In this study, we investigated the ultrastructure of aLC and LECs in HMC compared with ARC by TEM, to analyze the possible etiological mechanism of HMC. A lot of efforts have been devoted to explore the electron microscopic architecture of lens capsule since decades ago [6–15]. However, high-myopic cataracts haven’t been involved so far.
A sheet of epitheliums is located between anterior lens capsule and fiber cells, with region-specific features [4, 16, 17]. Though the central epitheliums we collected from routine phacoemulsification surgery are considered essentially quiescent in terms of cell cycle, they drive active metabolic and transporting machinery to guarantee the homeostasis of the whole crystallin lens [4, 16–19]. By TEM, we discovered some common pathologic features in both HMC group and ARC group, to a similar extent, such as pleomorphic cellular structure, vacuolated cytoplasm, swollen organelles, as well as degeneration and necrotic lesions, which were also confirmed by previous studies of other cataractic types [6–11, 13, 15]. One of the prevalent hypotheses on cataractogenesis postulates that endogenous and exogenous reactive oxygen species (ROS) are the common pathway in various types of cataract development [16, 17, 20–23]. LECs are considered as the initial target of oxidative stress damage [16, 17, 20–23]. Previous studies also demonstrated that with aging, the deposit of oxidized lens constituents together with the impairment of antioxidative function (which LECs play a role in) can bring about opacities in crystallin lens, resulting in caractogenesis [20–24]. Any disruption of morphologic structure might be implicated in dysfunction of LECs, which would accelerate cataract formation.
As to the underlying etiology of HMC, one of the possible mechanisms might be that the crystallin lens of high myope suffers from higher concentration of oxygen, which is resulted from impaired physical and chemical barriers of vitreous body [3, 24–26]. As a consequence, the surrounding ROS of crystallin lens add to the oxidative damage to LECs as well as the risk of cataract formation in high myopic eyes. What’s more, a number of significantly changed metabolites in aqueous humor (where the anterior lens capsule was bathed in directly) of high myopes were identified by Ji YH et al [27], but it is not verified yet whether or not the metabolic variations are involved in the morphological changes of LECs in high myopes.
In our study, there exist more extensive extracellular spaces and sometimes interdigitation between LECs in HMC group (shown in Fig. 4, 5). Similar extracellular vacuolization was also reported in a TEM study of LECs in presenile cataract [13]. Since all specimens in our experiment went through the same surgical procedure, we consider that the greater extracellular spaces in HMC group are irrelevant to the surgical stimulation. We proposed that the larger extracellular spaces might be the consequence of cell contraction and cytoskeletal reorganization. Firstly, an inverted microscopic research by Andjelic, et al demonstrated that LECs possessed the ability of contraction, and under non-specific stimulation, the intercellular width changed from barely noticeable to several microns [28]. Furthermore, larger extracellular spaces were also observed in LECs of a transgenic mouse model expressing Rho GTPase inhibitor compared with wild-type mouse LECs under TEM, while Rho GTPase was considered as a crucial point in actin cytoskeleton arrangement [29–31]. Researchers have also suggested that an inappropriate accumulation of transforming growth factor-β (TGF-β) induced remodeling of lens cytoskeleton through Rho family of GTPases [30, 31]. In the meanwhile, Zhu XJ and his copartners found significant elevated TGF-β2 concentration in the aqueous humor of highly myopic eyes, in proportion to axial length [32]. Therefore, it might be worthwhile to further explore the expression of TGF-β and Rho GTPase in surgical capsule samples of HMC as well as their association with lens epithelial cytoskeleton. In addition, the interdigitation between LECs was also observed in cataracts with retinal pigmentosa and uveitis [7, 9]. Andjelic, et al also reported similar interdigitation between adjacent LECs at the basal side but they were not related to the type of cataract (the specific cataract types were not provided though) [6]. However, in our study, the interdigitation of LECs were rarely found in ARC group.
Another significant characteristic observed in five high-myopic specimens (Patient 12, 13, 15, 17, 18, aged 48, 52, 59, 68 and 71 respectively) is the protuberances of extracellular matrix (ECM) towards LECs (shown in Fig. 5). This phenomenon was not reported by Andjelic and colleagues, whose experiment focused on the LECs attachment to aLC in various cataract types by a combination of TEM, scanning electron microscopy and confocal microscopy [6]. However, Hawlina M et al discovered similar protuberances with degraded cellular materials inside at the epithelium-basal lamina border in 86.6% samples of intumescent cataract and in 34.3% samples of nuclear cataract [33]. In their opinion, the function of the protuberances was to capture degrading LECs debris and after that, new layer of epithelium would be deposited upon the gap space. Different from this study, we didn’t find captured cellular debris in the protuberances, and we observed distorted epithelium with notable extracellular spaces upon the protuberances rather than newborn LECs (shown in Fig. 5). Richiert DM, et al and Zhang XH, et al respectively observed elevated secretion of ECM components (fibronectin and laminin) in animal model LECs treated by TGF-β, which was proved to increase in aqueous humor of highly myopic eyes [34–36].Hence, the specific components and origin of the protuberances along epithelium-basal lamina border in HMC need further investigation.
There are several limitations in this study: the crucial ones are the small sample size and the qualitative description; what’s more, our hypothesis about the etiology of extracellular spaces among LECs and protuberances in aLC cannot be merely established by morphologic observation, so immunohistochemistry and other confirmable methods are needed. Therefore, our next step will be expanding sample size and finding objective evidence to explain our observation of lens capsule in HMC.