د. مها السماك
Lec.2Morphological changes
The primordial germ cells ( male and female germ cells ) appear in the wall at the yolk sac of human embryo at the third week , from there they migrate to the developing gonad , reaching them during the fifth week.
Oogenesis : in female once the germ cells invade the gonads , they differentiate into oogonia which undergo a number of mitotic division , some of them differentiate into primary oocytes that will enter the prophase at the first meiotic division , each primordial oocyte is surrounded by a layer of flat epithelial cells forming the primordial follicle.
At the time of birth the primary oocytes finished the prophase and enter a resting stage till reaching puberty (i.e. they will not finish their first meiotic division).
Note: the germ cells number increase in the first five months in embryo reaching seven million , then decrease in the next months and after birth , reaching 40000 at puberty .
At puberty : with each ovarian cycle a number of follicles begins to mature , but usually only one reaches full maturity , the others degenerate.
Stages of maturation of the follicles :
Primordial follicle.Primary follicle : The primordial follicle converted to primary follicle in which the primary oocyte increase in size , the surrounding epithelia cells become cuboidal and known as follicular cells , and a layer of mucopoly saccharides will deposit on the oocytes surface called zona pellucida.
secondary or growing follicle :primary follicle grow more and the follicular cells increase in number forming many layer around oocyte , then small fluid filled spaces appear between Them , then these spaces united giving rise to a single space called follicular antrum.
mature or graafian Tetiary follicle : the growing follicle show increase the size of the follicular antrum,
and the follicle cells surrounded the oocyte remain intact forming cumulus oophorus , the mature follicle is surrounded by two layers of connective tissue: theca interna and theca externa and having a cilia of 12-18 mm . once the follicle become mature and shortly before ovulation , the primary oocyte finish the first meiotic division forming secondary oocyte and first polar body , the secondary oocyte enter the second meiotic division , the moment that it shows the spindle formation , ovulation occurs and the oocyte is shed from the ovary , the second meiotic division is completed if oocyte fertilized , if not it will degenerated after 24 hours.
Spermatogenesis in male : the differentiation Of the primordial Germ cells after their arrival to gonads begins at puberty . from the fourth month to puberty the testis is composed of sex cords rather than tubules contain the primordial Germ cells and sustentacular sertoli cells .
At puberty the cords acquire a lumen and become the seminiferous tubules , the primordial Germ cells differentiation To spermatogonia that differentiate to primary spermatocytes which will enter the first meiotic division giving rise to two secondary spermatocytes , that will complete the second meiotic division forming four spermatids . the spermatids by aprocerr called spermiogenesis undergo a series of changes resulting in the production of the spermatozoon . and these changes are :
Formation of the acrosome ( which will cover half of the nuclear surface ).
Condensation of the nucleus.
Formation of neck middle piece , tail.
Shedding of most of the cytoplasm.
The time required for a spermatogonium to form a mature spermatozoon is 61-64 days .
Abnormal gametes
abnormal meiotic division : some time separation does not occur (nondisjunction ) , As a result one cell receive 24 chromosome and the other 22 chromosome Instead of the normal 23 chromosome , so when fertilization occur with 23 chromosome , in the first case we will have 47 chromosome (trisomy) and in the second case 45 chromosome (monosomy).
abnormal shapes :
one ovarian follicle contains two or three primary oocytes → usually degenerated.
One primary oocyte contains two or three nuclei → usually degenerated .
dwarf , giant , joined sperms.
If 10% or less of sperms are abnormal , fertility will not be affected.