Posterior capsule opacification: What's in the bag?

Cataract, a clouding of the lens, is the most common cause of blindness in the world. It has a marked impact on the wellbeing and productivity of individuals and has a major economic impact on healthcare providers. The only means of treating cataract is by surgical intervention. A modern cataract operation generates a capsular bag, which comprises a proportion of the anterior capsule and the entire posterior capsule. The bag remains in situ, partitions the aqueous and vitreous humours, and in the majority of cases, houses an intraocular lens (IOL). The production of a capsular bag following surgery permits a free passage of light along the visual axis through the transparent intraocular lens and thin acellular posterior capsule. Lens epithelial cells, however, remain attached to the anterior capsule, and in response to surgical trauma initiate a wound-healing response that ultimately leads to light scatter and a reduction in visual quality known as posterior capsule opacification (PCO). There are two commonly-described forms of PCO: fibrotic and regenerative. Fibrotic PCO follows classically defined fibrotic processes, namely hyperproliferation, matrix contraction, matrix deposition and epithelial cell trans-differentiation to a myofibroblast phenotype. Regenerative PCO is defined by lens fibre cell differentiation events that give rise to Soemmerring's ring and Elschnig's pearls and becomes evident at a later stage than the fibrotic form. Both fibrotic and regenerative forms of PCO contribute to a reduction in visual quality in patients. This review will highlight the wealth of tools available for PCO research, provide insight into our current knowledge of PCO and discuss putative management of PCO from IOL design to pharmacological interventions.

The lens as a model for fibrotic disease

Fibrosis affects multiple organs and is associated with hyperproliferation, cell transdifferentiation, matrix modification and contraction. It is therefore essential to discover the key drivers of fibrotic events, which in turn will facilitate the development of appropriate therapeutic strategies. The lens is an elegant experimental model to study the processes that give rise to fibrosis. The molecular and cellular organization of the lens is well defined and consequently modifications associated with fibrosis can be clearly assessed. Moreover, the avascular and non-innervated properties of the lens allow effective in vitro studies to be employed that complement in vivo systems and relate to clinical data. Using the lens as a model for fibrosis has direct relevance to millions affected by lens disorders, but also serves as a valuable experimental tool to understand fibrosis per se.

Oligonucleotide microarray analysis of human lens epithelial cells: TGFbeta regulated gene expression

PURPOSE

Transforming growth factor beta (TGFbeta), a pro-fibrotic cytokine has been proposed a causative factor in the progression of lens pathologies including posterior capsule opacification (PCO), a condition that occurs after cataract surgery. This study employs oligonucleotide microarrays to provide a global profile of gene expression in FHL 124 cells, to identify changes in gene expression following treatment with TGFbeta1 and TGFbeta2, and to enable putative genes relating to TGFbeta regulation and PCO to be identified.

METHODS

Routinely cultured FHL 124 cells maintained in serum free Eagle's Minimum Essential Medium (EMEM) were treated with either TGFbeta1 or TGFbeta2 at 10 ng/ml for 24 h then total RNA extraction was carried out. Total RNA (16 microg) was used to analyze gene expression by spotted oligonucleotide microarray hybridization. The spotted oligonucleotide microarrays employed contained 13,971 oligonucleotide probes, each designed to be specific for an individual gene. Array images were analyzed using GenePix Pro 3.0, followed by raw data import into GeneSpring 7.0 where a cross gene error model (CGEM) filter was applied. Data was subjected to LoWess normalization prior to comparison of the different treatment groups. Quantitative real-time polymerase chain reaction (QRT-PCR) was used to validate the oligonucleotide microarray data, using a select number of genes exhibiting differential expression.

RESULTS

A total of 301 genes were up-regulated by more than 1.5 fold in FHL 124 cells by both TGFbeta1 and TGFbeta2. Many of these up-regulated genes had biological functions relevant to lens epithelial cells including roles in contraction, transdifferentiation and as extracellular matrix (ECM) components. A total of 164 genes were down-regulated by more that 1.5 fold in FHL 124 cells by both TGFbeta1 and TGFbeta2. Many of these down-regulated genes have biological functions including roles in apoptosis, signaling, and as anti-oxidants. Following treatment with TGFbeta1 and TGFbeta2, QRT-PCR successfully validated the differential changes in gene expression detected by oligonucleotide microarrays.

