Download Epithelium - VCU eCurriculum

Dr. Randall E. Merchant
[email protected]
EPITHELIAL TISSUE: LINING AND COVERING
Objectives
1.
Define the criteria by which the various types of epithelial membranes are classified.
2.
Understand the many functions of the surface epithelium and know the classification of
epithelium associated with each function.
3.
Identify the structures on the CD listed in the laboratory guide.
4.
Identify and differentiate the various classifications of epithelium.
5.
Know the characteristics that are common to all the surface epithelium.
6.
Know the difference between a basal lamina and a basement membrane.
7.
Identify microvilli and cilia with the light microscopy and be able to describe their
ultrastructural appearance and functions.
I. General Features and Considerations
A.
Characteristics
1.
Lines internal and covers external surface of body
2.
Consists of a single or multiple layers of cells - simple or stratified
3.
Little or no intercellular space - cells usually tightly packed
4.
Avascular - no blood vessels, nourished by diffusion of substances from the adjacent loose
CT
5.
Polarization - most epithelial cells have internal organization
6.
Keratin - an intermediate filament protein only found in epithelial cells
7.
Cell junctions - hold the delicate cells together and help maintain organization
8.
Separated from connective tissue by basement membrane
B.
Functions of the epithelial membranes
1. Protection - most important on outer surface (skin), also helps control temperature and guards
against mechanical damage, contact with toxins, and moisture loss
2. Absorption - many cells lining internal surfaces have this function, possess numerous microvilli
on the free surface
3. Secretion - individual cells or organized into endocrine and exocrine glands, most cells lining
internal surfaces show this function
4. Excretion - specialized epithelia modified to increase efficiency in transport of solutes and water
or elimination of substances
5. Sensation - receptor cells, termed neuroepithelia, for sight, hearing, taste, olfaction
6. Contraction - myoepithelial cells associated with some exocrine glands, cells resemble smooth
muscle, compress the secretory cells of a gland
II. Classification of Epithelia
A.
Terminology
1.
B.
Classification by number of layers
a. simple - a single layer of cells resting on a basement membrane
- offer little protection
- commonly associated with absorption and secretory surfaces
b. stratified - more than one layer thick with basal cells resting on a basement membrane
- primarily associated with protection
2.
Classification by shape of surface cells: squamous, cuboidal, or columnar
3.
Classification by structural specializations
a. pseudostratified
b. transitional (urinary)
c. surface structures and specializations (e.g. microvilli, cilia, keratinized)
Simple epithelia
1.
Simple squamous epithelium
a. single layer of flat cells, polygonal shape
b. lines surfaces across which metabolites or gases must move rapidly
• the layer presents little barrier to passive diffusion
c. endothelium - forms inner lining of blood vessels, lymphatics, and heart
• thrombosis occurs at sites of endothelial damage
d. mesothelium - lines body cavities (pericardium, pleura, and peritoneum)
• together with underlying CT form a serosa
• permits passage of tissue fluid into and out of body cavities
• damage causes adhesions - can obliterate cavity and reduce movement of viscera
2.
Simple cuboidal epithelium
a. single layer of cells which in profile look like a row of squares or low rectangles
b. cells may also take on a pyramidal shape in some exocrine glands (form acini)
3.
B.
Simple columnar epithelium
a. in profile are rectangular with nuclei usually at approximately the same level
b. most likely to show polarity
c. often function in absorption, secretion or both
d. may show extensive surface specializations such as cilia and microvilli
e. “psuedostratified” columnar - all cells touch the basement membrane (so it’s “simple)
but all do not reach the free surface so it looks like it is multilayered (discussed later)
Stratified epithelia
1.
Stratified squamous epithelium
a. usually 5-25 cell layers thick
b. cuboidal cells on the basement membrane and squamous cells at free surface
c. found on surfaces subject to injury and wear and tear
•
•
•
•
2.
D.
d. non-keratinized - lining inside surfaces
all cells including the surface cells viable
surface cells possess functional nuclei
e. keratinized - surfaces exposed to external environment
surface cells non-viable and do not possess nuclei
surface cells contain almost only keratin (eosinophilic protein)
Stratified columnar or cuboidal epithelium
a. deep cells small, irregularly polyhedral while superficial cells cuboidal or columnar
b. located at sites of transition from one type of epithelium to another
c. provides more robust lining than a simple type of epithelium
Specialized epithelia
1.
Pseudostratified columnar epithelium
a. like a simple epithelium, all cells contact the basement membrane but not all cells
reach the free surface
b. nuclei aligned at two or more levels (resembling a stratified epithelium)
2.
Transitional (urinary epithelium
a. a form of stratified epithelium
that changes in thickness due
to the stretch of the hollow
organ that it lines
b. when relaxed, the surface cells
cuboidal and when distended,
the surface cell become more
squamous in shape
c. found in component organs of
the urinary system – the
epithelium protects against
toxic substances in the urine
VI.
Surface Specializations
A.
Microvilli
1.
Large numbers on a cell surface constitute a brush or striated border by light microscopy
2.
1 :m X 0.1 :m evaginations of the luminal plasmalemma
3.
Composed of actin filaments, terminal web extends into cytoplasm
4.
Usually covered with a glycocalyx (sugar coat) on their exterior surface
5.
Functions - increase surface area for absorption, may also have associated enzymes that
facilitate digestion and absorption
6.
Stereocilia - very long microvilli
a. non-motile and filamentous core poorly developed
b. relatively rare
B.
Cilia
1.
Actively motile evaginations of luminal plasmalemma, 2-10 :m long
2.
Composed of a core of longitudinal microtubules called an axoneme
a. consists of nine pairs of doublet microtubules uniformly spaced around two single
microtubules (9 + 2 pattern)
b. arrangement covered by a plasmalemma
3.
Basal bodies
a. at base of cilia in apical portion of cell
b. formed of nine triplet microtubules (9 + 0 pattern) radially arranged like a pinwheel
(identical structurally with centrioles)
4.
Function in transport
V. Basal Lamina
A.
B.
C.
Components
1.
Acellular supportive structure that can be up to 100 nm thick
2.
Composed mainly of type IV collagen, laminin, and proteoglycans
Basement membrane vs. basal lamina
1.
Lamina densa + lamina lucida = basal lamina
a. produced by the epithelial cells
b. composed of type IV collagen, laminin, and proteoglycans
2.
Basal lamina + reticular lamina = basement membrane of light microscopy
a. reticular lamina produced by connective tissue cells
Functions
1.
Barrier and support
2.
Contains recognition and regulatory factors
3.
Carries a positive charge – thromobogenic
Epithelium - Laboratory Guide
Find the following among the images on the CD
Types of epithelial membranes
Simple squamous epithelium
Simple cuboidal epithelium
Simple columnar epithelium
Pseudostratified columnar epithelium
Stratified columnar epithelium
Stratified squamous epithelium, keratinized (dry)
Stratified squamous epithelium, non-keratinized (moist)
Transitional epithelium
Basement membrane
Reticular lamina
Basal lamina
Lamina densa
Lamina lucida
Surface specializations
Brush border
Cilia with basal bodies
Microvilli
Stereocilia