7. THE ULTIMOBRANCHIAL GLANDS
379
Tenenhouse (1968) have stressed the importance of calcium as an integrator of metabolic events within the cell.
The factors involved in calcium regulation in mammals have been discussed in recent reviews (Copp, 1968a, 1969a,b), They include the vast
reservoir of calcium in the skeleton, the continuous metabolic turnover
of bone (formation and resorption), and the action of the hormones
released by parathyroid and ultimobranchial cells. The former, when
stimulated by hypocalcemia, release parathormone which increases bone
resorption and calcium release, thus raising the plasma calcium. The
latter, which constitute the C cells in mammalian thyroid, respond to
hypercalcemia by releasing calcitonin which inhibits bone resorption. Together, the two glands provide a highly effective dual negative feedback
system which maintains the mammalian plasma calcium within narrow
limits.
B. Calcium Regulation in Fishes
For those interested in a more detailed coverage there are excellent
reviews on calcium metabolism (Fleming, 1967) and regulation (Urist,
1962, 1964a,b, 1966; Urist and Van de Putte, 1967) in fishes. This is closely
linked with the origin and development of the first vertebrates. Although
their ancestors came from the sea, it seems likely that the primitive fishes
developed in the freshwaters of lakes, streams, and estuaries in the
Ordovician and Silurian periods (see chapter by Conte, Volume I). Most
freshwater contains significant concentrations of calcium. Rawson ( 1939)
noted that biological productivity in freshwater is related to its calcium
content, and that a concentration of 2.5-3.2 mg % was required to support
full biological activity of plant and animal life. This is quite similar to the
ionic calcium concentration in the blood of higher vertebrates (4-5 mg %) .
The fist vertebrates were the ostracodenns-primitive jawless fishes
with well-developed dermal bone and often a bony endoskeleton as well.
This had many of the features of mammalian bone, with bone cells,
trabeculae, and vascular channels and with the same mineral, hydroxyapatite. True bone was also present in the placoderms-primitive jawed
fishes which are thought to be the ancestors of the bony fishes. As in
terrestrial vertebrates, this bone must have provided a useful storage
reservoir of calcium in an environment in which the availability of this
element might be limited.
When the ancestors of modem fishes returned to the sea with its high
calcium concentration, the bony reservoir of calcium was no longer
needed and in some cases bone was abandoned. The cyclostomes, chi-
379
Tenenhouse (1968) have stressed the importance of calcium as an integrator of metabolic events within the cell.
The factors involved in calcium regulation in mammals have been discussed in recent reviews (Copp, 1968a, 1969a,b), They include the vast
reservoir of calcium in the skeleton, the continuous metabolic turnover
of bone (formation and resorption), and the action of the hormones
released by parathyroid and ultimobranchial cells. The former, when
stimulated by hypocalcemia, release parathormone which increases bone
resorption and calcium release, thus raising the plasma calcium. The
latter, which constitute the C cells in mammalian thyroid, respond to
hypercalcemia by releasing calcitonin which inhibits bone resorption. Together, the two glands provide a highly effective dual negative feedback
system which maintains the mammalian plasma calcium within narrow
limits.
B. Calcium Regulation in Fishes
For those interested in a more detailed coverage there are excellent
reviews on calcium metabolism (Fleming, 1967) and regulation (Urist,
1962, 1964a,b, 1966; Urist and Van de Putte, 1967) in fishes. This is closely
linked with the origin and development of the first vertebrates. Although
their ancestors came from the sea, it seems likely that the primitive fishes
developed in the freshwaters of lakes, streams, and estuaries in the
Ordovician and Silurian periods (see chapter by Conte, Volume I). Most
freshwater contains significant concentrations of calcium. Rawson ( 1939)
noted that biological productivity in freshwater is related to its calcium
content, and that a concentration of 2.5-3.2 mg % was required to support
full biological activity of plant and animal life. This is quite similar to the
ionic calcium concentration in the blood of higher vertebrates (4-5 mg %) .
The fist vertebrates were the ostracodenns-primitive jawless fishes
with well-developed dermal bone and often a bony endoskeleton as well.
This had many of the features of mammalian bone, with bone cells,
trabeculae, and vascular channels and with the same mineral, hydroxyapatite. True bone was also present in the placoderms-primitive jawed
fishes which are thought to be the ancestors of the bony fishes. As in
terrestrial vertebrates, this bone must have provided a useful storage
reservoir of calcium in an environment in which the availability of this
element might be limited.
When the ancestors of modem fishes returned to the sea with its high
calcium concentration, the bony reservoir of calcium was no longer
needed and in some cases bone was abandoned. The cyclostomes, chi-