Chronologic versus Biologic Aging of the Human Choroid

Several aspects of chronologic and biologic aging in the human choroid are reviewed from the literature. They often reveal methodological problems for age-dependent changes of the following parameters: choroidal thickness, choroidal pigmentation, choroidal vasculature and blood flow, and choroidal innervation. On reinterpreting some data of studies concerning Bruch's membrane, changes observed at different age points seem more likely to be nonlinear. Concluding from the data presented so far, chronologic aging should not be used as a factor for physiological changes in the human choroid. Longitudinal study designs are necessary to further establish the impact of age. Meanwhile, a more biologic oriented model of aging processes in the choroid should be established, including specified conditions (e.g., light exposure and refractory state). This would help to define more individual strategies for prevention and early stages of a certain defined disease.


Introduction
Aging in our cultural setting is a complex of factors leading to disadvantages and potential diseases. Aging as a complex development leading to more knowledge and potential wisdom is widely neglected in the natural sciences. Aging is potentially dangerous and should be prevented if possible-but what does aging in a specific setting mean?
Even though ocular physiology has its primary focus on retinal light perception and transparency of the optic apparatus, a number of functions are known to be located in the choroid including nutrition of the outer retina, finetuning of the central fovea (short term during accommodation and long term during eye growth), and buffering of the intraocular temperature. All these conditions are crucial for proper vision and should remain functional throughout life. The present review focuses on the impact of chronologic and biologic aspects on the function of the choroid analysing the published literature. The key questions should clarify which aspects concerning aging are known at present in the human choroid and how can they lead our understanding of this process.

Data.
Thinning of the total choroid over age was described in several cross-sectional studies. Morphological postmortem observations revealed a decrease up to 57% ( [1] 95 donors, age range 6-100 years; [2] 45 donors, age range 17-84 years). The age-related thinning was also observed using optic coherence tomography in vivo ( [3]  The Scientific World Journal sectional design and they did not differ between subgroups of healthy volunteers (like gender, general health conditions, vision development, or ocular activity). Therefore, the relation to chronologic aging is not as yet verified. A longitudinal study design and a subgroup analysis are necessary for further studies covering this topic.

Choroidal Pigmentation
3.1. Data. In vivo, the pigmentation of the posterior eye fundus is a combination of the retinal pigment epithelium (RPE) and the choroidal melanocytes. Fluorescence optical measurements on paraffin sections of 38 donors (aged 2 to 90 years; 19 whites, 16 American blacks) showed a trend of decreasing content with aging in both RPE and choroidal melanocytes [10]. By contrast, biochemical measurements of melanin in the peripheral and macular choroid (11 donors, aged 17-88 years) revealed two-to-threefold higher levels in the macula region compared to the more peripheral regions, but no alteration with aging [11].

Discussion
. It remains to be determined whether melanin decreases physiologically with age. Its importance for normal eye function has been acknowledged for both RPE [12] and choroid [13], and a decrease of melanin has been described in numerous pathologic conditions.  [20] revealed significant lower levels in older (around 60 years of age; = 11) versus younger females (around 30 years of age; = 16) but not in males (same age groups; = 26).

Choroidal Vasculature and Blood Flow
The endothelial cells of the choroidal vessels showed no age-related changes regarding their cytoarchitecture ([21] 65 donors, age range 7-87 years).

4.2.
Discussion. As pointed out already for the choroidal thickness, most of the older studies had a cross-sectional design and did not differ among subgroups of healthy volunteers. Therefore, the relation of chronologic aging and vascular changes (function: reduced blood flow; structure: reduced choriocapillary density) is not as yet verified. Again, a longitudinal study design and a subgroup analysis are necessary for further studies covering this topic. New techniques [22] are on their way to make such studies possible.

Choroidal Innervation and Other Single
Aspects of Choroidal Tissue

Discussion.
Single descriptions suggest nerve fibre changes in the aging choroid and in the extracellular composition; these data have to be confirmed.

