Lecture 2 Part 1 (H) Histology and Human Development PDF

Unified Lecture 2 EPITHELIUM 1 HIHD-211 TERM-3 Basic Science Department COSHP, KSAU-HS, KSA Learning Outcomes By the end of this lecture the student must be able to : 1. Recognize the characteristics of epithelium 2. Define basal lamina and basement membrane 3. Describe intercellular junctions, cili...

Unified Lecture 2 EPITHELIUM 1 HIHD-211 TERM-3 Basic Science Department COSHP, KSAU-HS, KSA Learning Outcomes By the end of this lecture the student must be able to : 1. Recognize the characteristics of epithelium 2. Define basal lamina and basement membrane 3. Describe intercellular junctions, cilia and microvilli. 4. Describe the basic classification (Types) of epithelia: ▪ Epithelial membranes Epithelium ▪ Epithelium is layers of cells arranged in close apposition and sitting on the basement membrane ▪ It is innervated but not a vascular tissue (avascular) ▪ It is nourished by diffusion of nutrients across the basement membrane LO-1 Epithelium ▪ Epithelium is layers of cells arranged in close apposition and sitting on the basement membrane ▪ It is innervated but not a vascular tissue (avascular) ▪ It is nourished by diffusion of nutrients across the basement membrane LO-1 Epithelium ▪ It covers body surfaces and lines all body cavities and tubular organs ▪ Epithelium is regenerative ▪ All epithelia are bound together in sheets with intercellular junctions (repair by forming new cells) by process known as mitosis) ▪ Epithelium also forms the secretory portion (parenchyma) of glands LO-1 Polarity & Specializations of Epithelial Cells 1. Basal Surface Specializations The basal surface contacts the basal lamina. Because it is the surface closest to the blood supply, it often contains receptors for bloodborne factors such as hormones. 2. Apical Surface Specializations The cell’s apical surface is on the organ’s external or internal (lumen) surface. It is specialized to carry out functions that occur at these interfaces, including secretion, absorption, protection, and movement of luminal contents. Which includes , cilia, flagella, microvilli, and stereocilia. 3. Lateral Surface Specializations Epithelial cells attach tightly to one another by specialized intercellular junctions. Junctions occur in three major forms: zonulae are bandlike and completely encircle the cell; maculae are disklike and attach two cells at a single spot; and gap junctions are macular in shape but differ in composition and function. A junctional complex is a combination of intercellular junctions, typically lying near the cell apex. 6 Basement Membrane And Basal Lamina ▪ ▪ Epithelium is separated from the underlying connective tissue by a sheet of extracellular material called Basement Membrane The basement membrane is formed of two layers ▪ Basal lamina Reticular lamina Basal lamina is visible only with the electron microscope and is mainly formed by type IV collagen and glycoproteins. It is secreted by the epithelial cells. The basal lamina exhibits electron lucent and electron dense layers termed the lamina lucida (lamina rara) and lamina densa, respectively. Reticular lamina: In some sites, a layer of type III collagen fibers (reticular fibers), produced by the connective tissue cells and termed the reticular lamina, underlies the basal lamina. Basal laminae accompanied by reticular laminae are often thick enough to be seen with the light microscope as PAS positive layers, sometimes termed basement membranes. ▪ LO-2 Lateral surface specialization/ Intercellular Junctions ▪ Tight junctions are located immediately beneath the luminal surface of the epithelial cells forming a continuous band around the cells. These block the passage of molecules in-between adjacent cells hence separates the luminal space from the intercellular space and the connective tissue compartment. The Desmosomes: also provide strong attachment between adjacent cells. LO-3 Intercellular Junctions ▪ The zonula adherens: forms a continuous band around the cell just deep to the tight junctions and they give the epithelium strength by linking the cytoskeletons of adjacent cells Gap junctions: The gap junctions provide a channel for the passage of small molecules directly between adjacent cells Hemidesmosomes: binds the base of the cell to the underlying basement membrane LO-3 Focal Adhesions ▪ ▪ ▪ Microvilli are plasma membrane– covered cell surface extensions. Their cores (unlike those of cilia and flagella) contain many parallel actin By interacting with myosin, the microfilaments contract, shortening