Molecular BIology of Cancer Topics
There
are two general types of epithelium: simple and stratified. Simple epithelium
consists of a singel cell layer. According to cell shape, sinmple epithelium
is refered to as either squamous (thin, flat cells), cuboidal, columnar (tall
cells) or pseudostratified (cells have different hights). Stratified epithelium
has at least two cell layers, and the apical layer determines the type as either
squamous, cuboidal (rare), columnar (rare) or transitional (apical cells change
shape).
Normal epithelium is highly cellular, with little extra-cellular space. Cells keep in contact with one another thru structures like desmosomes and tighth junctions. Epithelial cells have polarity: the apical surface is towards the body exterior or cavity, while the basal surface is towards the blood vessels and organs.
In a stratified epithelium (for example the cervix) only the basal cells can proliferate but are not differentiated. Basal cells are stained dark blue by hematoxylin (stains negatively charged material such as DNA). Apical cells (usually squamous) are differentiated but cannot proliferate, thus are replaced by differentiated from the basal layer. The stroma contains blood vessels and fibroblasts that make the proteins of the extracellular matrix and connective tissue.
In normal epithelium, the blue intensity of a hematoxylin stain will be highest in a single layer of proliferating basal cells, clearly separated from stroma by a basement membrane.
In a bening carcinoma, basal cells will occupy a large vertical volume of the cell layers and mitoses can be seen beyond the basal layer. The basal cells also have larger nuclei than normal, but still remain clearly separated from the sstroma by the basement membrane.
A benign carcinoma contains highly displastic cells in every layer, no differentiated cells, and darkly stained, abnormally shaped nuclei. The epithelium has a high nucleus-to-cytoplasm ratio. As long as the the basement membrane remains intact, the carcinoma is noninvasive and nonmalignant.
A malignant carcinoma will invade the basement membrane and stroma. Under high magnification, the tissue appears poorly differentiated and highly vascularized.
Simple epithelium will undergo hyperplasia similarly to stratified epithelium except there is only one cell layer.
A common test for epithelial cancer is the cervical cytology known as Papanicolau (Pap) smear. Cervical cells are scraped, placed on a siled and stained. Normal cervical squamous cells will stain lightly because of their low nucleus-to-cytoplasm ratio. Displastic cells stain much darker because they are proliferating and thus contain more DNA. The nucley are also abnormally shaped. An abnormal smear is followed by a biopsy to confirm displasia. Pap smears detect 90% of cervical carcinomas.
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