CONCLUSIONS

TGFbeta1 and TGFbeta2 regulate the gene expression of genes that have important roles in human lens epithelial cell biology. Most importantly, TGFbeta induces the gene expression of a number of fibrotic markers which may have a role in promoting the development of PCO such as transdifferentiation markers, contractile factors, and ECM components.

Characterisation of TGF-β2 signalling and function in a human lens cell line

There is increasing evidence implicating Transforming growth factor beta (TGF-beta) in pathological states of the lens. However, the underlying signalling mechanisms in human cells have not been fully examined. We have therefore investigated in a human lens cell line, FHL 124, the signalling characteristics of TGF-beta and Smad proteins. Moreover, we have tested the effectiveness of a fully human monoclonal anti-TGF-beta2 antibody, CAT-152, in suppressing TGF-beta2 induced changes in a number of conditions. FHL 124 cells were routinely cultured in Eagle's minimum essential medium (EMEM) supplemented with 10% FCS. Characterisation of the cell line was determined using Affymetrix gene microarrays and compared to native human lens epithelium. Cells were serum starved for 24 hr prior to exposure to TGF-beta2 in the presence and absence of CAT-152. Non-stimulated cells served as controls. Smad 4 localisation was observed by immunocytochemistry. To study Smad-dependent transcriptional activity, cells were transfected with SBE4-luc, an artificial smad-specific reporter, using Fugene-6. Transcriptional activity was determined by luciferase activity. Gene expression was assessed using reverse transcriptase-polymerase chain reaction (RT-PCR). Proliferation was determined by 3H-thymidine DNA incorporation. Growth and contraction were assessed using a scratch and patch assay. Affymettrix gene microarrays identified 99.5% homology between FHL 124 cells and the native lens epithelium with respect to expression pattern of the 22,270 genes on the chip. Moreover, FHL 124 cells expressed phenotypic markers, alphaA-crystallin and pax6 along with lens epithelial cell specific marker FoxE3. Immunocytochemical studies revealed the presence of Smad 4 which following TGF-beta2 exposure accumulated in the cell nucleus. Furthermore, Smad-dependent transcriptional activity was also stimulated. TGF-beta2 enhanced the expression of mRNA levels of alpha smooth muscle actin (alphaSMA) and connective tissue growth factor (CTGF). Exposure to TGF-beta2 resulted in a relatively small inhibition of 3H-thymidine incorporation of FHL 124 cells. However, a more marked contractile effect was also observed. In serum-supplemented medium, growth rates and TGF-beta induced contraction were enhanced. Treatment with 0.1-10 microg ml(-1) CAT-152 dose-dependently inhibited 10 ng ml(-1) TGF-beta2 induced effects in the presence and absence of serum. Exposure of FHL 124 cells to TGF-beta therefore induces Smad translocation, transcription, expression of transdifferentiation markers and induces marked contraction. Treatment with CAT-152 can effectively inhibit these responses. TGF-beta2 induced changes can also persist long after the period of exposure and when in the presence of serum TGF-beta induced contraction is enhanced. The work presented therefore demonstrates a platform technology to study TGF-beta2 signalling in human lens epithelial cells and provides evidence to show TGF-beta2 can be a potent factor in the development of posterior capsule opacification following cataract surgery.

FGF: an autocrine regulator of human lens cell growth independent of added stimuli

PURPOSE

Posterior capsule opacification (PCO) arises because of a persistent growth of lens epithelial cells. Cultured human lens cells residing on their native collagen capsule and maintained in serum-free medium actively grow and thus show an intrinsic capacity for regulation. In the present study, the authors investigated the role of the putative FGF autocrine system in human capsular bags.

METHODS

Capsular bags were prepared from human donor eyes and maintained in a 5% CO(2) atmosphere at 35 degrees C. On-going observations were by phase-contrast microscopy. Cellular architecture was examined by fluorescence cytochemistry. De novo protein synthesis was determined by the incorporation of 35S-methionine. Basic fibroblast growth factor (FGF) and FGF receptor (R)-1 were detected using enzyme-linked immunosorbent assay (ELISA) and reverse transcription-polymerase chain reaction (RT-PCR) techniques. FGFR-1 inhibition was achieved using the specific antagonist SU5402.

