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HistologyHistology

Tissues

Cells work together in functionally related groups called tissuesTypes of tissues:

1. Epithelial – lining and covering2. Connective – support 3. Muscle – movement4. Nervous – control

Epithelial Tissue –General Characteristics & Functions

Covers a body surface or lines a body cavity Forms most glandsFunctions of epithelium

ProtectionAbsorption, secretion, and ion transportFiltrationForms slippery surfaces

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Special Characteristics of Epithelia

Cellularitycells are in close contact with each other with little or no intercellular space between them

Specialized contactsmay have junctions for both attachment and communication

Polarityepithelial tissues always have an apical and basal surface

Support by connective tissue at the basal surface, both the epithelial tissue and the connective tissue contribute to the basement membrane

Avascularnutrients must diffuse

InnervatedRegeneration

epithelial tissues have a high capacity for regeneration

Special Characteristics of Epithelia

Lateral Surface Features

Factors holding epithelial cells togetherAdhesion proteins link plasma membranes of adjacent cellsContours of adjacent cell membranes Special cell junctions

Tight JunctionsAdherens JunctionsDesmosomes

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Lateral Surface Features – Cell Junctions

Tight junctions (zona occludens) – close off intercellular space

Found at apical region of most epithelial typesSome proteins in plasma membrane of adjacent cells are fusedPrevent molecules from passing between cells of epithelial tissue

Tight Junction

Lateral Surface Features – Cell Junctions

Adherens junctions (zonula adherens) –anchoring junction

Transmembrane linker proteins attach to actin microfilaments of the cytoskeleton and bind adjacent cellsAlong with tight junctions, form the tight junctional complex around apical lateral borders of epithelial tissues

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Lateral Surface Features – Cell Junctions

Desmosomes – two disc-like plaques connected across intercellular space

Plaques of adjoining cells are joined by proteins called cadherinsProteins interdigitate into extracellular spaceIntermediate filaments insert into plaques from cytoplasmic side

Desmosome

Lateral Surface Features – Cell Junctions

Gap junctions – passageway between two adjacent cells

Let small molecules move directly between neighboring cellsCells are connected by hollow cylinders of protein

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Gap Junction

Basal Feature: The Basal LaminaNoncellular supporting sheet between the epithelium and the connective tissue deep to it Consists of proteins secreted by the epithelial cells Functions:

Acts as a selective filter, determining which molecules from capillaries enter the epithelium Acts as scaffolding along which regenerating epithelial cells can migrate

Basal lamina and reticular layers of the underlying connective tissue deep to it form the basement membrane

Epithelial Tissues

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First name of tissue indicates number of layers

Simple – one layer of cells

Stratified – more than one layer of cells

Classifications & Naming of Epithelia

Classification & Naming of EpitheliaLast name of tissue describes shape of cells

Squamous – cells wider than tall (plate or “scale” like)

Cuboidal – cells are as wide as tall, as in cubes

Columnar – cells are taller thanthey are wide, like columns

Naming EpitheliaNaming the epithelia includes both the layers (first) and the shape of the cells (second)

i.e. stratified cuboidal epitheliumThe name may also include any accessory structures

Goblet cellsCiliaKeratin

Special epithelial tissues (don’t follow naming convention)

PsuedostratifiedTransitional

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Simple Squamous Epithelium

Description single layer of flat cells with disc-shaped nuclei

Special types Endothelium (inner covering)

slick lining of hollow organsMesothelium (middle covering)

Lines peritoneal, pleural, and pericardial cavities Covers visceral organs of those cavities

Simple Squamous Epithelium

Function Passage of materials by passive diffusion and filtrationSecretes lubricating substances in serosae

Location Renal corpusclesAlveoli of lungs Lining of heart, blood and lymphatic vesselsLining of ventral body cavity (serosae)

Simple Squamous Epithelium

Simple squamous lining the walls of

the capillary

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Simple Cuboidal Epithelium

Descriptionsingle layer of cube-like cells with large, spherical central nuclei

Function secretion and absorption

Location kidney tubules, secretory portions of small glands, ovary & thyroid follicles

Simple Columnar Epithelium

Description single layer of column-shaped (rectangular) cells with oval nuclei

Some bear cilia at their apical surfaceMay contain goblet cells

Function Absorption; secretion of mucus, enzymes, and other substancesCiliated type propels mucus or reproductive cells by ciliary action

Simple Columnar Epithelium

Location Non-ciliated form

Lines digestive tract, gallbladder, ducts of some glands

Ciliated form Lines small bronchi, uterine tubes, uterus

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Pseudostratified Columnar Epithelium

