Relationships Among Morphologic Characteristics And Membrane Integrity Of Stallion Sperm

Abstract

Various measures of semen quality and fertility often yield either nonexistent or weak correlations. While it is generally assumed that morphologically normal sperm are progressively motile and capable of fertilization, some sperm may have membrane damage that could limit their fertilizing capacity. This study examined the relationship between morphology and membrane integrity of stallion sperm. Semen from 22 sexually active stallions underwent standard evaluation as part of a breeding soundness examination. Aliquots of gel-free semen were diluted to 200 x106 sperm/mL in semen extender then stained with SYBR-14 and propidium iodide (PI). Random fields were observed and digitally captured under 1250x magnification using epifluorescence, followed by differential interference contrast (DIC) microscopy to assess morphologic characteristics of sperm in the same microscopic field. Captured images were evaluated side by side to compare fluorescent staining to sperm morphology. Membrane integrity of sperm was based on SYBR-14 (green = membrane-intact) or PI (red = membranedamaged) labeling of the sperm head. Bivariate analyses were performed to determine the odds ratios (OR) of sperm having intact membranes within each morphologic classification compared to sperm without that morphologic characteristic. Morphologically normal sperm were equally likely to be membrane-damaged as membrane-intact (OR = 1.015). While sperm with abnormal heads (OR = 3.26, P<0.0001) were more likely to be membrane-damaged, 39% of sperm with abnormal heads were membrane-intact. Sperm with proximal droplets (OR = 4.95) and distal droplets (OR = 5.75) were more likely to be membrane-intact than sperm without droplets.