Epithelial Tissue PDF

_____________LESSON 6 _____________ EPITHELIAL TISSUE I. ANIMAL TISSUES: CONCEPT AND TYPES A tissue is an association of cells. The cells that constitute a tissue have similar morphological and functional characteristics: a tissue is therefore a morphological and functional association of cells th...

_____________LESSON 6 _____________ EPITHELIAL TISSUE I. ANIMAL TISSUES: CONCEPT AND TYPES A tissue is an association of cells. The cells that constitute a tissue have similar morphological and functional characteristics: a tissue is therefore a morphological and functional association of cells that are consistently arranged to perform a certain function. Cells are not the only component of tissues; the other component is the extracellular substance. There are four types of tissues: 1. Epithelial tissue 2. Connective tissue 3. Muscle tissue 4. Nervous tissue II. EPITHELIAL TISSUE Components of the epithelial tissue: epithelial cells and basal membrane II.1 Epithelial cells: General features of epithelial tissue cells The epithelial cells have a polyhedral shape, and therefore show an apical surface, two side surfaces and a basal surface. All surfaces can have the following cell membrane modifications: The apical surface: microvilli and cilia. 1 The side surfaces: junctional complexes with adjacent cells (zonula occludens, zonula adherens, desmosomes, interdigitations). The basal hemidesmosomes). surface: junctional complexes (here called Renewal of epithelial tissue cells The cells that make up the epithelial tissue are renewed throughout the life of the animal, although the rate of cell turnover varies with location and function. The time it takes for an epithelial tissue cell to be renewed remains constant in each epithelium. For example, the cells of the epidermis are renewed every 28 days and the cells of the mucosa of the small intestine, every 4 or 6 days. II.2. Basement membrane or basal lamina: It is the extracellular substance of the epithelial tissue. It is a narrow acellular region located between the basal surface of the epithelial tissue cells and the connective tissue that is always below them. The basement membrane has three functions: 1) Attach epithelial cells to the connective tissue beneath them in a flexible manner. 2) Serve as a molecular filter (this function is particularly relevant in epithelial tissues with absorption and secretion functions). 3) Serve as a "guide" during the processes of cell differentiation and migration that take place in the regeneration and repair of epithelial tissue. The basement membrane is not seen with the light microscope with the haematoxylin-eosin technique (routine staining technique), but it can be seen with special histochemical techniques, such as PAS and different silver impregnation techniques (silver techniques). After observation with the electron microscope, the basement membrane is divided into the basal lamina, produced by epithelial cells and containing proteoglycans, type IV collagen and glycoproteins such as laminin and fibronectin, and the reticular lamina, synthesized by cells of the underlying connective tissue and composed of type III collagen. Nutrition of epithelial tissue The epithelial tissue is AVASCULAR (except for the epithelium of the stria vascularis of the organ of Corti). The nutrition of the epithelial cells is carried out by physical diffusion of nutrients and oxygen from the blood capillaries located in the underlying connective tissue. 2 Functions of epithelial tissue Protection: acts as a barrier that protects the tissues beneath it from abrasions and traumatic injuries. Transcellular transport of molecules through epithelial cell layers. Secretion of different substances, such as mucus, enzymes, hormones, etc. Absorption of material from the lumen of hollow organs, such as the digestive tract or some renal tubules. Selective permeability, controlling the passage of materials between the different compartments of the body. Identification of sensations by the taste buds (taste), the retina (sight), specialized hair cells of the ear (hearing). Types of epithelial tissue Epithelial tissue is found in the body in two different forms: In the form of sheets of contiguous cells that cover the body on its external surface and line it on its internal surface, in which case it is called the lining epithelium. As isolated cells or in groups with only secretory functions, in which case it is called glandular epithelium. When these cells with only secretory functions are grouped together, they are called glands. III. LINING EPITHELIAL TISSUE The lining epithelial tissue has all the functions of the epithelial tissue except for secretion. There are several types of lining epithelial tissue. The two classification criteria are: The contact or not of all epithelial cells with the basement membrane. According to this criterion, two types of lining epithelia are distinguished: simple lining epithelium, made up of one or more layers of cells that contact the basement membrane, and stratified lining epithelium, made up of more than one layer of cells that can be morphologically and functionally homogeneous or heterogeneous between them. Some of these layers do not contact with the basement membrane. The morphology of the cells, which can be flat, cuboidal and columnar. The stratified epithelia are classified according the morphology of the cells of the superficial layer. 