Page 127 - Atlas of Histology with Functional Correlations

are cuboidal to columnar; these cells produce cells that migrate toward the

surface and become squamous. There are two types of stratified squamous
epithelia: nonkeratinized and keratinized.

Nonkeratinized epithelium exhibits live surface or luminal cells and covers
moist cavities, such as the mouth, pharynx, esophagus, vagina, and anal canal.
Keratinized epithelium lines the external surfaces of the body. The surface

layers contain nonliving, keratinized cells that are filled with the protein keratin.
The exposed epithelium that covers the palms and soles exhibits especially thick
layers of keratinized cells for added protection against abrasion.

Stratified cuboidal epithelium and stratified columnar epithelium have a

limited distribution in the body. Both types of epithelia line the larger excretory
ducts of the pancreas, salivary glands, and sweat glands. In these ducts, the
epithelium exhibits two or more layers of cells.

Transitional epithelium lines the minor and major calyces, pelvis, ureters,
and the bladder of the urinary system. Transitional epithelium changes shape

that can resemble either stratified squamous or stratified cuboidal epithelium,
depending on whether it is stretched or contracted. When transitional epithelium
is contracted, the surface cells appear dome shaped; when stretched, the
epithelium appears squamous and resembles the stratified epithelium of other

organs.

BASEMENT MEMBRANE/BASAL

LAMINA

Located between the epithelial cells and the underlying connective tissue is a

supportive noncellular layer called basement membrane or basal lamina. The
use of the terms basement membrane and basal lamina is inconsistent and
interchangeable in the literature. Using different tissue stains, basement

membrane was initially recognized and described with the light microscope.
With the advent of transmission electron microscopy (TEM), the basement
membrane was observed to consist of two major components, basal lamina and
reticular lamina. Basal lamina consists of fine fibrils and has direct contact with
basal poles of the epithelial cells. Reticular lamina is located beneath basal

lamina, is formed by collagen fibers, and is more diffuse. This layer supports the
basal lamina and is continuous with the connective tissue.

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