Biology Short Notes: Human Reproduction

It consists of :

• The primary sex organs i.e. a pair of testes
• The secondary sex organs i.e. the duct system and the associated glands
• External genitalia

Testes

• Scrotum – a pouch in which testes are situated
• Testicular lobules
• Seminiferous tubules – contains Spermatogonia and Sertoli cells- Leydig cells

Accessory Ducts

• Rete testis
• Vasa efferentia
• Epididymis
• Vas deferens
• Urethra

Accessory Glands

• A pair of seminal vesicles
• Prostate gland
• Bulbourethral gland

Secretions of these glands constitute the seminal plasma rich in fructose , calcium and certain enzymes .


Secretions of bulbourethral glands also help in lubrication of penis.

External Genitalia

• The penis is the external genitalia in human males .
• It is made up of special erectile tissue that helps in erection of the penis to facilitate insemination.

The enlarged tip of the penis is called glans penis covered by foreskin.

It consists of :

• The primary sex organ that is a pair of ovaries
• Secondary sex organs- the duct system consisting of a pair of fallopian tube , a uterus , cervix and vagina
• External genitalia
• Mammary glands

Ovaries

• Produce female gametes called ova
• Located in abdominal cavity
• Each ovary is almond shaped body
• Coved by a thin epithelium , enclosing the ovarian stroma
• Stroma is divided into 2 regions :

1.       Peripheral cortex


2.       Inner medulla

Fallopian Tube

• Part closer to ovary-funnel shaped infundibulum
• Infundibulum possesses finger like projections-fimbriae
• Wider part of oviduct –ampulla
• Last part of oviduct-isthmus

Uterus

• Covered by three layered wall:

1. Perimetrium – outer most layer
2. Myometrium- middle layer
3. Endometrium-inner most layer

External Genitalia

• Mons pubis – cushion of fatty tissues covered by skin and pubic hair
• Labia majora – fleshy folds of tissue extending down from mons pubis ,  surrounding the vaginal opening
• Labia minora – paired folds of tissue under labia majora
• Clitoris – tiny finger-like structure which lies at the upper junction of the two labia minora , above the urethral opening
• Hymen – a membrane covering the opening of vagina partially

Mammary Glands

• Consists of glandular tissue and variable amount of fat
• Glandular tissue divided into 15-20 mammary lobes
• Each lobe contains clusters of cells called alveoli opening into mammary tubules
• Mammary tubules join to form mammary duct
• Mammary ducts join to form mammary ampulla connected to lactiferous duct
• Milk secreted by cells of alveoli, stored in the lumen of alveoli

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 embeded 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 fertilisation 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 gets surrounded by more layers of granulosa cells-secondary follicles
• Secondary follicle transforms into a tertiary follicle characterised 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 under four phases

• Menstrual phase

1. It lasts for 3-5 days
2. It results due to breakdown of endometrial lining of uterus and its blood vessels

• Follicular phase

1. Endometrium is regenerated by proliferation of its cells
2. These changes are due to increased levels of FSH , LH, Estrogen
3. FSH controls follicular phase, stimulates growth of follicles , secretion of Estrogen
4. FSH and LH reach their peak level in the middle of the cycle

• Ovulatory phase

1. Peak level of LH induces rupture of mature graafian follicle and release of ovum-ovulation

• Luteal phase

1. Ruptured follicle transforms into corpus luteum
2. It secretes large quantities of progesterones

• 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 fertilisation.

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

Events during fertilisation :

• 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 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 , progesterones (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 – foetus develops limbs and digits
• 12 weeks (1st trimester) – major organ systems are formed
• 5th month – 1st movements of foetus and appearance of hair on head
• 24 weeks (2nd trimester) – body covered with fine hair , eye lids separate , eye lashes formed

Parturition and Lactation

• Gestation period – 9 months
• Parturition – the process of delivery of the foetus (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, causes stronger uterine contractions, which in turn stimulates further secretion of oxytocin.

Lactation

• The mammary glands undergo differentiation during pregnancy and starts 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 for new-borns.

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