Middle germ layer that ultimately forms the bones and nearly all the muscles of the body

1. Mesoderm that hugs the gut tube and all its derivatives (splanchnic=visceral);

2. Forms smooth muscle of the lungs, GI tract, and bladder

1. Mesoderm that forms on either side of the neural tube;

2. Congregates into somites (block-like shapes that align themselves on both sides of the neural tube);

3. At 6 weeks, human embryo has 44 pairs of somites (4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 8-10 coccygeal)

4. Forms dermatome, myotome, sclerotome

1. Cells of the somites differentiate and migrate to form specific structures: sclerotome, myotome, and dermatome(from paraxial mesoderm)

 2. Limb buds appear

1. The cells of the somite closest to the neural tube that grow towards the neural tube and fuse with the corresponding sclerotome cells on the other side of the neural tube and spread out to completely surround the neural tube

2. These cells form the vertebral bodies, ribs, bones of limbs

Cells of the somites just lateral to sclerotome cells that differentiate into epimere and hypomere

1. Forms the muscles of the spine

2. Innervated by dorsal rami

1. Forms the muscles of the body wall and the limbs

2. Innervated by ventral rami

1. Part of the somite closest to the overlying ectodome that forms the subcutaneous tissue (dermis and hypodermis) in a band-like distribution in the transverse plane

2. Innervated by spinal sensory nerve

Mesoderm found in the body wall that contributes to tissue formation of the bones and connective tissue of the limbs (cells from sclerotome, myotome, and somite also contribute to muscles and bones of the limbs and body wall)

1. Upper limb development is 2-3 days ahead of lower limb development

2. Each limb has dorsal (extensor) and ventral (flexor) muscle component

3. Limb bones form first as cartilage at about 6 weeks and then later develop islands of ossification;

4. Focal areas of apoptosis in the cartilage begin the process of joint formation

1. Upper limbs rotate externally 90 degrees, whereas lower limbs rotate internally 90 degrees

2. This rotation accounts for the odd twisting of the dermatomes in the region of the shoulders and buttocks and explains why extensor muscles are located on the posterior side of the upper limbs and the anterior side of the lower limbs

1. Hands and feet begin as paddle-shaped structures at the ends of the limb buds

2. Through cell proliferation and radially arranged lines of cell death, the fingers and toes are formed

1. Syndactyly

2. Polydactyly

3. Amelia

4. Gastroschisis

1. Fusion of two or more fingers or toes.

2. This results from incomplete apoptosis in one or more of the radially arranged columns in the distal limb buds, where apoptosis should occur.

3. Syndactyly may be only cutaneous (two or more normal digits enclosed in the skin) or bony (the bones of the digits are fused as well)

1. Excess fingers or toes, most commonly adjacent to the little finger or toe

2. Due to an additional radially arranged column of apoptosis in the distal limb bud

1. Complete lack of limb

2. Historically, a thalidomide-induced malformation

1. A portion of the anterior abdominal wall, usually just to the right of the umbilicus, fails to form

2. This allows herniation of bowel contents