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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.
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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.
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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
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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.
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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, ...