RESULTS

Human lens epithelial cells can maintain metabolic activity for more than 1 year in a protein-free medium. Basic FGF was shown to be present in capsular bags throughout culture and also in capsular bags removed from donor eyes that had previously undergone cataract surgery. Furthermore, FGFR-1 was identified. Inhibition of FGFR-1 caused a significant retardation of growth on the posterior capsule. On no occasion did any treated bag reach confluence, whereas all match-paired control samples did.

CONCLUSIONS

The results provide evidence that FGF plays an integral role in the long-term survival and growth of human lens epithelial cells, independent of external stimuli. Inhibition of FGFR-1 by specific synthetic molecules, such as SU5402, could provide a potential therapeutic approach to resolving PCO.

Hepatocyte growth factor function and c-Met expression in human lens epithelial cells

PURPOSE

Hepatocyte growth factor (HGF) and its receptor c-met perform a multitude of functions. However, despite the significant degree of study of HGF and c-met in numerous tissues and cell types, relatively few investigations have been performed on the lens. In the current study, therefore, the role of HGF and the receptor c-met in human lens epithelial cells was investigated.

METHODS

Anterior epithelium and capsular bags were prepared from human donor eyes and maintained in Eagle's minimum essential medium (EMEM) in a 5% CO(2) atmosphere at 35 degrees C. In addition, the human lens cell line FHL124, was routinely cultured and seeded onto glass coverslips (c-met immunodetection), 12-well plates (DNA and protein synthesis), and tissue culture dishes (migration). c-Met was detected by immunocytochemistry and fluorescence-activated cell scanning (FACS). HGF was measured using enzyme-linked immunosorbent assay (ELISA) techniques. Proliferation and protein synthesis were determined by [(3)H]thymidine and (35)S-methionine incorporation into DNA and proteins, respectively. Migration was assessed using a scratch-wound assay and time-lapse video microscopy.

RESULTS

HGF was detected at all stages of culture of capsular bags in protein-free medium. Moreover, c-met was present on the native epithelium and after mechanical trauma was seen to be upregulated. Immunolocalization and FACS analysis demonstrated c-met expression on FHL124 cells throughout the whole population. Furthermore, FACS analysis showed that serum-maintained cells sustained a higher level of receptor expression relative to serum-deprived cells. Additionally, HGF was found to stimulate proliferation, protein synthesis, and migratory responses.

CONCLUSIONS

c-Met receptors are expressed in native epithelium, capsular bag cultures, and FHL124 cells. Receptor is distributed across the entire cell population; however, this expression is environmentally and mechanically sensitive. HGF is also present in capsular bags at all stages of culture. In addition, HGF can stimulate migration, proliferation, and protein synthesis. It therefore appears that a multifunctional autocrine loop involving HGF and c-met is in place and could be important in the development of posterior capsule opacification.

Induction of matrix metalloproteinases 2 and 9 following stress to the lens

Matrix metalloproteinase 2 and 9 (MMP-2 and 9, also known as gelatinase A and B) have been implicated in a number of eye diseases, but their possible involvement in lens pathology is yet to be determined. In the present study, we therefore investigated a possible role of matrix metalloproteinases in cataract and posterior capsule opacification. Whole porcine lenses were removed from the eye and cultured in either Eagles Minimum Essential Medium (EMEM) or EMEM supplemented with 1 m M hydrogen peroxide. The medium was sampled and changed every 2 days. On some occasions a sham cataract operation was performed on cultured lenses. The resulting capsular bag was secured to a Petri dish and cultured in EMEM. Culture media from all preparations were analysed for MMP-2 and 9 activity by gelatin zymography. Media samples from lenses which maintained clarity over the 6 day culture period did not display any detectable gelatinolytic activity. However, media from cataractous lenses demonstrated a gelatinolytic band, which had similar molecular weights to the pro-form of MMP-2. In addition to this band, bands with a similar molecular weight to pro-MMP-9 and its dimeric form were also detected in samples obtained from capsular bag preparations within 24 hr. The data presented indicate that normal lenses have undetectable gelatinase activity. However, there is an associated expression of gelatinases with pathological states of the lens, and therefore gelatinase expression could play an important role in cataractogenesis and posterior capsule opacification.

