{"id":4422510,"date":"2025-01-11T21:51:38","date_gmt":"2025-01-12T03:51:38","guid":{"rendered":"https:\/\/myendoconsult.com\/learn\/topics\/parathyroid-anatomy\/"},"modified":"2025-01-13T06:44:46","modified_gmt":"2025-01-13T12:44:46","slug":"parathyroid-anatomy","status":"publish","type":"oen_topic","link":"https:\/\/myendoconsult.com\/learn\/topics\/parathyroid-anatomy\/","title":{"rendered":"Parathyroid Anatomy"},"content":{"rendered":"\n<h2 class=\"wp-block-heading\">HISTOLOGY OF THE NORMAL PARATHYROID GLANDS<\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Embryology and Number<\/strong>\n<ul class=\"wp-block-list\">\n<li>Derived from <strong>branchial pouches III and IV<\/strong>.<\/li>\n\n\n\n<li>Typically <strong>four<\/strong> parathyroid glands, though can range from <strong>two to six<\/strong>.<\/li>\n\n\n\n<li><strong>Lower glands<\/strong> are generally larger than the upper glands.<\/li>\n<\/ul>\n<\/li>\n\n\n\n<li><strong>Gross Anatomy<\/strong>\n<ul class=\"wp-block-list\">\n<li>Ovoid (bean-shaped) glands measuring about <strong>4\u20136 mm \u00d7 2\u20134 mm \u00d7 0.5\u20132 mm<\/strong>.<\/li>\n\n\n\n<li>Weigh approximately <strong>30 mg<\/strong> each.<\/li>\n\n\n\n<li>Color varies from <strong>yellow to tan<\/strong>, depending on <strong>vascularity<\/strong> and proportion of <strong>oxyphil cells<\/strong> and <strong>stromal fat<\/strong>.<\/li>\n<\/ul>\n<\/li>\n\n\n\n<li><strong>Childhood Gland Composition<\/strong>\n<ul class=\"wp-block-list\">\n<li>Sheets of closely packed <strong>chief cells<\/strong>, with little stroma.<\/li>\n\n\n\n<li><strong>Oxyphil (oncocytic) cells<\/strong> appear at <strong>puberty<\/strong>.<\/li>\n\n\n\n<li>Fat cells appear in the stroma in late childhood and increase with age.<\/li>\n<\/ul>\n<\/li>\n\n\n\n<li><strong>Adult Gland Composition<\/strong>\n<ul class=\"wp-block-list\">\n<li>Composed of <strong>cords, sheets, and acini<\/strong> of chief cells in a <strong>loose areolar stroma<\/strong> containing many mature <strong>fat cells<\/strong>.<\/li>\n\n\n\n<li>Chief cells may be in:\n<ul class=\"wp-block-list\">\n<li><strong>Active (dark) phase<\/strong>: prominent endoplasmic reticulum (ER) and Golgi<\/li>\n\n\n\n<li><strong>Resting (light) phase<\/strong>: less developed ER<\/li>\n<\/ul>\n<\/li>\n\n\n\n<li><strong>Oxyphil cells<\/strong> are scattered individually or in groups among the chief cells.<\/li>\n<\/ul>\n<\/li>\n\n\n\n<li><strong>Chief Cells<\/strong>\n<ul class=\"wp-block-list\">\n<li>~<strong>8 \u03bcm<\/strong> in diameter.<\/li>\n\n\n\n<li>Well-defined cell membrane, centrally located nucleus (~4\u20135 \u03bcm).<\/li>\n\n\n\n<li>Nuclei: either densely packed chromatin (almost pyknotic) or finely fibrillar with peripheral margination; nucleoli are rare.<\/li>\n\n\n\n<li>Cytoplasm is clear and amphophilic with <strong>H&amp;E<\/strong> stain.<\/li>\n\n\n\n<li><strong>Periodic acid\u2013Schiff (PAS)<\/strong>: abundant <strong>glycogen<\/strong>.<\/li>\n\n\n\n<li>Also contain abundant <strong>neutral lipid<\/strong> droplets (demonstrable by azure B, Erie garnet A, oil red O, or Sudan IV).