Ultrastructural immunocytochemical localization of luteinizing hormone-releasing hormone (LH-RH) in the median eminence of the guinea pig

Glucose responsiveness in a novel adult-derived GnRH cell line, mHypoA-GnRH/GFP: involvement of AMP-activated protein kinase

Glucose regulates energy homeostasis and reproductive function within the hypothalamus. The underlying mechanisms responsible for glucose regulation of GnRH gene transcription were investigated using a novel murine immortalized, adult-derived hypothalamic cell line, mHypoA-GnRH/GFP. Analysis of GnRH mRNA synthesis and secretion following agonist treatment demonstrated that the mHypoA-GnRH/GFP cell line is a representative model of in vivo GnRH neurons. c-fos mRNA levels, following glucose exposure, indicated that these neurons were responsive to low (0.5mM) and high (5mM) glucose, and high glucose stimulated GnRH mRNA transcription in a metabolism-dependent manner. Glucose inhibited AMPK activity, and was linked to the downstream stimulation of GnRH mRNA levels. The effect was confirmed with an AMPK antagonist, Compound C. Collectively, these findings demonstrate that glucose can directly regulate GnRH transcription, while implicating the AMPK pathway as an essential mediator of nutritional signaling in a novel GnRH neuronal cell model.

Three-dimensional properties of GnRH neuroterminals in the median eminence of young and old rats

The decapeptide gonadotropin-releasing hormone (GnRH), which regulates reproduction in all vertebrates, is stored in, and secreted from, large dense-core secretory vesicles in nerve terminals in the median eminence. GnRH is released from these terminals with biological rhythms that are critical for the maintenance of normal reproduction. During reproductive aging in female rats, there is a loss of GnRH pulses and a diminution of the GnRH surge. However, information about the specific role of GnRH nerve terminals is lacking, particularly in the context of aging. We sought to gain novel ultrastructural information about GnRH neuroterminals by performing three-dimensional (3D) reconstructions of GnRH neuroterminals and their surrounding microenvironment in the median eminence of young (4-5 months) and old (22-24 months) ovariectomized Sprague-Dawley female rats. Median eminence tissues were freeze-plunge embedded and serial ultrathin sections were collected on slot grids for immunogold labeling of GnRH immunoreactivity. Sequential images were used to create 3D models of GnRH terminals. These reconstructions provided novel perspectives into the morphological properties of GnRH terminals and their neural and glial environment. We also noted that the cytoarchitectural features of the median eminence became disorganized with aging. Quantitative measures showed a significant decrease in the apposition between GnRH terminal membranes and glial cells. Our data suggest reproductive aging in rats is characterized by structural organizational changes to the GnRH terminal microenvironment in the median eminence.

Neuroendocrine control of reproductive aging: roles of GnRH neurons

The process of reproductive senescence in many female mammals, including humans, is characterized by a gradual transition from regular reproductive cycles to irregular cycles to eventual acyclicity, and ultimately a loss of fertility. In the present review, the role of the hypothalamic gonadotropin-releasing hormone (GnRH) neurons is considered in this context. GnRH neurons provide the primary driving force upon the other levels of the reproductive axis. With respect to aging, GnRH cells undergo changes in biosynthesis, processing and release of the GnRH decapeptide. GnRH neurons also exhibit morphologic and ultrastructural alterations that appear to underlie these biosynthetic properties. Thus, functional and morphologic changes in the GnRH neurosecretory system may play causal roles in the transition to acyclicity. In addition, GnRH neurons are regulated by numerous inputs from neurotransmitters, neuromodulators and glia. The relationship among GnRH cells and their inputs at the cell body (thereby affecting GnRH biosynthesis) and the neuroterminal (thereby affecting GnRH neurosecretion) is crucial to the function of the GnRH system, with age-related changes in these relationships contributing to the reproductive senescent process. Therefore, the aging hypothalamus is characterized by changes intrinsic to the GnRH cell, as well as its regulatory inputs, which summate to contribute to a loss of reproductive competence in aging females.

