Tissues

Tissue Organization

Levels of structural organization

1. Molecular - organic molecules, ions, H₂O, metabolic reactions
2. Cellular - basic cellular structure & function, organelles
3. Tissues - aggregations of cells mutually cooperating to perform a group function. Composed of 1 or more cell types. Cells bound together by varying amounts of intercellular substance.
4. Organs - groups of tissues organized into functional units
5. System - groups of organs & tissues that are responsible for a major set of functions in the body
6. Organism
7. Society - groups of organisms

Basic Tissue Types

1. Epithelial - sheets of cells joined together with little intercellular space
   1. covering inner or outer free surfaces
2. Connective - cells with large amounts of intercellular space
   1. type determined by cell type, fiber type & amount of intercellular substance
3. Muscular - cells specialized for contraction, function in movement & contractility
   1. 3 types: skeletal, cardiac & smooth
4. Nervous - cells specialized for impulse conduction, integrative function

Epithelial characteristics

• Forms outer protective covering of body, glands, parts of sense organs, lines walls of body cavities, covers inner spaces & outer surfaces of organs
• All materials normally received or released by the body must pass thru epithelial layer
• Basement membrane - thin glycoprotein & fibrous layer forms boundary between epithelium & connective tissue, basal layer sits on basement membrane
• Cytoplasmic projections: short nonmotile microvilli that increase surface area of cell, apical
• Avascular

Cell shape

1. Squamous - flat
2. Cuboidal - cube or pyramidal
3. Columnar - rectangular

Cell arrangement

1. Simple - 1 layer thick, all cells touch basement membrane
   1. Pseudostratified - 1 layer with nuclei at various heights
2. Stratified - 2 or more layers, only lowest layer touches basement membrane
   1. Transitional - several layers of loosely packed cells in urinary system, cells flat when stretched & round when relaxed

Glandular epithelium

specialized for production & secretion of certain chemicals

• Endocrine glands - composed of secretory portion only
• Exocrine glands - composed of secretory portion & excretory duct
  • Classification by structure:
    • unicellular vs. multicellular
    • tubular (uniform diameter lumen) vs. alveolar (dilated secretory portion)
    • simple (1 undivided duct) vs. compound (divided duct)
  • Classification by secretion type:
    • serous - watery secretion often with enzymes
    • mucous - viscous glycoprotein
    • mixed - contains both, ex. salivary glands
  • Classification by mode of secretion:
    • merocrine - by exocytosis, ex. salivary glands, mucous glands
    • apocrine - apical cytoplasm & secretory vesicles ejected, mammary glands
    • holocrine - cell packed with vesicles explodes, sebaceous glands

Intercellular connections

specializations of cell membrane between adjacent cells

1. CAMs - (cell adhesion molecules) extend over large areas, transmembrane proteins bind membranes of adjacent cells or cell to basement membrane
2. Tight junctions - (zonula occludens) near apical surface, points of adjacent membranes fuse to form fluid-tight seal
3. Belt desmosome - (zonula adherens) band lying below z.o, thick layer of proteoglycan forms intercellular cement, condensation of microfilaments near inner membrane anchors junction to cytoskeleton, strengthen & stabilize cell shape
4. Spot desmosome - (macula adherens) spot welds, like z.a. only thinner layer of proteoglycan & intermediate filaments run cell-cell, resists stretching & twisting
5. Hemidesmosome - half desmosome, attaches cell to basement membrane
6. Gap junction - narrow gap between membranes crossed by interlocking protein channels (connexons), cell-cell communication

Connective Tissue characteristics:

• Cells & fibers embedded in ground substance
  • Ground substance - protein-polysaccharide complex (hyaluronic acid & proteoglycan), type and amount varies in different tissues
  • Fibers:
    1. Collagenous fibers - composed of tropocollagen, 1-12 microns thick, vary in length, occur in bundles, unbranched, flexible but resistant to pulling force
    2. Reticular fibers - collagen with different physical characteristics, very small branching fibers, form supporting networks for adipose tissue, liver, spleen, lymphoid, & in basement membrane
    3. Elastic fibers - composed of elastin protein, 0.2-1 microns thick, stretch but return to original length, form networks or sheets in arteries
  • Cells:
    1. Fibroblast - stellate shape, mitotic, synthesize tropocollagen, involved in healing
    2. Macrophage (histiocyte) - stellate but smaller, phagocytic
    3. Adipocyte - large central lipid droplet with thin layer cytoplasm & flattened nucleus, non-mitotic, forms from fiboblasts