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C.M. Wahl, T. Li, H.C. Howland; Growth of the Scleral Venous Sinus System in Chicks Exposed to Constant Light . Invest. Ophthalmol. Vis. Sci. 2003;44(13):1978.
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Purpose: Chicks raised in constant light (CL) have flatter, thicker corneas and shallower anterior chambers than normal (N) chicks raised in 12/12 cycles of light and dark. The scleral venous sinus (SVS) system is an anastomosing network of vessels within the sclera adjacent to the corneal margin and trabecular meshwork, forming part of the aqueous drainage system. We wished to see if there were morphological differences in the SVS system between growing CL and N chicks which could be functionally correlated with the CL effects. Methods: Eyes were collected and conventionally processed for serial paraffin sections along the naso-temporal axis of the anterior segment. Between 46 and 102 equatorial sections from eyes of 8, 16, 28, and 63 day old chicks (N= 3, 5, 5, and 2) were positioned on a Zeiss compound microscope equipped with a camera lucida. Temporal and nasal SVS systems were hand-traced and the tracings were copied using a Wacom graphics tablet into NIH Image software, where sinus perimeters and cross-sectional areas were computed. On the basis of dimensional analysis we computed a measure of outflow resistance of the SVS as a function of cross sectional areas and perimeters of SVS vessels, and investigated its correlation with the ratio of temporal sinus perimeter/(temporal sinus area).5 (TSPA ratio). Results: We found a significant positive correlation between computed resistance and the TSPA ratio (r = 0.51, p=0.0001). The temporal cross-sectional area of the SVS is much larger than the nasal cross-sectional area in the chick (temporal 0.0400 mm2, nasal 0.0086 mm2, p=0.0001). From 16 to 63 days of age, the TSPA ratio was significantly lower in the CL chicks than among N chicks (16Day, CL=11.0, N=13.3, p=0.0001; 28Day, CL=8.4, N=9.3, p=0.003; 63Day, CL=8.11, N=8.7, p=0.03). At 16 and 28 days this was attributable to smaller CL perimeters (16 days: CL=1.6 mm, N=1.9 mm, p=0.0001; 28 days: CL=1.6 mm, N=1.9 mm p=0.0005)and at 63 days it was attributable to larger CL cross-sectional areas (CL=0.075 mm2, N=0.059 mm2, p=0.0001). Conclusions: The temporal vessels of the SVS play a much larger role in aqueous drainage than the nasal vessels. The TSPA ratio, which is significantly correlated with computed outflow resistance, is lower in CL than N birds, indicating a lower resistance to aqueous outflow, and thus may play a role in corneal flattening.
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