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SPERMATOGENESIS. Definition : It is the different steps by which spermatogonia are transformed into spermatozoa in the testis. It begins at puberty (13-16 years) and continues into old age. The whole process takes about 64 days. Spermatogenesis.
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SPERMATOGENESIS Definition: It is the different steps by which spermatogonia are transformed into spermatozoa in the testis. It begins at puberty (13-16 years) and continues into old age. The whole process takes about 64 days.
Spermatogenesis • It is a process by which spermatogonia are transformed into spermatozoa. It begins at puberty. • At birth: germ cells in the male infant present in the sex cords of the testis are surrounded by supporting cells (Sertoli cells). • At this time the primordial germ cell divides BY MITOSIS to give spermatogonia. • Spermatogonia gives rise to 1ry spermatocyte. • 1ry spermatocyteby 1st meiotic division gives 2nd spermatocyte. • 2nd spermatocyte by 2nd meiotic division gives haploid spermatids (4 in no.).
- In spermiogenesis the spermatid contains two centrioles from which a flagellum develops (it will form the future axial filament). The anterior centriole persists while the posterior one becomes ring shape and moves along the flagellum. - The nucleus of the spermatid forms the future head of the sperm. - The Golgi apparatus forms the anterior cap of the head and the acrosome that is contained in the cap. - The mitochondria form the mitochondrial spiral sheath.
Spermiogenesis- It is the morphological changes through which the spermatid is transformed into a sperm.
The sperm Fig. 8A: Drawing showing the main parts of the human sperm (X1250). The head, composed mostly of the nucleus, is partly covered by the cap like acrosome (an organelle containing enzymes). The tail of the sperm consists of three regions: The middle piece, principal piece, and an end piece. Scanning electron micrograph of human sperms
OOGENESIS • Oogenesis means differentiation of the primordial germ cells (oogonia) that are present in the cortex of the ovary into mature ova. Oogenesis passes into two stages of maturation: prenatal and postnatal maturation:
1. Prenatatal maturation - The primordial germ cells(oogonia) undergo mitotic divisions then they are arranged in clusters surrounded by a layer of flat epithelial cells.Theflat cells are called follicular cells and are derived from the epithelium covering the ovary.
- Some oogonia differentiate into large primary oocytes where DNA replication occurs and they enter the prophase of the first meiotic division. - The surviving primary oocytes become surrounded by flat epithelial cells. Together they are called primordial follicles.
2. Postnatal maturation - Near birth, all the primary oocytes have started prophase of the first meiotic division then they are arrested till puberty. - The number of primary oocytes at birth ranges from 700,000 to 2 million. At puberty only 400,000 primary oocytes remain and about 400 only will be ovulated during the female life time.
- With the onset of puberty, 5-15 primordial follicles begin to maturate with each ovarian cycle. - The primary oocyte begins to increase in size and the surrounding flat cells change from flat to cuboid and proliferate to produce a stratified epithelium of granulosa cells.
- The follicle is now called the primary follicle. The granulosa cells rest on a basement membrane separating them from the stromal cells of the ovary that form the theca folliculi. - The granulosa cells and the oocyte secrete a layer of glycoprotein on the surface of the oocyte called the zonapellucida.
- Fluid-filled spaces appear between the granulosa cells then they coalesce together forming the follicular antrum and the follicle is now called the secondary follicle. . - The follicle enlarges and is called the Graafian follicle
- The primary oocyte continues the first meiotic division leading to two unequal daughter cells with 23 chromosomes each (22 autosomes + X). The large cell is called the secondary oocyte and the small one (having little cytoplasm) is the first polar body. The secondary oocyte and the first polar body enter the second meiotic division without DNA replication. - The second meiotic division is completed only if fertilization occurs to give fertilized oocyte (mature ovum) and a second polar body that soon degenerates.
- The primary oocyte continues the first meiotic division leading to two unequal daughter cells with 23 chromosomes each (22 autosomes + X). The large cell is called the secondary oocyte and the small one (having little cytoplasm) is the first polar body. The secondary oocyte and the first polar body enter the second meiotic division without DNA replication. - The second meiotic division is completed only if fertilization occurs to give fertilized oocyte (mature ovum) and a second polar body that soon degenerates.
The mature ovum - It is a large oval cell that varies from 117 – 142 µ in diameter. It has two membranes; the inner thin one is the vitelline membrane and the outer one is the zona pellucida. - The corona radiata is two or three layers of cells surrounding the zona pellucida externally when the ovum is shed from the follicle. .
Fig. 14: Drawings (A-C) illustrating the movement of the uterine tube that occurs during ovulation. Note that the fimbriated infundibulim of the tube becomes closely applied to the ovary. Its finger like fimbriae move back and forth over the ovary and sweep the secondary oocyte into the infundibulum as soon as it is expelled from the ovarian follicle and ovary during ovulation.
