Many people have reported a tapering off of LSJL results like our individual
who grew 1/4" an inch and another who also
grew 1/4" of an inch. The decrease in LSJL results over time could be a result of a loss of
mechanosensativity. I have grown 1 1/2" with Lateral Synovial Joint Loading, each increase in height occurring after an increase in load Bone(and probably chondrocytes, stem cells, and periosteum) loses mechanical sensitivity after loading but recovers 98% after 24 hours. Now, mechanical sensitivity in response to load of muscles is well documented and a number of pathways throughout the body are shared like the Wnt signaling pathway and actin filaments.
So, we can probably infer a lot about how to surpass plateau's in LSJL by how people surpass plateau's in muscle building. Some possible methods are
inhibiting myostatin, increasing the load, and strategic deconditioning. Let's look at how we can increase mechanosensitivity in stem cells.
Mechanosensitivity of the rat skeleton decreases after a long period of loading, but is improved with time off
"
After the initial adaptation to large mechanical loads, it appears as though the skeleton's responsiveness to exercise begins to wane. To counteract the waning effects of long-term mechanical loading,
“time off” may be needed to improve the responsiveness of bone cells to future mechanical signals and reinitiate bone formation{this may be true for chondrocytes and stem cells as well}. Fifty-seven female
Sprague–Dawley rats (7–8 months of age) were randomized to one of following groups: Group 1 loading was applied for 5 weeks followed by 10 weeks of time off (1 × 5); Group 2 loading was applied for 5 weeks, followed by time off for 5 weeks and loading again for 5 weeks (2 × 5); Group 3 loading was applied continuously for 15 weeks (3 × 5); Group 4 age-matched control group; and Group 5 baseline control group.
An axial load was applied to the right ulna for 360 cycles/day, at 2 Hz, 3 days/week at 15 N[so not lateral loading so no pressure gradient(laterally loading squeezes the bone thereby inducing fluid flow) like LSJL but still shows effects to sensitivity to load over time]. At the end of the intervention,
all three loaded groups showed similar increases in bone mass, cortical area, and IMIN in response to mechanical loading. Bone formation rate of the loaded
ulna was increased in the first 5 weeks of loading for all three loaded groups; however, during the last 5 weeks, it was only significantly increased in the group that had time off (2 × 5). The group that had time off (2 × 5) also showed greater improvements in work to failure compared to the group loaded for 5 weeks (1 × 5) and the entire 15 weeks (3 × 5). A second experiment showed that the waning effect of long-term loading on the
skeleton is not a result of aging.
mechanical loading of the rat ulna results in large improvements in bone formation during the first 5 weeks of loading, but continual loading decreases the osteogenic response. Having time off increases
bone formation and improves the resistance to fracture."
Note that the bone was loaded for 3 days a week which is well within the normal deconditioning period for the actin cytoskeleton(24-48 hours). The group that had 5 weeks off had the greatest improvements in bone parameters but that doesn't mean 5 weeks is needed to restore mechanosensativity.
"To counteract the waning effects of long-term mechanical loading, “time off” may be needed to improve the responsiveness of
bone cells to future mechanical signals and reinitiate
bone formation"<-the same principles may apply to chondrocytes and stem cells depending on what mechanism requires the time off. If it is a mechanism shared by all three cell types then it is likely that all three types may need the same deconditioning. And the mechanism of conditioning is important to know whether this adaptation to axial loading also applies to lateral loading(which increases intramedullary pressure).
"Short-term loading studies in
rats suggest that rest periods ranging from 10 to 14 s in between individual loading cycles, and from 1 to 8 h in between bouts of loading enhance the osteogenic response more than continuous uninterrupted loading"<-this may be an area of difference between stem cells and osteogenic cells.
"Mechanical loading imposed a strain of 3288 ± 83 μ
at the midshaft of the
ulna in the beginning of the experiment. The strain was decreased in all three loaded groups and age-matched controls due to periosteal
bone apposition that occurred over the 15-week period."<-so the adaptation can be due to a loss of relative strain(which decreases due to bone apposition as the stronger the bone is the less strain that is being applied at the same load). IF relative strain increased then adaptation would continue. This may not apply to LSJL in which increased pressure results in chondrogenic differentiation. However, these chondrocytes eventually undergo endochondral ossification not resulting in a permanent adaptation. Osteoblasts become osteocytes for a longer period time than chondrocytes which are transient as they undergo apoptosis before completing endochondral ossification.
