Human Reproduction: Events and Processes

Reproduction

• Reproduction is a biological process where offspring are produced, ensuring the continuity of species across generations.
• Without reproduction, species can face extinction.

Human Reproduction

• Human reproduction is characterized by sexual reproduction, viviparity (giving birth to live young), and sexual dimorphism (distinct physical differences between males and females).
• Reproductive events in humans begin after puberty.

Reproductive Events

• Gametogenesis: The formation of gametes (sperm and ova).
• Insemination: The process of transferring sperm to the female reproductive tract
• Fertilization: The fusion of sperm and ovum, resulting in the formation of a zygote.
• Development of Blastocyst: The zygote divides into multiple cells forming a structure called a blastocyst.
• Implantation: The blastocyst implants into the uterus's endometrial lining.
• Gestation: A period of about nine months when the fetus develops inside the uterus.
• Parturition: The process of giving birth.
• Lactation: The production and secretion of milk from the mammary glands after childbirth.

Gametogenesis Types & Differences

• Gametogenesis encompasses two processes: spermatogenesis (sperm formation) and oogenesis (ovum formation).
• Spermatogenesis occurs throughout a male's life, starting at puberty.
• Oogenesis starts during fetal life in females and stops after a certain age (menopause).

Sex Organs

• Primary sex organs are responsible for gamete and sex hormone production.
• Secondary sex organs aid in fertilization, implantation, and gamete transfer and development.

Male Reproductive System

• Located in the lower abdomen.
• Key components include primary and secondary sex organs, external genitalia, and accessory ducts and the glands.
• The primary sex organ in males is the testes, responsible for producing sperm and sex hormones.
• External Genitalia: The scrotum and penis.
• Accessory glands includes: seminal vesicles (2), bulbourethral glands (2), and a prostate gland (1).
• Accessory ducts include: vas deferens, epididymis, ejaculatory duct, vas efferentia, and the urethra.
• Testes are situated outside the abdominal cavity within the scrotum to maintain a lower temperature (2-2.5 degrees Celsius below body temperature) required for spermatogenesis.
• Testicular lobules (200-250 in number) contain seminiferous tubules (1-3 per lobule).
• Seminiferous tubules produce sperm.
• The cells present in seminiferous tubules are male germ cells (spermatogonia) and Sertoli cells (provide nutrition to germ cells).
• Interstitial spaces contain Leydig cells which produce androgens/testicular hormones and also immuno-competent cells

Male Reproductive System Flowchart

• Path of sperm: seminiferous tubules → rete testis → vasa efferentia → epididymis → vas deferens.
• Interstitial cells (Leydig cells) secrete testosterone.
• Sertoli cells provide nutrition to germ cells.
• Semen = sperm + seminal plasma.
• The penis facilitates insemination and contains erectile tissue.
• The prepuce or foreskin is a loose fold of skin covering the penis.

Accessory Ducts

• Assist in the storage and transport of sperm.
• The rete testis receives sperm from the seminiferous tubules.
• The vasa efferentia transports sperm to the epididymis.
• The epididymis temporarily stores sperm.
• The vas deferens carries sperm from the epididymis, passing over the urinary bladder to meet the ejaculatory duct.
• The ejaculatory duct is formed by the union of the vas deferens and seminal vesicle duct and opens into the urethra.
• The urethra serves as a shared pathway for urine and semen, known as the urinogenital duct.

Accessory Glands in Males

• Seminal vesicles secrete a fluid (60-70% secretion) containing fructose and prostaglandins, providing energy and aiding sperm motility.
• The prostate gland (25-30% secretion) secretes a fluid containing calcium.
• Bulbourethral glands/Cowper's glands (5-10% secretion) secrete mucus for lubrication.

Female Reproductive System

• Located in the lower abdomen and pelvic region.
• Includes a pair of ovaries, fallopian tubes, uterus, cervix, vagina, and external genitalia.

Female - Primary & Secondary Sex Organs

• Primary sex organ: ovaries (produce gametes and sex hormones).
• Secondary sex organs: accessory ducts, external genitalia, and mammary glands.
• Accessory ducts of the female reproductive system: fallopian tubes, uterus, vagina, and the cervix.
• The fallopian tube is 10-12 cm long and has three parts: infundibulum, ampulla, and isthmus.
• The infundibulum, closest to the ovary, possesses finger-like fimbriae for ovum collection.
• The ampulla is the site of fertilization
• The isthmus, a narrow part, joins the uterus.

