🧫 Week 1 · Beginning of development
fertilization → blastocyst
Fertilization occurs in the ampulla of the uterine tube. The secondary oocyte completes meiosis II, and the male and female pronuclei fuse to form a diploid zygote.
🔬 Sperm modifications
Capacitation ~7 h – removal of surface glycoproteins from the acrosomal membrane.
Acrosome reaction – release of hyaluronidase and proteases to penetrate the corona radiata and zona pellucida.
Cortical reaction – prevents polyspermy by hardening the zona pellucida.
Capacitation ~7 h – removal of surface glycoproteins from the acrosomal membrane.
Acrosome reaction – release of hyaluronidase and proteases to penetrate the corona radiata and zona pellucida.
Cortical reaction – prevents polyspermy by hardening the zona pellucida.
Cleavage and blastocyst formation
- Cleavage: rapid mitotic divisions produce blastomeres; the embryo remains within the zona pellucida.
- Morula (≈32 cells) forms by day 3–4.
- Blastocyst (day 5): fluid accumulates, forming a cavity. Inner cell mass (embryoblast) → embryo; outer cell mass (trophoblast) → placenta.
- Zona pellucida degenerates (hatching) to allow implantation.
Implantation window – normally in the posterior uterine wall, during the secretory (progestational) phase. The embryonic pole implants first.
Trophoblast differentiation
- Cytotrophoblast – mitotically active inner layer.
- Syncytiotrophoblast – multinucleated, erosive outer layer; no mitosis. Produces hCG.
📌 high yield: hCG from syncytiotrophoblast maintains the corpus luteum → progesterone production. Detectable in serum/urine early pregnancy.
🥯 Week 2 · Bilaminar embryo
epiblast · hypoblast
The embryoblast rearranges into two layers:
- Epiblast – columnar cells; gives rise to all three germ layers (via gastrulation) and forms the amniotic cavity.
- Hypoblast – cuboidal cells; forms the primary yolk sac (exocoelomic cavity).
Prechordal plate – a localized thickening formed by fusion of epiblast and hypoblast; marks the future cranial end and the future mouth.
Extraembryonic mesoderm (derived from epiblast) splits into:
• Somatic (parietal) – lines the trophoblast, covers the amnion, and forms the connecting stalk.
• Visceral (splanchnic) – covers the yolk sac.
• Somatic (parietal) – lines the trophoblast, covers the amnion, and forms the connecting stalk.
• Visceral (splanchnic) – covers the yolk sac.
Chorionic cavity – the extraembryonic coelom; its wall (chorion) is composed of extraembryonic somatic mesoderm + cytotrophoblast + syncytiotrophoblast.
Hematopoiesis – first occurs in the mesoderm surrounding the yolk sac (weeks 3–6), then liver, spleen, thymus, and finally bone marrow.
🌊 Week 3 · Gastrulation & germ layers
primitive streak · notochord
Gastrulation – the process that establishes the three germ layers: ectoderm, mesoderm, and endoderm. Begins with the formation of the primitive streak on the epiblast.
Epiblast cells→
ingress through primitive streak→
endoderm / mesoderm
- Ectoderm – surface ectoderm, neuroectoderm (neural tube), neural crest.
- Mesoderm – paraxial (somites), intermediate, lateral plate.
- Endoderm – gut tube and associated organs.
Notochord
Derived from the notochordal process (mesoderm); induces neural plate formation. Later forms the nucleus pulposus of intervertebral discs.
Clinical correlate – sacrococcygeal teratoma
Arises from remnants of the primitive streak; contains tissues from all three germ layers. Most common congenital tumor in neonates.
Arises from remnants of the primitive streak; contains tissues from all three germ layers. Most common congenital tumor in neonates.
🧠 Neurulation & neural tube formation
week 3–4 · new addition
The notochord induces overlying ectoderm to thicken into the neural plate (day 18). The plate folds into neural folds flanking a neural groove, and the folds fuse to form the neural tube — "zippering" that begins in the future cervical region and proceeds bidirectionally, cranially and caudally.
🔺 Neuropore closure
- Rostral (cranial) neuropore closes ≈ day 25.
- Caudal neuropore closes ≈ day 27–28.
- Failure of rostral closure → anencephaly.
- Failure of caudal closure → spina bifida spectrum (occulta, meningocele, meningomyelocele).
🧭 Patterning signals
- SHH from notochord/floor plate → ventralizes tube (motor neurons).
- BMP4 from roof plate/ectoderm → dorsalizes tube (sensory neurons).
- 3 primary vesicles (week 4): prosencephalon, mesencephalon, rhombencephalon.
- 5 secondary vesicles (week 5): telencephalon, diencephalon, mesencephalon, metencephalon, myelencephalon.
