Gametogenesis
Definition
Gametogenesis : Conversion of Germ Cells Into Male and Female Gametes i.e. sperm and ovum
Gametes
o Development begins with fertilization
o The male gamete, the sperm + female gamete the oocyte = Zygote.
o Gametes derived from germ cells in the wall of the yolk sac - fourth week of development.
o They migrate to the developing gonads - end of the fifth week.
o Mitotic divisions increase their number during their migration and also when they arrive in the gonad.
The process of gametogenesis
o In preparation for fertilization, germ cells undergo gametogenesis
o Gametogenesis includes meiosis to reduce the number of chrosomes and cytodifferentiation to complete their maturation
Oogenesis
o Begins before birth
o Primordial germ cells arrive in the gonad
o Differentiate into oogonia
o Meiosis takes place in the germ cells
o Meiosis requires two cell divisions : Meiosis I and meiosis II,
o Female gamete - with haploid number of chromosomes : 23 singles
o Some of the oogonia differentiate into primary oocytes
A primary oocyte together with its surrounding flat epithelial cells, is known as a primordial follicle.
Mitotic divisions 'clusters surrounded by a layer of flat epithelial cells called follicular cells
o Maturation continues at puberty.
o At puberty 5 to 15 primordial follicles begin to mature with each ovarian cycle
o Only one reaches full maturity
o Others degenerate and become atretic
o On maturing, the primary oocyte resumes the temporarily stopped meiosis i.e. the first meiotic division ' secondary oocyte and first polar body. The polar body lies between the zona pellucida and the cell membrane of the secondary oocyte
o The secondary oocyte completes the second meiotic division only if fertilized.
o If not fertilized the cell degenerates approximately 24 hours after ovulation.
o Three polar bodies are formed
Certain important considerations
o During meiosis one primary oocyte gives rise to four daughter cells
o Each with 22 + 1 (x) chromosomes.
o Only one of these develops into a mature gamete, the oocyte. The other three, the polar bodies , receive little cytoplasm and degenerate during subsequent development.
o Similarly one primary spermatocyte gives rise to four daughter(? Son) cells
o Two with 22 + 1 (x) chromosomes and two with 22 + 1 (y) chromosomes
o In contrast to oocyte formation, all four develop into mature gametes.
o Maturation of the gametes begins before birth.
Spermatogenesis
o Begins at puberty
o Primordial germ cells in the seminiferous tubules give rise to spermatogonia
o Spermatogonia ® mitosis ® primay spermatocytes ® meiosis I ® secondary spermatocytes ® meioss II ® spermatids ®spermiogenesis ® spermatozoa
o Spermatids are haploid because of meiotic division - i.e. reduction division.
Applied embryology
gene mutations account for about 8% of abortions.
Most common chromosome abnormalities are
o Turner Syndrome (One of the two X chromosomes normally found in females is missing or incomplete)
o Down Syndrome (Trisomy 21)
o Klinefelter's syndrome (trisomy - xxy - - xxxy may also occur)
In trisomy an extra chrosome is present ' total 47
