Reproductive Systems, Development & Genetics

Reproductive Systems Overview

• Gonads: Primary sex organs that produce gametes.

• Gametes: Sex cells (egg and sperm).

• Zygote: A single cell resulting from the fusion of an egg and a sperm.

• Secondary Sex Organs: Organs that aid in the growth and maturation of gametes and developing offspring.

Male Reproductive System

Structure of Sperm

• Size: About $0.06 ext{mm}$ long.

• Acrosome: Contains enzymes vital for fertilization.

• Head: Contains the genetic material (nucleus).

• Middle Piece: Contains numerous mitochondria, which provide energy through respiration for sperm motility.

• Cytoplasm: Little cytoplasm, leading to a short survival period; receives nourishment from semen.

Organs, Structure, and Function

• Testes:

  • Produce sperm (spermatozoa).

  • Located in the scrotum.

  • Suspended outside the body to maintain a temperature of approximately $ext{2}^ ext{o} ext{C}$ lower than body temperature, which is essential for sperm production and development.

  • Divided into compartments called lobules, which are filled with seminiferous tubules.

  • Seminiferous tubules are lined with cells that produce sperm cells.

  • These tubules join to form the epididymis.

  • Interstitial cells (Leydig cells) located between the seminiferous tubules secrete testosterone.

• Epididymis:

  • A highly folded tube (approximately $ext{5m}$ long) found at the rear of each testis.

  • Sperm are stored here for up to a month to mature.

  • Joins the vas deferens (sperm duct).

• Vas Deferens (Sperm Duct):

  • Loops around near the bladder.

  • Joins the urethra.

• Urethra:

  • Runs from the bladder to the end of the penis.

  • Transports both urine and sperm.

• Scrotum:

  • A skin-covered pouch containing two sacs (one for each testis).

  • Can contract when cold to bring testes closer to the body for warmth.

  • Can relax when warm to move testes away from the body for cooling.

• Semen (Fluid):

  • Fluid used to transport and nourish sperm.

  • Produced by three glands:

    • Seminal Vesicles: Secrete a thick fluid rich in sugars, contributing about ext{60%} of semen volume.

    • Prostate Gland: A donut-shaped structure that secretes a thin, milky, alkaline fluid.

    • Cowper's Glands (Bulbourethral Glands): Two small glands (size of a pea) that secrete a small amount of clear liquid that acts as a lubricant.

• Erectile Tissue:

  • Connective tissue with a rich blood supply.

  • Contains spongy spaces that fill with blood during sexual arousal, causing the penis to become stiff and erect.

Female Reproductive System

Anatomy Overview (Based on Diagrams)

• Uterine tube (Fallopian tube)

• Ovary

• Uterus (Body, Endometrium, Myometrium, Perimetrium)

• Cervix

• Vagina

• Clitoris

• Labium minus (Labia minora)

• Labium majus (Labia majora)

• Fimbriae

• Oocyte

• Follicle

• Mons pubis

• Urethral opening

• Hymen

• Vaginal opening

• Anus

• Vesicouterine pouch

Organs, Structure, and Function

• Ovary:

  • Composed of connective tissue called stroma.

  • Stroma is surrounded by a layer of germ cells, each enclosed in a follicle (which are at different stages of development).

  • A follicle matures and eventually releases an egg (oocyte) into the uterine tube.

• Fimbriae:

  • Finger-like structures at the end of the uterine tube.

  • Guide the egg from the ovary into the uterine tube.

• Uterine Tube (Fallopian Tube/Oviduct):

  • Contain beating cilia that help carry eggs from the ovary to the uterus.

  • Contraction of smooth muscle lining also aids in moving the egg along.

• Uterus:

  • Held in position by ligaments.

  • Made up of smooth muscle with a soft mucous membrane lining called the endometrium.

  • Protects and nourishes a developing foetus.

• Cervix:

  • The lower, narrow end of the uterus that connects to the vagina.