"An exercise regime that maintains the same loads for many years (i.e., long-distance running) may only be beneficial during the initial months of training. This phenomenon has already been identified in tennis players and exercise interventions in children.
a period of time off may be needed to reinitiate a response."<-Strain on the bone is required for TGF-Beta release by the osteocytes. TGF-Beta helps induce chondrogenic differentiation so time off from LSJL may be important. Stronger clamping may be another alternative to time off.
Loading alters actin dynamics and up-regulates cofilin gene expression in chondrocytes.
"
The actin cytoskeleton has been implicated in cell mechanics and mechanotransduction. Loading modulates actin dynamics and organisation with subsequent changes in gene expression for actin associated proteins. Chondrocytes were transfected with eGFP-actin, seeded in agarose and subjected to cyclic compression (10 cycles, 1 Hz, 0-15% strain).
Compression resulted in a subsequent reduction in cortical eGFP-actin intensity and a reduction in fluorescence recovery after photobleaching (FRAP), suggesting net cortical actin de-polymerisation, compared to unloaded controls.
Cyclic compression for 10 min up-regulated gene expression for the actin depolymerising proteins, cofilin and destrin.
mechanical loading alters cortical actin dynamics, providing a potential mechanism through which chondrocytes can adapt their mechanical properties and mechanosensitivity to the local mechanical environment."
Lateral Loading is compressive loading for the stem cells to differentiate into chondrocytes. Compressive loading reduces mechanosensitivity by upregulating actin depolymerising proteins. If you can re-polymerise the actin you may be able to restore mechanosensitivity in the chondrocytes(and your muscles, bones, etc.). Thus, the need for time off to allow actin to re-polymerise or increase the strength of LSJL clamping to make up for less sensative actin.
Membrane type-1 matrix metalloproteinase is induced following cyclic compression of in vitro grown bovine chondrocytes.
"Membrane type-1 matrix metalloproteinase (MT1-MMP)[MMP14 which is upregulated by LSJL] [responds] to cyclic compression of chondrocytes grown in vitro.
Cyclic compression (30min, 1kPa, 1Hz) was applied to bovine [articular] chondrocytes (6-9-month-old animals) grown on top of a biodegradable substrate within 3 days of initiating culture.
After cyclic compression, MT1-MMP showed a rapid and transient increase in gene expression. Elevated protein levels were detected within 2h of stimulation which returned to baseline by 6h. During cyclic compression,
phosphorylation of the mitogen activated protein kinase ERK1/2 increased significantly.
This was followed by increased gene and protein expression of the transcription factor; early growth factor-1 (Egr-1)[upregulated by LSJL] and Egr-1 binding to the MT1-MMP promoter. Blocking Egr-1 DNA binding with a decoy MT1-MMP oligonucleotide, downregulated MT1-MMP gene expression.
The ERK1/2 inhibitor U0126 also reduced Egr-1 DNA binding activity to MT1-MMP promoter sequences and subsequent transcription of MT1-MMP.
cyclic compression of chondrocytes in vitro upregulates MT1-MMP via ERK1/2 dependent activation of Egr-1 binding."
Since LSJL increased EGR-1 and MT1-MMP maybe LSJL also increased ERK1/2 phosphorylation? But we don't know Egr-1 and MMP14 binding only that they both increased in expression.
MT1-MMP contributes to the formation of cartilage canals. Cartilage canals being very important in the formation of growth plates and in turn surpassing your natural height. LSJL also upregulates early growth factor-1 which further accelerates MT1-MMP expression. The mechanosensitivity of MT1-MMP may be very important in LSJL induced height growth as MT1-MMP is crucial in the development of cartilage canals a possible mechanism for how LSJL works.
