Lec. 5 Medical Biology PDF

Medical Biology Lec.5 Epithelial Tissue Assist Lec. Heba Muaath Jassim  Epithelial Tissue Tissues are cells that are similar in structure and perform a common related function. There are four types of tissues:  Epithelial tissue  Muscle tissue...

Medical Biology Lec.5 Epithelial Tissue Assist Lec. Heba Muaath Jassim  Epithelial Tissue Tissues are cells that are similar in structure and perform a common related function. There are four types of tissues:  Epithelial tissue  Muscle tissue  Connective tissue  Nerve tissue Tissues are organized into organs, most organs containing all 4 types of tissue. Epithelial tissue is a sheet of cells that: 1. Covering and lining epithelium: forms the outer layer or a body surface such as skin and lines open cavities such as digestive and respiratory systems. 2. Glandular epithelium: surrounds glands within the body. Characteristics of epithelial tissue: 1- Polarity: all epithelia have an apical surface and a lower attached basal surface that differ in structure and function. Most apical surfaces have microvilli (small extensions of the plasma membrane) that increase surface area. For example, in epithelia that absorb or secrete substances, the microvilli are very dense giving the cells a fuzzy appearance called a brush border. Examples of this would include epithelia lining the intestine and kidney tubules. Other epithelia have motile cilia (hair like projections) that push substances along their free surface. 2- Specialized contacts: epithelial cells fit close together and form continuous sheets (except in the case of glandular epithelia). Such as Cell junctions are apply in epithelial tissues (tight junctions, gap junctions and desmosomes). 3- Supported by connective tissue: all epithelia are supported by connective tissue. For example, deep to the basal lamina is reticular lamina (extracellular material containing collagen protein fiber) which forms the basement membrane. The basement membrane reinforces the epithelium and helps it resist stretching and tearing. 4-Avascular: Epithelial layers contain no blood vessels, so they must receive nourishment via diffusion of substances from the underlying lamina propria in connective tissue, through the basement membrane. 5- Regeneration: epithelium has a high regenerative capacity and can reproduce rapidly as long as they receive adequate nutrition.  Functions of epithelial cells include secretion, selective absorption, protection, filtration, transport, and sensing. Classification of Epithelia: Based on the number of cell layers, epithelia can classify to simple and stratified. Simple epithelia: consist of a single cell layer. Stratified epithelia: are composed of two or more cell layers. Simple epithelial tissue: Depending on the shape of the cells, simple epithelial tissue can classified to: 1- Simple squamous epithelium: flat, thin scale like cells with disc nuclei, they are close fitting and flattened laterally. They’re found where filtration occurs Location: Bowman' s capsule in kidneys, lungs, endothelium of blood vessels. 2- Simple cuboidal epithelium: consists of a single layer of cup- like cells with central nuclei and same height and width. Functions include secretion and absorption. Location: in follicular cells of thyroid gland, kidney tubules and ovary surface. Simple cuboidal epithelial tissue Simple squamous epithelial tissue  3- Simple columnar epithelium: is a single layer of tall cell with oval basal nuclei, closely packed cells that line the digestive tract from the stomach to the rectum. Functions include absorption and secretion. They contain dense microvilli on their apical surface. Additionally, some simple columnar epithelia may display cilia on their free surface. Also there is goblet cell which produce mucous and protect lining of intestine. Location: Digestive system 4- Pseudostratified columnar epithelium: It looks like composed of several layers of cells, because the cells have different heights, and gives the illusion of multiple cell layers. All of their cells rest on the basement membrane and vary in height but only the tallest reach the apical surface. Most pseudostratified epithelia contain cilia on their apical surface and the goblet cells are found for secretion. Location: line the respiratory tract (trachea, bronchi). Simple columnar epithelial tissue Pseudostratified epithelial tissue  Stratified epithelial tissue Stratified epithelial tissue: It based on the shape of cells in the apical layer, stratified epithelia can classify to: 1- Stratified squamous epithelium: There are two types A- Non-keratinized Stratified squamous epithelium: Is the most widespread stratified epithelia. It’s composed of several layers and is perfect for its protective role. Its apical surface cells are squamous and cells of the deeper layer are either cuboidal or columnar while the intermediate cells are polyhedral. Location: Oral cavity, esophagus, pharynx, cornea and vagina. B- keratinized Stratified squamous epithelium: The surface cells are full with keratin, a protective protein and dead, basal cells are active in mitosis and produce the cell of the more superficial layers. Location: epidermis of the skin. Non-keratinized Str.sq.epi.tissue Keratinizes. Stra. sq. epi. Tissue 2- Stratified cuboidal epithelium: It is typically have two layers of cuboidal cells. Location: It’s mainly found in the ducts of glands (sweat glands, parotid and mammary glands). 3- Stratified columnar epithelium: is also rare in the human body. Location: pharynx, male urethra, and lining of some glandular ducts. Stratified cuboidal epithelium tissue Stratified columnar epithelium tissue 4- Transitional epithelium: forms the lining of hollow urinary organs, Transitional epithelium is a type of stratified epithelium composed of several layers of cells, with the morphology of cells varying depending on the function of the organ. In a relaxed state, the epithelium appears spherical or cubical, except for the apical layer, which may flatten when stretched. Location: Lines the ureter, urinary bladder and part of urethra. Transitional epithelium