Healthcare and Medicine Reference
In-Depth Information
Introduction: Stem Cell Types Overview
and Rationale for Labeling for Imaging
Cell labeling and tracking lend their roots to the development of methods
to stain specific cellular components for microscopic evaluation. One of the
best-known histological stains, hematoxylin, was originally used as a fabric
dye and adapted by an amateur microscopist in the mid-1800s to stain the
nuclear components in cells (Titford 2005). Subsequently, specific proteins
(e.g., antigens or enzymes) were used to localize specific cells or components
within the cell by microscopy. In 1939, Gomori developed a method to local-
ize phosphatase activity as a brown-black precipitate for light microscopy
(Gomori 1952). One of the earliest examples of translation of enzymatic his-
tological techniques to cell tracking was the injection of horseradish peroxi-
dase (HRP) into early-stage embryos. Transfer of the HRP into daughter cells
enabled tracking of cells into lineage-specific cells of mesoderm, ectoderm,
and endoderm (Balakier and Pedersen 1982; Cruz et al. 1991).
Antigen-antibody reactions for immunohistochemical staining were intro-
duced by Albert H. Coons, who adopted procedures developed for identify-
ing immune responses to tissue staining (Coons and Kaplan 1950). Coons'
original work involved binding a fluorescent dye to an antibody in vitro that
could form a complex with a specific antigen (i.e., an antigen-antibody com-
plex) on specific cells or structures that could be viewed visually with ultra-
violet light. This technique is still used for immunofluorescent staining of
cells for pathological microscopic evaluation. Fluorescein, a green dye under
ultraviolet light, in the form of fluorescein isothiocyanate (FITC) is one of
the most commonly used antibody fluorescent markers for cell identifica-
tion based on cell surface antigens (Figure I.1). Similar to enzymatic labeling
for cell tracking, fluorescent probes (e.g., CellTrackerâ„¢; Molecular Probes,
Invitrogen) are widely available for cell tracking and can now be used in
combination with histopathological fluorescent labeling (Figure I.1) One
advantage of these cell tracker fluorescent dyes is that they are less cytotoxic
than FITC.
The expansion of these techniques from histopathological examination
to cell tracking in vivo was precipitated by a number of different pathways
and clinical needs. The most widespread methods of cell tracking have
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