TISSUE LEVEL OF ORGANIZATION

4 BASIC TISSUE TYPES

• Epithelial

• Connective

• Muscle

• Nervous

Cell Connections

• Tight Junctions

• Adherens

• Desmosomes

• Hemidesmosomes

• Gap Junctions

Tight Junctionsmost apical part of cellfuse 2 adjacent membranes

with fibrous connectionsprevents passage of

molecules & ions between cells

if epithelium forms tube space in tube is-lumen

presence of tight junctions ensures that contents of lumen are isolated from basolateral cell surfaces

Adherens• dense layers of

proteins on inside of membrane

• attach membrane proteins to microfilaments of cell’s cytoskeleton

Desmosomeslocalized patches

holding cells together

allow tissues to resist twisting & stretching

stabilize cell shapesmost abundant in

superficial skin layers

links so strong that dead skin cells are shed in thick sheets-not individually

Hemidesmosomes• made of

proteins

• anchor cells to basement membrane

Gap Junctionsintercellular channels

permit passage of ions & small molecules

comprised of pore-like transmembrane proteins-connexons

help coordinate functions such as cilia beating

most abundant in cardiac & smooth muscle

coordinate muscle cell contraction

Epithelial Tissue • flat sheets of contiguous cells• line body surfaces & cavities • cover every exposed surface• skin & all passageways that

communicate with the outside world – Digestive– Reproductive– Urinary– Respiratory

EPITHELIA TISSUE CHARACTERISTICS

• Cellularity– made almost entirely of cells– packed together tightly with little extracellular space

• Polarity– cytoplasmic components of cells not evenly distributed– cells have one exposed face either to external world or to a lumen-

apical surface and basal surface which faces underlying connective tissue

• Attachment– bottom row of cells bound to basement membrane

• Avascularity– no direct contact of epithelial cells with blood vessels– nutrition comes via diffusion or absorption from underlying tissues

• Regeneration– able to repair and renew themselves– stem or germinative cells are found in deepest layer of epithelium near

basement membrane

FUNCTIONS• physical protection

– protect underlying cells from abrasion, dehydration and destruction

• control permeability– anything entering or leaving the body

must cross an epithelium• provide sensation

– some detect environmental changes & relay information to nervous system

• produce special secretions– primary function of glandular

epithelium

Specializations of Apical Surface

• Microvilli– finger-like projections– increases surface area 20X– specialized for absorption & secretion

• Cilia– longer with larger diameter – beat in coordinated fashion– movement of fluids across & through epithelia

Classification of Epithelia

• cell shape

• arrangement of cell layers

Arrangement of Layers• Simple

– one layer of cells• Pseudostratified

– one layer that looks like several layers

• all cells attach to basement membrane

• Stratified– several layers of cells

stacked on top of each other

Function & Classification of Epithelia• Simple

– each cell rests on basement membrane

– one surface faces either lumen or outside world

– cells are thin– same polarity– typically fragile– do not provide much protection

against mechanical damage– found only internally– absorption or secretion

• Stratified– basal layer of cells rests on

basement membrane– subsequent layers do not– stacked on top of basal layer– cells of only most superficial layer

have free surface– found in areas subjected to

mechanical or chemical stresses• skin & lining of mouth

Cell Shapes• Squamous

– flat & irregularly shaped

– often so thin that flattened nucleus bulges at cell surface

• Cuboidal – about as tall as wide

– look like cubes or hexagonal boxes– Nucleus-usually round & not flattened

• Columnar cells– taller than wide– look like columns– Nucleus-usually elongated & found in long axis of cell

• Transitional cells– go from squamouscuboidal & back again– all organs to change shape

TYPES OF EPITHELIA

Simple Squamous• one layer of

squamous cells• delicate• found in protected

regions• filtration or diffusion • slick, slippery

surfaces are needed to reduce friction

• substances can move quickly through

Simple Cuboidal• one layer of cuboidal

cells• specialized for

secretion & absorption• found in secretory

portion of glands• some cells may have a

dense border of microvilli

• found in kidney tubules, pancreas & salivary glands

Simple Columnar• one layer of columnar

cells• found where absorption &

secretion take place • small intestine• in small intestine

epithelium has goblet cells which secrete mucus to protect and lubricate

• found with cilia in oviducts & respiratory tract

Stratified Squamous• several layers of squamous cells• surface cells look squamous• lower ones appear more cuboidal

or columnar• found where body experiences

severe mechanical stresses• cells are worn away quickly • replaced rapidly by mitosis in lower

layers• outer layer of the skin- epidermis

– here mechanical stress and dehydration of the superficial layers is aided with keratin