3 There are two types of epithelia that can be classified as specials by their morphological characteristics: pseudostratified and transitional. Simple squamous epithelium Made up of a single layer of flat cells, very thin and identical to each other, with an oval nucleus near the center of the cell. This epithelium is found in the vascular system (in which it is called endothelium), in Bowman's capsule and loop of Henle of the kidney, pulmonary alveoli and lining the serous tunics of the pleura and peritoneum (in this case the epithelium receives the name of mesothelium). The main function of this epithelium is the transport of substances (liquids, crystalloids and some colloids at the level of the endothelium, liquids at the level of the kidney and mesothelium, and gases at the level of the lung alveoli), a function facilitated by the extreme thinness of the cytoplasm. Simple cuboidal epithelium Made up of a single layer of cells whose height and width are approximately the same, all identical or similar to each other. When they are observed in a perpendicular section to the surface, each cell has a square profile with a centrally located round nucleus. All nuclei are located at the same height. The cuboidal epithelium lines the excretory ducts of the glands, renal tubules and covers the choroid plexuses and ciliary bodies. Depending on their location, these cells can have a protective, secretory or absorptive function. Simple columnar epithelium Made up of a single layer of cells whose height is greater than the width, all identical or similar to each other. They present an oval nucleus located in the basal area. All nuclei are located at the same height. In general, the columnar epithelium covers organs that perform absorption functions (such as the small and large intestine) or secretion (as is the case of the bulbourethral gland). Columnar epithelial cells with absorption functions usually present microvilli, called “brush border” on their apical surface. 4 1 2 Figure 1: Simple squamous epithelium lining the Bowman capsule of the kidney. The nucleus bulges towards the lumen (arrows). Figure 2: Simple cuboidal epithelium lining mucous gland excretory ducts. Figure 3: Simple columnar epithelium lining the gallbladder. Stratified squamous epithelium As it is made up of several layers of cells (stratified), only the deepest layer is in contact with the basement membrane. It receives the second name (squamous) because the cells in the last layer have a flattened shape. There are two types of stratified squamous epithelium: Keratinized stratified squamous epithelium: this epithelium is in the epidermis, which is the most superficial layer of the skin, the lingual papillae, and some areas of the oral cavity, such as the cheeks and hard palate. It is called keratinized because it has keratin, a protein that makes this epithelium highly resistant to friction and waterproof. It has the following layers from the deepest to the most superficial: (1) Stratum basale: the deepest layer of cells, which is in contact with the basement membrane. These cells are cuboidal, with basophilic cytoplasm on light microscopy (due to it has many ribosomes) and a large nucleus. In this stratum many mitoses are observed because the cells that move to higher strata are being produced. For that reason, this layer is also called the germinal stratum. (2) stratum spinosum: it is formed by several layers of polyhedral cells with acidophilic cytoplasm and round nucleus that are strongly joined together by desmosomes. Desmosomes are seen under light microscopy as "spines" between cells, hence the name. (3) Stratum granulosum: it is constituted by several layers of flattened cells (variable number) with many basophilic cytoplasmic granules (keratohyalin granules, precursor of keratin) and a nucleus with signs of cell death (pyknosis). (4) Stratum lucidum: it is formed by several layers of flattened cells with acidophilic cytoplasm and nucleus with signs of cell death (karyorrhexis). This stratum only exists in hairless keratinized epithelia. (5) Stratum corneum: it is formed by a variable number of layers of flat cells without nucleus arranged like scales. These cells are already dead, and their nucleus and cytoplasm have been replaced by keratin. 5 3 Nonkeratinized stratified squamous epithelium: lines some areas of the mouth, pharynx, oesophagus and vagina. It is called nonkeratinized because it does not have keratin. It is constituted by the germinal, spinosum, corneum strata. The corneum is not a true stratum corneum because the flattened surface cells retain their nucleus and do not have keratin. 