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Biology 55
Tissues
TISSUES – group of specialized cells with a common function
4 Basic Tissue Types:
nervous tissue – detects changes and sends electrical signals to control
body functions
muscle tissue – moves the body and the contents of organs
connective tissue – supports and connects other tissues of the body
epithelial tissue – covers and lines surfaces and forms glands
Cell Junctions – contact points between cells that join them into a functional unit
tight junctions – web of membrane proteins on adjacent cells fuse together
forming a watertight seal (eg. cells lining digestive tract)
adhesion junction – plaques of dense membrane proteins held together by
velcro-like protein filaments and linked to cytoskeleton of cells
form strong connections between cells
gap junctions – membrane proteins of adjacent cells bind together forming
channels between cells
allows communication between cells
NERVOUS TISSUE – detects changes in internal & external environment and responds
by transmitting signals to other tissues to control body functions
found in brain, spinal cord, nerves
neurons – cells responsible for function of nervous system
neuroglia – cells that support, nourish, and protect neurons
MUSCLE TISSUE – uses ATP to contract (shorten), causing movement of body parts
Skeletal muscle – usually attaches to bone
striated (striped) appearance under microscope
very long cells with multiple nuclei
contractions are voluntary
Cardiac muscle – heart muscle
branched, striated cells with 1 nucleus
intercalated discs – gap junctions and adhesion junctions connecting cells
contractions are involuntary
Smooth muscle – walls of hollow organs and vessels
spindle-shaped with 1 central nucleus and no striations
contractions are involuntary
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EPITHELIAL TISSUE – tightly packed layers of cells held together by cell
junctions
basement membrane – layer of proteins and carbohydrates that anchors epithelium
tightly to underlying connective tissues
Functions of Epithelial Tissues
cover surfaces and line cavities
protect deeper tissues
excrete or absorb substances
form glands – produce secretions
Structure of Epithelial Tissue
free surface – faces body surface, body cavity, or cavity in organ
basal surface – adheres to basement membrane
epithelium is avascular (no blood vessels) – cells are constantly damaged by their
environment, are shed, and are replaced by division of stem cells
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 free surface
sweep mucus and other materials on surface
(eg. upper respiratory tract, uterine tubes)
microvilli – finger-like extensions of cell membrane on free surface
increase surface area for absorption or secretion
(eg. small intestine, kidney tubules)
TYPES OF EPITHELIUM
simple squamous – single, flat layer
thin layer where molecules can cross by diffusion
(eg. lining of lungs, blood vessels, ventral body cavities)
stratified squamous – many layers, layer on free surface is flat
lines surfaces exposed to damaging environment (especially abrasion)
(eg. skin, mouth, esophagus, vagina)
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simple cuboidal – single cuboidal layer
sites of secretion and absorption (may have microvilli)
(eg. most glands, kidney tubules)
stratified cuboidal – 2 or more cuboidal layers (gives additional strength)
lining large ducts of glands
simple columnar – single columnar layer
sites of much secretion and absorption (may have cilia or microvilli)
thickness provides some protection
(eg. lining digestive tract, uterine tubes)
stratified columnar – multiple layers, free surface layer is columnar
pseudostratified columnar – lines most of respiratory airways
thickness gives some protection, produces mucous secretions
cilia – sweep mucus and trapped debris out of airways
transitional – lines urinary bladder and ureters
number of layers and shape of cells varies depending on fullness
full – stretches out; appears flatter with fewer layers
empty – not stretched; appears large and plump with more layers
GLANDS – clusters of epithelial cells that produce secretions (most by exocytosis)
exocrine glands – secrete into ducts that empty onto epithelial surface
(eg. mucus, sweat, saliva, milk)
endocrine glands – ductless glands which secrete hormones into interstitial
fluid, from which it diffuses into bloodstream
goblet cells – single-celled glands found in columnar epithelium
(eg. respiratory airways, digestive tract)
CONNECTIVE TISSUE – connects, supports, and protects tissues and organs