<\/li>\n\n\n\n<li>Immunohistochemistry: stronger <strong><a href=\"https:\/\/myendoconsult.com\/learn\/parathyroid-hormone-pth-lab-assessment\/\" data-wpil-monitor-id=\"248\">parathyroid hormone<\/a> (PTH)<\/strong> staining than oxyphil cells.<\/li>\n<\/ul>\n<\/li>\n\n\n\n<li><strong>Oxyphil (Oncocytic) Cells<\/strong>\n<ul class=\"wp-block-list\">\n<li><strong>Larger<\/strong> than chief cells (12\u201320 \u03bcm diameter), polygonal shape.<\/li>\n\n\n\n<li>Cell membranes usually clear.<\/li>\n\n\n\n<li>Nucleus similar to chief cell nucleus.<\/li>\n\n\n\n<li>Cytoplasm: highly <strong>eosinophilic, fine granules<\/strong> (stain carmine with <strong>Bensley acid aniline fuchsin<\/strong> [BAAF], dark blue with <strong>phosphotungstic acid hematoxylin<\/strong>).<\/li>\n\n\n\n<li>Packed with <strong>mitochondria<\/strong> (hence \u201concocytic\u201d).<\/li>\n\n\n\n<li><strong>Little<\/strong> intracytoplasmic lipid or glycogen.<\/li>\n\n\n\n<li><strong>Transitional oxyphilic cells<\/strong>: smaller, less eosinophilic.<\/li>\n<\/ul>\n<\/li>\n\n\n\n<li><strong>Ultrastructure of Chief Cells<\/strong>\n<ul class=\"wp-block-list\">\n<li>Chief cells arranged in <strong>cords and nests<\/strong>, separated by a <strong>basal lamina<\/strong> from the interstitium.<\/li>\n\n\n\n<li>Plasma membranes: straight, with desmosomes linking adjacent cells.<\/li>\n\n\n\n<li><strong>Active phase<\/strong>:\n<ul class=\"wp-block-list\">\n<li>Enlarged Golgi apparatus, numerous vacuoles and vesicles in Golgi region.<\/li>\n\n\n\n<li>Many <strong>mature secretory granules<\/strong> (50\u2013300 nm).<\/li>\n\n\n\n<li>Granules are oval\/dumbbell-shaped, with a single membrane, a thin clear space inside, and a dense area of short rodlike profiles.<\/li>\n<\/ul>\n<\/li>\n\n\n\n<li><strong>Secretion pathway<\/strong>:\n<ul class=\"wp-block-list\">\n<li>Granules move from cell \u2192 basement membrane \u2192 pericapillary space \u2192 capillary basement membrane \u2192 fenestrated endothelium \u2192 bloodstream (liberating PTH).<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">PHYSIOLOGY OF THE PARATHYROID GLANDS<\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Primary Function<\/strong>: Regulate and maintain <a href=\"https:\/\/myendoconsult.com\/learn\/normal-pth-levels-by-age\/\" data-wpil-monitor-id=\"250\">normal serum <strong>ionized calcium<\/strong> levels<\/a> (8.9\u201310.1 mg\/dL).<\/li>\n\n\n\n<li><strong>Hormone<\/strong>: <strong>Parathyroid hormone (PTH)<\/strong>.<\/li>\n\n\n\n<li><strong>Regulation<\/strong>:\n<ul class=\"wp-block-list\">\n<li><strong>Ionized calcium<\/strong> in blood modulates PTH secretion.<\/li>\n\n\n\n<li>Low Ca\u00b2\u207a \u2192 \u2191PTH release; high Ca\u00b2\u207a \u2192 \u2193PTH.<\/li>\n\n\n\n<li><strong>Calcium-sensing receptors (CaSRs)<\/strong> on <a href=\"https:\/\/myendoconsult.com\/learn\/anatomy-of-the-parathyroid-gland\/\" data-wpil-monitor-id=\"249\">parathyroid glands<\/a> sense serum Ca\u00b2\u207a.<\/li>\n\n\n\n<li>CaSRs in kidneys also adjust tubular Ca\u00b2\u207a reabsorption.<\/li>\n<\/ul>\n<\/li>\n\n\n\n<li><strong>Importance of Calcium<\/strong>\n<ul class=\"wp-block-list\">\n<li>Critical for <strong>neuromuscular<\/strong> function, bone structure, signal transduction (e.g., cytosolic free calcium as second messenger).