Comparative immunohistochemical study of the distribution of neuropeptide Y, growth hormone-releasing factor and the carboxyterminus of precursor protein GHRF in the human hypothalamic infundibular area.

It is now well documented that various polyclonal antisera to the human growth hormone-releasing factor (hGHRF, somatocrinin) visualize in the brain by immunohistochemistry the classical hypothalamic hypophysiotropic neurons and also antigens present in otherwise characterized peptidergic neuronal systems. The nature of these antigens is still an open question. One of these hGHRF antisera, raised against an immunogen of hGHRF1-44NH2, labels in the arcuate nucleus of the human mediobasal hypothalamus the neuropeptide Y (NPY) containing neurons which for the most part constitute a tuberoextrainfundibular system. The identity of the hGHRF-like substance present in these neurons with true somatocrinin has been assessed by performing a comparative immunohistochemical study including sequential double and triple labeling using the antiserum to hGHRF1-44NH2 in conjunction with antisera to the carboxyterminus of preprosomatocrinin (CTPG) and to NPY. This made it feasible to dissociate the hGHRF1-44NH2-immunoreactive neurons into two major subpopulations costaining either for CTPG of NPY, and a minor neuronal group displaying simultaneously the three labelings. A subset of arcuate neurons also showed NPY staining only. These results suggest that (1) the hGHRF-like antigen present in the majority of the NPY neurons is not true somatocrinin, or alternatively that preprosomatocrinin undergoes a unique maturational processing in these neurons, and (2) a subset of tuberoinfundibular somatocrininergic neurons produces and releases NPY which may be involved in the multifactorial control of the pituitary function.

Ultrastructural characterization of prolactin-like immunoreactivity in rat medial basal hypothalamus.

Prolactin-like immunoreactivity has been reported in the medial basal hypothalamus at the light microscopic level, in hypophysectomized rats. Here, with preembedding immunocytochemistry at the electron microscopic level, we have observed prolactin-immunoreactive neurons and synapses in the hypothalamus. Reaction product was discovered in medial basal hypothalamic neurons, which had typical large nucleoli and received axosomatic synapses. In the cytoplasm, reaction product was distinctly granular. Immunoreactive neurons were usually surrounded by nonreactive cells. Reaction product was also seen in dendrites, some of which had spines. Some axons in the hypothalamus contained reaction product, usually surrounded by nonreactive axons, and immunopositive synapses were detected both in the hypothalamus and in the midbrain. In a small number of cases immunoreactive axons could be seen synapsing on immunoreactive dendrites.

[Serotonin contents in the hypothalamus of adult males, females and males castrated during the neonatal period]. / Soderzhanie serotonina v gipotalamuse vzroslykh samtsov, samok i neonatal'no kastrirovannykh samtsov.

The aim of the present work was to study the character of the change in serotonin level in the anterior and medial basal hypothalamus of adult rats after the effect of testicular hormones had been switched off on the first day of postnatal life. It was shown in our work that in males serotonin level was significantly lower than that in females by 67 and 46% in the anterior and medial basal hypothalamus, respectively. Castration of newborn males resulted in a significant increase in serotonin level in both anterior and medial basal hypothalamus-up to the level observed in females. It is supposed that the male sex hormones affect differentiation of serotoninergic system of the brain.

[LHRH in the human hypothalamus. Immunohistochemical mapping in normal newborn infants or in sudden death infants]. / LHRH dans l'hypothalamus humain. Etude cartographique immunohistochimique chez des nourrissons témoins ou décédés de "mort subite inexpliquée".

The topographical distribution of neurons containing LHRH has been investigated in newborn hypothalamus using the peroxidase anti-peroxidase technique. In control subjects, LHRH immunoreactive (LHRH-IR) perikarya have been mainly observed essentially in the infundibular nucleus. The preoptic region displayed a moderate density of LHRH-IR cell bodies. High LHRH innervation was observed in the anterior hypothalamus in the lamina terminalis and in the mediobasal hypothalamus in the median eminence, and in the peri- and paraventricular regions. In sudden death infant syndrome, a comparable mapping was observed, except a low density in the mediobasal peri- and paraventricular areas.

The bifunctional role of pro-opiomelanocortin derivatives in the mediobasal hypothalamus of the rat.

