anatomy 10.11

Week of skeletal muscle formation onset

The 4th week of development.

Embryonic tissue for skeletal muscle development

Paraxial mesoderm.

Structures from paraxial mesoderm that give rise to skeletal muscle

Somites.

Appearance of the first somites

Around day 20 in the cranial region of the embryo.

When the 44th pair of somites appear

Around day 30 of development.

Regions into which each somite differentiates

Sclerotome, dermatome, and myotome.

Formation of the vertebral skeleton

Sclerotome.

Formation of the connective tissue of the skin

Dermatome.

Formation of skeletal muscles

Myotome.

Cells that divide rapidly to initiate muscle formation in the myotome

Myoblasts.

Process by which myoblasts form myotubules

They migrate and fuse together.

Definition of myotubules

Multinucleated structures formed by fused myoblasts.

Timeframe for primitive myofilaments to extend through myotubules

About 9 weeks.

Growth in length of developing skeletal muscles

By the addition of more myoblasts.

Most skeletal muscles develop from this embryonic structure

The myotomes.

Skeletal muscles that do not develop from myotomes

Some muscles of the head.

Definition of axial musculature

Muscles that attach to the axial skeleton.

Developmental origin of axial muscles

Adjacent myotomes.

Muscles formed by thoracic and lumbar myotomes

Lower back and abdominal muscles.

Muscles formed by some cervical myotomes

Neck muscles.

Timing for portion of somites to migrate into developing limbs

The 5th week of development.

Myotomes that form upper limb muscles

Cervical and thoracic myotomes.

Myotomes that form lower limb muscles

Lumbar and sacral myotomes.

Two muscle masses into which migrating myotomes divide

Ventral muscle mass and dorsal muscle mass.

Muscles that develop from the ventral muscle mass

Future flexor muscles.

Muscles that develop from the dorsal muscle mass

Future extensor muscles.

Example of a muscle from the ventral muscle mass

Biceps brachii.

Example of a muscle from the dorsal muscle mass

Triceps brachii.

Timing for ventral and dorsal muscle masses to differentiate into individual skeletal muscles

During the embryonic period.

Relationship between ventral and dorsal muscle masses and future flexors and extensors

The ventral muscle mass develops into flexor muscles, while the dorsal muscle mass develops into extensor muscles of the limbs.