NEET Biology Human Reproduction Notes

This NEET Biology notes page on Human Reproduction covers asexual and sexual reproduction, the male and female reproductive systems with their glands, spermatogenesis and oogenesis, the menstrual cycle with the role of pituitary and ovarian hormones, fertilization, cleavage, blastula and implantation, early and late embryonic development, parturition, and mammary glands and lactation, along with the chapter's check point questions and answers.

HUMAN REPRODUCTION

Important points to pay special attention to while going through the chapter

Terms and definitions: 1. Cryptorchidism; 2. Follicular atresia; 3. Spermiogenesis; 4. Menopause; 5. Hysterectomy; 6. Parthenogenesis; 7. Colostrum; 8. Vasectomy; 9. Capacitation; 10. After birth.

Differences: 1. Epiboly and emboly; 2. Holoblastic and meroblastic cleavage; 3. Regulative eggs and mosaic eggs; 4. Ovoviviparous and viviparous animals; 5. Preformation theory and epigenesis theory; 6. Arrhenotoky and thelytoky; 7. Embryo and foetus; 8. Insemination and implantation.

Structures: 1. Inguinal canal; 2. Uterus masculina; 3. Corpus luteum; 4. Cortical granules; 5. Zona pellucida; 6. Allantois; 7. Blastocyst; 8. Placenta.

Topics: Female reproductive system, male reproductive system, human reproductive system, menstrual cycle, cleavages, blastula, early embryonic development, late embryonic and foetal development, fertilization, gametogenesis, egg, mammary glands and lactation, and parturition.

At a Glance

• Asexual Reproduction
• Sexual Reproduction
• Human Reproductive System
  • Male Reproductive System
  • Female Reproductive System
• Gametogenesis
  • Menstrual Cycle
• Human Development
  • Egg
  • Fertilization
  • Cleavages
  • Blastula
  • Early embryonic development
  • Late embryonic foetal development
• Parturition
• Mammary glands and Lactation

REPRODUCTION AND ITS TYPES

There are two basic categories of reproduction: asexual reproduction and sexual reproduction.

Asexual reproduction
A single parent is involved. The daughter individuals are genetically identical (clones) to the parent. Gamete formation is generally absent. There are three modes of asexual reproduction: (1) by fission, (2) by budding, and (3) by fragmentation.

(1) By fission It is of two types: binary fission and multiple fission.
(A) Binary fission: two daughter individuals are produced. Binary fission can be of three kinds:
(a) Simple binary fission: this fission is random or asymmetrical. The nucleus shows mitotic phases, but the nuclear membrane does not disappear at metaphase. e.g. Amoeba and Entamoeba.
(b) Transverse binary fission: the two daughter individuals are formed by transverse division. e.g. Paramecium (a ciliate protozoan). In Ceratium the binary fission is oblique. Transverse fission occurs in Planaria (Platyhelminthes).
(c) Longitudinal binary fission: in Euglena (a flagellate protozoan), the two daughter individuals are produced by longitudinal division of the parental organism.
(B) Multiple fission: this fission may occur without encystment, or after encystment if the environmental conditions are not favourable. In Plasmodium, schizogony is a type of multiple fission which occurs without encystment. In Amoeba, multiple fission occurs by sporulation.

(2) By budding It can be of two types: exogenous and endogenous budding.
(I) Exogenous budding: such buds are formed outside the body. It occurs in most of the Porifera and a few Coelenterates (Cnidaria). In most sponges and a few Coelenterates (e.g. Physalia), which are colonial, the colony is formed by budding. In chordates, budding occurs in the members of Tunicata (Urochordata).
(II) Endogenous budding: such buds are internal and are also called gemmules. They are produced in freshwater sponges like Spongilla and Ephydatia.

(3) By fragmentation The parental individual gets fragmented into a number of pieces. Each piece has the capacity of forming a complete daughter individual. Fragmentation occurs in both Hydra and Planaria.
Among lower non-chordates, asexual reproduction is absent in Ctenophores and Nematodes.

Check point (Match the animals of Column A with their type of reproduction in Column B)
Column A: 1. Hydra; 2. Euglena; 3. Entamoeba; 4. Earthworm; 5. Paramecium.
Column B: A. Oblique binary fission; B. Exogenous budding; C. Simple binary fission; D. Transverse binary fission; E. Only sexual reproduction; F. Endogenous budding; G. Longitudinal binary fission.
Answers: 1 - B; 2 - G; 3 - C; 4 - E; 5 - D.

Sexual reproduction
Two parents are involved. Gametes are formed. The daughter individuals are genetically different from both parents. The parental individuals may be unisexual (dioecious) or bisexual (hermaphrodite or monoecious), e.g. Paramecium, Plasmodium, Hydra, tapeworm and earthworm.
In Monocystis the gametes are identical, and their union is called isogamy. In other animals the two gametes are dissimilar, and their union is called anisogamy.
In Paramecium, sexual reproduction occurs by conjugation, in which gametic nuclei are exchanged between conjugating parents. Each Paramecium after conjugation produces four daughter individuals.
In tapeworms there can be self-fertilization also, while in earthworms it is reciprocal and cross-fertilization.
In roundworms, arthropods and members of higher groups, the two sexes can be morphologically differentiated (sexual dimorphism is present), whereas in echinoderms and some other lower organisms sexual dimorphism is absent.
Connecting concept: In most invertebrates the seminal vesicle is not a gland, but a storage structure for sperms.

HUMAN REPRODUCTIVE SYSTEM

Humans are dioecious with prominent sexual dimorphism. Males have facial hair (beard and moustaches), an Adam's apple, a low-frequency (pitch) voice and different body musculature. Females have the corresponding secondary sexual characters.

I. Male reproductive system
It consists of primary reproductive organs (testes) and accessory reproductive organs (penis, ducts and glands etc.).

