HISTOLOGYBy: Annabel Lapuz-Carungin, MD, RMT, PT, RN, RM, MAN Study of tissues

Tissues are collections of similar cells and the extracellular matrix surrounding them

The four primary tissue types are1. Epithelial tissue2. Connective tissue3. Muscle tissue4. Nervous tissue

Embryonic TissuePrimary tissue types are derived from the embryonic germ layersEndodermForms the lining of the digestive tract and its derivativesMesodermForms tissues such as muscle, bone, and blood vesselsEctodermForms the outermost layer of skin and the nervous systemGives rise to all tissues of the body

Epithelial TissueEpithelial tissue Covers surfacesHas little extracellular materialUsually has a basement membraneHas no blood vesselsEpithelial cellsHave a free, or apical, surface (not attached to other cells)A lateral surface (attached to other cells)A basal surface (attached to the basement membrane)

Fig. 4.1

Epithelial Tissue Functions1. Protecting underlying structuresExample: outer layer of skin and oral mucosa2. Acting as barriersExample: outer layer of skin3. Permitting the passage of substancesExample: epithelium in the lungs4. Secreting substancesExample: mucous glands and sweat glands5. Absorbing substancesExample: epithelial cells of the intestine

Epithelial Tissue ClassificationEpithelia are classified according to the number of cell layers and the shape of the cellsCell layersSimple epithelium has one layer of cellsStratified epithelium has more than one cell layerPseudostratified epithelium has one layer, but appears to have two or more layersTransitional epithelium is stratified epithelium that can be greatly stretchedCell shapeSquamous are flat and thinCuboidal are cubelikeColumnar are tall and thin

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Epithelial Tissue Structural and Functional RelationshipsCell Layers and Cell ShapesSimple epithelium is involved with DiffusionSecretionAbsorptionStratified epithelium serves a protective roleSquamous cells function inDiffusionFiltrationCuboidal or columnar cells, which contain cellular organellesSecreteAbsorb

Structural and Functional RelationshipsFree Cell SurfacesA smooth, free surface reduces frictionExample: Lining of blood vesselsMicrovilli are cylindrical extensions of the cell membrane that increase surface area (cells involved in absorption of secretion)Example: Lining of the small intestinesCilia propel materials over the cell surfaceExample: Lining of the nasal cavity and tracheaEpithelial Tissue Structural and Functional Relationships

Cell connectionsTight junctions bind adjacent cells together and form a permeability barrierDesmosomes mechanically bind cells togetherHemidesmosomes mechanically bind cells to the basement membraneGap junctions allow intercellular communicationFig. 4.2Epithelial Tissue Structural and Functional Relationships

Epithelial Tissue GlandsA gland is a single cell or a multicellular structure that secretesEndocrine glands do not have ductsSecrete hormones directly into the bloodExocrine glands have ductsSecretions are released onto a surface or into a cavitySweat glands and mammary glandsClassified byStructureHow products leave the cell

Epithelial Tissue GlandsFig. 4.3Structure of Exocrine GlandsSimple: have one ductCompound: have ducts that branch repeatedlyTubules: ducts end in small tubesAcini: ducts end in saclike structuresAlveoli: ducts end in hollow sacs

Epithelial Tissue GlandsFig. 4.4Exocrine Glands and Secretion TypesMerocrine no loss of cellular material (Ex. sweat glands)Apocrine part of the cell pinches off (Ex. mammary glands)Holocrine entire cell is shed (Ex. sebaceous glands)

Connective TissueConsist of cells seperated from each other by abundant extracellular matrixFunctionsEnclosing and separatingConnecting tissues to one another (Ex. Ligaments and Tendons)Supporting and moving (Ex. Bones and cartilage)Storing (Ex. Adipose tissue and Bones)Cushioning and insulating (Ex. Adipose tissue)Transporting (Ex. Blood)Protecting (Ex. Blood and Bones)

Connective Tissue CellsSpecialized cells of various connective tissues produce the extra cellular matrixEnd in suffixes that identify the cell functionsBlast (germ) cells form the matrixCyte (cell) cells maintain itClast (break) cells break it down

Connective TissueExtracellular matrix containsProtein fibersCollagen fibers are flexible but resist stretchingReticular fibers form a fiber networkElastic fibers recoilGround substanceProteoglycans in ground substance hold water, enabling connective tissues to return to their original shape after being compressedFluid

Connective Tissue ClassificationMesenchyme:embryonic connective tissue that gives rise to six major categories of connective tissue

Connective Tissue ClassificationLoose, or areolar, connective tissue is the loose packing material of the bodyFills the spaces between organsHolds organs in placeAdipose tissue (fat)Stores energyPads and protects parts of the bodyActs as a thermal insulatorDense connective tissue consists of a matrix containing densely packed fibersCollagen fibers (Ex. tendons, ligaments, and dermis of the skin)Elastic fibers (Ex. elastic ligaments and in the walls of arteries)

Connective Tissue ClassificationCartilage provides supportHyaline cartilage (Ex. covers ends of bones and forms costal cartilages)Fibrocartilage (Ex. disks between vertebrae)Elastic cartilage (Ex. external ear)Bone has a mineralized matrixforms most of the skeleton of the bodyCompact bone has more matrix than spacesCancellous bone has more spaces then matrixBlood has a liquid matrixFound in blood vesselsProduced in hemopoietic tissue (red bone marrow)

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Muscle TissueSpecialized to contract, or shorten, making movement possibleLength of muscle cells is greater than the diameterSometimes called muscle fibers because they often resemble tiny threadsThree types of muscle tissueSkeletalMeat of animalsConstitutes ~40% of a persons body weightAttaches to the skeleton and allows for movementVoluntary, multinucleated, and striated (banded)CardiacMuscle of the heartConnected to one another by intercalated disks (contain gap junctions)Involuntary, striated, and usually have one nucleus per cellSmoothForms the walls of hollow organs (except the heart) and also is found in the skin and the eyesInvoluntary, not striated, and have a single nucleus

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Nervous TissueNervous tissue is specialized to conduct action potentials (electrical signals)Neurons conduct action potentialsNeuroglia support the neurons

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MembranesMucous membranes line cavities that open to the outside of the bodyDigestive, respiratory, urinary, and reproductive tractsContain glandsSecrete mucusSerous membranes line trunk cavities that do not open to the outside of the bodypleural, pericardial, and peritoneal cavitiesDo Not contain glandsSecrete serous fluidSynovial membranes line freely movable joints

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