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 senescencein vivoin 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.

Autophagy is a degradative mechanism involved in the recycling and turnover of cytoplasmic constituents from eukaryotic cells. This phenomenon of autophagy has been observed in neurons from patients with Parkinson's disease (PD), suggesting a functional role for autophagy in neuronal cell death. On the other hand, it has been demonstrated that exposure to pesticides can be a risk factor in the incidence of PD. In this sense, paraquat (PQ) (1,1′-dimethyl-4,4′-bipyridinium dichloride), a widely used herbicide that is structurally similar to the known dopaminergic neurotoxicant MPP⁺ (1-methyl-4-phenyl-pyridine), has been suggested as a potential etiologic factor for the development of PD. The current study shows, for the first time, that low concentrations of PQ induce several characteristics of autophagy in human neuroblastoma SH-SY5Y cells. In this way, PQ induced the accumulation of autophagic vacuoles (AVs) in the cytoplasm and the recruitment of a LC3-GFP fusion protein to AVs. Furthermore, the cells treated with PQ showed an increase of the long-lived protein degradation which is blocked in the presence of the autophagy inhibitor 3-methyladenine and regulated by the mammalian target of rapamycin (mTOR) signaling. Finally, the cells succumbed to cell death with hallmarks of apoptosis such as phosphatidylserine exposure, caspase activation, and chromatin condensation. While caspase inhibition retarded cell death, autophagy inhibition accelerated the apoptotic cell death induced by PQ. Altogether, these findings show the relationship between autophagy and apoptotic cell death in human neuroblastoma cells treated with PQ.

Constant intense light causes apoptosis of photoreceptors in the retina of albino fish. However, very few studies have been performed on pigmented species. Tench (Tinca tinca) is a teleost inhabiting dimly lit environments that has a predominance of rods within the photoreceptor layer. To test the hypothesis that constant high intensity light can result in retinal damage in such pigmented epibenthonic teleost species, photodegeneration of the retina was investigated in the larvae and in juveniles of tench to assess whether any damage may also be dependent on fish age. We exposed both groups of animals to 5 days of constant darkness, followed by 4 days of constant 20,000 lx light, and then by 6 days of recovery in a 14 h light:10 h dark cycle. The results showed that the retina of the larvae group exhibited abundant photoreceptor cell apoptosis during the time of exposition to intense light, whereas that of juveniles was indifferent to it. Damaged retinas showed a strong TUNEL signal in photoreceptor nuclei, and occasionally a weak cytoplasmic TUNEL signal in Müller glia. Specific labelling of microglial cells with Griffonia simplicifolia lectin (GSL) histochemistry revealed that photoreceptor cell death alerts microglia in the degenerating retina, leading to local proliferation, migration towards the injured outer nuclear layer (ONL), and enhanced phagocytosis of photoreceptor debris. During the first days of intense light treatment, Müller cells phagocytosed dead photoreceptor cells but, once microglial cells became activated, there was a progressive increase in the phagocytic capacity of the microglia.