Testes
The testes are extra-abdominal and lie in the scrotum, where the body temperature is 2 to 5°C lower than the abdominal temperature. In carnivores and primates the testes are lodged permanently in the scrotum. In cetaceans (whales), sirenians (sea cows) and proboscideans (elephants) the testes are abdominal. In certain mammals the testes ascend and descend periodically between the scrotum and the abdominal cavity. In rodents, during the breeding season, the testes move into the scrotum where the sperms can develop. The condition in which the testes fail to descend from the abdominal cavity to the scrotum is called cryptorchidism.
[Figure: Fig. 3.1 L.S. of human testis showing tunica vaginalis, tunica albuginea, septum, mediastinum testis, testicular lobules, seminiferous tubules, tubuli recti, rete testis and vasa efferentia.]
Each testis is surrounded by a membranous covering of mesodermal origin, called the tunica vaginalis. The inner coating of the testis consists of white fibrous connective tissue and is called the tunica fibrosa or tunica albuginea. It is protective in nature.
Inside each testis there are about 250 compartments called testicular lobules (folliculi), each having 1 to 3 seminiferous tubules. Thus there are about 700 seminiferous tubules, each measuring about 5 m, in each testis. Sperms are produced inside these tubules and pass out of the testis through the tubuli recti, rete testis and vasa efferentia (10 to 12 in number).
The vasa efferentia open into a highly convoluted structure called the epididymis. It is differentiated into the caput (head) epididymis, corpus (body) epididymis and cauda (tail) epididymis. After the cauda epididymis, the epididymis uncoils to form the vas deferens (ductus deferens).
Connecting concept: Sperms are temporarily stored in the epididymis, where they become motile and mature in 1 to 2 months and acquire the capacity of fertilization. This is called capacitation. Capacitation also occurs in the female reproductive tract, where the viscous fluid secreted from the mucosa lining of the oviduct increases the motility of sperms.
Connecting concept: The cutting of the vas deferens and tying of its two ends separately, or bilateral ligation, is called vasectomy. It is a safe and convenient contraceptive procedure. About 50% of vasectomized men even develop antibodies against sperms.

Associated ducts and glands
The vas deferens enters the abdominal cavity through the inguinal canal (a passage between the abdominal cavity and the scrotum) and joins the duct of the seminal vesicle to form the ejaculatory duct. (In frogs the vas deferens is absent, and the sperms pass through Bidder's canal, a collecting duct in the kidney.)
The ejaculatory duct, together with the urinary duct, opens into the urinogenital sinus. The duct arising from the urinogenital sinus is called the urethra, and it passes through the penis to open at its enlarged tip, called the glans penis.
Over the glans penis there is loose skin called the prepuce or fore skin. Below the prepuce a waxy excretory secretion (smegma) gets deposited if personal hygiene is not maintained. This secretion is also a nutrient for bacteria. In Muslims, the prepuce is cut in childhood (circumcision) to prevent recurring infection. (Male circumcision has prevented about 60% of cases of AIDS and cancer in Africa.)
In a transverse section, the penis, the male copulatory organ, shows three spongy erectile tissues: two dorsolateral and one midventral. The dorsolateral structures are called corpora cavernosa, while the ventral spongy tissue is called corpus spongiosum. The urethra passes through the corpus spongiosum.
[Figure: Male reproductive system showing seminal vesicle, urethra, ejaculatory duct, prostate, bulbourethral gland, penis, artery, inguinal canal, ductus deferens, urinogenital sinus, ureter, corpus cavernosum, epididymis, prepuce, seminiferous tubules, scrotum, head, body and tail of epididymis, vas deferens, corpus spongiosum and T.S. of penis.]

Glands
1. Seminal vesicle: it is a remnant of the Mullerian duct and is also known as uterus masculina. It is paired and dorsal to the urinary bladder. It produces the bulk of the semen or seminal fluid (about 60%). Its secretion contains fructose, fibrinogen, hyaluronidase, prostaglandins and ascorbic acid.
2. Prostate gland: it is present around the urinogenital sinus and produces about 20% of the semen. Its secretion contains citric acid, fibrinolysin, phospholipids, calcium and acid phosphatase etc. The gland corresponding to the prostate is absent in females.
3. Cowper's gland (bulbourethral gland): it is comparable to Bartholin's gland of females. It comprises about 5% of the semen secretion.
The semen or seminal plasma, which mainly contains fructose, calcium and certain enzymes, is an alkaline fluid (pH 7.4) and contains about 100 million sperms per ml. About 3 to 5 ml of semen is produced per ejaculation.

Penis: the male copulatory organ, described above.

II. Female reproductive system
This system also contains primary reproductive organs (ovaries), accessory reproductive organs (uterus, ducts and glands) and external genitalia.
Ovary: oogenesis takes place inside the ovary. The mature Graafian follicles rupture (ovulation) to release the female gamete into the abdominal cavity. From the fimbriated funnel the gamete passes through the oviduct (uterine tube or fallopian tube). Fertilization occurs in the ampulla (the distal part of the uterine tube).
Uterus: it is a pear-shaped structure whose wall is composed of three layers: outer perimetrium, middle myometrium and inner endometrium. The myometrium is the thickest layer and contains areolar connective tissue and smooth muscle fibres. In the endometrium, the upper two-thirds has simple columnar epithelium, whereas the lower one-third has stratified squamous epithelium. The surgical removal of the uterus is called hysterectomy. The cutting of the oviduct and tying of its two ends separately, or tubal ligation, is called tubectomy. The percentage of failure in this contraceptive procedure is only 0.1%.
The neck of the uterus, called the cervix, opens into the vaginal tube (vagina), which lies between the urinary bladder and the rectum. The vagina is lined with stratified squamous epithelium. The bacteria in the vaginal passage secrete lactic acid to maintain a low pH (3.5 to 5), which inhibits the growth of pathogens. The vaginal opening is guarded by a perforated mucus membrane called the hymen.
[Figure: Fig. 3.2 Female reproductive system showing right ovary, left oviduct with infundibulum, funnel, ampulla and isthmus, uterus with perimetrium, myometrium and endometrium, cervix, vagina, hymen and vulva.]
Urethra (above) and vagina (below) both open into the vestibule, which is guarded by minor and major folds called labia minora and labia majora respectively. The labia majora extend downward and surround the vaginal opening.
At the top of the vestibule opening, above the junction of the labia minora, there is an erectile structure called the clitoris. It is homologous to the glans penis. Unlike the penis, the urethra does not pass through the clitoris.
The external genitalia of the female are called the vulva and include the mons pubis, clitoris, labia majora, labia minora and hymen.
Glands: Bartholin's glands lie on either side of the vagina and lubricate the passage for coitus (sexual intercourse). They are homologous to the Cowper's glands (bulbourethral glands) of males. (The glands corresponding to the prostate are lacking in females.)

