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.


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.


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.

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