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Germ Layers
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Germ Layers

The germ layers
Overview
* Ectoderm lines the amniotic cavity.
* Endoderm lines the yolk sac.
    • The vitelline duct connects the gut tube to the yolk sac.
* Mesoderm has both extraembryonic and intraembryonic portions.
    • The connecting stalk connects the embryo to the uterus.
    • The embryo lies within the chorionic cavity, which coalesces from pools of extraembryonic cavities.
Within the mesoderm, lies the:
  • The neural tube
The neural crest cells (we use an arrow to signify their migration*)
  • Notochord
The mesoderm divides (from medial to lateral) into:
  • The somites (paraxially)
  • The intermediate mesoderm
  • Lateral plate mesoderm.
The Germ Layers
Ectoderm
Epidermis:
  • The skin, specifically the surface layer (meaning NOT the dermis, the underlying layer)
  • The skin appendages (eg, the hair, nails, and other appendages).
The neural crest cell derivatives:
  • Select cranial nerves the pharyngeal arch derivatives (which are cranial nerves 5, 7, 9, and 10).
  • The dorsal root ganglia, which are the pseudounipolar sensory neurons.
  • The sympathetic chain ganglia, which supply the sympathetic portion of the autonomic nervous systems, responsible for "Fight or Flight".
  • The adrenal medullary cells, which are activated along with the sympathetic nervous system during stress.
  • The enteric nervous system, which is the intrinsic nervous system activator of the gut.
  • Additional nerve and cartilaginous derivatives.
Neural tube derivatives and the placodes (which are ectodermal thickenings):
From cranial to caudal, they are the:
  • Telencephalon
  • Diencephalon
  • Mesencephalon
  • Metencephalon
  • Myelencephalon
  • The caudal neural tube
* The Telencephalon and Diencephalon derive from the prosencephalon.
* The metencephalon and myelencephalon derive from the rhombencephalon.
  • Key placodes (which are areas of thickened surface ectoderm), which form CNs 1, 2, and 8 (the solely sensory set of CNs), from cranial to caudal
    • At the nasal prominences lies the olfactory placode, which derives the olfactory epithelium and olfactory nerve (CN 1).
    • The optic placode forms the optic nerve (CN 2); it originates from the diencephalon.
    • The otic placode forms the vestibulocochlear nerve (CN 8); it originates from the hindbrain.
Endoderm
The foregut
Derives the linings of many important gastrointestinal and respiratory structures:
  • The pharyngeal region
  • Divides distally into the esophagus and trachea and includes 4 key pharyngeal pouches of the head and neck.
  • Stomach
  • Proximal duodenum,
  • Liver buds, which ultimately form the liver,
  • The gallbladder,
  • The pancreas, which forms from the ventral and dorsal pancreatic buds (aka, diverticula).
  • The linings of the respiratory system (with a drawing of the trachea and lungs).
The midgut
  • Distal duodenum,
  • Jejunum,
  • Ileum,
  • Ascending colon,
  • Proximal 2/3 of the transverse colon.
The hindgut
Gives rise to the allantois before ending blindly at the cloaca.
  • Distal 1/3 of the transverse colon,
  • Descending and sigmoid colons, and,
The proximal 2/3 of the anorectal canal. The ectoderm gives rise to the distal 1/3 of the anorectal canal is derived from ectoderm; it invaginates the area around the proctodeum (aka, anal pit).*
mesoderm
From medial to lateral:
The somites (the paraxial mesoderm) form the axial musculoskeleton and dermis as follows:
  • Sclerotome derives the bone of the axial skeleton: the spine and the posterior base of the skull.
  • Myotome derives the paraspinal and abdominal musculature.
  • Dermatome derives the axial dermis.
Intermediate mesoderm:
  • The urogenital and reproductive systems
(we signify them with a kidney and ureter and an illustration of the uterus, a fallopian tube and ovary).
The lateral plate mesoderm derives
  • The cardiovascular system
  • The linings of the body walls and organs (the parietal and visceral pleura and peritoneum)
  • The appendicular musculoskeleton: the limb muscles and bones.
(We draw the heart tube inside of the chest to signify the cardiovascular system and the body walls and organ linings. And we draw an arm in flexion to signify the appendicular musculoskeleton.)

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