Neuroendocrine regulation of GnRH release in induced ovulators

GnRH is the key neuropeptide controlling reproductive function in all vertebrate species. Two different neuroendocrine mechanisms have evolved among female mammals to regulate the mediobasal hypothalamic (MBH) release of GnRH leading to the preovulatory secretion of LH by the anterior pituitary gland. In females of spontaneously ovulating species, including rats, mice, guinea pigs, sheep, monkeys, and women, ovarian steroids secreted by maturing ovarian follicles induce a pulsatile pattern of GnRH release in the median eminence that, in turn, stimulates a preovulatory LH surge. In females of induced ovulating species, including rabbits, ferrets, cats, and camels, the preovulatory release of GnRH, and the resultant preovulatory LH surge, is induced by the receipt of genital somatosensory stimuli during mating. Induced ovulators generally do not show "spontaneous" steroid-induced LH surges during their reproductive cycles, suggesting that the positive feedback actions of steroid hormones on GnRH release are reduced or absent in these species. By contrast, mating-induced preovulatory surges occasionally occur in some spontaneously ovulating species. Most research in the field of GnRH neurobiology has been performed using spontaneous ovulators including rat, guinea pig, sheep, and rhesus monkey. This review summarizes the literature concerning the neuroendocrine mechanisms controlling GnRH biosynthesis and release in females of several induced ovulating species, and whenever possible it contrasts the results with those obtained for spontaneously ovulating species. It also considers the adaptive, evolutionary benefits and disadvantages of each type of ovulatory control mechanism. In females of induced ovulating species estradiol acts in the brain to induce aspects of proceptive and receptive sexual behavior. The primary mechanism involved in the preovulatory release of GnRH among induced ovulators involves the activation of midbrain and brainstem noradrenergic neurons in response to genital-somatosensory signals generated by receipt of an intromission from a male during mating. These noradrenergic neurons project to the MBH and, when activated, promote the release of GnRH from nerve terminals in the median eminence. In contrast to spontaneous ovulators, there is little evidence that endogenous opioid peptides normally inhibit MBH GnRH release among induced ovulators. Instead, the neural signals that induce a preovulatory LH surge in these species seem to be primarily excitatory. A complete understanding of the neuroendocrine control of ovulation will only be achieved in the future by comparative studies of several animal model systems in which mating-induced as well as spontaneous, hormonally stimulated activation of GnRH neurons drives the preovulatory LH surge.

Hormonal and neurotransmitter regulation of GnRH gene expression and related reproductive behaviors

Gonadotropin-releasing hormone (GnRH), having a highly conserved structure across mammalian species, plays a pivotal role in the control of the neuroendocrine events and the inherent sexual behaviors essential for reproductive function. Recent advances in molecular genetic technology have contributed greatly to the investigation of several aspects of GnRH physiology, particularly steroid hormone and neurotransmitter regulation of GnRH gene expression. Behavioral studies have focused on the actions of GnRH in steroid-sensitive brain regions to understand better its role in the facilitation of mating behavior. To date, however, there are no published reports which directly correlate GnRH gene expression and reproductive behavior. The intent of this article is to review the current understanding of the way in which changes in GnRH gene expression, and modifications of GnRH neuronal activity, may ultimately influence reproductive behavior.

Targeting of gonadotropin-releasing hormone axons from preoptic area grafts to the median eminence

Implantation of normal GnRH neurons can reverse many of the reproductive deficiencies that characterize hypogonadal (hpg) mice. Since the GnRH axons follow a stereotyped trajectory to their target we investigated the possibility that host brain regions adjacent to the graft might provide signals that induced this directional growth. The role of the adenohypophysis in GnRH axonal outgrowth was studied in mice with co-grafts of fetal preoptic area (POA) and pituitary and in hypophysectomized hosts. When fetal pituitaries were grafted together with the POA, immunoreactive GnRH fibers did enter the glandular tissue but they also grew into the host median eminence. Surgical removal of the pituitary of hpg hosts prior to POA graft placement was also compatible with GnRH innervation of the host median eminence although in some individuals that innervation pattern was confined to the more caudal aspects. The results of these two experiments suggest that the anterior pituitary gland may be an attractive target for GnRH axons but that this tissue is not essential for directed GnRH axonal outgrowth to its target. To determine if the median eminence itself could direct the growth of GnRH axons, co-grafts of POA and a fetal medial basal hypothalamic (MBH) block, which was predominantly median eminence, were made. Immunocytochemistry showed that an intragraft mini-median eminence was formed with a highly organized and robust GnRH innervation. Ultrastructural analysis indicated that these axons terminated near fenestrated capillaries. However, even under these conditions some GnRH axons exited into the host median eminence. It now seems likely that a cellular component of the median eminence can provide a signal to attract GnRH axons. Whether this signal is produced by the specialized ependymal cells, by the endothelia, or by meningeal (pial) components must now be tested.