"MT1-MMP can activate MMP-13 and has been shown to be mechanosensitive"
"After 3 days of culture, cells were cyclically compressed for 30 min and 24 h later showed a significant (3.9-fold) increase in relative luciferase activity (RLU) (indicative of MT1-MMP expression) compared to unstimulated controls"<-About twice as much increase in fold as was observed in LSJL. And LSJL was 1 hour after loading whereas this was 24 hours after loading.
"the phosphorylated form of ERK1/2 (p-ERK1/2) increased 13.1-fold within the first 15 min of mechanical stimulation "
"Displacement of Egr-1 by Sp1 only occurs after phosphorylation at several different sites on Sp1, which increases its affinity for DNA binding. Sp1 is mechanosensitive as shear stress increased Sp1 activation in endothelial cells and inhibited MT1-MMP expression"
"In human chondrocytes, induction of Egr-1 by interleukin-1β resulted in repression of transcriptional activity of type II collagen.
Type II collagen expression, one of the main constituents of articular cartilage matrix is upregulated when Sp1 is bound, or inhibited when Egr-1 is bound and displacement of the Egr-1 protein by the Sp1 transcription factor reversed this inhibition"
Mechanical compression and hydrostatic pressure induce reversible changes in actin cytoskeletal organisation in chondrocytes in agarose.[Lateral Synovial Joint Loading induces both mechanical compression and hydrostatic pressure, thus LSJL induces reversible changes in actin cytoskeletal organization]
"In numerous cell types, the cytoskeleton has been widely implicated in mechanotransduction pathways involving stretch-activated ion channels, integrins and deformation of intracellular organelles. Studies have also demonstrated that the cytoskeleton can undergo remodelling in response to mechanical stimuli such as tensile strain or fluid flow. This study utilises the well-characterised chondrocyte-agarose model to demonstrate that both static and cyclic, compressive strain and hydrostatic pressure all induce remodelling of actin microfilaments. This remodelling was characterised by a change from a uniform to a more punctate distribution of cortical actin around the cell periphery. For some loading regimes, this remodelling was reversed over a subsequent 1h unloaded period. This reversible remodelling of actin cytoskeleton may therefore represent a mechanism through which the chondrocyte alters its mechanical properties and mechanosensitivity in response to physiological mechanical loading."
The actin skeleton of chondrocytes remodels to become less sensitive to mechanical loads. This is reversible over time due to unloading.
"Two different compressive loading regimes were investigated, namely cyclic strain between 0% and 15% at a frequency of 1 Hz and static strain of 15%"
"Pressure was applied in either a cyclic form between 0 and 5 MPa at 1 Hz or statically at 5 MPa. Control specimens remained unstrained and at atmospheric pressure throughout."
"Cells fixed 1 min after any of the four loading regimes exhibited notable differences in actin cytoskeletal organisation with a more pronounced punctate distribution than that observed in unloaded cells."
Static strain induced greater changes in actin cytoskeleton than cyclical strain. Actin cytoskeleton decreased to almost control in only 60 minutes after cyclic strain.
In contrast to cyclical hydrostatic pressure actin cytoskeleton organization increased after 60 minutes. In static hydrostatic pressure actin cytoskeleton organization decreased.
"Gross compression causes cell deformation from a spherical to an oblate ellipsoid morphology. The presence of a limited pericellular matrix at day 3 reduces the level of cell deformation"
"Cells were fixed in the unstrained spherical state to enable analysis of actin remodelling without the influence of changes in cell shap"
Mechanosensativity may also involve neural regulation:
Functional adaptation to loading of a single bone is neuronally regulated and involves multiple bones.
