The task of oogenesis: construct a large cell containing a large and complex dowry of resources for construction of the embryo before it can either make them on its own or obtain them from its environment. Organization is critical: Components may be spatially restricted to facilitate establishment of distinct embryonic regions.
Review the timing of oogenesis and compare to that of the male. Remember: most of the events of oocyte differentiation occur during prophase I of meiosis. In the mammal, all oogonial dicisions and transformation into oocytes is completed either before or shortly after birth. But, there is no oocyte growth until puberty. Then, cohorts of oocytes resume development during each cycle (See Browder et al., Fig. 3.2).
In some organisms, all stages of oogenesis can co-exist in the ovary (e.g. frogs).
Roles of accessory cells: production of steroid hormones, transportation of essential components to the oocyte, and formation of accessory layers surrounding the oocyte (later, the egg).
There are two categories of accessory cells: follicle cells and nurse cells. What is the major distinction between these two categories of cells? Nurse cells are found in a number of invertebrate species. We shall discuss insect nurse cells in some detail.
In the mammalian follicle, each oocyte is surrounded by a multilayered cohort of follicle cells, often referred to as granulosa cells. Between the oocyte and granulosa cells is the acellular zona pellucida, which is secreted by the oocyte. The zona is penetrated by many short microvilli from the surface of the oocyte and cytoplasmic processes from the follicle cells. Desmosomes and gap junctions form at the points where the cytoplasmic process contact the oocyte surface. The gap junctions function in transfer of nutrients and regulatory molecules into the oocyte.
Granulosa cells secrete the fluid that accumulates in the extracellular spaces and bathes the oocyte. The spaces coalesce to form the antrum (Fig. 3.4). Follicles with large antra are called Graafian follicles. In the Graafian follicle a cluster of granulosa cells surround the accentrically-located oocyte. They are collectively called the cumulus oophorus. The cumulus is shed along with the egg at ovulation.
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 (Fig. 3.6). One of these cells differentiates into the oocyte, and the rest become nurse cells. The nurse cells, which become highly polyploid (1024N), provide macromolecules (Fig. 3.8) and even organelles to the developing oocyte. 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 (Fig. 3.5), which also play significant roles in oogenesis, as we shall discuss later.
Polarity is a salient feature of oocytes. Consider, for example, the yolk-rich amphibian oocyte (Fig. 3.11). As we shall discuss later, not only are organelles localized in the egg, but macromolecules may also have specific domains.
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 (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 (Fig. 3.14).
The amount and distribution of yolk varies considerably in the animal kingdom. You should be able to distinguish among oligolecithal, telolecithal and centrolecithal eggs and name examples of each. How is an amphibian egg classified?
Yolk is formed in one of two ways: It is either synthesized within the oocyte (autosynthesis) or exogenously and transported into the oocyte (heterosynthesis). Vertebrate vitellogenesis is predominantly heterosynthetic. The yolk precursor (vitellogenin), which is synthesized in the liver, is incorporated via receptor-mediated endocytosis. Study this process in your textbook, particularly Figures 3.17-3.20. A similar process occurs in Drosophila. Yolk is synthesized in the fat body and ovaries.
Yolk is distributed in the amphibian oocyte in an asymmetrical pattern, with most platelets and the largest ones located in the vegetal hemisphere. The other hemisphere is the animal hemisphere. This polarity foreshadows the polarity of the embryo itself. The establishment of this polarity has been studied by monitoring the deposition of fluorescent-labeled vitellogenin (Fig. 3.22).
Young oocytes have a centrally located germinal vesicle (what is a germinal vesicle?) and uniformly-distributed yolk platelets; the latter are localized in the subcortical cytoplasm. Asymmetry results from a displacement of the yolk platelets from the animal hemisphere to the vegetal hemisphere (at a rate of 50 µm/day). The displacement process presumably involves the cytoskeleton.
The cortex is a semi-rigid gel (as distinct from the endoplasm, which
is fluid). The cortex resists centrifugation. The cortex may contain specialized
organelles, such as cortical granules and/or pigment granules (see Figs.
3.11 and 3.17). Cortical granules are membrane-enclosed organelles that
release their contents at fertilization. We shall discuss the roles of these
components later. The cortical granules are formed in the endoplasm. Precursor
molecules are apparently synthesized on the ribosomes of the rough endoplasmic
reticulum and transported within the E.R. to the Golgi apparatus, where
the granules are assembled (Fig. 3.24).
Browder, L.W., Erickson, C.A. and Jeffery, W.R. 1991. Developmental Biology. Third edition. Saunders College Pub. Philadelphia.
Xue, F. and L. Cooley. 1993. Cell 72: 681-693.
Browder, L.W. 1996. Oogenesis. In L.W. Browder (Ed.), Developmental
Biology, http://www.ucalgary.ca/~browder.
Copyright © 1996 Leon W. Browder. This material may be reproduced for
educational purposes only provided credit is given to the original source.
September 13, 1996