Unit 19 — General Embryology · Question Bank

Fertilisation is the single event that converts two haploid gametes into a diploid zygote — the cellular start-line of every human life. The examiner wants three things: a clear definition with site, a sequenced account of the cellular events (capacitation, acrosome reaction, cortical reaction, completion of meiosis II and pronuclear fusion), and an explicit statement of its three biological significances. Fertilisation happens in the ampulla of the oviduct, the lateral widening between fimbriae and isthmus, and must occur within roughly twenty-four hours of ovulation before the oocyte degenerates.

Preparing the sperm — capacitation

Sperm cannot fertilise straight after ejaculation. As they swim through the cervix, uterus and oviduct over a period of about a day, surface glycoproteins that were added in the epididymis are stripped away and membrane cholesterol is redistributed. This functional maturation is called capacitation, and only a capacitated sperm can bind the zona-pellucida glycoprotein ZP3 and undergo the next step. In assisted reproduction this step is mimicked with artificial capacitation media.

Penetration — the acrosome reaction

Binding to ZP3 triggers the acrosome reaction: the outer acrosomal membrane fuses with the sperm plasma membrane and releases hydrolytic enzymes — chiefly acrosin, with hyaluronidase — which dissolve a path through the corona radiata and the zona pellucida. The sperm head corkscrews through and its plasma membrane finally fuses with the oolemma. Only the sperm nucleus and centrioles enter the oocyte; the mid-piece and tail are left behind, which is why mitochondria are maternally inherited.

Blocks to polyspermy — the cortical (zona) reaction

The instant the membranes fuse, the oocyte mounts the cortical reaction: cortical granules just under the oolemma exocytose into the perivitelline space and release enzymes that denature ZP3 and harden the zona. This destroys further sperm-binding sites and ensures monospermy. Without this block the resulting zygote would be triploid, which is uniformly lethal.

Completion of meiosis and pronuclear fusion

The secondary oocyte has been arrested in metaphase II since ovulation. Sperm entry releases that arrest: meiosis II completes, extruding the second polar body and leaving a haploid female pronucleus. The sperm head decondenses to form a haploid male pronucleus. The two pronuclei migrate together; their envelopes break down and the maternal and paternal chromosomes align on a single mitotic spindle. At that instant the diploid number (46 chromosomes) is restored and the zygote exists.

Significance

Fertilisation does three things at once. First, it restores diploidy and combines parental genomes, creating the genetic individual. Second, it determines genetic sex — an X-bearing sperm gives 46,XX (female), a Y-bearing sperm gives 46,XY (male). Third, it initiates cleavage and development — the activation of the egg switches on the embryonic developmental programme and the first mitotic division follows within about a day.

Clinical anchor

Failure of capacitation or acrosome reaction is a cause of male-factor infertility, bypassed in vitro by intracytoplasmic sperm injection (ICSI). Failure of the cortical reaction allows polyspermy and lethal triploidy. If fertilisation succeeds but the conceptus stalls in the tube, an ectopic (tubal) pregnancy results — the commonest site is the ampulla itself, and rupture around 6–8 weeks causes life-threatening haemorrhage.

The first week of human development is the journey of a single zygote down the oviduct, dividing as it goes, until a hatched blastocyst arrives at the uterine wall ready to implant. The whole sequence happens inside the zona pellucida until the very end, so the conceptus does not grow — it just subdivides. An eight-mark answer needs to define each stage, give its day, describe the cellular features, and end with the developmental significance of the trophoblast / inner-cell-mass split.

Cleavage

Cleavage is the specialised mitosis of the zygote: rapid divisions without cell growth, taking place inside the rigid zona pellucida. The daughter cells, called blastomeres, get progressively smaller as their number doubles. The first cleavage produces two cells by about 24–30 hours after fertilisation, the second produces four, the third eight, and so on. Cleavage happens as the conceptus drifts through the oviduct toward the uterus, propelled by ciliary currents and peristalsis.

Morula — the solid mulberry

By day 3, with roughly sixteen blastomeres, the conceptus is a solid spherical cluster called the morula (from Latin morum, mulberry, which it resembles under the microscope). At this stage the outer cells flatten against each other and form tight junctions — the process of compaction — while the inner cells stay rounded. This is the embryo’s first overt commitment decision: the outer cells are committed to becoming trophoblast, the inner cells to becoming the embryo proper.