"Regulation of load-induced bone formation is considered a local phenomenon controlled by osteocytes [and]
may be neuronally regulated. Load-induced responses in the left and right ulnas and humeri were determined after loading of the right ulna in male Sprague-Dawley rats (69 +/- 16 days of age). After a single period of loading at -760-, -2000-, or -3750-microepsilon initial peak strain, rats were given calcein to label new bone formation. Bone formation and bone neuropeptide concentrations were determined at 10 days. In one group, temporary neuronal blocking was achieved by perineural anesthesia of the brachial plexus with bupivicaine during loading.
right ulna loading induces adaptive responses in other bones in both thoracic limbs compared with Sham controls and that neuronal blocking during loading [stopped] bone formation in the loaded ulna and other thoracic limb bones[bones relating to the chest]{but does this neuronal blocking stop chondrogenic induction?}. Skeletal adaptation was more evident in distal long bones{farther away from the body so the the tibia versus the femur} compared with proximal long bones. The
single period of loading modulated bone neuropeptide concentrations persistently for 10 days[so the most efficient period of loading for LSJL may be once every 10 days, HIT(High Intensity Training) recommended weight lifting every ten days. So LSJL once every 10 days may be most efficient if you're short on time but results will take much longer than if you load more frequently]. Functional adaptation to loading of a single bone in young rapidly growing rats is neuronally regulated and involves multiple bones. Persistent changes in bone neuropeptide concentrations after a single loading period suggest that plasticity[ability to undergo permanent changes] exists in the innervation of bone."
The study involves osteocytes but properties of chondrocytes can be inferred. If there is some neurological control then different means to restore adaptation may be used. Also, in the
Lateral Synovial Joint Loading study some increase in height was detected in the non-loaded contra-lateral limb. This indicates that there are neurological mechanisms at play in terms of stem cells and chondrocytes. For example, perceived strain may have a role on neurological adaptation but not at more local physical levels.
If you're short on time do LSJL once every 10 days. However, neuropeptide concentrations may still be sensitive to LSJL loading at much shorter intervals.
"The
periosteoum is densely innervated with a dense net-like meshwork of nerves, suggesting the existence of a sophisticated and
specialized neuronal regulatory mechanism optimized for detection of mechanical distortion of periosteum and bone. Peptidergic nerves are arranged into networks on the surface of bones, and are most numerous in the epiphysis and metaphysis."<-So it's possible that load on the fibrous capsule can induce a functional response in the epiphysis.
"Neuropeptides have pleiotrophic effects on bone cells, which express receptors for a range of neurotransmitters. In vitro, CGRP-α and SP act anabolically on bone cells, where as CGRP-β is not anabolic, and in vivo, CGRP+ and SP+ sensory nerves and sympathetic nerves may influence bone mass."<-Studying neurotransmitters such as SP may be a way to induce new height growth.
"In-vivo
load-induced bone formation was associated with
persistent decreases in bone CGRP, suggesting that the regulatory effects of CGRP+ sensory nerves on functional adaptation of bone are complex[CGRP may play a role in why the body may become less susceptible to LSJL over time]. Strain-dependent effects on bone concentrations of neuropeptides were most evident with CGRP.
As initial peak strain during loading increased, significant depression of bone CGRP concentrations occurred in as little as one hour after loading at high initial peak strain[CGRP concentration plays a role in mechanical sensitivity to load, of course we are looking for the neuropeptide associated with stem cells and chondrocytes]. This suggests that the peptideric innervation of bone is mechanosensitive and capable of a sophisticated response to changes in the loading environment of bone. In contrast, changes in SP and VIP concentrations in bone appeared much less mechanically sensitive. As initial peak strain increased, bone concentrations of these neuropeptides were higher, particularly at 10 days after loading. Such pleiotropic responses to mechanical loading by the innervation of bone again suggest the existence of a sophisticated mechanosensitive physiological pathway."<-We have to find out what neuropeptide is associated with stem cell/chondrogenic adaptation to loading. It's unlikely to be CGRP. However, Calcitonin[The C in CGRP stands Calcitonin] does induce responses in chondrocytes. However, it's unknown whether is an actual receptor chondrocytes or the effects of calcitonin are collateral affects from calcitonin on bone. But, if it is collateral effects that induce height growth than the mechanosensativity to CGRP is something worth pursuing.
A melacorticon[ACTH] may be the neuropeptide that is mechanosensative in chondrocytes.
A possible role for melanocortin peptides in longitudinal growth.
"The elevation of the melanocortin peptide, ACTH [is associated with] longitudinal growth.