In the present study, a polyclonal antibody against pro-opiomelanocortin derivatives was characterized biochemically. Its immunoreactivity with structures of the arcuate nucleus and the median eminence was investigated by means of the immunogold method and compared with its reaction on adenohypophyseal cells with and without pre-adsorption with pro-opiomelanocortin derivatives. The antiserum detects ACTH and its fragments, in particular alpha-MSH, and beta-endorphin. In the adenohypophysis gold particles are exclusively located on small secretory granules situated in the periphery of branched cells. In the perikarya of the arcuate nucleus gold particles are observed on terminal vesicles abutting from the cis-face of the Golgi apparatus, on granules in its direct vicinity and on small dense core vesicles preferentially located in the cell periphery. Immunoreactive gold-labeled fiber profiles are found in a sub- or intra-ependymal position as well as in the nuclear neuropil proper. Here axodendritic and axosomatic synapses are observed. In both situations the gold particles are mostly restricted to the small dense core vesicles and do not decorate the synaptic vesicles. In the median eminence gold labeled fibers are detected in all layers. The labeled fibers can be closely apposed to tanycytic processes, without, however, forming special contact differentiations. In direction to the perivascular layer of the external zone the labeled profiles are more frequently arranged in groups intermingled with unlabeled fibers. The axons decorated with gold particles can be freely exposed to the perivascular space or are found as single processes in close vicinity to the capillary wall. Subsequent to preincubation of the native antiserum with ACTH1-39 and ACTH18-39 (= CLIP) neither adenohypophyseal cells nor perikarya and fibers in the arcuate nucleus nor axons in the median eminence are decorated with gold particles. Preincubation of the native antiserum with alpha-MSH or beta-endorphin does not change the immunoreaction with the small, peripherally situated granules in the branched adenohypophyseal cells. In neurons of the arcuate nucleus and in fibers of the median eminence, however, the immunoreaction is completely extinguished when the antibody is pre-incubated with alpha-MSH, whereas subsequent to preincubation with beta-endorphin only the amounts of labeled structures are reduced.(ABSTRACT TRUNCATED AT 400 WORDS)

Localization of luteinizing hormone-releasing hormone in the forebrain of the pig.

The distribution of luteinizing hormone-releasing hormone (LHRH)-immunostained perikarya and processes was examined in the forebrains of six sexually mature female pigs by use of indirect biotin-avidin horseradish peroxidase immunocytochemistry. Two primary antisera (Drs. Y.F. Chen and V.D. Ramirez CRR11B73 and Miles-Yeda UZ-4) yielded positive staining. Adjacent sections treated either primary antiserum preabsorbed with LHRH or with normal rabbit serum substituted for primary antiserum lacked positive staining. The greatest proportion of LHRH-immunostained perikarya were found in the medial preoptic area adjacent to the organum vasculosum of the lamina terminalis. The LHRH-immunostained perikarya were also scattered rostrally in the diagonal band of Broca, and within the lateral hypothalamic area, paraventricular nucleus, periventricular zone, suprachiasmatic nucleus, and medial basal hypothalamus. LHRH-immunostained processes, which extended from the medial preoptic area, coursed either along the ventral surface to the median eminence or medially and ventrally along the third ventricular wall ventrally to the median eminence and caudally to the level of the mammillary bodies. Extrahypothalamic processes were located adjacent to the lateral ventricular floor and the third ventricle from the lateral septal area (stria terminalis) to the level of the habenular nucleus. LHRH-immunostained neurons were unipolar, bipolar, and multipolar. Close associations between individual LHRH-immunostained neurons were observed.

[Sexual dimorphism in the content of luliberin following deafferentation of the mediobasal hypothalamus]. / Polovoi dimorfizm v soderzhanii liuliberina posle deafferentatsii mediobazal'nogo gipotalamusa.

Luliberin (LH-RH) was assayed radioimmmunologically in the hypothalamic fragments taken from male and female rats, intact or with isolated mediobasal hypothalamus. An increased amount of LH-RH was found in the fragments containing organum vasculosum of the lamina terminals (OVLT) and taken from intact males. LH-RH amount was negligible in isolated parts of hypothalamic fragments containing arquate nucleus of median eminence. OVLT of females was mu;ch richer with LH-RH than that of the males. The females though were characterized by a lower blood lutropin. The inhibitory influences on the secretion of LH-RH seem to be more obvious in intact males than in females.

The postnatal developmental localization of pro-opiomelanocortin and alpha melanocyte stimulating hormone in the medio-basal hypothalamus of the rat.

