Biology Reference
In-Depth Information
amoeba,
Dictyostelium discoideum
, will form a colony (an aggre-
gate) of cells differentiated into two distinct types. Another form of
multicellularity is even more primitive. The normally unicellular
bacteria, such as
Escherichia coli,
also differentiate morphologically
and genetically one from the other, when they grow in colonies in
a poor medium. This differentiation depends on gene expression
and the position of the bacteria within the colony (Ohgiwari
et al
.,
1992; Ben Jacob
et al
., 1992; Shapiro, 1995).
These phenomena of multicellularity are simply explained by
ontophylogenesis: cells differentiate to adapt to their microenviron-
ment composed of other cells from the same organism. This mech-
anism of cellular adaptation is analogous to that of a living
organism that adapts to its environment through natural selection.
It is an extension of natural selection exerted within organisms
(Kupiec, 1986).
To understand it we can think of it as follows: imagine a colony
of cells growing on a solid medium containing its nutrient sub-
strate (Fig. 16A). Its growth is subject to an obvious environ-
mental constraint: the cells need to have access to the nutrient in
the substrate to survive and multiply. Only the cells of the first
layer in contact with the substrate have direct access to that nutri-
ent. For the cells to be able to proliferate vertically, the nutrient
has to diffuse vertically. The quantity that can diffuse is limited by
the initial concentration in the substrate, so a decreasing gradient
must be formed: the concentration of nutrient decreases vertically
through the colony. This decrease is a consequence of the physical
diffusion of the nutrient and of its consumption by the cells through
which it passes.
From a certain height, the nutrient is no longer sufficient for the
cells to be able to continue proliferating, and growth of the colony
ceases (Fig. 16B). However, in metabolising the nutrient the cells
produce metabolites. These metabolites likewise diffuse. In this
heap of cells there are then other secondary gradients corresponding
to the diffusion of the metabolites. Owing to these gradients, each
cell finds itself located in a different microenvironment determined