Overproduction of ACTH in familial glucocorticoid deficiency (FGD) is associated with increased growth and ACTH increases the differentiation of chondrocytes along the endochondral pathway in vitro[more differentiation of stem cells to chondrocytes = more height growth]. Using the leptin-deficient obese (ob/ob) mouse along with lean control littermates (n = 9-10), we investigated the effects of adrenalectomy[removal of the adrenal glands] (ADX)-induced elevated ACTH with and without peripheral administration of the MC3-R-specific agonist, gamma2-melanocyte stimulating hormone (gamma2-MSH), on longitudinal growth. Naso-anal and tibial growth were measured together with growth plate parameters; both total and zonal heights together with the proliferative index.
ADX significantly increased naso-anal length in lean mice and ADX plus gamma2-MSH administration significantly increased naso-anal length above ADX alone in ob/ob mice. gamma2-MSH administration to ADX lean and ob/ob mice significantly increased tibial length. In ob/ob mice, these changes occurred in the context of reduced food intake. Analysis of total and zonal growth plate heights suggest an increase in hypertrophic differentiation and an overall increase in growth plate turnover in ADX lean and ob/ob mice.
ADX enhances linear growth and the results of gamma2-MSH treatment suggest that the melanocortin system plays a role in linear growth."
ACTH is sold as a homeopathic remedy. The effectiveness is unclear.
Adrenal Corticot. (Potency: 12C)
Maybe Adult Height Increase should test it as part of their monster trial?
"ACTH also evokes transient elevations in intracellular free calcium [Ca2+]i and increases basal [Ca 2+]i and differentiation of chondrocytes along the endochondral pathway is associated with an incremental increase in basal [Ca2+]i"
So, ACTH may involve Calcium ions as well. Maybe CGRP and ACTH play a role in enhancing longitudinal growth with Calcium ions being the main factor.
L-Type Voltage Gates may play a role in mechanosensativity to these calcium ions.
Bone strength: current concepts.
"Bone is a multiphase material made up of a tough collagenous matrix intermingled with rigid mineral crystals. The mineral gives bone its stiffness. Without sufficient mineralization,
bones will plastically deform under load[we are looking for this deformation to increase hydrostatic pressure in the epiphysis to induce chondrogenic differentiation]. Collagen provides toughness to bone making it less brittle so that it better resists fracture. Bone adapts to mechanical stresses largely by changing its size and shape. Tissue is added in regions of high mechanical stress.
bone tissue possesses a mechanosensing apparatus. Several pathways [for mechanosensing] including
membrane ion channels, ATP signaling, second messengers, such as prostaglandins and nitric oxide, insulin-like growth factors, and Wnt signaling."
It's possible that numerous pathways play a role in bone cell response to mechanical loading.
"In cultured osteoblastic cells,
fluid flow increases intracellular calcium within minutes and this response is suppressed by gadolinium, a blocker of the stretch-activated calcium channel[Fluid flow is initiated by LSJL]. In addition, the
L-type voltage-operated calcium channel probably plays a role in bone cell mechanotransduction. Studies using bone explants showed that gadolinium abolished loading-related responses in osteocytes, while a blocker of L-type calcium channels inhibited loading-related responses in osteoblasts. In addition, two blockers of L-type calcium channels, verapamil and nifedipine, strongly suppress mechanically induced bone formation in rats."<-Chondrocytes do seem to be receptive to
calcium ions with Calcium ions encouraging MMP synthesis. So the L-type calcium channel sensitivity may play a role in chondrocyte driven height growth.
"Prostaglandins and nitric oxide are released from bone cells within minutes after dynamic mechanical loading. Blockade of prostaglandin synthesis using nonsteroidal anti-inflammatory drugs (NSAID) suppresses mechanically induced bone formation
in vivo, as does the nitric oxide synthesis inhibitor L-NAME."<-This is likely true of chondrocytes as well.
"In cultured osteoblasts, PTH(1-34) sensitizes cells to mechanical forces possibly by enhancing the mobilization of intracellular Ca
2+."PTH(1-34) is
teraparatide.
"Estrogen is another hormone that may interact with mechanical loading pathways.