Blastocyst — the hollow ball

By day 4–5, with the embryo now in the uterine cavity, fluid pumped into the morula by Na+/K+-ATPases in the outer cells coalesces into a single central cavity, the blastocoele. The conceptus now has two clearly distinct cell populations: a flat outer wall, the trophoblast, that encloses the blastocoele, and an eccentric clump on one pole, the inner cell mass (embryoblast). The pole of the blastocyst at which the inner cell mass lies is called the embryonic pole; the opposite end is the abembryonic pole.

Hatching from the zona

Before implantation can begin, the blastocyst has to escape the zona pellucida. Around day 5–6 proteolytic enzymes thin the zona until it ruptures, and the blastocyst emerges naked. Only now can the trophoblast actually contact the endometrium — and the embryonic pole adheres first, so that the inner cell mass is buried deep and the trophoblast spreads outward into the decidua.

Developmental significance — two lineages, two fates

The single most important event of the first week is the segregation of trophoblast vs embryoblast. The trophoblast will form the placenta and extra-embryonic membranes — everything that supports the pregnancy and is discarded at birth. The inner cell mass will form the embryo proper, plus a small amount of extra-embryonic tissue (yolk sac, amnion, allantois). Without that split there is no way to build both an embryo and the placenta needed to feed it.

Clinical anchor

Disorders of cleavage and the blastocyst include monozygotic twinning — a split at the two-cell stage gives dichorionic-diamniotic twins, a split inside the blastocyst at days 4–8 gives monochorionic-diamniotic twins (at risk of twin-twin transfusion), and a split after day 13 gives conjoined twins. Failure of hatching is a recognised cause of implantation failure in IVF; some clinics perform “assisted hatching” on the embryo before transfer.

Implantation is the moment the conceptus stops being a passenger and becomes a tenant: the blastocyst burrows into the endometrium, the trophoblast splits into two functional layers, the maternal stroma transforms into decidua, and hCG appears in maternal blood. An eight-mark answer needs the timing, the site, the two trophoblast layers with their roles, the decidua reaction, and at least one clinical anchor on what happens when implantation goes wrong.

Timing and site

Implantation begins around day 5–6 after fertilisation, when the hatched blastocyst contacts the secretory-phase endometrium, and is complete by about day 11–12, by which time the conceptus is entirely buried below the uterine epithelium and the overlying surface has healed over. The normal site is the upper posterior wall of the uterine body or the fundus, where the endometrium is thickest and best perfused. The polar trophoblast — the part directly over the inner cell mass — adheres first, ensuring the embryo proper is buried deep in the decidua and the trophoblast spreads outward into it.

Differentiation of the trophoblast

Almost immediately on contact the trophoblast splits into two layers. The inner cytotrophoblast retains discrete cells with clear boundaries and is mitotically active — it acts as the stem-cell pool. The outer syncytiotrophoblast forms when cytotrophoblast cells fuse, producing a multinucleate sheet with no internal cell membranes. The syncytiotrophoblast is the invasive front: it secretes proteolytic enzymes that erode the endometrium and the walls of maternal capillaries, and within its substance small spaces called lacunae appear and quickly fill with maternal blood. By the end of week 2 these lacunae have anastomosed with maternal spiral arteries and veins, establishing the first primitive utero-placental circulation.

Endocrine role of the syncytiotrophoblast

From day 8 the syncytiotrophoblast secretes human chorionic gonadotropin (hCG), a glycoprotein that binds LH receptors on the corpus luteum and rescues it from regression. The corpus luteum then continues to make progesterone, which maintains the decidua until the placenta itself takes over hormone production around week 8–10. Urinary hCG is the basis of the home pregnancy test, so detectable from about ten days after fertilisation. The syncytiotrophoblast also begins to produce hPL, oestrogen and progesterone as pregnancy progresses.

