Lecture 3- Epithelial Tissue PDF

Book: page 73-98 By Prof. Dr. Noha A Makhlouf ILOs Be able to classify epithelial tissues. List the characters of epithelial tissue. List different types of covering epithelium, describe their structure and list their sites. Classify glandular epithelium according to: mode o...

Book: page 73-98 By Prof. Dr. Noha A Makhlouf ILOs Be able to classify epithelial tissues. List the characters of epithelial tissue. List different types of covering epithelium, describe their structure and list their sites. Classify glandular epithelium according to: mode of secretion, type of secretion) Compare between mucous & serous acinus. Describe the structure of apical specializations and their functions. Describe the structure and function of LATERAL CELL SPECIALIZATION (junctions). Be able to correlate different types of epithelia to their functions. Give examples to abnormal transformation of epithelium. -The human body is composed of four basic types of tissues: - Epithelial, -Connective, -Muscular and -Nervous tissue - Tissues form different organs and systems  Epithelial Tissue Definition - A tissue that is composed mainly of cells. -with little intercellular substance. - These cells covers or lines the organs of the body (covering or lining epithelium) - or performs secretory functions (glandular epithelium).  Characters of epithelium ▪Covers surfaces (covering epithelium) and lines cavities (lining epithelium) ▪little intercellular substances ▪The cells lie on a basement membrane. ▪No blood vessels enter inbetween the cells (Avascular/ non vascular), but nerves can pass inbetween the cells. ▪The epithelia are continuously renewed and replaced Little intercellular substance Basement membrane  Types of Epithelium Covering epithelium (epithelial membranes) sheet like tissues that cover or line the surfaces, cavities, and organs of the body Glandular epithelium : perform secretory functions. Neuroepithelium : Modified to perceive sensation. Myoepithelium: contractile. Classification of epithelial membranes According to number of cell layers Simple: one layer. Stratified: more than one layer. According to shape of apical surface cells Squamous Cuboidal Columnar  Covering epithelium: Simple Epithelium: formed of one layer of cells resting on the basement membrane - Simple squamous - Simple cubical - Simple columnar - Pseudostratified columnar epithelium  Simple squamous epithelium It is formed of one layer of flat cells. Sites: - Lining the blood vessels (endothelium) - Lining the lung alveoli.  Simple cubical epithelium It is formed of a single layer of cubical cells. Each cell has a single rounded central nucleus Site: thyroid gland.  Simple columnar epithelium It is formed of a single layer of columnar cells ( height of the cell exceeds its width) Each cell has a single, basal oval nucleus sites: stomach. small intestine.  Pseudostratified columnar epithelium The cells are overcrowded, but they all rest on the basement membrane. Some cells appear short and not reaching the surface. Sites : - Trachea. Stratified epithelium The cells are arranged in two or more layers.  Stratified epithelium The cells are arranged in two or more layers Types: 1- Stratified squamous non-keratinized The basal cells are columnar (This layer is responsible for generation of other cell types.) The intermediate cells are polygonal which gradually flattens to become: thin squamous cells at the surface -Sites:It is found in wet surfaces of all openings leading to the skin e.g. Oral cavity Tongue Oesophagus 2- Stratified squamous keratinized as the previous type but, the top layer is covered by tough keratin. Sites: epidermis of the skin  Glandular Epithelium Epithelium characterized by its secretory function. When the cells are connected to the surface by duct it is called: Exocrine gland When the cells loose their connection to the surface it is called: Endocrine glands.  So according to the presence or absence of ducts glandular epithelium is classified into: - Endocrine glands: Loose their connection to the surface, Contain no ducts, their secretions are transported by blood. -Exocrine glands: They have ducts carrying their secretions to the surface e.g. Salivary glands  Exocrine glands: can be classified according to various criteria: According to the number of cells into: ❑ Unicellular e.g. goblet cells. ❑ Multicellular e.g. salivary glands.  