A circular 4 mm endothelial defect was induced by transcorneal freezing. The experimental damage and the healing took place in the living rabbit in 15 eyes, and in the isolated cornea in organ culture in further 20 eyes. The reparative process was studied by SEM, and proved to be the same in vivo and in vitro. The defect was covered with endothelial cells after 3 days. The normal hexagonal pattern was regained after 3 weeks. Both cell migration and cell division were involved in the reparative process. Only cells recruited from a zone close to the defect were active; the cells situated more than a few cell diameters from the original edge maintained their form and size unchanged. The first phase of cell division was the formation of a spherical cell with numerous blebs on its surface.

The rabbit cornea endothelium has been studied with specular and scanning electron microscopy after exposure with a commercially available ultraviolet lamp. The animals were plased 50 cm from the source, and most experiments were run with a 20 min exposure time. In the specular microscope, the endothelium revealed numerous circumscript reflex-free areas, which were observed from 4 days up to 8 months after the irradiation. A wide variety of changes were found in the scanning electron microscope. One day after the exposure the cells showed indistinct outlines, and they were bulging into the anterior chamber. In some regions the pattern was dominated by marked grooves preferentially along the cell demarcations, whereas other regions showed rough surfaced cells because of cytoplasmic projections and grainy deposits. At the 3 days stage there was a marked pleomorphy, and some cells were on their way to being rejected. After one week the cobblestone appearance of the endothelium was about to decline. Partly rejected cells were still common. In addition, distinct, ringshaped local impressions occurred imitating the previous described grooves. This phenomenon along with some single endothelial cells studied with microvilli were the abnormal findings 8 months after the damage. It is concluded that although ultraviolet radiation is largely absorbed in the cornea, the threshold dose for damage of the endothelium is not beyond those used for practical purpose. The endothelium was surprisingly resistant towards repeated radiation damage.

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The effects of keeping rat tympanic membranes with an artificially made pars tensa perforation in tissue culture were observed under a scanning electron microscope. After one day and onwards, spreading and thickening of the keratinizing, outer squamous epithelium (OE) was noted. In addition, ballooning of the innermost cells of the outer epithelium apposing the inner tympanic epithelium (IE) was seen. No appreciable reaction was noted in the connective tissue layer of the drum. The inner tympanic epithelium appeared to be swollen, containing spherical structures in the cytoplasm, especially close to the area of contact with the outer meatal epithelium. No complete cover of the drum defect was seen after 14 days of tissue culture. Hyperplasia and spreading of the keratinizing, outer squamous epithelium of the drum is not sufficient to achieve covering of a drum perforation and complete healing cannot take place unless supported by granulation tissue formation.

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