To observe the transparency and tissue structure changes of human corneal stromal lenticules after long-term cryopreservation and explore a simple and feasible method for long-term effective preservation of corneal stromal lenticules.

Two hundred samples of intact human corneal stromal lenticules from 200 eyes were obtained during femtosecond laser small-incision lenticule extraction (SMILE) in Hainan Eye Hospital, Zhongshan Ophthalmic Center from 2013 to 2020.The samples were divided into 1-month, 24-month, 60-month and 80-month group and were stored in an ultra-low temperature freezer for 1, 24, 60 and 84 months respectively at -80 ℃ according to grouping, with 50 samples in each group.Transmittance of the corneal lenticules at wavelength of 300-800 nm was measured with an ultra-micro spectrophotometer and every lenticule was measured for 10 times with a 50 nm interval.The histomorphology and collagen fiber structure of the corneal lenticules were examined by hematoxylin-eosin staining and Masson staining, respectively.The arrangement of collagen fibers and ultrastructure changes of keratocytes in the samples were inspected with a transmission electron microscope.The apoptosis rate of keratocytes was determined by TUNEL staining.The study protocol was approved by an Ethics Committee of Hainan Eye Hospital at Zhongshan Ophthalmic Center (No.2013-003). This study complied with the Declaration of Helsinki.Written informed consent was obtained from each subject before surgery.

The corneal lenticules were clear and intact in all groups and no significant difference in the transmittance within 450-800 nm wavelength was seen among the 4 groups (all at P>0.05). Masson staining revealed that the collagen fibers in the lenticules were neatly arranged and tightly packed in the 1-month group.In the 24-month group, interfibrous vacuoles were found in some collagen fibers.The arrangement of the collagen fibers was loose and more vacuoles were displayed in the 60-month group, and the loss of some collagen fibers appeared and the lenticules were thinned in the 84-month group.It was found through hematoxylin-eosin staining that the morphological changes of corneal stromal lenticules corresponded to the alterations of collagen fibers.Transmission electron microscopy showed that in the 1-month group, the collagen fibers of the corneal stroma lenticules were neatly arranged and regular, and the corneal stromal cells were elongated and spindle-shaped, and the nuclear membrane was intact and the cytoplasm was abundant.In the 24-month group, the collagen fibers showed slightly loose arrangement, and the corneal stromal cells were deformed with incomplete nuclear membrane.In the 60-month group, the collagen fibers were in loose and irregular arrangement, and the nuclei were atrophied and deformed.The 84-month group showed disorganized arrangement of collagen fibers, wrinkled and atrophied corneal stromal cells, discontinuous nucleus membrane and nucleoplasmic lysis.TUNEL staining showed that the percentage of apoptotic corneal cells in lenticules was (87.80±1.17)%, (89.50±1.05)%, (89.30±1.51)% and (90.20±1.47)% in the 1-month, 24-month, 60-month and 84-month groups, respectively, with no statistically significant difference found in overall comparison (F=4.525, P=0.053).

The disorder of collagen fibers and apoptosis of keratocytes occur in the human corneal stromal lenticules till 84 months after cryopreservation, however, the transparency and integrity remain excellent.The ultra-low temperature preservation technique provides an effective and simple solution for long-term storage of human corneal stromal lenticule.

Corneal diseases; Corneal transplantation; Corneal stroma; Preservation, biological; Cryopreservation; Ultrastructure; Apoptosis; Transparency