Expression of basal lamina protein mRNAs in the early embryonic chick eye

Laminin, collagen IV, collagen XVIII, agrin, and nidogen are major protein constituents of the chick retinal basal lamina. To determine their sites of synthesis during de novo basal lamina assembly in vivo, we localized their mRNA expression in the eye during maximum expansion of the retina between embryonic day (E) 2.5 and E6. Our in situ hybridization studies showed that the expression pattern of every basal lamina protein mRNA in the developing eye is unique. Collagen IV and perlecan originate predominantly from the lens epithelium, whereas collagen XVIII, nidogen, and the laminin gamma 1 and beta1 chains are synthesized mainly by the ciliary body. Agrin, collagen XVIII, collagen IV, and laminin gamma 1 also originate from cells of the optic disc. The only basal lamina protein that is synthesized by the neural retina throughout development is agrin with ganglion cells as its main source. Some of the mRNAs have short, transient expressions in the retina, most notably that of collagen IV and laminin gamma 1, both of which appear in the ventral retina between E4 and E5. That most retinal basal lamina proteins originate from extraretinal tissues infers that the basal lamina proteins have to be shed from the lens, optic disc, and ciliary body into the vitreous body. The assembly of the retinal basal lamina then occurs by the binding of these proteins by cellular receptor proteins on the vitreal endfeet of the retinal neuroepithelial cells.