Human Reproduction

Human Reproduction

Humans reproduce sexually and give birth to live young ones (viviparous). Let’s understand the key events and systems involved.

Key Reproductive Events

• Gametogenesis: Formation of sperms in males and ovum in females.
• Insemination: Transfer of sperms into the female genital tract.
• Fertilisation: Fusion of male and female gametes to form a zygote.
• Blastocyst Formation: Development of the zygote into a blastocyst.
• Implantation: Attachment of the blastocyst to the uterine wall.
• Gestation: Development of the embryo.
• Parturition: Delivery of the baby.

1.1 The Male Reproductive System

• Location: In the pelvis region.
• Components: Pair of testes (Primary sex organ rest are secondary), accessory ducts, glands, and external genitalia (Penis + Scrotum).

Testes

• Position: Outside the abdominal cavity in the scrotum.
  • Function: This position maintain lower temperature for sperm production (2-2.5°C below body temperature).
• Structure:
  • Oval shape, 4-5 cm long, 2-3 cm wide.
  • Covered by a dense layer.
  • Contains about 250 compartments/Testes, called testicular lobules.
  • Each lobule has 1-3 highly coiled seminiferous tubules for sperm production.

Seminiferous Tubules

• Lining Cells:
  • Male Germ Cells (Spermatogonia): Undergo meiosis to form sperms.
  • Sertoli Cells: Provide nutrition to germ cells thus also called nurse cells.
• Interstitial Spaces (Surroundings):
  • Contain small blood vessels & Leydig cells.
  • Leydig Cells: Secrete testicular hormones (androgens).

Accessory Ducts (tubes)

• Types: Rete testis, vasa efferentia, epididymis, vas deferens.
• Function: Store and transport sperms from the testis to the outside.

Penis

• Function: Male external genitalia, helps in insemination (Impregnation) by penetrating in female reproductive part i.e. vagina.
• Structure:
  • made of special spongy tissue, becomes erect when filled with blood.
  • Glans Penis: Enlarged end, covered by foreskin (loose fold of skin).

Male Accessory Glands

• Types: Seminal vesicles, prostate, bulbourethral glands.
• Secretions:
  • Form seminal plasma (Semen fluid).
    • Rich in fructose, calcium, and enzymes.
  • Lubricate the penis.

1.2 The Female Reproductive System

The female reproductive system helps with ovulation, fertilization, pregnancy, birth, and child care. It consists of several important parts.

Main Components

• Ovaries:
  • Primary female sex organs.
  • Produce ova (eggs) and ovarian hormones.
  • Located on each side of the lower abdomen.
  • About 2-4 cm long.
  • Connected to the pelvic wall and uterus by ligaments.
  • Covered by thin epithelium and have an ovarian stroma with cortex and medulla.
• Oviducts (Fallopian Tubes):
  • About 10-12 cm long.
  • Extend from ovaries to the uterus.
  • Infundibulum: Funnel-shaped part near the ovary with fimbriae (finger like projections) to collect the ovum.
  • Ampulla: Wider part of the oviduct.
  • Isthmus: Narrow part that joins the uterus.
  • The site of fertilization – Ampullary-isthmic junction of the fallopian tube.
• Uterus (Womb):
  • Single, pear-shaped organ also called the womb.
  • Supported by ligaments attached to the pelvic wall.
  • Opens into the vagina through the cervix.
  • Has three layers:
    • Perimetrium: Outer thin layer.
    • Myometrium: Middle thick muscle layer, strongly contracts during childbirth.
    • Endometrium: Inner glandular layer that changes during the menstrual cycle.
• Cervix:
  • Narrow part that connects the uterus to the vagina.
  • Contains the cervical canal which, along with the vagina, forms the birth canal.
• Vagina:
  • Tube that connects the external genitalia to the cervix.

External Genitalia

• Mons Pubis: Fatty tissue covered by skin and pubic hair.
• Labia Majora: Fleshy folds surrounding the vaginal opening.
• Labia Minora: Thin folds under the labia majora.
• Hymen: Membrane that partially covers the vaginal opening.
  • The hymen may tear during first intercourse or other activities like cycling, sports or tampon use, but its presence or absence doesn’t reliably indicate virginity or sexual experience.
• Clitoris: Small, highly sensitive structure (often tiny cap/finger like) at the upper junction of the labia minora, above the urethral opening.

Mammary Glands

• Function: Produce milk for feeding babies.
• Structure:
  • Paired structures called breasts.
  • Contain glandular tissue and fat.
  • Divided into 15-20 lobes with clusters of cells called alveoli.
  • Alveoli secrete milk stored in their cavities.
  • Milk flows through mammary tubules to mammary ducts, then to mammary ampulla, and finally through the lactiferous duct for sucking out.

These components work together to support reproductive processes in females.

