Abstract
SummaryBackgroundAdvanced mare age is associated with declining fertility and an increased risk of early pregnancy loss. Compromised oocyte quality is probably the primary reason for reduced fertility, but the defects predisposing to embryonic death are unknown. In women, advanced age predisposes to chromosome segregation errors during meiosis, which lead to embryonic aneuploidy and a heightened risk of miscarriage.ObjectivesTo evaluate the effect of advanced mare age on chromosome alignment and meiotic spindle morphology in in vitro‐matured (IVM) oocytes.Study designMorphometric and morphological analysis.MethodsTo investigate differences in spindle organisation and chromosome alignment between young and old mares, oocytes collected from slaughtered mares were divided into two groups depending on mare age (young, ≤14 years and old, ≥16 years), IVM and stained to visualise chromatin and alpha‐tubulin. Spindle morphology, morphometry and chromosome (mis)alignment were evaluated by confocal microscopy and 3D image analysis.ResultsOocytes from old mares showed a higher incidence of chromosome misalignment (47.4% vs. 4.5%; P<0.001) and a thicker metaphase plate (mean ± s.d.: 5.8 ± 1.0 μm vs. 4.9 ± 0.9 μm; P = 0.04) than oocytes from young mares. Although no differences in spindle morphometry were detected between old and young mares, an increased major spindle axis length was associated with chromosome misalignment (mean ± s.d.: 25.3 ± 6.1 μm vs. 20.8 ± 3.3 μm; P = 0.01) irrespective of age.Main limitationsThe oocytes were IVM and may not exactly reflect chromosome misalignment in vivo.ConclusionsAdvanced mare age predisposes to chromosome misalignment on the metaphase II spindle of IVM oocytes. The compromised ability to correctly align chromosomes presumably predisposes to aneuploidy in resulting embryos and thereby contributes to the age‐related decline in fertility and increased incidence of early pregnancy loss. The Summary is available in Portuguese – see Supporting Information
Highlights
Advanced mare age is associated with a decline in fertility [1,2,3]
No significant difference (P = 0.5) between the two groups was observed for the success of first polar body extrusion after maturation, with 75 of the 169 (44.4%) oocytes from young mares and 54 of the 134 (40.3%) oocytes from old mares reaching Metaphase II (MII)
25 MII oocytes were randomly selected to analyse spindle morphology and chromosome alignment
Summary
Advanced mare age (in particular beyond 14 years) is associated with a decline in fertility [1,2,3] This manifests as a lower success of fertilisation in older (81%) than younger mares (96%) [4], and a trebling of the incidence of pregnancy loss between Day 12 and Day 60 in mares aged 18 years or older (30%) compared with mares younger than 12 years (10%) [5,6,7,8]. The cause(s) of this decline in fertility and associated increase in the incidence of pregnancy loss are likely to be multifactorial; intrinsic oocyte defects are thought to be a major contributor, because transferring oocytes from old mares into the oviduct of younger recipients does not improve the likelihood of fertilisation or reduce the risk of pregnancy loss [5]. The purpose of this study was to determine whether advanced mare age affects spindle morphology and interferes with proper alignment of chromosomes in in vitro-matured MII oocytes
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