A 3-year-old girl is brought to her pediatrician for a well child checkup. She has met all of the appropriate developmental milestones. Her height corresponds to the 60^th percentile. Osteoblasts near the growth plates of her long bones secrete matrix material, and when they become trapped in the ossified matrix, they become known as osteocytes. These osteocytes remain connected to each other by:

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11ec160b_3797_a8bb_9249_d3484da4ecce_MD0001_00 Within a single Haversian system, the central canal is encircled by multiple concentric lamellae of bony matrix that each contains lacunae filled with osteocytes and extracellular bone fluid. Delicate canaliculi radiate from each lacuna to create a reticular network with adjacent lacunae, and the cytoplasmic processes of the osteocytes lie within these canaliculi. These cytoplasmic processes send signals to and exchange nutrients and waste products with the osteocytes within neighboring lamellae via gap junctions. The osteocytes serve to maintain the structure of the mineralized matrix and control the short-term release and deposition of calcium (i.e., calcium homeostasis). The plasma calcium concentration directly dictates the metabolic activity of osteocytes, while parathyroid hormone and calcitonin indirectly influence their metabolic activity. Osteocytes can also sense mechanical stresses and send signals to modulate the activity of surface osteoblasts, thereby helping to regulate bony remodeling. (Choice A) Tight junctions (zonula occludens) are observed at the apices of glandular cells and consist of two closely adherent cytoplasmic membranes without an intervening space. Tight junctions are the first component of the junctional complex. (Choice B) Hemidesmosomes are half desmosomes that extend from the basal surfaces of keratinocytes in the stratified squamous epithelium to attach to the basal lamina. (Choice C) Intermediate junctions (zonula adherens) are a delicate network of cytoplasmic filaments that radiate from the cell membrane to hold adjacent cells together. Intermediate junctions are the second component of the junctional complex. (Choice E) Desmosomes are small, circular, adherent patches circumferentially placed around cells that comprise the third component of the junctional complex. These patches are particularly common in stratified squamous epithelium and contribute significantly to the structural cohesiveness of tissues subject to mechanical stressors. Educational objective: Osteocytes have long intracanalicular processes that extend through the ossified bone matrix. These cytoplasmic processes send signals to and exchange nutrients and waste products with the osteocytes within neighboring lamellae via gap junctions. Osteocytes can sense mechanical stresses and send signals to modulate the activity of surface osteoblasts, thereby helping to regulate bony remodeling.

A 3-Year-Old Girl Is Brought to Her Pediatrician for a Well