Studies on the formation of germ cells in a compound ascidian Botryllus, primigenus oka

Abstract

Gametogenesis of a compound ascidian Botryllus primigenus was studied histologically. On either side of the zooid (stage 9), in the gonadal space between the epidermis and the atrial epithelium, either a single testis or a complex of an egg follicle and a testis can be formed. The egg follicle consists of a single ovum (occasionally two ova) and its accessory cells and is connected with the atrial epithelium by the follicle stalk. The eff follicle is always accompanied by the brood pouch, a diverticulum of the atrial cavity. The testis is equipped with a vestigial spermiduct and is attached to the atrial epithelium. Buds of stage 8 comprise, besides the developing testes and egg follicles, loose aggregations of hemoblasts and oocytes of early developmental stages, which are already accompanied by primary follicular cells. Both the oocytes and the primary follicular cells seem to arise from the hemoblasts. The young oocytes are isolated in the gonadal space of the buds and are transferred to buds of the succeeding generations until they finally mature. In the bud of stage 3, a compact mass of cells appears, attaching to the inner vesicle on either side of the body. It is derived from the hemoblasts lodged there in the preceding generation and presumably also from the circulating hemoblasts. When the cell mass receives a large oocyte derived from the preceding generation, part of the cell mass differentiates into egg envelopes, forming an egg follicle, and a follicle stalk and the remainder into a testis. When the cell mass receives no oocyte, it differentiates as a whole into a testis. In the egg follicle thus formed, the outer and inner follicular cells increase in number by mitotic division. Subsequently, initial test cells are derived from the inner follicle by migration across the developing chorion; then they increase in number by mitosis. In the testis, meiosis and spermiogenesis take place.

Colonial Allorecognition, Hemolytic Rejection, and Viviparity in Botryllid Ascidians

“…The blood vessels of botryllids are interconnected among the zooids, and hemocytes in the lumen circulate throughout the colony. These circulating hemocytes in botryllid ascidians are involved in budding (Oka and Watanabe, 1957), oogenesis (Mukai and Watanabe, 1976) and colonial allorecognition (Cf. Hirose et al, 2003).…”

Botryllus schlosseri: A model ascidian for the study of asexual reproduction

“…Their recycling through successive generations has been regarded as the necessary condition for their maturation (Mukai and Watanabe, 1976;Sabbadin and Zaniolo, 1979).…”

Body Muscle-Cell Differentiation from Coelomic Stem Cells in Colonial Tunicates

“…It is referred to as a hemoblast and is characterized by the presence of a large nucleus with a prominent nucleolus and narrow cytoplasm filled with polysomes (Wright, 1981). A variety of tissues and organs such as the gonad (Mukai and Watanabe, 1976;Sunanaga et al, 2006Sunanaga et al, , 2007, the pericardium (Nunzi et al, 1979), the pyloric gland (Kawamura and Nakauchi, 1986), and bud vesicle (Oka and Watanabe, 1957;Freeman, 1964;Sabbadin et al, 1975) differentiate from undifferentiated coelomic cells.…”