Tissues
• Definition
– A group of closely associated cells that work
together to perform a specific function
• Types
– Epithelial - covering
– Connective - support
– Muscle - movement
– Nervous - control
Epithelial Tissue or Epithelium
• Sheet of cells that covers a body surface or
lines a body cavity
• “epithe” = laid on, covering
Epithelium
• Occurs in the body as:
– Covering and lining epithelium
• Forms the outer layer of skin, dips into
and lines the open cavities of the
cardiovascular, digestive, and
respiratory system, and covers the
walls and organs of the ventral body
cavity.
– Glandular epithelium
• Forms the glands of the body.
Epithelium- Functions
•
•
•
•
•
Protection
Absorption
Filtration
Excretion
Secretion
– Exocrine glands – secretions released onto apical surface
– Endocrine glands – secretions into interstitial fluid or
blood (hormones)
• Sensory reception
• Has sensory nerves (smell, taste, sight, and
hearing)
Epithelium
Special Characteristics
• Polarity
– Apical surface – upper free surface that is exposed to
body exterior or the cavity of an internal organ.
• May have microvilli, cilia, or be smooth
– Basal surface – Lower attached surface
• Basal lamina – non cellular, adhesive sheet
(consists mainly of glycoproteins) that lies adjacent
to basal surface. Acts as a filter and a scaffolding
so cells can migrate to repair a wound
Epithelium– Special Characteristics
• Specialized Contacts
–Cells are fit very close together to
form continuous sheets
–Cells are bound together by
–A. tight junctions
–B. desmosomes
TIGHT JUNCTIONS
• Tight junction – a series of integral protein
molecules in the plasma membranes of
adjacent cells fuse together, forming an
impermeable junction that encircles the cells.
• Tight junctions help prevent molecules from
passing through the extracellular space
between adjacent cells.
EXAMPLE OF TIGHT JUNCTION
• Between epithelial cells lining the digestive
tract keep digestive enzymes and
microorganisms in the intestine from
seeping into the bloodstream.
• Although called “impermeable” junctions,
some are somewhat leaky and may allow
certain types of ions to pass.
DESMOSOMES
• “binding bodies” – are anchoring junctions
– mechanical couplings scattered like rivets
along the sides of abutting cells that
prevent their separation.
• On the cytoplasmic face of each plasma
membrane is a button like thickening called
a plaque. Adjacent cells are held together
by thin linker protein filaments that extend
from the plaques and interdigitate like the
teeth of a zipper in the intercellular space.
DESMOSOMES
• They not only bind neighboring cells together,
they also contribute to a continuous internal
network of strong “guy-wires.” This
arrangement distributes tension throughout a
cellular sheet and reduces the chance of tearing
when it is subjected to pulling forces.
• Desmosomes are abundant in tissues subjected
to great mechanical stresses, such as skin and
heart muscle.
Specialized Contacts
Epithelium– Special Characteristics
• Supported by Connective tissue
– All rest upon and are supported by connective tissue
– Basement membrane – made up of the reticular lamina
and the basal lamina
• Avascular but innervated
– Avascular – contains no blood vessels. Cells get
nourishment by substance diffusing from blood vessels
from the underlying connective tissue
– Innervated – supplied by nerve fibers
• Regeneration
– Reproduce themselves rapidly
Epithelial Tissue
• Classification
– Number of cell layers
• Simple – only a single layer of cells
• Stratified – Several layers of cells
– Shape of exposed cells
• Squamous – flattened/scale like
• Cuboidal - boxlike
• Columnar – tall and column
shaped
Glandular Epithelia
• Gland – consists of one or more cells that
make and secrete (export) a particular
product called a secretion.
• Secretion – an aqueous solution that
usually contains proteins
• Two types:
– Endocrine
– Exocrine
Glandular Epithelia - Endocrine
• Called ductless glands because they lose
their ducts
• Produce hormones that are excreted
directly into the extracellular space.
• From there the hormones enter the blood
or lymphatic fluid and travel to specific
target organs
• Examples: Thyroid, Pituitary, Adrenal, and
sex glands
Glandular Epithelia - Exocrine
• All secrete their products onto body surface or
into body cavities (ducts)
• Numerous and diverse
• Examples: sweat, sebaceous (oil), salivary
glands, liver (bile), pancreas (digestive
enzymes)
• Types:
–Unicellular
–Multicellular
Glandular Epithelia - Exocrine
• Unicellular
– Goblet cell –
– shaped like a goblet and produces mucin –
dissolves in water  forms mucus the
slimy coating that protects and lubricates
surfaces.
Glandular Epithelia - Exocrine
• Multicellular
–Two main parts – duct and a secretory unit
–Structural Classification (ducts)
• Simple – unbranched duct
• Compound – branched duct
Glandular Epithelia - Exocrine
• Multicellular
–Two main parts – duct and a secretory unit
–Structural Classification (secretory unit)
–Tubular – secretory cells form tubes
–Alveolar – secretory cells form small
flask-like sacs
–Tuboalveolar – secretory cells form
both
Glandular Epithelia - Exocrine
• Multicellular
Glandular Epithelia - Exocrine
• Multicellular
Glandular Epithelia - Exocrine
• Multicellular
Glandular Epithelia - Exocrine
• Multicellular
Glandular Epithelia - Exocrine
• Multicellular