Human Reproduction

The menstrual cycle consists of three phases; proliferative phase, secretory phase and menstrual phase. During menstrual phase the production of LH is considerably reduced. The withdrawal of this hormone causes degeneration of the corpus luteum and, therefore, progesterone production is reduced. The endometrium degenerates and breaks down. Thus menstruation begins.

Sertoli cells are present in the germinal epithelium of the seminiferous tubules. These cells nourish the developing sperms. These cells differentiate spermatogonia into sperms. They are under the influence of FSH released by anterior pituitary gland.

Grey crescent is the area just opposite to the site of entry of sperm into ovum. It marks the future dorsal side of the embryo.

Since corpus luteum degenerates so progesterone also decreases rapidly (progesterone is essential for maintenance of pregnancy). Also estrogen continues to curve growth of the endometrium which ultimately becomes thick enough to break down and cause menstruation. Hence choice (c) is incorrect as estrogen secretion does not decreases further. Primary follicles start developing.

Human chorionic gonadotropin is released by Anophoblast cells of the placenta. Estrogen and progesterone are also released by placental cells to maintain pregnancy. Prolactin is secreted from anterior lobe of pituitary. Placenta secretes Human Placental Lactogenic Factor which prepare mammary glands to secrete milk.

Ovulation (the release of secondary oocyte from the graafian follicle) takes place at the end of proliferative phase of menstrual cycle. During this phase, the follicle stimulating hormone (FSH) secreted by the anterior lobe of the pituitary gland stimulates the ovarian follicle to secrete estrogen. Estrogen stimulates the proliferation of the endometrium of the uterine wall. The endometrium becomes thicker by rapid cell multiplication and this is accompanied by an increase of uterine glands and blood vessels. This phase ends when the ovarian follicle ruptures and ovulation occurs and at the same time the production of estrogen stops.

An organizer is a signal aligning center that directs the development of embryo or part of embryo, e.g. in amphibians, it is dorsal tip of blastophore.

During cleavage, the zygote divides repeatedly to convert the large cytoplasmic mass into a large number of small blastomeres. It involves cell division without growth in size because cells continue to be retained within the zona pellucida. However, cell size decreases during cleavage.

Vitamin B₃ or Niacin is a constituent of NAD. Corpus luteum is temporary endocrine tissue developing from ruptured grafian follicle. Bundle of His is a part of the conducting system of heart conducting cardiac impulse from atria to ventricles. Sebum is the name given to the oil secreted by sebaceous glands in the skin.

Archenteron is known as the primitive gut that forms during gastrulation in the developing blastula. It develops into the digestive tract of an animal. The open end of the archenteron is called blastopore.