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Biology \Histology lec1 Epithelial tissue lecturer: Farah E. AlRawi Histology is the study of the tissues of the body and how these tissues are arranged to constitute organs. Tissue is an aggregate of cells in an organism that have similar structure and function. The human body is composed of only four basic types of tissue: epithelial, connective, muscular, and nervous. Tissues have two interacting components: cells and extracellular matrix (ECM). The main characteristics of these basic types of tissue are shown in this table Tissue cells Extracellular Main Functions matrix Nervous Elongated cells with Very extremely small Transmission fine amount of nerve impulses processes epithelial Aggregated Small amount Lining of surface polyhedral cells or body cavities; glandular secretion Muscle Elongated Moderate Strong contractile cells amount contraction; body movements connective Several types of Abundant Support fixed and wandering amount protection cells tissues/ organs 1 and of Biology \Histology lec1 Epithelial tissue lecturer: Farah E. AlRawi Epithelial Tissue: [Epithelium — a single tissue; plural, epithelia; adjective, epithelial] Epithelia are cellular sheets that line the cavities of organs and cover the body surface. The principal functions of epithelial tissues include the following: 1. Covering, lining, and protecting surfaces. Example: epithelial cells of the skin protect underlying tissue from mechanical injury, harmful chemicals, invading bacteria and from excessive loss of water 2. Absorption: certain epithelial cells lining the small intestine absorb nutrients from the digestion of food. 3. Secretion: in glands, epithelial tissue is specialized to secrete specific chemical substances such as enzymes, hormones and lubricating fluids 4. Contractility: myoepithelial cells. 5. Sensation: Specialized epithelial tissue containing sensory nerve endings is found in the skin, eyes, ears, nose and the tongue. 6. Cellular transport: Transport of molecules across epithelial layers. Characteristics features of epithelial tissue: Epithelium lines the surfaces of the body and is mainly located on the borders between the external and internal environments. Epithelium also lines all the internal body spaces that have a connection with the external environment at some stage. Epithelium plays an important role in homeostasis of the body and in maintaining the physiological parameters of the internal environment different from those outside the body. Epithelium is an avascular tissue and has no integral blood supply. 2 Biology \Histology lec1 Epithelial tissue lecturer: Farah E. AlRawi Epithelium develops in the embryo from all the three germ layers (Ectoderm, Mesoderm, and Endoderm). For example, the epidermis of the skin is derived from ectoderm, the epithelium lining the serous cavities (peritoneum, pleura, pericardium) is derived from mesoderm (and is often referred to as mesothelium), whereas the epithelium lining most of the intestinal tract is derived from endoderm. Most epithelia rest on connective tissue that contains the microvasculature bringing nutrients and oxygen to both tissues. The connective tissue that underlies the epithelia lining the organs of the digestive, respiratory, and urinary systems is called the lamina propria. The area of contact between the epithelium and connective tissue may be increased by irregularities at the interface in the form of small evaginations called papillae. Papillae occur most frequently in epithelial tissues subject to friction, such as the covering of the skin or tongue. Polarity– all epithelia have an apical pole (Apical domain, directed towards the exterior surface) and a lower attached basal pole (Basal domain, rest on the basal lamina anchoring the cell to underlying connective tissue) that differ in structure and function. Regions of cells that adjoin the neighboring cells are the lateral surfaces. 3 Biology \Histology lec1 Epithelial tissue lecturer: Farah E. AlRawi Epithelial cells have specific structural modifications on the apical surface (the free surface facing the lumen or external environment) include: microvilli, stereocilia, cilia or flagella. Epithelial cells adhere strongly to neighboring cells and basal laminae with no intercellular connective tissue, particularly in epithelia subject to friction or other mechanical forces. Lateral surfaces of epithelial cells exhibit several specialized intercellular junctions, which play roles in maintaining the integrity of the tissue. Junctional complexes of epithelial cells: 1. Tight junctions (zonulae occludens) form a seal between adjacent cells prevent passive flow of chemical materials or waste products between the cells and into the blood stream but are not very strong junction .eg: the lining epithelium of the urinary bladder. 