Chapter 210

Growth and development of the musculoskeletal system, in tandem with gross and fine motor development, are overshadowed perhaps only by intellectual development in terms of magnitude, complexity, and sophistication from conception to skeletal maturity. The pathway delineated by postnatal development defines infancy, childhood, and adolescence. In the simplest terms, the growth plates (or physes) of long bones are the “motors” of skeletal development. Their physical characteristics and function help separate the “child” from the “adult,” orthopedically speaking. In this chapter, we will briefly review musculoskeletal embryology, postnatal musculoskeletal development, and the structure and function of the growth plate.

Embryologic development may be thought of as having two phases: an embryonic period during which organ systems evolve and organize, and the fetal period, from the 12th week of gestation, during which these organ systems grow and mature.

Embryologically, the musculoskeletal system develops from the dorsal (somatic) mesoderm. Approximately 26 days after fertilization, the upper limb “bud” appears on the ventrolateral body wall, consisting of mesenchymal cell swelling covered by ectoderm. The tip of this ectodermal layer thickens to become the apical ectodermal ridge (AER), under which an area of rapidly proliferating, undifferentiated mesenchymal cells forms (the progress zone). The lower limb bud appears about 2 days after the upper; this slight delay relative to the upper limb in appearance, organization, and maturation continues throughout limb development. With continued growth and development, a chondrogenic core forms, beginning proximally and progressing distally (characteristic of both upper and lower limb formation). This chondrogenic core evolves to the point that the upper limb skeletal elements, except for the distal phalanges, are present by the seventh week (eighth for the lower limb). Subsequent to the development of the chondrogenic skeletal structure, nerve invasion of the limb occurs, followed quickly by skeletal muscle tissue formation. Joints form by programmed chondrogenic cell death (apoptosis) producing clefts within the chondrogenic skeleton. Further healthy joint development is dependent on movement, which is in turn dependent on the development and maturation of innervated skeletal muscle. By the eighth week, major tissue differentiation in the limbs is complete, and the majority of the remaining fetal period is dedicated to growth. Excellent detailed reviews of musculoskeletal embryology are available.1-3

Bone is a composite tissue, consisting of an organic component (osteoid) and inorganic matrix (primarily hydroxyapatite) that give bone its hardness. Bone is formed by one of two pathways: intramembranous and enchondral ossification. Intramembranousossification is characterized by direct ossification of fibrous primitive connective tissue; the clavicle and bones of the skull form intramembranously. During intramembranous ossification, dense nodules of mesenchymal cells convert to capillaries and osteoblasts. These osteoblasts secrete osteoid that subsequently becomes mineralized. The exact mechanism that induces mesenchymal cells to convert into osteoblasts is not known.

All other appendicular and axial bones form by enchondral ossification—that is, by gradual ossification of a cartilaginous anlage. Instead of converting directly into osteoblasts, ...

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