Magnetic Resonance Tracking of Stem Cells with Iron Oxide Particles - Intracellular Delivery: Fundamentals and Applications - page 471

on the relative amounts of the constituents, results in structures that range from
spheres to rods and toroids (Liu et al. 2001 ) and change dynamically in size over
several hours (Lai and van Zanten 2001 ). The complex formed by TA and SPIO
has a hydrated diameter that depends on the relative amount of its constituents and
may precipitate at certain ratios (Montet-Abou et al. 2007 ). Therefore, the use of
TA-mediated labelling requires careful optimisation of labelling conditions for each
cell type to be labelled, and has potential effects on cell physiology that are difficult
to predict.

4.6 Labelling Medium

During labelling of the cells, physicochemical properties of particles are influenced
by the microenvironment. Studies on the effects of the labelling medium have
shown that the use of 10% serum increases the uptake of ferumoxide compared to
serum-free medium (Rogers and Basu 2005 ). Similar observations were reported
when monocrystalline iron oxide particles (MION), opsonised by plasma, purified
and incubated with either macrophages or cancer cells, resulted in a six- and two-
fold increase in uptake respectively as compared to naïve MION (Moore et al. 1997 ).
It was suggested that opsonised MION were taken up by scavenger and complement
receptors of macrophages while naïve MION entered cells through fluid-phase
endocytosis. This suggestion was supported by reports that the macrophage
uptake of albumin-coated particles is C3b-complement-dependent (a component of
opsonisation) (Roser et al. 1998 ). In contrast, opsonised AMNP showed reduced
uptake by Hela cells (1 pg/cell) compared to naïve AMNP (16 pg/cell) (Wilhelm
et al. 2003 ). It was also reported that macrophage uptake of polystyrene and
phenylated polyacrolein between 0.5 and 4.5 mm was higher in serum-free media
(Tabata and Ikada 1988 ). The presence of serum in medium can thus alter uptake
of particles depending on the cell type. Indeed, it was observed that cell types with
large cytoplasm-nucleus size ratio are insensitive to the presence of serum during
uptake of poly-L-lysine complexed SPIO, as opposed to enhanced uptake by
hematopoietic-derived cells with small cytoplasm-nucleus ratio in serum-free media
(Arbab et al. 2003a ).

5 Transgenic Methods of Cellular Labelling

Encouraging cells to produce iron oxide particles intra-cellularly is an alternative to
labelling with exogenous particles. Living organisms possess specialised iron storage
mechanisms involving ferritin, transferrin and transferrin receptors. Expression of
these proteins in certain cells may be accompanied by an increase in intracellular iron
storage, which will allow the cells to be detected through MRI. One application for