Epithelial Tissue PDF

# Epithelial Tissue ## Epithelial Tissue - Dr. Malik Khamaysi - MD - Faculty of Medicine - AAUP ## Tissues and Tissue Types: - Tissues are: - Collections of specialized cells and cell products grouped to carry out specialized activities. - Basic types of tissues: - Epithelial tissue -...

# Epithelial Tissue ## Epithelial Tissue - Dr. Malik Khamaysi - MD - Faculty of Medicine - AAUP ## Tissues and Tissue Types: - Tissues are: - Collections of specialized cells and cell products grouped to carry out specialized activities. - Basic types of tissues: - Epithelial tissue - Connective tissue - Muscular tissue - Nervous tissue ## Properties of Tissues: - Properties of tissues are influenced by factors such kind of cells present, cellular arrangement, connection between cells and extracellular material (as types of fibers present). - Tissues may be hard, semisolid, or liquid. ## Table 4-1. Main Characteristics of the Four Basic Types of Tissues. | Tissue | Cells | Extracellular Matrix | Main Functions | |---|---|---|---| | Nervous | Intertwining elongated processes | None | Transmission of nervous impulses | | Epithelial | Aggregated polyhedral cells | Very small amount | Lining of surface or body cavities, glandular secretion | | Muscle | Elongated contractile cells | Moderate amount | Movement | | Connective | Several types of fixed and wandering cells | Abundant amount | Support and protection | ## Organ Components: - Most organs can be divided into two components: - **Parenchyma:** Composed of the cells responsible for the main functions typical of the organ. - **Stroma:** Supporting tissue. - Except in the brain and spinal cord, the stroma is made of connective tissue. ## A. Epithelial Tissue: - Epithelial tissue: - Form cellular sheets that cover the surface of the body and line its cavities (Covering epithelium). - Are arranged as three-dimensional secretory units (Glandular epithelium). - Epithelia are derived from all 3 major germ layers (i.e. ectoderm, mesoderm & endoderm). ## Functions of Epithelium: - Physical protection - Covering and lining of surfaces (skin, intestines) - Absorption (intestines) - Secretion (glands) - Sensation (gustative and olfactory neuroepithelium) - Contractility (myoepithelial cells) - Selective diffusion (Kidney glomeruli) ## Forms & Characteristics of Epithelial Cells: - Epithelium is composed of closely aggregated polyhedral cells with very little extracellular substance. - The forms of epithelial cells vary, ranging from: - Squamous - Cuboidal - Columnar - Epithelial cells nuclei also have distinctive shapes: - Flat, Spherical or Elongated (elliptical). - The form and number of nuclei are an important clue to the shape of cells and number of layers. - According to # of layers, they can be called: - Simple (one layer). - Stratified (2 & more layers). - Epithelia are characterized by polarity, i.e. has: - Apical pole (free surface). - Basal pole (bound surface). - The surfaces that face the neighbor cells are called lateral surfaces. ## Forms & Characteristics of Epithelial Cells (continued): - Almost all epithelial cells rest on a connective tissue. - In epithelia that line the cavity of internal organs, this layer of C.T. is often called: **Lamina Propria**. - Area of contact between epithelium and connective tissue is increased by small evaginations from the C.T. called: **Papillae**. - Occur mostly in epithelial tissues subject to stress, such as skin and tongue. - Most epithelial tissues are separated from the C.T by a sheet of extracellular material called: **Basal lamina**. - Visible only with the electron microscope. - Evidence suggests that it is formed by the epithelial cells rather than the C.T. - Consists of two layers: - **Lamina densa:** Electron-dense layer, network of very fine fibrils. - **Lamina lucida (or lamina rara): ** Electron-lucent layer, on one or both sides of the lamina densa. ## Forms & Characteristics of Epithelial Cells (continued): - Basal lamina is composed of: - Type IV collagen. - Glycoproteins (laminin and entactin). - Proteoglycans+. - Sometimes, C.T. reticular fibers are closely associated with the basal lamina, forming the **reticular lamina**. ## Forms & Characteristics of Epithelial Cells (continued): - **Basement membrane:** A periodic acid-Schiff (PAS)-positive layer visible with the light microscope. - Presents beneath some epithelia. - Usually formed by the association of: - Either two basal laminae - A basal lamina and a reticular lamina ## Functions of Basal Lamina and Basement Membrane: - Supports the epithelium - Acts as a selectively permeable filter. - Influences cell polarity. - Regulates cell proliferation and differentiation. - Influences cell metabolism. - Serves as pathways for cell migration. - Contains information necessary for certain cell-to-cell interactions. ## Forms & Characteristics of Epithelial Cells (continued): - Epithelial cells are extremely cohesive due to the presence of several membrane-associated structures that contribute to cohesion and communication between cells, as: - **Binding action** of a family of transmembrane glycoproteins. - **Interdigitations** between folds of membranes of neighbor cells. - **Intercellular junctions**. ## Intercellular Junctions: - In several epithelia are present in a definite order from the apex toward the base of the cell. - **Tight junctions (zonulae occludens):** Are the most apical of the junctions, band, completely encircling the cell, membrane fusions that close off the intercellular space. - Function to form a seal that prevents the flow of material between epithelial cells (paracellular pathway). - **Zonula adherens**. - **Gap (communicating) junctions**. - **Desmosomes (macula adherens)**. - **Hemidesmosomes**. ## Intercellular Junctions (continued): - Junctions between cells can be classified as: - **Impermeable junctions:** Zonula occludentes - **Adhering junctions:** Zonula adherentes, Desmosomes, Hemidesmosomes - **Communicating junctions:** Gap junctions ## Intercellular Junctions (continued) - **Cell-cell junction:** Symmetrical - same proteins on different cells interact. - Tight junction - Zonula adherens - Desmosome - Gap junctions - **Cell-matrix junction:** Asymmetrical - cell proteins interact with matrix - Hemidesmosome - Focal adhesion (actin-linked cell-matrix adhesion) ## Tight Junctions (zonulae occludens) - Are the most apical of the junctions. - **Zonula:** band, completely encircling the cell. - **Occludens:** membrane fusions that close off the intercellular space. - *Function:* to form a seal that prevents the flow of material between epithelial cells (paracellular pathway). - A number of proteins are involved in the binding of adjacent plasmalemmas in the zonula occludens region. The major proteins are **claudins & occludins**. ## Zonula Adherens: - Encircles the cell → provides for the adhesion of one cell to its neighbor. - Numerous **actin** filaments of the cytoskeleton in the terminal web are inserted into the a plaque on the cytoplasmic side of this junction and there are adhesion molecules **Cadherins**. ## Gap (communicating junctions) - Found in all mammalian tissues, except skeletal muscle. - Can occur almost anywhere along the lateral membranes of epithelial cells. - The individual unit of the gap junction is called a **connexon**. - Formed of 6 proteins called **connexins**. - Connexons of adjacent cells are aligned to form a **hydrophilic channel** between the two cells. - Connexons are arranged in circular patches. - Hormones, cyclic AMP and GMP, and ions can move through these gap junctions. ## Desmosome (macula adherens) - A complex disk-shaped structure at the surface of one cell that is matched with an identical structure at the surface of the adjacent cell. - Groups of **intermediate filaments** are inserted into an attachmet plaque on the cytoplasmic side of cell membran. - The attachment plaques contain mainly **cadherins**. - This adhesiveness needs Ca+. ## Hemidesmosomes: - Take the form of half a desmosome. - Bind the epithelial cell to the subjacent basal lamina. - The attachment plaques mainly contain **integrins**. - *A transmembrane protein that binds cell into matrix molecules - as laminin & collagen type IV*. ## Cellular Junctions: - A diagram showing cell junctions: hemidesmosome, gap junctions, desmosomes, zonula adherentes, zonula occludentes and the terminal web. ## General Biology of Epithelial Tissues: - **Polarity of Epithelial Cells**: Distribution of organelles and membrane proteins is different between basal and apical cells. - **Apical surface** of absorptive epithelial cells may contain digestive enzymes such as disaccharidases and peptidases. - Receptors for chemical messengers (as hormones neurotransmitters) are located at the **basolateral surface**. - **Nutrition:** Epithelial tissue is **avascular** (blood vessels do not penetrate the epithelium, metabolites diffuse through the basal lamina). - **Innervation:** Most epithelial tissues receive a rich supply of **sensory nerve endings** from nerve pl**exuses in the lamina propria**. ## General Biology of Epithelial Tissues (continued): - **Renewal of Epithelial Cells:** Epithelial cells are renewed continuously by means of mitotic activity (LABILE). Renewal rate is variable: - Intestinal epithelium - fast (every week) - Glands - slow. - In stratified epithelial tissues, mitosis only occurs within the basal layer in contact with the basal lamina that contain stem cells. - **Metaplasia:** Transformation of one type of an epithelial tissue into another type under certain circumstances. - E.g. Smoking transforms bronchial lining from ciliated pseudostratified columnar epithelium into stratified squamous epithelium. ## Specialization of the Cell Surface: - The free (apical) surface of many types of epithelial cells contains specialized structures, as: - Microvilli - Stereocilia - Cilia & Flagella - **Functions:** - Increase cell surface area - Cell movement or moving substances or particles stuck to the epithelium - *OR* ## Microvilli: - Finger-like extensions found mainly on the free cell surface. - Mainly found in **absorptive cells** (as intestine & proximal renal tubules epithelium). - Absorptive cells have a thicker **glycocalyx**, and the complex of microvilli and glycocalyx may be seen with the light microscope and is called the **brush (or striated) border**. - Contain **actin filaments** that are cross-linked to each other and to the plasma membrane by many proteins (as: fimbrin & villin). ## Stereocilia: - Long, nonmotile extensions of cells. - Mainly seen in the epididymis and ductus deferens. - Actually, they are long branched microvilli (not cilia). ## Cilia: - Long, motile structures at the surface of some epithelial cells (as respiratory airways, fallopian tubes). - They are composed of a central pair of isolated microtubules surrounded by nine pairs of microtubules (2+9 arrangement). - Cilia are inserted into **basal bodies** (cylindrical structures at apical pole of cell membrane with nine triplet arrangement of microtubules, centriole-like structure). - Cilia have a rapid forth & back (whip-like) movement that is coordinated to permit a current of fluid to be propelled in one direction over the surface. - ATP is the source of movement energy. ## Flagella: - Only found in human **spermatozoa**. - Have similar structure to cilium, but: - Much longer. - Limited to one **flagellum** per one cell. ## Types of Epithelia: - Epithelia are divided into two main groups according to their structure and function: - **Covering epithelia** - **Secretory (glandular) epithelia** ## Classification of Covering Epithelia: - **Simple**: Squamous, Cuboidal or Columnar, Pseudostratified - **Stratified**: Squamous, Cuboidal or Columnar, Transitional epithelium. ## Types of Squamous Epithelia - **Simple squamous**: E.g.: endothelium lining heart & blood vessels, alveoli of lungs, mesothelial lining of body cavities, thin section of loop of Henle, inner side of cornea... - **Stratified squamous epithelium**: - Actually, usually only the cells at or near the free surface are squamous. Deeper cells tend to be cuboidal or columnar. - Two forms: - **Keratinized**, - e.g. skin. - **Non-keratinized** line mucosal surfaces (wet cavities) - e.g. mouth, nasal passages & vagina, anus, rectum....This epithelium often (but not always) has mucous glands associated with it. ## Types of Cuboidal Epithelia - **Simple cuboidal**: E.g.: epithelial lining of proximal and distal convoluted tubules in kidney, thyroid gland, glands dusts... - **Stratified cuboidal**: Rare, found in some glandular ducts, E.g. sweat glands, sub-maxillary glands. ## Types of Columnar Epithelia - **Simple columnar**: e.g. epithelial lining of stomach, intestine, gallbladder, uterus & uterine tubes, collecting ducts of kidney... - **Pseudostratified epithelium**: Looks stratified due to varying positions of cell nuclei, but all cells are in contact with the basement membrane Some of the cells do not reach the free apical surface. The best example is ciliated pseudostratified columnar epithelium of respiratory airways. - **Stratified columnar**: Rare, only the cells at the free surface are columnar. E.g. parts of urethra, some ducts of the parotid gland... ## Transitional Epithelium - **Transitional epithelium**: Locations: Urinary bladder, renal pelvis, ureters. Functions: Permits expansion and recoil after stretching. - Lines inner lining of urinary bladder & ureters. - Intermediate form of cells → their shape changes from dome-shaped to flat cells according to the functional state of the bladder (permits distention). ## Secretory (Glandular) Epithelia: - Formed by cells specialized to produce secretions (as digestive enzymes, sweat, tears, milk...). - Molecules to be secreted are stored in small membrane bound vesicles called **secretory granules**. - Glandular epithelial cells may synthesize, store, and secrete: - **Proteins** (as: pancreas). - **Lipids** (as: adrenal, sebaceous glands). - **Complexes of carbohydrates and proteins** (salivary glands). - **Proteins, lipids, and carbohydrates** together (mammary glands). ## Types of Secretory Epithelia - **Unicellular glands:** Consist of isolated glandular cells, usually close to the covering epithelia. The most common example is the **Goblet cell** seen in epithelial lining of the small intestine or the respiratory tract. - **Multicellular glands:** Composed of clusters of cells. **Gland:** Large, complex aggregates of glandular epithelial cells. ## Formation of Glands - Glands arise during fetal life from covering epithelium by proliferation and invasion of epithelial cells into