This immunocytochemical study of the late postnatal development of the medio-basal hypothalamus revealed the presence of ACTH 1-39 like positivity in neurons of the arcuate nucleus form the begin of this study (day E 18-20) onwards. Alpha MSH positivity, on the contrary, is not present in cells of the same area before day P 16. No other areas in the developing medio-basal hypothalamus contain perikaryal positivity for alpha M-SH or ACTH 1-39. The pituitary contains ACTH 1-39 like positivity from the begin of this study (day E 18-20) onwards. Fibers are positive for alpha MSH during the fetal development of the medio-basal hypothalamus, demonstrating an overal reactivity without varicosities and restricted to bundles or neuropil areas. Towards P 16 the alpha MSH positivity diminishes in the whole medio-basal hypothalamus, remaining present only in large fibre systems like the fornix. ACTH 1-39 like fiber positivity is already distributed in arcuate and periventricular regions at days E 20-PO, reaching its mature extension at day P2. After P16 alpha MSH positive threads, possessing varicosities are restricted to the same areas as ACTH 1-39 like fiber positivity is.

Luteinizing hormone-releasing hormone (LHRH) neurons in the male and female rats at peripubertal period.

Luteinizing hormone-releasing hormone (LHRH) neurons were immunohistochemically studied in rats of both sexes at peripubertal ages. The number of immunoreactive LHRH neurons (irLHRH neurons) was counted in the brain region from the level of the septum-preoptic area to the level of the rostral part of the infundibulum in colchicine-treated male and female rats at 30 and 60 days of age. At 30 days, irLHRH neurons were more numerous in male rats than females. At 60 days, the number of irLHRH neurons in female rats increased to the level of male rats of the same age. In non-colchicine-treated rats, the count of irLHRH neuron was quite low. The difference in the number of irLHRH neurons between colchicine-treated and non-treated rats may be regarded as the activity of LHRH system. The difference in the number of irLHRH neurons was larger in male rats than in female rats at 30 days of age. On the contrary, at 60 days of age, the difference was larger in females than in males. LHRH contents were measured in the preoptic-anterior hypothalamic area (POA-AH), where LHRH neuronal perikarya are mainly located, and in the mid-hypothalamic area. LHRH content of the POA-AH in male rats at 60 days of age was not significantly different from that at 30 days of age. While, LHRH content in the POA-AH was greater in 60-day-old female rats at proestrous morning than that in 30-day-old females. At 30 days of age, male rats tended to contain more LHRH in the POA-AH than female rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Impaired growth hormone secretion in neonatal hypothyroid rats: hypothalamic versus pituitary component.

In 10-day-old rats made hypothyroid by giving dams propylthiouracil (PTU) in the drinking water since the day of parturition, simultaneous radioimmunoassay (RIA) determinations of basal and stimulated growth hormone (GH) secretion, hypothalamic GH-releasing hormone (GHRH)-like immunoreactivity (LI) content, immunocytochemical localization of somatotrophs, and hypothalamic GHRH-LI-positive structures were performed. The frequency of somatotrophs was also determined. One-day-old hypothyroid rats, whose mothers had been given PTU since the 14th day of pregnancy, were also used for comparison. In 10-day-old hypothyroid rats, pituitary and plasma GH levels and the number of somatotrophs were considerably lower and plasma TSH levels were significantly higher than those in age-matched control rats; however, GHRH-LI titers in the mediobasal hypothalamus and the morphology of GHRH-LI-positive structures were unaltered. In 1-day-old rats the only alteration present, in addition to elevated plasma TSH levels, was a clear-cut decrease in plasma GH levels. An acute challenge with GHRH (20 ng/100 g body wt, sc) or clonidine (15 micrograms/100 g body wt, sc) induced a clear-cut rise in plasma GH levels 15 min postinjection in 10-day-old control rats but failed to do so in age-matched hypothyroid rats. Both compounds failed to rise plasma GH in both hypothyroid and control 1-day-old rats. Taken together these data indicate that in neonatal and infant rats deprivation of thyroid hormones acts primarily to depress pituitary somatotroph function and that possible changes in GHRH-secreting structures represent a later postnatal event.

Pituitary receptors for GnRH and estradiol, and pituitary content of gonadotropins in beef cows. II. Changes during the postpartum period.

