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Animal tissues. Epithelium.


keratynized stratified squamous epithelium
Stratum corneum
Stratum granulosum
Stratum spinosum
Stratum basale
Connective tissue
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Keratinized stratified squamous epithelium
Organ: skin, epidermis.
Species: mouse (Mus musculus; mammal).
Technique: haematoxylin-eosin, 8 µm thick sections, paraffin embedding.

Image from thick skin.
Cursor over the mouse to see where the image comes from.

Keratinized stratified epithelium is typically observed in the epidermis of land vertebrates, but it is also found in the papillae of the tongue, oral palate and upper part of the esophagus. The strata of the epidermis can be clearly observed in the image above, which is from the thick skin of a mouse (see also Figure 1).

Keratinized stratified squamous epithelium
Figure 1. Keratinized stratified squamous epithelium.

The stratum basale (also known as stratum germinativum) is the inner layer of the epidermis. It is a one cell thick layer in contact with the basal lamina. Keratinocyte adult stem cells are found in this stratum. These undifferentiated cells proliferate and give rise to keratinocytes that will form the upper layers. In the large image above, several proliferating cells can be observed in different mitotic phases (see also Figure 2). Some stem cells remain as proliferating cells, but others start to differentiate by synthesizing keratin filaments (types 5 and 14), and migrate toward the upper layers. During the travel to the surface, new keratinocytes become part of the different layers of the epidermis. Actually, each of the epidermal strata is a layer of cells in a particular differentiation stage.

Figure 2. Stratum basale. Cells in different mitotic phases.

The stratum spinosum is the wider layer of the thick epidermis. It contains polygonal cells that, as they get closer to the upper layer, become flattened and synthesize new keratin isoforms (types 1 and 10). These cells show interdigitations that look like spines when observed at light microscopy. That is why it is called stratum spinosum. However, these spines are artifacts because the histological processing causes a general retraction of cell cytoplasm, but not at the places where desmosomes and interdigitations are present.

The stratum granulosum contains 3 to 5 layers of flattened cells that show their cytoplasms filled with basophilic keratohyalin granules. This stratum looks darker than the others after standard tissue staining, such as hematoxylin and eosin, because of its strong basophilia. The content of the granules is necessary for later cytoplasmic keratin aggregation.

Finally, the stratum corneum is composed of dead cells filled with keratin. It is well developed in the large image above. The function of the stratum corneum is to protect the skin against abrasion, dehydration, and pathogen invasion. Peeling continuously occurs in the upper part of this stratum, at similar rate than new cells are joined from the stratum granulosum. The thickness of the stratum corneum may change depending on the part of the body where the skin is located. In those parts of the body where mechanical stress is frequent, such as scrape or abrasion, a thicker stratum corneum can be found.

The main cell type of the keratinized stratified squamous epithelium is the keratinocyte. There are other much less abundant cell types intermingled with keratinocytes, such as melanocytes, that gives the dark color to the skin and found in the stratum basale, the Merkel cells, with sensory function, and the Langerhans cells, that performs immune roles.

Basal lamina
Figure 3. Keratinized stratified squamous epithelium from thin skin. The epithelial strata are hardly distinguishable. The basal lamina is clearly observed (black arrows). PAS-hematoxylin staining.

A sheet of specialized extracellular matrix covers the deeper part of the epithelium, referred as basal lamina. It is not observed in the image above, but it is shown in the Figure 3. The underlying tissue is called dermis, which is dense irregular connective tissue. The basal lamina attaches the epithelium to the connective tissue. It also regulates the proliferation of the stratum basale epithelial cells and the diffusion of substances from between the epithelium and the connective tissue.

More pictures

Thick skin
Thick skin from the palm of a rat. Keratinized stratified squamous epithelium is very thick, dermal papillae are highly developed, and the stratum corneum is well developed. The bluish color of the keratin is because of the staining.
Thin skin
Thin skin of the upper lip of a rat. The epithelium is so thin that differentiation of keratinocytes occurs almost in one layer of cells.The stratum corneum is visible.
Transition (arrow) from the keratinized stratified squamous epithelium of the outer part of the lid to the stratified squamous epithelium of the inner part of the lid.
The esophagus of animals eating hard food shows stratum corneum (arrows). This stratum is not present in the esophagus of animals eating softer food. Asterisk indicates the transition between esophagus and stomach.
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