Unit 5: Reproduction
Reproduction is the biological process by which new individual organisms ("offspring") are produced from their "parents". Human reproduction is sexual, involving a male and a female parent.
1. Male reproductive system of human
The male reproductive system is designed to produce and deliver sperm for fertilization.
- Primary Sex Organ:
- Testes (plural): A pair of organs held in the scrotum (which keeps them at a slightly lower temperature, optimal for sperm production). They produce sperm and the hormone testosterone.
- Accessory Ducts (The "Sperm Pathway"):
- Epididymis: A coiled tube on the back of each testis where sperm mature and are stored.
- Vas Deferens (Ductus Deferens): A tube that carries sperm from the epididymis up into the pelvic cavity.
- Urethra: A tube that runs through the penis, carrying both urine and semen (not at the same time) out of the body.
- Accessory Glands (Produce Seminal Fluid):
- Seminal Vesicles: Produce a fluid rich in fructose (energy for sperm).
- Prostate Gland: Adds a milky fluid that helps activate sperm.
- Bulbourethral Glands: Secrete a clear fluid that lubricates the urethra.
Semen = Sperm + Seminal Fluid (from all three glands)
2. Female reproductive system of human
The female reproductive system is designed to produce eggs (ova), receive sperm, provide a site for fertilization and implantation, nourish the fetus, and give birth.
- Primary Sex Organ:
- Ovaries (plural): A pair of organs that produce eggs (ova) and the hormones estrogen and progesterone.
- Accessory Ducts:
- Fallopian Tubes (Oviducts): Tubes that connect the ovaries to the uterus. The site of fertilization.
- Uterus (Womb): A thick-walled, muscular organ where the fertilized egg (embryo) implants and develops.
- Vagina: A muscular tube that receives the penis during intercourse and serves as the birth canal.
- External Genitalia (Vulva): Includes the labia and clitoris.
3. Gametogenesis, Human sperm and human ovum
Gametogenesis
Gametogenesis is the process of producing haploid gametes (sperm or eggs) from diploid cells through meiosis.
- Spermatogenesis: The production of sperm in the testes. One diploid cell (spermatogonium) undergoes meiosis to produce four small, equal-sized, haploid sperm.
- Oogenesis: The production of an ovum in the ovaries. One diploid cell (oogonium) undergoes meiosis, but the division is unequal. It produces one large, haploid ovum (egg) and 2-3 tiny, non-functional polar bodies that degenerate. This conserves all the cytoplasm and nutrients for the one viable egg.
Human Sperm (Spermatozoon)
- Head: Contains the haploid nucleus (the 23 chromosomes) and is tipped with the acrosome, a cap of enzymes used to penetrate the egg.
- Midpiece: Packed with mitochondria to produce ATP (energy) for swimming.
- Tail (Flagellum): A long, whip-like tail that propels the sperm.
Human Ovum (Egg)
- A large, non-motile, spherical cell.
- Contains the haploid nucleus (23 chromosomes).
- Rich in cytoplasm and yolk, which provide nutrients for the early embryo.
- Surrounded by protective layers, including the jelly-like zona pellucida.
4. Menstrual cycle
The menstrual cycle is the ~28-day hormonal and physiological cycle in the human female, preparing the uterus for a potential pregnancy. If no pregnancy occurs, the uterine lining is shed.
Phases of the Cycle:
- Menstrual Phase (Days 1-5):
- The cycle begins with menstruation (bleeding).
- Hormone levels (estrogen and progesterone) are low, causing the inner lining of the uterus (the endometrium) to break down and be shed.
- Follicular Phase (Days 6-13):
- The pituitary gland releases FSH (Follicle-Stimulating Hormone).
- FSH causes a follicle (containing an egg) in the ovary to mature.
- The maturing follicle produces estrogen, which causes the endometrium to rebuild and thicken.
- Ovulation (Day ~14):
- A surge in LH (Luteinizing Hormone) from the pituitary causes the mature follicle to rupture and release the egg from the ovary.
- This is the most fertile time in the cycle.
- Luteal Phase (Days 15-28):
- The empty follicle transforms into a structure called the corpus luteum.
- The corpus luteum secretes high levels of progesterone, which maintains the thick uterine lining, making it ready for a fertilized egg to implant.
- If no fertilization occurs, the corpus luteum degenerates, progesterone levels drop, and the cycle starts over at Day 1 (menstruation).
5. Fertilisation
Fertilisation is the fusion of the haploid sperm nucleus and the haploid egg nucleus to form a single diploid cell called a zygote (the first cell of a new individual).
The Process:
- Site: Fertilization normally occurs in the fallopian tube (oviduct).
- Sperm Penetration: After ejaculation, sperm swim up the vagina, through the uterus, and into the fallopian tubes. One sperm binds to the egg's outer layer (zona pellucida).
- Acrosome Reaction: The enzymes in the sperm's acrosome (head) are released and digest a path through the egg's protective layers.
- Fusion: The sperm's cell membrane fuses with the egg's cell membrane, and the sperm nucleus enters the egg's cytoplasm.
- Block to Polyspermy: As soon as one sperm enters, the egg's membrane instantly changes, preventing any other sperm from penetrating.
- Zygote Formation: The haploid nucleus of the sperm fuses with the haploid nucleus of the egg, restoring the diploid number of chromosomes (46).
This new zygote immediately begins to divide (cleavage) as it travels down the fallopian tube to implant in the uterine wall, beginning a pregnancy.