AN ULTRASTRUCTURAL STUDY OF THE MORPHOLOGICAL CHANGES OCCURRING IN THE HINDGUT EPITHELIAL CELLS DURING THE INTERMOLT CYCLE IN PORCELLIO RATHKEI, A TERRESTRIAL ISOPOD

The hindgut of Porcellio rathkei consists of a single layer of low columnar epithelial cells covered with a cuticle at the luminal surface. Ultrastructural differences were noted in the number and distribution of organelles, in the extent of apical and basal infoldings and in the relative sizes of the different layers of cuticle in the hindgut epithelial cells during the intramolt cycle. The shedding of the old cuticle does not occur simultaneously throughout the entire hindgut but proceeds from a posterior to an anterior direction and is asynchronous with the ecdysis of the exoskeleton. The molting of the hindgut cuticle is completed before molting of the posterior exoskeleton. The formation of the cuticle and the ultrastructural changes in the epithelial cells during molting of the hindgut epithelium of P. rathkei were very different from that reported during the molting of the exoskeleton of Oniscus asellus, but similar to that of the insect exoskeleton. During the initiation of the intermolt cycle after the termination of the endocuticle deposition, the development of numerous profiles of endoplasmic reticulum, dictyosomes, small membrane bound vesicles and large multivesicular bodies indicates the involvment of the hindgut epithelial cells in the secretion of proteinaceous materials, the ecdysial droplets, for the digestion of the old endocuticle and in the formation of the precursor materials for the protein part of the new cuticle. The maximum level of glycogen before the initiation of the new cuticle formation in the hindgut epithelial cells and its gradual disappearance through the succeeding stages of cuticle formation indicate the involvment of glycogen in cuticle formation either as precursor materials or as energy sources for cuticle secretion. The earlier concept of microtubular bundles in the hindgut epitheial cells with apical infoldings as aligning centers of mitochondria and other cell components is confirmed in the present work. The presence of extensive annulate lamellae in the rectal epithelial cells of P. rathkei suggest the possible value of these cells as a useful model for exploring the functions of annulate lamellae which remain unknown