Gametogenesis, as name signifies, is the procedure by which organism generates its gametes. Gametes are usually haploid. Reduction of chromosomes form diploid to haploid number is achieved in the procedure of meiosis. Though, meiosis is but one step in procedure of gametogenesis.
Gametogenesis in Animals:
In male animals the processes leading to formation of spermatozoa are together known as the procedure of spermatogenesis. Diploid cells destined to give rise to sperms are primordial germ cells, call SPERMATOGONIA (sing. - sper-maogonium). These cells are present in semi-niferous tubules of testes. Spermatogonia experience mitotic divisions, giving rise to meiocytes known as PRIMARY SPERMATOCYTES. As each spermatogonium is diploid primary spermatocytes are also diploid.
Primary spermatocytes undergo first meiotic division, each giving rise to two haploid SECONDARY SPERMATOCYTES. Each secondary spermatocyte in turn experiences second meiotic division giving rise to two haploid SPERMATIDS. Spermatid then experiences a metamorphosis comprising of morphological and intra-cellular changes culminating in formation of sperm. This procedure of metamorphosis from spermatid to sperm is known as SPERMIOGENESIS.
Oogenesis is formation of female gamete, and it takes place in ovary. Diploid primordial germ cell in female is known as OOGONIUM. It experiences mitosis to generate more oogoria. Each oogonium grows in size and matures in PRIMARY OOCYTE. Like primary spermatocyte, primary oocyte experiences meiosis-I giving rise to two haploid cells. But in this case, two cells aren't equal in size, unlike circumstances in spermatogenesis. The larger of the two cells is the SECONDARY OOCYTE and the smaller one is FIRST POLAR BODY.
The secondary oocyte experiences meiosis-II, giving rise to the large cell known as OOTID, and a small cell known as SECOND POLAR BODY. First polar body may undergo meiosis-II, giving rise to two polar bodies. Production of unequal-sized cells in meioses-I and I of oocytes is because of the fact that spindle in oocyte is displaced toward periphery of cell, rather than being in centre. The ootid undergoes the maturation procedure and becomes OVUM. In several animals process of oogenesis isn't completed until sperm has penetrated egg. For example, secondary oocyte may remain in metaphase-II or anaphase-II until it is penetrated by the sperm. In some insects, annelids, frog and mouse, it is the penetration by sperm that triggers completion of meiosis. In others like sea urchin meiosis is completed before sperm penetration. Despite of pattern of oogenesis, though, final act in procedure of fertilization, i.e. fusion of male and female nuclei doesn't occur until meiosis-II has been completed.
Gametogenesis in Plants:
Gametogenesis is rather different in plants. In plants there are sporophyte and gametophyte generatins. In anagiosperms (flowering plants), sporophyte is dominant and gametophyte is greatly decreased, and is dependent on sporophyte. Male gametophytes, called MICROGAMETOPHYTES are in anthers whereas female equivalent, MEGAMETOPHYTES are in ovary.
This is the procedure by which male gametes of plants, MICROSPORES are genertaed. The anther has pollen mother cells, MICROSPOROCYTES, that are diploid. Each microsporocyte undergoes meiosis to generate four microspores, that are haploid. Microspores mark starting of gametophyte generation.
Within each spore the nucheus undergoes mitotic division without cytokinesis. Spore therefore becomes cell with two haploid nuclei, one of which is TUBE NUCLEUS and other the GENERATIVE NUCLEUS. This cell with two nuclei grows in pollen grain that is MICROGAMETOPHYTE.
On stigma pollen grain germinates, generating pollen tube with two haploid nuclei. Inside the tube generative nucleus undergoes the mitotic division, giving rise to two functional SPERM NUCLIE. Therefore, microsporogenesis is like spermatogenesis in that each pollen mother cell (meiocyte) give rise to 4 microspores, each of which in turn grows in microgametophyte (pollen grain) which is the gamete.
The macrosporocyte (also known as megasporocyte or mother cell) is distinguished in nucellus inside ovule. It generates linear megaspore tetrad by division. Megaspores can stay together and some of them may degenerate. Based on how many megaspores form female gametophyte development of embryo sac is monosporic, bisporic or tetrasporic. Mature gametophyte comprises of 7 cells. During monosporic development remaining megaspore near to chalaza goes under mitotic division 3 times and generates 8 uniform nuclei. Nuclei move to their places and gametophyte becomes cellular. At micropylar end egg cell and 2 synergids form egg apparatus. At chalazal end 3 antipodals are situated that later help to give nutrition to embryo. At center of megaspore the remaining two polar nuclei fuse partly to create the large central cell. It develops secondary endosperm after fertilization. The stated process is also known as Polygonum type embryo sac development.
After double fertilization developing seedling engages in nutrition from surrounding gametophytic and sporophytic tissues. Seed is propagating structure having mature embryo and it is developed from ovule. Seed coat layers initiate from integument. Female sporophyte supplies embryo through funiculus but after a while it detaches and leaves scar known as hilum. Based on remaining food storage, seed can be either endospermic or perispermic (some of the nucellus remains but endosperm is consumed). If all nutrition from gametophytic and sporophytic tissues is taken up seed is supplied by cotyledons. Seed are surrounded by ovary. Wall of real fruits develops only from ovary, if other tissue (hypanthium or base of perianth) takes part in formation fruit is called false. Hence all fruits originated from inferior ovary are false. Ovary has more ovules and each one of them can form a seed after fertilization but this procedure needs more pollen grains and germinating pollen tubes.
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