Human Reproduction-3 (GAMETOGENESIS)

The process of formation of gametes is called gametogenesis
It is of two types:
1. Spermatogenesis in males
2. Oogenesis in females

1.  Spermatogenesis
The process of formation of sperms in males is called spermatogenesis

• The spermatogonia present on the inner wall of seminiferous tubules multiply by mitotic division and form primary spermatocytes.
• Primary spermatocytes undergo meiosis.
• Primary spermatocytes complete 1st meiotic division forms 2 equal haploid secondary spermatocytes
• Secondary spermatocytes undergo 2nd meiotic division to form 4 haploid spermatids
• Spermatids transform into spermatozoa (sperms) – spermiogenesis
• Sperm heads get embedded in Sertoli cells, finally released from seminiferous tubules -spermiation

Hormonal control of spermatogenesis

• Spermatogenesis initiated due to increase in secretion of gonadotropin-releasing hormone by hypothalamus
• Increase in GnRH act on anterior pituitary and stimulate secretion of two gonadotropins, LH and FSH
• LH acts on Leydig cells and stimulates them to secrete androgens.
• FSH acts on Sertoli cells stimulates secretion of some factors which help in spermiogenesis

Structure of Sperm

• Composed of head, neck, middle piece and a tail.
• Plasma membrane envelopes the whole body of sperm
• Sperm head contains an elongated haploid nucleus, the anterior portion of which is covered a cap-like structure, acrosome.
• Acrosome- filled with enzymes that help fertilization of ovum
• Middle piece possesses numerous mitochondria-energy for movement of tail
• Sperms released from the seminiferous tubules are transported by the accessory ducts.
• Secretions of epididymis, vas deferens, seminal vesicle, prostate – essential for maturation and motility of sperms
• Semen – seminal plasma along with sperms
• Functions of male accessory glands controlled by testicular hormones (androgens)

2.  Oogenesis

The process of formation of a mature female gamete is called oogenesis

• Oogonia start division, enter into prophase I of meiosis – primary oocytes
• Each primary oocyte gets surrounded by a layer of granulosa cells-primary follicle
• Primary follicles get surrounded by more layers of granulosa cells-secondary follicles
• Secondary follicle transforms into a tertiary follicle characterized by a fluid-filled cavity called antrum
• The primary oocyte within the tertiary follicle grows in size, completes 1st meiotic division
• It results in unequal division, formation of large haploid secondary oocyte and tiny 1st polar body
• Tertiary follicle changes into Graafian follicle
• Secondary oocyte forms a new membrane-zona pellucida
• Ovulation – Graafian follicle ruptures to release the secondary oocyte (ovum) from the ovary

Menstrual cycle
The cyclic changes that occur in the reproductive organs of primate females is called menstrual cycle
The events in a menstrual cycle can be studied in four phases

Menstrual phase

• It lasts for 3-5 days
• It results due to breakdown of endometrial lining of uterus and its blood vessels
• Follicular phase
• Endometrium is regenerated by proliferation of its cells
• These changes are due to increased levels of FSH, LH, Estrogen
• FSH controls follicular phase, stimulates growth of follicles, secretion of Estrogen
• FSH and LH reach their peak level in the middle of the cycle
• Ovulatory phase
• Peak level of LH induces rupture of mature Graafian follicle and release of ovum-ovulation
• Luteal phase
• Ruptured follicle transforms into corpus luteum
• It secretes large quantities of progesterone
• In absence of fertilisation, corpus luteum degenerates. This causes disintegration of endometrium
• Menstrual cycle cease at the age of 45 – 50  menopause.

Fertilisation and Implantation

The process of fusion of a sperm with an ovum is called fertilization.

Fertilisation can only occur if the ovum and sperms are transported simultaneously to the ampullary-isthmic junction.

Events during fertilization :

• Sperm comes in contact with zona pellucida
• Induces changes in membrane that block entry of additional sperms
• Secretions of acrosome help sperm enter the cytoplasm of ovum
• Meiotic division of secondary oocyte
• Haploid ovum and 2nd polar body formed
• Formation of zygote

Implantation

• zygote moves towards the uterus
• Blastomeres (2,4,8,16 daughter cells)
• morula (8-16 blastomeres)
• blastocyst
• Blastomeres in blastocyst arrange into an outer layer trophoblast and an inner group of cells attached to trophoblast
• Blastocyst gets embedded in endometrium of uterus (implantation)
• Uterine cells rapidly divide and cover the blastocyst

Pregnancy and embryonic development

• Inner layer grows out as finger-like projections called villi into the uterine stroma
• Chorionic villi and uterine tissue get interdigitated to form placenta
• Placenta secretes hormones like hCG, hPL, estrogens, progesterone (to maintain pregnancy)
• Inner cell mass differentiates into an outer layer called ectoderm and an inner layer called endoderm
• Mesoderm appears between ectoderm and endoderm
• Stem cells (undifferentiated embryonic cells)

Features of embryonic development

• The human pregnancy lasts for 9 months
• 1st month – embryo’s heart is formed
• 2nd month – fetus develops limbs and digits
• 12 weeks (1st trimester) – major organ systems are formed
• 5th month – 1st movements of fetus and appearance of hair on a head
• 24 weeks (2nd trimester) – body covered with fine hair, eyelids separate, eyelashes formed

Parturition and Lactation

• Gestation period – 9 months
• Parturition – the process of delivery of the fetus (childbirth)
• Signals for parturition originate from the fully developed fetus and placenta inducing mild uterine contractions called Foetal ejection reflex
• It triggers the release of oxytocin from maternal pituitary

Oxytocin acts on uterine muscle cause stronger uterine contractions, which in turn stimulates further secretion of oxytocin.

Lactation

• The mammary glands undergo differentiation during pregnancy and start producing milk towards the end of pregnancy by the process called lactation.
• The milk produced during the initial few days of lactation – colostrum
• It contains several antibodies essential to develop resistance to new-borns.