Sexual Reproduction: Meiosis, Germ Cells, and ... - U-Cursos

1290 Chapter 21: Sexual Reproduction: Meiosis, Germ Cells, and Fertilizationtheir cytoplasm, oocytes maintain their large size despite undergoing the twomeiotic divisions. Both of the polar bodies are small, and they eventually degenerate.In most vertebrates, oocyte maturation proceeds to metaphase of meiosis II,at which point they become arrested. At ovulation, the arrested secondaryoocyte is released from the ovary, ready to be fertilized. If fertilization occurs, theblock is lifted and the cell completes meiosis, becoming a mature egg. Becauseit is fertilized, it is also called a zygote.follicle cellnurse celloocyteOocytes Use Special Mechanisms to Grow to Their Large SizeA somatic cell with a diameter of 10–20 mm typically takes about 24 hours to doubleits mass in preparation for cell division. At this rate of biosynthesis, such acell would take a very long time to reach the thousand-fold greater mass of amammalian egg with a diameter of 100 mm. It would take even longer to reachthe million-fold greater mass of an insect egg with a diameter of 1000 mm. Yet,some insects live only a few days and manage to produce eggs with diameterseven greater than 1000 mm. Eggs must have special mechanisms for achievingtheir large size.One simple strategy for rapid growth is to have extra gene copies in the cell.Most of the growth of an oocyte occurs after DNA replication, during the prolongedarrest after diplotene in prophase I, when the diploid chromosome set isin duplicate (see Figure 21–23). In this way, it has twice as much DNA availablefor RNA synthesis as does an average somatic cell in the G 1 phase of the cellcycle. The oocytes of some species go even further to accumulate extra DNA:they produce many extra copies of certain genes. As discussed in Chapter 6, thesomatic cells of most organisms contain 100 to 500 copies of the ribosomal RNAgenes so as to produce enough ribosomes for protein synthesis. Eggs need evengreater numbers of ribosomes to support the increased rate of protein synthesisrequired during early embryogenesis, and in the oocytes of many animals theribosomal RNA genes are specifically amplified; some amphibian eggs, forexample, contain 1 or 2 million copies of these genes.Oocytes may also depend partly on the synthetic activities of other cells fortheir growth. Yolk, for example, is usually synthesized outside the ovary andimported into the oocyte. In birds, amphibians, and insects, yolk proteins aremade by liver cells (or their equivalents), which secrete these proteins into theblood. Within the ovaries, oocytes use receptor-mediated endocytosis to take upthe yolk proteins from the extracellular fluid (see Figure 13–46). Nutritive helpcan also come from neighboring accessory cells in the ovary. These can be of twotypes. In some invertebrates, some of the progeny of the oogonia become nursecells instead of becoming oocytes. Cytoplasmic bridges connect these cells tothe oocyte, allowing macromolecules to pass directly from the nurse cells intothe oocyte cytoplasm (Figure 21–24). For the insect oocyte, the nurse cells manufacturemany of the products—ribosomes, mRNA, protein, and so on—thatvertebrate oocytes have to make for themselves.The other accessory cells in the ovary that help to nourish developingoocytes are ordinary somatic cells called follicle cells, which surround eachdeveloping oocyte in both invertebrates and vertebrates. They are arranged asan epithelial layer around the oocyte (Figure 21–25, and see Figure 21–24), andthey are connected to each other and to the oocyte by gap junctions, which permitthe exchange of small molecules but not macromolecules (discussed inChapter 19). Although follicle cells are unable to provide the oocyte with preformedmacromolecules through these junctions, they can supply the smallerprecursor molecules from which macromolecules are made. The critical importanceof gap-junction communication has been elegantly demonstrated in themouse ovary, where the gap-junction proteins (connexins) involved in connectingfollicle cells to each other are different from those connecting follicle cellsto the oocyte. If the genes that encode either of these proteins are disrupted inmice, both the follicle cells and oocytes fail to develop normally and the femalemice are sterile. In many species, follicle cells secrete macromolecules that20 mmcytoplasmic bridgeFigure 21–24 Nurse cells and folliclecells associated with a Drosophilaoocyte. The nurse cells and the oocytearise from a common oogonium, whichgives rise to one oocyte and 15 nursecells (only 7 of which are seen in thisplane of section). These cells remainjoined by cytoplasmic bridges, whichresult from incomplete cell division.Eventually, the nurse cells dump theircytoplasmic contents into the developingoocyte and then kill themselves. Thefollicle cells develop independently frommesodermal cells.