21.14:
The Parathyroid Glands
The thyroid gland's posterior surface contains two embedded pairs of parathyroid glands. They are separated from the thyroid gland by a dense capsule.
The parathyroid glands primarily have two cell populations, namely oxyphil and principal parathyroid cells.
The oxyphil cells appear in late puberty, but their functions currently are unknown.
The principal parathyroid cells produce the parathyroid hormone, PTH or parathormone.
These cells monitor the circulating calcium concentrations.
When the calcium levels drop, the cells secrete the PTH. This hormone affects osteoblast and osteoclast activity in the bones to mobilize calcium.
The increased blood calcium levels enhance reuptake at the kidney and calcitriol secretion, enhancing the PTH activity.
21.14:
The Parathyroid Glands
The two pairs of parathyroid glands embedded within the posterior surface of the thyroid gland are restricted by a dense capsule around them. These glands comprise two distinct cell populations—parathyroid oxyphil and parathyroid principal cells- pivotal in calcium homeostasis.
Oxyphil cells, whose functions remain elusive, emerge during late puberty, adding a layer of complexity to the parathyroid gland's intricacies. In contrast, principal parathyroid cells undertake a vital role by producing the parathyroid hormone (PTH), also known as the parathormone. These cells actively monitor circulating calcium concentrations, exhibiting a dynamic response when levels decline.
Upon detecting decreased calcium levels, principal parathyroid cells release PTH, instigating a cascade of events to mobilize calcium. PTH regulates osteoblast and osteoclast activity in the bones. It initiates the secretion of the receptor activator of a growth factor called the nuclear factor-kappa-B ligament (RANKL) by osteoblasts. RANKL, in turn, amplifies osteoclast number and activity, intensifying mineral turnover and calcium release through bone matrix erosion.
Simultaneously, the heightened blood calcium levels trigger increased kidney reuptake and stimulate calcitriol secretion, reinforcing PTH activity. This feedback loop ensures a finely tuned regulation of calcium levels within the body, exemplifying the parathyroid glands' indispensable role in maintaining systemic mineral homeostasis.