Chapter 7: Human Reproduction

Introduction

Overview

• Humans reproduce sexually and are viviparous.
• Reproductive events: gametogenesis, insemination, fertilisation, zygote formation, blastocyst development, implantation, gestation, and parturition.
• Events occur after puberty with distinct male and female reproductive processes.
• Reproductive organs: primary (produce gametes and hormones) and secondary (conduct gametes, no gamete/hormone production).
• Secondary sexual characters are external features distinguishing sexes.

Memory Aids

• Primary = Produce gametes + hormones; Secondary = Conduct gametes only.
• Secondary sexual characters = External sex differences.

Quick Checkpoints

• What are the main reproductive events in humans?
• Define primary and secondary sex organs.
• What are secondary sexual characters?

Male Reproductive System

Structure and Location

• Located in the pelvis region.
• Includes paired testes, accessory ducts, accessory glands, and external genitalia.
• Testes are outside the abdominal cavity in the scrotum.

Memory Aids

• Testes in scrotum = outside abdomen for temperature control.

Quick Checkpoints

• Where is the male reproductive system located?
• What are the main components of the male reproductive system?
• Why are testes located outside the abdomen?

Scrotum

Function and Structure

• Connected to abdomen/pelvis by inguinal canals.
• Spermatic cord (artery, vein, nerve) passes through inguinal canal to testes.
• Maintains testes temperature at ~34°C, 2-2.5°C below body temperature.
• Temperature regulation via cremaster and dartos muscles adjusting scrotal position.
• Temperature receptors in scrotum control movement of testes towards/away from pelvis.

Memory Aids

• Scrotum = Temperature regulator for sperm production.
• Cremaster + Dartos muscles = Adjust testes position for heat control.

Quick Checkpoints

• How does the scrotum regulate testes temperature?
• What is the role of the spermatic cord?
• Why is testes temperature lower than body temperature?

Testis and Seminiferous Tubules

Structure and Function

• Testis: oval, 4-5 cm long, 2-3 cm wide; covered by tunica vaginalis, tunica albuginea, tunica vasculosa.
• Contains ~250 testicular lobules; each has 1-3 seminiferous tubules.
• Seminiferous tubules: site of spermatogenesis; lined by spermatogonia (male germ cells) and Sertoli cells (nutrition).
• Interstitial spaces contain blood vessels and Leydig (interstitial) cells producing androgens.
• Functions: sperm production and androgen secretion.
• Other immune cells present: lymphocytes, macrophages, mast cells.

Memory Aids

• Seminiferous tubules = Sperm production site.
• Sertoli cells = Support and nourish sperm cells.
• Leydig cells = Produce testosterone.

Quick Checkpoints

• What are the layers covering the testis?
• Where does spermatogenesis occur?
• What cells produce testosterone?

Male Accessory Ducts

Components and Functions

• Include rete testis, vasa efferentia, epididymis, and vas deferens.
• Seminiferous tubules open into vasa efferentia via rete testis.
• Vasa efferentia lead to epididymis (posterior testis surface), where sperm mature and are stored.
• Epididymis divided into caput, corpus, and cauda regions.
• Vas deferens ascends to abdomen, loops over bladder, receives seminal vesicle duct, and opens into urethra as ejaculatory duct.
• Urethra extends from bladder through penis to urethral meatus (external opening).

Memory Aids

• Epididymis = Sperm maturation and storage.
• Vas deferens = Sperm transport to urethra.

Quick Checkpoints

• What is the path of sperm from seminiferous tubules to urethra?
• What are the parts of the epididymis?
• Where does the vas deferens open?

External Genitalia (Penis)

Structure and Function

• Penis: male copulatory organ and urinal duct.
• Contains three erectile tissues: two dorsal corpora cavernosa and one ventral corpus spongiosum.
• Corpus spongiosum surrounds penile urethra and forms glans penis at tip.
• Glans penis covered by prepuce (foreskin).

Memory Aids

• Corpora cavernosa = Erectile tissue for erection.
• Corpus spongiosum = Protects urethra during erection.
• Prepuce = Covers glans penis.

Quick Checkpoints

• What erectile tissues are present in the penis?
• What is the function of the corpus spongiosum?
• What is the prepuce?

