Ovariectomy enhances mechanical load-induced solute transport around osteocytes in rat cancellous bone

Abstract

To test if osteoporosis alters mechanical load-induced interstitial fluid flow in bone, this study examined the combined effect of estrogen deficiency and external loading on solute transport around osteocytes. An in vivo tracer, FITC-labeled bovine serum albumin, was injected into anaesthetized ovariectomized and control female Sprague Dawley rats before the right tibia was subjected to a controlled, physiological, non-invasive sinusoidal load to mimic walking. Tracer movement through the lacunar-canalicular system surrounding osteocytes was quantified in cortical and cancellous bone from the proximal tibia using confocal microscopy, with the non-loaded tibia serving as internal control. Overall, the application of mechanical loading increased the percentage of osteocyte lacunae labeled with injected tracer, and ovariectomy further enhanced movement of tracer. An analysis of separate regions demonstrated that ovariectomy enhanced in vivo transport of the injected tracer in the cancellous bone of the tibial epiphysis and metaphysis but not in the cortical bone of the metaphysis. These findings show that bone changes due to reduced estrogen levels alter convectional transport around osteocytes in cancellous bone and demonstrate a functional difference of interstitial fluid flow around osteocytes in estrogen-deficient rats undergoing the same physical activity as controls. The altered interstitial fluid flow around osteocytes is likely related to nanostructural matrix-mineral level differences recently demonstrated at the lacunar-canalicular surface of estrogen-deficient rats, which could affect the transmission of mechanical loads to the osteocyte.

The effects of estrogen deficiency on cortical bone microporosity and mineralization

“…Tibiae were harvested and either put immediately in 10% neutral buffered formalin for 48 hours or frozen at −20 °C. The analysis focused on the cortical and cancellous bone of the proximal tibial metaphysis because our recent studies demonstrated alterations in the lacunar-canalicular porosity surrounding osteocytes in this region [21, 22]. Tibiae from a subset of animals were used for three analyses: a micro-CT analysis to assess cortical microporosity and cancellous microarchitecture, a backscattered electron imaging and nanoindentation analysis to assess mineralization, and a histomorphometry analysis to assess bone turnover, as detailed below.…”

“…We found that the increase in effective canalicular size measured using a small molecular weight tracer in ovariectomized rats was due to nanostructural matrix-mineral changes at the osteocyte lacunar-canalicular surface. We have also found changes in molecular transport due to mechanical loading in the rat ovariectomy model [22]. …”

Osteocyte Mechanobiology

“…Ciani et al used ovariectomy-induced estrogen-deficient rats to explore the transport of solute around osteocyte. They injected FITC-labeled bovine serum albumin tracer into rats’ tibia during loading and demonstrated that the load-induced solute transport around osteocyte was enhanced in ovariectomized animals (48). Another study reported that androgens inhibits osteogenic responses to mechanical loading in adult male mice (49) demonstrating a close relationship between sex hormones and skeletal mechanobiology.…”

Osteoblast migration in vertebrate bone

“…In another intriguing contribution, Smit, Burger & Huyghe () modelled strain‐induced fluid flow in the BMU and concluded that during compression a fluid influx along the reversal and closing zones and a fluid efflux along the cutting cone occurs. With respect to a potential pathophysiological relevance, altered fluid flow around osteocytes in the lacunar–canalicular network of oestrogen‐deficient mice suggested a role of abnormal fluid flow in osteoporosis (Sharma et al., ; Ciani et al., ). Another type of signal produced by mechanical load in bone is a time‐varying electric field (Otter, McLeod & Rubin, ).…”