Morphology of the vitreoretinal border region.

Abstract

Membrana limitans interna retinae (MLI) has been studied since 1871 and in structure and composition have been discussed since then. With the use of electron microscopes when studying the MLI a new terminology has been introduced, i.e. the vitreoretinal border region (VBR). In this survey, ther general concept of basement membranes has been applied to the VBR. The VBR consists of two major components. The inner: anchoring fibrils of the vitreous body and the outer: MLI. The MLI is further defined as composed of three structures: the fusing points of the anchoring vitreous fibrils, lamina densa and lamina lucida. Stress forces between the retina and the vitreous body are transmitted via this border region, and may cause severe clinical conditions such as retinal detachment. To investigate this border region morphologically, improvements in the conventional preparation technique for scanning electron microscopy (SEM) were found to be necessary in order to exhibit more details of the VBR. A new rapid procedure for desiccating frozen resin-cracked retinal tissue using hexamethyldisilazane was found to be appropriate. Sixteen pairs of normal eyes, 16 pairs of monkey eyes, 55 pairs of non-normal eyes from different animal species, enzyme digested monkey retinas and the retinas of two rat models with diabetes and hypertension respectively were investigated. In addition to SEM, the vitreoretinal border region was also investigated by means of light microscopy and transmission electron microscopy. The material was analyzed morphometrically. The human MLI increases markedly in thickness during the first months/years of life in the equatorial and macular regions. The thickness is stable from the second decade, and remains unchanged throughout subsequent decades. A regional difference in thickness of the MLI was found in all human adult eyes and in monkey eyes; it was thickest in the macular region. The length of vitreous fibrils close to the MLI also varied between the four regions in human eyes, the longest being in the ora serrata region, the second longest in the equatorial region, the next longest in the optic disc region and the shortest in the macular region. A morphological similarity in the appearance of the VBR was found in humans and monkeys. All other animals, except for cephalopods, showed a marked uniformity of the VBR. The enzyme-digested monkey retinas showed the fibrillar meshwork of the VBR to consist mainly of collagen fibers surrounded predominantly by hyaluronic acid. No firm correlation between thickness of the VBR and diabetes or hypertension could be demonstrated in the two animal models.