19: Musculoskeletal System Development Flashcards
Where is paraxial mesoderm + what is it continuous with?
Longitudinal columns along the developing neural tube, continuous with intermediate mesoderm
What is paraxial mesoderm derived from?
Primitive node
What is lateral plate mesoderm continuous with?
Extraembryonic mesoderm covering umbilical vesicle and amnion
When do somites form?
End of 3rd week
Sequence of somite formation
Craniocaudally
How many somite pairs are there initially -> by the end of the 5th week
28-40 -> 42-44 pairs
What do somites give rise to?
Most of axial skeleton and associated musculature and dermis
When does the sclerotome appear?
End of 4th week
What will the sclerotome develop into?
Vertebrae and ribs
What causes sclerotome to have cranial and caudal portions?
Gene expression and cell density
What forms the centrum?
Adjacent cranial + caudal segments of the sclerotome
Why do we have 8 cervical nerves and only 7 vertebrae?
The occipital bone is also formed by somites
What do ribs develop from?
Costal processes on T vertebrae
When do ribs ossify?
During fetal period (are cartilagenous in embryonic period)
Sternum development
Paired sternal bars move medially -> fuse at 10 weeks
What do the sternal bars arise from?
Lateral plate mesoderm (somatic division)
What do Hox boundaries correlate with in vertebrae?
Changes in vertebral shape/type of vertebrae
Thoracic outlet syndrome
A supernumerary rib on C7 can impinge the brachial plexus and/or subclavian A.
Pectus excavatum
Funnel chest; depressed sternum
Pectus carinatum
Pigeon chest; bilateral flattening of chest with anterior sternal protrusion
Steps in myogenesis
- Mesenchyme nuclei and cell bodies elongate -> myoblasts
- Myoblasts fuse -> myotubes
- Myofilaments develop in myotubes
- Myotubes + CT -> fascicles
When do skeletal muscles develop?
Most before birth; remaining ones are formed by one year
Poland syndrome
Absence of pec minor + partial loss of pec major -> ipsilateral breast hypoplasia
Two conditions Poland syndrome can be associated with
Syndactyly or brachydactyly
Population most at risk for Poland syndrome
Males
Prune belly syndrome
Partial or complete abs ente of abdominal musculature + cryptorchidism (failure of testes to descend) + malformation of urinary tract
