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VT 105
Comparative Anatomy and Physiology
Tissues
TISSUES – group of similar cells with a common function
Histology – study of tissues
Pathologist – doctor who examines tissues for changes in structure indicating
disease processes
Biopsy – removal of tissue for pathological examination
techniques – needle biopsy, surgical biopsy
3 primary germ layers – first tissues of embryo
ectoderm – dorsal layer
mesoderm – middle layer
endoderm – ventral layer
4 Basic Tissue Types:
nervous tissue – from ectoderm
muscle tissue – from mesoderm
connective tissue – from mesoderm
epithelial tissue – from all 3 layers depending on location
Cell Junctions – contact points between cells that bind them together into a functional
unit, form seals between cells, anchor cells in place, provide channels for
communication between cells
Tight junctions – web of membrane proteins on adjacent cells fuse together
forming a watertight seal (eg. cells lining digestive tract)
Desmosomes – plaque (dense area of proteins) on membrane with cell adhesion
molecules
desmosones of adjacent cells attach to each other like velcro
intermediate filaments link desmosomes to cytoskeleton of cell
contributes strength and stability to tissues
Hemidesmosomes – similar to desmosomes, but attach epithelial cells to a
basement membrane
Gap junctions – membrane proteins of adjacent cells bind together forming
channels between cells
certain particles can diffuse from one cell to the other, allowing
intercellular communication
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Neural Tissue – detects changes in internal and external environment and responds by
transmitting signals to other tissues to maintain homeostasis or cause a response
2 cell types:
neurons – detect stimuli, create nerve impulses; composed of a cell body,
with fibrous processes
neuroglia – support neurons
Muscle Tissue – use ATP to contract; cause body movements
3 muscle types:
Skeletal muscle – usually attaches to bone
striated (striped) appearance under microscope
very long, multinucleate cells
contractions are voluntary
Cardiac muscle – heart muscle
branched, striated cells with 1 nucleus
intercalated discs – junctions between cells with gap junctions
contractions are involuntary
Smooth muscle – walls of hollow organs and vessels
spindle-shaped with 1 central nucleus and no striations
contractions are involuntary
EPITHELIAL TISSUE
sheets of epithelial cells held together by cell junctions; anchored to a basement
membrane
basement membrane – fibrous layer which anchors epithelium to underlying
connective tissue
composed of protein fibers and adhesion proteins secreted by epithelium
and underlying connective tissue
Functions of Epithelial Tissues
cover and line surfaces
protect surfaces and deeper tissues
sites of excretion, absorption, or gas exchange
some modified to detect sensations
form glands
produce secretions
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Polarity of Epithelial Cells – they have directionality
basal surface – cell surface which adheres to basement membrane via
hemidesmosomes (basal layer = deepest layer)
apical surface – cell surface opposite basal surface; faces body surface, body
cavity, or lumen of organ or duct (apical layer = most superficial layer)
lateral surfaces – face adjacent cells and are attached by various cell junctions
epithelium is avascular (no blood vessels) so nutrients and wastes diffuse to and
from epithelium from blood vessels in connective tissue beneath
CLASSIFICATION OF EPITHELIUM
by number of cell layers:
simple epithelium – single layer of cells
stratified epithelium – 2 or more layers of cells
pseudostratified epithelium – 1 layer that appears multilayered
classification by cell shapes:
squamous – flat; nucleus in center
cuboidal – cubes or hexagons; nucleus in center
columnar – height greater than width; nucleus at base
transitional – shape varies due to tissue stretching (flat to cuboidal)
by cell modifications:
cilia – hair-like microtubule structures on apical surface
function in sweeping mucus and other materials on surface
(eg. upper respiratory tract, uterine tubes)
microvilli – finger-like extensions of cell membrane on apical surface
increase surface area for absorption or secretion
(eg. small intestine, kidney tubules)
TYPES OF EPITHELIUM
simple squamous – smooth, single, flat layer
mesothelium – lines body cavities and viscera
endothelium – lines inside of blood vessels
gas exchange surfaces of lungs
stratified squamous – many layers, apical layer is flat