Check point (Name the following)
(i) Outer membranous covering of testes.
(ii) The process of acquiring the capacity of fertilization.
(iii) The duct formed by the union of vas deferens and the duct of seminal vesicle.
(iv) The spongy tissue of penis through which the urethra passes.
(v) The reproductive gland that produces the maximum amount of semen.
(vi) Reproductive gland of males whose corresponding gland in females is absent.
(vii) The structure of the female reproductive system that is homologous to the glans penis.
(viii) The reproductive structure of males which is commonly called uterus masculina.
Answers: (i) tunica vaginalis; (ii) capacitation; (iii) ejaculatory duct; (iv) corpus spongiosum; (v) seminal vesicle; (vi) prostate gland; (vii) clitoris; (viii) seminal vesicle.

GAMETOGENESIS

Gametogenesis is the production of gametes from the gonads. The production of sperms (male gametes) from the testis is called spermatogenesis, and the production of ova (female gametes) from the ovary is called oogenesis.

A. Spermatogenesis
It consists of two parts: (1) formation of spermatids, and (2) metamorphosis of spermatids into sperms or spermatozoa (spermiogenesis).
Formation of spermatids is completed in three phases: (1) multiplication phase, (2) growth phase and (3) maturation phase.
The hormones which regulate spermatogenesis are FSH of the anterior pituitary and testosterone of the testis. Androgen binding hormone/factor (ABH) is secreted from Sertoli cells and concentrates testosterone in the seminiferous tubules.
Multiplication phase: the primordial germ cells divide several times by mitosis to produce a large number of cells called spermatogonia. Spermatogonia are differentiated into type A and type B. Type A spermatogonia continue as stem cells, while type B spermatogonia differentiate and enter the growth phase.
Growth phase: spermatogonia differentiate into primary spermatocytes. The size of the primary spermatocyte increases due to the accumulation of nourishing material obtained from the lumen of the seminiferous tubules. Preparation for meiotic division also begins. Both the spermatogonia of the multiplication phase and the primary spermatocytes of the growth phase are diploid (2n = 46).
Maturation phase: there are two divisions in meiosis, a first reduction division and a second equational division. The germ cells formed after the reduction division are haploid (n) and are called secondary spermatocytes. Each secondary spermatocyte then produces two spermatids. Thus, from each primary spermatocyte, four spermatids are produced.
The spermatids are non-motile, haploid cells, which get attached to the Sertoli cells for nourishment during metamorphosis.
[Figure: Fig. 3.3 A part of a seminiferous tubule showing spermatogenesis: primordial germ cell (2n), spermatogonium (2n), primary spermatocyte (2n), secondary spermatocyte (n), spermatids (n), almost mature spermatozoa, and the Sertoli cells.]

Spermiogenesis (metamorphosis of spermatid into sperm)
(i) Nucleus: it becomes compact due to the removal of nuclear sap and RNAs. From spherical, it changes into an anteriorly pointed structure.
(ii) Golgi bodies: all the Golgi bodies scattered in the cytoplasm of the spermatid assemble at the anterior side of the nucleus. The secretory granules or vacuoles fuse to form the acrosome, and the covering becomes the acrosomal vesicle. The vesicle protects the nucleus from the anterior side and is known as the head cap.
(iii) Centrosome: both centrioles of the centrosome move towards the posterior end of the nucleus and form a proximal (close to nucleus) and a distal (away from nucleus) centriole. The microtubules (9 sets) of the distal centriole elongate to form the axial filament, which later becomes part of the middle piece and tail.
(iv) Mitochondria: all these organelles fuse end-to-end to form a mitochondrial sheath, which becomes spirally twisted around the anterior part of the axial filament, forming the middle piece.
Connecting concept: During spermatogonial divisions cytokinesis is incomplete, and the cells remain connected through cytoplasmic bridges. The ions, molecules and gene products of one cell can readily diffuse into the cytoplasm of neighbouring cells. Spermatids finally lose their cytoplasmic connections and differentiate into spermatozoa. From spermatogonia to mature spermatozoa in humans takes about 65 days. During spermiogenesis, the histone proteins of the haploid spermatids are replaced by much smaller protamines (60% arginine amino acid) to reduce the overall weight of the spermatozoa.
[Figure: Fig. 3.4 Spermatogenesis with cytoplasmic bridges, residual bodies and sperm cell.]
A mature sperm is differentiated into three regions: (1) head, (2) middle piece and (3) tail. (The term 'neck' is sometimes used for the portion between the proximal and distal centrioles.)
Head: includes the acrosome, head cap, haploid nucleus and proximal centriole. The acrosome contains lytic agents and helps in penetration into the egg. The proximal centriole adds the 'mitotic division centre' to the fertilized egg (zygote). (If the proximal centriole is destroyed, fertilization will occur, but cleavage will not begin.)
Middle piece: includes the distal centriole and the mitochondrial sheath. The mitochondrial sheath provides energy for the movement of the sperm in the female reproductive tract (speed 2 to 3 mm per minute).
Tail or flagellum: formed by the axial filament, consisting of (9+2) sets of microtubules. The posterior part of it, called the end piece, does not have the covering of the plasma membrane.
[Figure: Fig. 3.5 Electron micrograph of mammalian sperm showing nucleus, proximal centriole, distal centriole, mitochondrial spiral, middle piece and axial filament.]
Connecting concept: The reproductive period in females ranges from 12 to 15 years to 45 to 50 years. The maximum number of eggs that can be released from the ovary, if there is no pregnancy, is about 400 (one egg per month, in the middle of the menstrual cycle).