Mechanical loading increases estrogen receptor alpha phosphorylation in osteoblasts through activation of ERK 1. In addition, mice deficient in estrogen receptor alpha expression show suppressed osteogenic responsiveness to mechanical loading. In contrast, others have shown that estrogen suppresses the anabolic effect of mechanical loading. These observations might be reconciled by considering that the effects of estrogen on the skeleton are site specific.
Estrogen suppresses bone resorption on trabecular and endocortical bone surfaces, thus preserving bone mass. Conversely, estrogen suppresses bone formation on the periosteal surfaces"<-These effects wouldn't seem to affect height growth. The suppression of bone resorption within the trabecular bone may be a problem for growth plate modeling.
"Pressure in the marrow cavity and/or fluid shear forces on marrow stromal cells (MSC) may stimulate nitric oxide synthase (NOS) activity and nitric oxide (NO) release"<-LSJL definitely increases pressure in the epiphyseal marrow cavity.
Staurosporine and cytochalasin D induce chondrogenesis by regulation of actin dynamics in different way.
"Actin cytoskeleton has been known to control and/or be associated with chondrogenesis.
Staurosporine and cytochalasin D modulate actin cytoskeleton and affect chondrogenesis[So straurosporine and cytochalasin D may have height increase applications]. We investigate the effect of staurosporine and cytochalasin D on the actin dynamics as well as possible regulatory mechanisms of actin cytoskeleton modulation. Staurosporine and cytochalasin D have different effects on actin stress fibers in that
staurosporine dissolved actin stress fibers while cytochalasin D disrupted them in both stress forming cells and stress fiber-formed cells[so they two compounds are complementary of each other].
Increase in the G-/F-actin ratio either by dissolution or disruption of actin stress fiber is critical for the chondrogenic differentiation[Thus staurosporine and cytochalasin D can help us grow taller]. Cytochalasin D reduced the phosphorylation of cofilin, whereas staurosporine showed little effect on cofilin phosphorylation. Either staurosporine or cytochalasin D had little effect on the phosphorylation of myosin light chain (MLC). These results suggest that staurosporine and cytochalasin D employ different mechanisms for the regulation of actin dynamics and provide evidence that removal of actin stress fibers is crucial for the chondrogenic differentiation."
Now just need to find a supplement that involves staurosporine and cytochalasin D. If you take these supplements you might not need a deconditioning period.
"Dynamic actin cytoskeleton is essential for diverse cellular processes such as the driving cell shape changes, cellular motility, adhesion, cytokinesis, and endocytosis"<-Processes which are all involved in chondroinduction.
"Cytochalasin D, a blocker of actin polymerization and elongation of actin, has been extensively used for the study of chondrogenesis. It has been known to induce chondrogenesis of limb mesenchymal cultures"
"Staurosporine, a broad spectrum protein kinase inhibitor, disrupts the actin stress fibers and restores the differentiated functions of dedifferentiated chondrocytes"
"Actin binding proteins regulate disassembly and assembly of actin filaments by sequestering G-actin and by depolymerizing actin. Formation of actin stress fibers is induced by myosin light chain (MLC) phosphorylation which regulates the activity of non-muscle myosin type II. Cofilin binds to both monomeric and filamentous actin and increase actin dynamics by depolymerizing filaments form their pointed ends. The activity of cofilin is regulated by phosphorylation of Serine 3. Phosphorylation of cofilin abolishes the ability of cofilin to bind to F-actin leading to loss of its ability to depolymerize F-actin"
"Staurosporine treatment [on undifferentiated cells] for 2 days induced the expression of type II collagen. Cytochalasin D treatment also resulted in disintegration of the stress fibers but F-actin exhibited an aggregated pattern. Cytochalasin D-treated cells also expressed collagen type II as in the case of staurosporine. "
"Cells which still [had] stress fibers, even thin and small in number, were not stained for collagen type II."
"the relative amount of filamentous (F)-actin, globular (G)-actin, and relative abundance of F-actin compared with G-actin [is the] F/G-actin ratio"<-So more G-actin is pro-chondrogenic.
