On the fine structure of the external glial layer in the isocortex of man

The surface of the external glial layer of the isocortex in the human temporal lobe is generally slightly undulated, with a few protrusions and indentations. The surface is formed by an uninterrupted basement membrane which is continuous over the surface no matter how tortuous it becomes. The overall thickness of the glial layer is generally 15 to 25 mum, but diminishes to about 5 mum immediately beneath blood vessels. It consists mainly of a variable number of stacked glial cell processes. Two groups of cell bodies are encountered particularly in the middle and lower levels of the glial layer. Most of the cells are specialized fibrous astrocytes. They are characterized by eccentrically placed, rounded nuclei with homogeneously dispersed chromatin, and electron-lucent cytoplasm rich in filaments. Lipofuscin pigment granules occupy large areas of the perikaryon. The astroglial cells give rise to four types of processes: foot-processes, tangential and radial processes, and processes irregular in outline. The foot-processes ascend towards the cortical surface and terminate as flat expansions spreading out immediately beneath the basement membrane. Contiguous terminal expansions are connected by gap junctions. The individual profiles are irregular in form and fit together like in a jig-saw puzzle. The plasmalemma beneath the basement membrane is underlined by a fuzzy material, which is penetrated by glial filaments. In the terminal expansions individual or groups of mitochondria are abundant. The tangential processes are straight and slender and form a lattice within the middle and deep level of the external glial layer. They contain numerous filaments, evenly distributed or fasciculated. The remainder of the lattice is filled up by a considerable number of processes irregular in outline and varying greatly in size. They contain fewer filaments than the tangential processes, coursing in all directions, and glycogen particles. In both types of processes only a few mitochondria are present. These processes are also connected by gap junctions and desmosomes, too. Large cytoplasmic areas of astroglial cells localized in the deepest portion of the glial layer protrude into the neuropil of the molecular layer, giving rise to several radiate processes, which extend deeper into the cortex. The second, heterogeneous group of cell bodies is characterized by elongated nuclei, ovoid or irregular in outline, which are smaller than those of astroglial cells, and contain blocks of condensed chromatin; a thin cytoplasmic rim generating a few appendages surrounds the nucleus. The first sub-type is characterized by a nucleus with large chromatin blocks bordering the inner nuclear membrane and a medium-dense cytoplasmic matrix. The second sub-type displays smaller chromatin condensations at the inner nuclear membrane and many microtubules are scattered throughout an electron-lucent cytoplasm.