B. Oogenesis
It occurs in the ovary and is also completed in three phases: (1) multiplication phase, (2) growth phase and (3) maturation phase. The germ cells in humans originate from the embryonic mesoderm and migrate via the yolk sac to the ovary. (In many other vertebrates these germ cells are endodermal in origin.) In oogenesis, the phase comparable to spermiogenesis is absent, and the ootid can be directly named as the ovum.
Connecting concept: The large number of loops around the chromosomes of primary oocytes look like the bristles of a brush, which were used in Greek times for cleaning the chimneys of lamp-posts. Hence these chromosomes were called lamp brush chromosomes.
[Figure: Fig. 3.6 Diagrammatic representation of oogenesis and C.S. of ovary showing surface epithelium, primary follicles, secondary follicles, tertiary follicles, Graafian follicle, first and second meiotic divisions, polar bodies, ootid, ovum, and corpus luteum formation.]
Multiplication phase: this phase, from beginning to end, is completed before birth (in the foetus). The primordial germ cells divide by mitotic division and form a large number of oogonia. These oogonia get surrounded by follicular cells derived from the germinal epithelium (better called surface epithelium) of the ovary. The follicle cells keep multiplying inside the ovary, and the size of the ovarian follicle increases. The mature follicles in the ovary of mammals are called Graafian follicles.
Growth phase: this phase begins during foetal life. The oogonia increase in size and are now known as primary oocytes. The number of primary oocytes at birth ranges from 3 to 4 lakhs. The growth phase continues up to puberty, and the number of follicles left at this stage is only about 2 lakhs. The rest of the follicles start degenerating (follicular atresia). Such ovarian follicles are disposed of by phagocytosis. Some cells in the connective tissue of the ovary (stroma), around the follicles, become active. These cells are called interstitial cells and secrete a small amount of androgen.
The follicle cells multiply by mitotic division to form several layers. The development of the antrum cavity among the follicle cells differentiates the inner follicular layer (cumulus oophorus) from the outer follicular layer. The fluid secreted in the antrum by the follicle cells is called liquor folliculi. The two follicular layers remain connected through a stalk or pedicel of follicle cells. The outer follicular layer is called the stratum granulosum (later, theca interna). Its cells (granulosa cells) secrete estrogen during the follicular phase of the menstrual cycle.
Outside the plasma membrane, the vitelline membrane is secreted between the primary oocyte and the inner follicular layer. In mammals the thick vitelline membrane is known as the zona pellucida. It is transparent, non-cellular and composed of glycoproteins. The space between the plasma membrane and the vitelline membrane is called the perivitelline space.
The size of the nucleus (amount of nuclear sap) and the amount of cytoplasm increase. The nucleus during the growth phase is so large that it is named the germinal vesicle.
Preparations for meiosis begin during the growth phase. The chromosomes remain in the diplotene stage for the longest duration. Due to uncoiling of DNA, transverse loops develop, and the chromosomes are now called lamp brush chromosomes. RNA transcription at the uncoiled loops of DNA is faster, and a large amount of RNA is synthesized. As the amount of rRNA increases, the number of nucleoli also increases (in frog oocytes the number of nucleoli is more than 600).
The synthesis of yolk (vitellogenesis) in vertebrates also occurs during this phase. In most vertebrates, yolk is synthesized in the liver and reaches the primary oocyte through the blood and follicle cells.
Maturation phase: during this stage the germinal vesicle loses nuclear sap and all RNAs, becomes smaller and compact. This diploid nucleus moves towards the periphery, and the first part of meiosis (reduction division) begins. This division is very unequal. The outer smaller cell, released into the perivitelline space, is called the first polar body (polocyte). The inner, larger cell, which has all the reserve food material, is called the secondary oocyte. Both the secondary oocyte and the polar body are haploid.
It is this secondary oocyte (with one polar body) that is released during ovulation. The second part of meiosis is an equational division, which takes place only after the contact of sperm. The nucleus of the secondary oocyte divides again to form a second polar body. The secondary oocyte now becomes the ootid. As there is no metamorphosis in the ootid, it may be called the ovum. Meanwhile, the first polar body may also divide into two polar bodies. The maximum number of polar bodies present with the ovum can be three.
The human ovum, besides having two or three polar bodies, also has a covering of zona pellucida and corona radiata, the latter being the innermost layer of the cumulus.

Check point (Name the following)
(i) Region containing mitochondria in sperms.
(ii) The 3 phases of spermatogenesis.
(iii) The germ cell formed with the 1st polar body.
(iv) Stage of longest duration in meiosis of oogenesis.
(v) The gamete released during ovulation.
(vi) Non-cellular and transparent membrane around the human egg.
(vii) First haploid cell during spermatogenesis.
(viii) The membrane that prevents polyspermy.
Answers: (i) middle piece; (ii) multiplication, growth and maturation phases; (iii) secondary oocyte; (iv) diplotene stage; (v) secondary oocyte; (vi) zona pellucida; (vii) secondary spermatocyte; (viii) fertilization membrane.

Comparison of oogenesis and spermatogenesis

Parthenogenesis is the development of an egg without fertilization. In such cases the male individuals are absent, and the whole population consists of female individuals only. Such individuals are produced either from eggs developed by mitosis (not meiosis), or from haploid eggs that have undergone ploidy changes. There are two main categories:
(i) Arrhenotoky: e.g. bees and wasps. In honey bees, the male individuals (drones) are haploid and develop parthenogenetically, whereas the female individuals (workers or queen) are diploid and are produced from fertilized eggs.
(ii) Thelytoky: e.g. the Russian lizard Lacerta saxicola and turkey birds.

