Biophysics

Collagen Clarity

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Science  17 Jan 2014:
Vol. 343, Issue 6168, pp. 230
DOI: 10.1126/science.343.6168.230-c
CREDIT: R. D. YOUNG ET AL., PNAS (2 JANUARY 2014) © 2014 NATIONAL ACADEMY OF SCIENCES

The cornea is not merely the front layer of the eye; it also provides the majority of the focusing power. It is an unusual kind of tissue because it is transparent to light; hence, it does not contain blood vessels. The bulk of the cornea consists of the stroma, which contains stacked layers of aligned collagen fibrils. In order to develop a three-dimensional picture of what happens as the stroma develops, Young et al. collected a series of scanning electron microscope images, where either a focused ion beam or an ultramicrotome was used to scrape away the surface. Corneas were harvested from 10- to 18-day-old chick embryos, and over that time period, the collagenous matrix increased from 20 to 70% of the cornea volume. But the authors were surprised to find that 20% of the cornea was occupied by keratocytes. These cells adopt a flattened morphology featuring extended cell membranes that align with the collagen fibrils. They observed an orthogonal network of actin filaments that resembled filopodia, with tubular membrane projections sometimes traveling more than 30 µm. Some clusters of cells attached to one set of collagen bundles, whereas others would bridge between two or more. The authors believe that this extensive network of cells and collagen bundles is key for the construction of the lamellar structure of the stroma.

Proc. Natl. Acad. Sci. U.S.A. 10.1073/pnas.1313561110 (2014).

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