4 5 epitelios estratificados cúbicos y cilíndricos se localizan en Figure 4: Los Keratinized stratified squamous epithelium (skin). zonas muy específicas del cuerpo, especialmente en los pasos Figure 5: Nonkeratinized stratified squamous epithelium (tongue). Stratified cuboidal and columnar epithelia are located in very specific areas of the body, especially in the transition from simple to stratified epithelia and in the transition from stratified to pseudostratified. They are found in the lacrimal sac and duct, in the distal urethra, and in the excretory ducts of the parotid and submandibular glands. Special epithelia Pseudostratified columnar epithelium As the name suggests, this epithelium appears to be stratified but is not. It is a simple epithelium, in which all cells contact the basement membrane, but these cells have different shapes and sizes. As some are taller than others, the nuclei are at different heights. It is this image of the nuclei that gives the impression that the epithelium is stratified. Also, not all cells reach the lumen. The pseudostratified ciliated epithelium may be ciliated or non-ciliated: the more abundant in the body is the ciliated, so called because the cells reaching the surface have cilia on their apical side. It is found lining most of the nasal cavity, trachea and the main bronchi. Pseudostratified columnar nonciliated epithelium is found in the male urethra, the epididymis, and the large excretory ducts of the glands. 6 Transitional epithelium It is considered a special form of simple epithelium located exclusively in the urinary system; hence it is also known by the name of urethelium. This epithelium lines hollow organs capable of undergoing considerable distention, as is the case of the urinary bladder. The shape and number of layers of the epithelium depend on the degree of distention of the urinary bladder. Thus, if there is little distention in the bladder, six or seven layers of cells are observed in the cushion-shaped epithelium. In this case, the most basal cells are small and cuboidal, the intermediate cells are polygonal and the apical cells are cuboidal with a rounded edge. When the urinary bladder is distended, one or two layers of cells are seen in the epithelium and the cells appear elongated and flattened. With the application of electron microscopy techniques, it is observed that all epithelial cells contact the basement membrane through the emission of thin cytoplasmic processes. Besides allowing distension of the organs of the urinary apparatus, the urethelium prevents the passage of water from the subepithelial connective tissue to the urine, which is hypertonic. 6 7 Figure 6: Pseudostratified columnar ciliated epithelium of the trachea. Figure 7: Transitional epithelium lining the urinary bladder. 7 IV. GLANDULAR EPITHELIAL TISSUE. It is made up of epithelial cells with secretory functions. These cells receive the generic name of gland. The secretory function has two parts: The synthesis: cells produce certain products intracellularly. These products are macromolecules that are usually stored in vesicles called secretory granules. The part of the gland that performs the synthesis is called the secretory unit (adenómero in Spanish). The excretion: cells send the synthetized products to their site of action. The part of the gland that carries out the transport is called the excretory duct. The glands are classified according to different structural and functional characteristics to facilitate their study (morphological characteristics, type of secretion product and mode of secretion). Morphological characteristics A . Unicellular glands. A.1. These consist of a single secretory cell embedded in a nonsecretory lining epithelium. As an example of a unicellular gland, we have the goblet cell. It is a specialized epithelial cell that produces mucin (mucus precursor), which is released on the epithelial surface. When this secretory substance is synthesized, it occupies the apical portion of the cell and stretches it, pushing the nucleus and organoids toward the thinner basal portion, thus giving the cell the shape of a calyx. Under the light microscope and with routine staining techniques, the mucus is weakly basophilic, but is the colour seen is magentared after performing the PAS technique. The scarce cytoplasm of the cell is very basophilic due to the large amount of rough endoplasmic reticulum it contains. A.2. Also, there are single-celled glands that synthesize hormones. These cells are found in the digestive and respiratory systems and are called diffuse neuroendocrine system cells or APUD cells. B . Multicellular glands. They are made up of more than one cell. B. 1. Endocrine glands. They are multicellular glands that do not have a duct system to transport their secretion product to the places of use. The secretion product, which is a hormone, is released directly into the intercellular fluid, from where it is transported to its place of action by blood and lymph. 