Male mice were raised in cohabitation with females from birth to 90 days. Testosterone was measured every 10 days in plasma and testes. Sex difference in body weight was related to the pre-pubertal increase of testosterone levels in males. The weight of the seminal vesicle was positively correlated with circulating testosterone levels between 1 and 40 days but not between 50 and 90 days Testosterone concentrations in the plasma and testes were high at birth: 630 pg/ml and 58.0 \ m=+-\ 17.7 ng/100 mg; they subsequently decreased during the first days of life and remained low until day 20: 240 \m=+-\ 110 pg/ml and 0.1 \ m=+-\0.03 ng/100 mg. The testosterone levels then increased rapidly between days 20 and 30 and especially between 30 and 40 reaching their maxima: 5770 \ m=+-\ 1720 pg/ml and 123.7 \ m=+-\ 18.3 ng/100 mg testis. This increase was transitory and testosterone levels fell after day 40. By 90 days, the testosterone levels, 440 \ m=+-\ 65 pg/ml and 43.2 \ m=+-\ 5.5 ng/100 mg testis, were comparable to those measured at birth. Plasma testosterone and age were positively correlated between 1 and 40 days, and negatively between 50 and 90 days. The first fertile matings occurred at age 40 days.In spite of a large body of research, the physiological mechanisms responsible for sexual maturation remain poorly understood (Odell 8c Swerdloff 1976). One important factor for their understanding involves a systematic study of the secretion of sex hormones from birth to puberty. The present report deals with the variations of plasma and testicular testosterone in mice between 1 and 90 days of age and the correlations between the levels of this hormone and the development of seminal vesicle, spermatogenesis and the appearance of fertility. MATERIALS AND METHODSMice of the Swiss strain (CD-I, Charles River) were raised in the laboratory under identical conditions of temperature (20 ± 1°C), lighting (daylight) and nutrition (com¬ plete pelleted chow and water given freely). Immediately after delivery the litters were reduced to 6 pups, 3 males and 3 females. After weaning on the 20th day, the 6 pups stayed in cohabitation until sacrifice. Litters which appeared in the cages were immediately removed. 15 males were sacrificed every 10 days from birth to 90 days. Blood was obtained by cardiac puncture without anticoagulant under light ether anaes¬ thesia. Testosterone was measured in the right testis of each male. Testosterone was determined on pooled samples of plasma (15 animals) on the one day and ten day old mice, in individual samples for the other stages. Testosterone was also measured in females, in pooled samples of plasma in the one day (100 animals), 10 -20 -30 and 40 (15 animals each) day-old animals.Testosterone was measured by the radioimmunoassay method previously described (Veyssiere et al. 1976). The antibody was prepared in rabbits against testosterone 3-0-carboxy-methyl-oxime bovine albumine. The major cross reacting steroid, 5a-dihydrotestosterone (74°/o) was separated by chromatography on a celite colum...

Abstract. Plasma testosterone, LH and FSH levels were determined and correlated with reproductive organs growth, testicular differentiation, fighting and mounting behaviour in maturing rabbit. An infantile phase of development extends from birth to 40 days, characterized by low testosterone and FSH levels, decreasing LH levels (until 20 days) and by a slow growth of testis and seminal vesicle. The peripubertal phase starts abruptly around day 40. It is marked by simultaneous events: the appearance of mature Leydig cells in the testis, a striking increase in testosterone and FSH levels, a small rise in LH levels and an acceleration of testicular growth. The phase of rapid growth of seminal vesicle and the first meiotic divisions start around day 70, in presence of high circulating levels of FSH and testosterone. Fighting (3 months) and mounting behaviour (146 ± 13 days) occur lately after a long period of high circulating testosterone levels.

The levels of testosterone (T) and dihydrotestosterone (DHT) in the epididymis, vas deferens and preputial gland were assessed in mice from 1 to 90 days. The weight increase of these 3 organs was proportionately greater than that of the whole body until 50 or 60 days, and they attained their adult histological appearance approximately 20 days prior to puberty. Expressed in ng/g, the concentration of androgens (T+DHT) in the epididymis (14.3 to 36.5), vas deferens (6.6 to 24.0) and preputial gland (1.5 to 4.7) were higher than in plasma (0.2 to 3.6 ng/ml). The concentration of either androgen varied little during sexual maturation and was not correlated with circulating levels. The highest concentration of androgen (T+DHT) was observed at birth suggesting that the neonatal period is crucial for development of the accessory sexual organs. In the epididymis and preputial gland T was the predominant androgen during the infantile phase of development, whilst DHT predominated thereafter. In the vas deferens concentrations of T were always equal to or higher than those of DHT. These results suggest that the ability of the accessory sexual organs to accumulate androgens appears to be more important than the circulating concentration of androgens in determining their growth and differentiation.