Pregnant beef heifers (n = 24) were assigned randomly to four groups and slaughtered at day 1, 15, 30 or 45 postpartum. The day prior to slaughter blood samples were taken from each cow every 15 min for 8 hr. The anterior pituitary gland, preoptic area (POA) and medial basal hypothalamus (HYP) were collected from each cow. Contents of gonadotropin-releasing hormone (GnRH) in extracts of POA and HYP, and luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in extracts of anterior pituitary were quantified by radioimmunoassay. In the anterior pituitary gland, membrane receptors for GnRH were quantified by a standard curve technique and cytosolic receptors for estradiol were quantified by saturation analysis. Concentrations of LH, FSH and prolactin in serum were quantified by radioimmunoassay. Only one cow of eight had a pulse of LH during the 8 hr bleeding period on day 1 postpartum. This increased to 8 pulses in 6 cows on day 30 postpartum. Contents of GnRH in POA (15.0 +/- 3.2 ng) and HYP (14.0 +/- 2.0 ng) did not change significantly during the postpartum period. Pituitary content of LH was low following parturition (.2 +/- .1 mg/pituitary) and increased significantly through day 30 postpartum (1.2 +/- .1 mg/pituitary). Pituitary content of FSH did not change over the postpartum period. Receptors for both GnRH (.9 +/- .2 pmoles/pituitary) and estradiol (5.0 +/- .9/moles/pituitary) were elevated on day 15 postpartum, possibly increasing the sensitivity of the anterior pituitary gland to these hormones and leading to an increased rate of synthesis of LH that restored pituitary content to normal by day 30 postpartum.

The effect of hypothalamic deafferentation on rat growth hormone-releasing factor (rGRF)-like immunoreactivity content in the medial basal hypothalamus of rats.

Rat growth hormone releasing factor (rGRF)- and somatostatin (SRIF)-like immunoreactivities (LI) were determined by radioimmunoassay in the medial basal hypothalamus (MBH) of the rat with either complete deafferentation (CD) or a sham operation. Two weeks after the surgery the mean amount of SRIF-LI in the isolated MBH was about 70% less than that in the sham-operated animals. On the other hand, the mean rGRF-LI in the MBH decreased only approximately 30% as compared to the levels in the sham-operated animals, the difference being statistically insignificant. These findings are consistent with anatomical evidence that the majority of the GRF perikarya are located in the arcuate nucleus, but a few are found outside the MBH, whereas the majority of the SRIF perikarya are located outside the MBH.

A model for combined morphological and functional investigations on the isolated mediobasal rat hypothalamus.

We have developed a model for combined morphological and functional in vitro studies of the isolated mediobasal hypothalamus (MBH) by considering two prerequisites: (1) the tissue must be well preserved, free of morphological artefacts and functionally unimpaired until the end of the in vitro incubation, and (2) the tissue must be processed for morphology in optimal conditions. To test our model we have studied some aspects of the luteinizing hormone-releasing hormone (LHRH) system in 4-month-old male Sprague-Dawley rats. After decapitation the MBH was isolated and put in a flask containing 0.5 ml Hepes-buffered Locke's medium gassed by 5 ml/min of O2/CO2 (95%/5%) and shaken in a water bath at 37 degrees C. After a 10-min washing, the medium was changed twice at an interval of 20 min. After the in vitro incubation the tissue was satisfactorily preserved as judged by light- and electron-microscopic analysis. LHRH, somatostatin and thyrotropin-releasing hormone could be demonstrated by alkaline phosphatase or peroxidase-antiperoxidase immunohistochemistry on semithin sections and by immunogold technique on thin sections. The LHRH secretion was close to basal values after 30 min of incubation (22.1 +/- 4.8 pg/MBH) and then remained constant for another period of 20 min (17.6 +/- 2.6 pg/MBH). During the second 20 min of incubation LHRH secretion increased in presence of 61.6 mM K+ (110.7 +/- 8.7 pg/MBH). Thus the isolated hypothalamus was excitable until the end of the in vitro incubation. We conclude that this model can be successfully used for combined morphological and functional studies.

Morphological evidence that luteinizing hormone-releasing hormone neurons participate in the suppression by estradiol of pituitary luteinizing hormone secretion in ovariectomized rats.

