Quantitative regional associations between remodeling, modeling, and osteocyte apoptosis and density in rabbit tibial midshafts

Abstract

Evidence suggests that osteocyte apoptosis is involved in the adaptive response of bone, although the specific role of osteocytes in the signaling mechanism is unknown. Here, we examined and correlated regional variability in indices of remodeling, modeling, osteocyte apoptosis, and osteocyte density in rabbit tibia midshafts. Histomorphometric analysis indicated that remodeling parameters (BMU activation frequency, osteon density, forming osteon density, and resorption cavity density) were lower in the cranial region compared to other quadrants. In addition, pericortical subregions displayed less remodeling relative to intracortical and endocortical ones. Modeling indices also demonstrated regional variability in that periosteal surfaces exhibited a greater extent of bone forming surface than endosteal ones across all anatomic quadrants. In contrast, endosteal surfaces demonstrated significantly greater surface mineral apposition rates compared to periosteal surfaces in caudal, medial, and lateral but not cranial quadrants. Using TUNEL analysis to detect osteocytes undergoing apoptosis, the density of apoptotic osteocytes was found to be lower in cranial quadrants relative to medial ones. In addition, the densities of osteocyte lacunae, empty lacunae, and total osteocytes were higher in lateral fields relative to caudal quadrants. There was a strong, statistically significant linear correlation between the remodeling indices and apoptotic osteocyte density, supporting the theory that osteocytes undergoing apoptosis produce signals that attract or direct bone remodeling. In contrast, the modeling parameters did not exhibit a correlation with apoptotic osteocytes, although there was a strong correlation between the modeling indices and the density of empty osteocyte lacunae, corroborating previous studies that have found that osteocytes inhibit bone formation. It was found that osteocyte density and osteocyte lacunar density did not significantly correlate with modeling or remodeling parameters, suggesting that cell viability should be examined in studies correlating bone turnover parameters with the functional role of osteocytes in bone adaptation.

Osteocyte Wnt/β-Catenin Signaling Is Required for Normal Bone Homeostasis

“…Osteocyte apoptosis has been shown to result in increased osteoclast recruitment and bone resorption (1,16,20,22,34,49,61). We therefore determined the levels of expression of the apoptosis facilitators Bcl2-like 11/Bim and BCL2-associated X protein (Bax) (38,65) in femoral diaphyses of 2-month-old mutant and control female mice.…”

Microscopic Evaluation of Necrotic Cell Death in the Cartilage Destined for Experimental Tracheal Allografts: Lyophilization vs Cryopreservation

“…During the last stages of programmed cell death, osteocytes break up into apoptotic bodies that may be less than 2-μm in diameter. Such bodies are likely to remain undetectable to light microscopy and may well be interpreted as empty lacunae [12]. On the other hand, apoptotic bodies are phagocytosed by neighboring cells or macrophages, thereby preventing the retention of cellular debris in the extracellular space.…”

“…The main cell death pathway after rehydrating or thawing tracheal segments would then be necrosis. "ccording to Hedgecock [12], if large numbers of osteocytes undergo apoptosis at the time of tissue harvest, many other osteocytes likely underwent apoptosis previously, becoming shrunken and fragmented and disappearing from their lacunae. Empty lacunae may represent osteocytes that previously died by apoptosis.…”

An Investigative Study Into the Relationship of Bone Morphogenetic Protein Antagonist Expression and Osteocyte Density by Region and Quadrant

“…All incubation times for VectorLabs products were also performed at suggested times; again excluding blocking serum incubation time, which was increased from 20 to 30 minutes to ensure blocking. Other procedures follow the general steps of immunohistochemical staining described in literature (such as those of Hedgecock, et al (2007) and Haudenschild, et al (2004)) but specifics were developed in house as needed.…”