An ultrastructural study of spermatogenesis and the mature spermatozoon of the ascidian Pyura stolonifera

Abstract

In Pyura stolonifera, there is an orderly arrangement of the male germ cells within the testicular follicles. Cells in early stages are located at the outer surface of the follicle while those in later stages of differentiation, including mature spermatozoa, lie nearer to the lumen. Developing spermatocytes are connected by cytoplasmic bridges which persist up to the late spermatid stage. The cytoplasmic bridge is reinforced on each side by an osmiophilic layer. Microtubules appear in the cytoplasm around the outside of the nuclear membrane in the mid-spermatid stage. The mature spermatozoon consists of an elongate, spindle-shaped head and a long tail. The head, which is about 9 µm in length, is slightly curved and tapers anteriorly. It comprises an elongate, eleclron-dense nucleus, a single large mitochondrion and a thin layer of cytoplasm. There is no middle piece. The mitochondrion is disposed lateral to the nucleus. Granules (presumably glycogen) occur in the cytoplasm between the nucleus and the mitochondrion. Neither an acrosome nor any other structure corresponding to it was observed in sections of the apical region. A ‘fuzzy’ material, however, surrounds the apex. The tail is about 40 µm in length and 0.2 µm in diameter. The axoneme has the 9 + 2 arrangement of microtubules.