Quantitative and ultrastructural analysis of the chondriome in ovogenesis and embryogenesis of the sea urchin Paracentrotus lividus.

The dynamics of chondriome in the ovogenesis of the sea urchin Paracentrotus lividus was studied. Growing oocytes 20-30, 50-60 and 90-100 microm in diameter ("small", "medium-sized" and "large", respectively) and mature eggs were used for the ultrastructural and stereological analysis of mitochondria. Linear parameters of mitochondria (length and thickness) were measured on 3-D reconstructions of serial ultrathin sections using the software developed in the laboratory. The following transformations of chondriome structure were shown to occur during ovogenesis: (1) the number of mitochondria (MT) increases with the growth of cytoplasmic compartment; (2) the modal length of MT increases from 0.5 microm in small oocytes to 1 microm in large ones and decreases again to 0.5 microm in the egg; this process is accompanied by changes in the relative number of spherical MT which decreases in medium-sized oocytes and subsequently rises again in the egg; (3) in medium-sized oocytes, dumbbell-shaped MT appear first, the number of these MT reaching the maximum to the stage of large oocytes. In mature eggs, the dumb-bell-shaped MT are absent; (4) in small and medium-sized oocytes, the orthodox conformation of MT is observed, in contrast to MT with a condensed matrix in large oocytes and eggs; (5) in mature eggs, mitochondrial clusters containing 10 to 20 MT of various size are formed. Based on the data obtained, we suggested that during ovogenesis of the sea urchin, specific differentiation of the chondriome is induced which leads to the increase in the quantity of MT via multiple division acts, while restricting the MT growth and variability of their shape.