1.3 Gametogenesis

• Gametogenesis: Process of forming gametes (sperms and ova) in humans.
  • Testis: Produces sperms in males.
  • Ovaries: Produce ova in females.

Spermatogenesis (Formation of Sperms)

• Location: Happens in the testis (seminiferous tubules).
• Starting Point: Begins at puberty.
• Process:
  • Spermatogonia (immature male germ cells) multiply by mitosis.
  • Some spermatogonia (46 chromosome) become primary spermatocytes (46 ch.) and undergo meiosis.
  • First Meiosis: Forms two haploid secondary spermatocytes (23 ch.).
  • Second Meiosis: Produces four haploid spermatids (23 ch.).
  • Spermiogenesis: Spermatids transform into spermatozoa (sperms).
  • Spermiation.: After spermiogenesis, sperm heads are embedded in Sertoli cells and then released from seminiferous tubules

Hormonal control of Spermatogenesis:

• GnRH (Gonadotropin Releasing Hormone) by Hypothalamus: Stimulates anterior pituitary to release LH and FSH. (LH = luteinizing hormone, FSH = follicle-stimulating hormone.)
  • LH: Stimulates Leydig cells to produce androgens (Testosterone).
  • FSH: Acts on Sertoli cells to support spermiogenesis.

Structure of Sperm

• Parts: Head, neck, middle piece, and tail.
  • Head: Contains haploid nucleus and acrosome (a cap-like structure filled with hydrolytic enzymes such as acrosin and hyaluronidase).
  • Middle Piece: Contains mitochondria for energy.
  • Tail: Helps in motility.

Semen

• Components: Sperms + Seminal plasma (fluid).
• Normal Male Fertility: Requires 60% normal sperms and 40% motile sperms in semen.

Oogenesis (Formation of Ova)

• Location: Happens in the ovaries.
• Starting Point: Begins very early, during the embryonic development of a female fetus.
• Process:
  • Oogonia (female germ cells) form in fetal ovaries.
  • Oogonia (46 ch.) become primary oocytes (46 ch.) and start meiosis but stop at prophase I and later gets transformed into different follicular stages at puberty as follows.
    • Primary Follicles: Primary oocytes surrounded by granulosa cells called 1° follicle.
    • Secondary Follicles: Primary follicles with more granulosa cells and theca layers.
    • Tertiary Follicles: Have a fluid-filled cavity called antrum.
      • First Meiotic Division: of primary oocyte occurs at 3° follicle stage, forms secondary oocyte and first polar body.
    • Graafian Follicle: Final mature follicle that releases the secondary oocyte during ovulation.

Key Differences Between Spermatogenesis and Oogenesis

Feature	Spermatogenesis	Oogenesis
Process Initiation	Begins at puberty	Begins before birth but pauses temporarily (arrest).
Process Continuity	Continuous process	resumes at puberty
End Products	Four spermatozoa from one spermatogonium	One ovum and polar bodies from one oogonium
Location	Testes	Ovaries
Duration	Takes about 64 days to complete	Spans over years (decades)
Cell Division	Equal cytokinesis	Unequal cytokinesis
Gametocyte	Spermatogonium (Sperm)	Oogonium (Ovum)
Meiotic Division	Symmetrical	Asymmetrical (producing one large ovum and smaller polar bodies)
Hormonal Regulation	Testosterone and follicle-stimulating hormone (FSH)	Estrogen, progesterone, luteinizing hormone (LH), and FSH
Number of Gametes	Continuous production of millions of sperm	Limited number of oocytes, each cycle typically releases one mature ovum
Cytoplasmic Content	Sperm have minimal cytoplasm	Ovum retains most of the cytoplasm
Genetic Variation	Results in genetic variation in sperm	Results in genetic variation in ova

1.6 Pregnancy and Embryonic Development

Implantation and Placenta Formation

• Implantation: After implantation, chorionic villi (finger – like projections) appear on the trophoblast.
  • Surrounded by: Uterine tissue and maternal blood.
• Placenta: Formed by the interdigitation of chorionic villi and uterine tissue.
  • Function:
    • Supplies oxygen and nutrients to the embryo.
    • Removes carbon dioxide and waste.
• Umbilical Cord: Connects placenta to embryo, aiding in transport of substances.

Placenta as Endocrine Tissue

• Hormones Produced:
  • Human chorionic gonadotropin (hCG)
  • Human placental lactogen (hPL)
  • Estrogens and progestogens
• Ovary: Produces relaxin during later pregnancy.
• Other Hormones: Increased levels of cortisol, prolactin, thyroxine, etc.
• Increased Levels: of Estrogens, progestogens, cortisol, prolactin, thyroxine in maternal blood during pregnancy.

