Expression of cellular senescence and neural stem cell markers in a case of advanced retinal fibrosis

Fibrosis in the posterior segment of the eye tends to be the final stage in the resolution of lesions in the retina and choroid. Two of the diseases that most often cause this type of fibrosis are diabetes and age-related macular degeneration (AMD).1 The main aetiopathogenetic mechanisms involved in the development of fibrosis are hypoxia and inflammation. In the case of AMD, retinal pigment epithelium dysfunction leads to accumulations of substances (drusen) with angiogenic activity on Bruch's membrane, this becoming thicker and limiting oxygen diffusion to the most external layers of the retina. It causes two types of lesions: dry or atrophic lesions, and wet or exudative (angiogenic) lesions.1,2 On the other hand, diabetes leads to hypoxic lesions due to microvasculopathy associated with glycosylation of the vascular wall. Both processes can lead to blindness. In AMD, cellular senescence and stem cells may play a key role in the progression of the lesions. The progressive accumulation of cell damage over time promotes the induction of senescence in which cells block their capacity to divide and acquire functions for biosynthesis and secretion of various different cytokines (senescence-associated secretory phenotype), which in turn leads to immunological and remodelling responses of the extracellular matrix, associated with repair mechanisms in damaged tissues.1-3 The inflammatory damage may, however, remain and increase, despite a reparatory inflammatory response. With age there is stem cell deficiency and hence an inability to regenerate the damaged structures.4 The progressive accumulation of senescent cells and stem cell depletion with aging may be the basis of the deterioration of the mechanisms of tissue homeostasis activated in response to damage and, consequently, responsible for fibrosis accumulation.


| INTRODUC TI ON
Fibrosis in the posterior segment of the eye tends to be the final stage in the resolution of lesions in the retina and choroid. Two of the diseases that most often cause this type of fibrosis are diabetes and age-related macular degeneration (AMD). 1 The main aetiopathogenetic mechanisms involved in the development of fibrosis are hypoxia and inflammation. In the case of AMD, retinal pigment epithelium dysfunction leads to accumulations of substances (drusen) with angiogenic activity on Bruch's membrane, this becoming thicker and limiting oxygen diffusion to the most external layers of the retina. It causes two types of lesions: dry or atrophic lesions, and wet or exudative (angiogenic) lesions. 1,2 On the other hand, diabetes leads to hypoxic lesions due to microvasculopathy associated with glycosylation of the vascular wall. Both processes can lead to blindness.
In AMD, cellular senescence and stem cells may play a key role in the progression of the lesions. The progressive accumulation of cell damage over time promotes the induction of senescence in which cells block their capacity to divide and acquire functions for biosynthesis and secretion of various different cytokines (senescence-associated secretory phenotype), which in turn leads to immunological and remodelling responses of the extracellular matrix, associated with repair mechanisms in damaged tissues. [1][2][3] The inflammatory damage may, however, remain and increase, despite a reparatory inflammatory response. With age there is stem cell deficiency and hence an inability to regenerate the damaged structures. 4 The progressive accumulation of senescent cells and stem cell depletion with aging may be the basis of the deterioration of the mechanisms of tissue homeostasis activated in response to damage and, consequently, responsible for fibrosis accumulation.  Figure 1B). There was even dense collagenous connective tissue extending along the curvature of the retina seeming to take its place ( Figure 1C). In several places, the retina had detached from the choroid and adhered through the pigment epithelium to loose connective tissue with abundant capillary vascularisation and extensive haemorrhage ( Figure 1D). In these tissues, there was notable expression of the marker of cellular senescence p16 INK4 (Figure 1E), both in the fibroblastic and the endothelial components of the neoformed capillaries. Along with these lesions, there was an inflammatory exudate which mainly contained macrophages, many of which were loaded with haemosiderin ( Figure 1F).

| C A S E PRE S ENTATI ON
At the level of the optic papilla, we found a glial scar ( Figure 1G) in highly vascularised connective tissue. Its cells expressed a marker of neural stem cells, Sox-2, which was also observed in the peripheral retinal remanent. This was not observed, however, in the rest of the tissue with fibrosis and haemorrhage ( Figure 1H).

| D ISCUSS I ON
The most significant lesions observed in this case report may be ora serrata. Some of the positivity for Sox-2 at this site corresponds to cells segregated from the tissue, located in the haemorrhagic lesions. This deficiency may be contributing to the damage, failure to regenerate tissue and accumulation of fibrosis. Senescence accumulation and deficiency in stem cell activity may be contributing to the damage and fibrosis accumulation in AMD.

CO N FLI C T S O F I NTE R E S T
The authors declare no conflicts of interest.

AUTH O R CO NTR I B UTI O N S
Study design, experiments and drafting of the manuscript: Moreno-Valladares. Interpretation of results and substantive revision: Matheu.
Both authors have read and approved the manuscript.

E TH I C S A PPROVA L A N D CO N S E NT TO PA RTI CI PATE
The brother of the patient signed informed consent form approved by the Institutional Ethical Committee. The study was approved by the ethics committee of Hospital Donostia.