Ultrastructural Studies of the Development of Nerves in Hydra

Three types of mature epidermal neurons and several of theirdifferentiating stages aie presented in this ultrastructuralstudy. Each of the three types, neurosensory, neurosecretory,and ganglionic cells, is derived from interstitial cells, (i)Mature neurosensory cells contain elongated nuclei, a well-developedcilium in each cell, and membrane-bounded neurosecretory droplets(700–1300 A in diameter). There may be two or more neuritesin which are numerous microtubules, glycogen particles, ribosomesand many neurosecretory droplets, (ii) Mature neurosecretorycells closely resemble neurosensory cells, except that no ciliumis present. The perikarya contain small, dense nuclei, neurosecretorydroplets (850–1300 A in diameter), mitochondria, glycogenparticles, and microtubules. Active Golgi complexes are presentin both cell types. The nemites are similar to those describedfor neurosensory cells, (iii) Mature ganglionic cells are bipolaror multipolar. The small, dense nuclei are surrounded by a smallamount of cytoplasm. The neurites contain mostly microtubules;a few mitochondria, ribosomes, and glycogen particles are alsopresent, but there are no secretory droplets. To date, only neurosensory and neurosecretory cells have beenobserved in the gastiodermis. They are structurally indistinguishablefrom their epideimal counterparts. A significant finding is that three types of synapses—neuromuscular,neuronematocyte, and interneuronal—are identified in boththe epidermal and gastrodermal neurons.