Spermatogenesis, Oogenesis, Menstrual Cycle

 SPERMATOGENESIS 

• Spermatogenesis is a process of formation of sperms from spermatogonia.
• The process begins after puberty in seminiferous tubules of testis.
• It includes – Spermatocytosis & Spermiogenesis.

Spermatocytosis :-

• It is the process of formation of spermatocytes.

Testosterone stimulate primordial germ cells to differentiate into dark type A spermatogonia (46,XY)

Dark type A spermatogonia undergo mitosis to form pale type A spermatogonia (46,XY)

Pale type A spermatogonia undergo mitosis to form type B spermatogonia (46,XY)

Type B spermatogonia undergo mitosis to form primary spermatocytes (46,XY)

Primary spermatocyte undergoes meiosis I to form secondary spermatocytes (23,X & 23,Y)

Secondary spermatocytes undergo meiosis II to form spermatids (23,X; 23,X & 23,Y; 23,Y)

Spermiogenesis :-

It is the process of metamorphosis of spermatids into spermatozoa.

Nucleus of the spermatid condenses and moves towards one pole 

Golgi apparatus forms an acrosomal cap and covers 2/3^rd of the nucleus

Centromere divides into proximal & distal centrioles which give rise to axial filament

Mitochondria develop spiral sheath around the axial filament

Most of the cytoplasm of spermatid is shed off as residual body

Finally, spermatozoa are formed and stored in the lumen of seminiferous tubules (spermiation) 

• Spermatogenesis takes 64 days to form a sperm.
• Sperms acquire full motility with the prostatic secretions in the urethra.
• Spermatogenesis requires 2⁰C less temperature than the body temperature.
• In a single ejaculation 200-300 million sperms are emitted through semen.
• Sperms survive 48 hours after ejaculation.

Aspermia- complete lack of semen.

Azoospermia- absence of sperms in semen.

Oligozoospermia- sperm count less than 15 million/ml of semen.

Structure of Spermatozoon :-

• Matured spermatozoon has 4 parts –
• Head, Neck, Middle piece/Body, Tail.

Head –

• It is pyriform in shape with less cytoplasm.
• It contains oval nucleus having haploid number of chromosomes.
• 2/3^rd of the nucleus is covered by acrosomal cap.
• Acrosomal cap contains digestive enzymes like hyaluronidase & acrosine.

Neck –

• It contains proximal centriole at the junction of head and neck.
• Proximal centriole forms transverse and longitudinal cylinders of axial filament.
• Longitudinal cylinder has nine thick filaments which continue into the body and tail.

Middle piece / Body –

• It contains distal centriole at the junction of middle piece and tail in the form of annulus.
• Axial filament continues into the body and tail from the neck.
• The axial filament is surrounded spirally by mitochondria.

Tail –

• It shows 2 parts- proximal principal piece & distal end piece.
• Principal piece contains axial filament surrounded by plasma membrane and fibrous sheath.
• End piece does not contain fibrous sheath.

 

OOGENESIS 

• Oogenesis is a process of formation of mature ovum from primordial germ cells.
• It occurs in ovarian cortex in 3 developmental phases- before birth, after puberty, after fertilization.

Before Birth :-

By 3^rd month of intra-uterine life, primordial germ cells undergo mitosis to form oogonia (46,XX)

By 7^th month of intra-uterine life, oogonia undergo mitotic divisions to form primary oocytes (46,XX)

At birth, primary oocytes enter into meiosis I and get arrested at dictyotene stage till puberty.

• Primary oocytes are surrounded by flattened epithelial cells forming primordial follicles.
• 2 million primary oocytes are present at birth, out of which 4 lakhs persist up to the puberty but only 500 ovulate.

After Puberty :-

Primary oocyte undergoes maturation and increase in size.

Surrounding layer of flattened follicular cells become cuboidal and form primary follicle.

Single layered follicular cells become multilayered granulosa cells and form secondary follicle.

A glycoprotein layer, zona pellucida, is formed between the primary oocyte and granulosa cells.

Secondary follicle acquires follicular antrum filled with liquor folliculi and form tertiary follicle.

Antrum separates the granulosa cells into outer stratum granulosum and inner cumulus oophorous layers

Stromal cells surrounding the follicle condense to form inner vascular theca interna & outer fibrous theca externa layers

Now the follicle is called mature Graafian follicle.

