by dbui89


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Four Types of Tissue
Epithelial, Connective, Muscle, and Nervous
Locations of epithelial tissue
Exterior surfaces, cavity linings, secretory portion of glands and ducts
Characteristics of epithelial tissue
Only tissue type that has a free surface such as an external free surface (skin) or a luminal surface (intestinal lining).
It has very little ECM, making the tissue densely packed by cells. Very strong adhesion between cells.
Avascular
Domains of epithelial tissue
Apical domain, lateral domain, and basal domain
Apical domain
Forms the external/luminal surface of the cell. Can have villi, microvilli, or stereocilia.
Microvilli
.01 micrometers in size, microvilli increase the surface area of the apical domain. This is important in areas of absorption/secretion such as the small intestine or kidneys where microvilli can be found.
Formed by a core of actin filaments anchored into the membrane by proteins such as myosin I.
Too small to see alone under light microscope, will be seen in groups as a striated border
Stereocilia
Modified microvilli, unrelated to cilia. Formed by an actin filament core, playing a role in the absorption of fluids in the epididymis. A heavily modified version of stereocilia is also present in the cochlear of the ear as a sensory receptor.
Cilia
0.5-2.5 micrometers, formed by microtubules, its function is to move substances and is located in areas where transport is necessary (respiratory tract where it moves mucus up.
Axoneme
Forms the core of the cilia, 9 microtubule doublets in a circle surrounding a double at the center.
Basal body
The centriole at the base of each cilia. Formed by 9 microtubule triplets in a circle with no central pair.
Primary ciliary dyskinesia
Also known as immotile ciliary syndrome, genetic efect of ciliary proteins cause malformation in skeleton of cilia. Cilia fail to beat properly. Causes embryological pathologies such as dextrocardia, impaired development of skull air sinuses, prone to severe chest infections (no mucous removal from lungs), and infertility in males (sperm can't move)
Lateral Domain
Presence of cell junctions that allow the epithelial tissue to function.
Occluding junctions
Also known as tight junctions, present in the apical portion of the lateral domain of epithelial cells. Found in cells with absorptive or secretory roles. These junctions prevent the diffusion by blocking paracellular pathways.
Zonula occludens
The only type of occluding junction. Found in lateral domain of epithelial cells. belt-like junction formed by transmembrane proteins that bind membranes of two adjacent cells.
Anchoring junctions
Junctions designed to provide stability and mechanical strength to tissue. Connects cytoskeleton of one cell to the cytoskeleton of an adjacent cell.
Zonula adherens
Belt-like anchoring junction found in basal domain, connects actin filaments (the cortex) of two cells together. Contains an actin-binding protein, a peripheral protein, and a transmembrane protein (cadherin).
Cadherin
A family of calcium-dependent transmembrane proteins that are important in the function of anchoring junctions in the lateral domain (zonula adherens, macula adherens). Loss of cadherins results in the invasive behavior of tumor cells.
Macula adherens
Also called desmosome, spot-like anchoring junction that connects intermediate filaments of two cells together. Contains transmembrane protein component from the cadherin family of proteins.
Terminal bars
Sites of junctional complexes that can be visualized under light microscope.
Pemphigus
A disease of cell junctions.The body produces abnormal antibodies to the desmosome junctions in the skin. Prevents normal cell adhesion causing skin tissue to blister and desmosomes to fall apart.
Communicating Junctions
Junctions that allow diffusion of particles between the cytoplasm of adjacent cells.
Connexon
Communicating/Gap junction formed by two connexons (one from each cell. Made up of 6 connexin subunits. Provides a narrow channel through which cells may communicate with each other.
Basal domain
Faces the basement membrane, important for interactions with underlying tissue
Basement membrane
Separates epithelium from underlying connective tissue, formed by basal lamina (Type IV collage and laminin) and a deeper reticular lamina (Type III collagen)
Focal adhesions
Anchoring junction in the basal domain, attaches actin cytoskeleton to the ECM of basement membrane. The transmembrane component is part of the Integrin family of proteins.
