Embryology - Musculoskeletal

paraxial mesoderm, lateral plate mesoderm, and cells from neural crest

What does the skeletal system develop from?

paraxial mesoderm, give rise to sclerotome (ventromedial part), and dermamyotome (dorsolateral part) (see figure 9.1)

Somites arise from what, give rise to what?

Formation of three primary germ layers

Gastrulation, what is it?


What are the somites of the head region called?

cranial to caudal fashion

What order do somites develop in?

Begin to closely grow around neural tube

What do sclerotome cells do in early development?

axial skeleton below skull (ribs and vertebral column)

What do sclerotomes develop into?

Spina bifida

What happens when the sclerotome doesn't develop?

growing spinal nerves from neural tube grow through sclerotome segments through Non Ebner's fissure, cranial end will go up to meet superior segment's caudal end. accounts for cranial nerve C8

What happens during the resegmentation of the sclerotome?


What does the nucleus pulposis derive from?


From what do dermis and trunk skeletal muscles derive?

epimere (proximal to primitive spinal cord), intermuscular septum, hypomere

What does the dermotome differentiate into?

Deep back muscules and extensors (erector spinae and transversespinalis), innervated by dorsal primary ramus

What does the epimere develop into?

outer, intermediate muscular layers, innervated by ventral primary ramus

What does the hypomere develop into?

Ventral inducer from notochord, induces formation of sclerotome

What is sonic hedghog?

dorsal inducer from dorsal neural tube and surface ectoderm, induces formation of dermomyotome

What is Wnt?

lateral plate mesoderm

Where does the appendicular skeleton come from?

dermatome, myotome

What does the dermomyotome divide into?

Week 4 (upper limbs start). By week 8, fingers visible

When do the upper and lower limbs begin to develop?


Where does the upper limb bud initially develop?


Where does the lower limb bud initially develop?

appendicular skeleton, ligaments, limb vasculature

What are the musculoskeletal derivatives of the lateral plate mesoderm?


Where does the limb musculature come from?

spinal ganglia, Schwann cells, melanocytes of dermis

What do migrating neural crest cells give rise to in the limbs?

Ridge at the distal end of limb bud. Produces FGF and Wnt7

What is the apical ectodermal ridge, and why is it important?

stimulates mitosis of underlying mesoderm that makes skeleton to lengthen limbs

What does fibroblast growth factor do?

helps organize things along the dorsal-ventral axis

What does the Wnt7 gene do?

zone of polarizing activity at the base of the limb bud

What directs organization of limbs along the anterior-posterior (cranial-caudal) axis?

directs cranial-caudal development of limb, through production of retinoic acid and sonic hedgehog

What is the zone of polarizing activity?

the mesoderm of the area

What determines what type of limb develops?

amelia, absence of limb growth.

What does absence of the apical ectodermal ridge produce?

endochondral ossification, intramembranous ossification

What are the two types of osteogenesis?

Mesoderm --> chondrocytes --> cartilage --> bone (characteristic of limb bones and most bones)

What is endochondral ossification?

mesoderm or neural crest cells condense into sheets of connective tissue, which then directly ossify (flat bones of skull, facial bones)

What is intramembranous ossification?

posterior and anterior condensations

How do cells of the myotomes condense after migration into the limb buds?

upper - extensors and supinators in the upper limb; lower - extensors and abductors in the lower limb

Posterior condensation of nerves innervate what in upper and lower limbs?

upper - flexors and pronators, lower - flexors and abductors

Anterior condensation of nerves innervate what in upper and lower limbs?

upper limb laterally, lower limb medially. occurs midway through limb development. results in spiral dermatome pattern in lower limb

Rotation of limbs?

Primary axial artery supplies, and a peripheral marginal sinus drains

What is the initial limb vasculature?


Lower limb innervations?

gains conenction with 7th thoracic intersegmelta artery, axial artery forms subclavian, axillary, and brachial arteries

How does the upper limb vasculature develop?

joins 5th lumbar intersegmental artery, but most of axial artery degenerates. new branch from the 5th lumbar arise as the external iliac artery and supplies most of the leg.

How does the lower limb vasculature develop?

inferior gluteal and sciatic arteries

What are the remnants of the lower limb axial artery?