Biomaterials Science Blog

Calcium phosphate (CaP) based materials have long been popular choices for a range of medical applications including bone replacement and drug delivery.  However, recent studies indicate a need for a closer look at how cells react to small, degraded CaP particles that find their way into the cell’s interior.  In a recent study, a research team from the University of Birmingham demonstrated that when CaP particles with a diameter larger than 1.5μm penetrated the interior of the cell but were not sequestered by the cell’s lysosomes, a series of events eventually leading to cell death could be observed.

Within the past few years, an increased focus on the end results of CaP degradation have shown that the eventual cytotoxicity of these materials is heavily dependent on the total volume of internalized material.  A study by Motskin et al. in 2009 illustrated that many of these degraded particles are localized to the lysosomes, cellular structures whose acidic environments are responsible for the degradation of waste products which, in the case of CaP, results in the generation of calcium ions.  It was suggested that the generation of a surplus of these ions could interfere with the cell’s signalling pathways, potentially triggering apoptosis.

In a study published recently in Biomaterials Science, Williams et al. quantified the volume and size distribution of CaP material within cells by chemically grafting a fluorescent probe to the surface of silicon-substituted hydroxyapatite (SiHA), and then exposing a mouse osteoblast precursor cell line to the labelled particles.  Following exposure, the group observed changes in cell behaviour that were indicative of the onset of cell death including alterations in cell morphology, an increase in the number of lysosomes, and cellular detachment from the underlying substrate.  As the cells transitioned from the early stages of cell death (morphology changes) to later stages (detachment from the substrate), there was a marked increase in the amount of SiHA within the cytoplasm but outside of the lysosomes.  Moreover, the onset of cell death was correlated with SiHA aggregates within the cytoplasm that were 1.5μm in diameter or larger.

The group hypothesized that this may have been due to a destabilization of the lysosome membrane which prevented undissolved CaP from remaining within the lysosomes or, alternatively, the destabilization of endosomes which were responsible for delivering particles to the lysosomes.  Regardless, this study proves a need for additional research into the size-dependent effects of CaP particles on cell health and suggests a new design concern for CaP based medical materials.

Check out the full article here:
Quantification of volume and size distribution of internalised calcium phosphate particles and their influence on cell fate
by Richard L. Williams, Isaac Vizcaíno-Castón, and Liam M. Grover

Ellen Tworkoski is a Web Writer for Biomaterials Science and a graduate student in the department of Biomedical Engineering at Northwestern University.

The “holy grail” of neural tissue engineering is to develop functional tissue constructs in an effort to reconnect nerves that have been previously disconnected from disease or injury (i.e. spinal cord injury).  One technique for developing personalized regenerative cell therapies is to use adult somatic cells from a patient (such as skin cells) and re-program them to an induced pluripotent stem cell (iPSC) state. After isolating iPSCs, patient-specific neurons may be generated and transplanted back into the patient, with the hope that the newly formed neurons will form connections in the tissue and restore function.

At the University of Victoria, Montgomery et al. have developed a method to differentiate mouse iPSCs into neurons within a fibrin gel. Fibrin has been regarded as a traditional biomaterial to effectively differentiate embryonic stem cells into neurons. In this study, fibrin was utilized to evaluate the efficiency of two iPS differentiation protocols. The first protocol tested was the traditional “4-/4+” method, involving no treatment to iPS embryoid bodies for 4 days of culture, then adding retinoic acid (RA) for the subsequent 4 days of culture. The second protocol tested was the newer “2-/4+” method, involving no treatment to iPS embryoid bodies for 2 days of culture, then adding RA and purmorphamine for the subsequent 4 days of culture. Individual embryoid bodies were then isolated and placed within a 3D fibrin gel to observe differentiation of neurons.

Interestingly, the overall differentiation efficiency was increased using the 6 day 2-/4+ method compared to using the traditional 4-/4+ method after seeding the treated iPS cells in 3D fibrin gels. Phenotypes were characterized using immunofluorescence staining and RT-PCR, with increases in expression of the neuronal markers TUJ1 and nestin at Day 14 of culture using the 2-/4+ method compared to the 4-/4+ method. The SOX2 pluripotent marker remained low at Day 14, indicating cells were differentiating to a neuronal state.

Taken together, this study demonstrates the ability to differentiate neurons from mouse iPSCs using simple differentiation and encapsulation protocols. Future work will need to be conducted to see if these protocols can be implemented to achieve neuronal differentiation using human iPSCs.

Check out the full article:
Engineering personalized neural tissue by combining induced pluripotent stem cells with fibrin scaffolds, by Amy Montgomery, Alix Wong, Nicole Gabers and Stephanie M. Willerth

Brian Aguado (@BrianAguado) is currently a Ph.D. Candidate and NSF Fellow in the Biomedical Engineering department at Northwestern University. He holds a B.S. degree in Biomechanical Engineering from Stanford University and a M.S. degree in Biomedical Engineering from Northwestern University. Read more about Brian’s research publications here.

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