Ultrastructural features of the shark brain

Abstract

The ultrastructural characteristics of elasmobranch central nervous system have not been previously reported. This study presents a general assessment of aldehyde perfused brain and spinal cord in three species of shark: tiger, hammerhead and Atlantic nurse. The same distinct cell types are present in the shark that exist in mammals and similar criteria may be used for their differentiation. Neurons have frequently round nuclei with a prominent nucleoli; cytoplasm is abundant and filled with formed elements; somatic synapses and subsurface cisterns are rare. Astrocytes are smaller, and have less cytoplasm and slightly fewer organelles. Glial fibrils occur, but are not invariably present. Separating astrocytes from neurons is the most difficult identification problem. Oligodendrogliocytes are smaller, and have denser cytoplasm and a dark nucleus. The striking feature of capillary morphology is the presence of an appreciable perivascular space containing collagen; many tortuous evanginations of this space occur into surrounding glial processes which completely invest the capillaries. Astrocyte cell bodies frequently lie immediately next to vessels, and capillaries are occasionally totally surrounded by a single astrocyte process, thus being endocellular. Smaller pericapillary processes may be either astrocytic or ependymal. Dendrites, synapses, axons, and myelin have no obvious special characteristics. Sodium, visualized by precipitation techniques, is prominent in the astroglia and neurons.