AN ULTRASTRUCTURAL-STUDY OF THE CENTROSOME AND CENTRIOLES IN GAMETOGENESIS AND EARLY EMBRYOGENESIS OF LYMNAEA-STAGNALIS L .2. CENTROSOME AND CENTRIOLES IN OOGENESIS AND EARLY EMBRYOGENESIS

The fine structure of astral poles during both meiotic and early mitot ic divisions of Lymnaea stagnalis eggs is studied. It is shown that bo th meiosis I spindle poles contain single centrioles due, apparently, to uncoupling of the nuclear and centriolar cycles before meiosis: DNA replication proceeds, but reduplication of the centrioles is absent. The outer pole of the meiosis II spindle also possesses one centriole and the inner pole contains a sperm basal body. The inner pole centros ome of the meiosis II spindle reveals a complex structure. Its periphe ry is formed as a result of transformation of the inner pole centrosom e of the meiosis I spindle, and the center includes the sperm basal bo dy. Thus, the zygote centrosome is a hybrid consisting of both materna l and paternal material. Both poles of the first mitosis spindle conta in single centrioles. At the proximal end of each centriole an electro n dense globule is found. The sperm basal body has not been found. The refore, formation of the centrioles de novo during the first interphas e may be proposed. At the second mitosis a pair of centrioles is obser ved at each spindle pole. Two rounds of centriole duplication appear t o occur during the second interphase. As a result, the correlation bet ween DNA content and number of centrioles typical for proliferating an imal cells is restored. Later, such a correlation is maintained: at th e third mitosis each spindle pole contains a diplosome. (C) 1994 Acade mic Press, Inc.