Neural crest cells
Delaminate from the neural fold tips and migrate extensively to form: PNS ganglia (dorsal root, cranial nerve, autonomic), Schwann cells, melanocytes, adrenal medulla chromaffin cells, pia/arachnoid, pharyngeal arch cartilage, odontoblasts, and part of the aorticopulmonary septum/endocardial cushions. CNS macroglia (astrocytes, oligodendrocytes) and neurons instead arise from the neuroepithelium itself; microglia are mesoderm-derived (yolk-sac macrophages).
⚠️ Teratogen link: Folate deficiency, valproic acid, and carbamazepine all raise the risk of neural tube defects — periconceptional folic acid supplementation is protective.
🌀 Embryonic folding
week 4 · new addition
During week 4 the flat trilaminar disc folds into a cylindrical 3-D embryo, driven mainly by the rapid longitudinal growth of the neural tube.
↕️ Cephalocaudal folding
- Forms a head fold and a tail fold.
- Head fold brings the septum transversum, buccopharyngeal membrane, heart tube, and pericardial cavity into the future thorax.
- Tail fold brings the cloacal membrane, allantois, and connecting stalk to the ventral surface.
↔️ Lateral folding
- Draws the lateral edges of the disc together ventrally.
- Incorporates part of the yolk sac into the embryo, forming the primitive gut tube (foregut, midgut, hindgut).
- Midgut stays temporarily connected to the yolk sac via the vitelline duct (yolk stalk).
- The amnion expands to envelop the whole embryo; the connecting stalk becomes the umbilical cord.
⚠️ Clinical correlate: A persistent vitelline duct → Meckel's diverticulum (true diverticulum; "rule of 2s"). Failure of the gut to return to the abdomen with a peritoneal covering intact → omphalocele.
🩸 Fetal membranes, yolk sac, placenta & umbilical cord
support structures · new addition
🥚 Yolk sac & allantois
- Primary yolk sac (day 8–9) is replaced by the secondary/definitive yolk sac.
- Site of origin of primordial germ cells, which migrate from its wall to the genital ridge in week 4.
- Site of the earliest hematopoiesis (blood islands, weeks 3–6).
- Allantois – outpouching into the connecting stalk; contributes to the bladder. Its remnant is the urachus (median umbilical ligament in the adult) — a patent urachus leaks urine from the umbilicus.
🌐 Amnion & chorion
- Amnion forms the amniotic cavity and fluid.
- Chorion = extraembryonic mesoderm + cytotrophoblast + syncytiotrophoblast; its villi form the fetal part of the placenta.
- Amnion + chorion fuse into the amniochorionic membrane (the "bag of waters").
Placenta
- Maternal component = decidua basalis (from endometrium); fetal component = chorion frondosum.
- Placental barrier layers: syncytiotrophoblast → cytotrophoblast → villous connective tissue → fetal capillary endothelium (cytotrophoblast becomes discontinuous later in pregnancy, thinning the barrier).
Umbilical cord
- 2 umbilical arteries — carry deoxygenated blood from fetus to placenta.
- 1 umbilical vein — carries oxygenated blood from placenta to fetus.
- Wharton's jelly — mucoid connective tissue that cushions the vessels.
⚠️ Clinical correlate: A single umbilical artery is associated with other congenital anomalies (cardiac, renal) and warrants further workup.
👯 Twinning & multiple gestations
timing of split · new addition
Dizygotic (fraternal) twins arise from two eggs fertilized by two sperm — always diamniotic/dichorionic and genetically distinct, like any siblings.
Monozygotic (identical) twins arise from a single zygote that splits; chorionicity and amnionicity depend entirely on the timing of the split:
| Timing of split | Result |
|---|---|
| 0–4 days (morula) | Diamniotic, dichorionic (di/di) |
| 4–8 days (blastocyst, before amnion forms) | Diamniotic, monochorionic (di/mono) |
| 8–12 days (after amnion has formed) | Monoamniotic, monochorionic (mono/mono) — highest risk, cord entanglement |
| > 13 days (after primitive streak forms) | Incomplete splitting → conjoined twins |
⚠️ High-yield: Monochorionic twins share placental vascular anastomoses and are at risk for twin–twin transfusion syndrome.
☣️ Teratogens & critical periods
susceptibility windows · new addition
Weeks 1–2: all-or-none→
Weeks 3–8: major structural risk→
Week 9–birth: growth & functional risk
- Pre-embryonic period (weeks 1–2) – "all or none": the conceptus either survives unaffected or is lost; gross defects are rare.
- Embryonic period (weeks 3–8) – organogenesis; the period of greatest susceptibility to major structural malformations.
- Fetal period (week 9–birth) – organs continue growing/maturing; teratogens now more often cause growth restriction or functional deficits rather than gross malformations (the CNS remains vulnerable throughout).
High-yield teratogens
- ACE inhibitors – fetal renal damage.
