Epithelial Tissue

Composed of one or more layers of closely packed cells

Covers body surfaces, lines body cavities, forms majority of glands

Cellularity

    Composed almost entirely of tightly packed cells

Polarity

    Apical surface

        Exposed to environment of internal body space

    Basal surface

        Attachment to connective tissue at basement membrane

Avascularity

        Lacks blood vessels

        Nutrients absorbed across exposed surface or internal body space

Extensive Innervation

    Richly innervated; detect changes in environment

High regeneration capacity

    Continual replacement of lost cells

Physical protection

    Protects from dehydration, abrasion, destruction

Selective permeability

Secretions

    Some cells are specialized to secrete

Sensations

    Supplies info to the nervous system

Classifications

    Determined by number of cell layers

    Simple epithelium

        One layer thick, all cells contact basement membrane

        Filtration, absorption, or secretion is primary function

    Stratified epithelium

        Two or more layers

        Only basal layer is in contact with basement membrane

        In areas subjected to mechanical stress

    Pseudostratified epithelium

        Appears layered

        All cells contact basement membrane, but may not reach apical surface

Classification by Shape

    Squamous

        Flat, wide, irregular

        Nucleus flat

    Cuboidal

        About as tall as they are wide

        Nucleus spherical and in center

    Columnar

        Slender and taller than they are wide

        Nucleus oval, oriented lengthwise in basal region

    Transitional

        Can change shape, depending on stretch of epithelium

Classified based on apical surface layer

Simple Squamous Epithelium

    Single layer of flat cells

    Spherical to oval nucleus

    Thinnest barrier

    Allows rapid movement of molecules across surface

    Lines air sacs of lungs, vessel walls, serous membranes

Simple Cuboidal

    Single layer of uniformly shaped cells

Simple Columnar

    Single layer of cells

Glands

    Individual cells or multicellular organs composed of epithelial tissue

    Endocrine or exocrine

Endocrine

    Lack ducts

    Secrete hormones into blood

Exocrine Glands

    Invaginated epithelium in connective tissue

    Connected to epithelial surface by duct

        Epithelium-lined tube for gland secretion

    Unicellular Exocrine

        Do not contain duct

        Located close to epithelium surface

        Goblet cell is most common type

    Multicellular Exocrine Glands

        Numerous cells

        Acini - cell clusters that produce secretions

        Ducts transport secretions to epithelial surface

        Surrounded by fibrous capsule

    Exocrine Gland Classifications by Anatomic Form

        Simple - single unbranched duct

        Compound - branched ducts

        Tubular - secretory portion and duct same diameter

        Acinar - secretory portion forms expanded sac

        Tubuloacinar - both tubules and acini

    Exocrine Gland Classification by Secretion Method

        Merocrine

            Package secretions into vesicles, released by exocytosis

            Lacrimal (tear) or salivary glands

        Apocrine

            Apical membrane pinches off and becomes secretion

            Mammary or ceruminous glands

        Holocrine

            Ruptured cell becomes secretion

            Sebacious (oil) glands