• Vagina:

  • A muscular structure with a mucous membrane.

  • Can stretch up to $ext{10cm}$ in length.

  • Receives the penis during sexual intercourse and extends during childbirth.

  • The external opening is partially covered by a fold of tissue called the hymen, which may stretch or tear during first sexual intercourse.

• Vulva:

  • The exterior region of the female genitalia.

  • Labia Majora: Two fleshy folds of skin, composed of fat and fibrous tissue, containing glands that secrete oils.

  • Labia Minora: Two smaller folds of pinkish skin, without fat, located within the labia majora.

  • Clitoris: Contains erectile tissue, nerves, and blood vessels. Highly sensitive to touch and fills with blood when stimulated.

Spermatogenesis & Oogenesis

Gametogenesis

• The process by which gametes (egg and sperm) mature.

• Two types:

  • Spermatogenesis: Formation of spermatozoa (sperm).

  • Oogenesis: Formation of ova (eggs).

Spermatogenesis (Sperm Formation)

• Location: Occurs in the seminiferous tubules of each testis.

• Initial Cells: The tubules are lined with immature spermatogonia cells (diploid, $2n$).

• Mitosis: From birth, spermatogonia divide by mitosis, providing a continuous source of new cells.

• Maturation Process:

  • From puberty, some spermatogonia daughter cells undergo growth to become primary spermatocytes.

  • Primary spermatocytes undergo Meiosis I.

  • This produces two secondary spermatocytes (haploid, $n$).

  • Secondary spermatocytes undergo Meiosis II to produce two spermatids (haploid, $n$) each (total of four spermatids from one primary spermatocyte).

  • Spermatids then mature into functional spermatozoa (sperm).

• Spermatid Maturation Time: Takes about $ext{72 days}$.

• Sperm Structure Recap:

  • Tail forms.

  • Nourished by cells in the seminiferous tubules.

  • About $ext{0.06mm}$ long.

  • Acrosome contains enzymes.

  • Head contains genetic material.

  • Middle piece contains mitochondria for energy.

  • Little cytoplasm for short survival, nourished by semen.

Oogenesis (Egg Formation)

• Initial Cells: Millions of oogonia develop in the ovaries of an unborn female baby.

• Mitosis: At birth, females have several thousand oogonia, created by mitosis.

• Before Birth Stages:

  • Oogonia grow.

  • These growing oogonia become primary oocytes.

  • Primary oocytes begin (but do not complete) prophase I of Meiosis I.

  • Each primary oocyte is surrounded by a single layer of cells to form a primary follicle.

• Puberty Stages:

  • From puberty, follicles grow and mature, completing Meiosis I.

  • Meiosis I produces two cells of unequal size:

    • Secondary oocyte: Receives half the number of chromosomes ($n$) and the majority of the original cell's cytoplasm.

    • First polar body: Receives half the number of chromosomes ($n$) and very little of the original cell's cytoplasm.

  • The secondary oocyte then starts Meiosis II but stops at metaphase II.

  • Ovulation: When ovulation occurs (egg released), the mature follicle ruptures, releasing the secondary oocyte and the first polar body into the uterine tube.

  • Fertilization: If the secondary oocyte is fertilized by a sperm, Meiosis II is quickly completed.

    • This completion again produces two cells of unequal size:

      • One cell (an ootid) develops into a mature ovum (egg).

      • The other cell develops into a secondary polar body.

    • The first polar body may also undergo Meiosis II to produce two additional secondary polar bodies.

  • Polar Body Fate: All polar bodies eventually disintegrate at different stages.

Comparison of Oogenesis and Spermatogenesis

Hormone Regulation and Puberty

Hormonal Control of the Reproductive System

• The reproductive system is controlled by the release of hormones from endocrine glands into the blood.

• Hormones are proteins coded for by genes.

• Hormones effect:

  • Growth and development.

  • Body processes.

