Lecture 2 Flashcards
(10 cards)
Gastrulation
a bilayer embryonic disk is formed, consisting of epiblasts and hypoblasts. Primitive streak is formed at caudal end of epiblast; defining body axes. On day 14-15, ingressing epiblast epiblast cells displace hypoblast and form definive endoderm. On day 16, ingressing epiblast cells migrate between endoderm and epiplast layers to form intraembryonic mesodern. Three germ layers formed
Migration of mesoderm
Intraembryonic mesoderm cell migrate bilaterally from the primitive streak and form different types of mesoderms. Mesodermal cells migrate cranially at the midline to form a midline cord (notocord). Cells migrating from just lateral to the notocord become the proximal mesoderm.
Neural plate formation
Day 18, thickened neural plate is formed cranial to the primitive streak. Neural plate forms in 4th week to form neural tube precursor to the NS
paraxial development
in the head, paraxial mesoderm remains unsegmented. Caudal to the occipital region, paraxial mesoderm becomes segmented into somites.
somite
give rise to most of the axial skeleton (incl. vertebral column), skeletal muscles of the trunk, limbs and dermis of the body. Appear in craniocaudal sequence of ~3 pairs/day. By 5th weel, there are 42-44 pairs. The first occipital and last 5-7 coccygeal somites disappear. The remaining somites become axial skeleton, voluntary muscles and dermis.
somite differentiation
pre-somatic mesoderm arrange themselves into a ball of spithelial cells with a lumen. Shortly after, they form sclerotome (to become bone and cartilage) and dermomyotome (to become muscles and dermis). Cells from both muscle precursor groups migrate ventral to the dermatome to create the dermomyotome. Cells in the dermomyotome migrate and form most of the musculature for body wall and limb muscles. Those that migrate anteriorly form limb and trunk muscles (hypaxial muscles) and posteriorly give rise to intrinsic back musckes (epaxial muscle). Each somote form its own sclerotome, myotome and dermotome (myotome and dermotome has own segmental nerve component). Each myotome and dermatome retains its inntervation from its segment of origin no matter where cells migrate.
Intramembrane ossification
develop directly from mesenchyme. At Week 8, mesenchymal cells aggregate to form an ossification centre. They differentiate into osteoprogenitor cells and later osteoblasts. Osteoblasts make bone and as this happens, the osteoid undergoes mineralisation and entrapped osteoblasts become osteocytes. Osteocytes separate as matrix is produced but they have processes that communicate with each other and osteoblast. Newly formed tissue has a microscopic structure of immature (woven) bone with thick trabecular lineed by osteoblasts and endosteal cells. Further bone growth and remodelling replace woven bone by compact bone in the periphery and spongy bone between them. Spaces between trabeculae become occupied by bone marrow cells that arrive with blood vessels
Endochondrial ossification
Mesenchymal cells differentiate into chondrocytes which then produce cartilage matrix. Cartilage model can grow in length and width then calcify. Blood vessels erode and invade cartilage model creating a marrow cavitiy. As a primary ossification centre develops, the endochondrial bone is formed on spicules of calcified cartilage. Grow in length of long bones depends on the presence of epiphysial growth plate. A secondary messenger of ossification is established in the epiphysis. With continued growth of the long bone, the distal epiphyseal cartilage disappears. With cessation of growth, the proximal epiphyseal cartilage cartilage disappears and the metaphysis then becomes continuous with the epiphesis. Epiphyseal limes remain where the epiphyseal plate last existed.
Synovial joint development
Mesenchyme of the interzone between chondrifying bone primordia differentiates into fibroblastic tissue. At both proximal and distal ends, fibroblastic tissue differentiates into atricular cartilage covering the surfaces of the two opposing bone primordia. Proximally and distally, the connective tissue in the central region condenses to form the synovial tissue that will line future joint cavities. Vacuoles form within connective tissue and coalesce to form the synovial cavity that is filled with synovial fluid for lubrication. Central zone can also give rise to menisci and internal joint ligaments. Koint capsual also arises from integration
Determinants of childhood growth rates
Nutrition, hormones, genes
