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Learning Outline


This is a brief review of the concept of tissues in the context of human biology. You will not be expected to be able to identify them by sight in the onlines tests or exam. That will come later, in your A&P lab course.


  • Review Levels of Organization
  • Tissues are the “fabric” that make up organs
  • Four main types of tissues: epithelial, connective, muscle, nervousbook-icon-lionden-fw

Extracellular matrix (ECM)

Complex, nonliving material filling the spaces between cells

  • Some tissues (epithelial) has little ECM and a lot of cells; some tissues (connective) have a lot of of ECM and few cells

Made up of

  • Water
  • Proteins
  • Glycoproteins – part carbohydrate, part protein
  • Proteoglycans – protein backbone with carbohydrates attached


  • Help hold tissues together
  • Communication between ECM and cell or among cells
Extracellular matrix

Extracellular matrix (ECM)
Immunofluorescent staining of fibroblast cells ( their nuclei are greenish here) reveals the extracellular matrix (bluish and turquoise areas)

Epithelial tissues book-icon-lionden-fw


  • Cover and line (form membranes)
  • Secrete (form glands) – “secretory tissue” slide_icon
    • Endocrine glands (ductless glands) secrete hormones into blood
    • Exocrine glands (ducted glands) secrete fluids onto surface (incl. into tubes)


  • Cellular junctions
    • Cohesion—sticking together
    • Review of junctional complexes
      • Desmosomes – “tangled filaments” hold cells together as in Velcro fasteners
      • Gap junctions – channels in adjacent plasma membranes form “tunnels” that hold cells together structurally and functionally
        • Syncytium – group of cells held together by gap junctions and acting (in some ways) as a single, giant cell
        • These occur mainly in cardiac muscle and nervous tissue—but are discussed here for the sake of convenience
      • Tight junctions – rows of connecting proteins, like “snaps” on a jacket form a collar-like seal all the way around a cell (as the plastic yoke on a six-pack)
tight junction
  • Basal surface of each cell is anchored; apical surface of each cell is is “free”
    • Basal layer attached to underlying connective tissue by glue-like basement membrane
  • Epithelial types are named for
    • Number of layers
      • Simple = one layer of cells
      • Stratified = more than one layer of cells (from stratum = layer)
      • Pseudostratified = looks like more than one layer, but because all cells touch the basement membrane it’s really just one layer
    • Shape of cells in outermost (“free surface”) layer (as seen in cross-section)
      • Squamous = flat, scale-like cells
      • Cuboidal = about as tall as wide
      • Columnar = taller than wide
      • Transitional = shape depends on how stretched the membrane is
  • Examples of epithelial tissue types
    • Simple squamous
    • Simple cuboidal
    • Simple columnar
    • Pseudostratified columnar
    • Stratified squamous
      • Keratinized (has outer layers of dead cells that have filled with tough, waterproof protein called keratin—as in skin)
      • Nonkeratinized
    • Transitional (stratified
epithelial categories

Connective tissues book-icon-lionden-fw


  • Connects body parts in any of several ways
  • Many different functions, really


  • Dominated by extracellular material (“extracellular matrix” or ECM)—with few, scattered cells
  • Matrix contains fibers (made by fibroblast cells) and other materials
    • Elastic fibers – made of elastin and stretch easily, then recoil (also called yellow fibers)
    • Collagen fibers – made of collagen and do not stretch (much) but are strong and flexible (also called white fibers or collagenous fibers)

Major types of connective tissues

  • Fibrous connective tissues
    • There are several ways to classify fibrous connective tissues—most commonly, they are categorized by the structure of the matrix (type and arrangement of fibers)
    • Loose (areolar) connective tissue has collagen and elastin fibers scattered loosely
    • Adipose tissue forms when fat-storing cells in areolar tissue enlarge as they accumulate more [triglyceride] fat
    • Dense fibrous connective tissue has a dense arrangement of collagen fiber bundles
      • Regular – has rougly parallel bundles of collagen fibers
      • Irregular – has hodgepodge, irregular arrangement of collagen fibers
    • Reticular tissue is a netlike meshwork of fine collagen fibers that helps hold tissues in place and sometimes helps to filter particles from fluids passing through it
3 connective types

Three types of fibrous connective tissue.
The third image is regular dense fibrous connective tissue.


  • Chrondrocytes make cartilage matrix
    • Found within spaces called lacunae (sing. lacuna = “lake”) giving cartilage a “Swiss cheese” appearance
  • Hyaline cartilage – some collagen in matrix
  • Elastic cartilage – some collagen and some elastin in matrix
  • Fibrocartilage – dense collagen in matrix
swiss cheese

Swiss cheese.
A model for cartilage. Compare this image to that of hyaline cartilage (below).

  • Bone
    • Osteocytes (within lacunae) surrounded by collagen fibers encrusted with calcium salts
    • Spongy bone – irregular beams of bone surrounded by red bone marrow (soft, blood-forming tissue)
    • Compact bone – denser type of bone made up of cylindrical units composed of concentric layers (lamellae) of bone matrix
  • Blood
    • Liquid matrix (blood plasma) and circulating blood cells
    • Blood cells are also called “formed elements” because plasma is “unformed” (taking the shape of its container)
    • RBCs = red blood cells, WBCs = white blood cells, platelets = thrombocytes
blood, bone, and cartilage

Blood, bone and cartilage
Bone is sometimes called “osseous” which means “bony” (compact bone is shown)

Muscle tissue book-icon-lionden-fw


  • Contraction
  • May be “voluntary” or “involuntary”


  • Cylindrical cells or “muscle fibers”
  • Muscle fibers have highly organized cytoskeleton that “slides together” like a split deck of cards to contract the fiber

Three types: skeletal, smooth, cardiac

  • Skeletal muscle
    • Also called “striated” because of striped appearance of overlapping filaments of cytoskeleton
    • Voluntary muscle
    • Connected to skeleton
  • Cardiac muscle
    • Faintly striated
    • Involuntary
    • In heart wall
    • Branched fibers held together end-to-end by gap junctions, forming intercalated disks that connect cardiac fibers into a network
      • Forms a syncytium, so that a region of the heart wall will contract at one time, as if one giant cell
  • Smooth muscle
    • Not striated (cytoskeleton organized differently)
    • Involuntary
    • In walls of hollow organs (except heart)
3 types of muscle tissue

Muscle tissue types.

Nervous tissue book-icon-lionden-fw


  • Conducting cells
  • Connected at synapses that act as switches, allowing information storage and processing

Glia (also called neuroglia)

  • Support cells (functional and structural support)
    • Involved in modulating neuron function
brain tissue

Brain tissue.

Click the image to see a larger view, where a mixture of neurons and glia can be seen.

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Last updated: March 27, 2019 at 21:02 pm