Germ cell development in larval and juvenile carp (Cyprinus carpio L.)

This thesis describes the development of larval and juvenile gonads of a teleost fish, the common carp, with special attention to the differentiation of the primordial germ cells. The early gonadal development has received relatively little attention, hitherto, as the research on fish reproduction has been focussed mainly on germ cell maturation in adult fish species. The early development of gonads and germ cells, however, is highly important, as in that period a.o. sex determination occurs.In the early larval period the gonadal tissue is gradually formed around the primordial germ cells (PGCs) which are already located at the sites of the future gonadal primordia. The PGCs in the developing larval gonads increase in size between week 2 and week 4 after fertilization. Concomitantly, ultrastructural changes occur in the cytoplasm and nuclei of these cells, which is supposed to be a preparation for the rapid proliferation of PGCs after week 6.An important part of the investigations was devoted to the production of monoclonal antibodies (MAbs). In general, MAbs may recognise differentiation stages accurately. This enables the investigation of the factors which may influence such differentiation processes. The MAbs were raised against primary and early secondary spermatogonia. The procedures are presented how germ cells were isolated from testis tissue and how the percentage of these cells (diameter>10 μm) could be enriched with a percoll gradient from an initial 4% in the early spermatogenic testis to more than 70%.One of the produced MAbs recognised antigens, appearing first in the cytoplasm and subsequently on the surface membrane of the enlarging PGCs in the larval gonads between week 2 and week 4. This supports the hypothesis that the enlarging PGCs are differentiating cells, possibly in preparation for the fast proliferation of PGCs after week 6. Four of the produced MAbs reacted with PGCs from the onset of PGC proliferation (after week 6) up to and including early (primary) spermatogonia in the adult testis (or oogonia in ovaries). This indicates that specific differentiation antigens are present on these cells. These MAbs may be used for the recognition of primary spermatogonia after isolation of germ cells for cell culture which is necessary for the investigation of factors which stimulate or inhibit specific steps, a.o. the onset of meiosis.Furthermore, the possible stimulation of gonadal development in larval or juvenile carp by injection with homologous pituitary extract containing a.o. the gonadotropic hormone (GTH) was investigated. It was found in juvenile carp that the gonads were greatly enlarged and that precocious spermatogenesis occurred after injection of pituitary extract (confirming data obtained in other fish species after injection of GTH). However, also gonadal development and PGC proliferation in larval fish was stimulated. Such effect of pituitary extract on larval gonads and germ cells has not been reported previously. The evidence obtained indicates a function of GTH in this stimulation. The question, however, whether one of the MAb-recognised determinants is involved awaits elucidation