Male Accessory Glands

Types and Secretions

• Include paired seminal vesicles, prostate gland, and paired bulbourethral (Cowper's) glands.
• Seminal vesicles: sac-like, near bladder base; ducts join vas deferens to form ejaculatory duct.
• Seminal vesicle secretion (60-70% semen volume): alkaline, contains fructose (energy), prostaglandins (stimulate uterine contractions), clotting proteins, and calcium.
• Prostate gland: chestnut-shaped, surrounds urethra; secretes alkaline milky fluid (20-30% semen volume) with citric acid (nutrient), enzymes, and prostaglandins (activate sperm motility).
• Bulbourethral glands: pea-sized, on either side of membranous urethra; secrete alkaline fluid to neutralize urine acidity and mucus to lubricate penis and urethra.

Memory Aids

• Seminal vesicles = Energy + uterine stimulation.
• Prostate = Nutrients + sperm activation.
• Bulbourethral glands = Neutralize acidity + lubrication.

Quick Checkpoints

• What are the main male accessory glands?
• What does seminal vesicle secretion contain?
• What is the function of bulbourethral glands?

Semen

Composition and Function

• Mixture of secretions from seminal vesicles, prostate, bulbourethral glands, and sperm from testes.
• Ejected during ejaculation; contains 200-300 million sperms per ejaculation.
• Alkaline pH (7.35-7.50) to protect sperm and neutralize urine acidity.
• Provides fluid medium for sperm transport and nourishment.

Memory Aids

• Semen = Sperm + gland secretions for protection and nourishment.

Quick Checkpoints

• What is semen composed of?
• Why is semen alkaline?
• How many sperms are typically in one ejaculation?

Common Male Reproductive Disorders

Cryptorchidism and Inguinal Hernia

• Cryptorchidism: failure of testis descent into scrotum due to low fetal testosterone.
• Inguinal hernia: intestine protrudes into scrotum due to torn inguinal tissue.

Memory Aids

• Cryptorchidism = Undescended testis.
• Inguinal hernia = Intestine in scrotum.

Quick Checkpoints

• What causes cryptorchidism?
• What is inguinal hernia?

Gametogenesis

Overview

• Primary sex organs (testis in males, ovaries in females) produce gametes (sperms and ova).
• Process of formation of male and female gametes is called gametogenesis.

Memory Aids

• Gametogenesis = formation of gametes (sperm + ovum).

Quick Checkpoints

• What organs produce gametes?
• Define gametogenesis.

Spermatogenesis

Phases and Process

• Occurs in seminiferous tubules of testes, starts at puberty.
• Multiplication phase: Spermatogonia divide mitotically to form many spermatogonia.
• Growth phase: Spermatogonium grows into primary spermatocyte (diploid, 46 chromosomes).
• Maturation phase: Primary spermatocyte undergoes meiosis I to form two secondary spermatocytes (haploid, 23 chromosomes each).
• Secondary spermatocytes undergo meiosis II to form spermatids.
• Each primary spermatocyte produces four spermatids (haploid).

Memory Aids

• Spermatogenesis phases: Multiply, Grow, Mature.
• 1 primary spermatocyte → 4 spermatids.

Quick Checkpoints

• Where does spermatogenesis occur?
• What are the phases of spermatogenesis?
• How many spermatids are formed from one primary spermatocyte?

Formation of Spermatozoa

Structure and Process

• Transformation of spermatids into spermatozoa is called spermiogenesis or spermateliosis.
• Sperm parts: head, neck, middle piece, tail.
• Head contains haploid nucleus with acrosome (enzymes for fertilisation).
• Middle piece has mitochondria for energy; tail has 9+2 microtubule arrangement for motility.
• About 200-300 million sperms ejaculated per coitus.
• Spermiation: release of spermatozoa from seminiferous tubules.
• Semen = sperm + seminal plasma from accessory glands.

Memory Aids

• Spermiogenesis = spermatids to spermatozoa.
• Sperm structure: Head (nucleus + acrosome), Middle (mitochondria), Tail (movement).

Quick Checkpoints

• What is spermiogenesis?
• Name the parts of a sperm.
• What is spermiation?

