In this article we will discuss about the control of reactive ability of tissues by the genotype.
While the initial step in the differentiation of cells is made by the cytoplasm, it can be shown that the final result cannot be achieved without the involvement of hereditary factors, the nuclear genes. This is true in particular in the case of differentiations caused by induction.
Inducing substances are not highly specific, and tissues of one species of animal may induce structures in another species even if the two are, systematically, very far apart. The structure induced, on the other hand, is as a general rule one that is peculiar to the reacting species.
It was indicated that the balancer of salamander larvae develops as a result of an induction, the stimulus being given off probably by the roof of the archenteron, and the reacting system being the epidermis in the neurula stage. Balancers are found in most newts and salamanders, but a few species do not possess them. Experiments have been performed to determine the cause of this difference.
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If, in the neurula stage, the epidermis of a species possessing a balancer (viz., Triturus taeniatus) is taken from any part of the body and transplanted to the side of the head of a species which does not have balancers (viz., Ambystoma mexicanum), the transplanted epidermis will develop a balancer in exactly the same position in which it is usually found, that is, underneath the eye near the angle of the mouth.
A reverse transplantation, that is, transplantation of epidermis from Ambystoma mexicanum to the site of balancer development in Triturus taeniatus, results in the absence of a balancer on the operated side of the head.
These results show that the absence of balancers in Ambystoma mexicanum is due to failure on the part of the epidermis to react to the stimulus of the inductor, or in other words, that the epidermis lacks an appropriate competence. The inducing stimulus, on the other hand, is present both in species possessing balancers and in species not having these organs.
As the inability of A. mexicanum to develop balancers is genetically fixed, we may conclude that the hereditary factors (genes or their combinations) responsible for this particular peculiarity of A. mexicanum directly affect the competence of the ectoderm, while the inducing systems remain unchanged.
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Another example illustrating the same principle is found in the development of the mouth in anurans and urodeles. The larvae of urodeles (salamanders and newts) have typical teeth consisting of the pulp and the layers of dentine and enamel.
The teeth are situated inside the mouth and are attached to the jaws and the bones of the palate. The tadpoles of frogs and toads have no true teeth; instead, the edges of the jaws are covered by horny sheaths, and rows of small horny teeth and epidermal papillae are developed on an oral disc surrounding the mouth.
The ectodermal mouth parts develop under the influence of an induction from the oral endoderm. The inductor, however, determines only the position of the ectodermal oral invagination, not its specific peculiarities. It is possible to transplant ectoderm from an early frog embryo to a salamander embryo in such a way that the grafted ectoderm covers the mouth region. As a result, the graft is induced to develop a mouth. This mouth is, however, in every respect the mouth of a frog, with horny jaws and rows of horny teeth and oral papillae.
It is thus evident that the influence of the mouth inductor is similar in salamanders and frogs, or at least sufficiently similar for frog ectoderm to be able to react to a salamander’s inductor. What is different is the competence of the ectoderm; to the same stimulus, the ectoderm of different animals reacts in its peculiar way. It is the competence or nature of reaction of the ectoderm that is affected by the hereditary factors responsible for the differences in the development of the mouth in urodeles and anurans.
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The power of the inductor is limited; it can evoke only such differentiations as are provided by the hereditary constitution of the reacting cells. Since this constitution is embodied in the structure of the nuclear DNA, it may be concluded that induction eventually is consummated by changing the activity of the genes in the reacting cells.
It is also possible, however, for the hereditary factors to modify an inducing system without changing the competence of the reacting tissues. It has been proved that the dorsal fin fold in amphibians is induced by the neural crest (The Fin Fold). In most salamander larvae, the fin fold reaches anteriorly almost to the occipital region, but in Eurycea bislineata it is present only on the tail, the trunk being devoid of a fin fold.
This peculiarity is not a result of the inability of the trunk epidermis to develop a fin fold. If the trunk epidermis of Eurycea bislineata is transplanted to the back of an embryo of Ambystoma maculatum (a species having a fin fold in the trunk region), the transplanted epidermis will participate in the formation of a fin fold. The peculiarity of the fin fold of Eurycea is thus due to failure of the trunk neural crest to act as an inductor.
We have seen that the position of the external gills found in amphibian larvae, and the very fact of their development, is dependent on the endoderm, while both the epidermis which covers the gills and the mesenchyme which forms the connective tissue and the blood vessels of the gills react to the inducing influence of the endoderm. The number of pairs of external gills in tadpoles of frogs and toads varies in different species from one to three. In the South African toad, Bufo carens, there are three pairs of external gills, while in another species, Bufo regularis, only two pairs are formed.
If the presumptive epidermis of Bufo regularis is transplanted to an embryo of Bufa carens, and if it lies in the branchial region, it will form gills in response to an induction from the host endoderm. It was found that three gills were formed on the operated side, the number typical for the host and not for the reacting epidermis. In the reciprocal transplantation, that is, transplantation of the presumptive epidermis from Bufo carens to Bufo regularis, two external gills develop, again the number typical for the host.
It is evident that the epidermis can form any number of external gills, and the number that actually develops is determined not by the reacting system (grafted epidermis) but by the inductor, the host endoderm. The ability of a part of the embryo to act as inductor may thus be affected by the specific genetic constitution of an animal.
There is no contradiction between these results and the experiments showing that an induction can produce only such differentiations as are provided for by the genotype of the reacting cells. Although in the last two experiments the inductor was responsible for some features of the induced structure (extent of fin fold, number of gills), the result of the induction did not go beyond changing the spatial distribution of certain structures. The heteroplastic inductor did not produce any qualitatively new differentiations that were foreign to the species supplying the reacting tissue.