Calcium cell signalling and cataract: role of the endoplasmic reticulum

The lens possesses an impressive array of G-protein receptors that are coupled to the release of intracellular calcium. They include members of the muscarinic, adrenergic and purinergic families and activation of the former has been implicated in cataract for some time. There are several possible mechanisms whereby activation of such receptors could give rise to cataract. A prolonged increase in intracellular calcium would be expected to activate proteases such as calpain and so could induce unscheduled and irreversible breakdown of important structural proteins. It has recently been shown that activation of G-protein receptors also modulates lens cell growth, and any interference with the highly controlled pattern of cell growth and development within the lens is also likely to have catastrophic consequences. If the calcium store is totally inactivated in lens cells, for example by exposure to thapsigargin, then growth ceases. This finding provides a means of inhibiting the lens cell growth which leads to posterior capsular opacification (PCO). For example, it has been shown that thapsigargin-coated intraocular lenses totally inhibit lens cell growth within cultured capsular bags, and if this technology could be transferred to the clinic then it could provide a simple and relatively inexpensive means of preventing PCO.

Ultraviolet irradiation (UVB) interrupts calcium cell signaling in lens epithelial cells

A preliminary study was undertaken to establish whether low-dose UV irradiation (UVB) affects calcium cell signaling in rabbit lens epithelia. In a suspension of lens epithelial cells (line NN1003A), changes in intracellular Ca2+ were measured by Fura-2 fluorescence in response to exogenously added ATP. The cellular response to ATP, referred to as the calcium signal, is characterized by a brief increase and subsequent decrease in cytosolic Ca2+ levels. Ultraviolet B irradiation (1.8-9 mJ/cm2) was found to reduce the magnitude of the Ca2+ signal in a dose-dependent manner. A 5 min UVB exposure (9 mJ/cm2) completely altered the biphasic nature of the calcium signal, causing only an immediate and steady rise in cytosol Ca2+ levels. Lower fluences of UVB irradiation (2 min exposure times or 3.6 mJ/cm2) induced a 50% reduction in the calcium signal. When irradiated cells were returned to culture for 3 h after irradiation, calcium signals induced by ATP were normal. In view of the photooxidative nature of UVB irradiation, the oxidative state of cells was assessed by measuring glutathione (GSH) levels. Ultraviolet B irradiation caused a rapid 20% decline in GSH levels that returned to near-control values after a 3 h postirradiation incubation. The results of this study indicate that fluences lower than previously found to be cataractogenic can perturb calcium cell signaling in cultured lens epithelial cells.

Transferrin in after-cataract and as a survival factor for lens epithelium

The Fe-transport protein, transferrin (Tf), is synthesized and secreted by whole lenses and cultured lens epithelial cells. Because of Tf's central role in cell growth and proliferation, its participation in lens cell proliferation following cataract extraction was explored using a rabbit model of after-cataract. Varying amounts of the central anterior lens capsule were removed (0, 35, or 80%) following extraction of the lens. The Tf content of and secretion by after-cataract lens capsular sacs containing regenerated lens tissue was determined ex vivo at 0, 3, 5, 7 and 9 weeks post-surgery. In all cases Tf content of and secretion by the lens sacs was higher than that of their contralateral controls (whole lenses). Tf secretion was up to 5-fold higher and metabolic labeling studies indicated secretion of newly synthesized Tf. The sacs contained up to 10 times the concentration of Tf as the control lenses. Human lens after-cataract capsular bags also secreted Tf. The function of Tf as a survival factor was tested on cultured lens epithelial cells. Cells cultured in serum-free medium had a survival rate of only 20-34% if the medium was changed each day. If the medium was never changed during this period, the survival rate was 43-52%, suggesting secretion of essential growth factors by these cells. Addition of 200 microg ml-1 Tf to the medium during each daily change increased survival to levels attained when the medium was not changed. Addition of Tf antibodies to the culture medium during each daily change decreased cell survival to 14%. Apparently Tf acts as a survival factor for lens epithelia and its synthesis is up-regulated in after-cataract lens sacs. These factors suggest that Tf may play an important role in the pathogenesis of lens epithelial cell proliferation and after-cataract formation following cataract surgery.

Phacoemulsification versus extracapsular cataract extraction: a comparative study of cell survival and growth on the human capsular bag in vitro

AIMS/BACKGROUND

Phacoemulsification is rapidly replacing conventional extracapsular cataract extraction (ECCE) as the method of choice for cataract surgery in the Western world. However, posterior capsule opacification (PCO) still remains the major postoperative complication, affecting 20-50% of patients, and results from persistent cell growth of epithelial cells remaining after surgery. This study aimed to compare cell survival and growth on capsular bags following ECCE and phacoemulsification surgery using an established human capsular bag culture system.