Hormonal Control of Male Reproductive System

Hormonal Regulation

• Puberty triggers GnRH secretion from hypothalamus.
• GnRH stimulates anterior pituitary to release LH and FSH.
• LH stimulates Leydig cells to produce testosterone.
• FSH stimulates Sertoli cells to secrete androgen binding protein (ABP) and inhibin.
• Testosterone and inhibin provide negative feedback on LH and FSH secretion.

Memory Aids

• GnRH → LH + FSH → Testosterone + ABP + Inhibin.
• Negative feedback controls hormone levels.

Quick Checkpoints

• What hormones regulate spermatogenesis?
• Role of LH and FSH in male reproduction?
• What feedback controls hormone secretion?

Oogenesis

Process and Stages

• Formation of mature female gamete (ovum).
• Begins during embryonic development with oogonia formation; no new oogonia after birth.
• Oogonia enter prophase I of meiosis and arrest at diplotene stage (primary oocyte).
• Primary oocyte surrounded by granulosa cells forms primary follicle.
• Many follicles degenerate; 60,000-80,000 remain at puberty.
• Primary follicles develop into secondary follicles, then tertiary follicles with fluid-filled antrum.
• Primary oocyte completes meiosis I forming large secondary oocyte and small first polar body.
• Secondary oocyte surrounded by zona pellucida; undergoes meiosis II after fertilisation.
• One oogonium produces one ovum and three polar bodies.
• Ovulation: release of secondary oocyte from Graafian follicle.

Memory Aids

• Oogenesis stages: Oogonia → Primary oocyte → Secondary oocyte → Ovum.
• One oogonium → 1 ovum + 3 polar bodies.
• Ovulation releases secondary oocyte.

Quick Checkpoints

• When does oogenesis start?
• What is the fate of primary oocytes before puberty?
• What cells are released during ovulation?

Hormonal Control of Female Reproductive System

Hormonal Regulation

• GnRH from hypothalamus stimulates anterior pituitary to secrete LH and FSH.
• FSH promotes follicle growth and estrogen production; completes meiosis I in oocyte.
• LH stimulates corpus luteum to secrete progesterone.
• High progesterone inhibits GnRH, reducing LH, FSH, and progesterone secretion (negative feedback).

Memory Aids

• GnRH → LH + FSH → Estrogen + Progesterone.
• Progesterone inhibits GnRH and gonadotropins.

Quick Checkpoints

• Which hormones regulate female reproduction?
• Role of FSH and LH in oogenesis?
• How does progesterone regulate hormone levels?

Additional Notes

Key Points

• Vitellogenesis: yolk deposition in primary oocyte during prophase I.
• Mature ovum is spherical, contains nucleus, and is the largest cell in the body.

Memory Aids

• Vitellogenesis = yolk formation in oocyte.
• Ovum = largest cell, spherical with nucleus.

Quick Checkpoints

• What is vitellogenesis?
• Describe the mature ovum.

Menstrual Cycle

Phases and Overview

• Reproductive cycle in female primates (monkeys, apes, humans).
• First menstruation at puberty called menarche.
• Cycle repeats every 28/29 days on average.
• Cycle from one menstruation to next is menstrual cycle.
• Four phases: menstrual, follicular, ovulatory, luteal.

Memory Aids

• Menstrual cycle phases: M-F-O-L (Menstrual, Follicular, Ovulatory, Luteal).
• Menarche = first menstruation at puberty.

Quick Checkpoints

• What triggers the start of the menstrual cycle?
• How long is the average menstrual cycle?
• What are the four phases of the menstrual cycle?

Menstrual Phase

Key Features

• Marks cycle start; lasts 3-5 days.
• Day 1 of menstruation = day 1 of cycle.
• LH production drops, corpus luteum degenerates.
• Progesterone and estrogen levels fall rapidly.
• Endometrium breaks down; uterine tissues and blood shed via vagina (menstruation).
• Hygiene during menstruation is crucial.

Memory Aids

• Menstruation = shedding of uterine lining.
• Low LH → corpus luteum degeneration → hormone drop → menstruation.

Quick Checkpoints

• What hormone changes cause menstruation?
• How long does the menstrual phase last?
• What marks day 1 of the menstrual cycle?

