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Tissues Organs Systems

Learning Outline

Tissues, Organs, and SystemsA&P 1

This outline is for your own review of tissues (in the context of organs and systems) and is provided as a brief overview of tissues.

Although you will learn this material in the lab course
(and again, bit by bit, in the lecture course),
I will not test you specifically on this material now.


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

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
  • Proteoglycans


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

Extracellular matrix
Extracellular matrix (ECM)
Immunofluorescent staining of fibroblast cells reveals the extracellular matrix (bluish and turquoise areas)

Epithelial tissues


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


  • Cellular junctions
    • Cohesion
    • 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
      • 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)
  • Basal surface is anchored; apical surface is “free”
    • Basal layer attached to underlying connective tissue by glue-like basement membrane (BM)
  • 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


  • 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.

  • Cartilage
    • 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); also called cancellous bone
    • 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


  • 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
    • Branched fibers held together end-to-end by gap junctions, forming intercalated disks
    • Involuntary
    • In heart wall
  • 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


  • 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.

lion trackClick here to explore a comprehensive visual review of tissues

Organs and systems

leonardo's "organs of a woman"Organs

Organs are made up of different tissues

Organs are organized into different body systems

Organs may belong to more than one system
Different biologists organize them differently

  • human nervous systemExample
    • Nervous system could be subdivided
      • Central nervous system (brain and spinal cord)
      • Peripheral nervous system (nerves)
    • Nervous system could be joined to other systems
      • Neuroendocrine system (nervous and endocrine systems)
      • Neuroskeletomuscular system (nervous, skeletal, and muscular systems)

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Last updated: October 22, 2019 at 13:32 pm