Thursday, July 28, 2011
8:07 PM
1.
Discuss the characteristics of epithelium that make it a unique tissue.
1.
Closely opposed; close proximity and adhere to each other by cell junctions, cell to cell
adhesion molecules
2.
Exhibit polarity; they have an apical, lateral, and basal domain
3.
Their basal surface is attached to a non-cellular basement membrane
2.
Describe the composition and functions of the basement membrane.
Composed of Collagen (mostly type IV), Laminin (initiates basal lamina assembly and has binding
sites for integrin), and Proteoglycans (cross-links Collagen to Laminin and is mostly made up of
Perlecan). Fibronectin is an additional protein cross-linking collagen to laminin.
Function of basement membrane is to anchor epithelium to underlying connective tissue.
3.
Describe the structural features and functions of intercellular junctions.
(Lateral Junctional Complexes) - Connect via Cell Adhesion Molecules (CAM)
1) Occluding Junctions
I. Zonula Occludin (tight junction) - Occludins and claudins seals adjacent cells together near
apical zone. Function is:
i. Barrier - limits movement of water and other molecules between cells;
maintains physiochemical separation of tissue compartments
ii. Recruits signal molecules
iii. Paracellular transport - Claudins transport H2O, ions, and small molecules
2) Anchoring Junctions
I. Zonula Adherens - reinforcement of junction below tight junction. Cadherin connects
adjacent cell membranes; is anchored to intracellular actin filaments. Protein interaction
from adjacent cells is mediated by Calcium.
II. Macula Adherens (desmosome) - localized to lateral domain, very strong linkage, and
provides anchoring sites for intermediate filaments via dense, desmosomal attachment
plaque on cytosomal (cytoplasmic) side. On extracellular side are Cadherins that interact
like a zipper and require Calcium for binding.
III. Cell to Extracellular Matrix Junctions - (Cell to Extracellular Matrix Junctions)
i. Focal Adhesions - anchor actin filaments of cytoskeleton to basement
membrane. Interact with basement membrane via integrin molecules.
Coordinate remodeling of actin cytoskeleton. And alter adhesion-mediated
functions in response to external mechanical stimuli (demonstrate
mechanosensitivity).
ii. Hemidesmosomes - anchor intermediate filaments of cytoskeleton to
basement membrane. Attachment plaque on cytoplasmic side similar to
desmosome, but integrin molecules form transmembrane proteins distinct
from desmosomes. Provides strong connection preventing separation from
underlying connective tissue.
3) Communicating Junctions
I. Gap Junctions - permits direct passage of signaling molecules between cells for
coordinated activity via transmembrane channels consisting of 2 half channels (connexons);
6 connexins make up one connexon; 2 connexons form a gap junction. Connexon
conformational changes (opening and closing) are mediated by Calcium, or other
mechanisms.
4.
Describe the microscopic criteria used to classify covering epithelium.
1) By layer I. Simple- One layer of cells
II. Stratified - One or more layers of cells
i. Keratinized -(for stratified squamous) Most apical layer of cells are dead and
lose their nucleus and cytoplasm. However, they contain a tough, resistant
protein called Keratin, which makes the epithelium waterproof.
ii. Transitional - Term applied to the epithelium lining the lower urinary tract. Is
stratified epithelium with specific morphologic characteristics that allow it to
distend.
2) By cell shape I. Squamous - width of the cell is greater than its height, can appear tear-drop shaped
II. Cuboidal - square-shaped
III. Columnar - Tall, thin rectangular (can be ciliated)
i. Pseudostratified Columnar -simple columnar epithelial cells whose nuclei
appear at different heights giving the misleading impression that the epithelium
is stratified when viewing a cross-section.
6.
Describe the structure and function of microvilli, cilia and stereocilia. (Apical
Domain Specializations)
1) Microvilli - Cytoplasmic projections from the apical surface (of epithilia) that vary in number,
length, and density. Are filled with actin filaments that bind to a web of actin filaments below the
base of the microvilli, called a terminal web. Microvilli can contract, decreasing the diameter of
villus and increasing intervillous space. The presence of myosin II and tropomyosin in the terminal
web allows the microvilli to contract.
2) Stereocilia - look like thin, irregularly distributed microvilli. Because of their length they were
initially mistaken for cilia. They are non-motile and are found only in the epididymis, ductus
deferens, and sensory (hair) cells of the inner ear (where they function as mechanoreceptors in
hearing).
3) Cilia- are present on nearly every cell of the body and most are designed for motility (but not
all). Epithelia that need to move substances across their surface, like mucous in the air passages,
have cilia inserted in the cytoplasm by basal bodies. Consist of an internal core of microtubules
arranged in a 9+2 pattern; axoneme (9 doublets of circularly arranged microtubules with 2 central
microtubules). Cilia beat in synchronous movement coordinated by orientation of basal foot.
I. Primary Cilia -are non-motile, arranged in a 9+0 pattern, and passively bend to flow of
fluid. They also transduce various signals from environment into internal chemical signals
that modify the cell's response to stimuli.
7.
Describe the functions of the various epithelial cells and examples of where
they are found
1) Secretion - columnar epithelium of the stomach and gastric glands
2) Absorption - in the columnar epithelium of the intestines and proximal convoluted tubules of
the kidney
3) Transportation - transport of materials or cells along the surface of an epithelium by motile cilia
or in the transport of materials across an epithelium to and from connective tissue
4) Protection - stratified squamous epithelium of the skin (epidermis) and the transitional
epithelium of the urinary bladder
5) Receptor Function - receive and transduce external stimuli; e.g. taste buds of the tongue,
olfactory epithelium of the nasal mucosa, and the retina of the eye
8.
Characterize endocrine vs exocrine glands.
1) Endocrine gland - Lacks a duct system and secretes hormones into connective tissue which
travels via bloodstream to reach target tissue, however paracrine secretions only affect
neighboring cells.
2) Exocrine gland - Secretes products onto a surface directly or through epithelial ducts connected
to surface.
9.
Compare the three modes of exocrine gland secretion.
1) Merocrine - no loss of cytoplasm; most common mode (MN: mero/zero). Found in the
2) Holocrine - entire cell lost (MN: holo/whole)
3) Apocrine - apical cytoplasm lost (MN: apo/apical side/half)
10.
Describe/identify the various structures of exocrine glands
See Slide 61 for Chart
11.
Describe clinical correlates of epithelial tissue discussed in lecture or otherwise
indicated