lines surfaces exposed to damaging environment (especially abrasion)
skin, mouth and esophagus, vagina, anus
simple cuboidal – single cuboidal layer
sites of secretion and absorption
many glands, kidney tubules
may have cilia or microvilli
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stratified cuboidal – 2 or more cuboidal layers
lining large ducts of glands
simple columnar – single columnar layer
thickness provides some protection
capable of secretion and absorption
lining digestive tract, uterine tubes
may have cilia or microvilli
stratified columnar – multiple layers, apical layer is columnar
protective and secretory
rare
pseudostratified ciliated columnar
lines most of respiratory airways
has goblet cells which produce mucus
traps and sweeps debris out of airways
transitional – found in urinary bladder and ureters
variable number of layers and shape of apical layer depending on fullness
of organ
full – stretches out; appears squamous with fewer layers
empty – not stretched; appears cuboidal to round with more layers
GLANDS – epithelia that produce secretions; most invaginate to form clusters of cells
beneath the covering layers of epithelium
exocrine glands – secrete into ducts that empty onto covering or lining
epithelium (eg. mucus, sweat, saliva)
endocrine glands – ductless glands which secrete (hormones) into interstitial
fluid, from which it diffuses into bloodstream
Structural classification of glands:
multicellular glands – multiple cells
simple, branched, compound
tubular – tube-shaped
acinar – sac-like
unicellular glands – 1 cell
goblet cells – modified columnar epithelial cells
secrete mucus for lubrication and protection
(eg. upper respiratory tract, digestive tract)
Functional classification of glands – how secretion is released
merocrine glands – most common type; secretory vesicles containing
product empty via exocytosis (eg. salivary glands)
holocrine glands – product accumulates in cytoplasm and is released as
cell ruptures (eg. sebaceous glands)
apocrine glands – product accumulates in apical region of cell and cell
pinches of apex to release (mammary glands)
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CONNECTIVE TISSUE
connects, supports, and protects body and organs; store energy, provide immunity,
allow transport of materials between tissues
most have a rich vascular supply
2 Basic Components of Connective Tissue
1) Specialized Cells – all derived from embryonic mesoderm
variety of cells types depending on specific tissue
fixed cells – remain in a specific connective tissue
wandering cells – come and go in many types of tissues
leukocytes – white blood cells
mast cells – inflammatory cells
macrophages – tissue phagocytes
2) Matrix – extracellular material (outside of cells)
protein fibers and ground substance
secreted between cells by –blast cells
matrix determines a tissue’s physical characteristics
Protein Fibers – fibrous proteins; strengthen and support matrix
collagen fibers – thick, strong white fibers
resistant to stretch, but flexible
made of collagen protein (25% of body protein)
(plentiful in bone, cartilage and tendons)
reticular fibers – collagen proteins form thin, branching
networks forming a framework to support soft
organs such as liver and spleen
elastic fibers – contain the protein elastin
strong but elastic (can be stretched but returns to
original shape)
(found in lungs, skin, blood vessels)
Ground substance – amorphous materials secreted between cells
and fibers; supports and binds cells and fibers
determines consistency of tissue (liquid, gel, solid)
water – major component in all ground substance
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glycosaminogycans (GAGs) - trap water molecules to
form a gel
hyaluronic acid – viscous, slippery substance
chondroitin sulfate – thick gel
mineral salts – form solid matrix
mostly calcium and phosphates
TYPES OF CONNECTIVE TISSUES
Embryonic Connective Tissue
mesenchyme – relatively undifferentiated mesoderm
gives rise to all adult connective tissues
mesenchymal cells; soft gel ground substance, delicate reticular
fibers
ADULT CONNECTIVE TISSUES
Connective Tissue Proper
Loose Connective Tissue – many cells with loosely arranged matrix
areolar connective tissue – most common; least specialized
“packing material”; cushions and supports organs
main fixed cells – fibroblasts – large, branched cells that
secrete the matrix
many wandering cells
fibers – collagen, elastic, and reticular
ground – soft gel
(found in subcutaneous (subQ) layer beneath skin)
adipose connective tissue – fatty tissue