- Alcohol – fetal alcohol syndrome.
- Isotretinoin – craniofacial & CNS defects.
- Lithium – Ebstein anomaly.
- Tetracyclines – discolored teeth, bone growth inhibition.
- Thalidomide – limb defects (phocomelia).
- Valproate / carbamazepine – neural tube defects.
- Warfarin – bone & cartilage defects.
- Maternal diabetes – caudal regression syndrome, cardiac defects.
- Methotrexate – multiple congenital anomalies.
🚑 Clinical correlations
high yield
Ectopic pregnancy
- Tubal (ampullary) – most common; risk factors: endometriosis, PID, tubal surgery, DES exposure.
- Abdominal – often in the rectouterine pouch (pouch of Douglas).
- Signs: abdominal pain, vaginal bleeding, positive hCG, culdocentesis with blood, sonographic findings.
Gestational trophoblastic disease
- Complete mole – 46,XX (paternal origin); no embryo; high hCG; risk of choriocarcinoma (~20%).
- Partial mole – triploid (69,XXY); maternal + two paternal sets; fetal tissue may be present.
hCG interpretation
• Low levels → spontaneous abortion, ectopic.
• High levels → multiple gestation, molar pregnancy, choriocarcinoma.
• Low levels → spontaneous abortion, ectopic.
• High levels → multiple gestation, molar pregnancy, choriocarcinoma.
🫘 Indifferent gonad · weeks 4–7
Embryology
The gonads arise from the urogenital ridge, a thickening of intermediate mesoderm. Initially they are bipotential and morphologically identical in both sexes.
Key components
- Primordial germ cells — migrate from the yolk sac wall (week 4) and colonise the gonad.
- Primary sex cords — extensions of surface epithelium that grow inward and receive the germ cells.
- Mesonephric (Wolffian) ducts — precursors of male internal ducts.
- Paramesonephric (Müllerian) ducts — precursors of female internal ducts.
Bipotential fate
- Without SRY → ovary (WNT4 pathway active).
- With SRY (Y chromosome) → testis via testis-determining factor (TDF).
- SRY activates SOX9 and FGF9, promoting testis development.
📌 Clinical correlate: SRY mutations → 46,XY complete gonadal dysgenesis (Swyer syndrome) – female phenotype.
⚥ Testis & ovary – differentiation
🧬 Testis (male)
- SRY gene → TDF → SOX9 upregulation.
- Sertoli cells secrete Müllerian-inhibiting factor (MIF) → regression of paramesonephric ducts.
- Leydig cells produce testosterone → stabilises Wolffian ducts (epididymis, vas deferens, seminal vesicle).
- DHT (from testosterone via 5α-reductase) → male external genitalia.
🌸 Ovary (female)
- No SRY → WNT4 and RSPO1 drive ovarian development.
- Absence of MIF → Müllerian ducts persist → fallopian tubes, uterus, cervix, upper vagina.
- Estrogen later supports female tract maturation.
- Ovarian cortex forms from secondary sex cords (cortical cords).
Indifferent gonad➜
SRY (+) → testis↔
SRY (−) → ovary
🔬 High-yield: MIF is a member of the TGF-β family; its absence leads to persistent Müllerian structures (e.g., in 46,XX males with SRY translocation).
🧬 Meiosis – two divisions, one goal
Meiosis reduces chromosome number from diploid (46) to haploid (23). Occurs in both testis and ovary, but timing and outcome differ.
Meiosis I (reductional)
- Synapsis – homologous chromosomes pair (tetrad formation).
- Crossing over – exchange of segments between non-sister chromatids (genetic diversity).
- Disjunction – homologous pairs separate; each daughter cell receives 23 chromosomes (each still duplicated).
Meiosis II (equational)
- No synapsis, no crossing over.
- Centromere splitting – sister chromatids separate → four haploid cells.
⚠️ Errors: Nondisjunction in meiosis I → gametes with 24 or 22 chromosomes (e.g., trisomy 21, Turner syndrome).
- Prophase I – leptotene, zygotene (synapsis), pachytene (crossing over), diplotene, diakinesis.
- Metaphase I – homologous pairs align at equatorial plate.
- Anaphase I – homologous chromosomes separate (reductional).
- Telophase I – cytokinesis produces two haploid cells.
- Prophase II – no chromosome replication.
- Metaphase II – chromosomes align.
- Anaphase II – centromeres split, sister chromatids separate.
- Telophase II – four haploid gametes.
🧪 Spermatogenesis – continuous production
Begins at puberty and continues throughout life. Takes ~74 days. Occurs in seminiferous tubules.
- Spermatogonia (2n) → mitotic proliferation → primary spermatocytes (2n) → meiosis I → secondary spermatocytes (n, duplicated) → meiosis II → spermatids (n) → spermiogenesis → mature spermatozoa.
