Human Embryonic Development
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Direct answer
Fertilisation occurs at the ampullary-isthmic junction of the fallopian tube, where a capacitated sperm undergoes the acrosomal reaction, releases enzymes to penetrate the corona and zona pellucida, and fuses with the egg membrane; the resulting cortical reaction hardens the zona pellucida and blocks any second sperm — the guarantee of monospermy. The zygote cleaves into a morula as it travels to the uterus, hollows into a blastocyst of trophoblast, inner cell mass and blastocoel, and implants in the endometrium; chorionic villi of the trophoblast interdigitating with uterine tissue form the placenta, which exchanges nutrients and wastes by diffusion and secretes hCG, hPL, oestrogens, progesterone and relaxin. After roughly nine months' gestation, the foetal ejection reflex triggers oxytocin-driven labour that delivers the baby.
What you must remember
- Site and setup: the ovum is released at ovulation and meets sperm at the ampullary-isthmic junction; sperm must first undergo capacitation in the female tract to become fertilisation-competent.
- Monospermy machinery: the acrosomal reaction digests the egg's envelopes; sperm entry induces the cortical reaction, with cortical granules altering the zona pellucida so no further sperm can bind — a one-sperm policy enforced chemically.
- Early staging: cleavage gives a 16-cell morula in the tube; the morula becomes a blastocyst (trophoblast outer layer, inner cell mass, fluid-filled blastocoel) before implantation in the uterine endometrium.
- Placenta business: chorionic villi (foetal) interdigitate with uterine (maternal) tissue; nutrients, oxygen and wastes cross by diffusion; the umbilical cord connects foetus to placenta.
- Placental hormones: human chorionic gonadotropin, human placental lactogen, relaxin, oestrogens and progesterone — hCG maintains the corpus luteum in early pregnancy and is the molecule urine pregnancy kits detect.
- Stem-cell note: cells of the inner cell mass are pluripotent stem cells, the "embryo proper" that forms all body tissues.
- Parturition cascade: a fully developed foetus signals via mild uterine contractions; the foetal ejection reflex prompts maternal oxytocin release, and oxytocin-driven contractions in positive feedback end in delivery, followed by the placenta as afterbirth; first milk, colostrum, is antibody-rich.
The first fortnight, step by step
Count the days like an embryologist. Day zero: fertilisation at the ampullary-isthmic junction restores diploidy and the zygote forms; completion of the female's second meiotic division occurs only at this moment, releasing the second polar body. Days one to three: cleavage divisions with no growth subdivide the cytoplasm as the embryo drifts through the isthmus, reaching the compacted morula. Around days four to five: fluid enters and the blastocyst cavitates — an outer trophoblast destined to become placenta and the inner cell mass destined to become the embryo. About day six to seven: the hatched blastocyst orients and implants in the receptive endometrium, trophoblast invading and differentiating; soon after, its chorionic villi lace into maternal lacunae and hCG rises steeply enough to flag a pregnancy test on a missed period. From there the disc-shaped embryo gastrulates into three germ layers whose fates — ectoderm to nervous system and skin, mesoderm to muscle, skeleton and blood, endoderm to gut and glands — occupy the organogenetic weeks that follow.
Where candidates slip
Three recurring errors: placing fertilisation in the uterus instead of the ampullary-isthmic junction; crediting the zona block to a membrane potential rather than the cortical reaction's chemical modification; and listing hCG among ovarian hormones — it is placental and exists to rescue the corpus luteum so progesterone keeps the endometrium alive. NEET statement sets also test that labour begins with a foetal signal (the foetal ejection reflex) and that oxytocin from the maternal pituitary amplifies it as positive feedback. Finally, keep placenta and umbilical cord roles distinct: exchange by diffusion happens in the villi, not the cord, and the placenta is a joint foetal-maternal organ.
Frequently asked questions
Where does fertilisation normally occur in humans?
At the ampullary-isthmic junction of the fallopian tube, where the ovulated secondary oocyte meets capacitated sperm.
How does the egg prevent entry of more than one sperm?
Sperm entry triggers the cortical reaction, releasing granules that modify the zona pellucida into a barrier against additional sperm — the block to polyspermy.
Which cells of the blastocyst form the placenta, and which form the embryo?
The outer trophoblast (with its chorionic villi) builds the placenta, while the inner cell mass, pluripotent stem cells, forms the embryo proper.
Which hormone do pregnancy tests detect, and what is its function?
Human chorionic gonadotropin secreted by the placenta; it maintains the corpus luteum so progesterone and oestrogen production continues in early pregnancy.
What initiates parturition at term?
Signals from the fully developed foetus and placenta start mild uterine contractions, and the foetal ejection reflex releases oxytocin from the maternal pituitary, driving labour in a positive-feedback cascade.