The cornea can be divided into five layers, with the endothelium being the innermost layer and the most “disease-prone” of the five layers. Many people may wonder why it is not the outermost layer of the cornea that is susceptible to disease, but the innermost layer. What happens when the inner corneal layer gets sick? When doctors refer to endothelial transplantation, do they only transplant the endothelial layer? Let’s first understand why the endothelium is susceptible to disease and what does it mean when the endothelium loses its compensations? The endothelium is composed of a layer of endothelial cells, which cannot regenerate and will gradually decrease in number with age. Moreover, trauma, viral infections and even eye surgery can cause a decrease in endothelial cells. At the beginning of the decrease, the body enlarges the neighboring cells and fills the gaps with reduced density (compensation). The endothelium functions as a corneal-atrial water barrier, acting like a pump, constantly pumping water molecules out of the stroma, leaving it in a dehydrated state while remaining transparent. Therefore, its proper function is related to whether the entire cornea can be transparent or not. When the number of endothelial cells is reduced to a certain level due to disease, it is not enough to pump out the excess water, i.e. the endothelium loses its ability to compensate. At this point, the cornea becomes edematous, and in severe cases, large blisters bulge from the corneal surface. Once the large blisters on the cornea are worn out, the patient will have symptoms such as eye pain, grinding, photophobia and tearing, and will need a corneal transplant. If there is endothelial cell loss, is the endothelial layer transplanted alone? The so-called corneal endothelial graft is not just a graft of the endothelial layer, which is made up of individual endothelial cells that have to be attached to the posterior elastic layer; the posterior elastic layer is very thin and difficult to separate from the stroma, so some graft pieces will also have some stroma. Depending on whether the graft has a stroma or not, and the thickness of the stroma, the endothelial grafting procedure is different. In one approach, the donor endothelium and posterior elastic layer, plus a certain thickness of stroma, are peeled off and transplanted into the patient’s eye using a tiny endothelial knife (called DSAEK, or automated keratome extraction endothelial graft). The common practice abroad is to graft the endothelium and posterior elastic layer alone (called DMEK, i.e., posterior elastic layer graft), which is relatively thinner, but the posterior elastic layer is too thin and prone to curling, resulting in a certain failure rate during retrieval, which is not a good choice in our country when materials are scarce. Ultra DSEK (ultra-thin endothelial keratoplasty), a method with an ultra-thin stroma, is currently being performed. The thickness of the graft is 50-70 microns, which is thinner than that of the traditional DSAEK procedure, while avoiding the curling problem caused by grafting the endothelial and posterior elastic layers alone.