• skin is said to be keratinized• Non-keratinized stratified

squamous epithelium – found in mouth, pharynx &

esophagus

Stratified Cuboidal• typically only 2 cell

layers of cuboidal cells

• not a great quantity found in

• human body• large ducts of sweat &

mammary glands

Stratified Columnar• rare

• found where 2 other types of epithelia meet

• large ducts

• in pharynx, epiglottis, anus & urethra

Pseudostratified Epithelium• looks like stratified

columnar

• appears layered but is not

• nuclei are at different levels but all cells rest on basement membrane

• not all same height

• often contains cilia & goblet cells

• found lining most of respiratory tract

Transitional Epithelium• multi-layered• goes from cuboidal

squamous and back again• thicker, multilayered

epithelium • found in bladder• tolerates great deal of

stretching• surface cells are more

muffin-shaped• cells are rounded when

organ is not filled and flattens as organ fills

Glanduar Epithelia• Gland

– cell or organ that secretes substances for use elsewhere in the body or releases them for elimination from body

– composed primarily of epithelia tissue

• Endocrine– ductless– release hormones into interstitial fluid– regulate or coordinate activity of other tissues, organs & organ

systems

• Exocrine– ducted– release secretions into passageways or ducts which empty onto

the skin or other epithelial surfaces– produce enzymes & perspiration

Exocrine Gland Classification• Unicellular

• Multicellular• Simple

–have single, unbranched duct

• Compound–have branched duct

Exocrine Gland Classification

• if duct & secretory part are equal in diameter-gland is tubular

• if secretory cells form sac-acinar• if secretory cells are found both in

tubular &acinar parts-tubuloacinar

• Exocrine Gland Structure– Unicellular

– Multicellular• Secretory sheets• Tubular• Alveolar (Acinar) • Tubuloalveolar

Merocrine Glands• most common

• sweat & mucus secreting

• release products via exocytosis

Apocrine Glands

• product accumulates in apical tip

• pinched off to secrete

• rest of gland repairs itself

Holocrine glands• entire cell becomes

packed with secretory product

• cell bursts releasing secretion and in so doing kills the cell

• further secretion depends on replacement of gland cell

• sebaceous or oil glands associated with hair follicles

Connective Tissue• widely spread throughout body• most diverse tissue type• never exposed to outside environment• highly vascularized-blood vessels are

present (except cartilage & tendons)• comprised of 3 basic components:

– specialized cells– extracellular matrix

• protein fibers• ground substance

Functions • structural framework

– binds muscle to bone, fat holds kidneys in place & fibrous tissues bind skin to underlying muscle

• supports the body• protection for delicate organs such as brain & lungs• immune protection defending body from

microorganisms• transports fluids & dissolved materials through

body• movement

– bones provide levers for body movement• stores energy & generates heat

Cells• each type of connective tissue has specialized cells at different stages of maturity

• Juvenile cells actively secrete matrix– have suffix blast

• Mature cells have suffix-cyte• Destructive cells have suffix

clasts• prefix is different for different

types of connective tissues• Cartilage-chondro• Bone-osteo• Blood-hemo

Protein Fibers• Collagen fibers• long, straight, unbranched & very

strong– each fiber-bundle of fibrous

protein subunits wound together like strands of rope

• Elastic fibers• contain elastin

– able to stretch & recoil without damage

• Reticular fibers• fine collagen fibers

– made of same protein subunits as collagen

– arranged differently forming a tough, flexible branching framework

Classification• Embryonic

– mesenchyme & mucous types– found in embryo from third gestational month to birth– tissue from which all connective tissue originates

• Mature– Loose– Dense– Cartilage– Bone– Liquid

Loose Connective Tissue• packing material

• fills spaces between organs, cushions & stabilizes cells in organs & supports epithelia

• surrounds & supports blood vessels and nerves & stores lipid

• areolar, adipose & reticular

Areolar Connective Tissue• consists of an open

framework• ground substance accounts

for most of its volume• forms soft-pliable-packing

material around tissues– muscles, blood vessels &

glands• absorbs shock• loose organization allows it

to distort without damage• presence of elastic fibers

makes it able to return to original shape

• forms layer separating skin from deeper structures

Adipose Tissue• composed mainly of

adipocytes• little matrix• cells have large vacuoles

filled with fat• fat droplet compresses

cytoplasm around edges of cell

• organelles are squeezed to side

• insulation• slows heat loss through skin• shock absorber around

organs

Reticular Connective Tissue

consists of network of reticular fibers & cellsFound-spleen, lymph nodes & liver

Dense Connective TissueDense regular– collagen fibers regularly arranged in

parallel– forms ligaments which connect

bone to bone & tendons which connect muscle to bones

• Dense irregular – collagen fibers found in irregular

arrangements forming interwoven meshworks

– provides strength & support for areas subjected to stress from many directions

– found in skin where it gives strength to lower layer

– forms sheath around cartilages-perichondrium & bones-periosteum

– forms thick, fibrous capsule around internal organs such as liver, kidney and spleen

