Development of the mesoderm and its differentiation

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Introduction

Mesoderm is formed during gastrulation on the 3rd week of embryonic development and it is the middle layer of the 3 primary germ layers (endoderm, mesoderm, and ectoderm). Mesoderm gives rise to adrenal cortex, ureters, dura mater, kidneys, spleen, bone, muscle, cartilage, and connective tissue.

Formation of the Intraembryonic Mesoderm

Origin during Gastrulation

The intraembryonic mesoderm originates from epiblast cells that migrate through the primitive streak and primitive node during gastrulation.

• Epiblast cells undergo epithelial-to-mesenchymal transition (EMT)
• Migrating cells interpose between the ectoderm and endoderm
• This newly formed layer constitutes the intraembryonic mesoderm

Mesoderm formation begins cranially at the primitive node and extends caudally along the primitive streak. As gastrulation progresses, the primitive streak regresses, leaving behind differentiated mesodermal tissues.

Organization of the Mesoderm

After its formation, the mesoderm is organized along the cranial-caudal and medial-lateral axes into distinct regions, each with specific developmental fates.

From medial to lateral, the intraembryonic mesoderm differentiates into:

1. 

   Organisation of the Mesoderm
   Axial mesoderm
2. Paraxial mesoderm
3. Intermediate mesoderm
4. Lateral plate mesoderm

Axial Mesoderm

• Location: central region of the mesoderm
• Components: notochord and prechordal mesoderm

Notochord Formation

The axial mesoderm forms the notochord, which originates from cells migrating through the primitive node.

Functions of the notochord:

• Establishes the longitudinal axis of the embryo
• Acts as a major inductive signaling center
• Secretes Sonic hedgehog (Shh), crucial for neural tube and vertebral patterning

Fate:

• Most of the notochord degenerates
• Remnants persist as the nucleus pulposus of the intervertebral discs

Paraxial Mesoderm

• Location: A cylindrical region of mesoderm positioned on either side of the notochord
• Constituents: Somites

Process:

• During week 4, the paraxial mesoderm undergoes segmentation into paired, rounded cell masses known as somites along the length of the neural tube
• By the beginning of week 5, approximately 42–44 pairs of somites have formed in a cranial-to-caudal sequence
• Several somite pairs subsequently regress, leaving 35–37 pairs
• As the earliest segmental units of the embryo, somites establish the fundamental pattern of body segmentation

Somite differentiation:

• Sclerotome: Cells migrate medially toward the notochord, and the sclerotomes from each somite pair fuse
• Dermomyotome:
  • Dermatome: Cells migrate toward the surface ectoderm to form the dermis
  • 

    Somites Development
    Myotome: Differentiates into dorsal epaxial and ventral hypaxial skeletal muscles, which contribute to the musculature of the lateral and anterior regions of the thorax and abdomen

Intermediate Mesoderm

• Location: lateral to the paraxial mesoderm
• Components: urogenital fold, consisting of the nephrogenic ridge and genital ridge

It gives rise to the urogenital system, and the development is followed in a cranial to caudal sequence, with transient embryonic kindey systems preceding the permanent metanephros.

Intermediate Mesoderm

Lateral Mesoderm

• Location: lateral to the intermediate mesoderm
• Sequence of events
  • In the second week of development, small folds develop in lateral aspect of the mesoderm.
  • These folds split horizontally to form two components.
    • Somatic (parietal) mesoderm: the dorsal layer that underlies the ectoderm and differentiates into the lining of the pleural, pericardial, and peritoneal cavities.
    • Splanchnic (visceral) mesoderm: the ventral layer that overlies the endoderm and differentiates into the visceral lining of internal organs.
  • The space between these components is called the coelom
  • The two coeloms from either side fuse at the end of the lateral folding of the embryo to form one large cavity, the intraembryonic coelom, which will differentiate into the thoracic and abdominal cavities (see “Morphogenesis” above).

Cardiogenic Mesoderm

A specialized region of splanchnic mesoderm, the cardiogenic mesoderm, forms cranial to the neural plate.

• Gives rise to:
  • Heart tube
  • Great vessels
• Heart development begins in week 3, making it the first functional organ system