144
UNIT 1
Organization of the Body
4
Check Your Understanding
21.
What are the three main steps of tissue repair?
22.
Why does a deep injury to the skin result in abundant scar
tissue?
For answers, see Appendix H.
Developmental Aspects
of Tissues
Indicate the embryonic origin of each tissue class.
Briefly describe tissue changes that occur with age.
One of the first events of embryonic development is the forma-
tion of the three
primary
germ layers
, which lie one atop the
next like a three-layered cellular pancake. From superficial to
deep, these layers are the
ectoderm
,
mesoderm
(mez
9
o-derm),
and
endoderm
(Figure 4.13)
. Tese primary germ layers then
specialize to form the four primary tissues—epithelium, ner-
vous tissue, muscle, and connective tissues—that make up all
body organs.
By the end of the second month of development, the primary
tissues have appeared, and all major organs are in place. In gen-
eral, tissue cells remain mitotic and produce the rapid growth
that occurs before birth. Te division of nerve cells, however,
stops or nearly stops during the fetal period. A±er birth, the
cells of most other tissues continue to divide until adult body
size is achieved. Cellular division then slows greatly, although
many tissues retain some ability to regenerate.
In adults, only epithelia and blood-forming tissues are highly
mitotic. Some tissues that regenerate through life, such as the
glandular cells of the liver, do so through division of their mature
(specialized) cells. Others, like the epidermis of the skin and cells
lining the intestine, have abundant
stem cells
, relatively undiffer-
entiated cells that divide as necessary to produce new cells.
Regenerative Capacity of Different Tissues
²issues vary widely in their capacity for regeneration. Epithelial
tissues, bone, areolar connective tissue, dense irregular connec-
tive tissue, and blood-forming tissue regenerate extremely well.
Smooth muscle and dense regular connective tissue have a mod-
erate capacity for regeneration, but skeletal muscle and cartilage
have a weak regenerative capacity. Cardiac muscle and the nervous
tissue in the brain and spinal cord have virtually no
functional
re-
generative capacity, and they are routinely replaced by scar tissue.
However, recent studies show that some unexpected (and highly
selective) cellular division occurs in both these tissues a±er dam-
age, and efforts are under way to coax them to regenerate better.
In nonregenerating tissues and in exceptionally severe wounds,
fibrosis totally replaces the lost tissue. Over a period of months, the
fibrous mass shrinks and becomes more and more compact. Te
resulting scar appears as a pale, o±en shiny area composed mostly
of collagen fibers. Scar tissue is strong, but it lacks the flexibility
and elasticity of most normal tissues. Also, it cannot perform the
normal functions of the tissue it has replaced.
Homeostatic Imbalance
4.3
Scar tissue that forms in the wall of the urinary bladder, heart,
or other muscular organ may severely hamper the organ’s func-
tion. Te normal shrinking of the scar reduces the internal
volume and may hinder or even block substances from mov-
ing through a hollow organ. Scar tissue hampers muscle’s abil-
ity to contract and may interfere with its normal excitation by
the nervous system. In the heart, these problems may lead to
progressive heart failure. In irritated visceral organs, particu-
larly following abdominal surgery,
adhesions
may form as the
newly forming scar tissue connects adjacent organs together.
Such adhesions can prevent the normal shi±ing about (churn-
ing) of loops of the intestine, dangerously obstructing the flow
of foodstuffs. Adhesions can also restrict heart movements and
immobilize joints.
Mesoderm
Endoderm
16-day-old embryo
(dorsal surface view)
Epithelium
(from all three
germ layers)
Nervous tissue
(from ectoderm)
Muscle and connective tissue
(mostly from mesoderm)
Inner lining of
digestive system
(from endoderm)
Ectoderm
Figure 4.13
Embryonic germ layers and the primary tissue types they produce.
The three
embryonic layers collectively form the very early embryonic body.
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