Gemetogenesis : easy and brief handout

Gemetogenesis: easy and brief description 

Definition: it is the process of formation and development of male and female gametes

·         Gametes are derived from primordial germ cells.

·          It is formed in the epiblast during the 2^nd  week

·         Then  move to the wall of the yolk sac  during the 4^th week ,

·         these cells begin to migrate from the yolk sac to the developing gonad, where they arrive by the end of the 5^th week

Meiotic cell division :

Importance of meiotic cell division:

1. Restores the normal dioploid number of chromosomes from generation to   generation
2. crossovers- interchange of chromatid segments between paired homologous chromosomes

      3. Allows random assortment of maternal & paternal chromosomes between gametes

Oogenesis :

 Definition: it is the sequence of events by which oogonia are transformed into mature oocytes. 

This maturation begins before birth and completed after puberty and continues to the menopause (the permanent cessation of the menses)

Maturation of oocytes:

Before birth:

Primordial germ cells (arrived in the gonad by the end of the 5^th week)

Differentiate into oogonia ( at 5^th month its number reaches maximum about 7 million)

Primary oocytes : oogonia enlarge to form primary oocytes.

Primordial follicle – a primary oocyte with its surrounding flattened follicular epithelium.

Near the birth, the primary oocyte remain arrested in prophase of meiosis I(diplotene stage) and do not finished meiosis I before puberty

At puberty:

Primary follicle - follicular cell became cuboidal and zona pellucida begin to form

Growing follicle: follicular cells forming the stratified layer of granulose cells and well-defined zona pellucida

Secondary follicle: fluid filled spaces developed within the granulosa cells .  

Grafiaan follicle : single cavity formed within granulosa cell

Shortly before ovulation the primary oocyte completes the first meiotic division and became secondary oocyte .

The division is unequal, the secondary oocyte receives almost all the cytoplasm but the 1^st polar body receives very little.

The secondary oocyte begins 2^nd meiotic division but arrest at metaphase.

Meiosis II is completed only if the oocyte is fertilized.

Spermatogenesis

Definition: It is the sequence of events by which spermatogonia are transformed into mature sperms or spermatozoa.

This maturation process begins at puberty and continues into old age

In male the primordial germ cells remain dormant until puberty.

At puberty, after several mitotic divisions of the spermatogonia, primary spermatocytes (diploid)  are formed. They are the largest cells of seminiferous tubules and enters a prolonged prophase (22 days)

Each primary spermatocyte is completed meiosis I to form two haploid secondary spermatocytes

Subsequently, two secondary spermatocytes complete the meiosis II to form four spermatids   

Spermiogenesis :

Definition: The spermatids are gradually transforms into mature sperm or spermatozoa by a process known as spermiogenesis

These changes include:

1. loss of cytoplasm
2. formation of acrosomes which covers half the nuclear surface, derived from the golgi region of spermatid contain enzymes that assist penetration of corona radiata and zona pellucida during fertilization
3. condensation of nucleus
4. formation of neck, middle piece, and tail
5. the mitochondria arranged in the middle piece as mitochordial sheath

The time required for a spermatogonium to become a mature spermatozoon is approximately 74 days