was resumed. The monthly weight gain was 1.5% when the fillies reached puberty and 5.4% during the next month, (P < 0.05, Friedman test). During this second period of accelerated growth after the beginning of progesterone production at detectable levels (above 0.318 nmol L ), the parameters of skeletal growth did not differ (P > 0.05). The month of onset of puberty was the month of lowest weight gain in the life of the fillies, and it coincided with the highest insolation period. In conclusion, horses, like all other developed vertebrates, have a double pattern of development, with the acceleration observed at puberty depending on sex steroids, which in turn coincides with the highest insolation period. Gonadal activity characterized by serum progesterone levels was low from birth to the onset of puberty. After puberty the progesterone cycles were similar to those of adult animals with a mature hypothalamic-gonadal axis.