Mesoderm Derivatives

Lateral plate of the mesoderm gets split into __ and ___ at day 21
somatic (parietal) layer and splanchnic (visceral) layer
three sections of mesoderm
paraxial, intermediate, lateral plate
derivatives of paraxial mesoderm
somites: vertebrae and ribs, dermis of the dorsal skin, skeletal muscles of the back, skeletal muscles of the body wall, limbs
derivatives of intermediate mesoderm
urogenital structures
derivatives of lateral plate mesoderm
heart and blood vessels, body wall
somitomere
somite precursor, organizes from paraxial mesoderm at day 15
-somitomere precedes craniocaudally
-cranial somitomeres don’t make it to somites, contribute to head mesenchyme
-somitomeres in occipital region on down are organized into somites at ~day 20
each somite forms:
sclerotome: ribs, vertebrae, rib cartilage
myotome: muscles of the back, body wall (intercostals) and some limb muscles
dermatome: dermis of the back
ventrolateral cells (subset): most of musculature for body wall (obliques, transversus abdominus) and limbs
stages of somite development
-balls of somitic (paraxial) mesoderm undergo epithelialization, forming donut shape around a small lumen in each somite)
-later, they lose epithelial characteristic s and become mesenchymal/fibroblastic
pharyngeal arches
-derived from paraxial mesoderm (somitomeres and occipital somites) and neural crest cells
-covered with ectoderm, lined by endoderm
-contribute to formation of face, neck, mouth, larynx, pharynx
-forms muscles, arteries, connective tissue, cartilage, parts of skeleton
pharyngeal grooves/clefts (ectodermal)
between pharyngeal arches externally
pharyngeal pouches (endoderm)
outpocketings of pharyngeal gut
intermediate mesoderm
form urogenital structures (kidneys, gonads, and their respective duct systems)
formation of kidneys
-3 overlapping kidney structures are formed cranial to caudal
formation of genital structures from genital ridge
-primordial germ cells (form in epiblast and migrate to yolk sac wall)
-migrate from the wall of the yolk sac to the urogenital ridges
-induce formation of ovaries or testes from genital ridges
lateral plate mesoderm
cavities coalesce and split lateral plate into 2 layers:
1. somatic/parietal mesoderm
2. splanchnic/visceral mesoderm
lateral plate mesoderm splits into:
1. parietal layer (lines intraembryonic cavity) parietal + ectoderm forms lateral and ventral body wall
2. Visceral layer (surrounds organs) visceral + endoderm forms wall of gut
vasculogenesis
-formation of new blood vessels from lateral plate (splanchnic) and extraembryonic mesoderm
-de novo formation of endothelial tubes
-starts with individual precursor cells, form a tube
angiogenesis
-formation of new blood vessels from lateral plate (splanchnic) and extraembryonic mesoderm
-sprouting of tubes off existing vessels
capillary hemangiomas
-occur in 10% of all births
-can occur anywhere, but usually on face
-can be focal or diffuse
changing sites of hematopoeisis
week 3: extraembryonic- blood islands of yolk sac
then: aorta-gonad-mesonephros region (AGM)
2-7 months: liver-colonized by cells from AGM
7 months: bone marrow: colonized by cells from liver
limb bud formation
-limb buds are outpocketings from the ventrolateral body wall
-forelimbs appear 1-2 days before hindlimbs
limb buds have a mesenchymal core (derived from parietal/somatic LPM) that will form bones and CT, as well as a covering of cuboidal ectoderm
x

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