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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.

Cells on the ependymal surface of hypothalamic third ventricle of hamsters exhibit surface, ultrastructural, and phagocytic features of macrophages. The object of this investigation was to identify functions that a resident phagocytic system of the ventricles of the brain may have normally. We found that supraependymal macrophages in two- to three-week-old hamsters were associated with intact and degenerating intraventricular neuronal processes, ependymal cytoplasmic blebs, and degenerating intraventricular cells. These macrophages frequently contained cellular debris within phagosomal vacuoles. In the vicinity of neuronal ganglion on the floor of the third ventricle, there were numerous structures identified as growth cones, with organelles in varying stages of alteration and degeneration. Supraependymal macrophages apparently remove cellular debris resulting from ependymal metabolic processes and from cellular and neuritic degeneration. Such activity may be part of a normal process of remodeling, involving cell growth, renewal, or cell death within the third ventricle of developing animals.

An intraventricular neuronal complex has been identified with scanning and transmission electron microscopy at the base of the lamina terminalis of the mouse. The raspberry-shaped complex protrudes from a thickened bulge on the ependymal surface of the lamina terminalis or adjacent rostral floor of the third ventricle. Neurons and occasional ependymal cells cover the surface of the complex. Its core is made up of neurons, ependymal cells, and neuronal processes, which are usually compactly arranged. The core is continuous, through a breach in the ependymal layers, with the subependymal neuropil of the lamina terminalis. Within the core of the complex are large numbers of axodendritic synapses. Axonal varicosities and synaptic terminals are filled with vesicles and mitochondria. Synaptic endings have one of two populations of vesicles: exclusively clear, small, round or flattened vesicles. In view of the known structural and functional characteristics of the lamina terminalis, it is possible that the neuronal complex may participate in neurohormonal regulatory systems of the hypothalamus and hypophysis or in the network of circumventricular organs mediating angiotensin effects.