Many capillaries are embedded in the cytoplasm of large supramedullary neurons (about 200 to 300 μ in diameter). Fewer such capillaries are seen in the smaller neurons (about 80 to 100 μ in diameter). The neuron clusters remain unstained after vital staining with trypan blue indicating presence of the blood‐brain barrier. Electron microscopic studies show that the capillary lumen is lined by nonfenestrated endothelial cells, pericytes, and a continuous basement membrane. Glial cell processes consisting mostly of astrocytes abut onto the capillary basement membrane separating the capillary from the neuron. The continuous glial investment around blood vessels may play a role in maintenance of the blood‐brain barrier. The nucleus in supramedullary cells is roughly circular and contains one large and several small nucleoli. The perikaryon contains neurofilaments, diffusely distributed elements of endoplasmic reticulum, clusters of RNP granules, Golgi complex, mitochondria, multivesicular bodies, and inclusion bodies; but is devoid of elementary neurosecretory granules. The cell body of the neuron is entirely covered by glial cells, often with processes which deeply indent the soma surface. Neither synapses nor arborizing dendrites are found on the cell body.
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