What are the 3 major germ layers?
Ectoderm
Mesoderm
Endoderm
What are the 3 main cavities in early embryo (transverse section)?
Dorsal → Amniotic cavity
Ventral → Yolk sac cavity
Within embryo → Intra-embryonic coelomic cavity
What are the components of Mesoderm?
- Axial Mesoderm → Notochord
- Paraxial Mesoderm → Somites
- Intermediate Mesoderm → Genitourinary system
- Lateral Plate Mesoderm (Splits into):
Somatic mesoderm (parietal)
Splanchnic mesoderm (visceral)
Together they enclose → Intra-embryonic coelom - Intra-embryonic Coelom → Future pericardial, pleural, peritoneal cavities
What does the Notochord become?
Remnant: Nucleus pulposus of intervertebral disc
Functions:
Axial support
Induces neuroectoderm to form neural tube
What is the fate of each mesodermal region?
Region Derivatives
Paraxial Mesoderm Somites → muscles, vertebrae, dermis
Intermediate Mesoderm Kidneys, gonads, reproductive tracts
Lateral Plate Mesoderm
Somatic layer → body wall, limbs
Splanchnic layer → heart, blood vessels, gut wall
|Intra-embryonic Coelom|Body cavities: pericardial, pleural, peritoneal|
What part of the skull is formed by neural crest cells (NCC)?
Anterior and anterolateral skull bones
(e.g. frontal bone, sphenoid, zygomatic, mandible, maxilla)
NCCs also contribute to:
Pharyngeal arch bones (malleus, incus, stapes, mandible, hyoid)
Odontoblasts (→ dentin)
Dermis of head & neck
What forms the posterior skull and vertebrae?
Paraxial mesoderm
Occipital bone
Vertebrae
Formed via Sclerotome
What are the 3 parts of a somite (paraxial mesoderm)?
- Sclerotome → vertebrae, ribs, posterior skull
- Myotome → skeletal muscles
- Dermatome → dermis of skin (esp. back)
🧠 Mnemonic: “Sclero-Myo-Derma” = Bone-Muscle-Skin
What does the notochord become in the adult?
Nucleus pulposus of intervertebral disc
What are the two layers of lateral plate mesoderm (LPM)?
LPM Layer Location Forms
Somatic (Parietal) Dorsal Parietal layers of pleura, pericardium, peritoneum, limb skeleton
Splanchnic (Visceral) Ventral Visceral layers, heart muscle, smooth muscle of gut & respiratory system
Which layer forms the intraembryonic coelom (body cavities)?
Space between somatic & splanchnic LPM
Future → Pleural, peritoneal, pericardial cavities
What’s the origin of the detrusor muscle (urinary bladder)?
Intermediate mesoderm
Which smooth muscles are NOT from LPM?
Smooth Muscle Source
Iris muscles (sphincter & dilator pupillae) Neural plate ectoderm
Myoepithelial cells (sweat, mammary glands) Surface ectoderm
Detrusor (bladder wall) Intermediate mesoderm
What does intermediate mesoderm form?
Kidneys
Testes, ovaries
Genitourinary smooth muscles (e.g., detrusor)
What are the three major subdivisions of intraembryonic mesoderm?
Paraxial, Intermediate, and Lateral Plate mesoderm.
What structures arise from paraxial mesoderm?
Somites, which differentiate into:
Sclerotome → vertebrae, base of skull
Myotome → skeletal muscles
Dermatome → dermis of skin
What does the notochord represent?
Axial mesoderm — forms nucleus pulposus of intervertebral disc.
Which part of somite forms vertebrae?
Sclerotome
Which part of somite forms skeletal muscles?
Myotome
Which part of somite forms dermis of skin?
Dermatome
What part of skull is formed by paraxial mesoderm?
Posterior skull, esp. occipital bone.
What are the derivatives of intermediate mesoderm?
Genitourinary system – kidneys, gonads, ducts, and associated smooth muscle (e.g., detrusor).
Which muscle of the bladder comes from intermediate mesoderm?
Detrusor muscle.
What are the two layers of lateral plate mesoderm?
Somatic (parietal) LPM
Splanchnic (visceral) LPM
