GAMETOGENESIS Zillan Neiron Gametogenesis describes the formation of both the male and female gametes, spermatogenesis is the production of sperm in the male testes and oogenesis is the production of ova within the ovaries. Spermatogenesis and oogenesis depend on mitosis and meiosis. Mitosis: Mitosis is the cell division that produces two genetically identical daughter cells with the same chromosome number and DNA content as the original cell. Growth of any multicellular organism occurs through mitosis. Mitosis is divided into four stages after DNA has been replicated: Prophase – chromosomes condense into (chromatids) held together by a centromere, nuclear envelope breaks, centrioles replicate. Metaphase – mitotic spindles develop from the centrioles at the cells poles, chromosomes are aligned at the center of the cell. Anaphase – chromatids separate and are pulled to opposite poles by the spindle fibers. Telophase – nuclear envelope reforms around the chromosomes and uncoil. Cytoplasm divides to form two daughter cells. Figure 1. Mitosis. The example cell (A) has one chromosome, however the cell is diploid so it contains two copies of the chromosome, one from each parent. Mitosis begins with the duplication of each chromosome producing two sister chromatids (B). The sister chromatids line up along the center of the cell and separate to the edge of the cell. The cell membrane constricts (C) to form a separate membrane around each new diploid cell, called daughter cells. Meiosis: Meiosis is the cell division process of gametes, consisting of two chronological cell divisions: Meiosis I (reductional division) Meiosis II (equational division because it does not reduce in chromosome numbers). The end result is the production of cells containing half the amount of chromosomes (23). During this process there is an exchange of genetic material therefore the daughter cells are not genetically identical. Before meiosis I DNA is replicated. Maternal and paternal chromosomes pair cross over and then separate during meiosis I. In meiosis II the chromatids separate resulting in 4 haploid daughter cells each genetically different from each other. Prophase I – chromosomes condense and cross over, nucleolus disappears and nuclear envelope disintegrates. Chromosomes have replicated. Chromosomes come together in pairs. Each chromosome is similar in structure but would have originally come from a different parent. Late Prophase – homologous pairs twist round each other and chromatids may cross over. Breaks occur at these cross-overs (chiasmata) and pieces of chromatids are exchanged. Crossing over means that the chromosomes exchange stretches of DNA. Chiasmata represent the points where earlier non-sister chromatids had swapped section. The segments exchanged by each non-sister chromatid are identical, however may carry different alleles. Crossing over requires great precision so that neither chromatid gains or loses any genes. The exchange is like genetic shuffling enabling the species to have a rich diversity of phenotypical expressions from generations to generations in a given family. Metaphase I – chromosomes are aligned along equator. Anaphase I – one of each paired chromosomes move to the poles Telophase I – division of the cytoplasm, nuclear envelope forms around the chromosomes. Daughter cells are haploid in chromosome number but diploid in DNA content. After meiosis I, meiosis II begins, the cell goes through prophase, metaphase, anaphase and Telophase again. This time the daughter cells are haploid in DNA content. Figure 2. Meiosis. Male and females meiosis differ in terms of cytoplasm distribution. Male: end result is four identical spermatids, which differ in genetic information. Female: ovum receives an unequal share of the cytoplasm and remains functional while the polar bodies are pockets of genetic information with minimal cytoplasm and therefore non-functional. Timing of meiosis is also different for males and females. Spermatogenesis: Spermatogenesis occurs continuously from puberty where spermatogonia divide into spermatozoa in the testes. The seminiferous tubules produce haploid cells. It involves several steps including mitosis and meiosis. It takes about 74 days in humans. 300 million sperm cells are produced per day. Spermatogenesis is divided into three phases: Spermatogonial phase (mitosis) Spermatocyte phase (meiosis) Spermatid phase (spermiogensis or sperm maturation). Along the seminiferous tubules there is a range of cells at various levels of maturation from spermatids to true spermatozoa. Spermatogonial phase: Mitotic division. The spermatogonia divide and replace themselves to provide a committed population of spermatogonia. Spermatocyte phase: Meiotic division. Primary spermatocytes undergo meiosis to reduce chromosome number. The mitotic division of spermatogonia produces primary spermatocytes. After the first meiotic division, primary spermatogonia become secondary spermatogonia that enter the prophase of the second meiotic division. At the end of the second meiotic division the nuclear membranes reform and two spermatids are formed from each secondary spermatocyte. Spermiogenesis phase: the spermatids mature/differentiate to produce sperm. Spermiogensis consists of four phases: The Golgi phase: glycoproteins accumulate in the golgi complexes and combine into a membrane bound vesicle. The Cap phase: the acrosomal vesicle develops further to form the acrosomal cap. The Acrosome phase: the spermatid re-orientates itself so it is embedded in the sertoli cell. The flagellum develops and extends into the lumen of the seminiferous tubule. The nucleus flattens and elongates with the cytoplasm organized posteriorly. Cytoplasmic microtubules become organize in a cylindrical sheath, called the manchette, which becomes the mid-piece of the flagellum. The Maturation phase: excess cytoplasm is pinched off and phagocytosed by the sertoli cells. Figure 3. Spermatogenesis. The ovary is where gametogenesis and folliculogenesis occur (the growth of the follicle, which is the structure containing the oocyte). Oogenesis: Oogenesis occurs by meiosis, however females do not make millions of ova since a female does not carry millions of fetuses. No more than one ovum is produced per month therefore females do not have constant mitosis of their germ cells occurring. Oogenesis begins during fetal life. During the first trimester of pregnancy the oogonia (primordial germ cells) of the female fetus divides continuously, mitotically. Each oogonia has 46 chromosomes. The oogonia reach a population of 7 million. These oogonia begin the first meiotic division but stop in prophase I of meiosis I as a primary oocyte and lie within the ovaries surrounded by other cells, follicular cells (together they comprise the primary follicle). Prior to birth there is a huge reduction in primary oocytes to only 2 million. Meiosis does not continue again until puberty, when menarche begins. At puberty only 400,000 oocytes remain in the ovaries. A monthly surge of LH causes the primary oocyte to continue meiosis from where it left off (prophase I). Every month, one primary oocyte is hormonally stimulated to begin to enlarge and complete meiosis. The end result is a secondary oocyte and a polar body. The primary oocyte finishes the first meiotic division by dividing its genetic material appropriately for meiosis I and undergoing cytokinesis (separation of daughter cells into two separate cells via a cleavage furrow). In oogenesis cytokinesis does not occur down the middle of the parent cell but unevenly. Only one of the daughter cells can become the ovum, as only one ovum is released per month. Therefore the one cell that becomes the ovum out of the four daughter cells receives all the nutrients. The polar bodies die quickly. The ovulated secondary oocyte begins meiosis II but stops at metaphase II. Secondary oocyte contains 23 chromosomes, is released from the ovary. Travels down the uterine tube. If it does not encounter a spermatozoan, it never undergoes the second meiotic division but will go through atresia. If it encounters a spermatozoan it rapidly completes Anaphase II and Telophase II to form the ovum and another polar body. The ovum is then ready to fuse with the spermatozoan. Only 400 oocytes are used over the reproductive life span of the female. Figure 4. Oogenesis Hormones involved in Oogenesis: FSH, LH, Estrogen and Progesterone released in a cyclical fashion. 1. After menstruation, an ovarian follicle begins to develop. 2. 14 days later, Estrogen levels peak, and 3. 24hrs later there is a surge in FSH and LH levels causing the release of an ovum from the ovaries (ovulation). 4. Estrogen and progesterone levels rise around day 24 5. If fertilization does not take place, the lining and the ovum is shed during menstruation.