Embryonic Development

• Inner Cell Mass:
  • Differentiates into ectoderm (outer layer), endoderm (inner layer), and mesoderm (middle layer).
  • These three layers develop into all the tissues and organs of the developing baby’s body.
  • Stem Cells: found in inner cell mass, capable to form all tissues and organs.

Embryonic Development Stages

• 1 Month: Heart formation; heartbeat detectable.
• 2 Months: Development of limbs and digits.
• 12 Weeks (First Trimester): Formation of major organs, limbs, and external genital organs.
• 5 Months: First fetal movements; hair appears on the head.
• 24 Weeks (Second Trimester): Fine hair covers the body, eyelids separate, eyelashes form.
• 9 Months: Fetus fully developed and ready for delivery.

Fun Fact

• Pregnancy Duration = Gestation Period.
  • Humans: 9 months.
  • Dogs, elephants, cats: (Look up for fun!)

Key Points

• Placenta: Essential for nourishment and waste removal.
• Hormones: Crucial for supporting pregnancy.

1.7 Parturition and Lactation

Parturition (Childbirth)

• Gestation Period: Human pregnancy lasts about 9 months.
• Parturition: The process of delivering the baby or simply Childbirth.
  • Caused/induced By: Strong contractions of the uterus.
  • Mechanism: Complex neuroendocrine process.
• Triggers for Parturition
  • Signals for Parturition: Come from the fully developed fetus and placenta.
  • Foetal Ejection Reflex: Mild uterine contractions start the process. This also stimulates/triggers maternal Pituitary gland to release Oxytocin.
  • Oxytocin: Released from the maternal pituitary gland.
    • Effect: Causes stronger uterine contractions.
    • Feedback Loop: Stronger contractions stimulate further more oxytocin release.
• Delivery Process :
  • Delivery: Baby is expelled through the birth canal.
  • After Delivery: The placenta is also expelled.

Lactation (Milk Production)

• Mammary Glands: Differentiate and start producing milk during pregnancy.
• Colostrum: The first milk produced after birth.
  • Content: Rich in antibodies.
  • Importance: Essential for developing the baby’s immune system.
• Breastfeeding: Recommended for the baby’s health during initial growth.

Key Points

• Oxytocin: Key hormone in inducing labor and contractions.
• Colostrum: First pale yellow colored milk, crucial for newborn immunity.
• Breastfeeding: Essential for a healthy start for the baby as it contains antibodies.

Chapter Summary:

• Humans are sexually reproducing and viviparous.
• The male reproductive system includes:
  • A pair of testes
  • Male sex accessory ducts
  • Accessory glands
  • External genitalia
• Each testis has about 250 compartments called testicular lobules.
• Each lobule contains one to three highly coiled seminiferous tubules.
• Seminiferous tubules are lined by:
  • Spermatogonia
  • Sertoli cells
• Spermatogonia undergo meiotic divisions to form sperms.
• Sertoli cells provide nutrition to dividing germ cells.
• Leydig cells outside the seminiferous tubules:
  • Synthesize and secrete testicular hormones called androgens.
• The male external genitalia is called the penis.
• The female reproductive system includes:
  • A pair of ovaries
  • A pair of oviducts
  • A uterus
  • A vagina
  • External genitalia
  • A pair of mammary glands
• Ovaries produce:
  • Female gamete (ovum)
  • Steroid hormones (ovarian hormones)
• Ovarian follicles are in different stages of development in the stroma.
• Female accessory ducts include the oviducts, uterus, and vagina.
• The uterus has three layers:
  • Perimetrium
  • Myometrium
  • Endometrium
• Female external genitalia include:
  • Mons pubis
  • Labia majora
  • Labia minora
  • Hymen
  • Clitoris
• Mammary glands are a secondary sexual characteristic in females.
• Spermatogenesis produces sperms, transported by male sex accessory ducts.
• A normal human sperm has a head, neck, middle piece, and tail.
• The process of forming mature female gametes is called oogenesis.
• The reproductive cycle in female primates is called the menstrual cycle.
• The menstrual cycle starts after puberty.
• During ovulation, one ovum is released per menstrual cycle.
• Cyclical changes in the ovary and uterus are due to changes in pituitary and ovarian hormone levels.
• After coitus, sperms reach the junction of the isthmus and ampulla.
• Fertilization occurs here, forming a diploid zygote.
• The sperm’s X or Y chromosome determines the embryo’s sex.
• The zygote undergoes mitotic divisions to form a blastocyst.
• The blastocyst implants in the uterus, resulting in pregnancy.
• After nine months, the fully developed fetus is ready for delivery.
• Childbirth, or parturition, is induced by a neuroendocrine mechanism involving cortisol, estrogens, and oxytocin.
• Mammary glands differentiate during pregnancy and secrete milk after childbirth.
• The newborn baby is fed milk by the mother (lactation) during initial growth months.