Week 2

Characteristics of Epithelial tissue:

-closely apposed cells without intervening intercullar substances/ densely pack together - avascular, but they grow on an underlying vascular connective tissue. ( they rely on diffusion of nutrients from underlying tissues) - the deepest layer of epithelia rest in amorphous material called basal lamina that provide structural support. - connective tissue and epithelium are separated by basement membrane - basal lamina seats on a layer of extra cellular material that contain reticular fibers known as lamina fibroreticularis.( product of connective tissue cells -fibroblast) - basal lamina amd lamina reticularis comprised the basement membrane which is thick enough to be appreciated by compound microscope. - epithelium covers all free surfaces of the body. - exhibits polarity, that basal, lateral and apical surfaces can be determined by the position of the surface modification.

2 parts of basement mmembrane

1. Basal Lamina- Nearest the epithelial cells is an electron-dense layer, 20-100 nm thick, consisting of a network of fine fibrils 2. Reticularis Lamina -beneath this layer is a more diffuse and fibrous

Specialization of the basal surface components that is responsible for the structural integrity of ET & hence organ's integrity

1. Hemidesmosomes- mediate the attachement pf the epithelial cells to the basal lamina --- exist in epithelial cells such as those in stratum basal of epidermis of skin ● Integrins- another group of proteins, allow the cell to attach to the matrix proteins of the basement membrane

classification of epithelia into 2 main group:

1. Lining Epithelia ----those that cover external surfaces and line the internal surfaces of the body 2. Glandular Epithelia----- those that are concerned with the production and secretions * certain cells can be classified as both

Most organs can be divided into:

1. Parenchyma -which is composed of the cells responsible for the organ's specialized functions, and the 2. stroma, the cells of which have a supporting role the organ. Except in the brain and spinal cord, the stroma is always connective tissu

Based on the number of cell layers

1. Simple - made of single layer 2. Stratified - made of more than 1 layer

Specialization of the lateral surface

1. Tight Junction( zonula occludens) --- most apically situated of the junctional complexes --- found just below the free surface that forms abond that completely surround the cells ---- the adkoining cell membranes fused with each other. where in the adjoining cells share a common cell membrane. ● Proteins inserted into the cell membranes of adjacent cells stitch the membranes of the cells together and provide and effective barrier to the diffusion of the substances from the outside of epithelium. ( called luminal side if the epothelium covers the surface of a tubular structure.

Special type of epithlial tissue

1. Transitional epithelium intermediate between stratified cuboidal amd stratified squamous. lines the urinary passages like renal calyxces, pelvis, ureter, urinary bladder and some parts of urethra can withstand stretching forces relaxed state- basal cells are cuboidal or columnar shape. several layers of polyhedral cells, and layer of superficial cells, which have a convex, dome, umbrella shape luminal surface distended state- only 1 o 2 layers of cuboidal cells are followed by a superficial layer of large, low cuboidal or squamous cells. epithelium resembles stratified squamous epithelium

In naming Lining/ covering epithelia we follow to criteria

1. based on the number of cell layers 2. base on the shape of the cells

2 types of stratified squamous epithelium:

1. keratinized stratified squamous epithelium/ dry epithelium flattened but unnucleated

Functions of Epithelial Tissue depends on location

1. protection - epthelial in skin from sunlight, bacteria physical damage 2. absorption - lining of small intestine, absorbing nutrients into blood 3. filtration - lining of kidney filtering wastes from blood plasma 4. secretion - different gland produce perspiration, oil, digestive enzymes and mucus.

Types of Simple Epithelia

1. simple squamous epithelia composed of single flattend cells, common in the body, nucleus occupies the thickest part of the cell which is near the center and will follow the shape of the cells( flattened) seen in sites where diffusion takes easily ( nephrons& lungs/alveoli) jigsaw puzzle when seen in the surface

based on the shape of the cells

1. squamous - flatened 2. cuboidal - equally tall and wide 3. columnar - tall than wide

Types of Stratified Epithelia

1. stratified squamous epithelium ° most common type composed of 2-20 layers, vary thickness depending on the specific organ the deepest layer, which are in contact with basemenr membarane are cuboidal or columnar in shape. • As cells matured they are pushed toward the surface and become flattened. - Can stand rubbing, friction

specialization of the basal surface

2. Basal infoldings of plasmalemma -- increase absorbing capacity ex. renal tubule

Specialization of the lateral surface

2. Intermediate Junction( Zonula adherenz) ---- is structurally not as well characterized as desmosomes ---- located just below zonula occludens ---- cell membrane of the adjoining cells are close to each other but neither adhere nor fused instead they are separated by intercellular space ----' Fibers of the cytoskeleton insert into the cytoplasmic sidr of this membrane specialization ---- typically appears as a close and consistent apposition ( 15-20nm) of teh cell membranes close to the apical cell surfaceand surrounding the entire cells.

