Mesoderm development

Summary

  • There are 3 main areas of the mesoderm which all develop separately
    • These are categorised by their relation to the notochord
  1. Paraxial mesodermforms somites which become the head and skeletal muscles, skin, and tendons
  2. Intermediate mesoderm: forms the urogenital system
  3. Lateral plate mesoderm:
    • Serous membranes – pleura, pericardium, peritoneum
    • Cardiovascular and lymphatic systems and cells
    • Adrenal cortex and spleen
  • Summary of mesoderm development:
Parts of the mesoderm at the start of development. Note the notochord, surrounded by the somites. Next is the intermediate and then the lateral plate mesoderm.
The pink layer next to the ectoderm is the parietal (aka somatic) layer, and the purple layer next to the endoderm is the visceral (aka splanchnic) layer
Summary of mesoderm development.

 


Paraxial mesoderm

  • This is organised into segmented blocks either side of the neural tube
    • These are called somitomeres (develop into somites)
    • These somites develop Cranio => Caudal

        The somites lie in pairs either side of the notochord. They are the most medial aspect of the mesoderm.
  • There are initially 50 pairs of somitomeres
    • 10 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 10 coccygeal
      • The first 7 occipital somitomeres form the skull/face
      • 1 Occipital and 5-7 coccygeal somitomereswill disappear
      • This leaves 35-37 pairs of somites
  • Differentiate into 4 different tissues in the 4th week (under influence of SHH)
    • Sclerotome which develops from cells forming the ventromedial somite wall (closest to notochord)
      • These migrate to surround the notochord and form the future cranium, vertebral column + ribs
      • These cells remain mesenchymal (develop a loosely woven tissue) and develop into fibroblasts, chondroblasts, and osteoblasts(also occurs from neural crest and lateral mesoderm cells)
    • Dermomyotome develops from dorsolateral somite
      • These form the skin and skeletal musculature
      • Split further  into dermatome and myotome
      • NB these cells retain their innervation from the segment they have migrated from
      • 2 types of axial muscle derive from the myotome
        • Epaxial muscle (aka epimere)
          • Forms pretty much everything 4cm each side of the spine
          • Dorsal/posterior muscle – Extensors of neck and spine
        • Hypaxial (aka hypomere)
          • Ventral/anterior and lateral axial muscle
          • Includes all muscle of the limbs
    • Syndetome – forms tendons

The diagram below illustrates the positions of the sclerotome, syndetome, and dermomyotome (from medial to lateral)

Development of the paraxial mesoderm.

Intermediate mesoderm

  • This initially connects the paraxial and lateral mesoderm
    • It later develops the excretory units of the urogenital system 

Lateral Plate Mesoderm

  • This forms:
    • The serous membranes (peritoneal, pleural, and pericardial membranes)
    • Cardiovascular system
    • Lymphatic system including the spleen
    • The adrenal cortex
  • The LP mesoderm first divides into two layers as cavities develop within it
    • Parietal mesoderm layer (somatopleuric mesoderm) – this lines the intraembryonic cavity/amnion and ectoderm
      • This will form the ventral and lateral body wall lining along with overlying ectoderm
      • Develops the parietal membranes of the peritoneal, pleural, and pericardial cavities
    • Visceral mesoderm layer (splanchnopleuric mesoderm) – lines the organs / yolk sac / endoderm
      • Forms the visceral membranes of the serous cavities
      • Develops into the gut smooth muscle
      • Develops into the cardiovascular/lymphatic system
      • Also forms the laryngeal cartilages
  • The space between the two is the intraembryonic cavity 
    • This later forms the serous cavities of the body
  • Both layers are continuous with the extraembryonic mesoderm
Langman’s Medical Embryology: The parietal and visceral LP mesoderm.

Summary

There are 3 main areas of the mesoderm forming different structures

  1. Paraxial mesodermforms Somites which forms the head and skeletal muscles, skin, and tendons
  2. Intermediate mesoderm: forms the urogenital system
  3. Lateral plate mesoderm:
    • Serous membranes – pleura, pericardium, peritoneum
    • Cardiovascular and lymphatic systems and cells
    • Adrenal cortex and spleen