Decidua reaction

The endometrium does not simply allow itself to be invaded — it transforms. Stromal cells swell and accumulate glycogen and lipid in their cytoplasm, becoming rounded decidual cells; uterine glands expand their secretion; the vascular bed dilates further. The whole functional layer of the endometrium is now called the decidua. By location it is divided into three named regions: decidua basalis, lying deep to the conceptus, which will form the maternal part of the placenta; decidua capsularis, covering it on the luminal side, which atrophies as the fetus grows; and decidua parietalis, lining the rest of the uterine cavity.

Clinical anchor

Implantation only succeeds when timing, site and signalling all line up. If the conceptus stalls in the oviduct, you get an ectopic pregnancy — tubal implantation in the ampulla being the commonest variety. The thin tubal wall cannot expand to house a growing placenta, so the pregnancy ruptures around 6–8 weeks with catastrophic intraperitoneal haemorrhage. Risk factors include prior chlamydial salpingitis and tubal surgery. If the blastocyst implants too low in the uterus, near the internal os, the resulting placenta covers the cervical opening — placenta praevia, presenting as painless third-trimester bleeding. Abnormal trophoblastic proliferation produces a hydatidiform mole, classically with grape-like villi and very high hCG.

The conversion of a bilaminar disc into a trilaminar disc is the single most important developmental event in the embryonic period — it is the moment when the body plan is laid down. An eight-mark answer needs the bilaminar disc of week 2 as background, the mechanism of gastrulation through the primitive streak in week 3, the identity of the three layers, and a tabulation of their major derivatives. Without this scaffold none of the systemic embryology that comes later makes sense.

Bilaminar disc — week 2

Through week 2 the inner cell mass reorganises into a flat plate only two cells thick. The dorsal cells become tall columnar epiblast; the ventral cells become small cuboidal hypoblast. Above the epiblast the amniotic cavity opens up, roofed by amnioblasts. Below the hypoblast a layer of cells migrates around the inside of the blastocoele to line the primary yolk sac. The bilaminar disc ends up suspended between these two cavities, hung in the larger chorionic cavity by the connecting (body) stalk — the future core of the umbilical cord. This is the “week of twos.”

Gastrulation — week 3

In week 3 a midline thickening of the caudal epiblast called the primitive streak appears and elongates cranially. Its cranial end expands into the primitive node; along its length runs the primitive groove, with a deeper primitive pit in the node. Epiblast cells stream toward the streak, dive down through the groove, and ingress between the existing layers. A first wave displaces the hypoblast and forms definitive endoderm. A second wave inserts itself between endoderm and epiblast to form mesoderm. The epiblast cells that never moved are renamed ectoderm. From the primitive pit a cord of cells migrates cranially in the midline to become the notochord, which induces neurulation. All three definitive germ layers therefore derive from the epiblast.

Ectoderm derivatives

Ectoderm splits early into surface ectoderm and neuroectoderm. Surface ectoderm gives the epidermis and its appendages (hair, nails, sweat and sebaceous glands, mammary glands), the lens of the eye, the inner ear epithelium, the anterior pituitary (Rathke pouch) and tooth enamel. Neuroectoderm folds into the neural tube — the entire central nervous system, retina and posterior pituitary — while cells at the crest of the closing folds delaminate as neural crest: peripheral ganglia, Schwann cells, melanocytes, the adrenal medulla, parafollicular C cells of the thyroid, odontoblasts, aorticopulmonary septum and most of the cartilage of the pharyngeal arches.

Mesoderm derivatives

Mesoderm organises paraxially, intermediately and laterally on each side of the notochord. Paraxial mesoderm segments into somites — sclerotome (vertebrae and ribs), myotome (skeletal muscle), dermatome (dermis of the back). Intermediate mesoderm forms the urogenital system — kidneys, ureters, gonads and ducts. Lateral plate mesoderm splits into a parietal sheet (body wall) and a visceral sheet (gut wall); from these come the heart, blood vessels, blood cells, smooth muscle and connective tissue of the gut, and the serous membranes that line the body cavities. The notochord itself persists as the nucleus pulposus of each intervertebral disc.