Relationship of glia to GnRH axonal outgrowth from third ventricular grafts in hpg hosts

The homozygous mutant hypogonadal (hpg) mouse lacks a functional gene for the neuropeptide gonadotropin releasing hormone (GnRH). The consequence of this defect is an infantile reproductive tract in adulthood. This condition can be reversed by the implantation of normal fetal preoptic area tissue that contains GnRH neurons. Reversal is always preceded by the outgrowth of GnRH axons into the host target tissue, the median eminence, by a stereotyped pathway. In the current experiments we investigated the cellular nature of the path taken by early emerging GnRH axons focusing on their relationship with astrocytic components and with the specialized ependymal population of this area, the tanycytes. In control tissue glial fibrillary acid protein (GFAP) immunoreactivity was confined to the exterior of cerebral blood vessels and glial limitans. Both GFAP and vimentin, another intermediate filament protein, marked the specialized ependymal cells of this region, the tanycytes. There was a robust reactive astrocytic response to the injury of transplantation in both the donor and host tissue within 5 days of implantation and the reactive astrocytes persisted for 60 days. These cells were GFAP-positive and were present in many areas of the host along the cannula tract and not confined to the area of GnRH axonal outgrowth. Vimentin, another intermediate filament, marked only the specialized ependymal cells of this region, the tanycytes, in both control and grafted tissue. Despite the profound reactive gliosis, GnRH axons were shown to exit the implant as early as 5 days after grafting suggesting that the gliotic process did not constitute a barrier to this phenomenon. At the light microscopic level, double label immunocytochemical studies did not reveal any specific association between GFAP or vimentin-positive cellular processes and these pioneer GnRH fibers. However, since normal GnRH axons had been reported to travel in tanycytic channels through the medial basal hypothalamus we reinvestigated the pattern of early emerging GnRH axons at the ultrastructural level. With this higher resolution, GnRH axons were found adjacent to glial elements along their entire traverse from the graft-host interface, through the host basal hypothalamus to their termination on the hypophysial portal capillaries. At the interface, GnRH-positive axons appeared to exit via glial channels similar to those described in other developing and regenerating systems. In the host, GnRH immunoreactive axonal profiles were surrounded by glial processes though the latter could not be further defined as tanycytic or astroglial. Other, immunonegative, axons were frequently seen in axonal bundles or fascicles and not necessarily in contact with glia.(ABSTRACT TRUNCATED AT 400 WORDS)

Ultrastructure and synaptic organization of luteinizing hormone-releasing hormone (LHRH) neurons in the anestrous ewe

Electron microscopic immunocytochemistry was employed to examine the ultrastructure of luteinizing hormone-releasing hormone (LHRH) neurons and their projections to the median eminence in the sheep brain. LHRH perikarya in the preoptic area of anestrous ewes are less innervated than nonimmunoreactive cells in the same sections, but still receive numerous synaptic inputs, primarily onto distal dendrites and small somatic protuberances. Axon terminals synapsing upon LHRH cells contain a combination of clear spherical vesicles and larger dense-core vesicles. Interestingly, LHRH cell bodies and dendrites are almost entirely surrounded by glial processes. These processes intervene between immunoreactive elements that at a light microscopic level appear to be in contact with each other. Thus no evidence was obtained at the ultrastructural level for contacts among adjacent LHRH cells or dendrites in the preoptic area. Synaptic inputs onto LHRH cell bodies and dendrites appear to penetrate this glial sheath. In contrast to the absence of contacts among LHRH cells in the preoptic area, individual LHRH terminals in the median eminence are often clustered in direct plasma membrane contact. Comparisons between animals of differing reproductive status are needed to determine whether alterations in synaptic inputs, glial ensheathment, or LHRH-LHRH appositions, may underlie seasonal changes in the activity of LHRH neurons.

Morphological adaptability at neurosecretory axonal endings on the neurovascular contact zone of the rat neurohypophysis

