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Simple Epithelium

الكلية كلية الطب     القسم  التشريح والانسجة     المرحلة 1
أستاذ المادة رنا أياد غالب محمد       23/01/2017 15:41:45
A- Simple Epithelium
Simple epithelium can be subdivided according to the shape and function of its cells. *Squamous (thin cells), *cuboidal (cells roughly as thick as they are wide) or *columnar (cells taller than they are wide).
1-Simple Squamous epithelium
Squamous cells have the appearance of thin, flat plates. The shape of the nucleus usually corresponds to the cell form and help to identify the type of epithelium. Squamous cells, for example, tend to have horizontal flattened, elliptical nuclei because of the thin flattened form of the cell. They form the lining of cavities such as the mouth, blood vessels, heart and lungs and make up the outer layers of the skin (Fig.1).
Fig.1 Simple Squamous epithelium
2- Simple cuboidal epithelium: As their name implies, cuboidal cells are roughly square or cuboidal in shape. Each cell has a spherical nucleus in the centre. Cuboidal epithelium is found in glands and in the
lining of the kidney tubules as well as in the ducts of the glands. They also constitute the germinal epithelium which produces the egg cells in the female ovary and the sperm cells in the male testes (Fig.2).

Fig.2.Simple cuboidal epithelium
3- Simple columnar epithelium
Cells of simple columnar epithelia are taller than they are wide. Such cells are usually highly specialized for absorption, with microvilli, and often have interspersed secretory cells or ciliated cells. Such epithelial cells always have tight and adherent junctional complexes at their apical ends, but are often loosely associated in more basolateral areas. This allows for rapid transfer of absorbed material to the space between the cells rather than transport the full length of the cells. The additional cytoplasm in columnar cells allows additional mitochondria and other organelles needed for absorption and processing. The examples shown here are from a renal collecting duct (a), the oviduct lining, with both secretory and ciliated cells (b), and the lining of the gall bladder (c) (Fig.3).


Fig.3. Simple columnar epithelium
B. Stratified Epithelium
1-Stratified squamous epithelia: have protective functions, protection against easy invasion of underlying tissue by microorganisms and protection against water loss. In the skin, protection against water loss and desiccation is particularly important and the epithelium is keratinized. As epidermal cells of the skin (a) differentiate they become filled with keratin and other substances and eventually lose their nuclei and other organelles. Epithelia lining many internal surfaces such as the esophagus (b), or covering the cornea (c) are considered nonkeratinized because the differentiating cells accumulate much less keratin and retain their nuclei (Fig.4).

Fig.4.Stratified squamous epithelia

2- Stratified cuboidal epithelium : is a type of epithelial tissue composed of multiple layers of cube-shaped cells. Only the most superficial layer is made up of cuboidal cells, and the other layers can be cells of other types. The main functions of stratified cuboidal epithelium is to protect areas such as the ducts of sweat glands, mammary glands, and salivary glands.
3-Stratified columnar epithelia; is a rare type of epithelial tissue composed of column shaped cells arranged in multiple layers.
Stratified columnar epithelia are found in the ocular conjunctiva of the eye, in parts of the pharynx and anus, the female s uterus, the male urethra and vas deferens. Also found in intralobular ducts in salivary glands. The cells function in secretion and protection.
Stratified cuboidal and stratified columnar epithelia are rare.
4- Transitional epithelium: which lines only the urinary bladder, the ureter, and the upper part of the urethra, is characterized by a superficial layer of domelike cells that are neither squamous nor columnar. These cells, sometimes called umbrella cells, are essentially protective against the hypertonic and potentially cytotoxic effects of urine. Importantly, the form of the surface cells changes according to the degree of distention of the bladder wall (Fig.5)
Fig.5. Transitional Epithelium

5- Pseudostratified Columnar Epithelium: Cells of pseudo-stratified epithelia appear to be in layers, but the basal ends of the cells are all in contact with the basement membrane, which is often very thick in these epithelia. The best example of this epithelial type is the pseudostratified ciliated columnar epithelium of the upper respiratory tract, which contains cell types which are irregularly shaped with their nuclei at different levels that give the false appearance of cellular stratification (Fig.6).
Fig.6.Pseudostratified Columnar Epithelium
MEDICAL APPLICATION
• In chronic bronchitis, common among habitual smokers, the number of goblet cells in the lining of airways in the lungs often increases greatly. This leads to excessive mucus production in areas where there are too few ciliated cells for its rapid removal and contributes to obstruction of the airways. The ciliated pseudostratified epithelium lining the bronchi of smokers can also be transformed into stratified squamous epithelium by metaplasia.


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