A cytoarchitectonic analysis is presented of a configuration of cells which extends through the medial preoptic and rostral part of the anterior hypothalamus of the guinea pig, rat, hamster, and mouse. Within that configuration, called here the sexually dimorphic nuclear complex of the medial preoptic-anterior hypothalamic area (SDNC-MPAH), there is nearly continuous variation in cellular distribution or packing density as one proceeds rostrocaudally, as well as variations between females and males at corresponding levels. The major cell groups comprising SDNC-MPAH are the medial preoptic nucleus (MP) and the preventricular portion of the periventricular nucleus (Pep) rostrally, and the anterior hypothalamic nucleus (AH), anterior portion of periventricular nucleus, (Pea), and bed nucleus of stria terminalis (ST) caudally. The most striking cytoarchitectonic dimorphisms usually involve MP and AH. In all four species examined, MP is larger and, except for the mouse, appears to be more densely cellular in the female than in the male. Most prominently in the rat and guinea pig, AH is densely cellular throughout in the female, including the subependymal zone, whereas in the male AH lies at a distance from the third ventricle, separated from the ependyma by a cell-poor zone. The significance of these findings may lie in the fact that the region exhibiting the sexual dimorphisms has been shown to be involved in the regulation of reproductive functions which are sexually differentiated and developmentally influenced by the fetal or perinatal hormonal milieu in some species.
Controversy exists in the neuropsychological literature concerning the existence of gender-associated differences in cognitive functioning and in hemispheric lateralization of cognitive functions. A recent study, based on 14 brains obtained at autopsy, reported sex differences in the splenium of the human corpus callosum and suggested that the larger splenium in females reflects less hemispheric lateralization, or "specialization," than the male brain for visuospatial functions. Our measurements of the human corpus callosum using magnetic resonance images of 37 living subjects failed to confirm reported sex differences in the splenium. A marginally significant sex-related difference in minimum body width and an age-related decrease in anteroposterior distance were found. Most striking were the large variations in callosal size and shape among individuals regardless of age or gender. Existing knowledge of the functions of the corpus callosum does not permit correlations between variations in callosal size and shape and variations in cognitive functions.
Numerous basal processes of ependymal cells extend peripherally into every cell group and area of the hypothalamus of the four species of mammals examined: mouse, rat, rabbit and cat. With Golgi-Cox impregnation and Nissl counterstaining, ependymal cell proccsses appear to have a variety of forms and of intricate structural relationships with neurons and capillaries within the hypothalamus. It is proposed that hypothalamic ependymal cells and their processes function as a communication system between the cerebrospinal fluid of the third ventricle and neurons and capillaries within the hypothalamus. The role that such a system may play in the regulation of adenohypophysial function is discussed.It was also noted that in all hypothalamic cell groups, dendrites of some neurons extend to the ependymal layer.An unusual cell, resembling microglial forms, within the infundibular nucleus appears to be selectively impregnated by the Golgi-Cox technique employed. The cell is characteristic of the infundibular nucleus and, with rare exceptions, is confined to that cell group. Some of these spider-cells lie on the ependymal layer and also have endfeet extending to the ventricular surface of the ependyma.
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