Gametogenesis and fertilization
In this article we shall look at both spermatogenesis and oogenesis. Sperm are continually produced as males need to be ready to utilise the small window of fertility of the female. The blood-testis barrier is formed by Sertoli cells and is important in preventing hormones and constituents of the systemic circulation from affecting the developing sperm, and also in preventing the immune system of the male from recognising the sperm as foreign — as the sperm are genetically different from the male and will express different surface antigens.
Sertoli cells also have a role in supporting the developing spermatozoa.
This replenishment of spermatogonia means that males are fertile throughout their adult life. Primary spermatocytes then undergo meiosis.
Each of these populations of spermatogenic cells will be at different stages of spermatogenesis. Oogenesis differs from spermatogenesis in that it begins in the foetus prior to birth. Once puberty begins, a number of primary oocytes begin to mature each month, although only one of these reaches full maturation to become an oocyte. The primary oocyte grows dramatically whilst still being arrested in meiosis I.
The follicular cells grow and proliferate to form a stratified cuboidal epithelium.
In each monthly cycle one of these secondary follicles becomes dominant and develops further under the influence of FSH, LH and oestrogen. The LH surge induces this stage and meiosis I is now complete.
Two haploid cells are formed within the follicle, but they are of unequal size. Both daughter cells then undergo meiosis II, the first polar body will replicated to give two polar bodies but the secondary oocyte arrests in metaphase of meiosis II, 3 hours prior to ovulation.
The LH surge increases collagenase activity so that the follicular wall is weakened, this combined with muscular contractions of the ovarian wall result in the ovum being released from the ovary and being taken up into the fallopian tube via the fimbriae finger-like projections of the fallopian tube.
The secondary oocyte will only complete meiosis II on fertilisation, giving off a third polar body once meiosis II is completed and a fertilised egg.
If fertilisation never occurs, the oocyte degenerates 24 hours after ovulation, remaining arrested in meiosis II. If the egg is fertilised however, the peristaltic movements of the fallopian tube move the egg to the uterus where it can implant into the posterior uterine wall.
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Oogenesis Oogenesis differs from spermatogenesis in that it begins in the foetus prior to birth. The primary oocytes undergo 3 stages: Pre-antral Antral Preovulatory Pre-antral Stage The primary oocyte grows dramatically whilst still being arrested in meiosis I.
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Fertilisation The secondary oocyte will only complete meiosis II on fertilisation, giving off a third polar body once meiosis II is completed and a fertilised egg. Modifications: vectorization CorelDraw.
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Spermatogenesis - Reproductive system physiology - NCLEX-RN - Khan Academy
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