Embryology: Human Development from Fertilization to Birth

Timeline

Period	Weeks	Key Events
Pre-embryonic	1-2	Fertilization, cleavage, blastocyst, implantation
Embryonic	3-8	Gastrulation, neurulation, organogenesis (critical period)
Fetal	9-38	Growth, maturation, rapid weight gain

Pre-Embryonic Period (Weeks 1-2)

Fertilization

Step	Location	Description
Capacitation	Female reproductive tract	Sperm membrane modifications
Acrosome reaction	Zona pellucida	Enzymatic digestion of zona
Sperm-egg fusion	Oolemma	Sperm enters oocyte
Cortical reaction	Oocyte	Blocks polyspermy
Second meiotic division	Oocyte	Ootid + second polar body
Pronuclear fusion	Cytoplasm	Zygote (2n, 46 chromosomes)

Site: Ampulla of uterine tube (oviduct)

Cleavage

Stage	Time	Cell count
Zygote	0 hours	1
2-cell	30 hours	2
Morula	3-4 days	16-32 (solid ball)
Blastocyst	4-5 days	~150 (hollow, with inner cell mass)

Blastocyst structure:

Structure	Fate
Inner cell mass (embryoblast)	Embryo
Trophoblast	Placenta (chorion)
Blastocyst cavity (blastocoel)	—

Implantation

Day	Event
5-6	Blastocyst hatches from zona pellucida
6-7	Apposition and adhesion to endometrium
7-10	Invasion of trophoblast (syncytiotrophoblast)
10-12	Complete embedding

Trophoblast differentiation:

• Cytotrophoblast (inner, cellular)
• Syncytiotrophoblast (outer, multinucleated, invasive)

Embryonic Period (Weeks 3-8)

Week 2 (Bilaminar Germ Disc)

Structure	Description
Epiblast	Columnar cells (dorsal)
Hypoblast	Cuboidal cells (ventral)
Amniotic cavity	Above epiblast
Yolk sac (primary)	Below hypoblast
Extraembryonic mesoderm	Between trophoblast and amnion/yolk sac
Chorionic cavity	Space in extraembryonic mesoderm
Connecting stalk	Future umbilical cord

Week 3 (Trilaminar Germ Disc - Gastrulation)

Primitive streak: Appears at caudal end of epiblast

Cell movements:

1. Epiblast cells invaginate through primitive streak
2. Replace hypoblast → definitive endoderm
3. Form intraembryonic mesoderm (between epiblast and endoderm)
4. Remaining epiblast → ectoderm

Germ layer derivatives:

Layer	Derivatives
Ectoderm	CNS, PNS, epidermis, lens, inner ear, adrenal medulla, neural crest
Mesoderm	Muscles, bones, heart, blood vessels, kidneys, gonads, dermis, serous membranes
Endoderm	GI epithelium, respiratory epithelium, liver, pancreas, thyroid, bladder

Notochord:

• Develops from primitive node (Hensen node)
• Extends cranially (notochordal process → notochord)
• Induces neural plate formation
• Forms nucleus pulposus of intervertebral discs

Neurulation (Weeks 3-4)

Stage	Timing	Description
Neural plate	Day 18	Thickening of ectoderm above notochord
Neural groove	Day 20	Invagination of neural plate
Neural folds	Day 21	Elevation of lateral edges
Neural tube closure	Day 22-26	Fusion begins at cervical region

Neural crest cells:

Structure	Derivatives
Dorsal root ganglia	Sensory neurons
Sympathetic chain	Autonomic ganglia
Adrenal medulla	Chromaffin cells
Enteric nervous system	Gut innervation
Schwann cells	Myelination
Melanocytes	Skin pigment
Parafollicular cells	Calcitonin
Odontoblasts	Dentin
Craniofacial skeleton	Pharyngeal arch mesenchyme

Pharyngeal Arches (Weeks 4-6)

Arch	Nerve	Skeletal	Muscles
1 (mandibular)	V3 (mandibular)	Malleus, incus	Mastication
2 (hyoid)	VII (facial)	Stapes, styloid	Facial expression
3	IX (glossopharyngeal)	Hyoid (greater horn)	Stylopharyngeus
4-6	X (vagus)	Laryngeal cartilages	Larynx, pharynx

Organogenesis (Weeks 4-8)

System	Key Events
Heart	Beating by day 22, 4-chambered by week 8
CNS	Brain vesicles (forebrain, midbrain, hindbrain) by week 4
Limbs	Upper limb buds (week 4), lower limb buds (week 5), digits (week 7)
Eyes	Optic vesicles (week 4), lens placode (week 5)
Ears	Otic placode (week 3), cochlea (week 8)
Face	Frontonasal + maxillary + mandibular prominences fuse
GI tract	Midgut herniates into umbilical cord (week 6-10)
Kidneys	Pronephros (vestigial), mesonephros (week 4), metanephros (permanent, week 5)
Lungs	Lung buds (week 4), branching (weeks 5-16)
Genitalia	Indifferent until week 7 (SRY gene determines testis)

Fetal Period (Weeks 9-38)

Week	Crown-Rump Length	Weight	Key Features
9	5 cm	—	Fingers/toes distinct, ossification begins
12	8 cm	14 g	External genitalia identifiable, urine production
16	12 cm	110 g	Fetal movements felt (quickening)
20	15 cm	320 g	Vernix caseosa, lanugo
24	23 cm	630 g	Survival possible (lung development critical)
28	27 cm	1,000 g	Eyes open, rhythmic breathing
32	30 cm	1,700 g	Subcutaneous fat accumulation
36	34 cm	2,400 g	Lanugo disappears, grasping reflex
38 (term)	36 cm	3,200 g	Ready for birth

Major Fetal Events

System	Timeline
Hematopoiesis	Yolk sac (week 3), liver (week 6), bone marrow (week 20)
Lung maturation	Surfactant production (week 24+)
Eye development	Eyelids fused (week 8-28)
Gut rotation	Midgut returns to abdomen (week 10)
Kidney function	Urine production (week 12+)
Immune system	IgG transfer from mother (week 16+)
Bone ossification	Primary centers (week 12), secondary (postnatal)

Fetal Circulation

See cardiovascular section for details. Key shunts:

• Ductus venosus: Umbilical vein → IVC
• Foramen ovale: RA → LA
• Ductus arteriosus: Pulmonary trunk → aorta

Placenta

Feature	Description
Shape	Discoid (15-20 cm diameter)
Weight	500-600 g (at term)
Thickness	2-3 cm
Maternal surface	Decidua basalis (15-20 cotyledons)
Fetal surface	Chorionic plate (umbilical cord insertion)

Umbilical cord:

Vessel	Function
2 umbilical arteries	Deoxygenated blood (fetus → placenta)
1 umbilical vein	Oxygenated blood (placenta → fetus)
Wharton jelly	Protective mucoid connective tissue

Placental functions:

• Gas exchange (O₂, CO₂)
• Nutrient transfer (glucose, amino acids, vitamins)
• Waste removal
• Hormone production (hCG, progesterone, estrogen, hPL)
• Immune barrier (maternal-fetal)