Epithelial tissue -Introduction -Tissue development -Cellular features -Tissue classification -Epithelial functions -Cell surface modification Byabato A. B. (2015)

Introduction Closely related cells with MINIMAL or NO intercellular substances. Cells occur forming membranes and as glands Cells in membrane occurrence cover external surfaces and line internal surfaces Glands grow from external surfaces by inward growing into underlying connective tissue. If connection to the surface remains throughout in life, a gland is called exocrine. If this connection is lost from th surface, a gland is called endocrine.

Tissue development Develop from all three germ layers. These are Endoderm, interior organogenesis Mesoderm, intermediate organogenesis Ectoderm, exterior organogenesis.

Cellular features Cells are in layers – single layer known as simple Two or more layers known as stratified. All basal cells are resting on the basal lamina (basement membrane) Lamina propria beneath the basal lamina for cell nutrition Avascular No blood or lymphatic vessels.

Tissue Classification Based on three criteria Shape of the cells Squamous, flattened cells Cuboidal, approximately equal sided cells Columnar, cells are taller than their width. Number of cell layers Simple, single layered cells Stratified, two or more layered cells.

Pseudostratified columnar transitional Epith. type Combined shape and number of layers. simple squamous Simple cuboidal Simple columnar Stratified squamous Stratified cuboidal Stratified columnar Pseudostratified columnar transitional

Squamous cells Squamous cells have the appearance of thin, flat plates. They fit closely together in tissues; providing a smooth, low-friction surface over which fluids can move easily. (a fluid is a substance which cannot resist any shear force applied to it) The shape of the nucleus usually corresponds to the cell form and helps to identify the type of epithelium. Eg hexagon .

Squamous cells……….. Squamous cells tend to have horizontally flattened, elliptical (oval or egg-shaped) nuclei because of the thin flattened form of the cell. Also nuclei are bulging towards the apical surface Classically, squamous epithelia are found lining surfaces utilizing simple passive diffusion such as the alveolar epithelium in the lungs, also form the lining of cavities such as the blood vessels (endothelium) and pericardium (mesothelium) and the major cavities found within the body

Squamous Epitheliar Cells

Each cell has a spherical nucleus in the centre. Cuboidal cells As their name implies, cuboidal cells are roughly cubic in shape, appearing square in cross section. Each cell has a spherical nucleus in the centre. Cuboidal epithelium is commonly found in secretive or absorptive tissue: for example the (secretive) exocrine gland the pancreas Also found in the (absorptive) lining of the kidney tubules as well as in the ducts of the glands. They also constitute the germinal epithelium that covers the female ovary.

Simple Cuboidal Epithelium

Columnar cells Columnar epithelial cells are elongated and column-shaped. Their nuclei are elongated and are usually located near the base of the cells. Columnar epithelium forms the lining of the stomach and intestines.

Columnar cells….. Some columnar cells are specialized for sensory reception such as in the nose, ears and the taste buds of the tongue. Goblet cells are found between the columnar epithelial cells of the duodenum. They secrete mucus, which acts as a lubricant.

Simple Columnar Epithelium

Pseudostratified columnar These are simple columnar epithelial cells with nuclei at different heights, giving the misleading (hence "pseudo") impression of stratification when the cells are viewed in cross section. When this epithelium possess fine hair-like extensions of their apical (luminal) membrane called ciliae, this epithelium is described as "ciliated" Pseudostratified epithelium.  

Ciliae cause the flow of mucus to outward direction Pseudo……… Ciliae cause the flow of mucus to outward direction Ciliae generate energy dependent pulsatile beating whose wafting effect produced will cause mucus secreted locally by the goblet cells to lubricate and to trap pathogens and particles. Ciliated epithelium is found in the airways (nose, bronchi), the uterus and Fallopian tubes of females where the cilia propel the ovum to the uterus.

Stratified epithelial cells Stratified epithelium is multilayered, therefore found where body linings have to withstand mechanical or chemical insult such that layers can be abraded and lost without exposing sub-epithelial layers. Cells flatten as their layers become more apical, though in their most basal layers the cells can be squamous, cuboidal or columnar, even hexagonal. Stratified epithelial tissue do not contain junctional complexes, so have their cells simply bound together only by desmosomes.

Stratified Epithelium

Transitional epithelium Transitional epithelia are found in tissues that stretch and it can appear to be stratified cuboidal when the tissue is not stretched or stratified squamous when the organ is distended and the tissue stretches. It is sometimes called the urothelium since it is almost exclusively found in the bladder, ureters and urethra.

Cilliated Epithelium

Glandular Epithelium

Epitherial functions P-rotection A-bsorption / diffusion S-ecretion S-ensory organs Other associated functions Cleaning e.g. nasal and respiratory tract Friction reduction e.g. vascular and capillary lining

Cellular surface modification Apical: keratinization, cilliae, microvilli Side: junction complexes Glycocalyx, proteins and other cementing factors for cell adhesion Types: i) jigsaw interlocking ii) charges interaction iii) junctional specializations: (zonula occludes/nexuses, gap junction, zonula adherens, macula adhaerens/desmosome). Basal: hemidesmosomes

Cellular junction