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 mesoderm: forms 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:

 

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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)

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Intermediate mesoderm

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

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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

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Summary

There are 3 main areas of the mesoderm forming different structures

1. Paraxial mesoderm: forms 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