Uterus

• The uterus is also known as the womb.
• Inverted pear-shaped structure attached to the pelvic wall by ligaments.
• The uterine wall has three layers: perimetrium (outer), myometrium (middle), and endometrium (inner).
• The myometrium consists of smooth muscle and generates contractions during delivery.
• The endometrium undergoes cyclic changes during menstruation.
• The vaginal and cervical canals form the birth canal facilitating childbirth.

External Genetalia (Female)

• Consists of the mons pubis, labia majora, labia minora, hymen, and clitoris.
• The mons pubis is a cushion of fatty tissue covered by skin and pubic hair.
• The labia majora are fleshy folds of tissue that extend from the mons pubis to the vaginal opening.
• The labia minora are paired folds of tissue under the labia majora.
• The clitoris, a finger-like structure, is found at the upper junction of the labia minora.
• The hymen covers the vaginal opening partially.
• The hymen is NOT a reliable indicator of virginity since it can be broken due to exercise, tampon insertion etc.

Mammary Gland's

• A characteristic feature of all female mammals, containing a pair of breasts.
• Glandular tissue and variable amounts of fat.
• Internally contains 15-20 mammary lobes, alveoli, mammary tubules.
• Structure: Mammary lobes → alveoli (milk secretion) → mammary tubules → mammary duct → mammary ampulla (milk storage) → lactiferous duct (milk release).
• Milk production is stimulated by prolactin, while milk ejection is stimulated by oxytocin.
• Colostrum, the initial milk produced, is rich in antibodies.

Gametogenesis: Spermatogenesis

• Spermatogenesis involves the formation of sperm, beginning at puberty and continuing throughout life.
• Process: spermatogonia (2n) → mitosis → primary spermatocytes (2n) → meiosis I → secondary spermatocytes (n) → meiosis II → spermatids (n) → spermiogenesis → sperm (n).
• Spermiogenesis is process of transformation of spermatids into sperm.
• Semen: Sperm + seminal plasma.
• During ejaculation 200-300 million sperm are released.
• 60% sperm in ejaculation must have normal shape and size.
• 40% sperm in ejaculation must have vigorous motility for male to be considered fertile.

Hormonal Regulation in Males

• Hypothalamus releases GnRH (Gonadotropin-releasing hormone).
• GnRH stimulates anterior pituitary gland.
• The anterior pituitary then releases LH and FSH.
• LH (Luteinizing Hormone) acts on Leydig cells (interstitial) which then releases testosterone (spermatogenesis).
• FSH (Follicle Stimulating Hormone) acts on Sertoli cells which then releases sperm maturation factor (spermiogenesis).

Sperm Structure

• Head: Contains the acrosome (enzymes for fertilization) and nucleus (genetic material).
• Midpiece: Contains mitochondria for energy production.
• Tail: Facilitates sperm motility during transport
• Plasma membrane covers all the parts of the sperm.

Gametogenesis: Oogenesis

• Is the process of formation of a mature female gamete or ovum
• Oogenesis starts during embryonic development stage
• Primary oocytes enter prophase-I and get temporarily arrested at that stage
• The primary oocyte is surrounded by a layer granulosa cells and called primordial follicles.
• Large number of follicles degenerate during the period from birth to puberty, therefore only 60,000-80,000 primary follicles are left in each ovary.

Oogenesis Steps

• oogonia (2n) → mitosis → primary oocyte (2n) → meiosis I (arrested at prophase I) → puberty → secondary oocyte (n) + first polar body (degenerates) → meiosis II (arrested at metaphase II) → sperm entry → ovum (n) + second polar body (degenerates).
• Meiosis II is completed after fertilization occurs.

Follicular Development

• Formation of primary follicle: Primary oocyte + layer of granulosa cells.
• Formation of secondary follicle: Primary oocyte + more granulosa layers
• Formation of tertiary follicle: Secondary oocyte (Theca interna and theca externa layer forms, also contain antrum fluid)
• Formation of graafian follicle: Tertiary follicle grow in to a mature follicle (contains zona pellucida)

Key Oogenesis Summary

• The Graafian follicle ruptures to release the secondary oocyte during ovulation.
• The remaining Graafian follicle transforms into the corpus luteum.
• Hormone regulation in females involves GnRH release, triggering FSH and LH secretion.
• FSH stimulates granulosa cell and follicular development.
• LH triggers ovulation and formation of the corpus luteum.
• The corpus luteum assists in the maintenance of pregnancy.

