Epithelium, Cell surface specializations, Cell junctions, and Glands Flashcards
Epithelial tissue
General information: covers the body surface or lines the body cavities; epithelial cells are closely placed with little intercellular space; cells lie on a layer of connective tissue, with the bottom layer of cells lying on a basal lamina; avascular, but cells derive nutrients via diffusion from blood vessels in the underlying connective tissue; cells perform various functions, depending upon the location: absorption (intestine), secretion (glands), protection (skin), sensory (neuroepithelium), and contractility (myoepithelial).
Basement membrane
Separates the epithelial tissue from the connective tissue; too thin to be seen at the light microscope; basal lamina and the adjoining connective tissue components, including collagen, glycoproteins (laminin and fibronectin), and a large proteo-glycan complex called heparin sulphate, form a layer called the basement membrane, which is PAS-positive and can be seen with a light microscope.
Classification of Epithelia
Simple: squamous, cuboidal, columnar
Stratified: squamous, cuboidal, columnar, and transitional
Pseudostratified: Usually columnar and ciliated with goblet cells
Simple epithelium
all cells lie on the basal lamina
Psuedo-Stratified epithelium
All cells lie on the basal lamina but appear stratified because their nuclei lie at different locations
Stratified epithelium
more than one layer, cells of the bottom layer lie on the basal lamina
Pseudostratified columnar ciliated
All cells lie on the basement membrane, but not all cells reach the lumen; nuclei are arranged in different layers and thus giving the illusion of more than one layer of cells in the epithelium.
Squamous
Simple: Nucleus is flat, and the cell has a flat appearance.
Example: Lining blood vessels (endothelium), lining body cavities (mesothelium), alveoli of lungs,
Stratified:More than one layers of cells, cells in the outermost layer have flat appearance
Example: lining oral cavity, epiglotis, anus, oesophagus/esophagus, vagina, vulva, glans penis,
Keratinized: Skin
The most prevalent epithelium is stratified squamous, which can be keratinized or non-keratinized.
Cuboidal
Simple: Nucleus is round, cells have similar width and height.
Example: collecting tubules of kidney, rete testis, small ducts of exocrine glands, surface of ovary
Stratified: More than one layer
example: larger ducts of exocrine glands
Columnar
Simple: Nucleus is elongated, height is higher than width.
example: Gallbladder, collecting ducts of kidney, endocervix
PseudoStratified example, cilliated: respiratory tract including nose and sinuses
Simple ciliated: Fallopian tubes
Transitional
Many cell layers, cells in the outermost layer have dome-shape appearance.
example: lower urinary tract: bladder
Metaplasia
A continuous abrasion or irritation can change the epithelium from one type to the other, usually stratified squamous non-keratinized to keratinized, as is the case in the esophagus when milk diet is replaced by roughage in young animals (physiological process); however, the change can be pathological; for example, pseudostratified columnar ciliated to stratified squamous in the respiratory tract because of continuous smoke inhalation
Renewal of Epithelia
Epithelial cells are continuously replaced, but the rate of replacement is variable. For example, replaced every week in the intestine, every 3-4 weeks in the skin, and very slow in the liver and pancreas.
Cell Surface Specializations
The apical cell surface modifications include: microvilli, stereocilia, cilia, and flagella.
Microvilli
are 1um high, 80 nm wide, extensions of the plasma membrane, increase the surface area of absorptive cells such as intestine and kidney by 15-40 times. An individual microvillus is difficult to see with the light microscope (LM), but they are grouped together and form a brush border (also called striated border) that is visible at the LM.
Stereocilia
Are long (10-20 um) and irregular microvilli, present in the epididymis, where their function is to absorb testicular fluid.
Cilia
Are hair like processes, 7-10 um high and 0.2 um diameter. Each cilium consists of 2 central microtubules and 9 peripheral microtubule pairs, which are surrounded by a cell membrane. Each peripheral microtubule pair has two little dynein arms, a motor protein that uses energy from ATP to hook up to the next pair of microtubules. Patients with Kartagener syndrome, also called immotile cilia syndrome, genetically lack dynein arms and thus are infertile and more prone to respiratory infections).
Flagellum
Is much larger than cilium (100 um long or longer) and is present in the sperm tail.
Intercellular Junctions
Provide adhesions between cells and restrict movement of materials into and out of lamina.
Junctional complex (called terminal bar at LM)
Is membrane-associated structures that function in cell-to-cell attachment of simple epithelia.
It consists of three distinct components visible at EM: Zonula occludens (ZO, tight junction), zonula adherens (ZA, intermediate junction), and macula adherens (MA, desmosome)
Zonula occludens (ZO, tight junction)
Surrounds the entire apical circumference of adjacent cells and is formed by fusion of the outer leaflets of the plasma membrane.
Function: prevents movement of substances into the intercellular space from the lumen or vice versa.
zonula adherens (ZA, intermediate junction)
Surrounds the entire circumference of adjacent cells, is located next to the ZO, and is characterized by 10-20 nm intercellular space occupied by filamentous material.
A variation is fascia adherens (FA), which is a ribbon like fusion between adjacent endothelial cells (does not cover the entire circumference). FO is also present between adjacent cardiac muscle cells. Function: binding of adjacent cells.
macula adherens (MA, desmosome)
Is a small, discrete, disk-shaped adhesive site. It is characterized by dense plaque of intermediate keratin filaments (tonofilaments) loops in the cytoplasm.
Function: Provides firm binding between cells; desmosomes are well developed in stratified epithelia, especially in the skin). A variation of desmosome is hemidesmosome (half desmosome), which binds the epithelium with the underlying connective tissue.
Gap junction (communicating junction, nexus)
is not a part of the junctional complex, but is present in all tissues, except skeletal muscle. Gap junctions couple adjacent cells metabolically and electrically (sites of low electrical resistance), is composed of six proteins called connexins, which leave a gap of 2 nm between opposing plasma membrane. Gap junctions permit between cells the exchange of signaling molecules such as ions, hormones, AMP and GMP, so that cells can act as a coordinated manner rather than as independent units (important in nervous system, smooth muscle and cardiac muscle).
