molecules. Like epithelium, connective tissue is clas-
sified according to its cells, extracellular matrix, and
appearance under the microscope (Table 3.3). Unlike
epithelium, most connective tissue has a rich supply of
blood vessels.
A class of connective tissue called
cartilage
is unique
because it has no blood vessels (Figure 3.25). Special-
ized cells called chondrocytes (
KON-dro-sitz
) secrete a
gel extracellular matrix that surrounds them. A chron-
drocyte or groups of chondrocytes form an “island” within
the surrounding space, which is called a
lacuna
(
la-KOO-
na
). Because there are no blood vessels within cartilage,
substances must diffuse through the extracellular matrix
into the chondrocytes. This diffusion is a relatively slow
process, which is why cartilage injuries take a long time to
heal. There are three types of cartilage that perform three
different functions: cushion joints, join structures, and
provide flexible shapes (see Table 3.3 and Figure 3.25).
Embryonic and fetal skeletons are initially made of car-
tilage that eventually develops into bone; some portions
develop in the womb, and the remainder develops after
birth during early childhood.
Classifications of connective tissues Table 3.3
Classification
Components
Function
Locations
Loose connective tissue
Areolar
Cells: fibroblasts, m acrophages,
plasm a cells, adipocytes, mast cells
Fibers: co llagen, elastic, and
reticular fibers
Provides strength, elasticity, and
support
Skin (subcutaneous layer); m ucous m em -
branes; around blood ve sse ls, nerves, and
body organs
A dip ose tissue
Cells: adipocytes
Reduces heat loss and provide
energy reserve, support, and protec-
tion
Skin (subcutaneous layer); m ucous m em -
branes; around heart, kidneys, yellow bone
m arrow , and joints; behind eye socket
Reticular
Cells: specialized fibroblasts
called reticular cells
Fibers: reticular fibers
Form s supporting fram ew ork of
organs, filters w orn-out blood cells
and m icrobes, binds sm ooth m uscle
cells together
Sp leen ; lym ph nodes; liver; red bone mar-
row; basem ent m em branes; around blood
v e sse ls and m uscles
Dense connective tissue
Dense regular
Cells: fibroblasts
Fibers: collagen fibers
Provides strong attachm ents
Tendons; ligam ents
Dense irregular
Cells: fibroblasts
Fibers: collagen fibers
Provides strength
Fascia; deep skin; bone; cartilage; joint
capsules; m em brane capsules around
organs (kidney, liver, testes); pericardium ;
heart valves
Elastic
Cells: fibroblasts
Fibers: elastic fibers
A llo w s stretching of variou s organs
Lu n gs; arterial w alls; trachea; bronchial
tubes; vocal cords; ligam ents
Cartilage
Hyaline
Cells: differentiated fibroblasts
called chondrocytes
Fibers: collagen fibers
Provides sm ooth surface for m ove-
ment, flexibility, and support
Jo in ts; ends of long bones; ribs; nose;
larynx; trachea; bronchial tubes; bronchi;
em bryonic/fetal skeleton
Fibrocartilage
Cells: differentiated fibroblasts
called chondrocytes
Fibers: collagen fibers
Provides support by join ing struc-
tures together
Pelvis; intervertebral discs; knee
Elastic
Cells: differentiated fibroblasts
called chondrocytes
Fibers: elastic fibers
Provides support by m aintaining
shape
Epiglottis; external ear; auditory tubes
Bone tissue
Several connective tissu es
(detailed in Chapter 5)
Provides mechanical support and
enables calcium and phosphate stor-
age, blood cell production, fat storage
Bones of the skeletal system
Blood and
lymph
Several cell types detailed in
Chapters 10 and 12
Enables transport of substances and
gases
Blood and lym ph vessels
78 CHAPTER 3
Cells and Tissues
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