Ultrastructure of Hypothalamic Neurons and of the Median Eminence

History of the Development of Knowledge about the Neuroendocrine Control of Ovulation-Recent Knowledge on the Molecular Background

The physiology of reproduction has been of interest to researchers for centuries. The purpose of this work is to review the development of our knowledge on the neuroendocrine background of the regulation of ovulation. We first describe the development of the pituitary gland, the structure of the median eminence (ME), the connection between the hypothalamus and the pituitary gland, the ovarian and pituitary hormones involved in ovulation, and the pituitary cell composition. We recall the pioneer physiological and morphological investigations that drove development forward. The description of the supraoptic-paraventricular magnocellular and tuberoinfundibular parvocellular systems and recognizing the role of the hypophysiotropic area were major milestones in understanding the anatomical and physiological basis of reproduction. The discovery of releasing and inhibiting hormones, the significance of pulse and surge generators, the pulsatile secretion of the gonadotropin-releasing hormone (GnRH), and the subsequent pulsatility of luteinizing (LH) and follicle-stimulating hormones (FSH) in the human reproductive physiology were truly transformative. The roles of three critical neuropeptides, kisspeptin (KP), neurokinin B (NKB), and dynorphin (Dy), were also identified. This review also touches on the endocrine background of human infertility and assisted fertilization.

Ultrastructural characterization of the central component of the medial preoptic nucleus

The fine structure of the sexually dimorphic central part of the medial preoptic nucleus was studied in male and female rats. Neurons of this subnucleus are characterized by having convoluted nuclear membranes and fibrillar nuclear inclusions. Within the perinuclear domain there is a well developed neurosecretory apparatus, i.e., Golgi complexes, assorted types of secretory granules, and lysosome-like bodies, whereas the marginal cytosol contains stacked cisterns of rough endoplasmic reticulum, nucleolus-like inclusions, and few mitochondria. The neuropil is made up of thick dendrites, fascicles of thin unmyelinated axons, and few small myelinated axons. Additionally four types of synaptic boutons were found according to the morphology of their vesicular contents. No apparent qualitative sex differences were noted. These cytologic features which suggest secretory activity, coupled with reports that have shown the presence of several neurohormones and peptides within the central part of the medial preoptic nucleus lend further support to the neuroendocrine nature of these neurons.

Ultrastructural changes in the hypothalamic ventromedial nucleus of ovariectomized rats after estrogen treatment

The changes produced in the hypothalamic ventromedial nucleus (VMN) of ovariectomized rats after administration of 100 microgram estradiol benzoate/kg body weight were studied using light and electron microscopy. Quantitative morphometric studies included number and size of VMN neurons and nuclei, size and density of terminals and synaptic contacts, spine-to-shaft ratio of postsynaptic elements and relative frequency of two types of synaptic vesicles. Evidence was obtained favoring the concept of heterogeneous composition of the VMN: in ovariectomized animals many cells appeared in a state of quiescence, but other neurons showed no major alterations. Estrogen administration to ovariectomized rats produced evidence of metabolic stimulation such as increase in rough surfaced endoplasmic reticulum, condensation of nucleolar material, enlarged Golgi and presence of pleomorphic mitochondria. The number of neurons in the VMN was not modified by estrogen treatment; however, neuron soma and nuclei were larger. In the ventrolateral division of the VMN terminals and synaptic contacts per unit area were increased after estrogen treatment, but synaptic contact length, terminal size and spine-to-shaft ratio were not modified. The possibility that the differences observed may be consequent to changes in synaptic organization of the VMN related to its estrogen-dependent functions is discussed.

Response of tanycytes to aging in the median eminence of the rat

Age-related changes in tanycytes of the median eminence were observed at both the light and electron microscopic level in male and female rats at ages 4 weeks to 14 months. The most obvious changes involved a marked progressive increase in number and size of droplets identified histochemically as neutral lipid and the development of complex degenerate bodies in the perivascular contact zone. These degenerate bodies consisted of large multiple complexes of myelin figures containing intercalated channels and pools of electron-dense material. They often occurred interspersed among large lipid droplets. The degenerate bodies (myelin figure complexes) and associated lipid droplets occupied much of the contact zone in the older animals. Evidence of axonal degeneration and tanycytic phagocytosis of deteriorating neural elements was also present. These age-related changes occurred independent of sex and were not affected by gonadectomy. This tanycyte response to aging in the median eminence is very similar to the response of neurohypophysial pituicytes to axonal degeneration, suggesting a phagocytic glial role for tanycytes.