Endoderm derivatives

Endoderm lines everything that handles food, air or urine. It forms the epithelium of the entire gastrointestinal tract from the pharynx to the upper anal canal, the lining of the respiratory tract from larynx to alveoli, the parenchymal cells of the liver and pancreas, the follicular cells of the thyroid, the parathyroids, the thymic epithelium, the epithelium of the bladder and most of the urethra, and the lining of the auditory tube and tympanic cavity.

Clinical anchor

Gastrulation defects matter clinically. If the primitive streak fails to regress, remnants can grow into a sacrococcygeal teratoma, the commonest tumour of the newborn, containing all three germ layers. Defects of paraxial mesoderm cause vertebral and rib anomalies; defects of intermediate mesoderm cause renal agenesis. The VACTERL constellation (Vertebral, Anal, Cardiac, TracheoEsophageal, Renal, Limb) clusters because the affected structures all arise around week 4.

The placenta is the temporary organ where fetal villi dipped in maternal blood do all the work of a lung, a gut and a kidney for the fetus. An eight-mark answer needs the dual fetal and maternal contributions, the circulation that brings maternal and fetal blood close without mixing, the four-layered placental barrier (and its term thinning), the principal hormonal functions, and a clinical anchor on placental pathology.

Fetal part — chorion frondosum

The fetal contribution to the placenta is the chorion frondosum — the pole of the chorion that faces the decidua basalis and develops lush villi. It is built around a chorionic plate of trophoblast plus extra-embryonic mesoderm, covered on its fetal surface by amnion. From the chorionic plate, stem villi branch into terminal villi bathed in maternal blood. The umbilical cord is attached to the fetal surface either centrally or eccentrically, and within it run two umbilical arteries and one umbilical vein embedded in Wharton jelly. The rest of the chorion, facing the decidua capsularis, has its villi regress to become the smooth chorion laeve.

Maternal part — decidua basalis

The maternal contribution is the decidua basalis, the part of the gravid endometrium that lies deep to the conceptus. Wedge-like placental septa project from the basalis up toward (but not reaching) the chorionic plate, incompletely subdividing the inter-villous space into 15–20 lobules called cotyledons, each containing a tuft of villi fed by its own spiral artery. The maternal surface of the delivered placenta is rough and cobblestoned by these cotyledons.

Placental circulation

Maternal blood and fetal blood come into intimate contact but never mix. Maternal blood arrives via the spiral arteries of the endometrium, jets into the intervillous space, percolates over the surfaces of the villi where gas and nutrient exchange occur, and drains away through endometrial veins. Fetal blood, by contrast, stays inside its own vessels throughout: deoxygenated blood travels from fetus to placenta in the two umbilical arteries, distributes into the villous capillaries, exchanges across the placental barrier, and returns oxygenated to the fetus through the single umbilical vein.

Placental barrier

The placental barrier is the set of layers separating maternal from fetal blood. From the maternal side inward there are four layers: (1) syncytiotrophoblast, (2) cytotrophoblast with its basement membrane, (3) loose connective tissue of the villous core, and (4) the endothelium of the fetal capillary with its basement membrane. As pregnancy advances the cytotrophoblast layer thins out and is almost lost, so by the third trimester the barrier is mainly syncytiotrophoblast applied directly to fetal capillary endothelium — this thinning increases diffusion efficiency to match the growing fetus’s demand.

Functions

Two big categories. Material exchange: oxygen, glucose, amino acids, fatty acids and immunoglobulin G pass to the fetus; CO₂, urea and metabolic waste pass back to the mother. Endocrine: the syncytiotrophoblast secretes hCG in the first trimester to maintain the corpus luteum, human placental lactogen (hPL) to drive maternal insulin resistance and make glucose available to the fetus, and rising amounts of oestrogen and progesterone, eventually taking over hormonal maintenance of pregnancy from the corpus luteum.

Clinical anchor

The placenta admits some drugs and pathogens — rubella, CMV, syphilis, alcohol, valproate, warfarin, retinoic acid — and teratogens act most strongly during organogenesis (weeks 3–8). Implantation low over the internal os gives placenta praevia (painless third-trimester bleeding). Premature separation of a normally sited placenta from the decidua basalis is placental abruption (painful third-trimester bleeding). Abnormal trophoblast proliferation gives a hydatidiform mole, with risk of progression to choriocarcinoma.