Week 2- Epithelial Tissue Flashcards
Epithelial Tissue aka Epithelium
-Composed of one or more layers of closely packed cells
-Contains little to no extracellular matrix between these cells
-covers body surfaces, lines the body and organ cavities, and forms glands
Common Characteristics
1) Cellularity
2) Polarity
3) Attachment to basement membrane
4) Avascularity
5) Extensive innervation
6) High regeneration capacity
Cellularity
-Epithelial tissue is composed almost entirely of tightly packed cells
-There is a minimal amount of extracellular matrix between cells
Polarity
Epithelium has an apical and basal surface
Apical surface
-Exposed either to the external environment or to some internal body space
-Can have either microvilli or cilia
-Lateral surfaces may contain membrane (intercellular) junctions
Microvilli
-Small membranous projections on apical surface of the cell that increase its surface area for secretion and absorption
-Individual microvilli can’t be distinguished with light microscopy, rather they appear collectively as a brush border
Cilia
Numerous, slightly longer, membranous projections that move fluid, mucus, and materials past the cell surface
Basal surface
-a fixed or deep surface
-Epithelium is attached to a basement membrane with underlying connective tissue
Basement membrane
-Binds basal surface of epithelial layer
-Consists of 3 molecular levels that can be viewed under electron microscope (using a light microscope, it appears as a single noncellular/molecular layer)
-These molecular layers are formed by secretions of both the epithelium and the underlying connective tissue
-Composed of collagen, glycoproteins, and proteoglycans
-Molecular structures of membrane act like glue to strengthen attachment and form a selective molecular barrier between the epithelium and underlying connective tissue
Avascularity
-All epithelial tissues lack blood vessels
-Get nutrients either directly across apical surface or by diffusion across the basal surface from blood vessels within underlying connective tissue
Extensive innervation
Richly innervated (supplied with nerves) to detect changes in the environment at that body or organ region
High regeneration capacity
-Undergo cell division frequently
-Allows tissue to regenerate at high rate, a necessary condition for a tissue that is often exposed to the environment and lost by abrasion and damage
-Continual replacement occurs through cell division of the deepest epithelial cells (stem cells), which are adjacent to basement membrane
Functions of Epithelial Tissue
1) Physical protection
2) Selective permeability
3) Secretions
4) Sensations
Physical protection
Protects both external and internal surfaces from dehydration, abrasion, and destruction by physical, chemical, or biological agents
Selective permeability
-Exhibits range of permeabilty
-May be relatively non-permeable to some substances, while promoting/assisting passage of other ions/molecules
-All substances entering and leaving the body must pass through epithelium
-Acts as body’s gatekeepers
Secretions
-Some are specialized to produce and release secretion
-These cells form glands
-Glands may be individual cells scattered among other cell types in an epithelium (ex: goblet cells) or arranged in small, organized clusters within a multicellular gland
Sensations
-Innervated by sensory nerve endings to detect or respond to a stimulus
-These nerve endings and those in underlying connective tissue continuously relay sensory input to CNS concerning touch, pressure, temp, and pain
What are the two things used to classify epithelial tissue types?
1) Number of cell layers
2) Shape of cells at apical surface
Simple epithelium
-One layer of cells
-All cells are in direct contact with basement membrane
-Found in areas where stress in minimal and secretion, absorption, or filtrations is primary function
Stratified epithelium
-Contains 2 or more layers of cells
-Only cells in basal (deepest) layer are in contact with basement membrane
-Provides either more structural support or better protection for underlying tissue
-Cells in basal layer are continuously regenerated as cells in the apical layer are lost due to abrasion or stress
Pseudostratifed epithelium
-Appears layered because cells’ nuclei are distributed at different levels between apical and basal surfaces
-All cells are attached to basement membrane, but some of them do not reach its apical surface
Squamous Cells
-Flat, wide, and somewhat irregular in shape
-Cells are arranged like floor tiles, and nucleus is somewhat flattened
Cuboidal Cells
-About as tall as they are wide
-Don’t resemble perfect cubes because edges are somewhat rounded
-Cells nucleus is spherical and located in center of cell
Columnar cells
-Slender and taller than they are wide
-Nucleus is oval and usually oriented lengthwise and in the basal region