Formation of the germ layers

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Introduction

The germ layers are the primary embryonic layers that form during gastrulation and give rise to all tissues and organs of the body. Their formation is a pivotal event in the third week of human development, establishing the body plan along the cranial-caudal, dorsal-ventral, and left-right axes. Human development involves three primary germ layers:

1. Ectoderm – external layer
2. Mesoderm – middle layer
3. Endoderm – internal layer

Additionally, the extraembryonic mesoderm and specialized derivatives (like neural crest cells) provide critical contributions to organogenesis, making germ layer knowledge essential for understanding congenital anomalies, tissue engineering, and regenerative medicine.

All three germ layers (ectoderm, mesoderm, and endoderm), as well as the amniotic cavity and therefore the entire embryonic tissue, arise from the epiblast. The extraembryonic mesoderm and the yolk sac are derived from the hypoblast.

Gastrulation: The Origin of Germ Layers

The trilaminar embryonic disc (composed of endoderm, mesoderm, and ectoderm) forms through the migration of epiblast cells. Epiblast cells move through the primitive streak, situated between the epiblast and hypoblast layers, creating an intermediate layer known as the intraembryonic mesoderm. The epiblast also replaces the hypoblast, giving rise to the endoderm, while the remaining epiblast forms the ectoderm.

• Origin: Primitive streak, a longitudinal thickening of epiblast cells
• Outcome: Formation of the trilaminar embryonic disc (mesoderm, endoderm, ectoderm)
• Timing: Occurs during weeks 3–4 (around day 15-16)

View of a 17-day-old embryo (left) and three-dimensional cross-sectional overview (right)

key steps:

1. Primitive streak formation
   • A linear thickening appears in the caudal epiblast
   • Node (Hensen’s node) at the cranial end organizes cranial structures
   • Signaling pathways: Nodal, Wnt, BMP, FGF regulate streak formation
2. Epiblast cell migration
   • Epiblast cells migrate through the primitive streak in an epithelial-to-mesenchymal transition (EMT)
   • Cells displace the hypoblast → form endoderm
   • Cells occupy the space between epiblast and endoderm → form intraembryonic mesoderm
   • Remaining epiblast → ectoderm
3. Cranial-caudal and left-right polarity
   • Node-derived signals (e.g., Shh, Lefty) establish left-right asymmetry
   • Primitive streak regression occurs from day 20 onward

Mesoderm development (intraembryonic mesoderm):

• • Epiblast cells detach from the epiblast layer above the primitive streak, forming the primitive pit
  • Mesoderm cells migrate through the primitive pit
  • Cells proliferate on both sides of the pit, establishing the mesoderm, a new layer situated between the epiblast and hypoblast

The chordamesoderm and paraxial mesoderm form the axial skeleton, whereas the intermediate mesoderm forms the kidneys and gonads, and the lateral plate mesoderm forms the circulating systems, body wall, and limbs (except for the musculature). nt neural tube.

Organization of Mesoderm:

The mesoderm are organized along the cranial-caudal axis into:

1. Paraxial mesoderm – forms somites
2. Intermediate mesoderm – forms urogenital system
3. Lateral plate mesoderm – splits into:
   • Somatic (parietal) layer → body wall, limbs, dermis
   • Splanchnic (visceral) layer → heart, blood vessels, gut wall

Somites and Derivatives

• Sclerotome → vertebrae, ribs
• Dermatome → dermis of back
• Myotome → skeletal muscles of trunk and limbs

Specialized mesoderm derivatives

• Cardiogenic mesoderm → heart and great vessels
• Nephrogenic mesoderm → kidneys and gonads
• Extraembryonic mesoderm → chorion, connecting stalk, and umbilical vessels

Endoderm development:

• Epiblast cells migrate from the primitive streak and primitive node into the hypoblast layer
• They replace hypoblast cells, generating an additional germ layer known as the endoderm

Endoderm Derivatives

Derivatives

• Epithelial lining of gastrointestinal tract (from pharynx to cloacal membrane)
• Respiratory tract epithelium (larynx, trachea, bronchi, lungs)
• Liver and pancreas (from hepatic and pancreatic buds)
• Thyroid and parathyroid glands
• Epithelial parts of the auditory tube and tonsils
• Urinary bladder epithelium (except trigone)

Ectoderm development:

• The epiblast cells that remain after migration for the endoderm, mesoderm, and notochord differentiate into ectoderms

Subdivisions:

• Surface ectoderm (Epidermis, hair, nails, Glands: sweat, mammary, sebaceous, Anterior pituitary (Rathke’s pouch), Lens of the eye, Enamel of teeth)
• Neuroectoderm (Forms the neural tube → CNS Brain: forebrain, midbrain, hindbrain Spinal cord. Neural tube closure: cranial neuropore (day 25), caudal neuropore (day 27))
  

  Neuroectoderm
  Neural Crest Cells (NCC): migratory population from the dorsal neural tube
  • Derivatives:
    • Peripheral nervous system: sensory, sympathetic, parasympathetic ganglia
    • Schwann cells and satellite cells
    • Melanocytes
    • Facial cartilage and bones (pharyngeal arches)
    • Adrenal medulla (chromaffin cells)
    • Outflow tract of the hear

Germ Layer Interactions

• Organogenesis depends on ectoderm–mesoderm–endoderm interactions
  • Mesoderm induces ectodermal differentiation → e.g., neural tube induces overlying ectoderm to form skin
  • Endodermal signaling via FGFs, Wnts, BMPs → induces mesodermal patterning
• Neural crest cells exemplify intermediate identity, contributing to ectomesenchyme (ectodermal origin but mesodermal function)