Quantitative and ultrastructural analysis of the chondriome in ovogenesis and embriogenesis of the sea urchin Paracentrotus lividus. 2. Growth and proliferation of mitochondria in embriogenesis of the sea urchin Paracentrotus lividus

The dynamics of structural changes of the chondriome in the early developme nt of the sea urchin Paracentrotus lividus was studied, Mature eggs and emb ryos at various stages of cleavage were used for quantitative and ultrastru ctural analysis based on computerized 3D reconstruction from serial ultrath in sections. The following structural transformations of the chondriome wer e shown to occur in the course of embryogenesis: 1) 15 min after fertilizat ion the disintegration of mitochondrial clusters takes place, and mitochond rial division is induced. At two-blastomeres stage a population of mitochon dria increases twofold; 2) the mitochondria divide by means of the contract ion of both outer and inner membranes. The forming furrow divides the "mate rnal" mitochondrion into two equal "daughter" parts; 3) at the four-cell st age the division is ceased, and a prolonged growth of mitochondria begins, that leads to the increase in the mitochondrial length; 4) cellular differe ntiation stimulates elongation of rod-shaped mitochondria and brings about changes in the rod/spherical-shaped mitochondria ratio; 5) in unfertilized egg, the mitochondria of condensed form are present: after fertilization al l the population of mitochondria is transformed to orthodox form. Modem con cepts of chondriome proliferation in eukaryotic cells are discussed.