Reproduction and Fertilization

Human Reproduction

• Human reproduction involves the process of producing offspring.
• Dioecism (separate sexes) is more common in animals than plants because of movement and less wastage of gametes in animals.
• In mammals, sex determination is genetic, whereas in some reptiles it is environmental.
• The Y chromosome has a specific region (SRY gene) that determines the development of testes.
• Human reproduction involves complementary reproductive organs for internal fertilization.

Male Reproductive System

• Function: sperm production and delivery, and production of testosterone to initiate and maintain secondary sexual characteristics.
• Structure: testes (singular = testis), genital ducts, accessory glands (seminal fluids, prostate gland, bulbourethral glands), and penis.
• Seminiferous tubules: are inside the testes, contain Sertoli cells (supportive function) and germinal epithelial cells (germ cells) which differentiate into spermatozoa (sperm).
• Testosterone is produced by Leydig cells.
• Sperm travels through genital ducts: Epididymis (sperm storage and maturation), Vas deferens (to urethra), Urethra (urinary and reproductive system).

Female Reproductive System

• Function: ovum production, passage to fallopian tubes for fertilization and implantation, embryo development, and milk production.
• Structure: ovaries that produce ova, fallopian tubes, uterus for embryo development, vagina as passage, vulva containing external genitalia, and mammary glands.
• Ovaries: contain ovarian follicles with developing primary oocytes and smaller granulosa cells.
• Oviducts: contain ciliated cells which move the ovum, and secretory cells which nourish the ovum.
• Uterus: composed of endometrium, myometrium and perimetrium, where embryo develops.
• Vagina: provides passage from uterus to the exterior, and cervix secretes mucus, during intercourse.
• External genitalia: vulva (labia majora, minora, and clitoris).

Gametogenesis

• Spermatogenesis: in males, the production of sperm.
• Spermatogonium divide mitotically. Produces primary spermatocytes which then divides to produce secondary spermatocytes, which divide again to form spermatids, which mature into sperm.
• Oogenesis: in females, the production of ova.
• Oogonia are produced during fetal development. They mature into primary oocytes paused in Prophase I of meiosis.
• Each cycle after puberty, a primary oocyte will continue meiosis.
• After ovulation, the secondary oocyte completes meiosis II only if fertilised, forming an ovum and additional polar bodies.

Menstrual Cycle

• Follicular Phase: Hypothalamus releases GnRH to the anterior pituitary stimulating FSH and LH, maturation of follicles, increased oestrogen levels.
• Ovulation: LH surge causes release of ovum from follicle, follicle develops into corpus luteum.
• Luteal Phase: Corpus Luteum secretes oestrogen and progesterone which prepare uterine lining for implantation. If no fertilization, the corpus luteum degenerates and menstruation occurs.

Fertilization

• Process of combining male and female gametes to form a diploid zygote. Happens in the oviduct (fallopian tube).
• Sperm must penetrate protective layers (corona radiata, zona pellucida) on the ovum to fertilize it.
• Fusion of nuclei occurs when sperm nucleus enters the ovum to create a zygote.

Cleavage

• Rapid cell division of the zygote following fertilization.
• Blastomeres (smaller cells) form a solid ball (morula), which develops into a hollow ball (blastocyst).

Implantation

• Blastocyst embeds in the uterine lining (process of implantation),
• Trophoblast cells form placenta.
• Inner cell mass becomes the embryo.

Human Development (Gestation)

• Average gestation period is 266 days (9 months).
• Early stages involve cleavage, blastocyst formation, implantation, and gastrulation leading to organogenesis.
• Three trimesters reflect the distinct stages of development and growth.
• The zygote undergoes its first cleavage about 30 hours after fertilization, then undergoes rapid divisions to form different cell layers within the blastocyst, which is differentiated to a blastula.
• After implanting in the uterus, the blastocyst is known as a blastula.
• A trophoblast develops that leads to the formation of the placenta which nurtures the developing embryo.

Placenta

• The placenta is an organ that connects the fetus to the uterine wall to provide it with nutrients, gases and eliminate waste products, etc.
• It undergoes hormone changes (which are responsible for maintaining progesterone levels).
• It is responsible for gas exchange, nutrient supply, and waste removal between mother and fetus.
• Chorion forms part of placenta.
• Yolk sac, Allantois are extraembryonic membranes supporting its functions, etc.

Birth (Parturition)

• Hormones regulate labour: Progesterone decreases and Oestrogen and Oxytocin increase to contract the uterine muscles, leading to labour.
• Labour stages occur for delivery: amniotic sac rupture, contractions which widen the cervix, and birth of baby, and expulsion of the afterbirth.

Nursing (Lactation)

• Milk production (lactation) in mammary glands is stimulated by prolactin (anterior pituitary).
• Baby suckling triggers oxytocin release (posterior pituitary), which causes milk ejection (milk letdown).
• Colostrum is the initial milk, rich in antibodies for passive immunity.

This lesson explores spermatogenesis, fertilization, and hormonal roles in reproduction. It covers sperm development, the importance of hydrolytic enzymes, meiotic divisions, and the sperm pathway. Also, the educational content includes pregnancy hormones and in vitro fertilization.