Unicellular glands Goblet cells -The gland is formed only of one cell. -Sites: in trachea. Light Microscopy(L/M): -Flask in shape; slender base and expanded apex due to presence of mucous granules Electron microscopy (E/M): - Basal part contains the nucleus, rER and Golgi complex. - Apical part rich in mucous secretory granules  Exocrine glands Classification: According to the type of secretion: Mucous glands: Secretes a viscous thick secretion Serous glands: secretes a watery secretion Mixed glands (Seromucous acinus): Secretes both mucous and serous secretions Mucous acini Serous acini Size Relatively large in diameter Relatively smaller than mucous acinus Lumen Wide Narrow In a section Formed of a few number of cuboidal cells Formed of a relatively larger number of that have well-defined boundaries pyramidal cells Cytoplasm Pale basophilic Dark Basophilic Each cell contains a single flattened and Each cell contains a single rounded nucleus , Nucleus basal nucleus shifted towards the base Mucous ( a viscous thick secretion) Watery whey-like containing enzymes Secretion Seromucous acinus (Mixed acinus) It is formed of a mucous acinus, surrounded partially by a small cap of serous secretory cells.  Myoepithelium The cell surrounds the secretory unit like a basket, so it is called basket cells The cytoplasm contains contractile elements ( actin, myosin) so it is called myoepithelial cells By contraction, it squeezes the secretions from the secretory unit to the duct  Neuroepithelium It is a special type of epithelium which is modified to receive sensations. Sites: - Taste buds for taste sensation - Olfactory mucosa  Epithelial polarity The region of the cell that faces the connective tissue is called basal poles. The opposite pole facing the space: apical pole. The surface facing the neighboring cell is called : lateral surface.  Specializations of the Epithelial Cell surface Apical Surface Microvilli Cilia Lateral Surface Cellular Junctions Basal Surface -Basement membrane Specialization of the apical surface A- kinocilia - They are hair like mobile structures. - Each cilium is covered by plasma membrane - - Cilia contain microtubules. - It is composed of: Basal body shaft (axoneme)  Each cilium is formed of: shaft (called axoneme). basal body,  Cilia 1-The basal body: 2- The shaft or axoneme, (Gr. axon, axis; nema, thread): nine pairs (doublets) of microtubules surrounding two central (singlets) microtubules (9+2 pattern). Pairs of arms formed of the protein dynein, are responsible for the cilia movement.  Cilia Each cilium is about 0.2 um in diameter and 5-10 um in length. The main function of the cilia is to sweep (move) fluid from the surface of the cell sheets in one direction. - Present in: - epithelium lining the respiratory tract. B- Microvilli They are finger-like projections of the apical cell membrane. They contain actin microfilaments in its middle. Sites: The absorptive columnar cells lining the small intestine. Function: They increase the surface area for absorption  Specialization of lateral surface Lateral surface specialization answers the question: ( How the cells are held together?) Lateral surface specialization ARE: Cell junctions. Cell junction 1. Occludens (tight) junctions: to prevent flow of material between the cells (paracellular transport). 2. Adherens (anchoring) junctions: are sites of adhesions (holding together cells of the epithelium). 3. Communicating (gap) junctions: channels for communication between cells 1- Occludens (tight) junctions: membrane fusions that close the intercellular space. The cell membranes of the two cells fuse together several times like the teeth of a zipper. The fusion is done by membrane protein called occludin. 2- Anchoring (adherent) junctions An adherens junction is are sites of cell adhesion. They can appear as a belt encircling the whole cell (so called: zonula adherens) or as spots (macula adherens or desmosome). 2- Anchoring (adherent) junctions a) Zonula adherens ( Belt desmosomes) It surrounds the entire perimeter of the cell like a belt. The intercellular space is filled with glycoproteins (called cadherin) which is the binding force between the cells. b- Macula adherens (desmosomes) It is circular or spot-like adherent junction. The intercellular space is occupied by (members of cadherin family called desmoglein and desmocollin). Site: stratified squamous epithelium (epidermis ) of the skin.  3- Communicating junction (gap junction): The space between the two cells is crossed by multiple channels called connexons contains protein called connexin. These channels allow the passage of molecules from one cell to another This makes the cells act together as one unit. Sites: Cardiac muscles.  Abnormal Changes (transformation) in the epithelium Neoplasia: abnormal growth that may or may not lead to cancers. Metaplasia: one type of epithelium is transformed into another type. Ex: in heavy cigarette smokers pseudostratified epithelium of the bronchi may be transformed into stratified squamous epithelium. Carcinoma: malignant tumor of epithelial origin. Adenocarcinoma: malignant tumor of glandular origin.( the most common tumor in adults). Functions of the epithelium - protection (e.g. skin) - selective permeability (e.g. endothelium) - secretion (e.g. glands) - absorption (e.g. intestinal epithelium) - transport along their surface (e.g. motile cilia) or transcellular - detection of sensations (e.g. taste bud)