stores energy, insulates, and cushions organs
main fixed cells – adipocytes – round cells filled with large
fat droplets
white fat – main type in adults
brown fat – newborn and hibernating animals
rich blood supply and mitochondria; generates heat
reticular connective tissue – reticular fibers and fibroblast cells
forms stroma (network) which supports soft organs
(liver, spleen, lymph nodes)
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Dense Connective Tissue – many thick fibers and fewer cells and matrix
connect body structures and resist stress
dense regular connective tissue – bundles of parallel collagen
fibers, all running in the same direction
fibroblasts between bundles
very resistant to stretch in 1 direction, (eg. tendons)
dense irregular connective tissue – many, irregularly woven
collagen fibers, with fibroblasts
resists pulling forces in multiple directions
(eg. joint capsules, skin)
elastic connective tissue – branching elastic fibers with
fibroblasts between
allows stretching but returns to original shape
(eg. lungs, arteries, vocal cords)
Specialized Connective Tissues
Cartilage – rigid, yet flexible
matrix – dense fiber network in thick gel ground
fixed cells
chondroblasts – immature cells that secrete matrix
chondrocytes – mature cells trapped in matrix
lacunae – spaces within matrix where cells live
perichondrium – dense irregular c.t. membrane covering surface
of most cartilage
hyaline cartilage – most common (eg. joint surfaces, nose,
trachea, fetal skeleton)
fine collagen fibers in a firm gel ground
numerous chondrocytes
provides support with some flexibility, reduces friction,
absorbs shock
fibrocartilage – dense bundles of collagen in thick gel ground
few chondrocytes; no perichondrium
very tough to absorb shock
(eg. intervertebral discs, menisci [pads] of stifle)
elastic cartilage – thread-like network of elastic fibers in gel
ground with chondrocytes
strong but flexible (eg. ear, epiglottis)
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Growth and Repair of Cartilage
no vascular supply (blood vessels); growth and repair are
very slow in adults
Bone (Osseous) Tissue – supports of skeleton
stores calcium and phosphorus
matrix – numerous collagen fibers in mineral salt ground
main fixed cells – osteocytes within lacunae
periosteum – dense irregular c.t. membrane covering bone surface
Blood – fluid connective tissue
blood plasma – fluid matrix composed of water with solutes
red blood cells (erythrocytes) – carry oxygen to cells and carbon
dioxide away
white blood cells (leukocytes) – involved in phagocytosis,
immunity, and inflammation
platelets (thrombocytes) – involved in blood clotting
Lymph – mainly fluid; variable cell and solute content
MEMBRANES – flat, pliable sheets of tissue; cover or line a body part
Epithelial Membranes – composed of epithelium and underlying connective
tissue
Mucous membranes (mucosa) – line body cavities open to the exterior
(eg. digestive, respiratory, reproductive tracts)
protective barrier against pathogens, chemicals, and physical injury
secrete mucus - stops desiccation, lubricates, traps foreign material
may function in absorption or secretion of other products
may have cilia, microvilli, and goblet cells
lamina propria – areolar connective tissue layer
Serous membranes (serosa) – line body cavities not open to the exterior
and cover organs within cavities
areolar connective tissue covered by mesothelium (simple
squamous epithelium)
2 layers – parietal layer covers wall & visceral layer covers organs
(eg. pleura, peritoneum, pericardium)
serous transudate (watery fluid) lubricates surfaces
reduces friction and prevents adhesions
Cutaneous membrane – skin (next lecture)
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Connective Tissue Membranes – composed of connective tissues, no epithelium
Synovial membranes – line joint spaces, bursae and tendon sheaths
areolar connective tissue with connective tissue cells
secrete synovial fluid – thick, viscous fluid which lubricates
articular cartilage
Meninges – cover and protect brain and spinal cord
TISSUE REPAIR
Regeneration – reconstruction of injured tissue by parenchymal cells
(functioning part of tissue or organ) – fully functional repair
stem cells – undifferentiated cells in parenchyma that divide and
differentiate to replace cells
some tissues don’t regenerate well (eg. muscle, nervous tissue)
Fibrosis – repair of injured tissue by fibroblasts (scarring)
replace parenchyma with connective tissue (not functional)
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