Fractones and other basal laminae in the hypothalamus.

The physiological role of basal laminae (BL) and connective tissue (meninges and their projections) in the adult brain is unknown. We recently described novel forms of BL, termed fractones, in the most neurogenic zone of the adult brain, the subependymal layer (SEL) of the lateral ventricle. Here, we investigated the organization of BL throughout the hypothalamus, using confocal and electron microscopy. New types of BL were identified. First, fractones, similar to those found in the lateral ventricle wall, were regularly arranged along the walls of the third ventricle. Fractones consisted of labyrinthine BL projecting from SEL blood vessels to terminate immediately beneath the ependyma. Numerous processes of astrocytes and of microglial cells directly contacted fractones. Second, another form of BL projection, termed anastomotic BL, was found between capillaries in dense capillary beds. The anastomotic BL enclosed extraparenchymal cells that networked with the perivascular cells coursing in the sheaths of adjacent blood vessels. Vimentin immunoreactivity was often detected in the anastomotic BL. In addition, the anastomotic BL overlying macrophages contained numerous fibrils of collagen. We also found that the BL located at the pial surface formed labyrinthine tube-like structures enclosing numerous fibroblast and astrocyte endfeet, with pouches of collagen fibrils at the interface between the two cell types. We suggest that cytokines and growth factors produced by connective tissue cells might concentrate in BL, where their interactions with extracellular matrix proteins might contribute to their effects on the overlying neural tissue, promoting cytogenesis and morphological changes and participating in neuroendocrine regulation.

Anatomy of the brain neurogenic zones revisited: fractones and the fibroblast/macrophage network.

Cytogenesis in adult peripheral organs, and in all organs during development, occurs nearby basal laminae (BL) overlying connective tissue. Paradoxically, cytogenesis in the adult brain occurs primarily in the subependymal layer (SEL), a zone where no particular organization of BL and connective tissue has been described. We have reinvestigated the anatomy of the area considered the most neurogenic in the adult brain, the SEL of the lateral ventricle, in zones adjacent to the caudate putamen, corpus callosum, and lateral septal nucleus. Here, we report structural (confocal microscopy using laminin as a marker) and ultrastructural evidence for highly organized extravascular BL, unique to the SEL. The extravascular BL, termed fractones because of their fractal organization, were regularly arranged along the SEL and consisted of stems terminating in bulbs immediately underneath the ependyma. Fractones contacted local blood vessels by means of their stems. An individual fractone engulfed in its folds numerous processes of astrocytes, ependymocytes, microglial cells, and precursor cell types. The attachment site (base) of stems to blood vessels was extensively folded, overlying large perivascular macrophages that belong to a fibroblast/macrophage network coursing in the perivascular layer and through the meninges. In addition, collagen-1, which is associated with BL and growth factors during developmental morphogenetic inductions, was immunodetected in the SEL and particularly regionalized within fractones. Because macrophages and fibroblasts produce cytokines and growth factors that may concentrate in and exert their effect from the BL, we suggest that the structure described is implicated in adult neurogenesis, gliogenesis, and angiogenesis.