Development of Bones of the Limbs

Development of Bones of the
All appendicular bones (except clavicle) develop by the endochondral
(intracartilaginous) method of osteogenesis. Mesenchymal models of limb
bones are formed during the’ 5th week of development. However, by the end
of the 6th week entire limb skeleton becomes cartilaginous.Limbs:Ossification of the long bones begins in the 7th week by the appearance of primary centers
of ossification in the middle of the cartilaginous rrodels. primary centers
of ossification are present in all the long bones of each limb by the end of
the 12th week of the intrauterine life. However, ossification of most of thecarpal and tarsal bones begins during the first year after birth.

Development of Muscles of the Limbs
As mentioned earlier, it has now been established that the limb muscles
do not develop from the local mesoderm but are derived from the mesoderm of the somites related to the limb buds cells derived from the dermomyotome region of the somites migrateinto the developing limb buds during the 4th week of development. As long bones begin to develop, the cells of somatic region give rise to myoblasts which form masses of muscles tissue located ventral and dorsal to the developing bones. The ventral muscle mass gives rise to the flexors and pronators of the upper limb and flexors and adductors of the lower limb, whereas the dorsal muscle mass gives rise to the extensors and abductors of the lower limb. The limb muscles are innervated by branches of the ventral primary rami of the spinal nerves (C5 to T1 for the upper limb and L4 to S3 for the lower limb).
Muscles derived from the ventral muscles mass are innervated by ante,
rior divisions of the ventral primary rami. Similarly, muscles originating in the dorsal muscles mass receive their nerve supply from the posterior divisions of the ventral primary rami