Follicular Phase or Proliferative Phase

Key Features

• Occurs days 6-13/14 in 28-day cycle.
• Primary follicles mature into Graafian follicle.
• Gradual rise in LH and FSH secretion.
• FSH stimulates follicle growth and estrogen secretion.
• Estrogen causes endometrium proliferation.

Memory Aids

• Follicle grows → estrogen rises → endometrium thickens.
• FSH and LH increase gradually.

Quick Checkpoints

• Which hormones rise during follicular phase?
• What happens to ovarian follicles in this phase?
• How does estrogen affect the uterus?

Ovulatory Phase

Key Features

• Occurs around day 14 of cycle.
• LH and FSH peak simultaneously.
• LH surge causes rupture of Graafian follicle.
• Ovulation: release of ovum from follicle.

Memory Aids

• LH surge = trigger for ovulation.
• Ovulation occurs mid-cycle (~day 14).

Quick Checkpoints

• What hormone surge triggers ovulation?
• When does ovulation occur in the cycle?
• What is released during ovulation?

Luteal Phase or Secretory Phase

Key Features

• Occurs days 15-28 in 28-day cycle.
• Time between ovulation and next menstruation.
• LH transforms ovulated follicle into corpus luteum.
• Corpus luteum secretes progesterone.
• Progesterone thickens endometrium for implantation.
• Uterine glands produce watery mucus.
• Uterine wall prepared to nourish blastocyst if fertilization occurs.
• No menstruation during pregnancy; cycle stops.
• Without fertilization, corpus luteum degenerates → endometrium breaks down → menstruation starts new cycle.
• Menstrual cycles cease around age 50 (menopause).
• Cyclic menstruation occurs between menarche and menopause.

Memory Aids

• Corpus luteum = progesterone source post-ovulation.
• Progesterone prepares uterus for pregnancy.
• Menopause = end of menstrual cycles (~50 years).

Quick Checkpoints

• What hormone does corpus luteum secrete?
• What happens to the endometrium during luteal phase?
• When does menstruation resume if no fertilization occurs?

Estrous Cycle

Phases and Overview

• Sexual cycle in mammals other than primates (no menstruation).
• Four phases: anestrous, proestrous, estrous, metestrous.
• Anestrous: resting phase, no sexual cycle.
• Proestrous: follicle development, endometrium starts developing under estrogen.
• Estrous: period of heat, Graafian follicle develops under gonadotropins.
• Metestrous: corpus luteum active, produces progesterone; regresses if no pregnancy.

Memory Aids

• Estrous phases: A-P-E-M (Anestrous, Proestrous, Estrous, Metestrous).
• No menstruation in estrous cycle.

Quick Checkpoints

• What is the resting phase in estrous cycle?
• Which phase is called the period of heat?
• What hormone does corpus luteum produce in metestrous?

Fertilisation and Implantation

Process and Site of Fertilisation

• Insemination: Semen released into female vagina during copulation.
• Secondary oocyte released from mature Graafian follicle enters fallopian tube.
• Ciliated cells in infundibulum propel oocyte to ampulla.
• Ampullary-isthmic junction is fertilisation site.
• Fertilisation requires simultaneous arrival of sperm and ovum at ampulla.
• Sperm aided by uterine and tube contractions, then swim at 1.5-3 mm/min.

Capacitation and Acrosomal Reaction

• Capacitation: Activation of sperm by female tract secretions removing surface coatings.
• Exposes sperm receptors; takes 5-6 hours.
• Acrosomal reaction releases lysins: hyaluronidase, corona penetrating enzymes, acrosin.
• Enzymes digest follicle cells, corona radiata, and zona pellucida.
• Plasma membranes of sperm acrosome and oocyte fuse.
• Cortical reaction blocks entry of additional sperms, ensuring monospermy.

Fertilisation and Zygote Formation

• Sperm entry triggers completion of secondary oocyte meiosis II.
• Produces second polar body and haploid ovum (ootid).
• Fusion of haploid sperm and ovum nuclei forms diploid zygote.
• Secondary oocyte viable for fertilisation only 24 hours post-ovulation.
• Fertilizin (egg) and antifertilizin (sperm) interaction causes species-specific sperm adhesion (agglutination).

