CORNIFIED CELL-ENVELOPE ASSEMBLY - A MODEL-BASED ON ELECTRON-MICROSCOPIC DETERMINATIONS OF THICKNESS AND PROJECTED DENSITY

In stratifying squamous epithelia, the cornified cell envelope (CE), a peripheral layer of crosslinked protein, is assembled sequentially fr om precursor proteins initially dispersed in the cytoplasm. Its major component is loricrin (37 kDa in mouse), which contributes from approx . 60% to >80% of the protein mass in different tissues. Despite its im portance to the mechanical resilience and impenetrability of these tis sues, detailed information has not been obtained on CE structure, even on such basic properties as its thickness or uniformity across a give n CE or from tissue to tissue. To address this issue, we have studied CEs isolated from three murine epithelia, namely epidermis, forestomac h and footpad, by electron microscopy of metal-shadowed specimens and scanning transmission electron microscopy (STEM) of unstained specimen s. The former data reveal that the cytoplasmic surface is smoothly tex tured whereas the extracellular surface is corrugated, and that the av erage thickness is 15.3+/-1.2 nm, and strikingly uniform. Measurements of mass-per-unit-area from the STEM images yielded values of approx, 7.0+/-0.8 kDa/nm(2), which were remarkably consistent over all three t issues. These data imply that the mature CE has a uniquely defined thi ckness. To explain its uniformity, we postulate that loricrin forms a molecular monolayer, not a variable number of multiple layers. In this scenario, the packing density is one loricrin monomer per 7 nm(2), an d loricrin should have an elongated shape, 2.5-3.0 nm wide by approx. 11 nm long. Moreover, we anticipate that any inter-tissue variations i n the mechanical properties of CEs should depend more on protein compo sition and cross-linking pattern than on the thickness of the protein layer deposited.