Usually, the immediate end products of meiosis are not fully developed gametes. A period of maturation commonly follows meiosis. In plants, one or more mitotic divisions are required to produce reproductive spores, whereas in animals the meiotic products develop directly into gametes through growth and/or differentiation. The entire process of producing mature gametes or spores, of which meiotic division is the most important part, is called gametogenesis. Review Figs. 1-3 and 1-5 for details of mitotic and meiotic divisions as needed.
Animal Gametogenesis (As Represented in Mammals)
Gametogenesis in the male animal is called spermatogenesis [Fig. 1-6(a)]. Mammalian spermatogenesis originates in the germinal epithelium of the seminiferous tubules of the male testes from diploid primordial cells. These cells undergo repeated mitotic divisions to form a population of spermatogonia. By growth, a spermatogonium may differentiate into a diploid primary spermatocyte with the capacity to undergo meiosis. The first meiotic division occurs in these primary spermatocytes, producing haploid secondary spermatocytes. From these cells, the second meiotic division produces four haploid meiotic products called spermatids. Almost the entire amount of cytoplasm then extrudes into a long whiplike tail during maturation and the cell becomes transformed into a mature male gamete called a sperm cell or spermatozoan (spermatozoa, plural).
Gametogenesis in the female animal is called oogenesis [Fig. 1-6(b)]. Mammalian oogenesis originates in the germinal epithelium of the female ovaries in diploid primordial cells called oogonia (oogonium, singular). By growth and storage of much cytoplasm, the oogonium is transformed into a diploid primary oocyte with the capacity to undergo meiosis. The first meiotic division reduces the chromosome number by half and also distributes vastly different amounts of cytoplasm to the two products by a grossly unequal cytokinesis. The larger cell thus produced is called a secondary oocyte and the smaller is a primary polar body. In some cases, the first polar body may undergo the second meiotic division, producing two secondary polar bodies. All polar bodies degenerate, however, and take no part in fertilization. The second meiotic division of the oocyte again involves an unequal cytokinesis, producing a large ootid and a secondary polar body. By additional growth and differentiation, the ootid becomes a mature female gamete called an ovum or egg cell.
The union of sperm and egg is called fertilization and reestablishes the diploid number in the resulting cell called a zygote. During fertilization, the head of the sperm enters the egg, but the tail piece (the bulk of the cytoplasm of the male gamete) remains outside and degenerates. Subsequent mitotic divisions produce the numerous cells of the embryo that become organized into the tissues and organs of the new individual.
Plant Gametogenesis (As Represented in Angiosperms)
Gametogenesis in the plant kingdom varies considerably between major groups of plants. The process as described below is that typical of many flowering plants (angiosperms). Microsporogenesis (Fig. 1-7) is the process of gametogenesis in the male part of the flower (anther, Fig. 1-8), resulting in reproductive spores called pollen grains. A diploid microspore mother cell (microsporocyte) in the anther divides by meiosis, forming at the first division a pair of haploid cells. The second meiotic division produces a cluster of four haploid microspores. Following meiosis, each microspore undergoes a mitotic division of the chromosomes without a cytoplasmic division (karyokinesis). This requires chromosomal replication that is not illustrated in the karyokinetic divisions of Fig. 1-7. The product of the first karyokinesis is a cell containing two identical haploid nuclei. Pollen grains are usually shed at this stage. Upon germination of the pollen tube, one of these nuclei (or haploid sets of chromosomes) becomes a generative nucleus and divides again by mitosis without cytokinesis (karyokinesis II) to form two sperm nuclei. The other nucleus, which does not divide, becomes the tube nucleus. All three nuclei are genetically identical.
Megasporogenesis (Fig. 1-9) is the process of gametogenesis in the female part of the flower (ovary, Fig. 1-8), resulting in reproductive cells called embryo sacs. A diploid megaspore mother cell (megasporocyte) in the ovary divides by meiosis, forming in the first division a pair of haploid cells. The second meiotic division produces a linear group of four haploid megaspores. Following meiosis, three of the megaspores degenerate. The remaining megaspore undergoes three mitotic divisions of the chromosomes without intervening cytokineses (karyokineses), producing a large cell with eight haploid nuclei. Remember that chromosomal replication must precede each karyokinesis, but this is not illustrated in Fig. 1-9. At one end of the sac there is an opening called the micropyle through which the pollen tube will penetrate. Three nuclei orient themselves near the micropylar end and two of the three (synergids) secrete products that attract the pollen tube. The third of these nuclei develops into an egg nucleus. Another group of three nuclei moves to the opposite end and degenerates (antipodals). The two remaining nuclei (polar nuclei) unite near the center forming a single diploid fusion nucleus. The mature embryo sac (megagametophyte) is now ready for fertilization.
Pollen grains from the anthers are carried by wind or insects to the stigma. The pollen grain germinates into a pollen tube that grows down the style. The pollen tube enters the ovary and makes its way through the micropyle of the ovule into the embryo sac (Fig. 1-10). Both sperm nuclei are released into the embryo sac. The pollen tube and the tube nucleus degenerate. One sperm nucleus fuses with the egg nucleus to form a diploid zygote, which will then develop into the embryo. The other sperm nucleus unites with the fusion nucleus to form a triploid (3n) nucleus, which, by subsequent mitotic divisions, forms a starchy nutritive tissue called endosperm. The embryo, surrounded by endosperm tissue becomes the familiar seed. Since two sperm nuclei are involved, this process is termed double fertilization. Upon germination of the seed, the young seedling utilizes the nutrients stored in the endosperm for growth until it emerges from the soil, at which time it becomes capable of manufacturing its own food by photosynthesis.
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