Epithelium

A tissue forming a lining or covering. Its normal organization and differentiation state matter when comparing epithelial tumors with adjacent tissue.

The retinal pigment epithelium, or RPE, is a single layer of specialized epithelial cells at the back of the retina, closely apposed to the light-sensitive photoreceptors. It supports photoreceptor metabolism and participates in the visual cycle, the chemical regeneration needed to sustain light detection. Support of a receptor and detection by that receptor are distinct cellular tasks.^[ref]

A 1998 genetic experiment made that dependence concrete. Mice lacking RPE65 showed altered vitamin-A-related chemistry and lacked normal rod cell responses measured by electroretinography, although cone responses remained. They also lacked rhodopsin despite retaining its opsin protein component. The comparison connected defective regeneration chemistry with loss of functional visual pigment rather than simply absence of all photoreceptor protein.^[ref]

An illuminating experimental comparison therefore measures both sides: a chemical assay tests the available pigment-related molecules, while an electrical recording tests the retina's response to light. Either observation alone leaves more possible explanations. The result also shows why a layer that does not itself provide the main light-triggered signal can be essential to vision.

The cellular lining of blood and lymphatic vessels. Its polarized surfaces and junctions participate in vascular lumen formation.

Transient blood-forming endothelium that generates haematopoietic cells through an endothelial-to-haematopoietic transition.

An epithelium organized into multiple cell layers. Cell production, differentiation and loss must be balanced across the layers.

An epithelium characterized by flattened surface cells. Stratified examples contain renewing basal populations and differentiating suprabasal layers.

Progression away from the basal compartment toward specialized surface layers. Differentiation, delamination and subsequent cell loss need not occur simultaneously.

Removal of a cell from an epithelial sheet through coordinated behaviour of that cell and its neighbours. Extrusion and apoptosis are distinct events and need not occur in the same order.

An epithelial cell producing enamel during tooth development. Loss of the enamel-producing cellular layer after eruption limits replacement of worn crown enamel.