Development of the Musculoskeletal System Flashcards
Where are cells that form the paraxial mesoderm derived from?
the primitive node
- thick, bilateral, longitudinal columns of cells along the developing neural tube
- eventually gives rise to structures of the head and somites
paraxial mesoderm
Describe the process of intermediate and lateral mesoderm formation from paraxial mesoderm:
- each column of paraxial mesoderm is continuous w/ intermediate mesoderm, which gradually thins into a layer of lateral mesoderm
- lateral (plate) mesoderm is continuous w/ the extraembryonic mesoderm covering the umbilical vesicle and amnion
What are the derivatives of:
- paraxial mesoderm:
- intermediate mesoderm:
- lateral plate mesoderm:
- chorda-mesoderm (notochord):
- paraxial mesoderm: head and somites, somites > sclerotome, syndetome (tendons), myotome (skeletal muscle), dermatome (dermis, skeletal muscle), endothelial cells (dorsal aorta)
- intermediate mesoderm: kidneys, gonads
- lateral plate mesoderm: splanchnic (circulatory system), somatic (body cavity, pelvis, limb bones), and extra-embryonic
- chorda-mesoderm (notochord): notochord > nucleus pulposus portion of intervertebral disc
Describe the process of somite formation:
- at the end of the 3rd week, paraxial mesoderm differentiates, condenses, and forms paired somites
- form in a craniocaudal sequence
- first appear in the future occipital region
- 38-39 pairs develop initially, 42-44 pairs form by end of the 5th week
- give rise to most of the axial skeleton and associated musculature, as well as dermis
- form as a result of segmentation clock
What are the different subdivisions of somites that they develop into shortly after forming?
- ventromedial part of somite undergoes EMT > forms the scelrotome
- dorsal portion becomes dermamyotome > further divides into dermatome and myotome
- intermediate portion > syndetome (between sclerotome and myotome)
(EMT = something that is well organized (epithelia), becomes more disorganized (mesenchyme)
What does the sclerotome develop into?
- in general:
- ventral region:
- dorsal region:
- lateral region:
- in general: vertebrae and ribs
- ventral region: migrates to surround the notochord > forms vertebral body
- dorsal region: surrounds the neural tube > forms vertebral arch and vertebral spine
- lateral region: forms tranverse processes and ribs
How are portions of the central sclerotome divided?
Why is it important?
- sclerotomes are subdivided into cranial and caudal portions due to gene expression and cell density (cranial is loosely packed, caudal is densely packed); these portions are divided by intrasegmental boundaries (von Ebner’s fissures)
- the intrasegmental boundaries are important breaks within the sclerotome to allow nerves to pass through
Describe resegmentation of the central sclerotome:
- at the end of the 4th week, sclerotome cells appear as paired condesations around the notochord
- some caudal cells (densely packed) move cranially and form the intervertebral disc
- remaining densely packed cells fuse w/ the cells of the next caudal sclerotome, forming the centrum
- centrum develops from adjacent cranial and caudal segments, becoming an intersegmental structure
- spinal nerves lie near intervertebral discs, extending from the spinal canal to innervate the myotome
- intersegmental arteries lie on each side of the vertebral bodies
How are intervertebral discs formed?
- notochord expands to form the nucleus pulposus
- nucleus pulposus becomes surrounded by caudal cells of the sclerotome > forms circularly arranged fibers that comprise the annulus fibrosus
tumor from remnants of the notochord
chordoma
Where are ribs derived from?
Where is the sternum derived from?
- ribs: develop from costal processes of the thoracic vertebrae; cartilaginous during embryonic period and ossify during fetal period (costovertebral joints become more distinct throughout development, start off as fused and become a joint); occurs day 35
- sternum: paired sternal bars arise in the body wall from somatic layer of lateral plate mesoderm; ventrolateral and move medially; fuse (10 weeks) to form cartilaginous models of the manubrium (some cells from NCC’s), sternebrae (segments of sternal body), and xiphoid process; occurs day 43
What genes are responsbile for vertebrae deferentiation?
- Hox genes
- pattern of expression boundaries along cranial-caudal axis
- boundaries correspond to changes in vertebral shape; changes in expression cause transformations of vertebrae
What will occur if you add or remove Hox10?
- add: all vertebrae will be lumbar, no ribs will form
- remove: normal cervical vertebrae, remaining vertebrae will be thoracic, even in place of lumbar/sacral areas, ribs will grow that should not be there
What symptom may occur with extra cervical rib?
- usually unilateral and attached to C7 (1% of population)
- may impinge on brachial plexus and/or subclavian artery (thoracic outlet syndrome)
- depressed sternum, sunken posteriorly
- accounts for 90% of chest wall anomalies, more common in males
- thought to be caused by extra cartilage on dorsal aspect of developing sternum
pectus excavatum
- bilateral flattening of the chest w/ anterior sternal protrusion
- “pigeon chest”
- unknown etiology
pectus carinatum
Describe the process of myogenesis:
- myogenic precursors (myotome) undergo epithelial-mesenchymal transformation (EMT) to form skeletal muscle of the trunk and limbs
- myogenesis begins w/ elongation of the nuclei and cell bodies of mesenchymal cells > differentiate into myoblasts
- myoblasts fuse > myotubes (elongated, multinucleated, cylindrical structures)
- myofilaments and myofibrils develop in the cytoplasm of myotubes, and are connected w/ connective tissue (endomysium, perimysium, and epimysium) to form muscles
(most skeletal muscles develop before birth, almost all remaining muscles are formed by end of 1st year)
*TLDR: mesenchymal cells > myoblasts > postmitotic myoblasts > myotubes > myofilaments + myofibrils > muscular fibers + connective tissue > muscle
What are the 2 derivatives of myotome?
What are their derivatives? lol
- epimere (expaxial division): extensor muscles of neck and vertebral column
- hypomere (hypaxial division): cervical myotomes (scalene, prevertebral, geniohyoid, and infrahyoid Ms.), thoracic myotomes (lateral/ventral flexor Ms. of vertebral column), lumbar myotomes (quadratus lumborum), sacroccocygeal myotomes (pelvic floor Ms., striated Ms. of anus and sex organs)
(epaxial muscles: erector spinae, transversospinal muscles (multifidus, semispinalis and rotatores), splenius and suboccipital muscles)
(hypaxial muscles: some vertebral muscles, the diaphragm, the abdominal muscles, and all limb muscles)
- absence of pectoralis minor and partial loss of pectoralis major
- ipsilateral breast hypoplasia
- a/w syndactyly or brachydactyly
- more frequent in males, 1:20,000
poland syndrome
Diagram of somite derivation:
(good study tool)