- Each primary spermatocyte yields 4 functional sperm.
- Regulated by FSH, LH, testosterone.
🧪 Clinical pearl: Infertility may result from defects in spermatogenesis (e.g., Y chromosome microdeletions, Sertoli-cell-only syndrome).
🥚 Oogenesis – discontinuous & finite
- Begins before birth: oogonia (2n) enter meiosis I → primary oocytes (arrested in prophase I) at birth (≈ 1–2 million).
- At puberty, each menstrual cycle: a cohort of primary oocytes resumes meiosis I → one ovum (and a polar body).
- Meiosis II is completed only upon fertilisation.
- One primary oocyte yields 1 ovum + 2–3 polar bodies (unequal cytokinesis).
⏳ Age & aneuploidy: Maternal age ↑ → risk of nondisjunction ↑ (especially meiosis I). Follicular atresia reduces oocyte pool.
📊 Spermatogenesis vs Oogenesis
| Feature | Spermatogenesis | Oogenesis |
|---|---|---|
| Location | Seminiferous tubules | Ovarian follicles |
| Initiation | Puberty | Embryonic (then arrested) |
| Continuity | Continuous (daily) | Cyclic (monthly) |
| Number of gametes per meiosis | 4 functional sperm | 1 ovum + polar bodies |
| Meiosis I completion | Continuous | Ovulation (each cycle) |
| Meiosis II completion | Before release | After fertilisation |
| End product size | Small, motile | Large, non-motile |
🗂️ Germ layer derivatives
organ systems
| Ectoderm | Mesoderm | Endoderm |
|---|---|---|
| Epidermis, hair, nails, sweat glands | Muscle (smooth, cardiac, skeletal) | Epithelium of GI tract (foregut, midgut, hindgut) |
| Lens of eye, cornea, inner ear | Bone, cartilage, connective tissue | Lower respiratory (larynx, trachea, lungs) |
| Enamel of teeth | Blood, lymph, cardiovascular system | Urinary bladder, urethra, lower vagina |
| Anterior pituitary (Rathke's pouch) | Adrenal cortex, gonads, kidneys | Pharyngeal pouches (thyroid, parathyroids, thymus) |
| Neural tube (CNS, retina, pineal) | Dura mater, notochord (nucleus pulposus) | Liver, pancreas, gallbladder |
| Neural crest: adrenal medulla, ganglia, Schwann cells, melanocytes | Endocardial cushions, aorticopulmonary septum | Submandibular & sublingual glands |
Extraembryonic structures – yolk sac gives rise to primordial germ cells and early blood cells/vessels.
💡 Clinical pearls & high-yield facts
exam essentials
Embryology
- SRY is on Yp11.3; translocation → XX male (SRY+) or XY female (SRY-).
- MIF (AMH) also important for testicular descent.
- WNT4 duplication can cause 46,XX testicular DSD.
Gametogenesis
- Crossing over occurs in prophase I — but oocytes arrest at that stage for decades.
- Nondisjunction in meiosis I → both homologs go to same daughter (trisomy).
- Oogenesis polar bodies are not functional gametes.
- Fertilization – ampulla of the uterine tube. Cortical reaction prevents polyspermy.
- Implantation – posterior uterine wall, day 6–7. Syncytiotrophoblast produces hCG.
- Week 2 – bilaminar disk: epiblast (amniotic cavity) and hypoblast (yolk sac).
- Gastrulation – primitive streak; three germ layers by week 3.
- Notochord – induces neurulation; remnant is nucleus pulposus.
- Teratomas – sacrococcygeal (primitive streak remnants); chordomas (notochord).
- Molar pregnancy – complete mole 46,XX (paternal); partial mole 69,XXY. High hCG.
💡 clinical pearl: The first missed menstrual period corresponds to week 3, when the embryo begins organogenesis — the most critical period for teratogenesis.
⚡ Exam nugget: Testosterone is needed for Wolffian duct maintenance; DHT for external virilisation. MIF is only from Sertoli cells.
📅 Full embryology timeline · weeks 1–38
- Week 1 – fertilization, cleavage, morula, blastocyst, implantation.
- Week 2 – bilaminar disk, amniotic cavity, yolk sac, chorionic cavity.
- Week 3 – gastrulation, primitive streak, notochord, neural plate.
- Weeks 4–8 – organogenesis; folding, neurulation, heart begins beating, neural tube closes, limb buds appear. Period of greatest teratogenic risk.
- Week 9–end of trimester 1 – embryo becomes a fetus; genitalia begin to differentiate; rapid growth in length.
- Trimester 2 (weeks 13–27) – organs mature and grow; quickening (fetal movement felt); viability threshold approached late in this trimester.
- Trimester 3 (weeks 28–birth) – rapid weight gain, pulmonary surfactant production, CNS maturation continues.