128
UNIT 1
Organization of the Body
4
White blood cells
(neutrophils, eosinophils, lymphocytes),
and other cell types that are concerned with tissue response
to injury.
Mast cells
, which typically cluster along blood vessels. Tese
oval cells detect foreign microorganisms (e.g., bacteria, fungi)
and initiate local inflammatory responses against them. In
mast cell cytoplasm are secretory granules (
mast
5
stuffed full
of granules) containing chemicals that mediate inflammation,
especially in severe allergies. Tese chemicals include
Heparin
(hep
9
ah-rin), an anticoagulant chemical that
prevents blood clotting when free in the bloodstream
(but in human mast cells it appears to regulate the action
of other mast cell chemicals)
Histamine
(his
9
tah-mēn), a substance that makes capil-
laries leaky
Proteases
(protein-degrading enzymes)
Various other enzymes
Macrophages
(mak
9
ro-fāj
0
-es;
macro
5
large;
phago
5
eat),
large, irregularly shaped cells that avidly phagocytize a broad
variety of foreign materials, ranging from foreign molecules
to entire bacteria to dust particles. Tese “big eaters” also dis-
pose of dead tissue cells, and they are central actors in the im-
mune system. Macrophages, which are peppered throughout
loose connective tissue, bone marrow, and lymphatic tissue,
may be attached to connective tissue fibers (fixed) or may
migrate freely through the matrix. Some macrophages have
Connective Tissue Cells
Each major class of connective tissue has a resident cell type
that exists in immature and mature forms (see ±able 4.1). Te
immature (undifferentiated) cells, indicated by the suffix
blast
(literally, “bud” or “sprout,” but the suffix means “forming”),
are actively mitotic cells that secrete the ground substance and
the fibers characteristic of their particular matrix. As listed in
the third column of ±able 4.1, the primary blast cell types by
connective tissue class are (1) connective tissue proper:
fibro-
blast
; (2) cartilage:
chondroblast
(kon
9
dro-blast
0
); and (3)
bone:
osteoblast
(os
9
te-o-blast
0
). Te
hematopoietic stem cell
(hem
0
ah-to-poy-et
9
ik), which is the undifferentiated blast cell
that produces blood cells, is not included in ±able 4.1 because it
is not located in “its” tissue (blood) and does not make the fluid
matrix (plasma) of that tissue. Blood formation is considered in
Chapter 17.
Once they synthesize the matrix, the blast cells assume their
mature, less active mode, indicated by the suffix
cyte
(±able 4.1,
third column). Te mature cells maintain the health of the ma-
trix. However, if the matrix is injured, they can easily revert to
their more active state to repair and regenerate the matrix. (Te
blood-forming stem cells are always actively mitotic.)
Additionally, connective tissue is home to an assortment of
other cell types, such as
Fat cells
, which store nutrients.
Macrophage
Fibroblast
Lymphocyte
Fat cell
Mast cell
Neutrophil
Capillary
Cell types
Extracellular
matrix
Fibers
Collagen fiber
Elastic fiber
Reticular fiber
Ground substance
Figure 4.7
Areolar connective tissue: A prototype (model)
connective tissue.
This tissue underlies epithelia and surrounds
capillaries. Notice the various cell types and three classes of fibers
(collagen, reticular, elastic) embedded in the ground substance.
(See Figure 4.8a for a less idealized version.)
previous page 162 Human Anatomy and Physiology (9th ed ) 2012 read online next page 164 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off