Vbones Of The Limbs And Limb Girdles

A. Development in general (see Chapters 19 and 20)

1. The bones of the limb and limb girdles develop from condensations of lateral plate mesoderm within the limb buds. The limb buds are visible at week 4 of development; the upper limb appears first. The limbs are well differentiated at week 8.

2. The limb tip contains the apical ectodermal ridge, which exerts an inductive influence on limb growth and development.

B. Clinical correlations

1. Amelia, an absence of one or two extremities, may result from the use of the teratogen thalidomide.

2. Polydactyly is an autosomal dominant disorder that is characterized by the presence of extra digits on the hands and feet.

Upper Limb Bones Moore

Figure 17-5. (A) Boy with achondroplasia. Note the short stature, short limbs and fingers, normal length of the trunk, bowed legs, relatively large head, prominent forehead, and deep nasal bridge. (13) Girl with Marian's syndrome. Note the unusually tall stature, exceptionally long limbs, and aiachnodaciyly (elongated hands and feec with very slender digits). (C) Infant with osteogenesis imperfecta. Note the short, bowed lower limbs. (D) Radiograph of the lower limbs in a patient with osteogenesis imperfecta. Note the multiple fractures that result in an accordion-like shortening of the limb. (A, from Moore KL, Persaud TVN: The Developing Human: Clinically Oriented Embryology, 6th ed. Philadelphia, WB Saunders, 1998, p 179. B, C, and D, from Salmon MA, Lindenbaum RH: Developmental Defects and Syndromes. Aylesbury, England, HM & M Publishers, 1978, pp 172 and 251.)

Figure 17-5. (A) Boy with achondroplasia. Note the short stature, short limbs and fingers, normal length of the trunk, bowed legs, relatively large head, prominent forehead, and deep nasal bridge. (13) Girl with Marian's syndrome. Note the unusually tall stature, exceptionally long limbs, and aiachnodaciyly (elongated hands and feec with very slender digits). (C) Infant with osteogenesis imperfecta. Note the short, bowed lower limbs. (D) Radiograph of the lower limbs in a patient with osteogenesis imperfecta. Note the multiple fractures that result in an accordion-like shortening of the limb. (A, from Moore KL, Persaud TVN: The Developing Human: Clinically Oriented Embryology, 6th ed. Philadelphia, WB Saunders, 1998, p 179. B, C, and D, from Salmon MA, Lindenbaum RH: Developmental Defects and Syndromes. Aylesbury, England, HM & M Publishers, 1978, pp 172 and 251.)

3. Syndactyly (webbed fingers or toes), the most common limb anomaly, results from failure of the hand or foot webs to degenerate between the digits.

4. Holt-Oram syndrome (heart-hand syndrome), an autosomal dominant condition associated with chromosome 12, causes anomalies of the upper limb and heart.

VI- OSTEOGENESIS. Osteogenesis occurs through the conversion of pre-existing connective tissue (mesoderm) into bone, a process called ossification. During development, two types of ossification occur:

A. Intramembranous ossification occurs in the embryo when mesoderm condenses into sheets of highly vascular connective tissue, which then directly forms a primary ossification center. Bones that form via intramembranous ossification are the frontal bone, parietal bones, intraparietal part of the occipital bone, maxilla, zygomatic bone, squamous part of the temporal bone, palatine, vomer, and mandible.

B. Endochondral ossification occurs in the embryo when mesoderm first forms a hyaline cartilage model, which subsequently develops a primary ossification center at the dia-physis. Bones that form via endochondral ossification are the ethmoid bone, sphenoid bone, petrous and mastoid parts of the temporal bone, basilar part of the occipital bone, incus, malleus, stapes, styloid process, hyoid bone, bones of the limbs, limb girdles, vertebrae, sternum, and ribs.

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