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).
Glands
Are epithelial invaginations into the underlying connective tissue.
Exocrine glands secrete via a duct on the epithelium of its origin; endocrine glands disconnect from the parental epithelium; so, they do not have a duct; secrete into the bloodstream.
Paracrine and autocrine glands are endocrine glands, but they secrete into the local extracellular space;
Example: Pituitary, adrenal, thyroid, parathyroid.
paracrine secretion acts on nearby cells otherI than those cells which produce them;
Example: Immune cells.
conversely, autocrine secretion acts on the same cell that secretes it.
Example: Liver regeneration in response to injury.
Classification of exocrine glands
Based on number of cells:
Based on the nature of secretion:
Based on how the secretion leaves the cell
Unicellular exocrine glands
mucous cells and goblet cells( example: intestinal goblet cell)
Multicellular exocrine glands
Composed of many cells and is further classified according to duct branching: simple (duct does not branch), compound (duct branches).
Further classification is based upon shape of the secretory unit: acinar or alveolar (sac or flask-like) or tubular (straight, branched, coiled).
Examples:
Simple tubular –> large intestine
Simple coiled tubular –> sweat gland
Simple branched tubular –>Stomach
Simple branched acinar –> Penile Urethra
Compound tubular –> Salivary Gland
Compound acinar –> Brunner’s gland
Compound tubular acinar–> Pancreas
Exocrine glands that secrete mucus
Is a viscous material that protects or lubricates cell surfaces.
that secretion is sticky and is rich in carbohydrates.
example:
acini in the esophagus
Exocrine glands that secrete serous
Is watery secretion that is often rich in enzymes
that secretion is watery and rich in proteins
example:
acini in the pancreas
Exocrine glands that secrete mixed
Secretion contains both mucus and serous.
example: mandibular salivary gland: type of acinar cell
Merocrine
Cells release only secretory granules by exocytosis with no loss of cytoplasm;
99% of cells are this
apocrine
Cells release secretory product with some apical cytoplasm
Ex: sweat gland, mammary gland
Holocrine
Cells release secretory product with all cytoplasmic contents (in other words, holocrine cells die).
Ex: sebaceous gland
Myoepithelial cells
Are epithelial in organization (lie between the epithelial cells and the basal lamina) and are muscular in function (contain both actin and myosin).
example: heart?
Cell Cycle
Divided into four phases: G1, S, G2, and M.
Phases G1, S and G2 are collectively called interphase.
G1 (cell cycle)
is the pre-duplication phase during which cell performs its specific functions;
S (cell cycle)
is the synthesis phase in which centrioles and DNA duplicate;
G2 ( cell cycle)
is the post-duplication phase during which cell finishes replicating its centrioles and synthesis of enzymes that control cell division;
M (cell cycle)
is the mitotic phase, in which a cell forms two new daughter cells.