8 Classification of endocrine glands The endocrine glands sometimes form the parenchyma of organs specialized in the synthesis and excretion of substances, such as the thyroid, hypophysis and liver. The endocrine glands are classified according to different criteria. According the nature of the secretory product: proteins and steroids. Cells that synthesize protein hormones have general characteristics different from those that synthesize steroid hormones. According to the morphology of the secretory unit: cords and follicles. B. 2. Exocrine glands. These glands develop a system of ducts through which their secretion products are transported to the places of use. The exocrine glands have two parts: unit. The set of cells holding the synthesis of the product, called secretory The set of cells that are responsible for transporting the product to the lining epithelium, which is called excretory duct. The exocrine glands can be: B. 2.1. Simple glands, constituted by one or more secretory units flowing into a single, unbranched excretory duct. B .2.2. Compound glands. They are composed by many secretory units flowing into a system of branched excretory ducts. B.2.1. Simple glands They are classified according to the shape of the secretory unit. B.2.1.1. Simple tubular glands: the secretory unit a straight tubule that opens directly to the epithelial surface, such as crypts of Lieberkünh of the small and large intestine, or flows into a short excretory duct. B.2.1.2. Simple coiled tubular glands: the secretory unit is arranged in a spiral and flows into a single excretory duct, as sweat glands. B.2.1.3. Simple branched tubular glands: have several secretory units that flow into a single short excretory duct, as in the case of the uterine glands. B.2.1.4. Simple acinar or alveolar glands: the secretory unit has a spherical shape and flows into an excretory duct. The lumen of the acinus is small, and the lumen of the alveolus is wide. For example, sebaceous glands. 9 B.2.1.5. Simple branched acinar and alveolar glands: two or more acinus or alveolus appear together, and the secretory product is poured through a common excretory duct. For example, sebaceous glands. B. 2.1.6. Simple tubuloacinar and tubuloalveolar glands: they appear only in their branched form and have secretory units formed by a tubular portion and a terminal acinus or alveolus. For example, salivary glands. 9 8 10 Figure 8: Simple tubular gland. Crypts of Lieberkünh (large intestine). Figure 9: Simple coiled tubular glands. Sweat gland of the skin. Figure 10: Simple branched acinar gland. Sebaceous gland of the skin. B.2.2. Compound glands Compound glands are made up of the same types of secretory units as simple glands, but they have a system of excretory ducts that repeatedly branch. They are also classified according to the shape of their secretory units into tubular, acinar (pancreas), alveolar (mammary gland, prostate), tubuloalveolar and tubuloacinar. The terminology “compound branched gland” is not correct because each excretory duct leads to several secretory units and, therefore, it would be a redundancy. Type of secretion product units. They are classified into mucous, serous and seromucous secretory The serous glands have a transparent and watery secretion. The cells of the secretory units generally have a spherical nucleus, in the basal position, numerous cisternae of rough endoplasmic reticulum and Golgi complex in the supranuclear position, and the apical cytoplasm is occupied by small 10 secretory granules (zymogen granules, which are precursors of enzymes). For example, pancreatic acini. The mucous glands produce a thick and viscous secretion that forms a protective layer over the lining of the hollow organs that communicate with outside of the body. They are full of mucin, the precursor to mucus, which is slightly basophilic with routine staining techniques. Nucleus and organoids are displaced towards the basal part of the cell and compressed against the cell membrane. For example, salivary glands. The seromucous glands contain both serous and mucous cells. 12 11 Figure 11: Serous secretory units (pancreas). Figure 12: Mucous secretory units (salivary glands). Mode of secretion Merocrine: the product is released in the form of small secretory granules, released by exocytosis. For example, pancreatic acini. Apocrine: with the secretion product a cytoplasm portion from the secreting cell is detached. For example, sweat glands and mammary glands. Holocrine: the secretory cell disintegrates and constitutes the secretion product. For example, sebaceous glands. Cytocrine: transfer of secretory granules to another cell. For example, melanocytes when they transfer melanin to other cells, pigmenting them. Myoepithelial cells: in some glands there are myoepithelial cells interposed between the base of the secretory cells and the basement membrane. They are epithelial cells that have the capacity to contract. When stimulated, they 11 contract and push the secretion product into the excretory duct system. These cells are well developed in the sweat and mammary glands. 15 Figure 13: Secretion granule pouring out the contents by exocytosis (merocrine secretion). Figure 14: Elevation of the apical part of the secretory epithelium that will be part of the secretion (apocrine secretion). Figure 15: Secretory units of sebaceous glands (holocrine secretion). MIXED EXOCRINE AND ENDOCRINE GLANDS Pancreas, ovary, testicle. 12