Hemidesmosome
Anchoring junction in the basal domain, connects intermediate filaments to the ECM. The transmembrane component is part of the Integrin family of proteins.
Classifications of Epithelial Tissue
By number of layers (simple, pseudostratified, stratified), by shape (squamous, cuboidal, columnar), and by apical cell modifications (ciliated or non-ciliated)
Simple squamous epithelium
Flattened nuclei, provides a barrier and allows diffusion of oxygen and carbon dioxide. Found in parietal layer of Bowman's capsule of kidneys, alveoli of the lungs, body cavity linings, vascular system linings.
Simple cuboidal epithelium
Have nearly spherical nuclei, provides a barrier (ducts), allows for secretion (thyroid) and absorption (kidneys). Found in ducts of various glands, kidney tubules, and thyroid follicles.
Simple columnar epithelium
Have oval-shaped nuclei. Can be ciliated or non-ciliated. Allows for absorption, secretion, and lubrication. In the case of ciliated simple columnar, also provides transport.
Non-ciliated have microvilli and are found in the GI tract and walls of the gallbladder.
Ciliated have cilia, found in walls of the oviduct (moves ova)
Pseudostratified columnar epithelium
Similar to simple columnar, but there are spherical cells that do not reach the surface. These cells form the basal layer. It acts as replacement cells for the columnar cells. Can be ciliated or non-ciliated.
Ciliated form found in the respiratory tract, have numerous cilia, used for transportation of mucus.
Non-ciliated form has stereocilia, found in the epidymis, used for absorption.
Metaplasia
Some epithelium can change into another type of epithelium when influenced by certain factors. This is common in heavy smokers, which causes the normally pseudostratified columnar epithelium to become stratified squamous epithelium. Mucus then accumulates and there is inflammation of the lungs.
Stratified squamous epithelium
Used as a barrier and for protection, found in keratinized or non-keratinized form.
Keratinized form found in outer layer of the skin.
Non-keratinized layer found in the oral cavity and esophagus as well as the vagina.
Stratified cuboidal epithelium
Acts as a barrier and as a conduit. Usually on two layers. Very rare. Found in ducts of sweat glands.
Stratified columnar epithelium
Acts as a barrier and as a conduit. Usually only two layers. Very rare. Found in ducts of parotid and submandibular salivary glands.
Transitional epithelium
Restricted to the urinary system, composed of multiple layers. Surface layer has large "dome" cells, often binucleated. Has ability to stretch when the organ becomes dilated. Found in the urinary bladder, ureter, and in the pelvis and calyces of the kidney.
Glandular epithelium
Secretory glands formed by invaginations of the epithelial tissue. Classified into endocrine and exocrine glands.
Endocrine
Glands that secrete products which diffuses and travels by blood.
Exocrine
Glands that secrete their product into ducts, usually cells of these types of glands are arranged into tubules or acini.
Merocrine Secretion
Method of secretion in which membrane-bound vesicles carying product release their content by exocytosis with the vesicle fusing into the plasma membrane. Serous, Mucous, and Mixed seromucous glands fall into this category.
Serous gland
Secretes thin, watery protein-rich secretions. Very euchromatic, a lot of RER and ribosomes in cytoplasm, stains very dark.
Mucous glands
Secrets mucins, carbohydrates that act as lubrication and protection. Stains very lightly with very heterochromatic nuclei on the sides of the cells.
Mixed seromucous glands
Have both serous and mucous glands. Mucous glands form tubules capped by serous cells that form serous demilunes.
Apocrine Secretion
Type of secretion involving pinching of the apical cytoplasm with the formation of secretory vesicles
Holocrine Secretion
Type of secretion involving cell destruction. Cell accumulates product until it undergoes apoptosis. Upon cell death, product is released.
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