Epithelial Tissue PDF

Epithelial tissue  Cover internal and external body surfaces, line cavities and form glands Cells tightly linked together by junctional complexes, there is hardly any extracellular space. Avascular tissue: nourished from the underlying connective tissue Separated from the underlying connec...

Epithelial tissue  Cover internal and external body surfaces, line cavities and form glands Cells tightly linked together by junctional complexes, there is hardly any extracellular space. Avascular tissue: nourished from the underlying connective tissue Separated from the underlying connective tissue by the basement membrane Functions of the epitelial tissue: Delimitation – cover all body surfaces – Lining Epithelium Protection Diffusion - regulate exchange of molecules between compartments Absorption Filtration Excretion Reception of stimuli Secretion - synthesize and secrete glandular products (enzymes, hormones) – secretory glands  Polarity Epithelial cells are polarized (structural Apical plasma membrane and funcional polarity) and resting on a basal lamina: basal domain - attaches to the basement membrane; apical domain – faces the lumen of the cavity or external environment; Lateral lateral domains - face the sides of plasma adjacent cells in the epithelium. membrane In each of these areas the cell will present different structures depending on its function.. Basal plasma membrane  Apical membrane specializations Apical domain of the cell - is the one that is directed towards the lumen. Presents abundant ion channels and transport proteins and it is the place where secretion products will be released. In this domain several structures may appear that will be related to the function of the epithelium: Mv Cilia Microvilli Cilia Stereocilia Stereocilia Keratin Keratin Microvilli Membrane-covered extensions of the cytoplasm, formed by a cytoskeleton of actin filaments. Rigid structures that increase the cell surface – its main function is transport and absorption. They appear mostly in the small intestine and renal tubules.  Microvilli Microvilli Enterocytes Microvilli (X 30.000)  Cilia Membrane-covered extensions, but much larger than the microvilli. Composed of 9 microtubule doublets and a central pair of microtubules. At its base appears the basal body formed by 9 triplets of microtubules that supports the entire structure. Flexible structures that cover the whole apical surface of the cell. They move substances from the medium by rapid and rhythmic oscillations. They appear mostly in the respiratory tract. Cilia x 800  Stereocilia Epididymis Modified microvilli, converted into sensory structures (mechanoreceptors) They increase the absorption surface, but also help the movement of the medium. They only appear in the male reproductive system and the cochlea (inner ear). Inner ear  Keratin Keratin is a protein produced by epithelial cells and is located in the cytoplasm of these cells. It provides rigidity to the cells, making the epithelium more resistant to external damages. When the cell becomes stiff, its vital functions will be hindered, and the cell will eventually die. The filaments of keratin that remain once the cell disappears, cover the epithelium providing protection before they flake and are replaced by new cells of the epithelium.  Lateral membrane specializations Lining epithelia are characterized by having very little extracellular matrix and t heir cells are tightly joined by junctional complexes. Microvilli Microvilli Types of junctions depending on the space they occupy: - Zonula: union in the shape of continuous band around the entire perimeter of the cell. - Macula: punctual union. Types of junctions depending on the space they leave between cells: - Occlusive: does not leave space between membranes. - Adherent: there is a small space between membranes. Zonula occludens or tight junction Zonula adherens or adherens junction Desmosomes (adherent macula) Communicating, GAP junctions or nexus  Zonula occludens: tight junction Occludins Claudins - Most apical junction between epithelial cells - Forms a continuous band around the entire perimeter of the cell - Seals completely the space between cells - The membranes of the two cells are fused at several points due to transmembrane proteins - occludins and claudins - Does not allow the passage of membrane proteins - Prevents the passage of molecules between the cells  Zonula adherens: adherens junction Cadherins Zonula occludens Zonula adherens - They are located next to the tight junctions. - They also form a belt that surrounds the entire perimeter of the cell. - They leave a space of about 15-20 nm between the membranes of both cells. - Desmosome The space between the membranes is occupied by the extracellular part of membrane proteins - cadherins. The cadherins are joined together between cells. - The intracellular side of the cadherins binds to actin filaments  Macula adherens: desmosome - Point junctions randomly distributed across the plasma membrane. - Located below the zonules. - They leave an intercellular space of about 30nm also occupied by cadherins - The cytosolic part of the junction has a junction plate in which intermediate filaments of keratin are attached. Cadherins  Communicating or GAP junctions or nexus - Circular union zones distributed randomly in the plasma membrane, always below the zonules. - Contain pores. - Present in epithelial tissues, heart muscle, smooth muscle and neurons. - Six proteins - connexins - bind to form transmembrane channels. This structure is called connexon. - The connexon binds to a connexon in the other cell, joining the two membranes and leaving a pore between them that allows the passage of ions and small molecules. Basal membrane specializations Basement membrane: membrane located at the border between epithelia and the underlying connective tissue. It works as an anchor for the epithelium and filters substances that pass from the epithelium to the inside of the body (or vice versa). Folds: invaginations of the plasma membrane that increase the surface of the cell. There is more surface for contacts or transport through the membrane. Hemidesmosomes  Hemidesmosomes - Binding structures that join the cell membrane to the basement membrane - They appear in the basal cells of the epithelium - Its structure resembles half desmosome - In the intracellular part they have a dense plate attached to intermediate filaments - The extracellular part has proteins that bind to the basement membrane - integrins  Classification of Epithelia Lining