PubMed · 8090446
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P U Fechner. 1994. Polymegalism?. https://doi.org/10.1016/s0161-6420(94)31148-1
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OBJECTIVE: To investigate the ultrastructural and physiologic effects of exposure of the human corneal endothelium to mitomycin at concentrations of 20 micrograms/mL and 200 micrograms/mL using electron microscopy and in vitro specular perfusion techniques. METHODS: Four pairs of corneas (with one cornea of each pair receiving balanced salt solution [BSS Plus, Alcon Laboratories, Fort Worth, Tex] and the other receiving BSS Plus with 20 micrograms/mL of mitomycin) suitable for transplantation, except for extremes of age or systemic disease, underwent perfusion with corneal thickness measured serially every 15 minutes followed by fixation for electron microscopy. Mean corneal swelling rate was calculated for all four experiments, and the control group that received BSS Plus was compared with the group that received mitomycin using a paired t test. Electron micrographs were examined in a masked fashion. Similar studies were performed using two pairs of corneas that received 200 micrograms/mL of mitomycin. RESULTS: The mean swelling rate for corneas perfused with 20 micrograms/mL of mitomycin (-4.1 microns/h) was not significantly different from that seen in tissue perfused with BSS Plus (-4.2 microns/h). No consistent ultrastructural changes could be attributed to exposure to 20 micrograms/mL of mitomycin. Perfusions of mitomycin at 200 micrograms/mL resulted in prompt corneal swelling with marked ultrastructural alterations compared with tissue perfused with BSS Plus. CONCLUSION: Human corneal endothelium may be exposed to undiluted (200 to 500 micrograms/mL) mitomycin with inadvertent entry into the anterior chamber during dissection of the scleral flap bed in trabeculectomy followed by application of mitomycin. This will result in prompt destruction of the endothelium. Exposure to 20 micrograms/mL of mitomycin, a level exceeding the concentration that may be present in the aqueous humor after its proper application, appears nontoxic in this system.
Plasma formation with nanosecond and picosecond Nd:YAG laser pulses was investigated as a function of laser pulse energy to find possibilities for a spatial reduction of the laser effects. The threshold for plasma formation (optical breakdown) in distilled water is 200 microJ for ns pulses and 15 microJ for ps pulses. At a pulse energy of 1 mJ, the plasma length is 80 microns for ns pulses and 250 microns for ps pulses. For the same pulse energy, ps plasmas are on average three times larger than ns plasmas. The plasma length is approximately proportional to the cube root of the laser pulse energy for both ns and ps laser pulses. The decrease in the breakdown threshold achieved by using ps pulses renders photodisruption with a smaller pulse energy (in the microJ range) possible, and therefore a reduction in the side effects. The results are compared with the "moving breakdown" model.
The ultra-microstructure of the human corneal endothelium was examined by scanning and transmission electron microscopy. The samples were corneas of 26 human eyes taken from fetuses of 5-22 weeks gestation and a newborn 10 months after birth. Corneal endothelium appeared at the seventh or eighth week after gestation. Initially, they formed an irregular structure of two to three layers. From the 17th week of gestation they formed a single layer, and assumed the form of a cuboidal epithelium. Scanning and transmission electron microscopy showed numerous microvilli protruding towards the anterior chamber. Within the microvilli uniform microtubules were observed. By the 20th week, these microvilli disappeared, and subsequently, a single long narrow cilium appeared in the center of each cell. Each cilium had an axial filament complex structure. This cilium involuted as development progressed, and was barely visible by the 10th month after birth. The significance of this cilium is not clear.