Prune Belly Syndrome

Preaxial side of limb V

Postaxial side of limb

Preaxial side of limb V

Postaxial side of limb

Human Limb Bud
Figure 9.5 Limb buds with their segments of origin indicated. With further development the segmental pattern disappears; however, an orderly sequence in the dermatome pattern can still be recognized in the adult. A. Upper limb bud at 5 weeks. B. Upper limb bud at 6 weeks. C. Limb buds at 7 weeks.
Myotomes Upper Limbs
Figure 9.6 Scanning electron micrograph of a mouse upper limb bud, showing spinal nerves entering the limb. Asterisks, spinal ganglia.

and ventral nerves. Thus the radial nerve, which supplies the extensor musculature, is formed by a combination of the dorsal segmental branches, whereas the ulnar and median nerves, which supply the flexor musculature, are formed by a combination of the ventral branches. Immediately after the nerves have entered the limb buds, they establish an intimate contact with the differentiating mesodermal condensations, and the early contact between the nerve and muscle cells is a prerequisite for their complete functional differentiation.

Spinal nerves not only play an important role in differentiation and motor innervation of the limb musculature, but also provide sensory innervation for the dermatomes. Although the original dermatomal pattern changes with growth of the extremities, an orderly sequence can still be recognized in the adult (Fig. 9.5).

Clinical Correlates

Partial or complete absence of one or more muscles is rather common. One of the best known examples is total or partial absence of the pectoralis major muscle (Poland anomaly). Similarly, the palmaris longus, the serratus anterior, and the quadratus femoris muscles may be partially or entirely absent.

Partial or complete absence of abdominal musculature results in prune-belly syndrome (Fig. 9.7). Usually the abdominal wall is so thin that organs are visible and easily palpated. This defect is often associated with malformations of the urinary tract and bladder.

Prune Belly Syndrome
Figure 9.7 Prune belly syndrome: a distended abdomen from aplasia of abdominal wall musculature.

Cardiac Muscle

Cardiac muscle develops from splanchnic mesoderm surrounding the endothe-lial heart tube. Myoblasts adhere to one another by special attachments that later develop into intercalated discs. Myofibrils develop as in skeletal muscle, but myoblasts do not fuse. During later development, a few special bundles of muscle cells with irregularly distributed myofibrils become visible. These bundles, the Purkinje fibers, form the conducting system of the heart.

Smooth Muscle

Smooth muscle in the wall of the gut and gut derivatives is derived from splanchnic mesoderm surrounding the endoderm of these structures. Vascular smooth muscle differentiates from mesoderm adjacent to vascular endothe-lium. Sphincter and dilator muscles of the pupil and muscle tissue in the mammary gland and sweat glands originate from ectoderm.

Summary

Most muscles arise from the mesoderm. Skeletal muscles are derived from paraxial mesoderm, including (a) somites, which give rise to muscles of the axial skeleton, body wall, and limbs, and (b) somitomeres, which give rise to muscles of the head. Progenitor cells for muscle tissues are derived from the dorsolateral and dorsomedial portions of the somites. Cells in the dorsolateral portion express MYO-D and migrate to form hypomeric muscle; cells in the dorsomedial portion express MYF5, migrate ventral to the dermatome to form the myotome, and ultimately form epimeric musculature. By the fifth week muscle precursor cells are divided into a small dorsal portion, the epimere, innervated by a dorsal primary ramus, and a larger ventral portion, the hypomere, innervated by a ventral primary ramus. Myoblasts from epimeres form extensor muscles of the vertebral column, while those of the hypomere form limb and body wall musculature. Connective tissue derived from somites, somatic mesoderm, and neural crest (head region) provide a template for establishment of muscle patterns. Most smooth muscles and cardiac muscle fibers are derived from splanchnic mesoderm. Smooth muscles of the pupil, mammary gland, and sweat glands differentiate from ectoderm.

Problems to Solve

1. Muscle cells are derived from what two regions of the somite? Which region forms the epimere and which the hypomere? What muscles form from each of these regions?

2. In examining a newborn female, you note that her right nipple is lower than the left and that the right anterior axillary fold is nearly absent. What is your diagnosis?

3. Patterning of muscles is dependent on what type of tissue?

4. How do you explain the fact that the phrenic nerve, which originates from cervical segments 3, 4, and 5, innervates the diaphragm in the thoracic region?

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Responses

  • staci
    What is the preaxial part of the human body?
    8 years ago

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