Figure 1.

Light microscopic localization of cartilage oligomeric protein (COMP) during early human bone and joint development. (a) The basement membrane zone of the dermal-epidermal junction is positive in a human embryo at (gestational week) gw 8 (arrows); the loose mesenchyme is not stained. (b) The same is true for the apical ectodermal ridge (AER), the starting point of limb development. Also, the condensed mesenchyme at this developmental stage is not stained. (c) At gw 10, the matrix of developing bones is positive for COMP. (d) Later, at gw 12, during joint development, COMP staining is restricted to the outer margins of the developing epiphysis (arrows), whereas the developing acetabulum shows still less staining (asterisks). (e) Pronounced staining for COMP (arrows) is seen adjacent to the developing joint space. The arrowhead indicates the area from which the high-magnification micrograph was taken (inset). The arrowhead in the inset indicates COMP staining. (f) At gw 12, COMP staining is found in the outer regions of the diaphysis and is mainly pericellular (inset). Bars = 70 μm in (f), as for (a)-(e), and 40 μm in inset (f), as for inset (e).