Stratified epithelium

Stratified

Epithelium

Stratified epithelium: is defined as epithelia consisting of two or more layers of cells.

• It have a protective function
• the degree and nature of the stratification is related to the kinds of physical stresses to which the surface is exposed.
• In general stratified epithelia are poorly suited for the function of absorption and secretion by virtue of their thickness, although some stratified surface are moderately permeable to water and other small molecules.

Stratified

squamous
epithelium

Stratified squamous epithelium: consist of a variable number of cells layers which exhibit transition from a cuboidal basal layer to a flattened surface layer with rounded central nucleus.
The basal cells divide continuously, their offspring being progressively pushed towards the free surface where they are ultimately shed. lines the:
• oral cavity,
• pharynx,
• esophagus,
• anal canal,
• uterine cervix and vagina.


Fig (1)Stratified squamous epithelium

Stratified squamous keratinizing epithelium

This specialized form of stratified squamous epithelium constitutes the epithelial surface of the skin (epidermis) and is adapted to withstand the constant abrasion and desiccation to which the body surface is exposed
During maturation , the epithelial cells accumulate cross-linked cytoskeletal protein in a process called keratinization resulting in the formation of a tough, non-living surface layer consisting of the protein keratin
keratinization may be induced in normally non- keratinizing stratified squamous epithelium such as that of the oral cavity when exposed to excessive abrasion or desiccation.

Stratified

cuboidal
epithelium

Stratified cuboidal epithelium is a thin, stratified epithelium which usually consists of only two or three layers of cuboidal or low columnar cells.
This type of epithelium is usually confined to the lining of the larger excretory ducts of exocrine glands such as the:
• salivary glands
• ducts of sweat glands
Fig (2).

Fig (2)Stratified cuboidal epithelium

Stratified
columnar
epithelium

Stratified columnar epithelium :the basal is cuboidal or polyhedral and the most superficial one is made up of columnar cells like in the:
• fornix of Conjunctiva,
• Cavernous urethra
Fig (3).

Fig (3)Stratified columnar epithelium

Ciliated stratified columnar epithelium

Transitional

Epithelium

Transitional epithelium is a form of stratified epithelium almost exclusively confined to the urinary tract in mammals where it is highly specialized to accommodate a great degree of stretch and to withstand the toxicity of urine.
This epithelial type varies in appearance depending on the condition of the organ Fig (4). between stratified cuboidal and stratified squamous epithelia in the relaxed (contracted) state or (empty bladder),

In relaxed state: The basal cells are roughly cuboidal, the intermediate cell layers are irregular polyhedral and the surface is large and rounded and may contain two nuclei these cells are called umbrella cells.



In the stretched state: (full bladder), transitional epithelium often appears only (2-3) cell layer thick (although the actual number of layers remains constant) and the intermediate and surface layers are extremely flattened (Squamous).








Fig (4)Stratified transitional epithelium

The basal lamina

All epithelial tissues have at their bases a sheet like extracellular structure called the basal lamina.
Under E.M. the basal lamina appears as dense layer 20-100 nm (10-9 meter)thick consisting of :
*a dark layer called (lamina densa)
*a light layer present on one side or on both sides of lamina densa called (lamina rara) or (lamina lucida).

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