Bruch's Membrane
6.1. Data. The most widely documented age-related changes of the choroid are in human Bruch's membrane and include thickening and changes in their composition (for reviews see [28,29]).
The thickening of Bruch's membrane seemed to start in the periphery; in the macular region first thickening was observed from 45  . This is in some contrast to the observations of Newsome, who reported an increase of collagen and elastin in the macula region [30].
Further changes in Bruch's membrane include accumulation of glycoconjugates and glycosaminoglycans [30], accumulation of degraded gelatinase (mmp2 and mmp9; [37] The Scientific World Journal 6.2. Discussion. While considering chronologic aging as a factor for changes in Bruch's membrane two incisions seem to take place: one is around forty and the other one around sixty. In reevaluating some data proposing linear changes over the whole age range, it becomes apparent that between 40 and 60 years of age some parameters like the diffusion of macromolecules (e.g., [52]) rather show a steady state than a linear constant decrease. Assuming that there is no complete linear chronologic aging in Bruch's membrane, three characteristic periods could be hypothesized. The first period (up to 40 years of age) is characterized by a peripheral thickening of Bruch's membrane caused by accumulation of lipids, changes in the enzymatic profile of extracellular material regulators, and a subsequent decrease of Bruch's membrane solubility and diffusion. The second period (between 40 and 60 years of age) is characterized by thickening of the macular Bruch's membrane, further accumulation of lipids, a more frequent presence of drusen, but some lingering of Bruch's membrane permeability. The third period (older than 60 years of age) shows no further lipid accumulation but further increase of drusen and decrease of permeability. This last period shows the greatest variability making it difficult to establish agerelated aspects in this subgroup.
Even with the greater number of studies, a longitudinal study design is almost not present. Only the Waterman study reported a follow-up of choroidal parameters after 5 years proofing that the morphological appearance of drusen is time sensitive; a clear increase of the number of drusen could not be demonstrated within the five-year period. It is still important to remember that even profound morphological changes are reversible to some extent.
Summarizing the discussions of the different aspects of chronologic age on changes of the choroid mentioned above, it becomes apparent that chronologic aging might not be helpful to understand the "normal" changes and variations of the choroid during time.
Having these difficulties in mind, a more biologic aging was suggested by Sarks characterizing different stages between normal and pathologic conditions. In this study, donor eyes were grouped according to their appearance of Bruch's membrane ( [58] 216 donors, age range 43-97 years, 6 groups). Although there was some correlation with chronologic age, individual factors of aging were considered as being more important. A second study followed the rules of Sarks ([59] comparing the data of Sarks with a Japanese population) but it did not became a standard for further age-related aspects. The problem with such a grouping is the implicit idea that aging is a prestage of disease and therefore necessary to prevent.
In terms of nonpathological changes of the choroid during life we might follow the ideas of Gerontologist Aubrey Nicholas Jasper De Grey, who pointed to the body's basal activity of metabolism leading to some kind of useless end products [60]. Accumulation over time might finally lead to pathological conditions or diseases, depending on the body's capability to deal with these often toxic end products. However, if the body changes its strategies or supply, these accumulations are reversible as is aging to a certain degree. De Grey's seven damaging events and their transduction to the choroid are as follows.
(1) Tissue Stiffening. Two functional aspects lead to the aspects of stability of the choroid-one is the sponge-like character due to the intense vascularisation allowing variation of the total thickness and the other is the elastic counterforce during accommodation. The first aspect is structurally realized by a loosely arranged network of collagen fibres and nonvascular smooth muscle cells. This dynamic system seems to remain fairly constant during chronologic aging [27,61], while collagen thickening and changes in the density and arrangement of the smooth muscle cells can be observed in nonhealthy conditions (e.g., own unpublished observations in glaucomatous eyes). There is a high interindividual variation of a certain subgroup of nonvascular smooth muscle cells around the macular region [61] which has not yet been assigned to certain conditions. The second aspect is supported by elastic fibres within Bruch's membrane and the stroma of the choroid (reviewed by [62]). Since elastic fibres cannot be replaced during life, they are structures undergoing irreversible changes with age (e.g., calcification of Bruch's membrane elastic fibres; reviewed by [63] (2) Extracellular Debris. Although the choroid is a highly vascularized tissue, its main capillary bed is the choriocapillaris showing strong interaction to Bruch's membrane and the RPE. In addition, the choroid is specific in having no true lymphatic drainage [67]. To cope with this fact, older eyes show numerous debris in the inner sclera which might act as a trash can for the choroid in that respect and explain the loss of scleral permeability with age [68]. Accumulation of extracellular debris towards Bruch's membrane also occurs as a general thickening and as the described Drusen formation, which seems to be a temporary effect at the first place [57], showing also some genetic predispositions [69]. With these two borders absorbing and accumulating extracellular debris, the choroid itself remains balanced over a long time concerning its extracellular composition. A completely unknown area is the influence of extraocular conditions like nutrition on debris formation: first loose observations were published for lipids [35] and in an animal model for Zinc [70], but much more effort should be made in this respect.
(3) Intracellular Debris and (4) Mitochondrial Defects. Almost no data exists about intracellular changes of choroidal cells, which include fibrocytes/fibroblasts, melanocytes, immune cells (mainly macrophages and mast cells), and vascular cells (endothelial cells, pericytes, and smooth muscle cells). Melanocytes might change their type of melanin as seen in cell culture experiments [71] and as reviewed for melanocytes in general [72,73]. Macrophages can change their state of activity as seen in early pathology [74]. Mast cells show diversity in the human uvea [75] and some general agerelated characteristics (reviewed by [76]). Specific studies for the human choroid do not exist. In contrast, changes in the adjacent human RPE are broadly documented.
(5) Cell Overproliferation. Since the inner eye has a very strict capacity, proliferation as a tool to cope with metabolic imbalance is negligible. However, looking at conditions considered as pathologic, fibroblasts are able to induce proliferation of endothelial cells and neovascularisation ( [77][78][79]). This activity is restricted towards Bruch's membrane and the RPE. In this careful observation of the data concerning age and changes in the choroid it becomes evident that quite a number of factors are hypothesised, but not sufficiently demonstrated yet. To preserve metabolic activity and function, the choroid develops a number of strategies which were touched above but need much more research to establish. Although a number of changes occur frequently correlated with age in the populations studied, we should beware of restricting this data to a linear, irreversible cascade of events leading to pathological conditions. Longitudinal study designs are necessary to further establish the impact of age. Meanwhile, age should not be propagated as an unswayable factor for diseases or predisease conditions. This would help to define more individual strategies for prevention and early stages of a certain defined disease.