METHODS

Sham ECCE and phacoemulsification cataract operations were performed on pairs of human donor eyes. Capsular bags were dissected free, pinned flat on a petri dish, and incubated with Eagle's minimum essential medium (EMEM) alone or EMEM supplemented with 10% fetal calf serum (FCS). Ongoing observations were made using phase contrast microscopy.

RESULTS

Cell growth was observed across the posterior capsule of all preparations studied. It was found that there was no significant difference in the rate of cell growth on the posterior capsule with the two extraction methods, such that 50% confluency was achieved in 7.0 (SD 1.8) (n = 7) days for ECCE and 7.43 (2.1) (n = 7) days for phacoemulsification surgery. The physical changes to the capsule as a result of cell growth, such as wrinkling and capsular tensioning, were also seen in both groups.

CONCLUSIONS

Cell survival and growth is dependent on the donor, rather than the surgical technique performed. There is no significant difference between phacoemulsification and ECCE surgery on the rate and nature of cell growth on the posterior capsule in vitro.

Thapsigargin-coated intraocular lenses inhibit human lens cell growth

Cataract is responsible for rendering several million people blind throughout the world and is also by far the most common cause of low visual acuity. Although cataract surgery is common, routine and effective, posterior capsule opacification (PCO) occurs in 30-50% of patients following modern cataract surgery. This condition arises from stimulated cell growth within the capsular bag after surgery. The resulting decline in visual acuity requires expensive laser treatment, and PCO therefore prevents modern cataract surgery from being carried out routinely in underdeveloped countries. The present study, using a human lens capsular bag culture system, has confirmed that cells from a wide age range of donors proliferate in the absence of added serum protein and explains why PCO is such a common problem even in aged patients. This study also provides one possible solution for PCO by using polymethylmethacrylate (PMMA) implanted intraocular lenses as a drug delivery system. PMMA lenses coated with thapsigargin, a hydrophobic inhibitor of endoplasmic reticulum (ER) (Ca2+)-ATPase, greatly reduced cell growth in the capsular bag at relatively low coating concentrations (200 nM) but, more significantly, induced total cell death of the residual anterior epithelial cells at higher concentrations (>2 microM).

Human lens epithelial cell proliferation in a protein-free medium

PURPOSE

The ocular humors are relatively low in protein, yet cell growth in the human capsular bag still occurs after extracapsular cataract extraction (ECCE) surgery. This resilient growth gives rise to posterior capsule opacification (PCO) in a significant proportion (30%) of patients. This study compared the ability of human lens cells to proliferate in serum-supplemented and protein-free medium.

METHODS

Sham cataract operations were performed on human donor eyes. The capsular bag was dissected free, pinned flat on a petri dish, and incubated in Eagle's minimal essential medium (EMEM) alone or in EMEM supplemented with 10% fetal calf serum. Observations were made by phase-contrast microscopy. At the endpoint, capsules were studied by fluorescence or electron microscopy. Mitotic activity was identified using Bromo-2-deoxyuridine labeling and detection techniques. When required, an intraocular lens was implanted when surgery was performed.

RESULTS

It was found that human lens cells from a wide age spectrum of donors proliferate and migrate on the lens capsule in the absence of added protein. The rate of growth was age-dependent, such that the posterior capsule was completely confluent after 8.0 +/- 0 days (n = 3) and 24.4 +/- 5.3 days (n = 3) for donor lenses aged < 40 years and > 60 years, respectively. The outgrowth of epithelial cells gave rise to capsular contraction, wrinkling, and increased light scatter. Growth on the anterior surface of the intraocular lens was less prolific than on the posterior capsule.

CONCLUSION

The protein-free model replicates many features of clinically-observed PCO. The resilient cell growth on the natural collagen capsule explains the high prevalence of PCO, especially in younger patients, and suggests that inflammation and external growth factors are not necessary for PCO. Furthermore, the protein-free capsular bag system can be used to explore fundamental questions concerning the autocrine control of lens epithelial cell survival and growth.

A study of human lens cell growth in vitro. A model for posterior capsule opacification

PURPOSE

After intraocular lens (IOL) implant surgery for cataract, cell growth on the posterior capsule is responsible for renewed visual impairment in approximately 30% of patients. The authors have, therefore, developed a human lens capsule system to study this growth in vitro.