<\/li>\n\n\n\n<li><strong>Hypercalcemia<\/strong> can cause muscle weakness, anorexia, constipation, confusion, coma.<\/li>\n\n\n\n<li><strong>Hypocalcemia<\/strong> can cause anxiety, muscle twitching, carpopedal spasm, seizures, stridor, bronchospasm.<\/li>\n<\/ul>\n<\/li>\n\n\n\n<li><strong>Calcium Balance<\/strong>\n<ul class=\"wp-block-list\">\n<li>Daily dietary Ca\u00b2\u207a: <strong>300\u20131500 mg<\/strong>; net GI absorption ~<strong>200 mg\/day<\/strong>.<\/li>\n\n\n\n<li>Urinary Ca\u00b2\u207a excretion ~<strong>200 mg\/day<\/strong> (about 2% of filtered load).<\/li>\n\n\n\n<li>Stool Ca\u00b2\u207a excretion varies greatly with body needs\/diet (normally 500\u2013700 mg\/day).<\/li>\n<\/ul>\n<\/li>\n\n\n\n<li><strong>Phosphate Metabolism<\/strong>\n<ul class=\"wp-block-list\">\n<li>Dietary phosphate: ~<strong>800\u2013900 mg\/day<\/strong>.<\/li>\n\n\n\n<li>GI absorption enhanced by <strong>1,25-dihydroxyvitamin D<\/strong>.<\/li>\n\n\n\n<li>Fecal excretion ~30% dietary intake; renal excretion varies inversely with PTH.<\/li>\n<\/ul>\n<\/li>\n\n\n\n<li><strong>Homeostatic Mechanisms<\/strong>\n<ul class=\"wp-block-list\">\n<li><strong>Low serum Ca\u00b2\u207a<\/strong> \u2192 parathyroid releases PTH \u2192 increased renal 1\u03b1-hydroxylation of 25(OH)D \u2192 forms <strong>1,25(OH)\u2082D (calcitriol)<\/strong> \u2192 \u2191intestinal Ca\u00b2\u207a absorption.<\/li>\n\n\n\n<li>PTH enhances <strong>renal tubule Ca\u00b2\u207a reabsorption<\/strong> &amp; <strong>bone resorption<\/strong> \u2192 net rise in serum Ca\u00b2\u207a.<\/li>\n\n\n\n<li><strong>High serum Ca\u00b2\u207a<\/strong> \u2192 suppressed PTH secretion \u2192 less 1\u03b1-hydroxylation \u2192 less Ca\u00b2\u207a absorption; also more renal Ca\u00b2\u207a excretion &amp; reduced bone resorption.<\/li>\n<\/ul>\n<\/li>\n\n\n\n<li><strong>Hypoparathyroidism<\/strong>\n<ul class=\"wp-block-list\">\n<li>If parathyroids are absent or nonfunctional:\n<ul class=\"wp-block-list\">\n<li>Serum Ca\u00b2\u207a can fall to ~7 mg\/dL, but seldom below 5 mg\/dL (body\u2019s skeletal reservoir stabilizes).<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<\/li>\n\n\n\n<li><strong>Effects of Excess PTH<\/strong>\n<ul class=\"wp-block-list\">\n<li>Osteoclast activation \u2192 bone matrix resorption \u2192 mobilization of Ca\u00b2\u207a and phosphate.<\/li>\n\n\n\n<li><em><a href=\"https:\/\/myendoconsult.com\/learn\/bone-turnover-markers-in-osteoporosis\/\" data-wpil-monitor-id=\"251\">Bone turnover<\/a> response<\/em>: osteoclasts &amp; subsequent osteoblastic repair \u2192 raised serum alkaline phosphatase.<\/li>\n\n\n\n<li>PTH-induced bone resorption is also linked to <strong>GI absorption<\/strong> of calcium &amp; phosphate (via upregulated 1,25(OH)\u2082D).<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">BONE REMODELING UNIT<\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Overview<\/strong>\n<ul class=\"wp-block-list\">\n<li>Bone = collagen matrix + hydroxyapatite (Ca\u2081\u2080(PO\u2084)\u2086(OH)\u2082).<\/li>\n\n\n\n<li><strong>Modeling<\/strong> (childhood): changes in bone size\/shape.