In meiosis II, the chromosomes line up in single file along the metaphase plate and the 2 chromatids of each chromosome separate. The 4 haploid cells resulting from meiosis II are called spermatids. Therefore 1 primary spermatocyte produces 4 spermatids. Unlike other cell division in the body, spermatocytes do not complete cytokinesis and they remain in contact via cytoplasmic bridges through their entire development. The final stage is the development of haploid spermatids into sperm. Developmental changes include; formation of an acrosome , elongation of the nucleus and development of a tail or flagellum. Sperm are then released from their connections to the sertoli cells into the lumen of the seminiferous tubules in an event called spermination and travel to the epididymis to mature and be stored. Each day millions of sperm complete the process of spermatogenesis. The major parts of the sperm are the head, body & tail. The head contains the nucleus with 32 condensed chromosomes. Covering the anterior 2/3 rds of the nucleus is the acrosome which is a cap-like vesicle filled with enzymes that help sperm penetrate an ovum. The tail (mid piece) contains mitochondria arranged in a spiral and provides energy for locomotion.
Oogenesis
Oogenesis is the formation of ova. In contrast to stallions where spermatogenesis begins at puberty, oogenesis begins in mares before birth. Oogenesis occurs in the same manner as spermatogenesis, meiosis takes place and then the resulting germ cells undergo maturation. During early fetal development, primitive germ cells migrate from the yolk sac to the ovaries where they differentiate within the ovaries into oogonia. These are diploid stem cells that will divide mitotically to produce millions of germ cells. Many will degenerate before they become anything. A few however develop into larger cells called primary oocytes that enter prophase I during fetal development but do not complete that phase until after puberty. At birth each ovary will contain roughly 200,000-2,000,000 primary oocytes. During this arrested stage of development, each primary oocyte is surrounded by a single layer of flat follicular cells and the entire structure is called a primordial follicle. With continued maturation, the primordial follicle develops into a secondary follicle as it becomes surrounded by many layers of granulosa cells. The secondary follicle becomes larger eventually turning into a mature Graffian follicle. It is within the graffian follicle that the diploid primary oocyte completes meiosis I, producing 2 haploid cells of unequal size, each with 32 chromosomes. This occurs just prior to ovulation. The smaller cell produced by meiosis I is called the 1 st polar body. It is essentially a pack of discarded nuclear material. The larger cell is known as the secondary oocyte. Once it is formed
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