The appendicular skeleton begins at day 26 with upper limb buds, which appear as paired outpouchings along the lateral wall of the developing embryo. Cartilage, connective tissue, striated muscle, bone, and blood vessels all take their origins from the mesoderm. 1 The mesoderm is also responsible for most of the structures in the appendicular skeleton. These are the primordia of the future axial skeleton and all associated muscles. 1 After the formation of the notochord and neural tube, the mesodermal tissue alongside forms the paraxial mesoderm, which subsequently divides into 38 paired somites. The Embryonic Phase of Prenatal DevelopmentĪt the end of the third week of development, the three primary germ layers-ectoderm, endoderm, and mesoderm-have formed and will give rise to all tissues in the developing embryo. The complex genetic code and signaling pathways, which direct the development of the human hip, are mostly unknown. The hip joint is fully mature at 35 weeks post conception. The fetal phase, from 8 weeks to birth, consists of refinement of the vascular supply, ossification of the diaphysis of long bones, and formation of intra-articular structures. Vascular insults, confining pathology, and teratogens, which act during this phase, have the greatest likelihood of interfering with the development of healthy functional symmetric lower extremities. During this embryonic phase, the entire musculoskeletal system develops in the following ways: both upper and lower limb buds appear, along with the cartilaginous anlage of the osseous limbs joints, longitudinal growth plates, and vascular supply to the bones are formed and, ultimately, the complex articulated human mobility system is fully differentiated and formed. Following blastocyst, the period of primary tissue differentiation is weeks 2 through 8. The blastocyst phase refers to fertilization through the end of the second week post conception. Intrauterine life is divided into three phases: blastocystic, embryonic, and fetal. This chapter will elucidate normal hip embryology and some of the postnatal pathology that can influence the ultimate shape and motion of the mature adult hip. After birth, the hip joint remains immature and the intimate relationship between the femoral head and the acetabulum is crucial for appropriate postnatal development. Normal hip development is the result of a carefully orchestrated series of prenatal events, which act in concert to form the acetabulum, the femoral head, intra-articular structures of the hip, and the blood supply to the hip joint. The resultant deformity may cause significant alterations in gait and joint mechanics and may lead to early arthritis. Slipped capital femoral epiphysis may result when shear forces across the adolescent physis overwhelm its strength. Depending on the age of the child at presentation and the degree to which the lateral epiphysis is involved, significant residual deformity may lead to early arthritis. Legg-Calvé-Perthes disease results from a vascular insult in the immature femoral epiphysis.Developmental dysplasia occurs when the intimate fit between the developing femoral head and the developing acetabulum is disrupted if untreated, this may lead to poor hip joint congruity and early arthritis.The primordium of the hip joint is an amorphous cartilage model in which the two sides of the joint are indistinct until the autolytic process at the interzone creates a “joint space” during the seventh week post conception.Lower limb development occurs in utero in its entirety during the fetal stage of development.
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