B6.012 - Early Development of Skeletal System Flashcards
ectoderm derivatives
neuraal crest originating from neural tube
mesoderm derivatives
paraxial and lateral plate
where does muscle tissue originate from
mesodermal germ layer
where do skeletal muscle, smooth muscle and cardiac muscle originate from
skeletal - paraxial mesoderm
smooth - splanchnic/visceral LPM
Cardiac - splanchnic/visceral LPM
describe skeletal muscle early development
skeletal muscle arises from somitomeres and somites (paraxial mesoderm derivatives).
describe development of skeletal muscle in the head
The head muscle comes from 7 somitomeres in occipital region, because it forms so quickly it doesn’t undergo epithelialization like body wall and limb muscle
describe development of skeletal muscle in the body wall and limbs
somitomeres undergo epithelialization to form balls of epithelial cells with cavities (somites) extending from occipital region to tail bud
what do each paraxial mesoderm derived somite form
sclerotome (ribs, vertebrae, rib cartilage) myotome (muscles of back and body wall) dermatome (connective tissue of dermis of back) ventrolateral cells (most of musculature for body wall)
describe formation of the sclerotome and dermatome
ventral region of somite becomes mesenchymal again to form sclerotome dorsal region of somite becomes dermatome and 2 muscle forming regions (Dorso-medial and ventro-lateral); cells migrate ventrally
describe the formation of the dermamyotome
its formed from migrating DM and VL muscle cells migrating ventral to dermatome to form the myotome
describe formation of skeletal muscles
myotome cells form muscles of the back, shoulder girdle and intercostal m.
Subset of ventro-lateral muscle cells migrate into the lateral plate mesoderm (parietal layer) to form the infrahyoid, abdominal wall and limb muscles
what muscles are formed by the ventro lateral muscle cells that migrate to the lateral plate mesoderm
infrahyoid, abdominal wall, limb muscles
what is the LSF (Lateral somitic fronteir)
separates 2 domains of muscle precursor cells (based on origin) and defines molecular signaling
what are the 2 domains separated by the LSF
primaxial and abaxial
what is the primaxial domain
region around the neural tube; contains only somite (paraxial mesoderm) derived cells signaing from neural tube and notochord
what is the abaxial domain
parietal layer of LPM plus somite cells from ventrolateral regions of myotome that migrated across LSF (signaling from LPM)
describe skeletal muscle formation from myoblasts
muscle precursors (myoblasts) fuse and form long, multinucleated muscle (myo) fibers, wrapped in CT (endomysium)
myofibrils appear in cytoplasm, cross striations appear at end of 3rd month
bundles of myofibers wrapped in CT (epimysium) are called fascicles, partitions of CT (perimysium) form septa
CT contains blood vessels and nerves
describe cardiac muscle formation
in the 4th week
develops from the lateral plate (splanchnic) mesoderm (mesenchyme) surrounding endothelial heart tube
myoblasts adhere to each other by special attachments that develop into intercalated discs, growth of myofibers occurs by formation of new myofilaments
unlike skeletal muscle myoblasts DO NOT FUSE
describe myofibers
mono or binucleated
some bundles of muscle cells with irregularly distributed (fewer) myofibrils and larger diameters form purkinje fibers
describe smooth muscle formation
multiple origins: LPM, SMC of some blood vessels
proepicardial cells and neural crest: SMC of coronary arteries
surrounding LPM (splanchnic) SMC of wall of gut and its derivatives
ectoderm: SMC of pupil sphincter and dilator muscles, myoepithelial cells in mammary and sweat glands
describe differentiation of smooth muscle cell formation
differentiation of mesenchymal cells begins with development of elongated nuclei in spindle shaped byoblasts
myoblasts do NOT fuse and remain mononucleated
in later development more SMC are formed by division of existing myoblats
filamentous but non sarcomeric contractile elements develop in cytoplasm
smooth muscle fibers develop into sheets or bundles
describe the origin of tendons
axial - dorsolateral sclerotome (PA mesoderm) derivatives, lie adjacent to myotomes and ant. & post. somite borders
limb - lateral plate mesoderm and dorsolateral sclerotome
describe the origin of ligaments
scleraxis is marker
exact origin from somite not clear
describe deep fasica origins
originates from mesenchymal (mesodermal) undifferentiated CT
present in embryo from week 21
runs parallel to skin below subcutaneous adipose tissue, projections extend superficially to organize adipose tissue, and deeply to embed muscle