MENSTRUAL CYCLE

The menstrual cycle in humans is found in the females of eutherian mammals of the order Primates, including humans, apes and monkeys. In non-primate mammals there is an estrous cycle. During the breeding season, in the estrous cycle, the level of estrogen rises in the blood, which causes sex urge. This period is also known as the heat period. The number of breeding seasons may differ between animals. The suspension of the estrous cycle is called anestrum.
The menstrual cycle in humans starts at the age of 12 to 15 years (menarche) and continues up to about 45 years. Around 45 years of age, the maturation of Graafian follicles stops, and due to suppression of estrogen secretion the menstrual cycle discontinues. This phase is called menopause.
In humans the menstrual cycle, though called a monthly cycle, covers about 28 days. It consists of three phases: follicular phase (proliferative phase), luteal phase (secretory phase) and menstrual phase. However, the counting of days begins from the beginning of the menstrual flow.
Follicular phase: there is development of follicles in the ovary. It is also called the proliferative phase, as the endometrium of the uterus proliferates.
Luteal phase: there is development of the corpus luteum in the ovary, and mucus secretion from the mucus glands of the uterus. It is therefore also known as the secretory phase.
Menstrual phase: lasts 3 to 5 days. The follicular phase lasts about 7 to 10 days and the luteal phase about 12 to 14 days.
Ovulation occurs between the follicular and luteal phases, i.e. on the 14th day or in the middle of the menstrual cycle.

Role of pituitary hormones (gonadotropins)
(i) FSH stimulates the maturation of Graafian follicles (discovered by R. de Graaf), and its secretion slowly increases during the middle of the menstrual cycle.
(ii) LH secretion increases during the middle of the menstrual cycle, and it is responsible for rupturing the Graafian follicle to release the female gamete, i.e. the secondary oocyte. This process is called ovulation, and in a normal case it occurs on the 14th day of the cycle.
The remaining part of the Graafian follicle is called the corpus haemorrhagicum. From this structure, LH stimulates the formation of a yellow endocrine body called the corpus luteum.
Connecting concept: The menstrual blood is mainly arterial (75%) and contains fibrinolysin and prostaglandins. Due to fibrinolysin (secreted from the endometrium), this blood does not normally clot. The amount of blood is usually 30 to 40 ml, but it may vary depending upon diseases, medication or the thickness of the endometrium.

Role of ovarian hormones
(i) Estrogen secretion, though it continues throughout the reproductive period of females, increases slightly before the middle of each menstrual cycle. Before ovulation, estrogen is secreted from the theca interna (granulosa cells) of the Graafian follicles, but after ovulation the secretion continues from the corpus luteum. The rising level of estrogen causes the thickening of the endometrium of the uterus. This lining of the uterus is richly supplied with blood vessels and endometrial glands. Increasing estrogen, by feedback mechanism, inhibits the secretion of FSH and gonadotropin releasing hormone.
(ii) Progesterone is secreted from the corpus luteum under the influence of LH and maintains the endometrium for implantation of the blastocyst. Progesterone, commonly called the pregnancy hormone, prevents the contraction of the uterus and the development of new follicles in the ovary. In the presence of progesterone, the mucus glands and blood vessels of the endometrium become highly branched. The rising level of progesterone, by feedback mechanism, inhibits the secretion of LH and gonadotropin releasing hormone.
When the progesterone level drops, the corpus luteum begins to degenerate and changes from yellow to white, becoming the corpus albicans. The persistence of the corpus luteum for about 12 to 14 days (if fertilization does not occur) is called the luteal phase. When the corpus luteum degenerates fully, the endometrium begins to break down. The blood vessels and mucus glands rupture, and the blood, mucus and endometrial cells all pass out of the uterus as the menstrual flow. This takes 3 to 5 days, and after this the maturation of the Graafian follicle under FSH restarts.
Connecting concept: After about 4 months, before the corpus luteum degenerates, the secretion of progesterone begins from the placenta, and pregnancy is maintained. If the corpus luteum degenerates, or the ovaries are removed, the pregnancy is still maintained and the development of the foetus remains normal till birth.
If fertilization occurs, the cleavage is completed as the egg descends through the oviduct into the uterus. The blastocyst then gets implanted into the uterine wall, and the placenta is formed. (Implantation occurs on the 6th to 9th day after fertilization, i.e. before the corpus luteum changes into the corpus albicans.) The hCG secreted from the placenta takes over the function of LH and keeps stimulating the corpus luteum to secrete progesterone for about 4 months, instead of 12 days. Due to the continuous production of progesterone, there is neither maturation of a new Graafian follicle, nor ovulation, nor menstrual flow during pregnancy.
[Figure: Fig. 3.8 Role of gonadotropins and ovarian hormones in the menstrual cycle, showing blood levels of pituitary hormones (gonadotropins) and ovarian hormones (estrogen and progesterone) with the uterine endometrium thickness, the phases of development of follicle and corpus luteum, and menstruation over days 2 to 28.]

Check point (Which hormone is responsible for)
(i) Thickening of endometrium of the uterus.
(ii) Descending of testes into the scrotum.
(iii) Ovulation.
(iv) Maturation of Graafian follicle.
(v) Secretory phase of the menstrual cycle.
(vi) Pregnancy.
Answers: (i) estrogen; (ii) testosterone; (iii) LH; (iv) FSH; (v) progesterone; (vi) progesterone (maintained by hCG and placental hormones).