To compare the effects of a variety of acute and chronic stimuli that bring about or terminate hormone release the ultrastructure of nerve terminal contact at the basal lamina of the neurohypophysial neurovascular contact zone was examined quantitatively in young adult rats of the following treatment groups: untreated virgin females, untreated male rats, prepartum (day 21 of gestation), postpartum (on the day of parturition), lactating (14 days of suckling), mothers 10 days after their pups were weaned, 48 h water-deprived males, males given 2% saline solution (dehydrated) for 10 days, males given 2% saline as described then given tap water to rehydrate for 2 or 5 weeks. Morphometric analysis of electron micrographs revealed that all stimuli leading to increased hormone release were accompanied by both increased occupation of the basal lamina by nerve terminals as well as decreased enclosure of neurosecretory processes by pituicyte cytoplasm. Neural occupation of the basal lamina remained significantly elevated 10 days post-weaning and at 2 weeks (but not 5 weeks) of rehydration following 10 days of dehydration. Pituicyte enclosure of neurosecretory axons had returned to control values in the postweaning and 5 week (but not 2 week) rehydrated animals. The mean length of individual nerve terminal contact with the basal lamina was found to increase under some, but not all, conditions associated with increased hormone release (i.e. parturition, acute and chronic dehydration, but not during lactation) and to decrease below control values in prepartum females and after 5 weeks of rehydration.(ABSTRACT TRUNCATED AT 250 WORDS)

Luteinizing hormone releasing hormone containing synapses in the diagonal band and preoptic area of the guinea pig

Light microscopic immunocytochemical processing of the region of the ventral diagonal band of Broca (vertical limb) and the medial preoptic area of the guinea pig for LHRH revealed complex axonal bundles. The axons were of several different shapes and widths and showed varicosities of varying form. When such tissue was embedded and processed for electron microscopic localization of immunoreaction product, numerous LHRH-positive synapses were seen. The synapses were made by large varicosities. They were all axodendritic and showed a well-defined synaptic cleft, as well as a clustering of immunoreactive large granules and immunonegative small, clear vesicles. Postsynaptic densities were either very small or absent. These data show that LHRH is present in direct synapses within the CNS and is, therefore, in a position to be released and act as a neurotransmitter/neuromodulator as well as a neurohormone.

CNS regulation of reproduction: peptidergic mechanisms

The coordinated activities of several networks of peptidergic neurons contribute to the overall neural regulation of reproduction and associated behaviours. Key elements are rostral hypothalamic cells that synthesize and release LHRH from their median eminence nerve terminals, subject to modulation by a variety of endogenous transmitters and neuropeptides including VIP, CCK, opioids and somatostatin. In the CNS, LHRH may also participate in intercellular communication to facilitate the expression of estrogen-sensitive sexual behavior. Vasopressin and oxytocin also appear to modulate maternal and reproductive behavior. In addition, 'oxytocinergic' neurons recorded during lactation and milk ejection display unique bursting activity patterns deemed important for efficient release of hormone. However, endogenous opioid peptides appear able to dissociate this stimulus-secretion coupling mechanism, possibly through interference with a calcium sensitive component in nerve terminals.

Horseradish peroxidase: a tool for study of the neuroendocrine cell and other peptide-secreting cells

The versatility of horseradish peroxidase is its usefulness both as an antigenic marker and as a probe molecule. We have demonstrated in the neuroendocrine cell that an HRP-bound antibody offers a high order of resolution for determining in which cellular compartment an antigen is located and where it is not. When native peroxidase is applied as an intracellular probe, it labels organelles associated with endocytosis in retrograde axonal transport and with the lysosomal system in both retrograde and orthograde axonal transport. The investigation that remains is the application of lectin-bound HRP to determine the pathways of membrane flow at the time when the neuroendocrine cell is stimulated to synthesize, transport, and secrete its peptide. For example, we are interested to know (1) whether internalized axon terminal membrane tagged with wheat germ agglutinin-HRP is channeled to all Golgi saccules engaged in the production of secretory granules in salt stimulated supraoptic neurons; and (2) if internalized cell membrane of the supraoptic cell body is tagged with wheat germ agglutinin-HRP and channeled to GERL, will this membrane be transferred from GERL to secretory granules, lysosomes in the cell body and axon, the axonal endoplasmic reticulum, and to autophagic/crinophagic vacuoles in axon terminals of salt-stressed supraoptic neurons? These additional studies should provide a more comprehensive, morphological picture of membrane flow in a neuroendocrine cell that is responding to the metabolic demands placed upon it.