  • Behaviour (especially related to reproduction).

• The pituitary gland secretes hormones (e.g., FSH, LH) that stimulate the reproductive system or specific target organs (testes and ovaries).

• Testosterone (males) and oestrogen (females) are the primary hormones that initiate puberty and the development of secondary sexual characteristics.

Secondary Sexual Characteristics at Puberty

Key Reproductive Hormones

Hormone Pathways

Hormones and the Female Reproductive System

• FSH (Follicle-stimulating hormone) stimulates the growth of ovarian follicles in the ovary.

• Developing follicles secrete oestrogens, which stimulate the growth and thickening of the endometrium (uterine lining).

• A surge in LH (Luteinising hormone) and FSH levels in the middle of the cycle induces ovulation (release of the egg).

• After ovulation, the ruptured follicle develops into the corpus luteum.

• The corpus luteum secretes oestrogens and progesterone.

• Progesterone maintains the endometrium, making it ready for implantation of a fertilized egg.

• If the egg is fertilized:

  • The egg enters the uterine tube and implants in the uterine wall.

  • The developing embryo secretes chorionic gonadotropin (HCG), which maintains the corpus luteum.

  • This ensures continued secretion of oestrogen and progesterone to support pregnancy.

• If the egg is NOT fertilized:

  • The corpus luteum degenerates.

  • Levels of oestrogen and progesterone decrease.

  • This leads to the breakdown of the endometrium and menstruation.

  • A new cycle then begins.

Hormones and the Male Reproductive System

• FSH (Follicle-stimulating hormone) acts on the seminiferous tubules to stimulate spermatogenesis (the development of immature sperm cells into mature spermatozoa).

• LH (Luteinising hormone) acts on the interstitial cells of the testes.

• Interstitial cells produce and secrete testosterone.

• Testosterone is responsible for the maintenance of male reproductive organs and stimulates sex drive.

Ovarian and Menstrual Cycles

Ovarian Cycle

• Duration: Typically lasts about $ext{28 days}$.

• Stages of Follicle Development:

  • Oogonia: Germ cells present before birth.

  • Primary follicle: Contains a primary oocyte surrounded by a single layer of cells (at prophase I of meiosis).

  • Secondary follicle: Contains a secondary oocyte (which completes Meiosis I during puberty).

  • Mature follicle (Graafian follicle): Grows and moves towards the surface of the ovary.

• Ovulation:

  • The mature follicle bursts, releasing the egg (secondary oocyte) into the uterine tube.

• Corpus Luteum Formation and Function:

  • The ruptured follicle collapses, and the blood within it clots.

  • The remaining follicle cells absorb the clot and enlarge to form a cream-coloured body called the corpus luteum.

  • The corpus luteum secretes hormones (oestrogen and progesterone) that affect the development of the uterus lining.

• Fate based on Fertilization:

  • If the ovum is NOT fertilized:

    • The corpus luteum reaches maximum development approximately $ext{8-10 days}$ after ovulation, then degenerates into scar tissue called the corpus albicans, which eventually disappears.

    • Another ovarian cycle begins.

  • If the ovum is fertilized:

    • The corpus luteum matures and continues to produce hormones, preventing breakdown of the endometrium.

    • The ovarian cycle ceases.

    • Its development continues for about $ext{3 months}$ into pregnancy, remaining present through childbirth.

    • The ovarian cycle typically resumes after breastfeeding has ceased.

Menstrual Cycle

• Duration: The bleeding phase (menstruation) lasts about $ext{7 days}$.

• Changes in the Endometrium:

  • While the ovarian follicle is maturing, the endometrium (lining of the uterus) becomes thicker and softer in preparation for the implantation of a developing embryo.

  • After ovulation, the endometrium continues to thicken, and glands within it secrete a glycogen-rich mucus.

• If Egg Not Fertilized:

  • The corpus luteum degenerates.