FERTILIZATION

There are three prominent stages in fertilization.
(1) Contact of gametes: in many non-chordates there is chemical attraction (chemotaxis) between the gametes. In humans or frogs, the contact between sperm and egg is random (by chance). The contact fixes the sperm onto the egg surface because of the reaction between fertilizin of the egg surface and antifertilizin of the sperm.
(2) Activation of the egg: after the sperm contact, the egg is activated, and both physiological and physical changes occur.
A. Physiological changes: the consumption of oxygen, the release of carbon dioxide and the production of ammonia all increase during activation of the egg. With the higher metabolic rate, the permeability of the plasma membrane also increases.
(i) Acrosomal reaction: the most important physical change. Influx of Na⁺ occurs, and the acrosome ruptures to release lytic agents containing the following enzymes, which help in penetration of the sperm: (a) hyaluronidase, which dissolves hyaluronic acid (the cementing material of the follicle cells); (b) corona-digesting enzyme, which digests the corona radiata; (c) acrosin or zona lysin, which digests the zona pellucida, a non-cellular and transparent layer.
(ii) Formation of fertilization cone: when the acrosomal content is released over the plasma membrane of the ovum, the cytoplasm of the ovum is raised to receive the sperm. This conical structure is called the fertilization cone or reception cone.
(iii) Cortical reaction: when free Ca⁺⁺ ions are liberated in the egg cytoplasm from their bound state, the cortical granules present in the outer part of the egg cytoplasm burst, and their contents fuse with the zona pellucida to form the fertilization membrane. This membrane is very thick (thicker than the vitelline membrane or zona pellucida) and prevents polyspermy (the entry of more than one sperm) in frogs.
In frogs, both the male and female nuclei, now in the interphase stage, are called pronuclei. The male nucleus (haploid), with the proximal centriole and mitochondria, moves into the egg cytoplasm along the penetration path. The whole structure, including nucleus, centriole and mitochondria, then rotates through 180° and moves further into the cytoplasm. This path of the sperm, after rotation, is called the copulatory path. The two pronuclei fuse, and a common nuclear membrane is formed. In most animals fertilization occurs in the centre of the egg. In frogs, however, it occurs towards the animal hemisphere, as the vegetal hemisphere contains yolk.
Connecting concept: The hen's egg yolk is a rich source of proteins and fat. It contains 16% proteins (phosvitin and lipovitellin), 32% phospholipids and 52% neutral fats.

Types of eggs
On the basis of the amount of yolk: (i) Microlecithal or oligolecithal eggs, with a very small amount of yolk compared with cytoplasm, e.g. marsupials, eutherian mammals, echinoderms and Cephalochordates. (ii) Mesolecithal eggs, with a moderate amount of yolk, e.g. cyclostomes and amphibians (frog); in frog the yolk is concentrated towards the vegetal pole. (iii) Macrolecithal, megalecithal or polylecithal eggs, with a huge amount of yolk, e.g. insects, fishes, lizards, birds and Prototherian mammals.
On the basis of the distribution of yolk: (i) Homolecithal or isolecithal eggs, where yolk is sparse and thinly or uniformly distributed throughout the cytoplasm, e.g. metatherians, eutherian mammals, echinoderms and Cephalochordates. (ii) Centrolecithal eggs, where the yolk is concentrated towards the centre and surrounded by cytoplasm, e.g. insects. (iii) Telolecithal eggs, where yolk is concentrated at one end (vegetal pole), e.g. cyclostomes, fishes, amphibians, reptiles, birds and prototherians.
In frogs the eggs are mesolecithal and telolecithal. In hens they are megalecithal and telolecithal. In cockroaches the eggs are megalecithal and centrolecithal. In humans they are microlecithal and homolecithal.
Connecting concept: The size of the egg depends upon the amount of yolk, and not on the size of the animal. For example, the eggs of the wall lizard, frog, mouse and human are compared, the egg of the wall lizard is the biggest, due to its megalecithal nature.

CLEAVAGE

Cleavage is a series of mitotic divisions which occur after fertilization and divide the entire egg, or part of it, into smaller cellular units.
Characteristics of cleavage: a single-celled structure (zygote) is converted into a multicellular structure (blastula). Growth in between the divisions is absent. The size of the embryo during cleavage does not change (zygote, morula and blastula are of the same size). There is no cytoplasmic displacement, and the external morphology of the embryo remains the same. Nucleoli are lacking in cleaving cells (there is no synthesis of fresh RNA). Protein synthesis continues, on the basis of stored RNA formed during the growth phase of oogenesis. The nucleo-cytoplasmic ratio increases during cleavage, because the volume of the nucleus per cell remains the same but the amount of cytoplasm decreases.

Holoblastic cleavage: the cleavage passes through the whole of the egg.
a. Equal holoblastic cleavage, e.g. in microlecithal eggs. Equal-sized cells are produced.
b. Unequal holoblastic cleavage, e.g. in mesolecithal eggs. Unequal-sized cells are produced.
Meroblastic cleavage (partial cleavage): the cleavage does not pass through the entire egg, but only a part of the egg undergoes segmentation.
a. Discoidal cleavage, e.g. in reptile and bird eggs. All cleavages occur in a disc-shaped portion of the cytoplasm (blastodisc).
b. Superficial cleavage, e.g. in insect eggs. Cleavages occur on the surface of the egg, and the centre (yolky part) remains undivided.
[Figure: Fig. 3.9 Meroblastic cleavage: (A) discoidal cleavage in chick, (B) superficial cleavage in an insect.]
On the basis of the fate of the cells:
1. Indeterminate cleavage, e.g. in the eggs of echinoderms and chordates (deuterostomes). The fate of the cells in early cleavages is not determined, and if the cells at the 2- or 4-celled stage are separated, each can form a complete (though small) embryo. Animals with indeterminate cleavage can produce identical twins.
2. Determinate cleavage, e.g. in the eggs of nematodes, annelids and molluscs (protostomes). The fate of the cells in early cleavages is fixed or determined. If even a single cell in the 4-celled stage is damaged, the embryo will be defective, because the structures to be formed by the damaged cell will not develop. Animals with such cleavage cannot produce identical twins.
On the basis of the arrangement (pattern) of the cells in early cleavage:
Radial cleavage, e.g. in the eggs of deuterostomes. The cells of the upper and lower tiers lie on the same vertical axis, or at the same radius.
Spiral cleavage, e.g. in protostomes. The cells of one tier lie at the junction of the cells of the other tier, so the cells of different tiers are spirally arranged. Sometimes the cells of the upper tier rotate clockwise or anticlockwise to assume a spiral pattern.
The eggs which show indeterminate cleavage or radial pattern are called regulative eggs, and the eggs which show determinate cleavage or spiral pattern are called mosaic eggs.
[Figure: Fig. 3.10 (A) Radial cleavage, (B) Spiral cleavage.]

