(a) Describe the stages of fertilization in chick with diagram. (20 marks) (b) Draw structure of cholesterol and explain its role…

(a) Stages of fertilization in the chick

Fertilization in the domestic fowl (Gallus gallus domesticus) is internal and takes place in the infundibulum of the left oviduct (the right ovary and oviduct are vestigial). It proceeds in the following stages.

• Copulation and insemination: in the "cloacal kiss" the everted cloacae of cock and hen are pressed together and semen is transferred to the hen's vagina.
• Sperm selection and storage: most sperm are expelled, but motile sperm ascend the vagina and enter sperm storage tubules (SSTs) at the uterovaginal junction; a few are also stored in the infundibulum. Stored sperm remain fertile for about two to three weeks, so one mating can fertilise a series of eggs.
• Sperm ascent: sperm are released gradually from the SSTs and move up the oviduct to the infundibulum, the only site where fertilization can occur.
• Ovulation: the mature ovum, a huge yolk-filled secondary oocyte arrested at metaphase II with a germinal disc (blastodisc) at the animal pole, is released and captured by the funnel-shaped infundibulum.
• Sperm binding and acrosome reaction: within about 15 minutes of ovulation, before any albumen is laid on, sperm bind to the inner perivitelline layer (the avian homologue of the zona pellucida, rich in ZP1 and ZP3 glycoproteins). Binding triggers the acrosome reaction, and acrosomal enzymes hydrolyse holes in this layer, most densely over the germinal disc.
• Sperm entry and physiological polyspermy: several sperm normally enter the germinal disc. Birds lack a fast electrical block and a cortical reaction; instead, an outer perivitelline layer secreted by the infundibulum soon covers the ovum and blocks further binding.
• Completion of meiosis and pronuclear fusion: sperm entry activates the oocyte to complete meiosis II and extrude the second polar body, forming the female pronucleus. One male pronucleus fuses with it (syngamy) to form the diploid zygote; the supernumerary sperm nuclei migrate to the periphery of the disc and degenerate.
• Onset of cleavage: the zygote passes through the magnum (albumen, about 3 hours) into the isthmus (shell membranes), where the first cleavage begins about 5 hours after ovulation. Cleavage is meroblastic and discoidal, confined to the germinal disc, and continues during the roughly 20 hours the egg spends in the shell gland (uterus) before it is laid through the vagina and cloaca.

(b) Structure of cholesterol and its role in health and disease

Cholesterol (C₂₇H₄₆O) is a sterol built on the cyclopentanoperhydrophenanthrene (steroid) nucleus: three six-membered rings (A, B and C) fused to one five-membered ring (D). It carries a 3β-hydroxyl group on ring A, a double bond between C5 and C6 in ring B, two angular methyl groups (C18 and C19), and an eight-carbon branched side chain at C17. The small polar hydroxyl head and the rigid, hydrophobic ring system with its tail make the molecule amphipathic, which underlies most of its functions.

Roles in health:

• Membrane structure: cholesterol inserts between phospholipids with its hydroxyl near their head groups. It prevents tight packing at low temperature and restricts movement at high temperature, buffering membrane fluidity, reducing permeability to small solutes and organising lipid rafts.
• Steroid hormones: it is the precursor of progesterone, cortisol, aldosterone, testosterone and oestradiol, the first step being side-chain cleavage to pregnenolone in mitochondria.
• Bile acids: the liver converts it (via 7α-hydroxylase) into cholic and chenodeoxycholic acids, whose salts emulsify dietary fat.
• Vitamin D: 7-dehydrocholesterol in the skin is converted by ultraviolet light into vitamin D₃ (cholecalciferol).
• Synthesis and transport: it is made mainly in the liver from acetyl-CoA, with HMG-CoA reductase as the rate-limiting enzyme, and carried in plasma by LDL (to tissues) and HDL (reverse transport to the liver).

Roles in disease:

• Atherosclerosis: excess LDL cholesterol enters the arterial intima, becomes oxidised and is taken up by macrophages, which form foam cells and fatty streaks that mature into plaques. Plaque rupture and thrombosis cause myocardial infarction and stroke; high HDL is protective.
• Familial hypercholesterolaemia: mutations in the LDL receptor gene (LDLR), or in APOB or PCSK9, raise plasma LDL greatly and cause premature coronary disease and tendon xanthomas.
• Cholesterol gallstones: bile supersaturated with cholesterol relative to bile salts and lecithin precipitates stones, causing biliary colic and cholecystitis.
• Treatment: statins inhibit HMG-CoA reductase; ezetimibe blocks intestinal absorption and PCSK9 inhibitors increase LDL-receptor recycling.

(c) Role of ovarian hormones in female reproduction

The ovary secretes steroid hormones (oestrogens, progesterone and some androgens) and peptide hormones (inhibin, activin, anti-Müllerian hormone and relaxin). Their actions are best followed through the cycle.

• Follicular phase: under the two-cell, two-gonadotrophin model, LH stimulates theca cells to make androgens, which FSH-stimulated granulosa cells aromatise to 17β-oestradiol. Oestradiol stimulates proliferation of the endometrium and thin, watery cervical mucus, and together with FSH induces LH receptors on granulosa cells of the dominant follicle.
• Ovulation: at low levels oestradiol exerts negative feedback on GnRH, FSH and LH, but a sustained high mid-cycle level switches to positive feedback, producing the LH surge that causes ovulation about 36 hours later.
• Luteal phase: the corpus luteum secretes progesterone and oestradiol. Progesterone converts the endometrium into a secretory, receptive layer, thickens cervical mucus, raises basal body temperature and, with oestradiol, exerts negative feedback on GnRH and gonadotrophins.
• Menstruation or pregnancy: if fertilization does not occur, the corpus luteum regresses and the fall in progesterone causes the endometrium to shed (menstruation in humans and other higher primates; in most other eutherians the endometrium is resorbed within an oestrous cycle). If implantation occurs, hCG rescues the corpus luteum, and progesterone keeps the myometrium quiet and maintains early pregnancy until the placenta takes over.
• Peptide hormones: inhibin B from granulosa cells in the follicular phase and inhibin A from the corpus luteum selectively suppress FSH, which helps select a single dominant follicle; anti-Müllerian hormone restrains follicle recruitment; relaxin softens the cervix and pubic symphysis before parturition.
• Other roles: oestrogen promotes secondary sexual characters, breast duct growth and bone density; progesterone develops breast alveoli; and oestrogen with oxytocin increases uterine contractility at term.
• Non-mammalian vertebrates: in the hen oestrogen stimulates hepatic synthesis of yolk precursors (vitellogenin), medullary bone formation, and oviduct growth with albumen secretion; progesterone from the largest follicle triggers the pre-ovulatory LH surge and induces avidin secretion in the oviduct. There is no menstruation, and the cycle is tied to the passage of each egg.
• Clinical links: in polycystic ovary syndrome excess ovarian androgens and anovulation disturb this feedback, and falling oestrogen after menopause leads to osteoporosis and vasomotor symptoms.