  • The reduction in hormone levels (oestrogen and progesterone) causes the endometrium to stop thickening and begin to break down.

  • Approximately $ext{14 days}$ after ovulation, blood, mucus, and the cells lining the endometrium are shed and released through the vagina. This process is called menstruation or a menstrual period.

Phases of the Menstrual Cycle

• Menarche: The first menstruation at the beginning of puberty.

• Menopause: The stage where the menstrual cycle becomes irregular and eventually ceases.

Interplay of Hormones in the Female Cycle

• FSH stimulates the growth of ovarian follicles, which in turn secrete oestrogen.

• As follicles develop, the rising oestrogen levels stimulate the growth of the endometrium.

• The levels of LH and FSH peak in the middle of the cycle, with a sudden surge in LH just before ovulation, which induces ovulation.

• After ovulation, the ruptured follicle develops into the corpus luteum.

• The corpus luteum secretes progesterone, which maintains the endometrium, preparing it for implantation.

• In the absence of fertilization, the corpus luteum breaks down into the corpus albicans, and progesterone levels decrease.

• This decrease in progesterone leads to the disintegration of the endometrium and the occurrence of menstrual flow.

Sexual Intercourse and Fertilisation

Sexual Intercourse

• The method by which the egg is brought into contact with the sperm.

• Ejaculation: The rhythmic contractions of the seminal vesicles, prostate gland, Cowper's glands, epididymis, and vas deferens (sperm duct) that occur during penetration of the vagina.

• Orgasm: The climax of sexual excitement.

  • In males, it involves the ejaculation of semen.

  • In females, it does not involve ejaculation but may lead to increased secretion of cervical mucus.

• Insemination: The ejaculation of sperm into the vagina.

Fertilisation

• Sperm Journey:

  • Sperm travel up the cervix, through the uterus, and into the uterine tubes within minutes.

  • This process is aided by muscular contractions of the uterus and uterine tubes.

  • While $ext{250-300 million}$ sperm are released during ejaculation, only a few thousand reach the egg due to high sperm mortality.

• Location: Fertilisation typically occurs in the uterine tube.

• Egg Covering:

  • The mature egg is surrounded by a layer of follicle cells called the corona radiata, which are held together by an acidic matrix.

  • The enzymes in a single sperm are usually insufficient to break down this acid; instead, thousands of sperm work together to weaken the cells of the corona radiata.

• Sperm Penetration:

  • Usually, only one sperm successfully penetrates the egg.

  • Upon penetration, the egg is stimulated to form a membrane around itself, preventing any further sperm from entering (polyspermy).

• Nuclear Fusion:

  • When the sperm enters the egg, only its head (containing the male pronucleus) moves through the cytoplasm.

  • This penetration also stimulates the completion of the egg's second meiotic division (the secondary oocyte completes Meiosis II).

  • The nucleus of the egg develops into the female pronucleus, which then fuses with the male pronucleus to form the zygote (a diploid cell, $2n$).

Development of an Embryo

Stem Cells

• A fertilized egg (zygote) is a totipotent stem cell, meaning it has the potential to create any type of cell required for development, including the embryo itself and the embryonic membranes.

• Stem cells are capable of repeated mitosis (proliferation) and can differentiate into specialized cells.

• Identical (monozygotic) twins occur when two totipotent cells separate early in development and each develops into a separate, identical embryo.

• During embryonic development, the outer layer of cells forms the placenta (for support and development), while the inner cells develop into the foetus.

• Differentiation into specialized daughter cells occurs through the activation of different genes, even though all cells contain the same DNA.

Types of Stem Cells

• Totipotent stem cells: Possess the ability to form the entire embryo and all embryonic membranes (e.g., placenta).

• Pluripotent stem cells: Have the ability to form all cell types of the body but not the embryonic membranes.

• Multipotent stem cells: Have the ability to form into a more limited range of specialized cells within a specific lineage (e.g., blood stem cells).

