Reproductive systems produce and deliver gametes
Testes contain seminiferous tubules where sperm develop, supported by Sertoli cells. Leydig cells between tubules produce testosterone. Sperm mature and are stored in the epididymis, travel through the vas deferens and join glandular secretions that form semen.
Ovaries contain follicles with developing oocytes. Oviducts transport the ovulated secondary oocyte and are the usual site of fertilisation. The uterus supports implantation and development; its endometrium changes cyclically under ovarian hormones.
| Feature | Spermatogenesis | Oogenesis |
|---|---|---|
| Location | Seminiferous tubules | Ovarian follicles |
| Cytokinesis | Roughly equal | Unequal; polar bodies form |
| Products per primary cell | Four functional sperm | One large ovum lineage |
| Pattern after puberty | Continuous production | Cyclic maturation of a limited pool |
The ovarian and uterine cycles are coordinated
Early in the cycle, FSH supports follicle growth. Developing follicles release oestrogen, which promotes repair and thickening of the endometrium. At moderate levels, oestrogen contributes to feedback that limits FSH and helps favour one dominant follicle.
Sustained high oestrogen near mid-cycle produces positive feedback and an LH surge. LH triggers ovulation and transformation of the ruptured follicle into the corpus luteum.
Follicles grow and release oestrogen.
Endometrium proliferates.
High oestrogen triggers ovulation.
Progesterone supports the endometrium.
Without pregnancy, menstruation begins.
Progesterone stabilises the post-ovulation uterus
The corpus luteum secretes progesterone and some oestrogen. Progesterone maintains a thick, secretory endometrium and suppresses additional FSH and LH release, reducing the chance of another ovulation during the same cycle.
If implantation does not occur, the corpus luteum degenerates, ovarian hormone levels fall, spiral arteries constrict and the functional endometrial layer is shed. Falling inhibition also allows FSH to begin rising for the next cycle.
Fertilisation combines genomes and activates development
Capacitated sperm reach the oocyte in the oviduct. Enzymes released during the acrosome reaction help sperm penetrate surrounding layers. Fusion triggers changes that reduce polyspermy, and the secondary oocyte completes meiosis II.
Male and female pronuclei contribute haploid chromosome sets to the diploid zygote. Cleavage divisions produce a morula and then a blastocyst, which reaches the uterus and implants in the endometrium.
Gamete membranes and genomes unite.
Rapid mitoses increase cell number.
A compact cell ball forms.
Inner cell mass and outer layer appear.
The blastocyst embeds in endometrium.
Early pregnancy preserves hormonal support
Cells associated with the early embryo secrete hCG, which maintains the corpus luteum. Progesterone therefore remains high and menstruation is prevented. Later, the placenta becomes a major source of hormones supporting pregnancy.
The placenta allows exchange between maternal and fetal circulations without normally mixing the two blood supplies directly. Its large surface area, thin exchange barrier and blood flow support diffusion and transport of gases, nutrients and wastes.
The ideas to carry forward
- Gamete production differs in timing, cytokinesis and number of functional products.
- FSH supports follicles; the LH surge triggers ovulation.
- Progesterone maintains the post-ovulation endometrium.
- hCG preserves the corpus luteum during early pregnancy.
Answer first. Then reveal the marking logic.
01Why can high oestrogen trigger an LH surge even though ovarian hormones often inhibit pituitary activity?2 marks · show the biological link
Answer: Near mid-cycle, sustained high oestrogen changes to positive feedback, strongly stimulating the pathway that produces the LH surge.
02What is the immediate hormonal consequence if no implantation occurs?2 marks · show the biological link
Answer: The corpus luteum degenerates, progesterone and oestrogen fall, and the supported endometrial state breaks down.
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