"F-actin which was dissolved by staurosporine or cytochalasin D, showed a diffused or spotted distribution, respectively. The relative amount of G-actin was decreased with staurosporine treatment and increased with cytochalasin D treatment, which were reflected in the F/G actin ratio. Both of staurosporine and cytochalasin d reduced F-actin/G-actin ratio with more effect by cytochalasin D"
"Cytochalasin D decreases the phosphorylation of cofilin. Cytochalasin B itself has a weak severing effect. Hcytochalasin D reduces phosphorylation of ADF/cofilin which would activate cofilin and thereby depolymerize F-actin. cytochalasin D promotes F-actin depolymerization by reduction of cofilin phosphorylation in concert with the prevention of F-actin polymerization by its immediate effect on the organization of cytoskeletal networks."
"Inhibition of protein kinase C, protein kinase A, and Ca2+/calmodulin-dependent protein kinase with specific inhibitors did not induce the chondrogenesis of mesenchymal cells"
"ROCK controls actin dynamics by regulating MLC phosphorylation directly or indirectly through MLC phosphatase or by regulating cofilin phosphorylation through LIM kinases"
"Staurosporine reduced the activity of RhoA. RhoA regulates ROCK activity but inhibition of ROCK activity neither affects MLC phosphorylation nor induces chondrogenesis. RhoA uses other pathway than ROCK for the chondrogenic differentiation"
"cytochalasin D reduced phosphorylation of cofilin"
Cellular accommodation and the response of bone to mechanical loading.
"The ulnae of Sprague-Dawley rats were loaded in axial compression. The animals received loading for 15 weeks with progressively decreasing loads, increasing loads, or a constant load. The results showed the largest increases in geometry in the decreasing load group, followed by the constant load group. Bone formation rates (BFRs) were significantly greater in the decreasing load group during the first 2 weeks of the study as compared to all other groups (P<0.05)[the decreasing load group had the highest load first]. After the first few weeks of mechanical loading, the BFR in the loaded ulnae returned to the values of the nonloaded ulnae. After the initial weeks of loading, bone stopped responding so the degree of adaptation was proportional to the initial peak load magnitude."
Stem cells ability to differentiate into chondrocytes may stop responding as well especially if osteocytes release chemical signals that alter this mechanism. This means that you probably don't want to gradually work up but start with the maximum planned LSJL load right away followed by a deconditioning period.
"Bone cells must process loading information locally because bone tissue is poorly innervated and, unlike many mechanoreceptor cells, cannot rely on the central nervous system to integrate and distribute information about mechanical signals"
"the MES[minimum effective strain] may well be location dependent within bones"
"the groups that had the largest force applied at the start of the study saw the largest overall changes. After the initial weeks of the loading, bone stopped responding to mechanical loading"
"10 weeks of loading desensitized bone cells so no further adaptation response was possible in the last 5 weeks"
"significant trabecular bone formation occurring in response to loading within the first week of loading, with significantly less in weeks 2–4 of loading"
This next study will help answer our question of whether loading produces adaptations in marrow cells that would reduce their responsiveness to chondroinduction by LSJL. By studying unloading we can see if there's any indication of the sensitivity of genes increasing.
"Hind limb unloading (HU) [was performed on] the tibiae of young C57BL/6J male mice. We focused on the effects of HU in chondrogenic, osteogenic, and marrow mesenchymal cells.
We analyzed for expression of genes and proteins at two time points after HU (7 and 14 days), and at 14 days after recovery from HU. We studied genes involved in osteogenesis (alkaline phosphatase (AP), osteocalcin (OC){up in LSJL}, bonesialoprotein (BSP){up in LSJL}, membrane type1 matrix metalloproteinase (MT1-MMP)), in extracellular matrix (ECM) formation (procollagenases (BMP1), procollagenase enhancer proteins (PCOLCE)) and remodeling (metalloproteinase-9 (MMP9), RECK), and in bone homeostasis (Stro-1, CXCL12, CXCR4, CD146).