2. Adherent or anchoring junctions (zonulae adherence) are sites of string cell adhesion serve to stabilize and strengthen the circular occluding bands and help hold the cells together. 3. Desmosome or macula adherence form very strong attachment points and play a major role to maintain the integrity of an epithelium. most commonly seen in the epithelium of the skin and in cardiac muscle fibers 4. Gap junctions have little strength but serve as intercellular channels for flow of molecules and communication between adjacent cells. Hemidesmosomes bind epithelial cells to the underlying basal lamina. Basal laminae and basement membranes All epithelial cells have at their basal surface a sheet like extracellular structure called the basal lamina, separating them from the underlying connective tissue (lamina propria). The basal laminae, are 4 Biology \Histology lec1 Epithelial tissue lecturer: Farah E. AlRawi visible only by transmission electron microscopy, where are formed from an electron-dense layer (20-100 nm thick) composed of: 1. lamina lucidae: which appear to be transparent. 2. lamina densa: a delicate network of fine fibrils. Basal laminae are composed mainly of type IV collagen and a glycoprotein, called laminin. Basal laminae are not exclusive features of epithelia, but are also found associated with some other cell types. Basal lamina is sometimes attached to the underlying connective tissue by anchoring fibrils of unknown composition. In some instance, reticular fibers are closely associated with the basal lamina forming a layer termed the reticular lamina, the reticular fibers are produced by connective tissue cells and it is responsible for affixing the lamina densa to the underlying connective tissue thus the epithelial sheath is bound to the underlying connective tissue. Basement membrane: Is formed by the combination of a basal lamina and a reticular lamina and therefore, it is thicker. It is stained well by Periodic acid Schiff reagent and so it is seen by the light microscope. 5 Biology \Histology lec1 Epithelial tissue lecturer: Farah E. AlRawi Functions of basal lamina: 1. It is considered as a molecular filter and as a flexible, firm support for the overlying epithelium. 2. Provide a selective barrier between connective tissue and other cells. 3. The presence of the basal lamina around a muscle cell is necessary for the establishment of new neuromuscular junctions. 4. The ability to influence cell polarity. 5. Regulate cell proliferation and differentiation by binding with growth factors. 6. Influence cell metabolism. Classification of Epithelia Epithelia are divided into main groups according to their structure and function: 1. Covering epithelia or lining epithelia. 2. Glandular (secretory) epithelia. Covering epithelia Covering epithelia are tissues in which the cells are organized in layer that cover the external surface or line the cavities of the body. Covering epithelia are classified according to the number of cell layers to Simple epithelia contain one cell layer and stratified epithelia contain two or more layers. Covering epithelia are classified according to the cell morphology in the surface layer Based on cell shape, as squamous (thin cells), cuboidal (cell width and thickness roughly similar) or columnar (cells taller than they are wide). 6 Biology \Histology lec1 Epithelial tissue lecturer: Farah E. AlRawi Simple epithelia 1. Simple squamous epithelium: Squamous cells have the appearance of thin, flat plates. Squamous cells tend to have horizontal flattened nuclei because of the thin flattened form of the cell. They form the lining of cavities such as blood vessels (endothelium), and parietal wall of Bowmanns capsule in kidney glomeruli, also line the serous cavities of the body (peritoneum, pleura, pericardium) which known as Mesothelia. The main functions of simple squamous epithelium are filtration, diffusion, transport, secretion, and reduction of friction. 2. Simple Cuboidal Epithelium: Cuboidal cells are roughly square or cuboidal in shape. Each cell has a spherical nucleus in the centre. Cuboidal epithelium line many small ducts in the body. Examples include the urinary ducts of the kidney, the bile ductules of the liver, or the cells lining thyroid follicles. The main functions of simple cuboidal epithelium are: Protects ducts; transports and absorbs filtered material in kidney tubules. 