Cell Biology International, 1996, Vol. 20, No. 12, 831–838 MITOCHONDRIOGENESIS IN MATURING SEA URCHIN OOCYTES: A COMPUTERIZED RECONSTRUCTION ANALYSIS D. FAIS* 1 , I. KIREEV 2 , M. SOUKHOMLINOVA 2 , G. MORICI 1 and V. POLYAKOV 2 1 Dipartimento di Biologia Cellulare e dello Sviluppo, Università, Viale delle Scienze, 90128 Palermo, Italy 2 Belozersky Institute of Physico-Chemical Biology, Lomonosov Moscow State University, 119899 Moscow, Russia Accepted 19 August 1996 The dynamics of chondriome changes in oogenesis of the sea urchin Paracentrotus lividus were studied by electron microscopy. An oocyte-enriched fraction obtained by gonad mechanical dissociation without protease treatment was used. The shape, size and arrangement of mitochondria (Mt) in cells were quantitatively analysed on the basis of data from reconstruction experiments, with serial sections performed using a specific computer program. At all stages of oogenesis, the chondriome was shown to consist of rod-shaped Mt of various lengths and also of small amounts of globular Mt about 0.3 m in diameter. Chondriome transformation during oogenesis is shown to involve the following processes: (1) a 64-fold increase in number of Mt, with the ratio of cytoplasm to Mt volume quite constant in the course of oogenesis; (2) an increase in length of Mt to a maximum of 1.54 m in medium oocytes and successive considerable mitochondrial division; (3) changes in Mt ultrastructure; and (4) a clustering of Mt. In a mature egg, the modal value of Mt length was reduced and, unlike the oocytes, was more homogeneous, and the Mt were completely clustered.  1996 Academic Press Limited K: mitochondria; reconstruction analysis; division; oogenesis; sea urchin INTRODUCTION It is known that Mt are organelles of dynamic nature, i.e. they can grow, divide and interact among themselves and modify their own structure (Bakeeva and Chentsov, 1989). These processes determine those well-defined modifications of the mitochondrial mass that are the basic condition necessary to suit the production of energy to the needs of the cell (Attardi and Shatz, 1988). In some unicellular organisms, mitochondrial bio- genesis is shown by the growth of the pre-existent mitochondrial complexes, in other words through a simple adaptation process (Hayashi and Ueda, 1989; Broadwater and Scott, 1986; Tanaka and Kanabe, 1985). Another case is that of the differ- entiated cells of multicellular organisms, where the mitochondrial biogenesis is not exhausted with their growth and shows more complex pecu- liarities. For example, the differentiation of the striated muscle cells, such as the myocardiocytes, causes the formation of particular contact struc- tures between the membranes of each Mt (Bakeeva et al., 1978; Amchenkova et al., 1988); in some kinds of differentiated cells the chon- driome appears as mitochondrial clusters, but its functional meaning is not yet well understood (Bakeeva and Chentsov, 1989). During specific stages of the oogenesis of amphibia and echino- derms, the mitochondrial division affects the whole chondriome (Bakeeva and Chentsov, 1989). Many data in the literature indicate that during cell differentiation, greater functional modi- fications of the Mt may take place, not only their simple growth, and that a real process of mito- chondriogenesis may occur (Chen, 1988). Sea urchins have long represented a model for the study of cell differentiation (Giudice, 1973, 1986) and in which important discoveries on mitochon- drial genesis have been made (Rinaldi, 1981). Surprisingly, however, a quantitative ultrastruc- tural study of mitochondriogenesis during sea urchin oogenesis is still lacking. It is the purpose of this paper to describe such a study by electron *To whom correspondence should be addressed. 1065–6995/96/120831+08 $25.00/0  1996 Academic Press Limited