Epithelial Tissue

Lecture 2 notes

• Histology - study of tissues

  • Tissue: collection of cells that performs a specific function

  • Epithelial

    • Covers exposed surfaces

    • Lines internal passageways and chambers

    • Produces glandular secretions

    • Types:

      • Epithelia

        • Secretory cells, gland cells, scattered among others

      • Glands

        • Derived from epithelia, but are mostly secretory. Also has 2 types

          • Exocrine glands

            • Secrete onto external surfaces or into internal passageways (ducts) that connect to the exterior

          • Endocrine

            • Need to travel from where they are secreted to where they are used via the bloodstream

            • Hormones or precursors secreted into interstitial fluid

    • Functions:

      • Protection - cells are strongly cross linked together to form a barrier

      • Filtration

      • Absorption

      • Secretion

      • Sensation (nerve connected to hair or skin)

    • Classification – based on number of layers

      • Apical

        • Surface to the outside

        • Simple - just one layer

        • Squamous shape - flat

      • Basal

        • Surface to which apical cells are attached

        • Stratified - has a basal surface that leads up to the apical surface

        • Cells change as they move from basal to apical

    • Classification - based on Shapes:

      • IMPORTANT: shape is determined by the cells at the apical surface because that’s where the function is carried out

      • Squamous - flattened, most number of cells in the body

      • Cuboidal - as high as it is wide - least common cell in the body

      • Columnar - taller than it is wide - second most number of cells in the body

    • Cell walls often contain gap junctions to permit chemical communication between cells in order to control regulation

      • Formed by interlocking junctional proteins (connexons)

      • Nervous signals can pass through

    • Desmosomes are attachments between cells

      • Interlock adjacent cells, are very strong, and formed by cell adhesion molecules (CAMs) and intercellular cement (thin layer of proteoglycans, specifically hyaluronan)

    • Hemidesmosomes connect cells to the basal layer

      • Made of peripheral and transmembrane proteins

      • Attached at basal lamina - deepest epithelial cells

      • Basal lamina has 2 layers

        • Clear layer contains glycoproteins and fine protein filaments

        • Dense layers contains bundles of course protein fibers and gives strength to restrict diffusion

    • Adhesion belt locks cells together at one level - typically at the same point where the blood vessels flow

    • Occluding junctions form a barrier that isolates basolateral surfaces and deeper tissues from contents of the outside world

    • Simple squamous epithelium

      • Thin and flat, irregular shape

      • Found in protected regions where absorption or diffusion takes place

        • Ex. along kidney passages, inside eye, alveoli of lung

      • Slippery surface reduces friction

        • Lining ventral body cavity (mesothelium)

        • Lining heart and blood vessels (endothelium)

      • Interconnected, forms a protected sheet

    • Stratified squamous epithelium

      • Stratified = layers

      • Where mechanical or chemical stressors are severe

      • Series of layers to make a thicker protective layer

      • Ex. skin, mouth, throat, esophagus, rectum, anus, vagina

      • Has basal layer that grows upward to become squamous

      • Function happens in the squamous layer

      • After cells become squamous and flattened, they eventually slough off

      • Two types:

        • Keritinized

          • Tough, dried out, water resistant

          • Surface of the skin

        • Nonkeritinized

          • Resists absorption, stays moist

    • Cuboidal epithelium

      • Cells that look like hexagonal boxes with nucleus in the center

      • 2 types

        • Simple cuboidal epithelium – in the lining of the exocrine glands and ducts, Potions of kidney, secretory section of thyroid gland

        • Stratified cuboidal epithelium - rare, in ducts of some exocrine glands, like excreting sodium into urine

    • Transitional epithelium

      • Most rare

      • Located in the later parts of the urinary system

      • Unusual stratified epithelium that stretches and recoils

      • Bladder - expands with urine

    • Columnar epithelium

      • Cells appear rectangular when looking at a sectional view

      • Elongated nuclei near basal lamina

      • Types:

        • Simple columnar epithelium - located in stomach, intestine, uterine tubes, kidney ducts

        • Pseudostratified columnar epithelium - varying cell shapes and funcitons, 

          • Appears stratified because nuclei are located at different areas of the cell

          • All cells attached to basal lamina

          • Cells typically have cilia

            • Different from microvilli - microvilli absorbs nutrients, cillia sweeps mucus up toward the trachea

          • Located in nasal cavities, trachea, larger airways of lungs

          • Shape is determined by the apical surface

        • Stratified columnar epithelium

          • Relatively rare

          • Two or multiple layers

            • In multiple layers, only superficial cells are column

          • Located in large ducts of salivary glands and pancreas

          • Including salivary glands - prevents bacteria from entering basal layer through thick columnar cells

      • Glands

        • Collections of epithelial cells that produce secretions

        • Can be scattered or complex organs

        • 2 types

          • Endocrine glands

            • 3 types of secretions:

              • Merocrine secretions

                • Product released from secretory vesicles via exocytosis

                • Most common

                • Mucin - merocrine secretion taht mixes with water to form mucus

                • Used as a lubricant, protective barrier, and trap for foreign particles

              • Apocrine secretion

                • Loss of apical surface and cytoplasm with secretion

                • Ex. mammary glands

              • Holocrine secretion

                • Entire cell bursts, releasing secreation and killing the cell

                • Replaced by stem cell division

          • Exocrine glands

  • Connective

    • Fills internal spaces

    • Provides structural support

    • Stores energy

  • Muscle

    • Contracts to produce movement

  • Nervous

    • Conducts electrical impulses

    • Carries information