Sex Determination and Cleavage

• Female gametes carry X chromosome; male gametes carry X or Y (50% each).
• Fusion determines zygote sex: XX female, XY male.
• Zygote undergoes repeated mitotic divisions (cleavage) without volume increase.
• Cleavage forms 2, 4, 8, 16 blastomeres; 8-16 cell stage called morula.
• Morula develops into blastocyst with outer trophoblast and inner cell mass.
• Trophoblast attaches to endometrium; inner cell mass forms embryo.
• Implantation: blastocyst embeds in uterine lining, initiating pregnancy.

Cleavage Significance and Types

• Cleavage distributes cytoplasm among blastomeres.
• Increases protoplasm mobility for cell differentiation and germ layer formation.
• Restores cell size and nucleocytoplasmic ratio.
• Converts unicellular zygote into multicellular embryo.
• Human cleavage is holoblastic (complete division).
• Trophoblast cells near inner cell mass called Rauber cells; form extraembryonic membranes.
• Rauber cells do not form embryo but provide nutrition.
• Inner cell mass is embryo precursor.

Implantation and Zona Hatching

• Implantation occurs ~7 days after fertilisation.
• Zona pellucida prevents premature implantation and exposure of trophoblast cells.
• Blastocyst sheds zona pellucida via zona hatching before implantation.
• Blastocyst inner cell mass faces endometrium for implantation.
• Uterine cells cover blastocyst post-attachment, embedding it in endometrium.

Memory Aids

• Insemination = semen into vagina.
• Capacitation = sperm activation by female secretions.
• Acrosomal reaction = enzyme release to penetrate ovum layers.
• Cortical reaction = blocks polyspermy.
• Cleavage = mitotic division without growth.
• Morula = solid ball; blastocyst = hollow ball.
• Zona hatching = blastocyst escapes zona pellucida for implantation.
• Rauber cells = trophoblast cells near inner cell mass, form membranes.

Quick Checkpoints

• What is the site of fertilisation in the female reproductive tract?
• Define capacitation and its significance.
• Which enzymes are involved in the acrosomal reaction?
• What triggers the completion of meiosis II in the oocyte?
• How is sex determined at fertilisation?
• Describe the stages from zygote to blastocyst.
• What is the role of the trophoblast in implantation?
• Explain the function of zona pellucida during early embryo development.
• What is zona hatching and why is it important?

Pregnancy and Embryonic Development

Implantation and Placenta Formation

• Implantation initiates pregnancy.
• Trophoblast forms finger-like chorionic villi surrounded by uterine tissue and maternal blood.
• Chorionic villi and uterine tissue interdigitate to form the placenta.
• Placenta supplies oxygen and nutrients, removes wastes, and connects to embryo via umbilical cord.
• Placenta acts as endocrine tissue producing hCG, hPL, estrogens, progestogens, and relaxin (later stage).
• Pregnancy increases maternal blood levels of estrogens, progestogens, cortisol, prolactin, and thyroxine.

Memory Aids

• Placenta = Nutrient + Waste exchange + Hormone production.
• hCG, hPL, relaxin only during pregnancy.

Quick Checkpoints

• What forms the placenta?
• Which hormones are produced by the placenta?
• Role of umbilical cord?

Embryonic Layers and Development

• 8 days post-fertilization, trophoblast splits into syncytiotrophoblast and cytotrophoblast.
• Inner cell mass contains stem cells forming all tissues and organs.
• Inner cell mass differentiates into epiblast and hypoblast forming embryonic disc.
• Gastrulation transforms blastocyst into gastrula with three germ layers: ectoderm, mesoderm, endoderm.
• Epiblast and trophoblast differentiation forms amniotic cavity with amniotic fluid.
• Trophoblast forms extra-embryonic mesoderm, which splits into somatopleuric and splanchnopleuric layers enclosing extraembryonic coelom.
• Chorion formed by somatopleuric mesoderm and trophoblast; main embryonic placenta part and hCG producer.

Memory Aids

• Gastrulation = Formation of 3 germ layers.
• Chorion = Placenta + hCG source.

Quick Checkpoints

• What are the two layers of trophoblast?
• What forms the embryonic disc?
• Function of chorion?

