Fine structure of the developing avian corneal stroma as revealed by quick-freeze, deep-etch electron microscopy

Corneal transparency depends on the precise organization of the stromal extracellular matrix. The morphology of the extracellular matrix of the embryonic and adult avian secondary posterior cornea was studied in glutaraldehyde-fixed, quick-frozen, deep-etched replicas with the electron microscope. Although the collagen fibrils changed from a loose network to a more ordered parallel lamellar arrangement during development, their mean diameter remained constant between 30.3 and 31.2 nm. Besides collagen fibrils, other extracellular matrix components were observed: (i) straight or Y-shaped cross-bridging interfibrillar 8-10 nm filaments with 18-22 nm globules; (ii) relatively loose networks of 10-20 nm beaded filaments, with a mean periodicity of 107 nm, often running perpendicular to the collagen fibrils and adhering to the plasma membrane of stromal cells at early developmental stages; (iii) straight or curved 6-12 nm strands forming sheets within the stromal matrix that progressively disappeared, whereas similar structures persisted at the Descemet's membrane-stroma interface; (iv) dense networks of filaments with 6-8 nm filaments, sometimes polygonally arranged, and a substructure of 2-3 nm filaments with globular domains, which progressively disappeared with maturation but remained at the Descemet's membrane-stroma interface; (v) polygonal networks of 9-10 nm filaments with globular domains adherent to the surface of cell plasma membranes at early developmental stages. The temporal expression of deep-etched supramolecular structural assemblies is compatible with that of the so-called 'interstitial basement membrane components' previously described. The quick-freeze and deep-etching method can reveal important aspects of the in situ organization of the corneal extracellular matrix.