Abstract
During vertebrate embryonic development, cellular senescence occurs at multiple locations. To date, it has been accepted that when there has been induction of senescence in an embryonic tissue, β-galactosidase activity is detectable at a pH as high as 6.0, and this has been extensively used as a marker of cellular senescence in vivo in both whole-mount and cryosections. Such senescence-associated β-galactosidase (SA-β-GAL) labeling appears enhanced in degenerating regions of the vertebrate embryo that are also affected by programmed cell death. In this sense, there is a strong SA-β-GAL signal which overlaps with the pattern of cell death in the interdigital tissue of the developing limbs, and indeed, many of the labeled cells detected go on to subsequently undergo apoptosis. However, it has been reported that β-GAL activity at pH 6.0 is also enhanced in healthy neurons, and some retinal neurons are strongly labeled with this histochemical technique when they begin to differentiate during early embryonic development. These labeled early post-mitotic neurons also express other senescence markers such as p21. Therefore, the reliability of this histochemical technique in studying senescence in cells such as neurons that undergo prolonged and irreversible cell-cycle arrest is questionable because it is also expressed in healthy post-mitotic cells. The identification of new biomarkers of cellular senescence would, in combination with established markers, increase the specificity and efficiency of detecting cellular senescence in embryonic and healthy mature tissues.
Highlights
During vertebrate embryonic development, cellular senescence occurs at multiple locations
It has been reported that β-GAL activity at pH 6.0 is enhanced in healthy neurons, and some retinal neurons are strongly labeled with this histochemical technique when they begin to differentiate during early embryonic development
In the case of the nervous system, it has been proposed that β-GAL activity at pH 6.0 might be detected in differentiated neurons (Piechota et al, 2016, see below), and even at early stages of development. All these findings indicate that the reliability of the “so-called” SAβ-GAL assays is questionable because, since the enzyme is not always specific for cell-aging, this histochemical technique is insufficient to characterize cellular senescence
Summary
There is a strong SAβ-GAL signal which overlaps with the pattern of cell death in the interdigital tissue of the developing limbs, and many of the labeled cells detected go on to subsequently undergo apoptosis. Concomitant with the apoptotic process (Figures 1H,L), AER cells exhibit SAβ-GAL activity (Figure 1D) and express different markers of cellular senescence such as p21 (Storer et al, 2013; Muñoz-Espín et al, 2013; Lorda-Díez et al, 2015; Li et al, 2018).
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