Histological and ultrastructural study of the muscular and nervous systems in <i>Hydra</i>. II. Nervous system

Abstract

Abstract Nerve cells in hydra have been studied with the light and electron microscopes. Techniques for observing bi‐polar, tri‐polar, multi‐polar and sensory cells are described. Evidence is presented that these cells form a major part of the nervous system in hydra. The ultrastructural studies indicate the following: (1) There are three types of nerve cells: ganglionic, sensory and neurosecretory. (2) Nerve cells are concentrated at the base of tentacles, hypostome and basal regions. (3) Nerve cells are situated adjacent to the muscle fibers of the epithelio‐muscular cells. Criteria for identification of nerve cells at the ultrastructural level are presented. The nerve cells in hydra appear to be structurally different from similar cells in higher invertebrates and vertebrates. An important distinction is the lack of specialized synaptic vesicles in hydra. The appearance, dimensions, and location of secretory droplets in cells identified as neurosecretory cells are discussed. The droplets resemble closely neurosecretory droplets in nerve cells of higher invertebrates and vertebrates. The mechanism for the release of neurosecretory material in hydra appears to be similar to that suggested for higher organisms. It is suggested that one type of secretory droplet in hydra nerve cells may represent a growth stimulating principle that is believed to control growth and differentiation (Burnett, '66). The relationship of nerve cells to muscle processes is discussed.