Biomedical Engineering Reference
In-Depth Information
in their genomes. This remarkable sacrifice gained these cells the ability to co-
operate in the formation of complex spatial structures, which innovation opened
new ecological niches and allowed these organisms to develop entirely new
ways to make a living. The terminally differentiated somatic cells of plants and
animals are provided stable and rich environments within the organism they
constitute, and they entrust to their germline brethren the propagation of their
common genome (1).
The ecological niches thus made exploitable were opened by virtue of the
new functionality made possible by the formation of coherent spatial structures
among the cooperating cells. These tissues and organs enabled the utilization of
new food sources as well as the distribution of cellular resources to all constitu-
ents of the organism. This organizational innovation, in turn, created a new
niche for unicellular life. Single-celled organisms are able to take advantage of
the rich, climate-controlled environment of the multicellular organism even
without cooperating in the soma-germline pact. Some of these associations have
led to alternative versions of such a pact, and benefit both the host and the mi-
crobe, but not all of them have. There is ample opportunity for the non-
cooperating microbe to take advantage of the host internal milieu, and thereby
become parasitic.
Those plants and animals that are unable to protect themselves from such
parasitism suffer loss of replicative fitness relative to those conspecifics that are.
The rich diversity and elaboration of various forms of host defense that are
found throughout the multicellular phyla testify to the force of the host-
pathogen relationship as an agent of selection. Indeed, parasitism is credited,
according to one major hypothesis, as the primary reason for the advantage of
sexual reproduction (2).
The unicellular lifestyle confers a number of advantages to microbial para-
sites—they have compact genomes and short generation times (and lack of sen-
timentality about death). As a result, they are capable of enormous genetic
plasticity and rapid adaptation to uncertain and fluctuating environments. These
advantages are so powerful that multicellular host defense has developed its own
quasi-unicellular organization. The major components of innate immunity in
animals are motile cells that circulate throughout the body and monitor their
environment for various signs of infection and damage to host tissues. When
such signs are encountered, these host cells carry out the effector functions of
immunity: killing, disarming, or sequestering the pathogen.
The cells of the immune system also retain the advantages of multicellular
tissue organization, however. Immune cells, known in vertebrates as leuko-
cytes, are induced to abandon their strictly independent lives when signals of
danger and pathogens are detected, and to reorganize into aggregates,
coordinating their activities to a much higher degree than would otherwise be
possible. Such behavior is evident in the inducible formation of such complex
structures as granulomas in response to, e.g., schistosome ova (3) and myco-
