Morphological Changes During Maturation

Primordial Germ Cells

1. Appear in the wall of the yolk sac at the end of the third week of development
2. Migrate by amoeboid movement towards the developing gonads ( primitive sex glands)
3. Arrive at the end of 4^th or the beginning of 5^th week

Oogenesis

1. Prenatal maturation

• Oogonia – derived from primordial germ cell
• Undergo mitotic divisions
• Arranged in clusters by the end of 3^rd 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

Spermatogenesis

• 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

• Spermatogonia
• Primary spermatocytes (cells start with prophase of 1^st 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)

Spermiogenesis

Spermatids undergo a series of changes  resulting in the production of spermatozoa Changes are:

1. Formation of the acrosome extends half the nuclear surface
2. Condensation of the nucleus
3. Formation of neck, middle piece, and tail
4. 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