DescriptionAll cells originate at basement membraneOnly tall cells reach the apical surfaceMay contain goblet cells and bear ciliaNuclei lie at varying heights within cells

Gives false impression of stratification

Function secretion of mucus; propulsion of mucus by cilia

Pseudostratified Columnar Epithelium

LocationsNon-ciliated type

Ducts of male reproductive tubes Ducts of large glands

Ciliated variety Lines trachea and most of upper respiratory tract

Stratified Epithelia

Contain two or more layers of cellsRegenerate from belowMajor role is protectionAre named according to the shape of cells at apical layer

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Stratified Squamous Epithelium

DescriptionMany layers of cells – squamous in shapeDeeper layers of cells appear cuboidal or columnar Thickest epithelial tissue – adapted for protection

Stratified Squamous EpitheliumSpecific types

Keratinized – contain the protective protein keratinSurface cells are dead and full of keratin

Non-keratinized – forms moist lining of body openingsFunction

Protects underlying tissues in areas subject to abrasion

Location Keratinized – forms epidermisNon-keratinized – forms lining of esophagus, mouth, and vagina

Transitional EpitheliumDescription

Basal cells usually cuboidal or columnarSuperficial cells dome-shaped or squamous

Functionstretches and permits distension of urinary bladder

Location Lines ureters, urinary bladder and part of urethra

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Glandular EpitheliumDucts carry products of exocrine glands to epithelial surfaceInclude the following diverse glands

Mucus-secreting glands Sweat and oil glandsSalivary glandsLiver and pancreasMammary glands

May be: unicellular or multicellular

Unicellular Exocrine Glands (The Goblet Cell)

Goblet cells produce mucin Mucin + water mucusProtects and lubricates many internal body surfaces

Multicellular Exocrine GlandsClassified by structure (branching & shape) of ductCan also be classified by mode or type of secretion

Merocrine secretion – secretory vesicles released via exocytosis (saliviary glands)Apocrine secretion – apical portion of the cell is lost, cytoplasm + secretory product (mammary glands)Holocrine secretion – entire cell is destroyed during secretion (sebaceous gland)

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May also be classified by types of secretions from exocrine glands

Serousmostly water but also contains some enzymesEx. parotid glands, pancreas

Mucousmucus secretionsEx. sublingual glands, goblet cells

Mixesserous & mucus combinedEx. submandibular gland

Connective Tissues

Connective Tissue

Most diverse and abundant tissueMain classes

Connective tissue properBlood – Fluid connective tissueCartilageBone tissue

Components of connective tissue:Cells (varies according to tissue)Matrix

Protein fibers (varies according to tissue)Ground substance (varies according to tissue)

Common embryonic origin – mesenchyme

Supporting connective tissues

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Classes of Connective Tissue

Connective Tissue Proper - StructuresVariety of cells, fibers & grounds substances

Types of depend on useCells found in connective tissue proper

FibroblastsMacrophages, lymphocytes (antibody producing cells)Adipocytes (fat cells)Mast cellsStem cells

Fibers:Collagen – very strong & abundant, long & straightElastic – branching fibers with a wavy appearance (when relaxed)Reticular – form a network of fibers that form a supportive framwork in soft organs (i.e. Spleen & liver)

Ground substance:Along with fibers, fills the extracellular spaceGround substance helps determine functionality of tissue

Connective Tissue Proper -Classifications

Loose Connective TissueAreolarReticularAdipose

Dense Connective TissueRegularIrregularElastic

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Areolar Connective TissueDescription

Gel-like matrix with:all three fiber types (collagen, reticular, elastic) for supportGround substance is made up by glycoproteins also made and secreted by the fibroblasts.

Cells – fibroblasts, macrophages, mast cells, white blood cells, adipocytesHighly vascular tissue

Function Wraps and cushions organsHolds and conveys tissue fluidImportant role in inflammation Main battlefield in fight against infection

Areolar Connective Tissue

Location Widely distributed under epitheliaPackages organsSurrounds capillaries

Adipose TissueDescription

Closely packed adipocytes Have nucleus pushed to one side by fat droplet FunctionProvides reserve food fuelInsulates against heat lossSupports and protects organs

LocationUnder skin Around kidneys Behind eyeballs, within abdomen and in breasts

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Reticular Connective Tissue

Description – network of reticular fibers in loose ground substanceFunction – form a soft, internal skeleton (stroma) – supports other cell typesLocation – lymphoid organs

Lymph nodes, bone marrow, and spleen

Dense Irregular Connective TissueDescription

Primarily irregularly arranged collagen fibersSome elastic fibers and fibroblasts

