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 connect...

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 • Delimitation • Protection • Diffusion • Absorption • Filtration • Excretion • Uptake of stimuli • Secretion Polarity Apical plasma membrane Epithelial cells are resting on a surface. They have a basal surface, which attaches to the basement membrane, an apical surface, which faces the lumen of the cavity or external environment, and lateral surfaces, which face the sides of adjacent cells in the epithelium. Lateral plasma membrane Basal plasma membrane In each of those areas the cell will have different structures according to its function. That is why it is said that the epithelial cells are polarized. Apical membrane specializations The apical domain of the cell is the one that is directed towards the lumen. It 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 • Keratin Stereocilia Keratin Microvilli The microvilli are membranecovered cytoplasm extensions formed by a skeleton of actin filaments. They are rigid structures. They increase the surface of the cell a lot, that is why 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 They are also membrane-covered extensions, but much larger than the microvilli. They are made of a pair of microtubules located in the center and surrounded uniformly by 9 microtubule doublets. On its base appears the basal body formed by 9 triplets of microtubules that supports the entire structure. They are flexible and 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 Extremely long microvilli. They increase the absorption surface, but also help the movement of the medium. They only appear in the male reproductive system and in 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 In the lateral zones of the epithelial cells, we will find structures that intervene in the junctions between cells and that will be responsible for the cohesion of the epithelium: • Zonula occludens or tight junction • Zonula adherens or adherens junction • Desmosomes (adherent macula) • Communicating, GAP junctions or nexus 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: 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 - Desmosome - 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. 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 so in addition to joining the two cells, they also attach their cytoskeletons. Macula adherens: desmosome - Junctions of punctual shape distributed randomly in 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. They contain pores. In addition to the epithelial tissue, they also appear in the heart muscle, smooth muscle and neurons. Six proteins called connexins bind to form transmembrane channels. This structure is what we call connexon. The connexon binds to a connexon in the other cell and thus join the two membranes, leaving a pore between them that allows the passage of ions and small molecules. Basal membrane specializations • Basement membrane: is a 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: are invaginations of the plasma membrane that increase the surface of the cell. There is more surface for connections or transport through the membrane. • Hemidesmosomes Hemidesmosomes - They are binding structures that join the cell membrane to the basement membrane. - They appear in the basal cells of the epithelia. - Its structure resembles half desmosome. - In the intracellular part they have a dense plate attached to intermediate filaments just as it appeared in the desmosomes. - The extracellular part has proteins called integrins that bind to the basement membrane. 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 A single layer of thin, flattened cells. In addition to lining surfaces, it easily allows the filtration of substances. It 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 A single layer of cuboidal cells, with rounded nucleus in the center of the cell. This epithelium is often associated with absorption, secretion or excretion. They appear in the gland ducts, covering the ovary and also in some renal tubules. Simple columnar epithelium A single layer of tall cells, rectangular profile and oval nucleus located more towards the base of the cell. In addition to lining, this type of epithelium intervenes in the transport, absorption and secretion of substances. Appears largely in the digestive tract, gallbladder and large gland ducts. It can present microvilli, thus increasing 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 Formed by several layers of cells and only the deepest layer will contact the basement membrane. These basal cells usually have a cuboidal or cylindrical shape, in the intermediate layers there are polygonal cells, and the apical cells are flattened, hence the name of the epithelium. In addition to lining, also serves as protection. Covers the walls of the mouth, oropharynx, esophagus, vocal cords and vagina. These epithelia are usually protected by a mucous secretion, which is why they are always kept moist and are also known as mucosal epithelia. Stratified squamous keratinized epithelium It is similar to the stratified flat mucosal epithelium, but its apical cells are dead cells filled with keratin. That is why this epithelium is very resistant to abrasion and is also impermeable to water. It appears mainly in the epidermis of the skin. Stratified cuboidal epithelium It contains multiple layers of cuboidal cells and lines the ducts of some glands. They can be used for absorption and secretion. Stratified columnar epithelium The basal layers are polyhedral or cuboidal and the surface layer is cylindrical. It hardly appears in the organism, only in the conjunctiva of the eye, some large ducts of glands and some regions of the male urethra. In addition to lining, also provides protection and can serve to absorb and secrete. Pseudostratified columnar epithelium It consists of a single layer of 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. These cells are tall, that's why we talk about columnar epithelium. 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. It serves as lining, but also for transport, secretion and lubrication. Urothelium (Transitional epithelium) It appears only in the urinary tract. It consists of one or several layers of cells and usually has a pseudostratified appearance. The basal cells are cuboidal or cylindrical and the most apical cells are large with a rounded top. They are called cupuliform or umbrella cells because of the way they look. 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. The glands can be classified into two main 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. For example it is seen in the salivary glands. Holocrine: the secretory cell matures and dies releasing the product of secretion. It occurs for example in the 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. It occurs for example in the 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. They are for example the adrenal glands or the adenohypophysis. Follicular endocrine glands In this type of glands, the cells are organized forming hollow spherical structures that we call 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. The example of this type of glands is the 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.