Morphological characteristics of LHRH neurons identified by immunocytochemistry were studied using light and electron microscopy in female rats in which estradiol was replaced at the time of ovariectomy ('pseudo-intact' rats) or 3 weeks after ovariectomy (long-term ovariectomized, estradiol-treated). While estradiol levels were equivalent in these two groups, the rise in LH after ovariectomy was prevented by the immediate administration in the pseudo-intact rats, while the augmented plasma LH levels present three weeks following ovariectomy were only reduced by 50% as a result of delayed estradiol treatment. The LHRH content of the medial basal hypothalamus (MBH) including the median eminence (ME) was greater in pseudo-intact females than in untreated long-term ovariectomized control females or long-term ovariectomized, estradiol-treated females, both 1 and 14 days after estradiol exposure. Immunocytochemistry revealed fewer LHRH-immunopositive neuronal processes coursing throughout the MBH and terminating in the ME of long-term ovariectomized, estradiol-treated rats compared to those in pseudo-intact rats. However, within individual neurovascular terminals in the ME, image analysis revealed that the area of reaction product was greater in long-term ovariectomized, estradiol-treated animals. Equivalent amounts of LHRH were assayed in the MBH within each group of animals by several LHRH antisera regardless of their different binding requirements (R42, IJ29 and A-R743), suggesting that the predominant moiety present in neuronal terminals is the fully mature decapeptide. In contrast, in the preoptic area-anterior hypothalamus (POA-AH) these antisera assayed amounts of LHRH that varied as a function of binding characteristics, although the quantities did not vary with the estradiol treatment schedule. Immunocytochemical results paralleled these assay data; antisera requiring an interior sequence of amino acids (A-R743 and A-R419) detected approximately 3 times as many immunoreactive perikarya in the POA-AH as did an antiserum requiring the free amidated C terminal (IJ29). The estradiol treatment schedules had no effect on the total number of LHRH-immunopositive neurons detected by each antiserum or the distribution of LHRH-immunopositive neuronal perikarya. These data support the hypothesis that the predominant moieties present in neuronal cell bodies are precursor forms. The fine-structural characteristics of LHRH-immunopositive neuronal cell bodies are consistent with greater secretory and biosynthetic activity in LHRH neurons of long-term ovariectomized, estradiol-treated rats.(ABSTRACT TRUNCATED AT 400 WORDS)

Cholinergic agonist and antagonist drugs modulate the growth hormone response to growth hormone-releasing hormone in the rat: evidence for mediation by somatostatin.

Recently, data have been presented showing that muscarinic cholinergic agonists or antagonists can modulate, in opposite ways, GH-releasing hormone GHRH)-induced GH release in man. The aim of the present study was, first, to confirm these findings in the rat and, secondly, if confirmed, to investigate the mechanism(s) subserving the effect of cholinergic drugs. In adult male rats bearing chronic indwelling atrial cannulae, pretreatment with the cholinergic antagonists pirenzepine (0.5 mg/kg, i.v.) or atropine (0.5 mg/kg, i.v.) significantly reduced the rise in plasma GH induced by GHRH (2 micrograms/kg, i.v.), while pretreatment with the cholinergic agonist pilocarpine (3 mg/kg, i.v.) potentiated it. In rats with hypothalamic somatostatin (SRIF) depletion, i.e. rats with anterolateral deafferentation of the mediobasal hypothalamus or rats treated with cysteamine, the modulatory action of cholinergic drugs on the neuroendocrine effect of GHRH was completely lacking. In these two experimental models, an antiserum raised against SRIF failed to elicit a rise in plasma GH and measurement of hypothalamic SRIF content revealed a clear-cut reduction of the neuropeptide. Atropine (1 mumol/l) and pilocarpine (1 mumol/l), added to pituitary cells in vitro, failed to alter GHRH-induced GH release. The present results indicate that muscarinic cholinergic agonists and antagonists modulate GHRH-induced GH release in the rat and suggest that the effect of cholinergic modulation takes place through SRIF.

The influence of suckling on the hypothalamic and pituitary secretion of immunoreactive alpha-melanocyte stimulating hormone.

The effect of suckling on the secretion of alpha-melanocyte stimulating hormone (alpha-MSH) from the hypothalamus and pituitary was determined by a specific radioimmunoassay. There was an increase in the neurointermediate lobe (NIL) content of alpha-MSH 1 h after the onset of suckling. The values were restored to control levels within 3 h. The anterior lobe content of alpha-MSH was not affected by suckling. Plasma alpha-MSH levels were also unaffected by suckling, indicating that suckling probably affects the synthesis of NIL alpha-MSH, and not its release. Suckling lowered the alpha-MSH content in the mediobasal hypothalamus of lactating rats, and had no effect on the median eminence content of this peptide. In vitro, hypothalami from lactating rats released more alpha-MSH than hypothalami of random cycling females under basal, and stimulated (56 mM potassium) conditions. These results suggest that hypothalamic alpha-MSH may play a role in mediating some of the hormonal changes occurring during lactation.

Abbreviation of the period of sexual behavior in female guinea pigs by the progesterone antagonist RU 486.

