Bioprosthetic cornea: using polymeric templates for directed stem cell growth

角膜是我们通向世界的窗口，一旦受到伤害、疾病或年龄的损害，就会导致视力丧失。通过提高我们对角膜结构的了解和提供新的角膜移植方法，可以恢复更多的患者的视力。目前，角膜移植需要源源不断的健康供体角膜。然而，随着全球需求的增长，加上人口老龄化和激光眼科手术的快速增长(这可能会对供体组织移植的适合性产生负面影响)，如果要维持当前的角膜移植水平，寻找有效的工程化替代物是至关重要的。这项研究源于我们之前在了解支持角膜透明的分子结构、新型角膜生物材料的开发以及当前角膜干细胞移植技术的局限性方面所做的工作，特别是用于生长和向患者输送干细胞的材料。此前，我们已经量化了角膜的纳米结构，并将这种结构与保持角膜透明度联系在一起；此外，我们还将这些测量应用于能够支持角膜上皮干细胞分化和生长的新的角膜生物材料的设计。因此，我们将利用我们在角膜结构、角膜干细胞分离和培养以及新型生物材料方面的知识来设计出适合角膜移植的组织。为了做到这一点，我们将开发一种由易处理的规则间隔排列的聚合物(反映角膜纳米结构的自然状态)制成的模板。该模板将包含人类角膜干细胞可识别的蛋白质片段，使它们能够以高度有序和受控的方式附着。一旦附着，细胞将被化学诱导分化并产生胶原纤维。这些纤维的排列将由细胞取向来引导。然后，模板将慢慢失去其完整性，并通过角膜细胞释放的酶从细胞中分离，从而释放出排列整齐的胶原蛋白作为组织工程胶原垫。然后，这些垫子将被堆叠和压缩，以产生完全由人类蛋白质(主要是胶原蛋白)制成的坚固的生物材料，聚合物模板在加工过程中已被移除。然后将测试生物材料的机械强度和在其表面支持角膜上皮生长的能力。我们已经证明，压缩的鼠尾胶原蛋白垫子既具有机械强度，又是角膜上皮细胞生长的极佳基质。这项工作代表着生物材料发展的重要一步。我们的方法不是设计和使用生物相容聚合物来代表移植组织，而是仅仅将聚合物用作模板，允许细胞生产实际的生物材料。此外，由于模板很容易丢弃，基于细胞的生物材料代表了生物相容性的终极，因为它由人类蛋白组成，甚至可能来自患者自身的细胞。这项工作的受益者将是那些在聚合物化学领域工作的人，因为开发含有细胞识别和降解的蛋白质片段的排列聚合物并不是一件微不足道的事情。在蓬勃发展的生物材料领域，科学家将被赋予开发真正生物相容材料(即来自干细胞的材料)的新方向。组织工程师将被赋予一种新的工具，利用它他们可以创造类似的干细胞材料，用于骨、皮肤等修复，最终眼睛再生医学将从替代人类供体角膜组织的开发中受益。

The cornea is our window to the world, once compromised by wounding, disease or age, a loss of vision results. By improving our understanding of corneal structure and providing new methods of corneal transplantation the sight of many more patients can be restored. Currently, corneal transplantation requires a continuous supply of healthy donor corneas. However worldwide demand has grown and taken together with an aging population and the rapid rise in laser eye surgery (which can negatively affect the donor tissue suitability for transplantation) the search for an effective engineered replacement is essential if current levels of corneal transplantation are to be maintained. This investigation stems from our previous work in understanding the molecular structure underpinning corneal transparency, the development of novel corneal biomaterials and the limitations of the current corneal stem cell transplantation techniques, specifically the materials used to grow and convey the stem cells to the patient. Previously, we have quantified the nanostructure of the cornea and related this structure to the preservation of corneal transparency; furthermore we have applied these measurements to the design of new corneal biomaterials capable of supporting corneal epithelial stem cell differentiation and growth. Therefore, we will draw on our knowledge of corneal structure, corneal stem cell isolation and cultivation and novel biomaterials to engineer a tissue suitable for corneal transplantation. To do this we will develop a template made from tractable regularly spaced aligned polymers (reflecting the natural state of corneal nanostructure). The template will contain protein fragments recognisable by the human corneal stem cells enabling them to attach in a highly ordered and controlled manner. Once attached the cells will be chemically induced to differentiate and produce collagen fibres. The alignment of these fibres will be guided by the cells orientation. The template will then slowly lose its integrity and detach from the cells by way of enzymes released by the corneal cells, thus releasing the aligned collagen as a tissue engineered collagen mat. These mats will then be stacked and compressed to produce a robust biomaterial made solely from human proteins (mostly collagen), the polymer template having been removed during processing. The biomaterials mechanical strength and ability to support corneal epithelial growth upon its surface will then be tested. We have already shown that compressed mats of rat tail collagen are both mechanically robust as well as excellent substrates for corneal epithelial cell growth. This work represents a significant step forward in the development of biomaterials. Instead of designing and using bio-compatible polymers to represent tissue for transplantation our approach is to use the polymers merely as a template allowing the cells to produce the actual biomaterial. Furthermore, since the template is easily discarded the cell based biomaterial represents the ultimate in biocompatibility as it is comprised of human proteins possibly even derived from cells taken from the patient's own body. The beneficiaries of this work would be those working in the fields of polymer chemistry as the development of aligned polymers containing protein fragments that are both recognised and degraded by cells is not trivial. Scientists in the blossoming field of biomaterials would be given a new direction in the development of truly bio-compatible materials (i.e. ones derived from stem cells). Tissue engineers would be given a new tool with which they could create similar stem cell based materials for bone, skin etc. repair and finally ocular regenerative medicine would benefit from the development of a replacement human donor corneal tissue.