Overview
This lecture reviews the embryonic period (weeks 3–8), focusing on the derivatives of the three germ layers—ectoderm, mesoderm, and endoderm—formed during gastrulation, and their contributions to organs and tissues.
Embryonic Period Overview
- Embryonic period spans third to eighth week, with slight variations by textbook.
- Also called organogenesis, when germ layers form tissues and organs.
- Organs are immature at this stage, maturing during the fetal period.
- Embryo is highly sensitive to teratogenic agents during this time.
Ectoderm Derivatives
- Notochord induces thickening of ectoderm into the neural plate (neurulation).
- Neural plate forms neural tube, giving rise to CNS, retina, pineal gland, and neurohypophysis.
- Neural crest cells migrate to form facial bones, cranial nerve ganglia, adrenal medulla, melanocytes, parts of the heart, and more.
- Surface ectoderm forms epidermis, hair, nails, glands, mammary glands, adenohypophysis, enamel, and sensory placodes (otic for hearing/balance, lens for the eye).
Mesoderm Derivatives
- Paraxial mesoderm forms somites, differentiating into sclerotome (axial skeleton), myotome (skeletal muscle), and dermatome (dermis).
- Intermediate mesoderm forms most urogenital structures, except the bladder.
- Lateral mesoderm splits into somatic (parietal) and splanchnic (visceral) layers.
- Parietal forms body wall, limb bones, and somatopleura.
- Splanchnic forms visceral linings, heart muscle, and splanchopleura.
- Heart, blood vessels, and blood cells originate from splanchnic mesoderm, with blood islands forming via vasculogenesis and angiogenesis.
- Yolk sac is the first hematopoietic organ.
Endoderm Derivatives
- Endoderm lines the ventral embryonic disc, forming the primitive gut through embryonic folding.
- Forms foregut, midgut, and hindgut; foregut and hindgut are separated by oropharyngeal and cloacal membranes.
- Gives rise to epithelial linings of GI tract, respiratory tract, glands (thyroid, parathyroid, liver, pancreas), urinary bladder, urethra, tympanic cavity, and tonsil/thymus stroma.
Key Terms & Definitions
- Neurulation — Formation of the neural tube from the neural plate.
- Somites — Segmented blocks of paraxial mesoderm forming skeletal and muscular structures.
- Neural crest cells — Migratory ectoderm cells giving rise to diverse structures.
- Vasculogenesis — Formation of new blood vessels from blood islands.
- Angiogenesis — Formation of new blood vessels from existing ones.
- Somatopleura — Parietal mesoderm plus ectoderm (forms body wall).
- Splanchopleura — Visceral mesoderm plus endoderm (forms internal linings).
Action Items / Next Steps
- Review tables summarizing germ layer derivatives.
- Study embryonic folding and neurulation processes.
- Prepare for further lessons on external embryonic features during this period.