Epithelial Tissue Flashcards
General Character of Epithelium
- has an exposed free surface
- closely aggregated cells with little to no EC matrix, strong adhesion
- avascular, however make up the vascular system
Morphologic polarity
Epithelial cells have distinctive orientation in the organs. Thus, it is usually possible to identify the three main surfaces present in the cell: apical, lateral, and basal.
Apical domain
forms the external or luminal surface of the cell.
a. Apical surface often exhibits various modifications, such as microvilli, stereocilia, or cilia.
microvilli
i. The function is to increase the surface area, so they are found in the epithelia, where absorption is important, i.e. in the small intestine or kidney. Microvilli are characterized by the following features:
1. Size: 1 x 0.01 μm.
2. The core of a microvillus is formed by actin filaments that are bound together by actin-binding proteins, such as fimbrin and fascin, and are anchored into the membrane by the lateral anchoring proteins, such as myosin I.
3. The amorphous apex of the microvillus is formed by villin into which the actin filaments are anchored.
Stereocilia
are modified microvilli and are not related to cilia. These are long, sometimes branching projections with the core formed by actin filaments, similar to microvilli. Stereocilia in the epididymis play an important role in the absorption of fluid. Stereocilia in the inner ear cochlea are sensory receptors.
Cilia
are important to move substances (e.g. mucus) along the surface of the epithelium and are found in the areas where this transport is necessary, e.g. in the respiratory tract or the oviduct. The core of the cilium is formed by the axoneme. Cilia are thicker than microvilli and measure 2-10 x 0.25 μm.
1. At its base each cilium is attached to the centriole, known as a basal body. The skeleton of the basal body is formed by 9 triplets of microtubules without a central pair.
genetic defects of ciliary proteins
result in the malformation of the skeleton of cilia. This causes uncoordinated or absent ciliary beating and results in the primary ciliary dyskinesia (immotile ciliary syndrome). This abnormality may cause several consequences:
a. Embryological pathologies, such as dextrocardia.
b. Impaired development of skull air sinuses.
c. No mucus removal from lungs causes recurrent chest infections.
d. Infertility is common and is due to the inability of the flagella of spermatozoa to beat in males and failure of the cilia to move the ovum in the oviduct in females.
Secretory vesicles
are present in the epithelial cells involved in the production and secretion of macromolecules, such as enzymes and mucins.
Lateral domain
characterized by the presence of cell junctions that allow the tissue to function as a whole (J&C: p. 69). Three major classes of cellular junctions are present in the epithelium: occluding junctions, anchoring junctions, and communicating junctions. Junctions often occur together and form junctional complexes. In the light microscope these junctional complexes can be visualized as terminal bars.
Tight or occluding junctions
are mostly present towards the apical portion of the cell and are represented by a single type, zonula occludens. Occluding junctions are belt-like junctions formed by sealing strands of transmembrane proteins that bind membranes of two adjacent cells. Occluding junctions are found in cells with secretory role (e.g. stomach epithelia) or in cells with absorptive role (e.g. kidney tubule cells). Zonula occludens is virtually impermeable and its main function is to prevent diffusion by blocking paracellular pathways.
Anchoring junctions
are designed to provide stability and mechanical strength to the tissue and allow it to function as one unit. Anchoring junctions connect cytoskeleton of one cell to cytoskeleton of an adjacent cell or to the extracellular matrix. Two main types of anchoring junctions are found along the lateral surface of epithelial cells: zonula adherens and macula adherens (=desmosome).
Zonula adherens
belt-like junction that connects actin filaments of one cell to actin filaments of another cell. It is not as tight as the occluding junction and the cleft between two cells is usually about 20 nm. The adherens junction is composed of: a. Actin-binding proteins: Vinculin, α-actinin.
b. Peripheral protein: Catenin.
c. Transmembrane link protein: Cadherin (family), Ca2+ dependent.
cadherin and cancer
cadherin plays an important role in the control of cell behavior and its loss is often associated with an
acquisition of invasive behavior by tumor cells (metastasis), for example in gastric cancer.
