Superficial cells

Three principal cell types exist in the corneal epithelium, namely, superficial cells, wing cells, and basal cells. A single layer of cuboidal basal cells is the sole site of cell division in the corneal epithelium. By necessity, these cells have more prominent mitochondria and Golgi apparatuses and hence high levels of metabolic and synthetic activities. Immediately above the basal cells is a zone comprising two to three layers of wing cells that are in an intermediate state of differentiation. 

The superficial cells are irregular arrays of polygonal cells with a diameter of 40-60 pm and a thickness of 2-6 pm each. These cells, the most differentiated cells of the epithelium, possess microvilli in their apical surfaces, which are covered with a glycocalyx. It is, however, controversial whether mucus exists on their surface [58,59], As cell division occurs in the basal cells of the cornea, the daughter cells move toward the surface while becoming more differentiated. As the daughter cells migrate toward the outermost layer, the superficial cells are... 

While the lactate-H+ symporter and the K+/H+ exchanger are involved in acidification of the cell, the Na+/H+ exchanger present in the basal cells exports protons out of the cell in exchange for Na+ [139]. It was observed that removal of Na+ from the Ringer s solution decreased intracellular pH by 0.5 unit in basal cells, possibly due to inhibition of the Na+/H+ exchanger. As the basal cells are the precursors for the superficial cells of the corneal epithelium, it is quite likely that similar exchange processes are also present in the superficial layer, the principal barrier to ion and drug transport [99,103],... 

Because of the possible effects of active and carrier-mediated processes and metabolic biotransformation, the issue of tissue viability is important for in vitro buccal mucosal experiments. The barrier nature of the buccal mucosa resides in the upper layers of the epithelium, where unlike in the stratum corneum, the cells contain a variety of functional organelles [119, 122, 125, 150], and so tissue viability may be an important component of the barrier function of the tissue. Various methods have been employed to assess the viability of excised buccal mucosa, including measurement of biochemical markers, microscopic methods, and linearity of transport data [42], While biochemical methods, including measurement of adenosine 5 -triphosphate (ATP) levels and utilization of glucose, provide information on the metabolic activity of the tissue, this does not necessarily relate to the barrier function of the tissue. In excised rabbit buccal mucosa, levels of ATP were measured and found to decline by 40% in 6 h, and this correlated well with transmission electron microscopic evaluation of the tissue (intact superficial cells) [32], In addition, the permeability of a model peptide was unaltered up to 6 h postmortem, but at 8 h, a significant change in permeability was observed [32], These investigators therefore claimed that excised rabbit buccal mucosa could be used for diffusion studies for 6 h. 

At the corneal surface, two to four layers of polygonal, extremely flat, and terminally differentiated superficial cells can be found. The cell diameters are 40-60 /xm with about 2-6 /xm thickness. The corneal surface is populated with microvilli and microplicae, resulting in surface area enlargement. These structures are associated with the tear film. Superficial corneal epithelial cells are... 

In another approach, Parnigotto and coworkers reconstructed corneal structures in vitro by using corneal stroma containing keratocytes to which corneal epithelial cells from bovine primary cultures were overlaid [73], However, this particular corneal model did not contain an endothelial layer. This model was histochemically characterized and the toxicity of different surfactants was tested using MTT methods. This stroma-epithelium model has been reported to show a cornea-like morphology, where a multilayered epithelial barrier composed of basal cells (of a cuboidal shape) and superficial cells (of a flattened shape) is noted. Furthermore, the formation of a basement membrane equivalent and expression of the 64-kDa keratin were reported, indicating the presence of differentiated epithelial cells. The toxicity data for various surfactants obtained with this model correlate well with those seen by the Draize test [73], However, this corneal equivalent was not further validated or used as a model for permeation studies. 

Astringents act by precipitating proteins in superficial layers of cells and are used to diminish the excretion or exudation of superficial cells. They are also used as local haemostatics and mummifying agents (discussed elsewhere). The different types of astringents used in dentistry are ... 

It is an autoimmune disease which is characterized by marked increase in undifferentiated epidermal cell proliferation and can increase the number of superficial cells having abnormal keratinization. 

Thanks to specular microscopy and electron scanning microscopy, some cells with various aspects have been identified it could be different stages of differentiation and maturation of the superficial cells preceding their desquamation. Along their evolution, the metabolic activity of the superficial cells slows down until it stops. They are then evacuated into the lacrymal secretion. 

The most superficial cells are the most mature ones, with a low metabolic activity, and they are going to desquamate. In electron scanning microscopy, the corneal surface looks like a mosaic of flattened and polygonal cells, with uneven sizes. Their nucleus is reduced to a few condensed chromatin clods. 

Their cytoplasmic membrane is bristled up with microvillosities and micropilis. They increase the exchange surface with the lacrymal secretion and enable its fixing. Their lateral and basal cytoplasmic membranes have got junctional complexes uniting these superficial cells together. There are three types of junctional complexes ... 

These are cells of transition between the basal and superficial cells. They constitute the thickest and biggest layer. They are polygonal cells, with a convex front side and a concave back side. They are arranged on two or three layers at the center and five to six layers on the edge. Their nucleus is active and stretched out along the big axis of the cell. Their cytoplasm contains a very developed Golgi s apparatus as well as tonofilaments (microtubules and keratin filaments) connected to the desmosomes. Their cytoplasmic membranes are only united desmosomes and gap junctions that enable both the unity of intermediate cells and the union of intermediate and basal cells (Figs. 4.3. 5). 

The stratum granulosum is the most superficial cell layer of the viable epidermis and contains highly differentiated keratinocytes. The lamellar bodies, which have been formed in the stratum spinosum, migrate to the apical periphery of the uppermost granular cells and eventually fuse with the membrane of the keratinocyte. Via exocytosis their content is extruded into the intercellular spaces at the stratum granulosum-stratum corneum interface. The lipids derived from the lamellar bodies are essential for the formation of the stratum corneum barrier. 

The epithelium is the outermost layer and consists of five to six cell layers. These can be subdivided into one to two outermost layers of flattened superficial cells with microvilli on their anterior surface enhancing the cohesion and stability of the tear... 

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