In previous studies we have tested the hypothesis that the termination of the period of sexual behavior in female guinea pigs results from the loss of progestin receptors from hypothalamic cell nuclei. We have shown that hormonal manipulations that delay heat termination also delay loss of hypothalamic nuclear progestin receptors. In order to determine if accelerated nuclear receptor loss results in abbreviation of the period of sexual behavior, we tested the effect of 17 beta-hydroxy-11 beta-(4-dimethylaminophenyl)-17 alpha-(1-propyl)-estra-4,9-diene-3-one (RU 486), a progesterone antagonist, on heat termination. Ovariectomized guinea pigs were treated with estradiol benzoate. Forty hours later, they received progesterone followed 4 h later by injection of RU 486 or vehicle. RU 486 injected 4 h after progesterone caused heat abbreviation. We have found that RU 486 administration to estradiol-treated guinea pigs causes accumulation of progestin receptors in cell nuclear extract. Because this accumulation can be detected only when assay conditions are used that promote exchange of RU 486 progestin receptor complexes (15 degrees C incubation rather than 0 degree C); our routine assay conditions (at 0 degree C) can be used to measure primarily receptors that are occupied by progesterone. In order to confirm that RU 486 decreased progesterone-occupied nuclear progestin receptor levels when injected 4 h after progesterone, animals treated as in the behavioral experiment were killed 6 or 10 h after progesterone injection (2 or 6 h after RU 486), and nuclear progestin receptor levels were measured. RU 486 treatment resulted in lowered nuclear concentrations of hypothalamic progestin receptors at both times. These results support our hypothesis that the termination of the period of sexual receptivity in female guinea pigs is the result of loss of progestin receptors from hypothalamic cell nuclei.

Hypothalamic localization of multiunit electrical activity associated with pulsatile LH release in the rhesus monkey.

Abrupt increases in electrical activity can be recorded from the medial basal hypothalamus of the rhesus monkey and these bursts are correlated with a pulse of LH release from the anterior pituitary. In this study we have localized the tips of the electrodes from which such electrical activity can be recorded and have attempted to correlate the placement with the presence of immunoreactive LHRH neurons and axons. Electrodes associated with bursting activity were found as far rostral as the suprachiasmatic nucleus and as far caudal as the premammillary nuclei. The majority were found in the medial basal hypothalamus including the region of the arcuate nucleus, the retrochiasmatic zone and the dorsal aspect of the median eminence. The tips of the electrodes were associated with either LHRH neurons or axon bundles. Negative electrodes (those not associated with bursting electrical activity) which did not exit through the ventral surface of the brain nor enter the ventricle were found in the same regions of the hypothalamus as positive electrodes. Although it is tempting to associate the bursting electrical activity with the LHRH neurosecretory system, such a correlation cannot yet be made.

Immunocytochemical study of the development of vasotocin/mesotocin in the hypothalamo-hypophysial system of the chick embryo.

The hypothalamo-hypophysial system of the chick embryo has been studied with a monoclonal antibody which cross-reacts with arginine vasotocin and mesotocin, using thick (100 micron) sections in conjunction with a peroxidase-conjugated rabbit anti-mouse antibody. Although weakly stained perikarya occur occasionally in the tuberal region on embryonic days 6 and 7, the most consistent immunostaining of perikarya is found in the periventricular region of the caudal midhypothalamus at the level of the optic chiasm after embryonic day 8 1/2. Synthesis of peptides, therefore, takes place while the cells are close to their site of origin. Between embryonic days 9 and 10, beaded axons run along the anterior median eminence closely apposed to the adenohypophysis, thereby forming the anlage of the zona externa. The axons of the hypothalamo-neurohypophysial tract surround the neural lobe between embryonic days 11 and 12. The caudal to rostral wave of neuronal maturation that occurs during development appears to be due to a progressive differentiation of the periventricular zone, as well as the migration of perikarya. The early periventricular perikarya at embryonic day 8 1/2 send processes rostrally in a wing-shaped formation that extends both dorso- and ventrolaterally. From embryonic days 10 to 12, perikarya can be observed in the wing-like extensions, apparently migrating to rostral levels. The dorsolateral pathway gives rise at its midportion to the lateral cell group, whereas those perikarya migrating more laterally form the anlage of the external supraoptic nucleus. The ventrolateral wing-shaped extension of perikarya appears to be directed toward the ventral group and those lateral perikarya continuous with it. The location of mature neuronal cell groups is well established by embryonic day 17.