Types of Simple Epithelia

2. Simple Cuboidal Epithelium made of single layer of cube like cells nuclei are seen at the center and are round occurs in the small excretory ducts or many glands( major salivary glands& pancreas) the tubules of the kidney and on the surface of the ovaries

Types of Stratified Epithelia

2. Stratified cuboidal epithelia not common, consists of 2-3 layers of cuboidal cells seen in the ducts of sweat glands and major salivary glands

2 types of stratified squamous epithelium:

2. non-keratinized stratified squamous epithelium/ wet epithelium seen in wet areas of body: esophagus, oral cavity, and vagina superficial cells are flattened buy nucleated

special type of epithelia tissue

2. pseudostratified Columnar Epithelium composed of one layer of cells nuclei are located in various location all cells are in contact with the basement membrane but not all reached the surface ● Ciliated pseudostratified columnar epithelia are found in respiratory system ( trachea& bronchi) ( respiratory epithelium)

Specialization of the lateral surface

3. Desmosomes ---- meduate cell adhesions, protein inserted into the cells membrane of the adjacent cells form a protein zipper linking the cells. --- Fibers of the cytoskeleton attach to the cytoplasmic side of the desmosome to stabilize the area of contact. ---- cells are separated by wide intercelullar space this space is filled by protein such as cadherins to fill the gap.that why it appears densely pack ---- numerous in epidermis of skin ● A group of glycoprotein ( cadherins) inserted into the opposing plasma membranes mediate cell to cell adhesion at desmosomes and also at the adhesion zones or patches mentioned below.

Types of Simple Epithelia

3. Simple Columnar Epithelium the nuclei are usually located at the same height within cells - often close to base and are oval and form single row ex. lines the internal surface of the gastrointestinal tract( GIT) from cardiac of the stomach to rectum. ● Ciliated simple columnar epithelium lines the bronchus, uterus, and oviducts ○ cilia are specialized for transport of fluid or mucus. - example of cell modification found in apical of the cells

Another apical Modification

3. Stereocilia- are essentialy microvilli, as long as cilia, non-motile, ○ Microvilli & Stereocilia are found in epithelia active in absorption; main function is to increase teh surface area of Epithelia cells ● microvilli cobtains filaments, which are in contact with the terminal web of the cell location: Epidydimis

types of stratified epithelia

3. Stratified Columnar Epithelia consists of atleast 2 layer of columnar cells, sonetimes the cell im deeper layer are cuboidal found in excretory ducts of the mamary glands.

Specialization of the lateral Surface

4. Gap Junction - cells are closely apposed but not fused, separated by a wide intercellular space --- plasma membrane consist of tiny tubes known as connexons ---- found below the desmosomes ● Connexon allows exhange of ions and small molecules between the cells to happen ---Gap junction is wide in term of distribution

The basal surface of all epithelia rests on a thin extracellular, felt-like sheet of macromolecules and serves as the a semipermeable filter for substances reaching epithelial cells from below.

Basement Membrane

characterized by cells producing very abundant ECM

Connective tissue

line all external and internal surfaces of the body and all substances that enter or leave an organ must cross this type of tissue.

Epithelial tissue

composed of closely aggregated polyhedral cells adhering strongly to one another and to a thin layer of ECM, forming cellular sheets that line the cavities of organs and cover the body surface.

Epithelial tissues

Apical modifications

Flagella

zonula occludens and zonula adherens

are mostly develop in simple cuboidal and simple columnar epithelia collectively known as junctional compexes

simple squamous epethelial tissue that lines the surface of heart, blood and lymphatic vessels

endothelium

4 basic tissue types

epithelial, connective, muscle, nervous

projects from the surface of microvilli

glycoproteins and glycolipids that form an outer coating of the cell known as glyco calix

The connective tissue that underlies the epithelia lining the organs of the digestive, respiratory, and urinary systems is called the

lamina propria

simple squamous epithelial tissue that lines the large internal body cavities

mesothelium

another cell modification seen apically taht is short, fine, finger-like processes that protrude from the apical surface. It increase cell surface and is mainly used for absorption. typically found In epithelial cells of GIT particularly numerous in small intestines.can be seen under light microscope as fine, vertical lines known as the striated / brush borders.

microvilli

composed of elongated cells specialized for contraction and movement

muscle tissue

composed of cells with long, fine processes specialized to receive, generate, and transmit nerve impulses.

nervous tissue

is an assemblage of similarly specialized cells united in performing a specific function.

tissue

The principal functions of epithelial tissues include the following:

■ Covering, lining, and protecting surfaces (eg, epidermis) ■ Absorption (eg, the intestinal lining) ■ Secretion (eg, parenchymal cells of glands) Specific cells of certain epithelia may be contractile (myoepithelial cells) or specialized sensory cells, such as those of taste buds or the olfactory epithelium.