Macula adherens
spot-like junction that connects intermediate filaments of two cells. Desmosomes are extremely common in the epidermis of the skin and provide mechanical strength for that tissue, but they also occur in other types of tissue, such as muscle.
Intracellular plaque
made of desmoplakin. Intermediate filaments go through the plaque.
Transmembrane proteins
belong to the cadherin family.
disease of cell junctions: pemphigus
the body produces abnormal antibodies to the proteins forming desmosome junctions in the skin. This prevents normal adhesion between cells and causes widespread skin and mucous membrane blistering as the intraepidermal desmosomes fall apart
Communicating or gap junctions
allow diffusion of small molecules and ions between the cytoplasm of adjacent cells.
- The diffusion occurs through narrow intercellular channels that are 2.8 nm in diameter.
- Each channel is formed by a pair of connexons aligned with each other.
- Each connexon consists of 6 protein subunits, connexins.
- Gap junctions usually occur in “patches”. Gap junctions are most common in such tissues as epithelia, cardiac and smooth muscle, and embryonic tissues and allow communication between cells and have a role in the control of cell proliferation and differentiation.
Basal domain
faces the basal lamina and is important for its interactions with the underlying tissue (usually connective tissue). There are several important components of the basal domain including:
basal lamina, anchoring junctions
Focal adhesions
attach actin cytoskeleton of a cell to the extracellular matrix. These junctions are similar to the adherens junctions, but instead of attaching two cells to each other, they attach cells to extracellular matrix. Also notice that proteins that compose this junction are different from the adherens junction. These are the main components of the focal adhesion:
a. Actin-binding proteins (vinculin, α-actinin) attach to microfilaments.
b. Peripheral protein, talin, connects actin-binding protein to the transmembrane protein.
c. Transmembrane protein integrin belongs to the Ca2+-independent integrin family, instead of cadherins found in the zonula adherens.
Hemidesmosome
resembles the desmosome as it connects the intermediate filaments, but these junctions attach cell cytoskeleton to the extracellular matrix. Also, like in the case with focal adhesions, the transmembrane proteins are from the integrin family. These are the main components of the hemidesmosome:
a. Intracellular plaque made of desmoplakin. Notice that the intermediate filaments end on the plaque.
b. Transmembrane proteins (integrin family) attach to the extracellular matrix.
Location of simple squamous epithelium
vascular system, lines body cavaties, parietal layer of bowmans capsule, alveoli of lung
endothelium of heart
mesothelium of internal body cavaties
Plural Mesothelioma
an aggressive malignant tumor often caused by an exposure to asbestos.
Location of simple cuboidal epithelium
walls of ducts, kidney tubules, germinal epithelium of ovary, thyroid follicles, barrier, secretion, absorption
Location of simple columnar epithelium
have a striated border
non-ciliated: GI tract, gallbladder
ciliated: oviduct
Location of pseudostratified columnar epithelium
ciliated: respiratory epithelium
nonciliated: epididymis
Metaplasia
Under the influence of certain factors the respiratory epithelium can change into another type of epithelium, for example into stratified squamous. If the factors are removed the epithelium might change back. This is common in heavy smokers. The loss of cilia results in the accumulation of mucus in lungs and inflammation. Metaplasia can occur in other types of epithelia as well, e.g. in the simple columnar epithelium
Location of stratified squamous epithelium
Keratinized: skin
Non-keratinized: GI tract (mouth/esophagus), vagina
What does the presence of keratin in a tumor indicate?
Presence of keratin in a malignant tumor is indicative of its epithelial origin.
Location of stratified cuboidal epithelium
sweat glands of skin, glands
function as barrier and conduit
Transitional Epithelium
sometimes called urothelium, because it is restricted to the urinary system. This epithelium cannot be classified as squamous, cuboidal, or columnar. It is composed of multiple layers of cells of different shape. The surface layer often contains large rounded “dome” cells that are often binucleated. The most important feature of the transitional epithelium is its ability to stretch enormously, when the organ becomes dilated. When that happens, the shape of the epithelial cells changes dramatically. From rounded or cuboidal shape the cells change into flat, squamous cells and back, when the organ contracts again. The transitional epithelium is found in the urinary bladder, ureter, and in the pelvis and calyces of the kidney
Glandular epithelium
formed as a result of invagination of the epithelial tissue. Epithelial cells then start secreting various materials, such as mucins, steroids, etc.
