Somite Development Quiz

Somite Development

• Paraxial mesoderm differentiates into clusters of cells (somites) on both sides of the embryo
• Somites differentiate into occipital, cervical, thoracic, lumbar, sacral, and coccygeal somites

Somite Derivatives

• Sclerotome: cells migrate medially around the notochord to form the vertebral column and base of the skull
• Myotome: cells divide into two groups (epaxial and hypaxial) and form most skeletal muscles of the body
• Dermatome: cells migrate under the ectoderm (epidermis) to form the dermis and subcutaneous tissue

Body Cavities

• The intra-embryonic coelom is the primordium of the body cavity
• The body cavity will give rise to the pericardial cavity, pericardioperitoneal canals, and peritoneal cavity during week 4
• Each body cavity (pericardium, pleural, and peritoneal) is composed of both visceral and parietal serous membranes
• Visceral serous membrane is adhered to the surface of the organ and is derived from splanchnic mesoderm
• Parietal serous membrane lines the cavities and is derived from somatic mesoderm

Embryonic Development

• Week 2: uterine lining closes behind the implanting embryo
• Week 2: blastocyst has penetrated the endometrium about halfway, and continued growth of syncytiotrophoblast
• Week 2: formation of amnionic cavity, a fluid-filled space within the embryoblast

Formation of Bilaminar Disk

• Epiblast gives rise to the embryo proper and amnion
• Hypoblast gives rise to the yolk sac and allantois
• Exocoelomic membrane forms from hypoblast cells

Formation of Yolk Sac

• Hypoblast cells proliferate and migrate to cover the inner surface of cytotrophoblast (exocoelomic membrane)
• Primary yolk sac (lined by exocoelomic membrane) forms, and fluid accumulates to open up spaces within the extraembryonic mesoderm layer
• Spaces eventually fuse to form the extraembryonic coelom (or chorionic cavity) between two layers of extraembryonic mesoderm
• Pole of primary yolk sac farthest from the bilaminar germ disk pinches off and separates from the original yolk sac to form a remnant of the primary yolk sac and leaving the remainder to be called the secondary yolk sac
• Gastrulation results in the replacement of hypoblast by endoderm
• Definitive yolk sac (lined by endoderm) forms

Blood and Blood Vessels

• Vasculogenesis: mesodermal cells form clusters (blood islands) and blood islands form vessels
• Angiogenesis: vessels sprout from other vessels

Test your knowledge on the development of somites, including paraxial mesoderm, sclerotome, myotome, and dermatome. Learn about the formation of the vertebral column, skeletal muscles, and more.