Ultrastructural localization of LH-RH-immunoreactive synapses in the hamster accessory olfactory bulb

Electron microscopic immunocytochemistry was used to localize luteinizing hormone-releasing (LH-RH) immunoreactivity within the male golden hamster accessory olfactory bulb. Two LH-RH-immunoreactive fiber populations were identified in the accessory olfactory bulb. A superficial system of immunoreactive axons was localized to the vomeronasal nerve and glomerular layers, and a periventricular system appeared in granule cell and periventricular layers. LH-RH-immunoreactive varicosities were observed to contain large reactive vesicles (80-120 nm) as well as a variable degree of cytoplasmic reaction product. Additionally, small vesicles with unreactive lumens and mitochondria were often present. Intravaricose segments of immunoreactive fibers invariably displayed fewer reactive vesicles than did varicosities. Within both glomerular and periventricular layers, some LH-RH-immunoreactive varicosities were observed to form asymmetric contacts characterized by prominent postjunctional densities. In the glomerular layer, these junctions could be identified as synaptic by several features. The presence of LH-RH-immunoreactivity in presynaptic elements supports a neuromodulatory role for LH-RH. As the accessory olfactory system is critically involved in the initiation of mating behavior of the male golden hamster, LH-RH-immunoreactive synapses in the accessory olfactory bulb may function to regulate reproductive behavior.

A comparison of neuropeptide immunocytochemistry in fluid-fixed and freeze-dried brains

Immunocytochemical staining of luteinizing hormone-releasing hormone (LHRH), somatostatin, and neurophysin was compared in rat brains fixed with 1) formalin, 2) Bouin's solution, 3) freeze-dried (FD), or 4) freeze-dried + paraformaldehyde vapor perfused (FDV). The distribution of LHRH fibers was similar in all preparations; however, beads of granular reaction product often appeared finer and more numerous in the median eminence of FD- and FDV brains. Positively stained LHRH perikarya were not observed in any of the preparations. In contrast, somatostatin-immunoreactive perikarya were present in the fluid-fixed and FD brains, although few were observed in FDV brains. Somatostatin-immunoreactive fibers were present in all preparations, but appeared most numerous in the median eminence of FD brains. Staining of neurophysin-containing perikarya and fibers was similar in all preparations. These observations suggest that the FD brain can provide a suitable tissue substrate for immunocytochemistry, demonstrating staining comparable to or surpassing that of more conventional preparations. However, staining of antigens in FD brain was not uniformly successful and may depend on stereochemical characteristics of each antigen as well as properties of the primary antisera used in the staining procedure.

The fetal development of the luteinizing hormone-releasing hormone (LHRH) neuronal systems of the guinea pig brain

We have studied the distribution of LHRH-like immunoreactive material in fetal guinea pig brains beginning at day 25 of gestation. Cells and processes were first detected throughout the peripheral, intracranial and central course of the nervus terminalis at 28 (but not 25) days of gestation. The localization of LHRH in this structure preceded its appearance in the hypothalamus and coincided with the initial detection of immunoreactive LH in the pituitary gland. The possible role of the LHRH neuronal network within the nervus terminals in the development of reproductive function is discussed. Comparisons between the brains of littermates of both sexes were made at each age (days 28 through 60 of gestation) to determine possible differences between the sexes in the development of the LHRH neurosecretory systems. No sexually dimorphic features were evident in these systems throughout the prenatal period except at days 40 and 45. At these ages, differences in the number of LHRH neurons in the arcuate nucleus were found between the sexes in some but not all of the brains examined. These differences in LHRH concentrations may reflect the onset of testicular activity as indicated by an increase in serum testosterone levels. Increased serum testosterone concentrations were observed in the male fetuses beginning at 45 days of gestation. However, cell counts made within this nucleus from days 40 through 60 of gestation indicated no comparable sexual dimorphism in the total neuronal population which appeared to be relatively stable throughout this period of brain growth. The number of immunoreactive LHRH neurons visible throughout the brain increased from days 30 through 45 and fewer LHRH cells were seen on days 50 and 60 of gestation, particularly in the arcuate nucleus. The apparent decrease in visible LHRH neurons was concomitant with an increase in number and more extensive distribution of immunoreactive processes throughout the hypothalamus and in certain extrahypothalamic areas of the brain.

Simultaneous monoamine histofluorescence and neuropeptide immunocytochemistry: I. Localization of catecholamines and gonadotropin-releasing hormone in the rat median eminence

Adjacent tissue sections through the rat median eminence were examined for the distribution of gonadotropin-releasing hormone (GnRH) and catecholamines (CA). A simultaneous visualization technique was employed for this correlative neuroanatomical analysis. At rostral and mid-central levels of the median eminence the majority of GnRH terminals do not appear in coexistence with CA terminals; the latter were confined to the outer-most 10 micrometers of the median eminence while the densest concentration of GnRH terminals was located internal to this layer. However, individual GnRH fibers appeared to penetrate the outer CA zone wherein they were found in juxtaposition to portal capillaries. At caudal levels of the median eminence, there was an extensive overlap of CA and GnRH varicosities adjacent to the tubero-infundibular sulcus. In addition, numerous GnRH terminals were seen adjacent to portal vessels. The differences in the positions of CA and GnRH terminals between rostral and caudal median eminence may provide a morphological basis for the hypothesis of separate regulatory mechanisms for CA upon GnRH secretion at these two levels of the median eminence.