epithelia Glandular epithelia or glands Lining epithelia They are classified according to three features: - Number of layers of cells - Simple - Stratified - Pseudostratified - Cell shape (surface cells) - Flat/ squamous - Cuboidal - Columnar - Presence of specializations on apical surface - Microvilli - Cilia - Stereocilia - Keratin Lining Epithelium  Simple squamous epithelium Single layer of thin, flattened cells Function: lining of surfaces, filtration of substances Appears covering the pulmonary alveoli, blood and lymphatic vessels, some renal tubules and in the mesothelium of the pleural, pericardial and peritoneal cavities Mesothelium  Simple cuboidal epithelium Single layer of cuboidal cells, with rounded nucleus in the center of the cell Function: absorption, secretion or excretion Appears in the gland ducts, covering the ovary and also in some renal tubules  Simple columnar epithelium Single layer of tall cells, rectangular profile and oval nucleus located more towards the base of the cell Function: lining, transport, absorption and secretion of substances Appears largely in the digestive tract, gallbladder and large gland ducts. It can present microvilli, that increase its absorption surface. It can also present cilia as it occurs in oviducts, efferent ducts or small bronchi Simple columnar epithelium with microvilli  Stratified squamous epithelium Several layers of cells: flattened apical cells, polygonal cells in the intermediate layers and basal cells with a cuboidal or cylindrical shape. Only the deepest layer will contact the basement membrane. Function: lining and protection Covers the walls of the mouth, oropharynx, esophagus, vocal cords and vagina. These epithelia are usually protected by a mucous secretion that keeps moist - mucosal epithelia Stratified squamous keratinized epithelium Similar to the stratified flat mucosal epithelium, but its apical cells are dead cells filled with keratin. This epithelium is very resistant to abrasion and is impermeable to water. It appears mainly in the epidermis of the skin. Stratified cuboidal epithelium Multiple layers of cuboidal cells Function: absorption and secretion Lines the ducts of some glands  Stratified columnar epithelium Several layers of cells: the surface layer is cylindrical, and the basal layers are polyhedral or cuboidal Scarce in the organism: conjunctiva of the eye, some large ducts of glands and some regions of the male urethra Function: lining, protection, absorption and secretion  Pseudostratified columnar epithelium Single layer of tall/columnar cells, but it seems to be stratified. All the cells are in contact with the basement membrane, but only some of them reach the lumen. It usually contains cilia (ciliated). It appears mostly in the respiratory tract, but also in ducts of the male reproductive system and in some large ducts of glands. Function: lining, transport, secretion and lubrication. Urothelium (Transitional epithelium) Appears only in the urinary tract One or several layers of cells with a pseudostratified appearance. The basal cells are cuboidal or cylindrical and the most apical cells are large with a rounded top (cupuliform or umbrella cells). When the organ is filled with urine, the epithelium is more distended, cupuliform cells flatten and the epithelium becomes thinner. Glandular epithelia The glands originate from epithelial cells that leave the surface and penetrate the underlying connective tissue. These cells will have the same characteristics as the rest of the epithelial cells, but they will also be specialized in manufacturing some product of secretion. Types of glands types: - Exocrine: have ducts that communicate with the epithelial surface and release their secretion products to the external environment or the cavity in which they are. - Endocrine: do not have an excretory duct. The secretion will be collected by the capillaries and distributed throughout the body. Exocrine glands They can be classified according to several criteria: ▪ According to the excretory duct ▪ Simple: they only have one duct ▪ Compound: have more than one duct ▪ Branched: a single duct for several secretory units ▪ According to the shape of its secretory part ▪ Tubular: straight, tube shape ▪ Acinar: sac-shaped ▪ Alveolar: rounded shape  Type of secretion - Merocrine: they release their secretion products by exocytosis. It is the most used mechanism. Ex: salivary glands. - Holocrine: the secretory cell matures and dies releasing the product of secretion. Ex: sebaceous glands of the skin. - Apocrine: release a small portion of cytoplasm with the secretion product inside. It is used for lipidic products that do not solubilize in aqueous media. Ex: lipid fraction of milk.  Nature of the secretion Mucosae: secrete mugens that together with water form mucin (main component of mucus). Ex: goblet cells. Serous: they secrete enzymes. Ex: exocrine pancreas. Mixed: they contain serous and mucous secretion. Ex: major salivary glands. Mucosae Serous Mixed  Number of cells Unicellular: made of just one cell. For example, goblet cells that appear in the digestive or respiratory track. Multicellular: made of several cells, they have a duct and a secretory part. Goblet cell  Endocrine glands - They do not have excretory duct - They pour their secretion into the blood or lymphatic stream that will carry it to their target organs - Its secretion are mainly hormones - The endocrine glands can be classified into: - Cordonal - Follicular  Cordonal endocrine glands They are the most frequent. The endocrine cells are organized in cords or groups and between them will pass capillaries that will collect the secretion. Ex: adrenal glands, adenohypophysis.  Follicular endocrine glands Cells are organized forming hollow spherical structures -follicles. Inside these follicles, the hormone is stored inactive and when it is necessary, it is activated and is released into the bloodstream. Between the follicles there will be capillaries that will collect the secretion. Ex: thyroid.  Anficrins glands They are glands that produce exocrine and endocrine secretion. In the human body we have two examples of these glands: - Pancreas: secretes pancreatic juices that are released through ducts to the digestive tract to participate in digestion. It also secretes hormones into the blood such as insulin or glucagon. - Liver: secretes bile that through ducts will also reach the digestive tract to participate in digestion. On the other hand, they also secrete coagulation and growth factors that release into the blood.