stores energy, insulates and cushions, transports materials between tissues
most have a rich vascular supply
2 Basic Components of All Connective Tissues
1) Specialized Cells – cell type varies depending on type of tissue
2) Matrix – material that the cells secrete around themselves
matrix determines the tissue’s physical characteristics
protein fibers – strengthen and provide a framework
collagen fibers – composed to collagen protein
very strong yet flexible fibers
give strength to resist tearing
elastic fibers – contain the protein elastin
can be stretched, but it returns to itsoriginal shape
reticular fibers – thin, branching network of collagen protein
form a framework to support soft organs
ground substance – water plus various proteins, carbohydrates, and/or
minerals that surround and connect cells and protein fibers;
the type and amount of ground substance determines whether the
tissue is liquid, gel-like, or solid
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TYPES OF CONNECTIVE TISSUES
Fibrous Connective Tissues
fibroblasts – main cell of most fibrous connective tissues
matrix – gel-like ground substance plus variable amounts of protein fibers
Loose Connective Tissue – many cells with loosely arranged fibers
areolar connective tissue – soft “packing material” found between
most other tissues and organs
collagen and elastic fibers for strength and elasticity
immune cells and inflammatory cells give protection
adipose tissue – fatty tissue; stores energy, insulates, and cushions
adipocytes – fat, round fibroblasts, filled with fat droplets
little matrix seen
reticular connective tissue – framework supporting cells of soft
organs (e. liver, spleen, lymph nodes)
reticular fibers and a few fibroblasts (called reticular cells)
Dense Connective Tissue – many collagen fibers, less cells and matrix
connect body structures and resist tearing
dense regular connective tissue – parallel collagen fibers
fibers run in one direction, strong connection resists tearing
(eg. tendons and ligaments)
dense irregular connective tissue – network of collagen fibers
resists tearing forces in multiple directions
(eg. joint capsules, skin)
Specialized Connective Tissues – specialized cell types and ground substance
form tissues with specialized structures and functions
Cartilage – thick gel ground substance and many protein fibers form a
solid, yet flexible matrix
cartilage is avascular, so it heals slowly
chondrocytes – cartilage cells found in small cavities in solid matrix
lacunae – fluid-filled holes within matrix where cells live
3 types– based on type and amount of protein fibers in matrix
hyaline cartilage – fine collagen fibers (not visible)
most common type in body
(eg. joint surfaces, nose, trachea, fetal skeleton)
fibrocartilage – dense bundles of collagen fibers
very tough to absorb shock
(eg. intervertebral discs, menisci [pads] of knee)
elastic cartilage – elastic and collagen fibers
strong but flexible
(eg. ear, epiglottis)
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Bone (Osseous) Tissue – mineral salt ground substance plus collagen
fibers form an extremely hard matrix that supports the skeleton
osteocytes – bone cells found within lacunae
2 types of bone – compact bone and spongy bone
Blood – fluid ground substance and soluble protein fibers form a liquid
connective tissue that transports materials between cells
blood plasma – fluid matrix
red blood cells – transport oxygen
white blood cells – provide protection
platelets – cell fragments involved in blood clotting
MEMBRANES – sheets of tissue
Epithelial membranes – epithelium plus underlying connective tissue
mucous membranes (mucosa) – line body cavities open to the exterior
(eg. digestive, respiratory, reproductive tracts)
protective barrier against microbes, chemicals, and physical injury
secrete mucus – protects, lubricates, traps foreign material
serous membranes (serosa) – line body cavities not open to the exterior
2 layers – parietal layer covers wall & visceral layer covers organs
(eg. pleura, peritoneum, pericardium)
serous fluid (watery fluid) lubricates surfaces
reduces friction and prevents adhesions
cutaneous membrane – skin (next lecture)
Synovial membranes – line joint spaces
connective tissue membranes (no epithelium)
synovial fluid – thick,slippery secretion, lubricates joint surfaces
Meninges – cover brain and spinal cord
connective tissue membranes that protect nervous tissues
cerebrospinal fluid – cushioning fluid found within meninges
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