the subjacent connective tissue, followed by further differentiation. ## Types of Glands: - **Exocrine glands** - **Endocrine glands** - **Mixed glands** (as pancreas) ## A. Exocrine Glands: - Retain their connection with the surface epithelium from which they originated. - This connection is transformed into tubular ducts lined with epithelial cells. - So they have: - **Secretory portion:** which contains cells responsible for secretory process. - **Duct/s:** Which is responsible for transporting secretions to surface (skin) or internal cavities. ## Structure of Exocrine Glands: - Three features are used describe the structure of exocrine glands: - **Ducts:** - Simple: Have one undivided duct. - Compound: Have divided & branching duct/s. - **Shape of secretory part:** - Tubular: Tube shaped. - Alveolar or Acinar: Blind pockets. - **Relationship between a duct and the secretory part:** - Coiled - Branched: Several secretory areas sharing one duct. ## Methods of Secretion in Exocrine Glands: - According to **METHODS OF SECRETION**, exocrine glands can be classified into: - **Merocrine:** The secretory granules leave the cell by **exocytosis** with no loss of other cellular material. E.g.: Pancreas, salivary gland. - **Apocrine:** The secretory product is discharged together with parts of the apical cytoplasm. E.g.: Mammary gland. - **Holocrine:** The product of secretion is shed with the whole cell after the cell is filled with secretion and burst. E.g.: Sebaceous gland. ## Types of Secretions in Exocrine Glands: - Exocrine glands can also be classified according to **TYPES OF SECRETIONS**: - **Serous glands**: Produce watery secretions containing enzymes. E.g.: Parotid salivary glands, pancreas. - **Mucinous glands**: Secrete **mucins** (protein*a*ceous) secretion. E.g.: Sublingual salivary glands, submucosal glands of small intestine. - **Mixed exocrine glands**: Produce both serous and mucous secretions. E.g.: Submandibular salivary glands. ## Serous cells: - Acinar cells of the pancreas and parotid salivary glands are examples. - Their polarity is well defined: - Intense **basophilia** in the basal region due to accumulation of Golgi apparatus & mRNA (**basal or peripheral basophilia**). - Apex contains light-stained secretory granules due to accumulation of mature secretory granules (**apical or central acidophilia**). - These granules are called **zymogen granules** in cells that produce digestive enzymes. - Secretion occurs by **exocytosis**. ## Mucus-secreting cells: - Has numerous large, lightly staining granules containing strongly hydrophilic glycoproteins called **mucins**. - The best example is **GOBLET CELL**. - Pale secretory granules fill the apical pole of the cell. - The nucleus is located in the cell base. - Other mucus-secreting cells are found in several parts of the digestive tube, salivary glands, respiratory tract, and genital tract: - These cells are organized as **tubules** showing a pale cytoplasm and a darkly stained nucleus positioned at the cell base. ## Myoepithelial Cells: - They are stellate or spindle-shaped cells found in several exocrine glands (sweat, lacrimal, salivary, mammary). - Located between the basal lamina and the basal surface of secretory or ductal cells. - Cytoplasm contains numerous **actin and myosin** filaments. They also contain intermediate filaments that belong to the **cytokeratin** family, which confirms their epithelial origin. - They function to contract around the secretory or conducting portion of the gland to help propel secretory products toward the exterior. ## B. Endocrine Glands: - Connection with the surface is lost during development (ductless). - Their secretions (called **hormones**) are picked up and transported to site of action **via blood**. - Endocrine cells are arranged in two ways: - Anastomosing cords interspersed between dilated blood capillaries (as: adrenal & parathyroid glands). - Vesicles or **follicles** filled with non-cellular material (as thyroid gland). ## Diffuse Neuroendocrine System (DNES) - Endocrine cells interspersed among nonendocrine cells. - These endocrine cells contain epinephrine, norepinephrine, and serotonin. - Many, but not all, of these cells are able to take up amine precursors and exhibit amino acid decarboxylase activity → designed as APUD. - Some of these cells stain with silver salts → they are also called argentaffin and argyrophil cells. - As some of these endocrine cells do not concentrate amine precursors, the designation APUD has been replaced by DNES. - These cells are widespread throughout the respiratory, urinary, and gastrointestinal systems, the thyroid, and the hypophysis. - Have secretory granules in the basal region of the cell and the secretions are released in the blood. - Some DNES cells are known as **Paracrine cells** because they produce chemical signals that regulate the function of neighboring cells.