Primary oocyte completes meiosis I and converted into secondary oocyte (23,X) by releasing first polar body into perivitelline space.

Secondary oocyte enters into meiosis II and gets arrested at metaphase.

• Ovulation occurs by the rupture of Graafian follicle and release of secondary oocyte (ovum) into the abdominal cavity.
• Released secondary oocyte is surrounded by few cells of cumulus oophorous known as corona radiata.
• After ovulation graafian follicle collapse and forms corpus luteum.

After Fertilization :-

• If ovum gets fertilized, secondary oocyte completes meiosis II and releases second polar body into perivitelline space.
• If fertilization does not occur, secondary oocyte undergoes degeneration within 24 hours after ovulation.

Structure of Ovum :-

• Matured ovum consists of – Nucleus, Ooplasm, Vitelline membrane, Zona pellucida, Corona radiata.

Nucleus –

• It contains 23,X chromosomes.

Ooplasm / Cytoplasm –

• It contains lecithin-like substances that form deutoplasm.
• 2 centrioles are present near the nucleus but they disappear during fertilization.

Vitelline membrane –

• It is a cell membrane surrounded by perivitelline space.
• First polar body lies within the perivitelline space.

Zona pellucida –

• It is a glycoprotein coat that surrounds vitelline membrane.
• It is synthesized by the oocyte.
• ZP3 receptors of zona pellucida facilitate binding of sperms and induce an acrosomal reaction.
• Zona pellucida disappears on 5^th – 6^th day after fertilization to permit implantation.

Corona radiata –

• Few cells of cumulus oophorous remain attached to outer surface of ovum which form corona radiata.
• Hyaluronidase from acrosomal cap disintegrates corona radiata cells during fertilization.

 

MENSTRUAL  CYCLE

• The periodic structural change in the endometrium of uterus during the reproductive life is known as uterine / menstrual cycle.
• It is associated with uterine haemorrhage known as menstruation which persists for 3 – 4 days.
• The cycle is counted from the first day of one menstrual bleeding to the first day of next menstrual bleeding.
• The cycle is repeated at an interval of 28+5 days.
• Based on endometrial changes each menstrual cycle is described in 3 phases.
• Proliferative / Follicular phase
• Secretory / Progestational phase
• Menstrual phase

Proliferative / Follicular Phase :-

• It is characterized by repair and subsequent growth of endometrium following the previous menstruation.
• Repair work requires about 4 days.
• Rest of the phase shows endometrial growth about 1 – 3mm thickness.
• The endometrial & glandular epithelium shows active proliferation and becomes columnar.
• Uterine glands increase in length and remain straight.
• The cells of endometrial stroma are arranged in 3 layers-
• Superficial Stratum compactum
• Intermediate Stratum spongiosum
• Deep Stratum basale
• Ovulation takes place at the end of follicular phase.

Secretory / Progestational Phase :-

• The changes in this phase are due to the actions of progesterone and oestrogen of corpus luteum.
• It is characterized by further growth of endometrium about 5 – 7mm thickness.
• Uterine glands increase in length and become tortuous.
• The stratum basale takes no part in cyclical changes.
• At the end of this phase sudden regression of endometrium takes place due to diminished progesterone due to degeneration of corpus luteum.
• As the endometrial regression precedes menstrual bleeding, end of this phase is known as pre-menstrual phase.

Menstrual Phase :-

• It is characterized by shedding of stratum compactum and stratum spongiosum with some amount of blood due to sudden withdrawal of progesterone.
• In this phase endometrium further regresses and the spiral arteries become more coiled.
• Further vasoconstriction of spiral arteries leads to diminished circulation through endometrium.
• This is followed by dilatation of arteries and blood escapes through the damaged capillaries into intercellular spaces.
• Eventually small pieces of endometrium get detached.
• This phase lasts for 3 – 5 days.
• Average blood loss is 50 – 60 ml.
• Blood does not clot because of proteolytic enzymes.
• When the ovum is fertilized, the secretory phase of endometrium is prolonged by the action of progesterone from corpus luteum of pregnancy.
• Menstruation does not take place as long as period of gestation continues.