Human egg
The human egg is microlecithal, homolecithal (isolecithal) and regulative. Its size is less than 0.1 mm. The plasma membrane is surrounded by a non-cellular, transparent and secretory coating called the zona pellucida (in non-mammals this coating is called the vitelline membrane). The zona pellucida is surrounded by a cellular layer of follicle cells called the corona radiata.
[Figure: Fig. 3.11 Unfertilized egg showing zona pellucida (ZP) and corona radiata.]

Sperm entry and fertilization in humans
During coitus (sexual intercourse), the sperms released from the penis of the male are deposited into the vagina of the female. This process is called insemination. Being motile, the sperms ascend through the cervix and uterus to reach the fallopian tube. The female gamete (secondary oocyte) also reaches the fallopian tube through the fimbriated funnel. In humans, fertilization occurs in the ampullary region. Both the sperms and the eggs have short fertility and survival periods. If one of the two gametes is delayed in reaching the site, fertilization will not be successful.
The sperm first comes in contact with the cells of the corona radiata. Due to the enzymes released from the acrosome, the cells of the corona radiata separate, giving entry to the sperm. The zona lysins, also from the acrosome, break down a part of the zona pellucida and plasma membrane so that the sperm reaches the cytoplasm of the egg. Meanwhile, due to sperm contact, the nucleus of the secondary oocyte completes the second part of meiosis to form the ootid (ovum) and the second polar body. The zona pellucida, due to the sperm reaction, is modified into a thick fertilization membrane, which prevents polyspermy. Finally, the male and female pronuclei fuse to form the zygote nucleus, and the diploid number of chromosomes (2n = 46) is restored.

Cleavage in humans
The cleavage in the human zygote is equal holoblastic, and the fate of the cleaving cells is not predetermined (indeterminate type). The pattern of cleavage in the human egg is radial.
Connecting concept: Cleavage in mammals has the following peculiarities: (i) in the animal kingdom, they are slowest in the mammals; (ii) the orientation of early blastomeres is different in comparison to other vertebrates; (iii) early cleavages are asynchronous; (iv) unlike other vertebrates, the mammalian genome is activated during early cleavages; (v) after the 8-celled stage there occurs the phenomenon of compaction.
The first cleavage is normal meridional and forms two equal-sized cells. It occurs about 15 hours after fertilization. (As fertilization occurs at the ampullary-isthmic junction of the fallopian tube, cleavage also begins in this region and is completed when the egg reaches the uterus.)
In the second cleavage, one cell divides meridionally and the other equatorially. This type of cleavage is called rotational cleavage. Subsequent cleavages are asynchronous, i.e. all blastomeres do not divide at the same time, and an odd number of cells may occur. After the 8-celled stage, the blastomeres huddle together to form a compact mass of cells (compaction). An 8- to 16-celled structure, formed after 3 to 4 days of fertilization, is called the morula. It is a solid, ball-like structure resembling a mulberry fruit.
Connecting concept: The sex of a baby is determined at the time of fertilization. The male is heterogametic and produces two types of sperms: 50% carry the X chromosome and 50% carry the Y chromosome. In females (homogametic) all eggs are similar, each having one X chromosome. The Y chromosome is male-determining, and its absence in the zygote decides femaleness. The sex of the child is therefore decided by the sperm (father).

Blastula
During the morula stage, the peripheral cells separate from the inner cells of the embryo. The peripheral layer forms the trophoblast. The inner cell mass (a group of formative cells) is oriented towards the animal hemisphere, leaving a large cavity called the blastocoel. This is the blastula stage, which is about 5 days old. The number of cells in the blastula ranges from 32 to 64. In mammals the blastula is known as the blastocyst.
The formative cells (inner cell mass) form the embryo or foetus, whereas the trophoblast forms the extra-embryonic membrane called the chorion (a part of the placenta). The inner cell mass also contains stem cells that have the potency to give rise to all the tissues and organs of the body.
[Figure: Early development of human embryo showing (A) fertilized egg (zygote), (B) two-celled stage, (C) four-celled stage, (D) eight-celled stage (uncompacted), (E) eight-celled stage (compacted), (F) morula stage, (G) blastocyst (blastula stage), (H) zona pellucida removed, and (I) differentiation of amnion.]

Labels of Fig. 3.12: corona radiata (CR) and zona pellucida (ZP) are marked in stages (A) to (F); in the morula stage (F) the corona radiata is shown as CR (disintegrating). In (G) blastocyst: ZP, inner cell mass, trophoblast and blastocoel. (H) zona pellucida removed. In (I): amnion, amniotic cavity, epiblast, hypoblast and trophoblast.

Implantation
After the blastula stage, the zona pellucida erodes, and the blastocyst hatches out of it, exposing the trophoblast. The blastocyst reaches the uterus and gets attached to the endometrium of the uterine wall. This attachment is called implantation. It occurs when the embryo is 6 to 8 days old (after fertilization). The trophoblast becomes syncytial to form the syncytiotrophoblast, which digests the endometrial lining of the uterus so that the embryo is completely embedded in it. The uterine tissue below and above the embryo is differentiated into the decidua basalis and decidua capsularis. The lining of the uterus opposite the site of implantation is called the decidua parietalis.

