Ultrastructure and cytochemistry of vitellogenesis and the vitellocytes of the bothriocephalidean cestode Clestobothrium crassiceps (Rudolphi, 1819), a parasite of the teleost fish Merluccius merluccius (L., 1758) (Gadiformes, Merlucciidae)

Abstract

AbstractVitellogenesis and vitellocytes of the bothriocephalidean cestode Clestobothrium crassiceps (Rudolphi, 1819), a parasite of the teleost fish Merluccius merluccius (L., 1758), were studied by means of transmission electron microscopy (TEM) and cytochemistry. During vitellogenesis, four developmental stages were distinguished at the TEM level: (I) a stem cell stage of the gonial type; (II) an early differentiation stage, predominantly exhibiting lipid and protein synthetic activity; (III) an advanced differentiation or vitellocyte maturation stage, primarily exhibiting active glycogenesis still accompanied by an increase in lipid accumulation; and (IV) a mature vitellocyte stage. Vitellogenesis involves: (1) an increase in cell volume; (2) an extensive development of parallel, frequently concentrically arranged, cisternae of granular endoplasmic reticulum (GER) that produce dense, proteinaceous shell-gobules; (3) the development of Golgi complexes engaged in the packaging of this material; (4) an accelerated accumulation of unsaturated and saturated lipid droplets, along with their continuous enlargement and fusion; (5) the formation of individual β-glycogen particles and α-glycogen rosettes and their accumulation in the form of glycogen islands scattered among lipid droplets in the cytoplasm of maturing and mature vitellocytes; and (6) the rapid accumulation of large, saturated lipid droplets accompanied by dense accumulations of α- and β-glycogen along with proteinaceous shell-globules or shell-globule clusters in the peripheral layer during the advanced stage of vitellocyte maturation. Vitellogenesis in C. crassiceps generally resembles that previously described for three other bothriocephalideans, but differs from that of other cestode orders. Cytochemical staining with periodic acid-thiocarbazide-silver proteinate for glycogen indicates a strongly positive reaction for β-glycogen particles and α-glycogen rosettes, which form several large glycogen accumulations around the large, saturated lipid droplets of maturing and mature vitellocytes. Some hypotheses concerning the interrelationships between patterns of vitellogenesis, the possible modes of egg formation, embryonic development and life cycles in cestodes, and their phylogenetic implications are commented upon.