Skeletal morphogenesis begins in structures called limb buds 4 1/2 weeks after gestation. The limb buds, comprised of mesodermal cells, form at discreet locations beneath a layer of ectoderm. The initial stimuli for limb development are still vague, but FGF-8 and possibly retinoic acid are important. By week 5 the limb primordium begins to bulge from the body wall. Despite the bud’s simple structure it contains enough information to guide its development, for if transplanted to another site or cultured in vitro it will become a limb. At the apex of the bud is a thickened ectoderm (apical ectodermal ridge) and experiments have shown that continuous interactions between the AER and underlying mesoderm promote the initiation phase, outgrowth phase, and subsequent skeletal morphogenesis. The limb develops along three axis’s; proximodistal (base of limb to digits); anteroposterior (digits 1 to 5); dorsoventral (top of hand/foot to palm/sole). Each is under the influence of “morphogens” made in discreet signalling centres.
Skeletal elements of the limbs first appear in a core of mesenchymal cells, then in cartilage (weeks 5 1/2 to 6 1/2), and finally in bone (6 to 8 weeks). The condensed cells that form the precartilage express bone morphogenetic proteins (BMP-2 and –4). As development continues they are restricted to cells that will become periosteum or perichondrium.
In contrast, BMP-6 is only expressed in maturing cartilage within limbs. Another signalling molecule, Indian hedgehog, found in the same location is thought to induce the BMP-6 expression.
Joint formation is a late feature in limb differentiation. Around week 6, between condensations in the precartilage, there develop transverse bands of tightly packed, flat mesenchymal cells, called interzones. At this time, growth/differentiation factor-5, a member of the BMP family, is expressed in the condensed cells. The interzone consists of two regions of high cell density surrounding a region of low cell density. The former will become articular cartilage, while the latter disappears through apoptosis leaving behind a cavity. At the periphery of each interzone are mesenchymal cells and blood vessels. These arise from the primordial perichondrium and will become the future synovium and joint capsule.
The synovium consists of an incomplete lining (the intima) sitting on a vascularized matrix. After birth two cell types can be identified. One (the type A synoviocyte) arises in the bone marrow, passes into the circulation and enters the joint as a monocyte, but during its progression into the intima acquires attributes of a tissue macrophage (including the CD68 phenotype). The other lining cell is a specialised fibroblast, similar to bone marrow stromal cells that supports hematopoeisis (type B –fibroblast-like synoviocyte). However, these only acquire VCAM-1 and the typical FLS phenotype after birth.
The shafts of long bones (diaphysis) are completely ossified at birth, but the ends of the bones (epiphysis) are still cartilagenous. Only later, when secondary centres of ossification develop does this segment gradually become bone. Even then, however, a layer of cartilage persists between the epiphysis and the growing end of the diaphysis. In man chondrocyte proliferation continues in this growth plate for about twenty years and not until epiphyseal fusion does bone growth cease.
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