Function Withstands tensionProvides structural strength

LocationDermis of skinSubmucosa of digestive tractFibrous capsules of joints and organs

Dense Regular Connective TissueDescription

Primarily parallel collagen fibersFibroblasts and some elastic fibersPoorly vascularized

FunctionAttaches muscle to boneAttaches bone to boneWithstands great stress in one direction

LocationTendons and ligamentsAponeurosesFascia around muscles

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CartilageCharacteristics:

Firm, flexible tissueContains no blood vessels or nervesMatrix contains up to 80% waterCell type – chondrocyte

Types:HyalineElasticFibrocartilage

Hyaline CartilageDescription

Imperceptible collagen fibers (hyaline = glassy)Chodroblasts produce matrixChondrocytes lie in lacunae

FunctionSupports and reinforcesResilient cushionResists repetitive stress

LocationEnds of long bonesCostal cartilage of ribsCartilages of nose, trachea, and larynx Location

Elastic Cartilage

DescriptionSimilar to hyaline cartilageMore elastic fibers in matrix

Function Maintains shape of structureAllows great flexibility

LocationSupports external earEpiglottis

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FibrocartilageDescription

Matrix similar, but less firm than hyaline cartilageThick collagen fibers predominate

FunctionTensile strength and ability to absorb compressive shock

LocationIntervertebral discsPubic symphysisDiscs of knee joint

Bone TissueFunction

Supports and protects organsProvides levers and attachment site for musclesStores calcium and other mineralsStores fatMarrow is site for blood cell formation

Location Bones

Blood TissueDescription

red and white blood cells in a fluid matrix

Functiontransport of respiratory gases, nutrients, and wastes

Locationwithin blood vessels

CharacteristicsAn atypical connective tissueConsists of cells surrounded by fluid matrix

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Covering and Lining Membranes

Combine epithelial tissues and connective tissuesCover broad areas within bodyConsist of epithelial sheet plus underlying connective tissue

Types of Membranes

Cutaneous membrane – skin Mucous membrane

Lines hollow organs that open to surface of bodyAn epithelial sheet underlain with layer of lamina propria

Serous membrane – slippery membranesSimple squamous epithelium lying on areolar connective tissueLine closed cavities

Pleural, peritoneal, and pericardial cavities

Synovial membranes – lining joint cavitiesLoose connective (areolar) + simple squamous epitheliumSecretes fluid (synovial fluid) which lubricates, protects & cushions joint structures

Muscle Tissue

TypesSkeletal muscle tissueCardiac muscle tissue Smooth muscle tissue

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Skeletal Muscle Tissue

CharacteristicsLong, cylindrical cellsMultinucleateObvious striations

Function Voluntary movementManipulation of environmentFacial expression

LocationSkeletal muscles attached to bones (occasionally to skin)

Cardiac Muscle Tissue

FunctionContracts to propel blood into circulatory system

CharacteristicsBranching cellsUni-nucleateIntercalated discs

LocationOccurs in walls of heart

Smooth Muscle TissueCharacteristics

Spindle-shaped cells withcentral nucleiArranged closely to form sheetsNo striations

FunctionPropels substances along internal passagewaysInvoluntary control

LocationMostly walls of hollow organs

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Nervous TissueNervous Tissue

Nervous Tissue

FunctionTransmit electrical signals from sensory receptors to effectors

LocationBrain, spinal cord, and nerves

DescriptionMain components are brain, spinal cord, and nervesContains two types of cells

Neurons – excitatory cellsSupporting cells (neuroglial cells)

Tissue Response to Injury

Restoration involvesInflammationRegeneration (repair)

InflammationDue to something that damages/kills cells or fibers or in some way damage tissue, causing . . .

SwellingWarmthRednessPain

These common conditions are a result of mast cell activation – releases vasodilators such as histamine

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Tissue Response to Injury

Goal:Restore normal function to tissue

Process:Fibroblasts activated to produce fibrous tissueUsually remodeled over time

ChallengesSome tissues are non-vascular and will repair very slowlyIf excitable tissue is replaced by scar tissue – function is lost!

The Tissues Throughout LifeEarly on – Gastrulation

The most important time in your life!!This is when tissues differentiate – mess up here and you don’t develop correctly

At the end of second month of development:Primary tissue types have appearedMajor organs are in place

AdulthoodOnly a few tissues regenerateMany tissues still retain populations of stem cells

With increasing age:Epithelia thin Collagen decreasesBones, muscles, and nervous tissue begin to atrophyPoor nutrition and poor circulation – poor health of tissuesIncreased chance of developing cancer