VI.
DIFFERENTIATION OF VERTEBRATE PIGMENT CELLS
291
to do this in the presence of the internal cells. It must be stated that in
no case was the elicitation of pigment cells as perfect as with molecular
phenylalanine.
Further work (Wilde, unpublished) indicated that the ability to
respond to environmental phenylalanine matured at the time of
gastrulation. Pre-gastrula cells responded poorly, the response increasing
slowly until the time of gastrulation. The differentiation obtained was
incomplete in pre-gastrular cells but the degree of completeness itself
slowly improved until dramatically converted at gastrulation. This
leads to the inference that there is a slow development of the enzymatic
systems responsible for unique differentiations which in the case of the
neuro-epithelium is ready to respond to its morphogenetic substratum
(of which phenylalanine is probably only a key compound) from the
time of gastrulation onward.
Briggs and King (1957), have developed data, from experiments with
nucleus transplantation in anura, indicating that the morphogenetic
capacity of the nuclei of ventral cells is different from that of dorsal
cells in the gastrula and later stages. With regard to the elicitation of
dorsal types of differentiation (pigment cells) from ventral cells by an
environmentally supplied molecular morphogen, it must be stated,
that regardless of the morphogenetic capacity of ventral nuclei, these
cells remain equipped with response mechanisms which permit their
conversion to a dorsal type of differentiation. In view of the ability of
fragments of neuro-epithelial cells to carry out two overt acts of pigment
cell differentiation (melanogenesis and assumption of dendritic form) it is
tempting to consider that differentiation processes are in an immediate sense, the contemporary response of interaction between microenvironment and cytoplasm.
This heretical reduction of the nucleus to a somewhat secondary role
in immediate cellular differentiation can be buttressed, in part, by
recent experiments in the author's laboratory. The work covered the
ability of isolated undifferentiated cells to differentiate either singly or
in very small groups (one to fifteen cells) when placed in minute hanging
drops of nutrient medium. Twitty and Niu (1954), have reported
experiments with small numbers of already migrating melanoblasts
taken up in capillary tubes (vide supra) and Sanford et al. (1948, 1954)
have cloned fibroblast tissue culture lines by the single cell technique.
The experiments presently under discussion were carried out to test
whether isolated cells could carry out their differentiation from start to
finish with no neighbours or at the most very few. These data will be
published in detail elsewhere. The pertinent data to the present context
are that in no case did a single isolated cell undergo differentiation in a
microdrop, but that of two cells in a similar drop, one would undergo
DIFFERENTIATION OF VERTEBRATE PIGMENT CELLS
291
to do this in the presence of the internal cells. It must be stated that in
no case was the elicitation of pigment cells as perfect as with molecular
phenylalanine.
Further work (Wilde, unpublished) indicated that the ability to
respond to environmental phenylalanine matured at the time of
gastrulation. Pre-gastrula cells responded poorly, the response increasing
slowly until the time of gastrulation. The differentiation obtained was
incomplete in pre-gastrular cells but the degree of completeness itself
slowly improved until dramatically converted at gastrulation. This
leads to the inference that there is a slow development of the enzymatic
systems responsible for unique differentiations which in the case of the
neuro-epithelium is ready to respond to its morphogenetic substratum
(of which phenylalanine is probably only a key compound) from the
time of gastrulation onward.
Briggs and King (1957), have developed data, from experiments with
nucleus transplantation in anura, indicating that the morphogenetic
capacity of the nuclei of ventral cells is different from that of dorsal
cells in the gastrula and later stages. With regard to the elicitation of
dorsal types of differentiation (pigment cells) from ventral cells by an
environmentally supplied molecular morphogen, it must be stated,
that regardless of the morphogenetic capacity of ventral nuclei, these
cells remain equipped with response mechanisms which permit their
conversion to a dorsal type of differentiation. In view of the ability of
fragments of neuro-epithelial cells to carry out two overt acts of pigment
cell differentiation (melanogenesis and assumption of dendritic form) it is
tempting to consider that differentiation processes are in an immediate sense, the contemporary response of interaction between microenvironment and cytoplasm.
This heretical reduction of the nucleus to a somewhat secondary role
in immediate cellular differentiation can be buttressed, in part, by
recent experiments in the author's laboratory. The work covered the
ability of isolated undifferentiated cells to differentiate either singly or
in very small groups (one to fifteen cells) when placed in minute hanging
drops of nutrient medium. Twitty and Niu (1954), have reported
experiments with small numbers of already migrating melanoblasts
taken up in capillary tubes (vide supra) and Sanford et al. (1948, 1954)
have cloned fibroblast tissue culture lines by the single cell technique.
The experiments presently under discussion were carried out to test
whether isolated cells could carry out their differentiation from start to
finish with no neighbours or at the most very few. These data will be
published in detail elsewhere. The pertinent data to the present context
are that in no case did a single isolated cell undergo differentiation in a
microdrop, but that of two cells in a similar drop, one would undergo