Implantation

• After fertilization, the dividing embryo forms a blastocyst.

• Implantation is the process where the blastocyst sinks into the endometrium of the uterus, establishing contact with maternal glands and blood vessels to gain nourishment.

Corpus Luteum (Post-Fertilization Role)

• The corpus luteum continues to produce hormones (oestrogen and progesterone) to prevent the endometrium from breaking down until the developing placenta takes over this hormonal production.

Germ Layers, Embryonic Membranes, and the Placenta

Embryonic Membranes

• Amnion: A membrane surrounding the embryo.

  • It ruptures just before childbirth, releasing the amniotic fluid.

• Amniotic Fluid:

  • Protects the embryo by acting as a shock absorber.

  • Helps maintain a stable temperature for the embryo.

  • Allows for foetal movement and development.

• Chorion: Formed from the outer cells of the blastocyst.

  • Becomes the main part of the foetal portion of the placenta.

The Placenta

• Chorionic Villi:

  • Finger-like projections containing numerous foetal blood vessels that develop from the outer cells of the blastocyst.

  • They penetrate the endometrium and become surrounded by pools of the mother's blood.

  • Crucially, foetal and mother's blood do not mix; exchange occurs across the villi membranes.

• Umbilical Cord:

  • Attaches the foetus to the placenta.

  • Contains two umbilical arteries (carrying deoxygenated blood and waste products from the foetus to the chorionic villi).

  • Contains one umbilical vein (carrying oxygenated blood and nutrients from the placenta to the foetus).

• Uterine Arteries and Veins:

  • The mother's blood is transported to spaces within the placenta via the uterine arteries.

  • Waste materials are removed from the placenta by the uterine veins.

Roles and Functions of the Placenta

Embryo to Foetus: Key Milestones

$ext{3-4 Weeks}$ Gestation

• Neural tube begins forming (precursor to brain & spinal cord).

• Muscle segments (somites) start to appear.

• Primitive heart and liver begin development.

• A tail is visible.

• Pharyngeal arches and clefts appear (will develop into future face & throat structures).

$ext{5-6 Weeks}$ Gestation

• About $ext{30 pairs}$ of mesodermal blocks are present (precursors to future muscles & vertebrae).

• The brain shows early development.

• Arm and leg buds start to form.

• The heart is beating.

• The basic body plan is taking shape.

$ext{7-8 Weeks}$ Gestation

• The embryo takes on a recognizably human form.

• All major organs are present but are not yet fully functional.

• The head is disproportionately large, about half the body size; eyes move forward.

• Jaws, nose, and earlobes begin to form.

• Arms and legs become well-proportioned; fingers and toes are clearly formed.

• External sex organs become evident (allowing identification of male/female).

• At the end of the $ext{8th week}$, the embryo is now officially called a foetus.

$ext{3 Months}$ Gestation

• The foetus can begin to move its arms and legs.

• Fingers and toes are fully formed.

• Facial features become more distinct.

• External genitalia are clearly distinguishable.

• Kidneys begin functioning, and urine is excreted into the amniotic fluid.

• The foetus begins to make reflex movements.

Labour and Childbirth

Pregnancy, Labour, and Parturition

• Pregnancy (Gestation): The period from conception to birth.

• Labour: The stages of preparation for birth.

• Parturition: The act of birth itself.

Changes Leading to Labour

• Hormone Changes:

  • Cause ligaments of the pelvis to soften, making them more flexible.

  • Increase the uterus's response to stimuli.

  • Strengthen uterine muscle contractions.

• Position of Foetus (Engagement):

  • The foetal head settles onto the pelvis.

  • Knees are drawn up.

  • The head presses against the mother's bladder and bowel.

• Labour Pains:

  • Begin in the final $ext{3 months}$ of pregnancy as weak, irregular contractions, which gradually become stronger and more regular.

• Braxton Hicks Contractions:

  • Also known as