Independent deposition of collagen types II and IX at epithelial–mesenchymal interfaces

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Abstract
Previous studies have demonstrated the presence of type II collagen (in mature chickens predominantly a ‘carti-lage-specific’ collagen) in a variety of embryonic extra-cellular matrices that separate epithelia from mesen-chyme. In an immunohistochemical study using collagen type-specific monoclonal antibodies, we asked whether type IX collagen, another ‘cartilage-specific’ collagen, is coexpressed along with type II at such interfaces. We confirmed that, in the matrix underlying a variety of cranial ectodermal derivatives and along the ventro-lateral surfaces of neuroepithelia, type II collagen is codistributed with collagen types I and IV. Type IX collagen, however, was undetectable at those sites. We observed immunoreactivity for type IX collagen only within the notochordal sheath, where it first appeared at a later stage than did collagen types I and II. We also observed type II collagen (without type IX) beneath the dorsolateral ectoderm at stage 16; this correlates with the period during which limb ectoderm has been reported to induce the mesoderm to become chondro-genic. Finally, in older hind limbs we observed subepith-elial type II collagen that was not associated with subsequent chondrogenesis, but appeared to parallel the formation of feathers and scales in the developing limb. These observations suggest that the deposition of collagen types II and IX into interfacial matrices is regulated independently, and that induction of mesenchymal chondrogenesis by such matrices does not involve type IX collagen. Subepithelial type IX collagen deposition, on the other hand, correlates with the assembly of a thick multilaminar fibrillar matrix, as present in the notochordal sheath and, as shown previously, in the corneal primary stroma.