L1 The Covering Epithelium PDF

1 1 The Covering of Epithelium ILOs By the end of this lecture, students will be able to 1. Interpret the significance of general features of epithelium in relevance to its functional requirements. 2. Correlate structural adaptation of each type of epithelium t...

1 1 The Covering of Epithelium ILOs By the end of this lecture, students will be able to 1. Interpret the significance of general features of epithelium in relevance to its functional requirements. 2. Correlate structural adaptation of each type of epithelium to its function. 3. Recognize significance of epithelial in view of its structure & function. Epithelial tissue Epithelial tissue is present in two forms: (1) as sheets of contiguous cells (epithelia) that cover the body on its external surface and line the hollow organs on the internal surface, and (2) as glands, which originate from invaginated epithelial cells. Epithelial tissues have numerous functions Protection of underlying tissues of the body from abrasion and injury. Transcellular transport of molecules across epithelial layers. Secretion of mucus, hormones, enzymes, and so forth, from various glands. Absorption of material from a lumen (e.g., intestinal tract or certain kidney tubules). Control of movement of materials between body compartments via selective permeability of intercellular junctions between epithelial cells. Detection of sensations via taste buds, retina of the eye, and specialized hair cells in the ear. General features of epithelium 1. It is formed of closely packed cells with limited intercellular spaces. 2. The cells are related by intercellular junctions. 3. They are separated from the underlying connective tissue by an extracellular matrix, the basal lamina. 4. It has a high regenerative capacity through stem cells division. Epithelia possess a unipotent stem cell that is capable of division and differentiation to one type of epithelium only. 5. It is a non-vascular tissue but richly innervated. 6. The adjacent supporting connective tissue through its capillary beds supplies nourishment and oxygen via diffusion through the basal lamina. 1 Classification of Epithelial Membranes 2 Cell arrangement and morphology are the bases of classification of epithelium Simple; formed of a single cell layer. Stratified; formed of several cell layers. Types Shape of cell Site Function I- Simple 1. Squamous Flat cells forming thin Lining: pulmonary Limiting membrane, sheets alveoli, blood and fluid transport, lymphatic vessels. gaseous exchange. 2. Cuboidal Cuboidal Lining :Thyroid gland Secretion, absorption, follicles, kidney tubules. protection 3.Columanr Columnar Lining: oviducts, uterus, Transportation, much of digestive tract absorption, secretion, (stomach, intestine). protection 4.Pseudostratified All cells rest on basal lamina Lining: most of trachea, Secretion, absorption columnar but not all reach epithelial primary bronchi. lubrication, protection, surface; surface cells are transportation. columnar N.B. Columnar cells could be ciliated or non-ciliated (will be discussed later) Figure 1. Types of covering epithelium 2  3 Types Shape of cell Site Function II- Stratified 1. Non-keratinized Flat cells forming the most Lining: oral cavity, Form a physical Squamous superficial cell layer oesophagus & vagina. barrier for protection. 2. keratinized Flat cells form the most Skin. Protection Squamous superficial cell layer & covered by a keratin layer 3.Cuboidal Cuboidal cells form the Ducts of sweat glands Protection & secretion most superficial cell layer 4.Columnar Surface cells are columnar Conjunctiva of eye. Secretion & protection 5.Transitional Surface cells are dome Lining the ureters & Protection, distensible shaped urinary bladder Transitional Epithelium It is structurally adapted to volume changes. It has wide intercellular spaces allowing cells to glide and reduce number of cell layers in response to pressure by full bladder. Dome shaped cells form a barrier between urine and the blood vessels in the underlying connective tissue. 3