Amnion and Yolk Sac

• Amnion formed by amniogenic cells and somatopleuric membrane.
• Amniotic fluid acts as shock absorber.
• Hypoblast cells line blastocoel forming primary yolk sac (endodermal cells).
• Extraembryonic mesoderm and coelom reduce yolk sac to secondary yolk sac.
• Three germ layers (ectoderm, mesoderm, endoderm) form by cell movements in triploblastic animals.

Memory Aids

• Amniotic fluid = Shock absorber.
• Yolk sac shrinks from primary to secondary.

Quick Checkpoints

• What forms the amnion?
• Role of amniotic fluid?
• Difference between primary and secondary yolk sac?

Gastrulation and Germ Layers

• Primitive streak is a faint groove on epiblast dorsal surface during gastrulation.
• It defines embryo's head-tail and right-left sides.
• Epiblast cells invaginate below primitive streak (invagination).
• Invaginated cells displace hypoblast to form endoderm.
• Cells between epiblast and endoderm form mesoderm.
• Remaining epiblast cells form ectoderm.
• Three germ layers give rise to all tissues and organs with conserved fate across animals.

Memory Aids

• Primitive streak = Body axis + germ layer formation.
• Invagination = Epiblast cells move inward.
• Germ layers: Ectoderm (outer), Mesoderm (middle), Endoderm (inner).

Quick Checkpoints

• What is the primitive streak?
• Which germ layer forms first?
• What is invagination?

Germ Layer Derivatives

• Ectoderm forms skin, hair, nails, nervous system, parts of eyes and ears.
• Mesoderm forms muscles, bones, connective tissue, heart, kidneys, reproductive organs.
• Endoderm forms lining of mouth, stomach, glands, parts of ear and reproductive system.
• All major body structures arise from these three layers.

Memory Aids

• Ectoderm = External structures + nervous system.
• Mesoderm = Muscles + skeleton + organs.
• Endoderm = Internal linings + glands.

Quick Checkpoints

• Which germ layer forms the nervous system?
• What does mesoderm develop into?
• Endoderm gives rise to which organs?

Early Embryonic Development Stages

• Fertilization: Fusion of male and female gametes forming zygote.
• Cleavage: Rapid cell division forming morula (solid ball) by day 4.
• Blastocyst: Hollow structure with inner cell mass and cavity by day 7.
• Implantation: Blastocyst attaches to uterine wall around day 7.
• Late blastocyst: Trophoblast cells develop for implantation.
• Gastrulation (day 16): Formation of three germ layers.
• Organogenesis: Organs develop, umbilical cord forms, systems differentiate.
• Embryo growth (days 35-60): Development of eyes, ears, limbs, fingers, and toes.
• Transforms single cell into complex embryo.

Memory Aids

• Fertilization → Cleavage → Blastocyst → Implantation → Gastrulation → Organogenesis.
• Day 7: Blastocyst and implantation.
• Day 16: Gastrulation starts.

Quick Checkpoints

• What is the sequence of early embryonic stages?
• When does implantation occur?
• What happens during gastrulation?

Parturition and Lactation

Parturition

• Parturition is the expulsion of the full-term baby from the uterus at the end of gestation.
• Signals originate from the fully developed fetus and placenta, causing mild uterine contractions called fetal ejection reflex.
• Fetal ejection reflex triggers oxytocin release from maternal pituitary.
• Oxytocin stimulates strong uterine contractions to expel the baby.
• Relaxin hormone increases flexibility of pubic symphysis and ligaments.
• Relaxin also helps dilate the uterine cervix during labour.
• Forceful uterine contractions during birth are called labour.
• After birth, placenta and umbilical cord remnants are expelled by uterine contractions, called after birth.

Lactation

• Lactation is milk production from mammary glands.
• Mammary glands differentiate during pregnancy and start milk production near term.
• Initial milk produced for 2-3 days is colostrum.
• Colostrum contains antibodies essential for newborn immunity.

Memory Aids

• Fetal ejection reflex = fetus triggers oxytocin release.
• Relaxin = relaxes ligaments and dilates cervix.
• Labour = strong uterine contractions for birth.
• Colostrum = first milk rich in antibodies.

Quick Checkpoints

• What triggers parturition signals?
• Role of oxytocin in birth?
• Function of relaxin during labour?
• What is colostrum and its importance?
• Define after birth.