Ultrastructure of the sinus gland and lateral cephalic nerve plexus in the isopod Ligia oceanica (Crustacea Oniscoidea)

Abstract

Isopod crustaceans have two distinct cephalic neurohemal organs: the sinus gland (SG) and lateral cephalic nerve plexus (LCNP). The present study of Ligia oceanica was designed to ascertain the ultrastructure, during the moulting cycle, of the terminals constituting the SG and LCNP, both of which store and release neurosecretory material, and to trace these terminals to their probable origin in neurosecretory perikarya. The SG was observed to contain four types of terminals (I, II, III, and IV) assigned, on the basis of the appearance of their neurosecretory granules, to four types of neurosecretory cells in the protocerebrum (β 1, β 2, B 1 and B u). When the same morphological criteria were applied to the LCNP, two types of terminals were found—III′ and IV′. Type III′ was thought to originate in the B p plexus cells and in the B 2 cells of the suboesophageal ganglion. The origins of Type IV′ terminals were believed to be the B u and B m cells of this ganglion. Release from both the GS and LCNP occurred by exocytosis. The discussion attempts to relate the ultrastructural variations observed in the SG and LCNP with existing data on the neuroendocrine regulation of the moult. Such regulation involves the two antagonistic hormones (moult-inhibiting and moult-accelerating) which determine the circulating ecdysteroid level. It is also suggested that the plexus cells are the site of synthesis of a factor controlling the release of the exuviation factor.