METHODS

Sham cataract surgery, including anterior capsulorhexis, nucleus hydroexpression, and aspiration of lens fibers, was performed on donor eyes. In some cases, a polymethylmethacrylate IOL implant was placed in the capsular bag. The capsular bag was dissected free, pinned flat on a plastic culture dish, covered with Eagle's minimum essential medium supplemented with 10% fetal calf serum and observed by phase-contrast and dark-field microscopy for as long as 100 days. At the end-point, capsules were examined by fluorescence microscopy for actin, vimentin, and chromatin.

RESULTS

Within 24 hours, there was evidence of cell growth in the equatorial region. After 2 to 3 days, cells were normally observed growing from the rhexis onto the posterior capsule and across the anterior surface of the IOL, if present. Growth proceeded rapidly so that the posterior capsule, for example, was totally covered by a confluent monolayer of cells at 5.8 +/- 0.6 days and 7.2 +/- 0.7 days for capsules aged < 40 years and > 60 years, respectively. Total cover of the anterior IOL surface generally followed 4 to 5 days behind that of the capsule. Capsular wrinkles became increasingly apparent as time progressed, causing a marked rise in light scatter. An increase in capsular tension also occurred, and the actin filaments became more polarized near the wrinkles.

CONCLUSIONS

The model presented here for posterior capsule opacification shows many of the changes seen in vivo, including rapid lens cell growth, wrinkling, tensioning, and light scatter in the posterior capsule. It will be possible to develop strategies for inhibiting cell growth with this system.

Estimation of intracellular calcium activity in confluent monolayers of primary cultures of quail medullary bone osteoclasts

The present study was concerned with an investigation of the relative utilities of the fluorescent indicators fura-2 and fluo-3 for spectrofluorimetric estimation of the intracellular calcium concentration ([Ca2+]i) in confluent osteoclast monolayers that had been isolated from medullary bone of quail hens and maintained in tissue culture for 6-8 days. Additionally, we have determined the effects of raised extracellular calcium ([Ca2+]o) and chicken calcitonin (CT) on [Ca2+]i in this preparation. Relative to fura-2, fluo-3 was poorly incorporated into the osteoclasts and had a high apparent equilibrium binding constant (Kd) for Ca2+ binding (809 nM). The osteoclasts were only weakly sensitive to the calcium ionophore, ionomycin. It is concluded that fura-2 is of greater utility than furo-3 in this preparation. In contrast to its lack of effect in freshly isolated cells, elevated [Ca2+]o up to 20 mM stimulated a concentration-dependent increase in [Ca2+]i in cultured osteoclasts, but CT was without effect. These findings further support the idea that quail osteoclasts are able to acquire a Ca2+ sensor or 'receptor' and thus to respond to [Ca2+]o in a similar manner to mammalian osteoclasts when they are removed from the bone microenvironment, but retain a refractoriness to CT under these conditions.

MEDULLARY BONE AND AVIAN CALCIUM REGULATION

Medullary bone forms in egg-laying birds in response to gonadal steroids and is the most overtly oestrogen-dependent of all bone types. It acts as a labile reservoir for the supply of eggshell calcium. Previous studies indicate that feeding calcium- and vitamin-D-deficient diets to chickens results in resorption of cortical rather than medullary bone. More recent studies in calcium-stressed quail hens question this hypothesis and suggest that during the first 2 weeks of dietary calcium depletion the medullary bone is resorbed while cortical bone volume remains intact. The role of the osteoclast in bone resorption is the focus of much research that has recently included studies of medullary bone osteoclasts. The functional morphology of the avian cells, i.e. changes from quiescent to active osteoclasts with ruffled borders, reflects the rapid changes in medullary bone turnover that occur during the egg-laying cycle. Unlike mammalian osteoclasts, those from avian sources generally appear refractory to inhibitory factors such as calcitonin or raised extracellular calcium concentration. However, medullary bone osteoclasts cultured in vitro for several days recover their ability to respond to the latter factor by increasing their levels of free cytosolic Ca2+, reducing tartrate-resistant acid phosphatase secretion and reducing their cell spread area. It is suggested that factors such as ambient calcium levels and prostaglandins may form part of a system of rapid local control for medullary bone osteoclast activity.