Histology Chapter 2: Epithelial Tissue PDF

# Chapter 2 Epithelial Tissue ## Epithelial Tissue * LOs by the end of this chapter the students will be able to: * **Knowledge and understanding to:** * Describe the structural characteristics of the epithelial tissue. * Identify different types of epithelial tissue. *...

# Chapter 2 Epithelial Tissue ## Epithelial Tissue * LOs by the end of this chapter the students will be able to: * **Knowledge and understanding to:** * Describe the structural characteristics of the epithelial tissue. * Identify different types of epithelial tissue. * Recognize the structural specializations of the cell surfaces and epithelial cell types. * **Intellectual skills to:** * Differentiate between different types of epithelial tissue. * Relate the composition of epithelial tissue type to its specific function. * Predict the special type of epithelial cells from its components. * **Professional & practical skills to:** * Use the microscope efficiently and handle the histological glass slides and examine them using the maximum microscopic facilities. * Recognize and differentiate between types of epithelial tissue in histological slides. * Draw and label the structures they have seen in electron photomicrographs and under light microscope during practical classes. ## Epithelial Tissue * The human body is composed of four basic types of tissues: Epithelial, connective, muscular and nervous. * These tissues are formed of cells and extracellular matrix. * These tissues do not exist as isolated units but in association with each other in variable proportions forming organs and systems of the body. ## Characteristics of epithelial tissue: 1. **Origin:** Epithelium is derived from all three embryonic germ layers: 2. **Structure:** * It is composed of closely aggregated cells with very little intercellular substance (matrix). * Cells joined together with junctional complexes forming strong cellular sheets, firmly bound to the underlying connective tissue by a thin membrane called basement membrane (basal lamina). 3. **Blood vessels do not penetrate the epithelial tissue. (avascular)** 4. **Sensory nerve endings penetrate the epithelium providing it with the proper sensation.** 5. **Renewal:** * Epithelial cells are renewed continuously by mitotic activity. 6. **Functions of epithelial tissue are:** * Protection (covering and lining of surfaces e.g. skin). * Absorption e.g. small intestine and kidney tubules. * Secretion e.g. glands. * Sensation e.g. neuroepithelium. * Contractility e.g. myoepithelial cells. ## Types of Epithelium Epithelium is classified according to its structure and function into four types: 1. **Covering and lining epithelium** 2. **Glandular epithelium** 3. **Neuroepithelium** 4. **Myoepithelium** ## 1 - Covering and lining epithelium * The epithelial cells are present in layers that cover the external surface or line the body cavities. * They are classified according to the number of cell layers into: * **A- Simple epithelium** that consists of one layer of cells. * **B-Stratified epithelium** that consists of more than one layer. ### A- Simple epithelium (Fig. 2-1) * It can be further classified according to the cell shape into: 1. **Simple squamous epithelium:** * It is made of one layer of flattened plate-like cells with flattened nuclei. * It is present in places where interchange of substances (gases or liquids) is needed. * **Sites:** * Lungs (lining the alveoli). * Kidneys (lining the parietal layer of Bowman's capsule). * Endothelium (lining the blood and lymphatic vessels). * Mesothelium (lining the serous membranes; peritoneum, pericardium and pleura) 2. **Simple cuboidal epithelium:** * It is composed of a single layer of cuboidal cells with round central nuclei. * **Sites:** * Thyroid follicle. * Medium sized ducts of glands. 3. **Simple columnar epithelium:** * It is made of a single layer of tall rectangular cells with oval basal nuclei. It is subdivided into non-ciliated and ciliated varieties: * **a) Simple columnar ciliated epithelium:** * The cells have motile hair-like processes called cilia on their free surface. * **Sites:** * Conducting bronchioles of lungs. * Lining the central canal of spinal cord. * Together with the non-ciliated variety lining the uterus and fallopian tubes. * **b) Simple colunmar non-ciliated epithelium.** * **Sites:** * GIT * It lines the gastro intestinal tract (from the stomach to the rectum) * It lines the (gall bladder.) * It lines the large ducts of glands. * It lines the large ducts of glands. 4. **Pseudostratified columnar epithelium:** * It consists of one layer of overcrowded columnar cells. * All the cells are attached to the basal lamina but some of them do not reach the surface and their nuclei lie in various levels resulting in the false appearance of stratified epithelium. * This type may be ciliated or non-ciliated. * **a) Pseudostratified columnar ciliated epithelium** * This type is usually associated with goblet cells. * **Sites:** * Respiratory passages (trachea and bronchi) * **b) Pseudostratified columnar non-ciliated epithelium** * **Sites:** * Large ducts and vas deferens. ## B- Stratified Epithelium (Fig. 2-2) * It has a protective function. * The epithelium is made of more than one layer of cells. * The basal layer is columnar followed by a few layers of polyhedral cells. * It is classified according to the shape of the most superficial layer of cells into: 1. **Stratified squamous epithelium:** * The most superficial 1-2 layers are squamous or flattened * It is either keratinized or non-keratinized. * **a) Stratified squamous non-keratinized epithelium** * It lines (wet)surfaces. * **Sites:** * Digestive tract) it lines the mouth cavity, tongue, palatine tonsils, oropharynx, esophagus and anal canal, * Female genital system; it lines the vagina and vaginal part of cervix uteri. * **b) Stratified Squamous keratinized epithelium** * It covers dry surfaces. * In this type of epithelium, there is an additional superficial layer of dead comfied cells. This surface layer of cells helps to protect against water loss. * **Sites:** * Epidermis of skin. * All openings on the surface of the skin e.g. lips, nose, ears and anus. 2. **Stratified cuboidal epithelium** * The most superficial cells are cuboidal in shape. * **Site: Lining ducts of (sweat glands, ** 3. **Stratified columnar epithelium:** * The superficial cells are columnar. * There are two varieties: ciliated and non-ciliated. * **a) Stratified columnar non-ciliated epithelium** * ** Sites: ** * Fornix of conjunctiva * Large ducts of glands * **b) Stratified columnar ciliated epithelium** * **Sites:** * Fetal esophagus ## 4. Transitional epithelium * This epithelium lines the urinary passages and can change the number of layers according to the changes in diameter of the lumen of the organ (its fullness). * In the empty or relaxed condition, the epithelium is made of 3-8 layers. * The superficial layer is composed of large cuboidal dome shaped cells with a convex upper surface and a concave lower surface. Each cell may contain 1-2 nuclei. This is followed by few layers of polyhedral cells and the basal layer is cubical or columnar. * When the organs are distended, the epithelium becomes stretched and made up of 2-3 layers of cells. The superficial appear as squamous while the deeper is cubical. The causes of this change are the thin corrugated basement membrane. (basal lamina) * The free surface of the superficial cells is provided with a thick cuticular border which prevents the escape of urine between the cells during distension. It also protects the cells from the injurious substances present in the urine. * **Sites:** * It lines the pelvis of the kidney, ureter, urinary bladder, and upper part of the urethra. ## Fig. 2-1: Types of simple epithelium [Image Description: A diagram showing different types of simple epithelium. This diagram includes the following: simple squamous epithelium, simple cuboidal epithelium, simple columnar epithelium (ciliated and non-ciliated), pseudostratified columnar epithelium (ciliated and non-ciliated).] ## Fig. 2-2: Types of stratified epithelium [Image Description: A diagram showing different types of stratified epithelium. This diagram includes the following: stratified squamous epithelium (keratinized and non-keratinized), stratified columnar epithelium (ciliated and non-ciliated), transitional epithelium (empty bladder and distended bladder).]