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Connective Tissue
Model 1: Connective Tissue Constituents—example: loose connective tissue
Legend
basement
membrane
basement
membrane
Rule 1: Between every epithelium and a connective tissue there is a ‘basement’ membrane.
– Frank N. Low, 1953
A
B
C
From [L]: A, Photomicrograph of loose areolar CT; B, elastin fibers; C, reticular fiber network corresponding to the outside of a capillary.
(Figures A from On-line Lab Manual; Figures B and C from: T Ushiki: Arch. Histol. Cytol., 65:109-126, 2002.)
Take about 5 minutes to read the background reading on the next page. Then answer the questions
that follow the reading. Use the pictures above to aid in forming your answers.
CONNECTIVE TISSUE — background reading
Connective tissue comprises cells and extracellular matrix, just as in the other tissue types. In fact, connective
tissues harbor what is arguably the widest variety of forms and functions of cells. Nonetheless, it isn't cells that
we focus on when we turn our attention to the connective tissues. In connective tissues, the ratio of cells to
extracellular matrix (ECM) is lower than in other tissue types, which makes the ECM of singular importance to
connective tissues. Also noteworthy is the fact that unlike in muscle, nerves, and epithelia, the connective
tissue ECM is highly variable in both appearance and activity; this in turn reflects the wide varieties of
functions that connective tissues play in our bodies.
Cells:
principal connective tissue cells
fibroblasts
adipocytes
wandering blood cells
macrophages
plasma cells
mast cells
specialized CT cells
platelets
osteoblasts
1) Some connective tissue cells are specific to connective tissues. Thus, when you “see them” in a tissue
sample, it usually means you’re looking at a connective tissue sample. These include fibroblasts and
adipocytes.
2) Other connective tissue cells are found only in a specific form of connective tissue (e.g., osteocytes,
platelets); in these specialized forms of connective tissue, these highly differentiated cells have very distinct
functions.
3) Some connective tissue cells have migrated out of the vascular system into the “connective tissue space.”
These cell types typically have immune or defense functions, and include macrophages, neutrophils, and
plasma cells.
Extracellular matrix, in turn, is made up of two principal ingredients: fibers, and ground substance.
Fibers pretty much explain themselves — linear proteins are secreted by the cells and assembled by
extracellular enzymes, and then wind themselves into helical strands. This is very similar to how a climbing
rope is wound together from thousands of strands of smaller pieces of nylon. Extracellular fibers generally
come in three “flavors” — collagen fibers, elastic fibers, and reticular fibers (listed here in descending order of
size).
Ground substance is an admittedly dorky sounding name. The name reflects the fact that whenever early
scientists looked at connective tissues in the microscope, they could easily identify the cells and fibers in
whatever sample was there. Unlike the cells and fibers, it was hard to assign a potential function to the stuff
that was “just there” in the background. Thus: “ground substance.” In the past two decades, a much better
understanding of the composition and function of ground substance has emerged. We now know that what
was once thought of as inert and “in the background” is, in fact, a critical protein/carbohydrate lattice. This
lattice, or matrix, serves as a fluid reservoir (helps to store H2O) and is especially suited to permit cell
infiltration and migration, as in inflammation, wound healing, and developmental processes. Unlike the case of
fibers, the proteins in ground substance are:
1) either glycoproteins (Gr., glykos, sugar + protein) that signal from the matrix to the inside of the cell and
regulate attachment and migration of cells to and through the matrix, or
2) they are large, complex mixtures of proteins and sugars called proteoglycans, which form a repeating
pattern that organizes all of the other connective tissue components (e.g., fibers, adhesive glycoproteins, and
signaling proteins) into a weave or pattern, not unlike the way a carpet or rug has a noticeable pattern.
Questions
1. Place the fibers of the connective tissue in order from smallest to largest diameter.
a. reticular fibers
b. elastin fibers
c. collagen fibers
2. The ground substance has two important functions. What are they?
1) stores H2O, 2) permits cell migration & invasion.
3. Cells such as neutrophils, mast cells, and plasma cells are often seen in the connective tissue
space of, say, the dermis —where did they come from?
Neutrophils, mast cells and plasma cells are bone-marrow derived, and normally
circulate in the blood— in response to injury or infection, they migrate out of the
body to attack foreign materials or aid in the repair of damaged tissues.
4. Place the tissue types in order from MOST to LEAST extracellular matrix:
A
a. connective
D
b. epithelial
C
c. muscle
B
d. nervous
5. Look at the names of the cells that are normally are found in loose connective tissue —these
names should help you define their function. Using your experience with anatomical
terminology, fill out the table below.
Cell
Fibroblast
synthesis & maintenance of ECM fibers & ground substance
Function
Adipocyte
triglyceride (fat) “synthesis” and storage
Macrophage
lit. “big eater”
6. On Model 1, picture 1C consists of reticular fibers surrounding an “organ” — what is that
“organ”?
A capillary
7. Look at your answers to questions 2 and 3. What if foreign, “non-
self” cells (or your damaged, “self” ones) could not migrate/traverse
through your “connective tissue space” as effectively as your body’s
own healthy cells? How might this space be utilized to your
advantage following:
a. a cut/abrasion caused from a slide on a softball infield?
b. a respiratory infection caused by airborne bacteria?
the picture of a moat is used to suggest that by trapping invaders in the “watery”
tissue space, the body’s immune cells can attack them literally like fish in a barrel or
hapless English knights trapped in a moat.
Model 2: Predicting connective tissue functions from structure of cellular and ECM components
We have already looked at the loose kind of connective tissue — let’s now look at some other types.
Micrographs
A
B
C
D
Schematics
1
3
2
Connective tissue functions
Transmitting force (as in a tendon attaching muscle to bone to move it).
Absorbing force (as in a cartilage meniscus padding the space between the femur and tibia).
Protection — as in a connective tissue sheath surrounding muscle, nerve or bone.
Collagen fibers
Collagen fibers interact with other collagen fibers in distinct ways — this is dependent largely on the
type of the collagen molecules involved, and their particular locations.
Questions
8. Which portions of model 2 show the tightest packing organization of the ECM fibers?
Schematic 3 and micrograph B
9. Based on the schematics and micrographs above, which items would you choose to serve as:
schematic
micrograph
a. a boundary marker, (e.g., fence)
2
C
b. a climbing rope
3
B
c. a space occupying material, (i.e., filler)
1
D
10. FILL OUT THE CHART BELOW based on carefully observing model 2 and your answers to the
questions above.
Schematic #
None
IMAGE
A
type of CT
dense irregular CT
function
protection
sample location
perimysium
3
B
dense regular CT
force transmission
tendon
1
D
hyaline cartilage
2
C
loose regular
force absorption, fluid
retention
scaffolding for cell
adhesion/migration
articular surface of
bone
basement membranes
Two other special cases . . . Blood and bone are also connective tissues.
11. What is the extracellular matrix of blood? The ground substance of blood? Of bone?
A1 — the liquid portion (plasma)
A2— the serum (plasma, less the clotting (fibrillar) proteins)
A3— osteoid, (i.e., bone matrix)
12. What represents the fiber component of blood? What kind of fibers are these? What about
the fibers of bone?
A1 — the clotting proteins — clotting cascade
A2 — fibrillar, non-collagenous fibers
A3 — collagen fibers given bone tensile strength