Oogenesis:A Collaborative Effort

The insect egg chamber is found in so-called meroistic insects (e.g., Drosophila). Incomplete cytokinesis of the terminal oogonial divisions results in a cluster of 16 cells interconnected by cytoplasmic bridges.

(See Browder et al., Fig. 3.6; Gilbert, Fig. 22.27; Kalthoff, Fig. 3.15)

One of these cells differentiates into the oocyte, and the rest become nurse cells. The nurse cells, which become highly polyploid (1024N), provide macromolecules and even organelles to the developing oocyte.

(See Browder et al., Fig. 3.8; Gilbert, Fig. 22.28; Kalthoff, Fig. 3.18; Shostak, Fig. 7.28)

Oocytes grow rapidly from the support they receive from these 15 polyploid cells. Drosophila oocyte volume increases 90,000 fold in just 3 days (Xue and Cooley, 1993)! At the end of oogenesis, residual nurse cell cytoplasm is transferred into the oocyte. Its volume doubles in just 30 minutes.

The entire nurse cell-oocyte complex is surrounded by follicle cells, which also play significant roles in oogenesis, as we shall discuss later.

(See Browder et al., Fig. 3.5; Gilbert, Fig. 22.27; Kalthoff, Fig. 3.14)

Oocyte Structure and Morphogenesis

Polarity is a salient feature of oocytes. Consider, for example, the yolk-rich amphibian oocyte. As we shall discuss later, not only are organelles localized in the egg, but macromolecules may also have specific domains.

(See Browder et al., Fig. 3.11)

As with sperm, mitochondria are significant oocyte organelles. They may be present in great abundance to fuel the events of early development, during which the embryo has no facility to produce new mitochondria of its own. (Remember that mitochondria are self-replicating.)

An example is the amphibian Xenopus laevis. In somatic cells, the ratio of nuclear DNA:mitochondrial DNA is 100:1. In the fully-grown oocyte, that ratio has reversed to a range from 1:1 to 1:100. During mitochondrial replication in early oogenesis, mitochondria cluster around the nucleus to form a structure called the mitochondrial cloud.

(See Browder et al., Fig. 3.12)

The cloud also contain electron-dense material called granulofibrillar material (GFM). The mitochondrial cloud later disperses, forming large subcortical islands of mitochondria and GFM at one pole of the oocyte. These clusters resemble similar clusters in unfertilized eggs that are similar to the germ plasm, which is thought to function in germ cell determination

NEXT