We report the following patterns and timing of changes in gene expression induced by HU: 1) transient or stable down modulations of differentiation-associated genes (AP, OC), genes of matrix formation, maturation and remodelling, (BMP1, PCOLCEs MMP9) in osteogenic, chondrogenic and bone marrow cells; 2)
up modulation of MT1-MMP in these same cells, and uncoupling of its expression from that of AP; 3) transient down modulation of the osteoblast specific expression of BSP; 4) for genes involved in bone homeostasis,
up modulation in bone marrow cells at distal epiphysis for CXCR4{CXCR4 plays a role in stem cell migration}, down modulation of CXCL12, and transient increases in osteoblasts and marrow cells for Stro1{
Stro1 positive stem cells decline with age}.
14 days after limb reloading expression returned to control levels for most genes and proteins in most cell types, except AP in all cells, and CXCL12, only in bone marrow."
So temporary unloading periods can result in permanent decrease in CXCL12 positive cells. Unloading also decreased expression of procollagens in bone marrow that was not recovered by load.
"RECK is an endogenous membrane inhibitor of MMPs"
"CXCR4 is the receptor for Stromal cell-derived factor-1 (SDF-1 or CXCL12), which is constitutively secreted by osteoblasts and bone marrow stromal cells. "
"Dynamic levels of CXCL12 and CXCR4 expression play a key role in the homing of hematopoietic cells to the bone marrow"
"Activation of the CXCR4/CXCL12 pathway induces proliferation of hematopoietic and mesenchymal progenitors. CXCL12 also stimulates mononucleate cell fusion and TRAP activity and is a key factor in the normal homeostatic regulation of bone development and remodeling"
"Stro1 [is] a stage- and/or lineage-specific stromal antigen"
Do calcium fluxes within cortical bone affect osteocyte mechanosensitivity?
"Osteocytes directly participate in calcium homeostasis by regulating dissolution and deposition of calcium in the perilacuno-pericanalicular space. In the bulk of the canalicular space, the fluid flow due to chemical gradient generated by deposition or dissolution of calcium is negligible compared to the fluid flow due to hydraulic pressure. However, at the osteocyte proximity, the presence of calcium gradient generated sufficient fluid flow to induce significant changes in the shear stress on the osteocyte membrane. Calcium deposition and dissolution on the canalicular wall resulted in increased or decreased shear stress on the osteocyte membrane respectively. Strong calcium fluxes due to whole body calcium homeostasis may affect mechanical forces experienced by osteocytes."
"The pore space is filled by a pericellular matrix, which slows down fluid flow. The presence of chemical gradients (such as directional calcium fluxes between the bone matrix and the interstitial fluid) can result in osmotic fluid flow. In addition, the collagen fibers and the phospholipids present at the canaliculus and osteocyte walls, respectively, produce a negative surface charge density at these interfaces. As a consequence, these surfaces attract the cations present in the interstitial fluid, forming an electrical double layer composed of a layer of cations adsorbed by the charged surface (Stern layer) and a layer of mobile cations close to the interface (diffuse or Gouy–Chapmann layer), which may induce electro-osmotic fluid flow. Electrical phenomena in bone canaliculi and other nanoscale porous media have been shown to noticeably affect both ionic diffusion and fluid transport."
"Under the action of a chemical gradient, the chemical species move to compensate this difference in concentration, dragging with them some interstitial fluid through a viscous force."
"The interstitial fluid flows from the entrance to the exit in response to the hydraulic and electrical gradients (e.g., generated by blood pressure). In this situation, a calcium/phosphate deposition onto the canalicular wall (loss of chemical species in the fluid) corresponds to a higher concentration of calcium at the entrance of the canaliculus comparing to its exit, i.e. to a negative calcium gradient. As a consequence, the osmotic velocity is positive\ and concomitant to the hydraulic and electro-osmotic velocities"
"in the presence of a calcium/phosphate dissolution from the canalicular wall (gain of chemical species in the fluid), the amount of calcium within the canaliculus is lower at the entrance than at the exit. Thus, the calcium gradient is positive and the osmotic velocity is negative and opposite to the hydraulic and electro-osmotic velocities "
"If the hydraulic shear stress is too low to activate osteocyte's response, a concomitant calcium flux could increase the total shear stress up to values activating osteocyte's response. Conversely, if the hydraulic shear stress is such as to activate osteocyte's response, opposite calcium fluxes can decrease the total shear stress to values that osteocytes are not sensitive to."