Endocrine Glands
If the glands secrete their product into the intercellular space or underlying connective tissue and the product diffuses into blood, they are called endocrine glands. Cells within endocrine glands are usually arranged in cords that follow the capillaries that supply the gland. Endocrine glands do not have ducts.
Exocrine Glands
maintain their connection to the originating epithelium and secrete into ducts, or spaces lined with the originating epithelium. Cells within the exocrine glands are usually arranged into acini or tubules.
B. Exocrine glands can be classified based on the mechanism of secretion.
1. There are three major mechanisms of secretion: merocrine, apocrine, and holocrine.
merocrine glands
the secretory product is delivered in membrane-bound vesicles to the apical surface, where the vesicles fuse with the plasma membrane to release their content via exocytosis. Based on the type of product that is produced merocrine glands can be further subdivided into serous, mucous, and mixed.
Serous Glands
secrete a thin, watery, protein-rich secretion. The secretory vesicles are small and densely-staining. These cells have better developed rough ER than mucous cells, and consequently their cytoplasm stains more basophilic. The rough ER occupies the basal part of the cell, and as a result the nuclei are shifted towards the middle of the cell. These cells are usually pyramidal in shape and as a result serous acini are rounded in shape.
Examples: pancreas and parotid salivary gland.
Mucous glands
produce mucins that consist of long chains of negatively charged carbohydrates attached to a small protein core. Mucins are viscous and are important for lubrication and protection. The secretory granules are large and stain very lightly with hematoxylin and eosin. The rough ER is not well developed. The nuclei are heterochromatic and occupy basal position in the secretory cells of mucous glands. Mucous cells are usually columnar in shape and mucous acini typically have tubular shape. Examples: goblet cells, glands of the soft palate, Brunner’s glands
Mixed (or) seromucous glands
have both serous and mucous cells. Mucous cells form tubules, capped by serous cells that form serous demilunes. Example: submandibular salivary gland
Apocrine
type of secretion involves pinching of the apical cytoplasm with the formation of secretory vesicles. This mechanism is found in the mammary gland, where it is responsible for releasing large lipid droplets into milk.
Holocrine
type of secretion involves the whole cell, rather than its parts. As the cell accumulates the secretory product, it goes through apoptosis (programmed cell death), and both the cellular content and cell debris are released into the lumen of the gland. This type of secretion is typical for sebaceous glands of the skin.
Unicellular glands
are individual secretory cells scattered within the non-secretory epithelium.
a. Goblet cells represent the most common type of unicellular glands. Goblet cells, which are specialized for the secretion of mucus, are scattered within the columnar epithelial cells of the GI and respiratory tracts.
Multicellular exocrine glands
are composed of more than one cell. Most multicellular glands form tubular invaginations from the surface. Invaginating the epithelium allows to increase the surface area. The terminal part of the invagination is the secretory portion; the portion that connects the secretory cells to the surface serves as a duct. Secretory part can be shaped differently
Alveolar/acinar secretory part
flask shaped, serous
Tubular secretory part
mucous glands, tube shaped
Tubuloalveolar
If the secretory tube ends with a flask-shaped dilation, the gland is tubuloalveolar. These glands are always mixed, or seromucous
Myoepithelial cells
are contractile cells, present in some glands between the basal lamina and in the secretory portion of glands and ducts. These cells contain contractile proteins, such as myosin; contraction causes expulsion of the secretory product out of the gland. These cells are present in sweat glands, salivary glands, and in the mammary gland.
Cancer of epithelium.
Because epithelial tissues are on the surface they are exposed to a variety of carcinogens and are often sources of various types of malignant tumors: carcinoma and adenocarcinoma
Carcinoma
derived from barrier epithelium
Adenocarcinoma
derived from glandular epithelium