Testerone (T) concentrations in the plasma, gonads, and adrenals were measured by radioimmunoassay in 188 male and 160 female rabbit fetuses. Determinations were performed daily from the 20th to the 31st day of gestation and were correlated with the maternal plasma concentration of T. The T content of both testes remained relatively constant from the 20th (3040 pg) to the 26th day (3940 pg), and subsequently decreased until the 31st day (1630pg). the concentration of testicular T fluctuated only slightly from the 20th (1040 pg/mg) to the 23rd day (783 pg/mg), and thereafter decreased until the 31st day (138 pg/mg). The T levels in plasma of males (132-361 pg/ml) were significantly higher than those of females (21-116 pg/ml). Plasma T levels in males were relatively constant and did not exhibit any rise, which is similar to observations of the testis during differentiation of the genital tract. Testosterone concentrations were low in the adrenals (3.5-12.3 pg/mg) of both sexes and in the ovaries (1.5-20.4 pg/mg) of fetuses. These data provide the first evidence for testicular T secretion at the time of genital differentiation in the rabbit.

Plasma oestrogens (E1, E2) and gonadotrophins, and ovarian oestrogens, were determined in female rabbits from birth to 6 months. Increases in ovarian weight followed a curvilinear pattern, with a phase of slow growth (from 1 to 60 days) preceding a phase of rapid growth (from 60 to 90 days). At birth, E2 was already quantifiable in ovaries; it remained at very low levels up to 50 days. Ovarian E2 content increased until 6 months with two sharp rises between 50 days (46 \ m=+-\4 pg/2 ovaries) and 60 days (365 \m=+-\ 53 pg/2 ovaries) and between 80 days (326 \ m=+-\77 pg/2 ovaries) and 90 days (1118 \ m=+-\307 pg/2 ovaries). E1 appeared in ovaries at 50 days and then followed a pattern similar to that of E2. Oestrogens were never detected in plasma.At birth FSH and LH levels were similar to those measured in adult females but from 10 days the secretion of both gonadotrophins showed a clear dissociation. LH concentrations varied little during the period studied (between 1077 \m=+-\106 pg/ml to 2030 \ m=+-\466 pg/ml). Plasma FSH levels were considerably higher between 10 and 50 days than those measured thereafter, particularly in adulthood. The decline in FSH levels occurred simultaneously with the rise in ovarian oestrogens. The pubertal development involves many different maturational events including maturation of the hypothalamic-pituitary-gonadal axis and secondary sexual development which culminates in fertility.rabbit. The aim of the present prospective study was to measure longitudinally, from birth to adult¬ hood, ovarian and plasma oestrogens and circula¬ ting gonadotrophins. Materials and MethodsRabbits of the New Zealand strain were raised in the laboratory under identical conditions of temperature, lighting (daylight) and nutrition (complete pelleted chow and water given freely). Newborns of both sexes were raised by their mothers. When 30 day old, females were isolated 2-4 per cage until sacrifice and were housed in a room containing males and females of different ages. Under our breeding conditions, female rabbits first con¬ ceived at 110-120 days and first fertile matings occurred in males at the mean age of 146 ± 13 days (Berger et al. 1982). Females were killed, without anaesthesia, by sec¬ tion of the jugular and carotid vessels, every 10 days from birth to 90 days and at 4, 5 and 6 months. The blood plasma and both ovaries were frozen at -25°C until assayed.Oestrone (E^a nd oestradiol-17ß (E2) were determined in plasma and ovaries by radioimmunoassay. After addi¬ tion of 2000 DPM of tritiated Et and 2000 dpm of tritiated E2 to monitor procedural losses, plasma (1 to 5 ml) and ovaries (homogenized in 3 ml distilled water) were alkalinized with 0.1 ml 0.1 NaOH and extracted twice with 5-10 ml of freshly distilled diethylether. Ej and E2 were separated by chromatography on a celite column (Ro¬ bertson et al. 1972). After incubation overnight at 4°C, antibody-bound and free hormone were separated using dextran-charcoal.The recovery of the added radioactive standards was 80 ± 13% (mean ± SD) for E, a...