Summary - Spermatogenesis vs Oogenesis

• The primary sex organ in males are the testes and the primary sex organ in females is the ovary.
• The basic function of the primary sex organs produce gametes and secrete sex hormones.
• The testes produce sperm and testicular hormones and The ovaries responsible for the formation of ova and the secretion of estrogen and progesterone.

Menstrual Cycle

• Occurs in female primates.
• Consists of a cycle of events that repeats approximately every 28-30 days.
• Begins at puberty (menarche) and ceases at menopause.
• Uterine events: menstrual, proliferative, ovulatory, and luteal phases.
• Ovarian events occur simultaneously with the uterine events.

Menstrual Phases

• Menstrual Phase (Days 1-5): Endometrial lining sheds; blood vessels rupture; menstruation occurs.
• Follicular/Proliferative Phase (Days 6-13): Follicle development causes increase in estrogen production.
• LH surge at the peak of which is about Day 14 which causes the Graafian follicle to rupture releasing the ovum (ovulation).
• Luteal/Secretory Phase (Days 15-28): Development of corpus luteum; increased progesterone and estrogen levels.
• Without fertilization, the corpus luteum degenerates; progesterone, estrogen levels decline, restarting the menstrual cycle.

Menstrual Cycle Hygiene

• Maintain hygiene during menstruation to minimize infection risks.
• Use sanitary napkins or other products to absorb menstrual flow.
• Change sanitary pads every 4-5 hours.
• Dispose of used napkins properly by wrapping and discarding them.
• Wash hands with soap and water.
• Avoid flushing menstrual products down the toilet to prevent plumbing issues.

Fertilization

• During coitus semen is released in vagina, the sperms swim through the cervix and enters in to uterus towards the fallopian tube.
• The ovum also reaches ampulla from fimbriae through infundibulum.
• Only one of the sperms fertilizes the ovum.
• After the sperms reach the female's uterus, they release Hyaluronidase. This secretion is responsible for dissolving hyaluronic acid and breaking down the corona radiata layer.
• The Acrosome helps the sperms to penetrate the layer of Zona Pellucida
• Once the membrane is penetrated the ovum is fertilized.
• The sperm cell then enters the egg and stimulates it in order to complete the meiosis phase.
• Once the egg is fertilized it creates a Zygote. From that the female's chromosomes become fully functional.

Pregnancy

• After fertilization the Morulla develops that grows into a Blastocyst.
• During pregnancy the Blastocysts gets attache to the endometrium, this is called as implanatation
• Once cell attache it divides into 3 layers. Endoderm, Mesoderm, and Ectoderm.

Sex Determination

• Sex determination is the result of the fertilization of an ovum woth X carrying or Y carrying sperm.
• 50% probability of a male child birth.
• 50% probability of a female child birth.

Implantantation

• Attachement of the blastocyst to the endometrium is calle implantation.
• Trophoblasts get attache to the endometrium,
• During implantation, cells divide and the bastocyts penetrates inside it.

Gastrulation

• After implantation it divides into 3 layers known as blastocyst (endoderm, mesoderm, ectoderm) that turns into a fetus by organizing itself.
• Placenta is also develoepd which is connected to the mother and the fetus.
• Placenta acts as barrier between mother and fetus.
• It transport oxygen and nutrients to fetus.
• Also, releases waste from fetus through mother.

Placenta's Hormone

• HCG (Human Chorionic Gonadotropin)
• HPL (Human Placental Lactogen)
• Estrogen
• Progesteron
• Relaxin

Gestation Period of some common Mammals

• Cat and Dog(50-65 days)
• Elephant (18-22 Months)

Human - Trimester Growth

• 4th Week - Heart beat
• During 2nd month - limbs (hand and feet) get visible
• After 12 weeks - Most of the organs are formed , and external Genitalia are well developed.
• Fifth month - First movement of limbs and baby hair start to appear
• sixth months – Eyelids are formed.

Childbirth / Parturition

• Induces due to signals transmitted by fully grown and develoepd fetus.
• Oxytocin is released which cases the contraction of uterus.
• The Uterine contraction resullt with pushing of the baby by force which is also known as the fetal Ejection reflex.
• Placenta is also expelled out of the body after child brith.
• Oxytocin is continuously released by pituitary gland which causes stringer and quick contraction.