Early embryonic development
The outer cells of the inner cell mass, close to the trophoblast, differentiate into the amnion. The amnion secretes amniotic fluid, which later acts as a shock absorber and protects the embryo from mechanical injury and dehydration.
The trophoblast now differentiates into the chorion (another extra-embryonic membrane). The chorion develops chorionic villi, which penetrate the uterine tissue and branch to almost fuse with maternal tissue. The structure formed jointly by foetal and maternal tissue is called the placenta. Later, the allantois (another extra-embryonic membrane) arises as an outgrowth from the posterior part of the gut, and its membrane fuses with the chorionic membrane to form the allantochorion membrane. Because the allantois participates with the chorion, the placenta in humans is called an allantochorion placenta. The chorionic villi penetrate deep into the blood vessels of the uterus, and since they come in direct contact with blood, the placenta in human is also known as Haemochorial placenta.
After the differentiation of ectoderm from the outer part of the inner cell mass and endoderm from its inner part, the mesoderm forms between the two layers, and the structure becomes triploblastic (three-layered). All body structures develop from these three germ layers.

Late embryonic and foetal development
The embryonic period extends up to the end of the 8th week of pregnancy, and the development of organ systems is completed during this period. From the 9th week onward the embryo is called a foetus. During this (foetal) period there is growth and modelling in the embryo.
I trimester (1st, 2nd and 3rd months or 12 weeks): the pharyngeal arches and endocrine organs like the thyroid, parathyroid and thymus are well differentiated. The limbs with digits are formed. External genitalia are formed. The heart also develops and starts beating after 1 month. The crown-to-rump length (CR-length) of a 2-month-old embryo is about 2.5 cm.
II trimester (4th, 5th and 6th months or 24 weeks): movements in the body of the foetus begin during the 5th month. Head hair also appears in the 5th month, but body hair and eyelashes develop by the end of the II trimester. The eyelids also separate during this period.
III trimester (7th, 8th and 9th months or 36 to 38 weeks): there is fast growth. The CR-length at full term is about 30 cm.
The duration of about 9 months (266 days or 38 weeks) of pregnancy (gestation period) is divided into three trimesters.

Check point (Fill in the blanks with suitable word or phrase)
(i) In humans the embryo develops from ... of the blastula.
(ii) In human development the compaction occurs at the ... stage.
(iii) The attachment of the blastula with the uterine wall is called ... .
(iv) The foetus is formed after ... months of pregnancy.
(v) In humans the blastula is called ... .
(vi) The fertilization in humans occurs at ... .
(vii) The ... stage is a solid ball-like structure formed after 3 to 4 days.
(viii) Rotational cleavages occur in the members of class ... .
Answers: (i) the formative cells (the inner cell mass); (ii) 8-celled; (iii) implantation; (iv) 2 (from the 9th week onward); (v) blastocyst; (vi) the ampullary region (ampullary-isthmic junction) of the fallopian tube; (vii) morula; (viii) Mammalia.

PARTURITION

The expulsion or delivery of the foetus at the end of pregnancy is called parturition or childbirth. Before parturition there are vigorous contractions in the uterus (labour pains), which are controlled by a complex neuro-endocrine mechanism. The signals for parturition originate from the fully developed foetus and placenta, and induce mild uterine contractions called the foetal ejection reflex. This reflex triggers the release of oxytocin from the posterior lobe of the pituitary of the mother. Oxytocin makes the contraction of uterine muscles stronger, which stimulates further secretion of oxytocin. This stimulatory reflex between uterine contraction and oxytocin secretion continues until the child is expelled through the birth canal. The umbilical cord is then cut. The remaining part of the umbilical cord and the placenta are expelled from the uterus a little later, and this is called the after birth. To induce expulsion of the child, doctors may inject oxytocin.

Check point (Answer the following)
(i) How much is the gestation period in humans?
(ii) During which trimester do the body and head hair develop?
(iii) From where do the signals for parturition originate?
(iv) Which type of placenta is formed in humans?
(v) Which term is used for the expulsion of the placenta and part of the umbilical cord?
(vi) How many mammary lobes are present in each breast?
(vii) Which hormone is directly responsible for the expulsion of the child?
Answers: (i) about 9 months (266 days or 38 weeks); (ii) the II trimester (head hair appears in the 5th month, and body hair and eyelashes by the end of the II trimester); (iii) the fully developed foetus and placenta; (iv) allantochorion placenta; (v) after birth; (vi) 15 to 20; (vii) oxytocin.

MAMMARY GLANDS AND LACTATION

As in most other mammals, in humans the mammary glands are functional in females. These glands are pectoral and are composed of a mixture of fatty and glandular tissue interspersed with fibrous tissue. The glandular tissue of each breast is divided into 15 to 20 mammary lobes, each further divided into a number of lobules. These lobules contain alveoli, whose cells secrete the milk. The milk is stored in the lumen of the alveoli, which open into mammary tubules. The tubules of each lobe join to form a mammary duct. The mammary ducts of different lobes join to form a broader structure called the mammary ampulla. The ampullae are finally connected to the lactiferous duct, through which the milk is sucked by the baby.
Alveoli (lumen) to mammary tubules to mammary ducts to mammary ampullae to lactiferous duct.
During pregnancy the mammary glands undergo differentiation, and milk synthesis and storage begin. The actual flow or secretion of milk occurs after parturition. Milk is secreted whether the child is born premature or mature, alive or dead, and whether or not it is put to the breast. The suckling reflex is a neuro-endocrine mechanism that causes the release of oxytocin from the posterior pituitary. This results in the contraction of mammary alveoli and ducts, and the consequent ejection of milk. The hormone responsible for milk synthesis is prolactin of the anterior pituitary, which becomes active after the levels of estrogen and progesterone from the placenta drop at or after parturition.