Stratified squamous epithelium
Stratified epithelium is also a lining tissue which provides protection for underlying tissues. Stratified epithelia are normally found in places where there is a great deal of wear and tear. Frequently, the outer surface of these epithelia is sloughed off and replaced by cells below it. Thus, the deeper layers of such epithelia (those nearest the basement membrane) are mitotically active. One of the most common types of stratified epithelium is stratified squamous. There are two types of stratified squamous epithelium. In areas where the tissue is subjected to drying it may be keratinized. In keratinized stratified squamous epithelium the outermost layer is composed of tightly packed dead cells filled with the protein keratin. Keratin is the same protein which makes up hair and fingernails. The layer of keratin-filled cells acts as water-proofing. The stratified squamous epithelium you will look at in human skin is keratinized. Later in the semester you will look at non-keratinized stratified squamous epithelium in the esophagus.
Examine the slide of a cross section through human skin under low power. Examine the low magnification view, and note the pink stained dermis; notice that the cells of the dermis are widely separated. This layer is predominantly connective tissue, and also contains blood vessels and glands. The outer epidermis (stained purple) is the stratified squamous epithelium. In the high magnification image, note that the outermost layer of the epidermis, the stratum corneum, consists of dead, highly keratinized cells. The cells at the base of this layer are actively dividing to replace the dead cells which slough off. Use the high magnification image to identify the tissues described above.
What happens to the nuclei as cells migrate from the basement membrane outward to replace the sloughed surface? Using the high magnification image, compare the sizes of the cells nearest to and farthest from the basement membrane. Is there any difference in size?
You have already looked at slides of hair in a preceding lab. Find a hair follicle (hair shafts are visible in the low and mid magnification images) and notice that the cells that make it up are continuous with the epidermis. The follicle cells secrete the protein material (keratin) that forms hair. What functions does hair perform? Compare the relative roles of hair in humans and other mammals, such as rodents.
In what other organs would you expect to find stratified squamous epithelium?
What would you expect the microscopic appearance of a callous to be?