Epithelial Tissue

• Epithelial Tissue


  also referred to as epithelium, refers to the sheets of cells that cover exterior surfaces of the body, lines internal cavities and passageways, and forms certain glands.
• Classifications of Epithelial Tissue

  Simple Squamous, Simple Cuboidal, Simple Columnar, Stratified Squamous, Stratified Cuboidal, Stratified Columnar, Pseudostratified Columnar, Transitional Epithelium
• Simple Squamous 


  a simple thin layer of flattened cells with irregular boundaries; line the walls of blood vessels and air sacs of the lungs; are involved in physiological processes like osmosis and diffusion (ex of organ: lung)
• Simple Cuboidal 


  cells are almost tall as they are wide; contain a single layer of cuboidal cells with large spherical nuclei that are centrally located; mainly functions for absorption and secretion; examples are cells covering thyroid follicles, ovary, kidney etc. (ex. kidney cortex)
• Simple Columnar 


  longer than they are wide; specialized to provide a sensory function/input because the cells can be innervated; main function is protection; very good at absorbing and transporting nutrients from locations such as small intestine; found throughout our organ system; can be also found in the cornea, inner ear, and nose (ex. small intestine)
• Stratified Squamous


  has layers of flattened epithelial cells arranged on a basal membrane; forms the outermost layer of the skin and serves as protection from constant abrasions; found in the innermost lining of the esophagus, mouth and vagina
• Stratified Cuboidal


  consist of cube shaped cells; has several layers, but not all are cubes; for secretion and protection; occurs in the excretory ducts of sweat glands and salivary glands; also found in mammary glands and in the developing ovarian follicles
• Stratified Columnar


  made up of multiple layers of epithelial cells, wherein the apical layer is composed of columnar cells, and the deeper layer can be either be columnar or cuboidal; rarely found column-shaped epithelium, they are uncommon and can be found in the following locations in the human body: conjunctiva, male urethra, anus and pharynx; functions: perform secretion and for protection
• Pseudostratified Columnar 


  consist of a single layer of cells that gives the illusion of multiple layers; covers most of the upper respiratory tract; consist of column-like cells of varying heights; cells are arranged tightly adjacent to one other; due to the large number of organelles provided, they are a higher level of secretion and absorption than other cells
• 2 Types of Pseudostratified Columnar

  Ciliated Pseudostratified Columnar and Non-Ciliated Pseudostratified Columnar
• Ciliated Pseudostratified Columnar


  contains cilia on the apical surface
• Non-Ciliated Pseudostratified Columnar


  do not contain cilia but they have stereocilia, which are much longer and less motile than microvilli
• Transitional Epithelium


  composed of several layers of cells, with the morphology of cells varying depending on the function of the organ; known as urothelium
• Epithelial Membrane 


  consist of epithelial tissue and connective tissue, to which it is attached by way of the basement membrane; (example: skin)
• 3 Types of Membranes

  Mucous Membrane, Serous Membrane, Cutaneous Membrane
• Mucous Membrane


  these membrane line the body passageways or tracts that are exposed to the outside environment; they are "wet" membranes; found in digestive tract, respiratory tract, urinary tract, male and female reproductive tracts
• Serous Membrane 


  these membranes line the body cavities that are not directly exposed to the outside environment; they are "wet" and "moist" covered with a thin layer (serous fluid)
• Two Parts of Serous Membrane

  Visceral layer and Parietal layer
• Cutaneous Membrane 


  this covers the entire outermost surface of the body, it is only "dry" membrane encountered in the body and an important role is the prevention of water loss from the body; it is also a physical barrier and protective to deeper tissues; also called as skin
• Cell Junctions


  they are the classes of cellular structures consisting of multi-protein complexes that provide contact or adhesion between neighboring cells or between a cell and the extracellular matrix in animals
• Types of Cell Junctions

  Tight, Adherens, Gap, Desmosomes, Hemidesmosomes
• Tight Junctions


  composed of transmembrane proteins, such as occludin and claudins, which interact with cytoplasmic proteins to form a continuous seal between adjacent cells
• Two Types of Tight Junctions


  Occludin and Claudin
• Occludin-based

  are found in various epithelial and endothelial tissues
• Claudin-based 


  more prevalent in specific tissues, such as the blood-brain barrier
• Gap Junctions


  they are formed by the docking of two connexon channels, each composed of six connexin proteins; these connexin proteins assemble to create a pore-like structure that facilitates the passage of various molecules and ions between cells.
• Two Classfications of Gap Junctions

  Homotypic and Heterotypic
• Homotypic


  composed of the same connexin type
• Heterotypic


  composed of different connexin types
• Adherens Junctions


  also known as zonula adherens or adhesion belt; it is a a form of cell–cell adhesion structure observed in a variety of cell types, as well as in different animal species. it is characterized by a pair of plasma membranes apposed with a distance of 10–20 nm between them.
• Desmosomes


  they are found in various epithelial and cardiac tissues, where they connect adjacent cells and provide a strong, yet flexible, connection; mediates cell–cell adhesion and anchor the intermediate filament network to the plasma membrane, providing mechanical resilience to tissues such as the epidermis and heart
• Hesmidesmosomes


  they are specialized cell-matrix adhesion complexes that play a crucial role in anchoring epithelial cells to the underlying basement membrane. These structures are found in various epithelial tissues, such as the skin, mucous membranes, and certain internal organs.
• Two Types of Cell Surface

  Apical and Basal
• Apical Surface


  the top or exposed surface; depending on the tissue's location, which is frequently specialized for specific functions like secretion or absorption.
• Basal Surface


  the bottom surface of an epithelial cell that is attached to the basement membrane, supporting the cell structurally and allowing molecules to be exchanged with tissues underneath
• 3 Specializations of Apical Surface

  Microvilli, Stereocilia, Cilia and Flagella
• Microvilli


  these are non-motile cytoplasmic projections that arise from the apical surface of most epithelial cells as finger-like evaginations; they contain a core of actin filaments
• Stereocilia


  specialized in the inner ear and male reproductive system; they contribute to sensory perception in hearing and assist in sperm motility along the reproductive tract
• Cilia and Flagella


  generally, motile cytoplasmic projections that extend from the cell surface; cilia are longer than microvilli. Flagella resemble cilia in structure, but they are much longer and are single per cell e.g. flagellum of the spermatozoon
• 2 Specializations of Basal Surface

  Basal Infoldings, Basal Membrane