The role of parathyroïd hormone in mineral homeostasis and bone modeling in suckling rat pups

Calciotropic and phosphotropic hormones in fetal and neonatal bone development

There are remarkable differences in bone and mineral metabolism between the fetus and adult. The fetal mineral supply is from active transport across the placenta. Calcium, phosphorus, and magnesium circulate at higher levels in the fetus compared to the mother. These high concentrations enable the skeleton to accrete required minerals before birth. Known key regulators in the adult include parathyroid hormone (PTH), calcitriol, fibroblast growth factor-23, calcitonin, and the sex steroids. But during fetal life, PTH plays a lesser role while the others appear to be unimportant. Instead, PTH-related protein (PTHrP) plays a critical role. After birth, serum calcium falls and phosphorus rises, which trigger an increase in PTH and a subsequent rise in calcitriol. The intestines become the main source of mineral supply while the kidneys reabsorb filtered minerals. This striking developmental switch is triggered by loss of the placenta, onset of breathing, and the drop in serum calcium.

Bone development and mineral homeostasis in the fetus and neonate: roles of the calciotropic and phosphotropic hormones

Mineral and bone metabolism are regulated differently in utero compared with the adult. The fetal kidneys, intestines, and skeleton are not dominant sources of mineral supply for the fetus. Instead, the placenta meets the fetal need for mineral by actively transporting calcium, phosphorus, and magnesium from the maternal circulation. These minerals are maintained in the fetal circulation at higher concentrations than in the mother and normal adult, and such high levels appear necessary for the developing skeleton to accrete a normal amount of mineral by term. Parathyroid hormone (PTH) and calcitriol circulate at low concentrations in the fetal circulation. Fetal bone development and the regulation of serum minerals are critically dependent on PTH and PTH-related protein, but not vitamin D/calcitriol, fibroblast growth factor-23, calcitonin, or the sex steroids. After birth, the serum calcium falls and phosphorus rises before gradually reaching adult values over the subsequent 24-48 h. The intestines are the main source of mineral for the neonate, while the kidneys reabsorb mineral, and bone turnover contributes mineral to the circulation. This switch in the regulation of mineral homeostasis is triggered by loss of the placenta and a postnatal fall in serum calcium, and is followed in sequence by a rise in PTH and then an increase in calcitriol. Intestinal calcium absorption is initially a passive process facilitated by lactose, but later becomes active and calcitriol-dependent. However, calcitriol's role can be bypassed by increasing the calcium content of the diet, or by parenteral administration of calcium.

Calcium deficiency during lactation and in the first two weeks after weaning: decreased ash and increased magnesium in bone of rat pups

The mineral and skeletal status of offspring of calcium-deficient lactating dams was examined. At weaning pups of calcium deficient dams weigh less than controls and are hypocalcemic and hypermagnesemic, but have normal phosphorus levels. Bone ash expressed as percent dry weight is decreased, as is ash content of calcium and phosphorus, while magnesium is high. Histologically, except for thinner layers of lamellar bone, long bones and calvariae are unremarkable. Calcium-deficient pups subsequently fed a normal diet for two weeks gain weight rapidly, recover bone ash (increase of 62%) and normalize magnesium content of both blood and bone. At this point, bones from experimental and control animals are histologically indistinguishable. Weanlings of calcium-deficient mothers, themselves put onto the calcium-deficient diet for two weeks, show a further decline of blood calcium and a further decrease in bone ash. Blood and bone magnesium remain elevated. Long bone trabecular architecture and marrow cavity formation appear normal, but less compact bone is evident. These doubly deprived animals recover rapidly when placed on a normal diet. Within two weeks, mineral content of blood is in the control range, bone ash increases by 93%, and the slope of the weight gain curve parallels that of controls. However, in spite of the profound bone ash increase and linear weight gain, these animals remain deficient in both parameters when examined at 9 weeks of age. Similarly, bone mineral content, which also tended to normalize, fails to completely correct by this time point.