Talk:2010 BGD Practical 6 - Week 3

From Embryology

Within the embryonic disc lateral plate mesoderm a space (coelom) forms, it lies within the embryo and so is called the intraembryonic coelom. This single "horseshoe-shaped" space will form the 3 major body cavities: pericardial (around the heart), pleural (around the lungs) and peritoneal (around the GIT and visceral organs).

The mesoderm adjacennt to the endoderm is now called the splanchnic mesoderm which forms the connective tissue and muscular wall of the GIT.

Note intraembryonic coelomic cavity communicates with extraembryonic coelom (space outside the embryo) through portals (holes) initially on lateral margin of embryonic disc.

Week 3

Week 3 - Embryonic disc

Week 3

Mesoderm means the "middle layer" and it is from this layer that nearly all the bodies connective tissues are derived. In early mesoderm development a number of transient structures will form and then be lost as tissue structure is patterned and organised. Humans are vertebrates, with a "backbone", and the first mesoderm structure we will see form after the notochord will be somites.

Facts: Week 4, 22 - 23 days, 2 - 3.5 mm, Somite Number 4 - 12

View: This is a dorsal view of the human embryo, the amniotic membrane has been removed. Top embryo is an early stage 10, bottom is late stage 10.

Mesoderm organization: lateral plate - intermediate mesoderm - paraxial mesoderm - axial mesoderm - paraxial mesoderm - intermediate mesoderm - lateral plate

Axial Mesoderm

  • notochord
  1. mechanical role in embryonic disc folding
  2. molecular role in patterning surrounding tissues

Adult - contributes to the nucleus pulposis of the intervertebral disc

Paraxial Mesoderm

Stage 7 paraxial mesoderm
  • differentiates rostro-caudally (head to tail)
  • remains unsegmented in the head region.
  • segments in the body region to form pairs of somites along the length of the embryo.

Adult - contributes vertebral column (vertebra and IVD), dermis of the skin, skeletal muscle of body and limbs

Intermediate Mesoderm

Stage 7 intermediate mesoderm
  • named by position (between paraxial and lateral plate)
  • differentiates rostro-caudally (head to tail)
  • forms 3 sets of "kidneys" in sequence
  1. pronephros
  2. mesonephros
  3. metanephros

Adult - metanephros forms the kidney

Lateral Plate Mesoderm

Stage 7 lateral plate
  • a "horseshoe shaped" space forms in the middle
  • somatic mesoderm - closest to ectoderm
  • space - forms the 3 body cavities (pericardial, pleural, peritoneal)
  • splanchnic mesoderm - closest to endoderm

Adult - body connective tissues, gastrointestinal tract (connective tissues, muscle, organs), heart

Somite Development

stage 11 Embryo

Somite initially forms 2 main components

  • ventromedial- sclerotome forms vertebral body and intervertebral disc
  • dorsolateral - dermomyotome forms dermis and skeletal muscle


  • sclerotome later becomes subdivided
    • rostral and caudal halves separated laterally by von Ebner's fissure
  • half somites contribute to a single vertebral level body
  • other half intervertebral disc
  • therefore final vertebral segmentation “shifts”


  • Body - epaxial and hypaxial muscles
  • Limbs - flexor and extensor muscles


  • connective tissue underlying epidermis
  • begins as a dorsal thickening
  • spreads throughout the body

Mesoderm 001 icon.jpg Somite 001 icon.jpg Vertabra 003 icon.jpg