A Practical Approach, Second Edition=Ronald D. Ho.pdf

POSTNATAL DEVELOPMENTAL MILESTONES 1049to increase shortly after birth, reaching maximum concentrations on day 12, then declining graduallyto approximately 20% of the day-12 values by the end of the juvenile period (6,7,8). Plasmaluteinizing hormone (LH) levels are also higher in neonatal-infantile rats than in juvenile rats andare characterized by sporadic surges (6,7,8,10). These surges disappear completely by the juvenilephase and LH levels remain low during this period.During the infantile period, estradiol exerts relatively little negative feedback, due to highaffinitybinding to the high levels of alpha-fetoprotein (11), and aromatizable androgens play aleading role in the steroid negative feedback control of gonadotropin secretion. This changes froma predominantly androgenic control to a dual estrogenic-androgenic control during the juvenileperiod. During the juvenile period, there is an increase in secretion of dehydroepiandrosterone(DHEA) and DHEA sulfate associated with the maturation of a prominent adrenal zona reticularis.Occurring at day 20 in the rat (12), adrenarche, the maturation of a prominent zona reticularis, ischaracterized by a prepubertal rise in adrenal secretion of DHEA and DHEA sulfate that isindependent of the gonads or gonadotropins. The rise in DHEA and DHEA sulfate productionresults from the increased presence of 17alpha-hydroxylase/17,20-lyase and DHEA sulfotransferaseand the decreased presence of 3beta-hydroxysteroid dehydrogenase (13,14).Also during the juvenile period, morphological maturation of neurons in the hypothalamuscoincides with changes in the diurnal pattern of LH release and the time at which the hypothalamicpituitaryunit becomes fully responsive to estradiol positive feedback (15).The ovary is relatively insensitive to gonadotropin stimulation during the first week after birth,coming under strong gonadotropin control during the second week (16,17). Waves of folliculardevelopment and atresia occur during the juvenile period, but these follicles are not ovulated. Atthe end of the juvenile phase, the mode of LH release begins to change (9,10), animals are olderthan 30 days of age, their uteri are small (wet weight less than 100 mg), no intrauterine fluid canbe detected and vaginal patency is not yet achieved (18). Animals progressing to the next phasehave larger uteri with intrauterine fluid and the vagina remains closed (19). On the day of the firstproestrus, there is a large amount of intrauterine fluid, the wet weight of the uterus is greater than200 mg, the ovaries have large follicles, and the vagina is closed in most animals (18). On the dayof the first ovulation, uterine fluid is no longer present, fresh corpora lutea have been formed, thevagina is open and vaginal cytology shows a predominance of cornified cells. A common parameterof sexual maturity in rats is vaginal opening. Vaginal opening occurs on the day after the firstpreovulatory surge of gonadotropins, i.e. day 36-37 (20,21,22,13) but can range from 32 to 109days (24). Female rats may maintain their full reproductive potential to 300 days (1), however, thisis influenced by the strain of rat.As in the rat, there is an early postnatal rise in gonadotropin secretion in the human and monkey,followed by a reduction lasting from late infancy to the end of the prepubertal period (25,26,27,28).Until about 12-24 months in girls, the ovaries respond to the increased follicle-stimulating hormone(FSH) by secreting estradiol reaching levels that are not again achieved before the onset of puberty(29,30). The secretion of estradiol then decreases with a nadir, in the human, at about 6 years ofage (29). During this time, there is a reduction in the pulsatile release of gonadotrophin releasinghormone (GnRh) which appears to involve both gonadal and central nervous system restraints. Inthe human, the timing of adrenarche may vary from 5 to 8 years (31,32). However, several reportssuggest that adrenarche may be a gradual maturational process beginning at 7-8 years that continuesto 13-15 years and is not the result of sudden rapid changes in adrenal enzyme activities or adrenalandrogen concentrations (10,17,33,34). Interestingly, adrenarche appears to be absent in non-humanprimates (5,35,36). Although adrenal androgen concentrations are similar to those in humans,adrenal androgen levels throughout development in rhesus and cynomolgus monkeys and baboonsare similar to those in adulthood (37,38).Menarche, the onset of menstrual cyclicity, is regarded as an overt sign of the initiation ofpuberty, although endocrine profiles indicate significant hormonal changes years/months prior. Thefirst ovulation occurs sometime after the first cycle. This supports the “adolescent sterility” hypothesis© 2006 by Taylor & Francis Group, LLC