Human Physiology: Bone Formation and Metabolism

Vocabulary flashcards covering key terms, transport proteins, regulatory pathways, bone histology, ossification processes, and clinical conditions in bone formation and metabolism.

Calcitriol (1,25-Dihydroxycholecalciferol)

The principal biologically active steroid hormone metabolite of vitamin D, produced in the kidney by $1\alpha\text{-hydroxylase}$, that increases intestinal calcium and phosphate absorption.

Calcidiol (25-Hydroxycholecalciferol)

The major circulating storage form of vitamin D produced in the liver by $25\text{-hydroxylase}$, commonly measured in clinical settings to evaluate vitamin D status.

Parathyroid Hormone (PTH)

An $84\text{-amino acid}$ peptide secreted by parathyroid chief cells that increases plasma $\text{Ca}^{2+}$ and decreases plasma $\text{PO}_4^{3-}$ through direct actions on bone and kidney.

Calcitonin

A calcium-lowering peptide hormone secreted by the parafollicular cells (C cells) of the thyroid gland that inhibits osteoclast activity and bone resorption.

TRPV6

Transient receptor potential vanilloid type 6; an apical membrane calcium channel in intestinal enterocytes that mediates active calcium entry under the regulation of calcitriol.

Calbindin-D9k

An intracellular calcium-buffering transport protein inside intestinal enterocytes that binds $\text{Ca}^{2+}$ at the apical side and facilitates its movement to basolateral exit transporters.

CaSR (Calcium-Sensing Receptor)

A G-protein-coupled receptor on parathyroid chief cells that monitors extracellular free ionized $\text{Ca}^{2+}$ levels and suppresses PTH secretion when calcium concentration is elevated.

FGF23 (Fibroblast Growth Factor 23)

A bone-derived phosphaturic hormone that downregulates NaPi-IIa and NaPi-IIc cotransporters in the renal proximal tubule and inhibits renal $1\alpha\text{-hydroxylase}$ activity.

\alpha-Klotho

A transmembrane co-receptor protein in the renal proximal tubule required for high-affinity binding and functional signaling of FGF23.

Rickets

A disorder of defective bone mineralization occurring in children that affects the epiphyseal growth plate and developing bone, typically resulting from vitamin D deficiency.

Osteomalacia

A disorder of defective bone mineralization occurring in adults with closed growth plates, characterized by accumulation of unmineralized osteoid and increased fragility.

Osteoporosis

A disease characterized by a reduction in bone mass where bone resorption chronically exceeds formation, while the remaining bone matrix stays normally mineralized.

Osteopetrosis

A rare metabolic bone disease marked by excessively high bone density caused by defective osteoclast-mediated bone resorption.

Osteoblast

A single-nucleated bone-forming cell derived from the mesenchymal lineage that synthesizes and secretes organic bone matrix (osteoid).

Osteocyte

A mature, single-nucleated bone cell derived from an embedded osteoblast within a lacuna, functioning in mechanosensing and matrix maintenance.

Osteoclast

A multinucleated bone-resorbing cell derived from the monocyte/hematopoietic stem cell lineage that breaks down mineralized bone matrix.

RANKL

Receptor Activator of Nuclear Factor-\kappaB Ligand; a protein produced by osteoblasts and stromal cells that binds RANK on osteoclast precursors to promote osteoclastogenesis.

Osteoprotegerin (OPG)

A decoy receptor protein secreted by osteoblast-lineage cells that binds and neutralizes RANKL, preventing RANK activation and inhibiting osteoclast differentiation.

Cortical Bone

Dense, highly organized compact bone made of osteons that accounts for $80\%$ of total skeletal mass and exhibits a relatively slow metabolic turnover rate.

Trabecular Bone

Cancellous or spongy bone composed of porous plates and spicules, comprising $20\%$ of skeletal mass, with a high surface-area-to-volume ratio and a rapid turnover rate of $20\%$ per year.

Endochondral Ossification

An indirect method of bone formation where bone progressively replaces an initial hyaline cartilage model, forming long bones and most of the axial skeleton.

Intramembranous Ossification

A direct method of bone formation from mesenchymal precursor cells without a pre-existing cartilage template, producing the cranial vault, facial bones, and clavicles.

PTHrP (Parathyroid Hormone-Related Protein)

A paracrine factor sharing N-terminal homology with PTH that regulates tissue and cartilage development, and causes humoral hypercalcemia when systemically secreted by malignant tumors.

Pseudohypoparathyroidism

A condition characterized by target-tissue resistance to PTH due to defective G-protein signaling, resulting in hypocalcemia and hyperphosphatemia despite elevated circulating PTH levels.

Chvostek Sign

A clinical indicator of hypocalcemia and increased neuromuscular excitability elicited by tapping the facial nerve anterior to the ear, producing facial muscle twitching.

Trousseau Sign

A clinical indicator of hypocalcemia and increased neuromuscular excitability elicited by inflating a blood pressure cuff on the upper arm, producing carpal spasm with wrist/finger flexion and thumb adduction.