Parenchyme - stroma interaction
An asymmetric CA was introduced in the
previous chapter . Actually,
a CA by itself cannot become asymmetric. Any zygote of CA#600 will become
symmetric. In order to become asymmetric, it has to be supported by
a second CA. In order to grow an asymmetric CA, two zygotes have
to be planted, one will grow into an asymmetric CA and the other will support
it. Such a couple is depicted in the image below. The two zygotes
differentiate side by side. Their mutual impact is {0, 8} . Ca-1 does not interact with CA-2, while the other supports its asymmetry.
This relationship illustrates
an important biological phenomenon known as Parenchyme-Stroma interaction. For instance, in order to survive, epithelium of the skin (parenchyma) depends on trophic factors
produced by the dermis (stroma). Stroma
controls the differentiation of parenchyma. In the present experiment stroma makes
CA-1 asymmetric. Remove the second zygote (stroma) and the remaining zygote
will grow into a symmetric CA.
Imprinting
In the embryo parenchyma-stroma relationship may be temporary.
Like in the second experiment, when two zygotes were planted and grow side
by side. Their mutual impact is {10, 9}. As CAs mature, their mutual
impact becomes more effective, until
CA-2 (stroma) dies. CA-1 (parenchyme)
changes its structure, and continues living. During the short period
of co-existence, CA-1 was endowed with two new features: independence,
and new structure. This phenomenon is known as imprinting. CA-2 imprinted
CA-1 with these features.
Induction
The third experiment demonstrates an important phenomenon
of embryogenesis, called Induction. First,
two zygotes are planted and the parenchyme grows into an asymmetric
CA. At time = 40 a third zygote
is planted. It interacts solely with the second CA . impact ={11
, 0}. This impact inhibits the growth of CA-2 and it dies. Actually CA-1
was imprinted as in the previous experiment, and its changes became apparent
after the third zygote was planted. It
appears as if CA-3 induced them. Yet in the present setting such
is not possible, since the impact of CA-3 on CA-1 is zero. CA-3 did not induce these changes. Since
real experiments in embryology are too crude, such an experiment would be
interpreted as induction.
The image on the right is an intuitive illustration of induction. Imagine that at t = i. a stem cell starts dividing and generates a hand. At t = j the first finger is generated by induction. The inducing process is not depicted.