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

Chick fertilization Fertilization in the chick is internal and occurs in the infundibulum of the oviduct shortly after ovulation. Sperm deposited during copulation enter the oviduct and may be stored in the oviductal reservoir. When the mature secondary oocyte, bearing the germinal disc, enters the infundibulum, capacitated sperm penetrate the oolemma. In birds physiological polyspermy is common: several sperm may enter and their chromatin may decondense, but only one male pronucleus fuses with the female pronucleus. Before syngamy, the oocyte completes meiosis II, extrudes the second polar body and forms the female pronucleus. The selected male pronucleus decondenses, the two pronuclei approach, and their chromosomes align to form a diploid zygote. Polyspermy is limited by a fast block, a rapid membrane change that limits further sperm activation, and a slow block, in which cortical granule exocytosis modifies the vitelline membrane and prevents further pronuclear fusion. Cleavage then begins in the magnum as the blastoderm forms; because the yolk is large, cleavage is meroblastic and the early blastoderm remains a disc of cells on the animal pole. The diagram would show the oviduct sequence: infundibulum (fertilization), magnum (albumen and cleavage), isthmus (shell membranes), uterus (shell), vagina (laying). The germinal disc at the animal pole is labelled, with sperm entry, pronuclear fusion and first cleavage indicated in the infundibulum/magnum region.

Cholesterol structure and health Cholesterol is a sterol with a tetracyclic steroid nucleus: three six-membered cyclohexane rings (A, B and C) fused to one five-membered cyclopentane ring (D). It has a hydroxyl group at C3, a double bond in the B ring, and an alkyl side chain at C17. This amphipathic structure is central to its health roles. In cell membranes, cholesterol inserts between phospholipids, preventing tight packing at low temperature and restricting excessive phospholipid movement at high temperature, thereby maintaining membrane fluidity and permeability. It is also the precursor of steroid hormones such as cortisol, aldosterone, oestrogen and androgen, and it is converted in the liver into bile acids, which emulsify dietary fats for digestion and absorption. In disease, excess cholesterol, especially carried in low-density lipoproteins, promotes atherosclerosis by depositing in arterial intima and forming plaques that narrow vessels and can cause myocardial infarction or stroke. Familial hypercholesterolaemia, usually due to defective LDL-receptor function, causes very high plasma cholesterol and premature coronary disease. Cholesterol supersaturation in bile can precipitate cholesterol gallstones, causing biliary colic and cholecystitis.

Ovarian hormones Ovarian hormones coordinate the female cycle through follicular and luteal phases. During the follicular phase, developing follicular granulosa and theca cells secrete oestrogen, mainly 17β-estradiol, which thickens the endometrium in mammals, stimulates FSH receptor expression, and, at high mid-cycle levels, gives positive feedback to the hypothalamus and anterior pituitary to produce the LH surge that triggers ovulation. After ovulation, the corpus luteum secretes progesterone, with some oestrogen, in the luteal phase; progesterone makes the endometrium secretory and receptive, supports implantation, and maintains early pregnancy by suppressing uterine contractions and sustaining the endometrium. Ovarian inhibin, secreted by granulosa cells, follows a phase-specific pattern, suppresses FSH and helps select follicles. In the luteal phase, progesterone and oestrogen exert negative feedback on GnRH, FSH and LH; if implantation does not occur, corpus luteum regression lowers these hormones, causing menstruation in eutherians; in other mammals the same fall produces the oestrous cycle. In birds and other non-mammalian oviparous animals, the same hormones regulate reproduction differently: oestrogen stimulates oviduct growth, albumen secretion and reproductive behaviour, while progesterone supports oviduct and shell-gland function; there is no menstruation, and the cycle is tied to ovulation and egg passage. Thus ovarian hormones integrate follicle development, ovulation, implantation and pregnancy maintenance across vertebrates.