the microvilli. An absorptive cell’s apical surface is usually covered with microvilli, which greatly increase its surface area. Microvillus–covered epithelia, which exhibit a striated, or brush border, include the absorptive simple columnar epithelia lining the small intestines and the kidney’s proximal tubules. LO-3 Microvilli Cilia ▪ Cilia are cytoplasmic processes from the luminal surface of the cell containing bundle of microtubules ▪ Ciliated epithelia include ciliated pseudostratified columnar (respiratory) epithelium and ▪ The ciliated simple columnar epithelium of the oviducts. LO-3 Types Of Epithelium The epithelium is usually classified based on two factors: 1- The shape of the cells ▪ Flattened cells will form squamous epithelium ▪ Cuboidal cells will form cuboidal epithelium ▪ Tall cells will form columnar epithelium 2- The number of cell layers ▪ One layer of cells = simple epithelium More than one layer= stratified epithelium LO-4 Types Of Epithelium ▪ ▪ How to name stratified epithelium when it contains flat, cuboidal and columnar cells at the same time? stratified epithelium is named according to the shape of the outermost layer of cells.e.g. stratified squamous epithelium stratified cuboidal epithelium ▪ Stratified Squamous Epithelium The presence of surface modifications such as cilia and keratin add to the nomenclature of epithelia e.g. stratified keratinized squamous epithelium Columnar ciliated epithelium LO-4 Stratified Cuboidal Epithelium Stratified Keratinized Squamous Epithelium Columnar Ciliated Epithelium Simple Squamous Epithelium LO-4 Single layer of flat cells attached on the basement membrane forms the simple squamous epithelium It is most often found in the linings of the cavities e.g. peritoneal cavity, pleural cavity, and tubular organs e.g. blood and lymph vessels It provide for diffusion Stratified Squamous Epithelium Many layers of flat cells sitting on the basement membrane form the stratified squamous epithelium Shape of the surface layer of cells determines the type of the epithelium It is found in the organs where there is wear and tear e.g. skin, esophagus, vagina It provides protection and act as a barrier LO-4 stratified squamous epithelium non keratinised LO-4 stratified squamous epithelium keratinised Simple Cuboidal Epithelium Single layer of cubical cells sitting on the basement membrane forms the simple cuboidal epithelium It is found most commonly in the small ducts of the exocrine glands and form epithelium of some endocrine glands e.g. Thyroid follicles Stratified Cuboidal Epithelium Many layers of the cubical cells sitting on the basement membrane form the stratified cuboidal epithelium Shape of the surface layer of cells determines the type of the epithelium This type of epithelium is usually confined to the lining of the larger excretory ducts of exocrine glands such as the salivary glands and ano-rectal junction LO-4 Simple Columnar Epithelium ▪ Single layer of tall columnar cells sitting on the basement membrane forms the simple columnar epithelium ▪ It is most often found on absorptive surfaces such as in the small intestine. It may constitute the lining of secretory surfaces such as that of the stomach Pseudostratified Epithelium ▪ Pseudostratified epithelium is an special type of epithelium in which all cells rest on the basement membrane but some of the cells do not reach the free surface ▪ The nuclei of these cells, however, are disposed at different levels, thus creating the illusion of cellular stratification ▪ It is actually a simple epithelium but appears as stratified ▪ It is limited in its distribution and is found in respiratory tract like trachea, and in the epididymis LO-4 Transitional Epithelium ▪ ▪ LO-4 Transitional epithelium is a stratified epithelium in which the shape of the surface cells changes (undergoes transitions) depending on the degree of stretch. Is found exclusively in the excretory passages of the urinary system. It covers the lumina of renal calyces, pelvis, ureters, and bladder. Transitional epithelium: bladder (unstretched or relaxed). Stain: hematoxylin and eosin. High magnification. In the non-distended state, it appears to be six cell layers thick. The basal cells are roughly cuboidal, but the surface cells are dome shaped, sometimes containing two nuclei Transitional epithelium: bladder (stretched). Stain: hematoxylin and eosin. High magnification In the stretched state, it often appears only two or three cells thick, and the intermediate and surface layers are extremely flattened Thank you 22