Healthcare and Medicine Reference
In-Depth Information
shared by all cells in general (and the fertilized ovum and
stem cells in particular). For instance, all cells conduct
along their membranes, but nerve cells have become
excellent at it (at a cost, incidentally, to their ability to
contract or reproduce well). All cells contain at least some
actin, and are thus capable of contraction, but muscle
cells have become masters of the art. Epithelial cells also
contract, but very feebly, while they specialize in lining
surfaces and in the secretion of chemical products such
as hormones, enzymes, and other messenger molecules.
Connective tissue cells are generally less effective at
contraction (with one major exception explained later in
this chapter) and only so-so as conductors, but they
secrete an amazing variety of products into the intercel-
lular space that combine to form our bones, cartilage,
ligaments, tendons, and fascial sheets. In other words,
it is these cells that create the structural substrate for all
the others, building the strong, pliable 'stuff which
holds us together, forming the shared and communica-
tive environment for all our cells - what Varela 4 termed
a form of 'exo-symbiosis' - shaping us and allowing us
directed movement. (As an aside, we cannot let the
word 'environment' enter our discussion without
quoting from the master of the term, Marshall McLuhan: 5
'Environments are not passive wrappings, but are,
rather, active processes which are invisible. The ground-
rules, pervasive structure, and overall patterns of envi-
ronments elude easy perception.' This may go some
way toward explaining why the cellular environment of
the extracellular matrix has remained essentially
'unseen' for some centuries of research.)
According to Gray's Anatomy: 6
Connective tissues play several essential roles in the
body, both structural, since many of the extracellular
elements possess special mechanical properties, and
defensive, a role which has a cellular basis. They also
often possess important trophic and morphogenetic roles
in organizing and influencing the growth and
differentiation of the surrounding tissues.
We will leave the discussion of the defensive support
offered by the connective tissue cells to the immunolo-
gists. We will touch on the trophic and morphogenetic
role of connective tissues when we take up embryology
and tensegrity later in this chapter. 7 " 9 For now, we
concern ourselves with the mechanical support role the
connective tissue cell products offer the body in general
and the locomotor system in particular.
protein fibrils and soluble complexes composed of
carbohydrate polymers linked to protein molecules (i.e.
they are proteoglycans) which bind water. Mechanically,
the ECM has evolved to distribute the stresses of
movement and gravity while at the same time
maintaining the shape of the different components of the
body. It also provides the physico-chemical environment of
the cells imbedded in it, forming a framework to which
they adhere and on which they can move, maintaining an
appropriate porous, hydrated, ionic milieu, through which
metabolites and nutrients can diffuse freely} 0
This statement is rich, if a little dense; the rest of this
chapter is an expansion on these few sentences, pictured
in Figure 1.3.
Dr James Oschman refers to the ECM as the living
matrix, pointing out that 'the living matrix is a continu-
ous and dynamic "supermolecular" webwork extend-
ing into every nook and cranny of the body: a nuclear
matrix within a cellular matrix within a connective
tissue matrix. In essence, when you touch a human
body, you are touching an intimately connected system
composed of virtually all the molecules within the body
linked together.' 1 1
Taken altogether, the connective tissue cells and their
products act as a continuum, as our 'organ of form'. 1 2
Our science has spent more time on the molecular inter-
actions that comprise our function while being less thor-
ough on how we shape ourselves, move through
environments, and absorb and distribute impact in all
its forms - endogenous and exogenous. Our shape is
said to be adequately described by anatomy, but how
we think about shape results partly from the tools avail-
able to us. For the early anatomists, this was principally
the knife. 'Anatomy' is, after all, separating the parts
with a blade. From Galen through Vesalius and beyond,
it was the tools of hunting and butchery which were
applied to the body, and presented to us the fundamen-
tal distinctions we now take for granted (Fig. 1.4). These
knives (later scalpels, and then lasers) quite naturally
cut along the often bilaminar connective tissue barriers
between different tissues, emphasizing the logical dis-
tinctions within the extracellular matrix, but obscuring
the role of the connective tissue syncytium considered
as a whole (Figs 1.5, 7.15 and 7.29).
If we imagine that instead of using a sharp edge we
immersed an animal or a cadaver in some form of deter-
gent or solvent which would wash away all the cellular
material and leave only the connective tissue fabric
(ECM), we would see the entire continuum, from the
basal layer of the skin, through the fibrous cloth sur-
rounding and investing the muscles and organs, and the
leathery scaffolding for cartilage and bones (Fig. 1.6A
and B). This would be very valuable in showing us this
fascial organ as a continuum, emphasizing its uniting,
shaping nature rather than simply seeing it as the line
where separations are made (Fig. 1.7). this topic pro-
ceeds from this idea and this chapter attempts to fill in
such a picture.
We are going to refer, a bit improperly, to this body-
wide complex as the fascia, or the fascial net. In medi-
cine, the word 'fascia' is usually applied more narrowly
The extracellular matrix
The connective tissue cells introduce a wide variety of
structurally active substances into the intercellular
space, including collagen, elastin, and reticulin fibers,
and the gluey interfibrillar proteins commonly known
as 'ground substance' or more recently as glycosamino-
glycans or proteoglycans. Gray calls this proteinous
mucopolysaccharide complex the extracellular matrix:
The term extracellular matrix (ECM) is applied to the
sum total of extracellular substance within the connective
tissue. Essentially it consists of a system of insoluble
15
Search Pocayo ::




Custom Search