Morphological Changes During Maturation
Primordial Germ Cells
- Appear in the wall of the yolk sac at the end of the third week of development
- Migrate by amoeboid movement towards the developing gonads ( primitive sex glands)
- Arrive at the end of 4th or the beginning of 5th week
- Prenatal maturation
- Oogonia – derived from primordial germ cell
- Undergo mitotic divisions
- Arranged in clusters by the end of 3rd month
- Surrounded by a layer of flat epithelial cells
- Some differentiate into larger primary oocytes that will enter to the prophase of first meiotic division
- Fifth month- maximum number of germ cells (7,000,000)
- Seventh month- majority of the oogonia have degenerated
- Primordial follicle: surviving primary oocyte together with its surrounding flat epithelial cells
2. Postnatal Maturation
- Dictyotene stage- a resting stage during prophase characterized by a lacy network of chromatin.
- Primary oocytes remain in prophase and do not finish their first meiotic division before puberty is reached due to OOCYTE MATURATION INHIBITION (OMI)
- At puberty, a number of primordial follicles begins to mature with each ovarian cycle.
- PRIMARY FOLLICLE –Primary oocyte ( still in dictyotene stage) begins to increase in size, and flat epithelial cells change to cuboidal.
- ZONA PELLUCIDA– formed by thickened acellular material consisting mucopolysaccharides deposited on the surface of the oocyte
- FOLLICULAR ANTRUM– formed by coalition of fluid-filled spaces appear between the follicular cells
- CUMULUS OOPHORUS– follicular cells surrounding the oocyte remain intact.
- At maturity, the follicle is known as the tertiary or vesicular follicle, that is surrounded by:
- Thecainterna thecal gland – cellular, rich in blood vessels, the main source of estrogen
- Thecaexterna–merges with the ovarian stroma, fibrous
- Differentiation of germ cells in male begin at puberty
- Sustentacular or Sertoli cells
- Before puberty, the sex cords of the testis acquire a lumen and become seminiferous tubules
Primordial Germ Cell
- Primary spermatocytes (cells start with prophase of 1st meiotic division- lasted for 16 days)
- Secondary spermatocytes(second maturation or meiotic division result to production of 2 spermatids, each containing 23 chromosomes and n amount of DNA)
Spermatids undergo a series of changes resulting in the production of spermatozoa Changes are:
- Formation of the acrosome extends half the nuclear surface
- Condensation of the nucleus
- Formation of neck, middle piece, and tail
- Shedding of most of the cytoplasm
- 61 days- time required for a spermatogonium to develop into a mature spermatozoon
- Spermatozoa when fully formed, enter the lumen of seminiferous tubules
- Pushed toward the epididymis by the contractile elements in the wall of seminiferous tubules
Obtain Full Motility in the Epididymis
- Pharyngeal arches are rod-like thickenings of mesoderm present in the wall of the foregut.
- In the interval between any two arches, the endodermis pushed outwards to form endodermal or pharyngeal pouches.
- Opposite each pouch, the surface ectoderm dips inwards an ectodermal cleft. So,
★ Arch → Mesoderm;
★ Pouch → Endoderm;
★ Cleft → Ectoderm