The control mechanism that my colleagues
and I consider more likely is one mediated by
the parathyroid gland. The role of this gland is
to regulate the level of calcium ions in the
blood. A drop in the level of plasma calcium
causes the release of parathyroid hormone
from the gland, and the hormone brings about
a resorption of bone tissue through the agency
of the bone cells (osteoclasts and enlarged
osteocytes). Both organic matrix and bone
mineral are removed together, and the calcium
and phosphate are released into the blood. The
level of plasma calcium is thus restored; the
phosphate is excreted.
Bone resorption under the influence of
parathyroid hormone is largely due to an
increase in the number and activity of
osteoclasts. The histological picture observed
in the medullary bone of pigeons at the height
of eggshell calcification bears a strong
resemblance to the resorption of bone in rats
and dogs following the administration of
parathyroid hormone. Leonard F. Belanger of
the University of Ottawa and I have recently
shown that the histological changes in the
medullary bone of hens treated with
parathyroid hormone were very similar to
those occurring naturally during eggshell
formation.
It has been shown that the level of
diffusible calcium in the blood drops during
eggshell calcification in the hen; the stimulus
for the release of parathyroid hormone is
therefore present. The hypothesis that the
parathyroid hormone is responsible for the
induction of bone resorption associated with
shell formation is also consistent with the time
lag between the end of the calcification of the
eggshell of the first egg in the pigeon's clutch
and the resumption of medullary bone
formation.
When hens are fed a diet deficient in
calcium, they normally stop laying in 10 to 14
days, having laid some six to eight eggs.
During this period they may deplete their
skeleton of calcium to the extent of almost 40
percent. It is interesting to inquire why they
should stop laying instead of continuing to lay
but producing eggs without shells. Failure to
lay is a result of failure to ovulate; once
ovulation takes place and the ovum enters the
oviduct, an egg will be laid, with or without a
hard shell.
The question therefore becomes: Why do hens
cease to ovulate when calcium is withheld
from their diet? The most probable answer
seemed to us to be that the release of
gonadotrophic hormones from the anterior
pituitary gland is reduced under these
conditions. To test this hypothesis we placed
six pullets, which had been laying for about a
month, on a diet containing only .2 percent
calcium—less than a tenth of the amount
normally supplied in laying rations.
After five days on the deficient diet, when
each hen had laid three or four eggs, we
administered daily injections of an extract of
avian pituitary glands to three of the
experimental birds. During the next five days
each of these hens laid an egg a day, whereas
two of the untreated hens laid one egg each
during the five days and the third untreated
hen laid three eggs. We concluded that the
failure to produce eggs on a diet deficient in
calcium is indeed due to a reduction in the
secretion of pituitary gonadotrophic hormones.
The mechanism of pituitary inhibition under
these conditions has not been established. It is
possible that the severe depression of the level
of plasma calcium inhibits the part of the brain
known as the hypethalamus, which is known
to be sensitive to a number of chemical
influences. The secretion of gonadotro-phins
in mammals is brought about by hormone-like
factors released by the hypothalamus, but it is
not known if the same mechanism operates in
birds.
Plainly the laying of eggs with highly
calcified shells has profound repercussions on
the physiology of the bird. The success of
birds in the struggle for existence indicates
that they have been able to meet the challenge
imposed on them by the evolution of shell
making. Many facets of the intricate relations
between eggshell formation, the skeletal
mobilization of calcium, the ovary and the
parathyroid and anterior pituitary glands await
elucidation, but the general picture is now
clear.
The control mechanism that my colleagues
and I consider more likely is one mediated by
the parathyroid gland. The role of this gland is
to regulate the level of calcium ions in the
blood. A drop in the level of plasma calcium
causes the release of parathyroid hormone
from the gland, and the hormone brings about
a resorption of bone tissue through the agency
of the bone cells (osteoclasts and enlarged
osteocytes). Both organic matrix and bone
mineral are removed together, and the calcium
and phosphate are released into the blood. The
level of plasma calcium is thus restored; the
phosphate is excreted.
Bone resorption under the influence of
parathyroid hormone is largely due to an
increase in the number and activity of
osteoclasts. The histological picture observed
in the medullary bone of pigeons at the height
of eggshell calcification bears a strong
resemblance to the resorption of bone in rats
and dogs following the administration of
parathyroid hormone. Leonard F. Belanger of
the University of Ottawa and I have recently
shown that the histological changes in the
medullary bone of hens treated with
parathyroid hormone were very similar to
those occurring naturally during eggshell
formation.
It has been shown that the level of
diffusible calcium in the blood drops during
eggshell calcification in the hen; the stimulus
for the release of parathyroid hormone is
therefore present. The hypothesis that the
parathyroid hormone is responsible for the
induction of bone resorption associated with
shell formation is also consistent with the time
lag between the end of the calcification of the
eggshell of the first egg in the pigeon's clutch
and the resumption of medullary bone
formation.
When hens are fed a diet deficient in
calcium, they normally stop laying in 10 to 14
days, having laid some six to eight eggs.
During this period they may deplete their
skeleton of calcium to the extent of almost 40
percent. It is interesting to inquire why they
should stop laying instead of continuing to lay
but producing eggs without shells. Failure to
lay is a result of failure to ovulate; once
ovulation takes place and the ovum enters the
oviduct, an egg will be laid, with or without a
hard shell.
The question therefore becomes: Why do hens
cease to ovulate when calcium is withheld
from their diet? The most probable answer
seemed to us to be that the release of
gonadotrophic hormones from the anterior
pituitary gland is reduced under these
conditions. To test this hypothesis we placed
six pullets, which had been laying for about a
month, on a diet containing only .2 percent
calcium—less than a tenth of the amount
normally supplied in laying rations.
After five days on the deficient diet, when
each hen had laid three or four eggs, we
administered daily injections of an extract of
avian pituitary glands to three of the
experimental birds. During the next five days
each of these hens laid an egg a day, whereas
two of the untreated hens laid one egg each
during the five days and the third untreated
hen laid three eggs. We concluded that the
failure to produce eggs on a diet deficient in
calcium is indeed due to a reduction in the
secretion of pituitary gonadotrophic hormones.
The mechanism of pituitary inhibition under
these conditions has not been established. It is
possible that the severe depression of the level
of plasma calcium inhibits the part of the brain
known as the hypethalamus, which is known
to be sensitive to a number of chemical
influences. The secretion of gonadotro-phins
in mammals is brought about by hormone-like
factors released by the hypothalamus, but it is
not known if the same mechanism operates in
birds.
Plainly the laying of eggs with highly
calcified shells has profound repercussions on
the physiology of the bird. The success of
birds in the struggle for existence indicates
that they have been able to meet the challenge
imposed on them by the evolution of shell
making. Many facets of the intricate relations
between eggshell formation, the skeletal
mobilization of calcium, the ovary and the
parathyroid and anterior pituitary glands await
elucidation, but the general picture is now
clear.
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