Typically when we think of cells we envision opaque (or at best translucent) membrane-bound structures. However recently in a study of the developing avian cornea, Robert Young, Carlo Knupp, Christian Pinali, Kenneth Png, James Ralphs, Andrew Bushby, Tobias Starborg, Karl Kadler, and Andrew Quantock found a surprisingly large proportion (20%) of the volume is made up of cells. The cornea is where light enters the eye. It also provides the majority of the focusing power (the lens is for “fine tuning” of focus). If this study in birds also applies to primates, then we could be looking through many cells all the time!
The microscopic structure of the cornea is well known. It is an avascular specialized connective tissue assembled as a remarkably ordered array of superimposed collagenous lamellae, made of collagen fibrils, essential for optical transparency. It also must be biomechanically stable to restrain ocular pressure. In addition to immunocytochemistry and other conventional methods, Young et al. used volume-scanning electron microscopy to examine the ultrastructure of the cornea in three dimensions and in larger tissue volumes than were previously possible. The tissue volume was gradually decreased from the surface by two different methods: a focused ion beam essentially milled down the surface, or ultrathin sections were removed with an ultramicrotome. Scanning electron micrographs taken after each ultra-thin removal could be reconstructed for examination and analysis. The two different methods revealed unique features of the fibroblasts of the cornea, which are called keratocytes (Figure 1).
Figure 1 3D reconstruction of two keratocytes in embryonic chick corneal stroma at day 12 of development, obtained by 3View® serial block face scanning electron microscopy and IMOD segmentation. The cells and their processes exhibit orthogonal alignment in common with collagen fibrils (not shown in this image) in the developing extracellular matrix. Bar = 1 µm.
The surprising finding was that the volume of the cornea occupied by cells relative to extracellular matrix was 20%, higher than expected. The additional volume is apparently due to an extensive network of cellular processes that could not be fully appreciated using other methods. During development it appears as though relatively spherical cells are flattened as collagen is deposited, and as that happens the keratocytes extend long slender processes into the growing stroma. Young et al. showed that these tubular membrane extensions travel more than 30 microns from the main body of the cell. They also showed that these extensions were rich in actin, which is reminiscent of filopodia. These filopodia are much longer than usually described, but this study on fixed specimens could not provide evidence of motion. Young et al. propose that they be called “keratopodia” because they appear to be unique structures that have a cornea-specific function in orienting and organizing collagen into lamellae, crucial for transparency. Keratopodia permit a system of cell-matrix associations to be maintained at sites distant from the main secretory machinery of the cell during a period when collagen bundles increase and grow to fill the extracellular space. It will be interesting to see if similar structures exist within other tissues that form highly organized matrices.
