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Fig. 1.36 Mesenchymal cells from the paraxial mesoderm disperse
through all three layers of the embryo to form the reticular net, the
precursor and foundation for the fascial net, in order to maintain
spatial relationships among the rapidly differentiating cells.
Fig. 1.35 Gastrulation, a turning inside-out motion of the
embryonic sock, forms the trilaminar disc (ecto-, meso-, and
endoderm) between the two large sacs of the amnion and yolk
(transverse section). This action turns the double bag into a tube.
Notice the similarity in shape to Figure 1.33B.
fibers, but the fact remains: this is the source of our sin-
gular fibrous net, and the reasoning behind our favoring
of the singular 'fascia' over the plural 'fasciae'. While
we may, for analytical purposes, speak of the plantar
fascia, the falciform ligament, the central tendon of the
diaphragm, lumbosacral fascia, or dura mater, each of
these is a man-made distinction imposed on a net that
is in truth unitary from top to toe and from birth to
death. Only with a knife can these or any other indi-
vidual parts be separated from the whole. This fibrous
net can fray with age, be torn asunder by injury, or be
divided with a scalpel, but the fundamental reality is the
unity of the entire collagenous network. The naming of
parts has been one of our favorite human activities since
Genesis, and indeed a very useful one, as long as we do
not lose sight of the fundamental wholeness.
Once the three layers and the binding net of fascia are
established, the embryo performs a magnificent feat of
auto-origami, folding and refolding itself to form a
human being from this simple trilaminar arrangement
(Fig. 1.37A). The mesoderm reaches around the front
from the middle, forming the ribs, abdominal muscles,
and pelvis, creating and supporting the endodermal ali-
mentary canal within (Fig. 1.37B). It also reaches around
to the back, forming the neural arch of the spinal column
and the cranial vault of the skull, which surrounds and
protects the central nervous system (the fasciae
within these cavities were briefly described at the end
of the section on the fibrous net earlier in this chapter -
Fig. 1.37C). One of the last bits of origami is the fold
that brings the two halves of the palate together. Since
it is one of the last bricks in the wall of developmental
stages, if any brick below it is missing it could result in
a cleft palate, which explains why this is such a common
birth defect (Fig. 1.38). 8 4
Just lateral to the mesenchyme, near the edge of the
embryo, lie the tubes of the intraembryonic coelom. 8 5
This tube runs up each side of the embryo, joining in
front of the head. These tubes will form the fascial bags
of the thorax and abdomen. The very top part of the
coelomic tube will fold under the face and surround the
developing heart with the double bag of the endocar-
dium and pericardium (Fig. 1.39) as well as the central
part of the diaphragm. The upper part on either side
ment, is a very important moment. Up until this point,
most cells have been carbon copies of each other; very
little differentiation has taken place. Therefore, spatial
arrangement is not crucial. During this time, the
mucousy 'glue' among the cells and their intermembra-
nous gap junctions have sufficed to keep the tiny embryo
intact. Now, however, as increasing specialization takes
place, it is imperative that concrete spatial arrangements
be maintained while still allowing movement, as the
embryo begins to increase exponentially in size and
complexity.
If we look more closely at this middle layer, the meso-
derm, we see a thickening in the middle below the
primitive streak, called the notochord, which will ulti-
mately form the spinal column - vertebral bodies and
discs. Just lateral to this, in the paraxial mesoderm, is a
special section of the mesoderm called the mesenchyme
(literally, the mess in the middle). 8 2 Mesenchymal cells,
which are the embryonic stem cells for fibroblasts and
other connective tissue cells, migrate among the cells
throughout the organism, to inhabit all three layers (Fig.
1.36). There they secrete reticulin (an immature form of
collagen with very fine fibers) into the interstitial space. 8 3
These reticulin fibers bind with each other, chemically
and like Velcro®, to form a body-wide net - even though
the entire body is only about 1 mm long at this point.
As an aside, some of these pluripotential mesenchy-
mal cells are retained in the tissues of the body, ready to
convert themselves into whatever connective tissue
function is most called upon. If we eat too much, they
can convert to fat cells to handle the excess; if we are
injured, they can become fibroblasts and help heal the
wound; or if we are subject to a bacterial infection, they
can convert to white blood cells and go forward to fight
the infection. 8 2 They are a perfect example of the supreme
adaptability and responsiveness of this fibrous/connec-
tive tissue system to our changing needs.
The reticular fibers these mesenchymal cells generate
will gradually be replaced, one by one, by collagen
39
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