Epithelial Tissue

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Unit 2
Cells, Tissues & Membranes
SECTION 1: EPITHELIAL TISSUES
Anatomy & Physiology
NeQuelle DeFord

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• Epithelial tissue is a sheet of cells that covers a body
surface or lines a body cavity.
• Includes epithelia and glands
–Epithelia are layers of cells that cover internal or
external surfaces
–Glands are composed of secreting cells derived from
epithelia
Epithelial Tissue

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1. Cellularity: Composed almost entirely of cells (all cells
tightly bound by cell junctions), forming a continuous
sheet.
2. Avascular: Containing no blood vessels.
3. Innervated: Supports nerve fibers.
4. Regeneration: Epithelial cells are frequently damaged or
lost to abrasion, so they are replaced quickly
5. Polarity: Apical (free) and basal surfaces (attached to
underlying connective tissue)
Epithelial Tissues: Characteristics

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Epithelial Tissues: Characteristics

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1. Protection: Epithelial cells protect underlying tissue from
mechanical injury, harmful chemicals and pathogens and
dehydration.
2. Sensation: Changes in the environment are detected by
specialized epithelial cells found in the skin, eyes, ears,
and nose, and on the tongue.
3. Secretion: In glands, epithelial tissue is specialized to
secrete specific chemical substances such as enzymes,
hormones and lubricating fluids.
Epithelial Tissues: 8 Major Functions

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4. Excretion: Epithelial tissues in the kidney excrete waste
products from the body and reabsorb needed materials
from the urine. Sweat is also excreted from the body by
epithelial cells in the sweat glands.
5. Diffusion: Simple epithelium promotes the diffusion of
gases, liquids and nutrients. Because they form such a
thin lining, they are ideal for the diffusion of gases (e.g.
walls of capillaries and lungs).
Epithelial Tissues: 8 Major Functions

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6. Absorption: Certain epithelial cells lining the small
intestine absorb nutrients from the digestion of food.
7. Contractility: Some very specialized epithelial cells,
found in the lining of the heart and respiratory tract,
contain the contractile proteins myosin and actin, similar
to muscle.
8. Transportation: Some epithelial cells have cilia, short
hair-like projections that propel fluid or particulate matter
over tissue surfaces.
Epithelial Tissues: 8 Major Functions

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To be effective in protecting other tissues, epithelial cells
must remain firmly attached to the basement membrane
and to one another to form a complete cover or lining.
A cell junction is a specialized attachment site on a cell
that allows for support and communication.
Four common intercellular connections are tight junctions,
adhering junctions, gap junctions, and anchoring junctions
(desmosomes).
Epithelial Tissues: Intercellular Connections

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Tight junctions anchor cells to
each other
– prevents substances from
passing between cells
– materials must move through
cells, or are blocked from
moving past cells
– found in intestinal lining
Epithelial Tissues: Intercellular Connections
Tight
junction

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Adhering junctions are often
deep to tight junctions
– form all the way around the cell
– support apical surface
– allow passage between cells
below the apical surface
Epithelial Tissues: Intercellular Connections
Adhering
junction

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Gap junctions tunnel between
two plasma membranes
– found in muscles
– enables sharing of ions and
proteins
– enables fast communication
between cells
Epithelial Tissues: Intercellular Connections
Gap
junction

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Epithelial Tissues: Intercellular Connections
Anchoring
junction
Anchoring junctions attach
cells to each other
– bind epithelium together
– bind muscle cells
– resists shear force
– allow bending and twisting

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In some epithelial tissue, basal
cells are anchored to the
basement membrane with a
specialized type of anchoring
junction called a
hemidesmosome.
Epithelial Tissues: Intercellular Connections
Hemidesmosome

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Epithelial Tissues: Intercellular Connections

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The basement
membrane
has no cells
but is a
network of
protein fibers
that connect
adjacent
epithelial cells
to underlying
connective
tissues.
Epithelial Tissues: Basement Membrane

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• Epithelial cells may only survive for a day or two because
they are lost or destroyed.
• Replaced by continuous division of unspecialized cells
called stem cells.
• Stem cells are found in deepest layer next to basement
membrane.
Epithelial Tissues: Renewal and Repair

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Each epithelium tissue is given two names:
• The first name indicates the number of cell layers present.
– Simple- one layer (used in filtration)
– Stratified- more than one layer (wear and tear areas)
– Pseudostratified- one layer that looks like many (areas that need to stretch)
• The second describes the shape of its cells
– Squamous- cells are wider than tall
– Cuboidal- cells are as wide as tall
– Columnar- cells are taller than wide
– Transitional- cells have no specific shape
• Ciliated- If the apical edge of the epithelium has cilia
Epithelial Tissues: Classification

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Glandular Epithelia
Many epithelia contain gland cells that produce exocrine or
endocrine secretions
1. Exocrine Glands – produce exocrine secretions and discharge
their products through a duct/tube, onto some external or
internal surface. Gland can be unicellular (goblet cell) or
multicellular. Ex. sweat glands, oil glands, mammary glands,
liver, and pancreas
2. Endocrine Glands– are produced by ductless glands and
released into blood or tissue fluids. Ex. hormones (chemical
messengers that regulate or coordinate the activities of other
tissues, organ and organ systems).
Glandular Epithelia

20. • Merocrine Secretion – exocytosis,
product oozes out of cell
(ex. sweat gland)
• Apocrine Secretion – part of cell
pinches off, releasing secretion
(ex. mammary gland)
• Holocrine Secretion – superficial
cell fills with secretion, bursts
open and dies, releasing
secretion (ex. sebaceous gland) 20
Modes of Exocrine Secretion