FRONTIERS IN BIOSCIENCE; ULTRASTRUCTURAL APPEARANCE OF SQUAMOUS CELL CARCINOMAS
	Squamous cell carcinomas are tumors that are derived from squamous cells. These cells exist in the skin and squamous mucosas such as those in the mouth, pharynx, cervix, and vagina. In some instances these tumors derive from an epithelium that has undergone metaplasia such as those that occur in the respiratory tract and bladder. Squamous cells reside on a basement membrane and are attached to each other by desmosomes which exist among their interdigitating cell membranes called intercellular bridges. Each desmosome consists of an electron dense area on the membrane which are separated on the two opposing cell membranes by a gap normally about 300 nm in width. Within the squamous cells, one can observe tonofilaments which converge on the desmosomal structures. The tonofilaments are part of the cytoskeleton of the squamous cells which consists of intermediate filaments called cytokeratins. The cells within the well differentiated squamous cell carcinomas, gradually undergo apoptosis. This process is associated with progressive increase in keratinization of the cells which ultimately leads to the formation of the keratin pearls. Depending on the amount of these keratinization, the squamous cell carcinomas are divided into the well differentiated, moderately differentiated and poorly differentiated squamous cell carcinomas. The well differentiated tumors possess large numbers of keratinization and kertain pearl formation. On the other hand, the poorly differentiated tumors rarely exhibit these features. Therefore the pathognomonic ultrastructural features of squamous cell carcinomas consists of basement membrane, desmosomes and tonofilaments.