<\/li>\n\n\n\n<li><strong>Remodeling<\/strong> (lifelong): maintenance\/repair via coordinated osteoclast &amp; osteoblast cycles.<\/li>\n<\/ul>\n<\/li>\n\n\n\n<li><strong>Bone Cells<\/strong>\n<ul class=\"wp-block-list\">\n<li><strong>Osteoblasts<\/strong>: form bone, derived from mesenchymal stem cells.\n<ul class=\"wp-block-list\">\n<li>Respond to PTH, 1,25(OH)\u2082D, estrogen, etc.<\/li>\n\n\n\n<li>Secrete collagen &amp; osteoid; become osteocytes or line bone surface or undergo apoptosis.<\/li>\n<\/ul>\n<\/li>\n\n\n\n<li><strong>Osteoclasts<\/strong>: multinucleated, large cells from monocyte\/macrophage lineage; resorb bone via acid &amp; enzymes.<\/li>\n<\/ul>\n<\/li>\n\n\n\n<li><strong>Remodeling Cycle<\/strong>\n<ol class=\"wp-block-list\">\n<li><strong>Resorption<\/strong> (\u223c2 weeks)\n<ul class=\"wp-block-list\">\n<li>Osteoblasts release cytokines (RANK ligand, M-CSF) \u2192 osteoclast differentiation \u2192 degrade bone mineral &amp; collagen.<\/li>\n\n\n\n<li>Self-limited by local factors (high Ca\u00b2\u207a, TGF-\u03b2, etc.).<\/li>\n<\/ul>\n<\/li>\n\n\n\n<li><strong>Reversal<\/strong> (\u223c4 weeks)\n<ul class=\"wp-block-list\">\n<li>Mononuclear cells deposit a cement line &amp; recruit osteoblast precursors.<\/li>\n<\/ul>\n<\/li>\n\n\n\n<li><strong>Formation<\/strong> (\u223c16 weeks)\n<ul class=\"wp-block-list\">\n<li>Osteoblasts lay osteoid until resorbed cavity is refilled.<\/li>\n\n\n\n<li>Osteoid \u2192 mineralized.<\/li>\n\n\n\n<li>Cycle returns to quiescent state.<\/li>\n<\/ul>\n<\/li>\n<\/ol>\n<\/li>\n\n\n\n<li><strong>Defective Remodeling<\/strong>\n<ul class=\"wp-block-list\">\n<li><strong>Excessive osteoclastic resorption<\/strong> or <strong>incomplete osteoblastic refilling<\/strong> \u2192 net bone loss.\n<ul class=\"wp-block-list\">\n<li>Found in osteoporosis, hyperparathyroidism, etc.<\/li>\n<\/ul>\n<\/li>\n\n\n\n<li><strong>Impaired osteoclast function<\/strong> \u2192 overly dense bones (osteopetrosis).<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"<p>HISTOLOGY OF THE NORMAL PARATHYROID GLANDS PHYSIOLOGY OF THE PARATHYROID GLANDS BONE REMODELING UNIT<\/p>\n","protected":false},"featured_media":0,"template":"","oen_topic_chapter":[687],"class_list":["post-4422510","oen_topic","type-oen_topic","status-publish","hentry","oen_topic_chapter-parathyroid-gland","post-wrapper","thrv_wrapper"],"_links":{"self":[{"href":"https:\/\/myendoconsult.com\/learn\/wp-json\/wp\/v2\/oen_topic\/4422510","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/myendoconsult.com\/learn\/wp-json\/wp\/v2\/oen_topic"}],"about":[{"href":"https:\/\/myendoconsult.com\/learn\/wp-json\/wp\/v2\/types\/oen_topic"}],"version-history":[{"count":3,"href":"https:\/\/myendoconsult.com\/learn\/wp-json\/wp\/v2\/oen_topic\/4422510\/revisions"}],"predecessor-version":[{"id":4422513,"href":"https:\/\/myendoconsult.com\/learn\/wp-json\/wp\/v2\/oen_topic\/4422510\/revisions\/4422513"}],"wp:attachment":[{"href":"https:\/\/myendoconsult.com\/learn\/wp-json\/wp\/v2\/media?parent=4422510"}],"wp:term":[{"taxonomy":"oen_topic_chapter","embeddable":true,"href":"https:\/\/myendoconsult.com\/learn\/wp-json\/wp\/v2\/oen_topic_chapter?post=4422510"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}