Development Of MSK Flashcards
Cells derived from the primitive node form what?
Paraxial mesoderm which is a thick, bilateral longitudinal column of cells along the developing NT
Each column of paraxial mesoderm is continuous with what?
Intermediate mesoderm which gradually thins into a layer of lateral mesoderm
What is lateral (plate) mesoderm continuous with?
The extraembryonic mesoderm covering the umbilical vesicle and amnion
Each population of mesoderm contributes to the formation of what?
Numerous derivatives
What does the intermediate mesoderm form?
Kidneys and gonads
What does the notochord develop into in the adult?
Intervertebral discs (nucleus pulposus)
What does the paraxial mesoderm form?
The head and somites which further differentiate into sclerotome, myotome, syndetome, dermatome and endothelial cells
What does the lateral plate mesoderm form?
Splanchnic (CVS), somatic (body cavity, pelvis, limb bones) and extraembryonic structures
When does the paraxial mesoderm differentiate and condense to form paired somites?
End of the 3rd week
Briefly describe somite formation
Formed in a craniocaudal sequence and first appear in the occipital region
39-39 pairs of somites develop initially
42-44 pairs form by the end of 5th week
What do somites give rise to?
Most of the axial skeleton and associated musculature as well as to the dermis of the skin
What do somites form as a result of/
Segmentation clock
Shortly after forming, each somite divides into subdivisions that give rise to what?
Specific mesoderm components
What does the ventromedial part of a somite form after undergoing EMT?
The sclerotome
What does the dorsal portion of a somite form?
Dermomyotome which will further divide into the dermatome and myotome
What does the intermediate portion of a somite form?
Syndetome (between sclerotome and myotome)
What is the sclerotome?
Regionalized structures that will develop into vertebrae and ribs
The ventral region of a sclerotome migrates to surround the notochord and forms what structure?
Vertebral body (central region also contributes)
The dorsal cells of the sclerotome that surround the NT form what structure?
Vertebral arch and vertebral spine
What do the lateral cells of the sclerotome form?
Transverse processes and ribs
Sclerotomes are divided into cranial and caudal portions due to what?
Gene expression and cell density
What divides the cranial and caudal portions of the sclerotome?
Intersegmental boundary
Describe the cranial portion of the sclerotome
Loosely packed
Describe the caudal portion of the sclerotome
Densely packed
Describe resegmentation of the sclerotome
End of 4th week sclerotome cells appear as paired condensations around the notochord
Some caudal cells move cranially and form the intervertebral disc
Remaining densely packed cells fuse with the cells of the next caudal sclerotome forming the centrum
Centrum develops from adjacent cranial and caudal segments and becomes an intersegmental structure
Spinal nerves lie near intervertebral discs and extend from SC to innervate the myotome
Intersegmental arteries lie on each side of the vertebral bodies
Describe intervertebral disc formation
Notochord expands to form the nucleus pulposus
Nucleus pulposus becomes surrounded by caudal cells of the sclerotome which forms circularly arraigned fibers that comprise the annulus fibrosis
Describe development of ribs
Ribs develop from costal processes of thoracic vertebrae
Cartilaginous during embryonic period and ossify during fetal period
Describe development of the sternum
Paired sternal bars arise in the body wall from somatic layer of ventral plate mesoderm
Located ventrolaterally and will move medially
Fuse at 10 weeks to form cartilaginous models of the manubrium, sternebrae and xiphoid process
What do Hox genes code for?
Vertebrae identity
Describe Hox gene family members
Have a nested pattern of expression boundaries along cranial caudal axis
These boundaries correspond to changes in vertebral shape -> changes in expression cause transformations in vertebrae
Which gene is important for the thoracic/lumbar boundary?
Hox10
What happens with loss of Hox10 paralogs?
Lumbar and sacral -> thoracic identity and have ribs
What occurs with a gain of Hox10 paralogs?
Thoracic vertebrae become lumbar instead
What are cervical ribs?
A malformation when ribs attach to cervical vertebrae (typically C7)
May impinge on brachial plexus and/or subclavian artery (thoracic outlet syndrome)
What is pectus excavatum?
Depressed sternum and sunken posteriorly
Funnel chest
Underlying mechanism is extra cartilage formation of the internal aspect of the sternal bars and pulls them deeper into the eloping chest wall
What is pectus carinatum?
Bilateral flattening of the chest with anterior sternal protrusion
Pigeon chest
Myogenic precursors undergo what process to form which structures?
EMT to form skeletal muscle of the trunk and limbs
What does myogenesis begin with?
Elongation of the nuclei and cell bodies of mesenchymal cells -> differentiate into myoblasts
Describe myogenesis
Mesenchymal cells differentiate into myoblasts
Myoblasts fuse -> myotubes form which are elongated, multinucleated cylindrical structures
Myofilaments develop in the cytoplasm of the myotubes and become invested with CT to produce fascicles
When do skeletal muscles develop?
Most develop before brith and almost all the remaining muscles are formed by the end of the first year
Each myotome divides into what?
An epimere and hypomere
What is the epaxial (epimere) division of the myotome?
Extensor muscles of the neck and vertebral column
What is the hypaxial division (hypomere) of the myotome?
Cervical, thoracic, lumbar and sacrococcygeal myotomes
What is the cervical myotome?
Scalene, prevertebral, geniohyoid and infrahyoid muscles
What is the thoracic myotome?
Lateral and ventral flexor muscles of the vertebral column
What is the lumbar myotome?
Quadratus lumborum
What is the sacrococcygeal myotome?
Pelvic floor musculature, striated muscles of anus and sex organs
Every spinal nerve divides to innervate what?
Trunk musculature by either dorsal or ventral ramus
What does the dorsal ramus supply?
Epaxial muscles
What does the ventral ramus supply?
Hypaxial muscles
What is Poland syndrome?
Absence of pectoralis minor and partial loss of pec major
Ipsilateral breast hypoplasia
Associated with syndactylyl or brachydacytly
More frequent in males
What is prune bell syndrome?
Partial or complete absence of abdominal musculature
Cryptorchidism (failure of one/both testes to descend)
Malformation of urinary tract and bladder causing urethral obstruction and fluid accumulation that distends the abdomen causing muscle atrophy