Elastic Connective Tissuecontains great many elastin fibers

give tissue flexibility

found-vocal cords & ligaments which connect vertebrae

Supporting Connective Tissues-Cartilage

• strong, flexible• found throughout the body• Matrix firm gel containing chondroitin sulfate which

forms complexes with proteinsproteoglycans• cells are chondrocytes • found in chambers or lacunae• avascular, blood cells do not grow into it• three types : hyaline, elastic and fibrocartilage.• Hyaline

– covers ends of long bones– matrix consists of closely packed collagen fibers

which makes it tough & flexible– found connecting ribs to sternum, nasal cartilages,

respiratory tract and as a cover in opposing bone surfaces in joints such as the knees & elbows.

• Elastic cartilage– like hyaline-more elastin fibers making it flexible and

resilient– epiglottis & ear pinna

• Fibrocartilage– looks like dense regular connective tissue– matrix dominated by collagen fibers-densely

interwoven making it durable, tough & more compressible than other cartilages

– found as intervertebral discs– menisci of knees, between pubic bones, around or

in joints and tendons– resists compressions, absorbs shocks– prevents bone to bone contact

Supporting Connective Tissues-Bone

• osseous tissue• support & protection, fat storage

and blood cell formation• small amount ofground substance• Matrix-like cartilage but more rigid

because of calcium salt-CaPO4– remainder is collagen fibers– Ca salts make tissue hard &

brittle– Collage fibers make it strong &

flexible• Bone cells are called osteocytes

– found in lacunae– organized around blood

vessels that branch through the matrix

– osteocytes communicate with each other & blood vessels by canaliculi

Fluid Connective Tissue• Blood

– contains blood cells– formed elements– RBCs– WBCs– platelets– suspended in liquid matrix-

plasma which contains protein fibers for blood clotting

• Lymph

Membranes• physical barriers composed of epithelia &

supported by connective tissue

• cover & protect other tissues

• 4 types:– Mucous– Serous– Cutaneous– Synovial

Cutaneous Membranes• cover body surface• largest membrane in

body– skin– stratified squamous

epithelium + layer of areolar connective tissue reinforced by underlying dense connective tissue

• thick, relatively water proof & usually dry

Mucus Membranes• line cavities in communication

with outside• mucosa consists of two to three

layers• an epithelium• an areolar connective tissue

layer (lamina propia) • sometimes layer of smooth

musclemuscularis mucosae• absorptive, secretory &

protective functions• help keep epithelial surfaces

moist with mucus made by goblet cells

Serous Membranes• line sealed internal parts such

as ventral body cavities• simple squamous epithelium

resting on thin layer of areolar connective tissue

• produce watery serous fluid• pleura lines pleural cavity

and covers lungs• pericardium lines pericardial

cavity covering heart• each can be divided into

parietal partlines inner surface of cavity

• and visceral part-covers outer surface of organs

Synovial Membranes• surround joint cavities• Joints-articulations for

bones• allow for movement• surrounded by fibrous

capsule consisting of areolar tissue with matrix of interwoven collagen fibers, proteoglycans & glycoproteins

• space filled with synovial fluid

Muscle Tissue• specialized for movement & contraction• 3 types: skeletal, cardiac and smooth• all contract alike but have different internal organizations• Skeletal muscles have cells called fibers

– long & thin– multinucleated often containing several hundred nuclei– striated or striped due to repeating groups of cellular proteins

actin and myosin-responsible for contraction

• cells contract when stimulated by nerves• under voluntary control • can be called striated voluntary muscle

Cardiac Muscle• found only in the heart• striated like skeletal & arranged same• uninucleate-may have 1-5-centrally

located nuclie• Cardiocyte-smaller than skeletal m. cell• connected to one another via darkened

bands between themintercalated discs

• special areas locked together by desmosomes, gap junctions & intercellular cement

• Ions move through gap junctions which coordinates contractions

• do not need nerve activity to contract• pacemaker cells establish regular rate of

contraction• not under voluntary control • striated involuntary muscle

Smooth Muscle• Cells-small, spindle shaped

with tapering ends• contain actin & myosin-not

arranged in striated fashion• cells are uninucleate• found in digestive & urinary

organs, uterus & blood vessel walls

• not under voluntary control• non-striated involuntary

Nervous Tissue• consists of neurons (nerve

cells) & neuralgia cells• specialized to detect stimuli,

respond quickly & transmit information

• each nerve cell has soma or cell body

• one long process-axon-transmits messages

• many smaller projections-dendrites-receive information

• Exocrine Gland Structure– Unicellular

e.g. Goblet cell