Epithelial Tissue
BIOL241
Epithelial Tissue
• Overview:
– Characteristics and functions of epithelia
– Cell junctions
– Classification of epithelia
– Exocrine glands
Four tissue types in the body
• Groups of cells similar in structure and
function
• The four types:
– Epithelial
– Connective
– Muscle
– Nerve
Remember the levels of
organization
What is an Epithelium?
• Epi = “on” or “around”
• Thele = “nipple”
• Covers the external body surface (epidermis),
lines cavities and tubules, and generally
marks off our insides from our outsides
• Other examples?
Epithelial Tissues – two types
• Epithelia:
– layers of cells covering internal or external
surfaces
• Glands:
– structures that produce secretions
Characteristics of Epithelia
1. Cellularity: composed of cells bound by
cell junctions
2. Polarity: apical and basal surfaces
3. Attachment: via basal lamina to
underlying connective tissue
4. Avascularity: no blood vessels (but
richly innervated)
5. Regeneration: germinative cell division
Free Surface and
Attached Surface
• Polarity:
– apical and basolateral surfaces
Figure 4–1
Repairing and
Replacing Epithelia
• Epithelia are replaced by division of
germinative cells (stem cells)
• Near basal lamina
Functions of Epithelial Tissue
1.
2.
3.
4.
5.
Provide physical protection
Control permeability
Move fluids over the surface
Provide sensation (e.g. neuroepithelia)
Produce specialized secretions
(glandular epithelium)
Specializations
• Microvilli increase absorption or secretion
• Cilia (ciliated epithelium) move fluids
Indentations & Protrusions
Increase Surface Area
More Examples
Effective Barriers
• Physical integrity is maintained by:
– intercellular connections
– attachment to basal lamina
– maintenance and repair
Cell Junctions
Cell junctions
• Tight Junctions – surround cells,
waterproof
– Isolates wastes in the lumen
• Gap junctions – allow rapid
communication
• Desmosomes – tie cells together with
great strength (like rivets)
– Hemidesmosomes attachment
Figure 4–2b
Desmosomes
• CAMs, dense areas, and intercellular
cement
Figure 4–2d
Attachment to Basal Lamina
• Hemidesmosomes
Figure 4–2e
Classification of epithelia
• Cell shape
– Squamous: flat “square”
– Cuboidal: cubes (2-D: “square”)
– Columnar: tall
• Layers of cells
– Simple: one layer of cells (what is a function?)
– Stratified: many layers of cells (what is a
function?)
Classes of Epithelia
• Based on shape and layers
Table 4–1
Classification of Epithelia
• Simple or
stratified
Figure 4.1a
Classification of
Epithelia
• Squamous,
cuboidal, or
columnar
Figure 4.1b
Simple Squamous Epithelia
• Single layer of flattened cells with discshaped nuclei and sparse cytoplasm
Look like a fried egg from the top
• most delicate
• Diffusion, friction reduction
• Special names
Mesothelium:
• lines body cavities (e.g. peritoneum, pleura)
Endothelium:
• lines heart and blood vessels
Epithelia: Simple Squamous
Figure 4.2a
Simple Squamous Epithelium
Figure 4–3a
Epithelia: Simple Cuboidal
• Single layer of cube-like cells with large,
spherical central nuclei
• Function in secretion and absorption
• Present in kidney tubules, ducts and
secretory portions of small glands, and
ovary surface
Epithelia: Simple Cuboidal
Figure 4.2b
Simple Cuboidal Epithelium
• Kidney tubules
Figure 4–4a
Epithelia: Simple Columnar
•
•
•
•
•
Single layer of tall cells with oval nuclei
May have microvilli
Goblet cells are often found in this layer
Function in absorption and secretion
Line digestive tract and gallbladder, small
bronchi, uterine tubes, and some regions
of the uterus
Epithelia: Simple Columnar
Figure 4.2c
Epithelia: Simple Columnar
• Intestinal lining
Epithelia: Pseudostratified
Columnar
• Single layer of cells with different heights;
all touch the basal lamina but some do not
reach the free surface
• Nuclei are seen at different layers
• Function in secretion and propulsion of
mucus
• Present in the male sperm-carrying ducts
(nonciliated) and trachea (ciliated)
Epithelia: Pseudostratified Columnar
Figure 4.2d
Pseudostratified
Columnar Epithelium
• Trachea
Figure 4–5b
Epithelia: Transitional
• Several cell layers, basal cells are
cuboidal, surface cells are dome shaped
(or flat)
• Stretches to permit the distension of the
urinary bladder
• Lines the urinary bladder, ureters, and part
of the urethra
Epithelia: Transitional
Figure 4.2f
Epithelia: Transitional
• Urinary bladder
Figure 4.2f
Stratified epithelia
Epithelia: Stratified Squamous
• Thick membrane composed of several
layers of cells (the only one with more than
2 or 3 true layers)
• Functions in protection of underlying areas
subjected to abrasion
• Forms the external part of the skin’s
epidermis (keratinized cells), and linings of
the esophagus, mouth, and vagina
(nonkeratinized cells)
Epithelia: Stratified Squamous
Figure 4.2e
Epithelia: Stratified Cuboidal
• Quite rare in the body
• Found in some sweat and mammary
glands
• Typically two cell layers thick
• Only top layer is cuboidal
Stratified Cuboidal Epithelium
• Sweat gland ducts
Figure 4–4b
Epithelia: Stratified Columnar
• Limited distribution in the body
• Found in the pharynx, male urethra, and
lining some glandular ducts
• Also occurs at transition areas between
two other types of epithelia
Stratified Columnar Epithelium
• Rare
• Salivary gland duct
Figure 4–5c
Epithelia: Glandular
• A gland is one or more cells that makes
and secretes an aqueous fluid
• Classified by:
– Site of product release – endocrine or
exocrine
– Relative number of cells forming the gland –
unicellular or multicellular
Glandular Epithelia
• Endocrine and exocrine
glands
Figure 4–6
Glands
• Endocrine
– Ductless glands that produce hormones
– Secretions include amino acids, proteins, glycoproteins, and
steroids
• Exocrine
• More numerous than endocrine glands
• Secrete their products onto body surfaces (skin) or into
body cavities via ducts
• Examples include mucous, sweat, oil, digestive, and
salivary glands
• The most notable unicellular gland is the goblet cell
• EXAMPLES?
Goblet Cell
Figure 4.3b
Glands are classified based on mode of
secretion - 3 types
Modes of Secretion
• Merocrine – products are secreted by
exocytosis (e.g., pancreas, sweat, and
salivary glands)
• Holocrine – products are secreted by the
rupture of gland cells (e.g., sebaceous
glands)
• Apocrine – products accumulate in the top
of the cell and then it breaks down
Summary
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•
•
•
Epithelial tissue structures and functions
Cell junctions
Classification by cell shape and layers
Glands