VI.
DIFFERENTIATION OF VERTEBRATE PIGMENT CELLS
289
was shown to be an agent causing the metaplastic change of stratified
squamous epithelium of the chick embryo into ciliated, mucus secreting
rhinal epithelium. This was subsequently confirmed by Weiss and James
(1954), and the data further extended by Fell (1957).
An extended series of experiments in the author's laboratory (Wilde,
1955a, b, c, 1956) using similar techniques indicated that the amino acid
phenylalanine was the causal agent leading to the differentiation of
pigment cells and ectomesenchyme from the urodele neuro-epithelium.
In this case, environmentally supplied structural analogues were first
shown to be specifically inhibitory to pigment cell and ectomesenchyme
differentiation from explanted neuro-epithelium. This was followed by
the conversion of ventral ectoderm cells into pigment cells through
the agency of environmentally supplied phenylalanine. Early evidence
indicated that the structural analogues of phenylalanine fell into specific classes of inhibitory substances, depending upon the structural configuration of the molecule in its relationship to phenylalanine. This
indicated, but did not confirm, that the whole molecule was the essential
agent and also implicated several distinct chemical pathways in which
the molecule was used in following different differentiation pathways
appropriate to neuro-epithelial cells. Also it was early found that equimolar amounts of phenylalanine itself would serve to over-ride the
inhibition. The analogue compounds were found to be effective only
against neuro-epithelial derivatives and not to impede the differentiation
of other cell types from other sources, such as striated muscle.
If the whole molecule of phenylalanine is the essential agent in
particular metabolic sequences leading to the final differentiated state
of pigment cells and ectomesenchyme in the urodele, then it should be
possible to test the particular role played by the various parts and
atomic positions in the molecule. Some information of this kind was
developed by Markert (1955), in another pigmentary system, the chick
embryonic skin. This investigator was able to show, through judicious
radioautographic experiments, that tyrosine
1 4
C labelled in the sidechain of the molecule was not differentially segregated into differentiating melanoblasts, whereas randomly
1 4
C labelled tyrosine was.
Several inferences could be drawn from the data. One was, that since
the randomly labelled molecule was segregated in the differentiating
melanoblasts, the whole molecule was incorporated into the cell. On the
other hand, cleavage fragments of the molecule were fed generally into
the metabolic pool of all of the cells in the explant. Since
1 4
C was the
label, presumably the cleaved side-chain elements were one or two
carbon fragments, with or without deamination. With this clue, the
author (Wilde, 1956) was able to choose certain molecules from which
phenylalanine could possibly be formed and test mixtures for their
DIFFERENTIATION OF VERTEBRATE PIGMENT CELLS
289
was shown to be an agent causing the metaplastic change of stratified
squamous epithelium of the chick embryo into ciliated, mucus secreting
rhinal epithelium. This was subsequently confirmed by Weiss and James
(1954), and the data further extended by Fell (1957).
An extended series of experiments in the author's laboratory (Wilde,
1955a, b, c, 1956) using similar techniques indicated that the amino acid
phenylalanine was the causal agent leading to the differentiation of
pigment cells and ectomesenchyme from the urodele neuro-epithelium.
In this case, environmentally supplied structural analogues were first
shown to be specifically inhibitory to pigment cell and ectomesenchyme
differentiation from explanted neuro-epithelium. This was followed by
the conversion of ventral ectoderm cells into pigment cells through
the agency of environmentally supplied phenylalanine. Early evidence
indicated that the structural analogues of phenylalanine fell into specific classes of inhibitory substances, depending upon the structural configuration of the molecule in its relationship to phenylalanine. This
indicated, but did not confirm, that the whole molecule was the essential
agent and also implicated several distinct chemical pathways in which
the molecule was used in following different differentiation pathways
appropriate to neuro-epithelial cells. Also it was early found that equimolar amounts of phenylalanine itself would serve to over-ride the
inhibition. The analogue compounds were found to be effective only
against neuro-epithelial derivatives and not to impede the differentiation
of other cell types from other sources, such as striated muscle.
If the whole molecule of phenylalanine is the essential agent in
particular metabolic sequences leading to the final differentiated state
of pigment cells and ectomesenchyme in the urodele, then it should be
possible to test the particular role played by the various parts and
atomic positions in the molecule. Some information of this kind was
developed by Markert (1955), in another pigmentary system, the chick
embryonic skin. This investigator was able to show, through judicious
radioautographic experiments, that tyrosine
1 4
C labelled in the sidechain of the molecule was not differentially segregated into differentiating melanoblasts, whereas randomly
1 4
C labelled tyrosine was.
Several inferences could be drawn from the data. One was, that since
the randomly labelled molecule was segregated in the differentiating
melanoblasts, the whole molecule was incorporated into the cell. On the
other hand, cleavage fragments of the molecule were fed generally into
the metabolic pool of all of the cells in the explant. Since
1 4
C was the
label, presumably the cleaved side-chain elements were one or two
carbon fragments, with or without deamination. With this clue, the
author (Wilde, 1956) was able to choose certain molecules from which
phenylalanine could possibly be formed and test mixtures for their