An ultrastructural study of germ plasm in spermatogenesis ofXenopus lœvis

ABSTRACT Spermatogonia and primary diplotene and zygotene spermatocytes contain an electron-dense, finely granular substance which is usually closely associated with mitochondria; small patches of this substance also occur close to the nuclear membrane, often in the nuclear pores, and within the nucleus of primary spermatogonia. The fine structure of this substance is very similar to the fine structure of germ plasm in other stages of development, and since an ontogenetic continuity with germ plasm can be demonstrated, it was concluded that this substance is also germ plasm. The substance disappears about pachytene, earlier than in oogenesis where it persists until mid-diplotene, a difference which may be due to the fact that the oocyte stores large quantities of germ plasm in its cortex for the next generation of primordial germ cells. If the presence of the substance in the nuclear pores and within the nucleus is an indicator of synthesis of germ plasm, then synthesis stops in the secondary spermatogonium, which correlates with the subsequent absence of germ plasm from the pachytene spermatocyte stages. It is suggested that the function of the germ plasm in specifying germ line cells is carried out between the gastrula stage and the beginning of meiosis. The three events which take place during this period are (i) the migration of the presumptive primordial germ cells from the endoderm to the genital ridges, (ii) mitosis of the primordial germ cells and subsequently of the oogonia and spermatogonia in the developing gonads and (iii) preparations for meiosis. It is suggested that the mechanism of action of the germ plasm may be in the control of one or more of these processes. Other types of granular cytoplasmic deposits are also described, and their possible relationship to germ plasm discussed.