16. A C T I O N S OF H O R M O N E S
643
initial stimulation of the tibiae followed by retardation while the radii
were stimulated during the whole of the 4-day culture period. It was
thought possible that the variation in effect might be related to the size
of the explants and that the retardation of the larger tibiae could be
due to an exhaustion of nutrients. T o test this theory Lawson compared
the T 3 -effect on the tibia with those on the smaller third metatarsal,
radius, ulna and third and fourth metacarpal, but found that there was
no correlation between initial size of the rudiments and their response
to T 3 . The action of the hormone on the smaller bones varied from
severe retardation of the third metatarsal to a marked stimulation of
the fourth metacarpal. Bones of the same size, like the third metatarsal
and the radius, showed a profound difference in effect: the growth of
the metatarsal was inhibited while that of the radius was promoted.
However, if the rudiments were arranged in order of their response
the sequence closely corresponded to their growth rate in vivo and
in vitro. Lawson attempted to influence the T 3 effect by artificially
lowering the growth rate of the rudiments in vitro. This was achieved
by explantation into inadequate media, such as serum without embryo
extract or defined media, or by lowering the temperature of incubation
to 34°C. Under these conditions, the rate of growth was slowed down
in all rudiments but the differences between the individual bones and
their differential response to T 3 were maintained. It was concluded
that the T 3 effect was determined by the rate of development in vivo
and depended on differences in metabolism which are reflected in
their relative growth rates. The exact nature of this mechanism remains,
as yet, unknown.
c. T H E E F F E C T OF T H Y R O X I N E A N D A N T I T H Y R O I D
COMPOUNDS ON P I G M E N T F O R M A T I O N IN
VITRO
Thyroxine plays an important part in pigment formation in vivo.
Barred Plymouth Rock and other breeds of domestic fowls respond to
small doses of thyroxine with increased melanin formation and to large
doses with inhibition of pigmentation (Emmens and Parkes, 1940).
The amount of pigment and the colour was found to vary according to
the concentration of the hormone. Deficiency of thyroid hormones
change the colour of the plumage from white to black or from black
to white or orange, red or brown depending on the genotype of the
chicken (Parkes and Selye, 1937). Antithyroid compounds also inhibit
pigment formation or produce a change of colour in chickens (Juhn,
1946). The changes induced by thiouracil are very similar to those seen
in Brown Leghorn fowls after thyroidectomy and have been attributed
to hypothyroidism.
643
initial stimulation of the tibiae followed by retardation while the radii
were stimulated during the whole of the 4-day culture period. It was
thought possible that the variation in effect might be related to the size
of the explants and that the retardation of the larger tibiae could be
due to an exhaustion of nutrients. T o test this theory Lawson compared
the T 3 -effect on the tibia with those on the smaller third metatarsal,
radius, ulna and third and fourth metacarpal, but found that there was
no correlation between initial size of the rudiments and their response
to T 3 . The action of the hormone on the smaller bones varied from
severe retardation of the third metatarsal to a marked stimulation of
the fourth metacarpal. Bones of the same size, like the third metatarsal
and the radius, showed a profound difference in effect: the growth of
the metatarsal was inhibited while that of the radius was promoted.
However, if the rudiments were arranged in order of their response
the sequence closely corresponded to their growth rate in vivo and
in vitro. Lawson attempted to influence the T 3 effect by artificially
lowering the growth rate of the rudiments in vitro. This was achieved
by explantation into inadequate media, such as serum without embryo
extract or defined media, or by lowering the temperature of incubation
to 34°C. Under these conditions, the rate of growth was slowed down
in all rudiments but the differences between the individual bones and
their differential response to T 3 were maintained. It was concluded
that the T 3 effect was determined by the rate of development in vivo
and depended on differences in metabolism which are reflected in
their relative growth rates. The exact nature of this mechanism remains,
as yet, unknown.
c. T H E E F F E C T OF T H Y R O X I N E A N D A N T I T H Y R O I D
COMPOUNDS ON P I G M E N T F O R M A T I O N IN
VITRO
Thyroxine plays an important part in pigment formation in vivo.
Barred Plymouth Rock and other breeds of domestic fowls respond to
small doses of thyroxine with increased melanin formation and to large
doses with inhibition of pigmentation (Emmens and Parkes, 1940).
The amount of pigment and the colour was found to vary according to
the concentration of the hormone. Deficiency of thyroid hormones
change the colour of the plumage from white to black or from black
to white or orange, red or brown depending on the genotype of the
chicken (Parkes and Selye, 1937). Antithyroid compounds also inhibit
pigment formation or produce a change of colour in chickens (Juhn,
1946). The changes induced by thiouracil are very similar to those seen
in Brown Leghorn fowls after thyroidectomy and have been attributed
to hypothyroidism.