The luteinizing hormone-releasing hormone (LH-RH) neuronal networks of the guinea pig brain. I. Intra- and extra-hypothalamic projections

In the guinea pig brain, LH-RH-containing cell bodies are located not only within the classical hypophysiotrophic area but also in the medial preoptic area, septum and olfactory tubercle. LH-RH fiber tracts project not only to the primary portal plexus in the median eminence but also throughout the limbic forebrain and limbic midbrain regions. Using radiofrequency lesions in different brain regions, the projections of LH-RH cell bodies were determined. Cells in the medial preoptic area project ot the organum vasculosum of the lamina terminalis (OVLT), the suprachiasmatic nucleus, the mammillary body complex and the ventral tegmental area. LH-RH neurons in both the medial septal nucleus and medial preoptic area project via the stria medullaris to the medial habenular nucleus and from there via the fasciculus retroflexus to the interpeduncular nucleus of the midbrain. Other LH-RH neurons in the medial septal nucleus, nucleus of the diagonal band of Broca and olfactory tubercle are congregated in small clusters around large blood vessels which penetrate into this area, and they do not appear to send axons outside their immediate vicinity. The types of LH-RH axonal terminations and the roles of these peptide-containing neurons are discussed.

The luteinizing hormone-releasing hormone (LH-RH) neuronal networks of the guinea pig brain. II. The regulation on gonadotropin secretion and the origin of terminals in the median eminence

Cell bodies synthesizing LH-RH are located throughout the central nervous system including the hypothalamic arcuate nucleus, the medial preoptic area and medial septal nucleus. The contribution of each of these cell groups to the LH-RH terminals in the median eminence was assessed by immunocytochemistry following placement of radiofrequency lesions in male guinea pigs. Lesions in the arcuate nucleus resulted in an almost complete absence of LH-RH fibers in the median eminence. Lesions in the medial preoptic area or suprachiasmatic nucleus produced a decrease in the amount of immunoreactivity throughout the median eminence; but there was only a small decrease in the numbers of labeled fibers, which was localized to the internal zone of the median eminence and the external zone on the ventral and lateral surfaces of the infundibular stalk. Lesions of the mammillary bodies, medial amygdaloid nucleus, septal nucleus and fornix had no effect. The effects of these lesions on pituitary gonadotropin secretion was also assessed. Only lesions in the arcuate nucleus diminished plasma concentrations of luteinizing hormone (LH) and testosterone in the male guinea pigs. Similary only arcuate lesions prevented the postcastration rise in plasma LH in ovariectomized female guinea pigs. These data strongly suggest that the arcuate nucleus is the major but not the sole source of LH-RH terminals in the median eminence, and these are responsible for the neural regulation of tonic gonadotropin secretion.

[Immuno-cytologic study of hypothalamic LH-RH neurons of the human fetus]

The immunocytological study of LH-RH producing neurons was carried out on 3 newborns and 14 fetuses (from 10 to 36 weeks of age). Perikarya and fibers which were immunoreactive to anti-LH-RH IS were revealed by IF or IE in all the hypothalamus beginning with the 13th week. Important variations in neurons staining may translate "physiological" differences in their LH-RH charge but can be also the result of the diverse technical conditions. In three 16-week-old female fetuses, the large number of neurons (more than 150 per hypothalamus) permitted a good topographical and morphological study of them. They are scattered in vast areas of the anterior hypothalamus (lamina terminalis (LT) and septum), mediobasal and premammillary hypothalamus. The fibers which are particularly immunoreactive in semi-thin sections form a large hypothalamo-infundibular contingent in the posterior lip of the ME where they give rise to collaterals that terminate in contact with the capillaries of the mantelplexus, this taking place both before and after the apparition of the intra-eminential loops at the 16th week. Numerous in the LT, they terminate around the deep capillaries of the vascular organ or in contact with the ependymal epithelium. Some extra hypophyseal fibers go towards the epithalamus and the mesencephalon. To conclude, very early, in the human fetus the peptidergic LH-RH system resembles that described in adult primates; its role in the maturation and control of the gonadotropic cells is evoked.