21.14
De bijschildklieren
De twee paar bijschildklieren ingebed in het achterste oppervlak van de schildklier worden beperkt door een dichte capsule eromheen. Deze klieren best…
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.
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Q1: Where are the parathyroid glands located in the body?
The parathyroid glands are embedded within the posterior surface of the thyroid gland as two pairs of small endocrine structures. A dense capsule separates them from the thyroid tissue. These glands are part of the structures of the endocrine system and play a critical role in maintaining calcium homeostasis throughout the body.
Q2: What are the two main cell types found in the parathyroid glands?
Parathyroid glands contain two distinct cell populations: principal parathyroid cells and oxyphil cells. Principal cells produce parathyroid hormone and actively monitor circulating calcium levels. Oxyphil cells emerge during late puberty, but their specific functions remain unknown, adding complexity to parathyroid gland physiology.
Q3: How do parathyroid glands respond to low blood calcium levels?
When calcium levels drop, principal parathyroid cells detect this change and secrete parathyroid hormone (PTH). PTH then triggers a cascade of events to mobilize calcium from bone stores and increase kidney reuptake, restoring blood calcium to normal levels through chemical signaling in the endocrine system.
Q4: What is the role of PTH in bone calcium mobilization?
PTH regulates osteoblast and osteoclast activity in bones to release calcium into the bloodstream. PTH stimulates osteoblasts to secrete RANKL, which amplifies osteoclast number and activity, intensifying mineral turnover and calcium release through bone matrix erosion and restoring systemic calcium balance.
Q5: How does increased blood calcium reinforce PTH activity?
Elevated blood calcium levels trigger increased kidney reuptake of calcium and stimulate calcitriol secretion, which reinforces PTH activity and maintains calcium balance. This feedback mechanism ensures finely tuned regulation of calcium levels, exemplifying the parathyroid glands' indispensable role in maintaining systemic mineral homeostasis.
Q6: What is the relationship between parathyroid hormone and calcium homeostasis?
Parathyroid hormone is the primary regulator of calcium homeostasis in the body. Principal parathyroid cells continuously monitor circulating calcium concentrations and secrete PTH when levels decline, initiating coordinated responses in bone and kidney tissues to restore calcium balance and maintain physiological stability.
Q7: Why are oxyphil cells considered functionally enigmatic in the parathyroid glands?
Oxyphil cells appear during late puberty within the parathyroid glands but their specific functions remain elusive and unknown to researchers. Unlike principal cells, which clearly produce PTH for calcium regulation, oxyphil cells' physiological role in parathyroid function has not been definitively established or characterized.