In adult ovariectomized rhesus monkeys bearing hypothalamic lesions which reduced circulating LH and FSH to undetectable levels, sustained elevated gonadotropin concentrations were reestablished by the intermittent administration of gonadotropin-releasing hormone (GnRH) at the rate of 1 microgram/min for 6 min once every hour. The effects of varying either the frequency or the amplitude of these GnRH pulses on gonadotropin secretion were examined in such animals. Increasing the frequency of GnRH administration from the physiological one pulse per h to two, three, or five pulses h while maintaining a constant infusion rate and pulse duration resulted in gradual declines in plasma gonadotropin concentrations. These declines were most profound at the highest frequencies and the consequence of reduced pituitary responses to individual GnRH pulses. Decreasing the frequency of GnRH pulses from one per h to one every 3 h led to variable declines in plasma LH levels, but circulating FSH invariably rose. Reducing the GnRH infusion rate from 1 to 0.1 mg/min while maintaining constant frequency and pulse duration resulted in abrupt declines in plasma LH and FSH to immeasurable levels, although pulsatile increments in circulating GnRH concentrations without a concomitant reduction in plasma LH concentrations, which remained unchanged. An infusion rate of 0.5 microgram/min resulted in unstable plasma LH and FSH levels. These results demonstrate that changes in the frequency or amplitude of hypophysiotropic stimulation have profound effects on plasma gonadotropin levels as well as on FSH to LH ratios in the circulation. The physiological implications of these observations are discussed.

The aim of this review is to provide an integrative analysis of the role of FSH in the control of testicular function in higher primates, including man. Attention is focused on the action of FSH during neonatal development, puberty, and adulthood. Whether FSH is the major determinant of the adult complement of Sertoli cells and whether FSH is obligatory for the initiation, maintenance, and restoration of spermatogenesis is evaluated. The mechanism whereby the circulating concentration of FSH regulates spermatogonial proliferation to dictate the sperm production rate under physiological conditions in the adult is discussed in detail. Inhibin B is the major component of the testicular negative feedback signal governing FSH beta gene expression and FSH secretion, and the evidence for this view is presented. The review concludes with the presentation of a model for the operation of the FSH-inhibin B feedback control system regulating sperm production postpubertally in monkey and man, and with speculation on issues of clinical interest.

Gonadal quiescence prior to puberty in primates results from a diminished secretion of the pituitary gonadotropic hormones, follicle-stimulating hormone and luteinizing hormone, which, in turn, is occasioned by an interruption of pulsatile release of gonadotropin-releasing hormone (GnRH) from the hypothalamus during this phase of development. A discharge of GnRH may be provoked from the hypothalamus of prepubertal monkeys, however, by an i.v. injection of N-methyl-D-aspartate (NMDA), an analog of the putative excitatory neurotransmitter, aspartate. Since this action of NMDA is blocked by the specific NMDA receptor antagonist, DL-2-amino-5-phosphonopentanoic acid, the release of GnRH is likely mediated by NMDA receptors located either on the GnRH neurons themselves or on afferents to the GnRH cells. We report here that prolonged intermittent NMDA stimulation of GnRH neurons within the hypothalamus of the juvenile monkey for 16-30 wk results, with surprising ease, in the onset of precocious puberty with full activation of the hypothalamic-pituitary-Leydig cell axis and initiation of spermatogenesis. These rmdings demonstrate that, in primates, the network of hypothalamic GnRH neurons, which in adulthood provides the drive to the gonadotropin-secreting cells ofthe anterior pituitary gland, must now be viewed together with the pituitary and gonads as a nonlimiting component of the control system that governs the onset of puberty in these species.In the rhesus monkey and other primates including man, the hypothalamic-pituitary component ofthe control system that governs gonadal function attains a high degree of organization during fetal development (1). Thus, with loss of the inhibitory action of placental steroids at birth, the behavior of this neuroendocrine axis during early neonatal development is reminiscent of that associated with adulthood. Infancy in primates, however, is not followed by an immediate transition into a state of sexual maturity; instead, gonadotropin secretion declines, thus guaranteeing the protracted phase of gonadal quiescence that characterizes prepubertal development in these species. Puberty occurs several years later when the hypothalamic-pituitary axis is reawakened from its prepubertal dormancy (1).An interruption in the pulsatile release of gonadotropinreleasing hormone (GnRH), the hypothalamic-feleasing factor that provides the major drive to the gonadotropin-secreting cells of the anterior pituitary gland (gonadotrophs), appears to underlie the prepubertal hiatus in luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion (refs. 2-5, 7; 1). The seemingly dormant GnRH neurons of the hypothalamus of the prepubertal monkey, however, may be excited by peripheral injections of N-methyl-D-aspartate (NMDA) into producing an intermittent discharge of their releasing factor into the hypophysial portal circulation that results in a sustained hypophysiotropic drive to the gonadotrophs (8, 9). This and other central neural actions of NMDA, which is an analog of the p...

In rhesus monkeys with hypothalamic lesions (which appear to abolish the endogenous production of gonadotropin-releasing hormone), normal ovulatory mestrual cycles were reestablished by an unvarying, long-term replacement regimen consisting of one intravenous pulse of synthetic gonadotropic-releasing hormone per hour. This finding is in accord with the hypothesis that the pattern of pituitary gonadotropin secretion throughout the menstrual cycle (basal secretion interrupted, once every 28 days on the average, by a preovulatory surge) is not directed by alterations in hypothalamic gonadotropin-releasing hormone secretion but by the ebb and flow of ovarian estrogens acting directly on the pituitary gland.

Objective Determine the molecular characteristics of human spermatogonia and optimize methods to enrich spermatogonial stem cells (SSCs). Design Laboratory study using human tissues Setting Research institute Patient(s)/Animal(s) Normal adult human testicular tissue. Interventions Human testicular tissue was fixed or digested with enzymes to produce a cell suspension. Human testis cells were fractionated by FACS and MACS. Main Outcome Measure(s) Immunostaining for selected markers, human-to-nude mouse xenotransplantation assay. Results Immunohistochemistry co-staining revealed the relative expression patterns of SALL4, UTF1, ZBTB16, UCHL1 and ENO2 in human undifferentiated spermatogonia as well as the extent of overlap with the differentiation marker, KIT. Whole mount analyses revealed that human undifferentiated spermatogonia (UCHL1+) were typically arranged in clones of 1–4 cells while differentiated spermatogonia (KIT+) were typically arranged in clones of 8 or more cells. The ratio of undifferentiated to differentiated spermatogonia is greater in humans than in rodents. SSC colonizing activity was enriched in the THY1dim and ITGA6+ fractions of human testes sorted by FACS. ITGA6 was effective for sorting human SSCs by MACS; THY1 and EPCAM were not. Conclusions Human spermatogonial differentiation correlates with increased clone size and onset of KIT expression, similar to rodents. The undifferentiated to differentiated developmental dynamics in human spermatogonia is different than rodents. THY1, ITGA6 and EPCAM can be used to enrich human SSC colonizing activity by FACS, but only ITGA6 is amenable to high throughput sorting by MACS.