3. Simple Columnar Epithelium: Columnar epithelial cells are elongated and rectangular-shaped. The nuclei are elongated and are usually located near the base of the cells. It is either ciliated or non ciliated. Non ciliated columnar epithelium forms the lining of the stomach and gall bladder, the larger ducts near the urinary papilla of the renal pyramids. Ciliated epithelium (or called simple columnar with striated border or with brush border) is 7 Biology \Histology lec1 Epithelial tissue lecturer: Farah E. AlRawi usually found in the small intestine (jujenum), there are goblet cells between the columnar cells in the jujenum and the cilia is called brush border. The main functions of simple columnar epithelium are: Secretes protective mucus for stomach lining, absorption of nutrients in small intestine, protection and lubrication. 4. Pseudo stratified epithelium: Pseudo stratified epithelium is consists of a single layer of cells in which all cells attach to the basement membrane but not all cells reach the surface. It consist of different cell shapes (fusiform, columnar, and basal cells), the nuclei of these cells located at different levels so giving the tissue this pseudo stratified pattern. It either be ciliated or non ciliated. It is lining the passage of the respiratory system (e.g. trachea) which is the ciliated form. The non ciliated form is present in ducts of parotid glands. The main functions of pseudostratified epithelium are: Protection, secretion; cilia-mediated transport of particles trapped in mucus out of the air passages Stratified epithelia The stratified epithelial tissue is made of multiple layers of cells and the superficial cell layer determining the epithelial type 1. Stratified squamous epithelial tissue: Composed of many layers of cells, the basal layer composed of columnar or cuboidal cells, the middle cell layers composed of polygonal cells, and the superficial layer is squamous cells. 8 Biology \Histology lec1 Epithelial tissue lecturer: Farah E. AlRawi There are two types of stratified squamous epithelium: a) Stratified squamous non- keratinized type (moist): These exhibit live superficial cells contain nuclei. It lines the moist cavities of the mouth, esophagus, vagina and anal canal. The main functions of Nonkeratinized squamous are: protection, secretion and prevent water loss. b) stratified squamous keratinized type(dry): It lines the dry areas like the skin and contains non- living cells rich with keratin intermediate filament and containing no nucleus. The main functions of keratinized epithelium are protection against abrasion, bacterial invasion, and desiccation. 2. stratified cuboidal epithelium: It is Consist of two layers of cuboidal cells, the nuclei are large and central, is found in large excretory ducts of sweat and salivary glands. The main function of Cuboidal epithelium is providing protection for the ducts. 3. stratified columnar epithelium: It is present only in small areas in the human body such as the conjunctiva of the eye. The superficial cells are columnar and below there are polygonal cells and the basal cells either cuboidal or columnar. And the main function of stratified columnar epithelium is protection 4. Transitional epithelium: Transitional epithelium is found exclusively in renal calyces, renal pelvis, ureters, and bladder 9 Biology \Histology lec1 Epithelial tissue lecturer: Farah E. AlRawi Transitional epithelium is characterized by a superficial layer of large, dome-like cells sometimes called umbrella cells. These cells are specialized to protect underlying tissues from the hypertonic and potentially cytotoxic effects of urine (forms a protective osmotic barrier between urine in the bladder and the underlying connective tissue). The epithelium changes its cell shape in response to either stretching, as a result of fluid accumulation, or contraction during voiding of urine. In a relaxed, unstretched condition, the surfaces cells are usually cuboidal or appear dome-shaped. Frequently, binucleate (two nuclei) cells are visible in the superficial layers or surface cells of the bladder. While in stretched conditions, the surface epithelium appears squamous. Proliferation and regeneration Epithelia are present in vulnerable sites of the body, where they are continually exposed to the hazards of the external environment. Epithelial cells are constantly subjected to mechanical damage, destroyed, or sloughed off. Epithelium have a high regenerative capacity and can reproduce rapidly as long as they receive adequate nutrition and typically show many dividing cells (mitoses) in order to replace the cells lost and maintain the integrity of the tissue. In cases of trauma or wounds, the epithelia need to cover the lesion as rapidly as possible, repair the lining tissue and prevent damage to the underlying tissues. Most of the cancers of the body are the result of uncontrolled proliferation of epithelial cells. 11