Regional differences of mitochondrial size in the sea urchin embryo

Abstract

1. 1. Mitochondria are larger, compared to those in ectodermal cells, in invaginating vegetal and primary mesenchyme cells in early gastrulae of the sea urchin, Lytechinus anamesus. The average size ( μ 3), calculated from the profile dimensions in electron micrographs, of animal-cell mitochondria was 73 per cent that of vegetal-cell mitochondria in embryos fixed in buffered osmium solutions. In early gastrulae fixed by freeze-substitution the animalcell mitochondria averaged 52 per cent the size of those in invaginating vegetal cells and primary mesenchyme. 2. 2. A comparison of mitochondria in embryos fixed by freeze-substitution and in buffered osmium solutions indicates that the latter fixative extracts the matrix and probably causes swelling. Considering the unlikelihood of swelling of mitochondria in embryos fixed by freeze-substitution, it is concluded that regional size differences of mitochondria exist in the living embryo. In the majority of sections the cristae were more numerous in the vegetalcell mitochondria. 3. 3. A regional difference in mitochondrial size was found in the mesenchyme blastula stage and in early and late gastrulae. Mitochondria were found, however, to be uniform in size in all regions of the 16-cell stage. There is an indication of an overall increase in mitochondrial size with an increase in number of cristae per mitochondrion during early development. It is concluded that a differential “growth” of mitochondria occurs during, or more likely, after the cleavage period. 4. 4. The comparatively larger mitochondria are associated with the vegetalization of cells. Animal-half embryos contained significantly larger mitochondria in the cells, originally presumptive ectoderm, which had been converted to entoderm by treatment with LiCl.