Biomedical Engineering Reference
In-Depth Information
(''para''
near) the signal-releasing cell. Two neurons would be an example of a
paracrine signal. Only neighbor-neighbor interactions of osteocytes would permit
them to create an image of the bone state. This would be in the spirit of the
research by David G. Kendall, who in the 1970s discussed recovery of a structure
from fragmentary information [23, 24]. Such a behavior was observed in a colony
of bacteria or in another collective response to external disturbances by Howard
Bloom in 2007, cf. [25]. The bone marrow, distributed in many parts, acts and
reacts as one organ [26].
A quantitative experimentation is still needed for understanding the behavior of
regulatory actions that manifest themselves in many biological systems, explaining
principles of cellular information processing, and to advance predictive modeling
of cellular regulation.
The body is made up of cells that communicate with each other and with external
cues
via
receptors at their surfaces. To generate cellular responses, signaling
pathways are activated, which initiate movement of proteins to specific locations
inside the cells, notably the nucleus, where DNA is situated.
Enzymes called
kinases
are widely used to transmit signals and control processes
in cells. One particular pathway is the extracellular signal-regulated kinase (ERK)
pathway. ERKs act as messenger molecules by relaying signals that are received
from outside the cell to the administrative core, the nucleus. To do so, ERK must
move from the intracellular fluid to the nucleus of the cell and turn on several
genes while turning off others, which in turn finally makes the cell to divide or
differentiate. ERK's entry in the nucleus is unconventional, because the protein
lacks the ability to bind to the known nuclear import proteins. Recently, Lidke
and her colleagues showed that protein pairing - known as
dimer formation
-is
not necessary for ERK to move into the nucleus after all. Instead, the process was
found to be dependent solely on the rate at which stimuli activate the ERK. A delay
in activation triggers a delay in nuclear entry of ERK, indicating that ERK entry in
the nucleus is a direct consequence of activation [27].
Shekhter has applied the systems approach to the analysis of mechanisms
whereby CT is integrated into one functional system. The primary CT functions
in health and in disease (biomechanical, trophic, protective, reparative, and mor-
phogenetic) are carried out by means of cell-cell, cell- matrix, and intertissue
interactions based on feedback between these components. Both CT as a whole and
its cellular and extracellular components exhibit structural and functional hetero-
geneity, which increases the capacity of CT for adaptation. Shekhter's study supports
the concept of internal network regulation of the CT composition, functions, and
growth through intercellular interactions at different levels of structure [28].
There is another analogy in which a bone structure resembles the mechanical
structure of a tree trunk. In plants, the carbohydrate cellulose is the most important
constituent of the wall cells, while in bones the hard skeleton in the exterior of bone
cells is created by osteocytes. The resulting mechanical effect - a strong sponge-like
structure - is similar. In biology, convergence of analogous (but without apparent
common origin) structures is known as
homoplasy
.
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