Histological and endocrine characterisation of the annual luteal activity in Eurasian lynx (Lynx lynx).

Lynx presents a unique sexual cycle with persistent corpora lutea (CLs) and elevated serum progesterone (P₄) throughout parturition and lactation. In other mammals, CLs normally disintegrate after parturition, therefore the aim of our study was to characterise the annual life cycle of lynx CLs. Ovaries from Eurasian lynxes were obtained from the National Veterinary Institute in Sweden, where tissues from killed lynx were stored at -20 °C. Ovaries from 66 animals were weighed; each corpus luteum was segmented for histology and hormone analysis. Ovary and CLs weights were constant throughout the year, peaking during pregnancy. In non-pregnant lynxes, the seasonal level of intraluteal steroids was steady for P₄ (3.2±1.9 s.d. μg/g, n=53) and total oestrogens (18.3±15.5 s.d. ng/g, n=53). Within histology slides, structurally intact luteal cells were found throughout the year with the highest incidence in March/April; evidence of luteal regression was predominantly found in post-breeding season. Ovaries from pregnant animals contained two types of CLs. Group A was bigger in size with large luteal cells (P₄, 72.3±65.4 s.d. μg/g; oestrogen, 454.0±52.4 s.d. ng/g). In contrast, group B were smaller, with greater luteal regression and lower steroid concentrations (P₄, 8.3±2.9 s.d. μg/g; oestrogen, 31.5±20.4 s.d. ng/g). Our results suggest that structural luteolysis proceeds throughout the year and into next breeding cycle, resulting in two CLs types on the same ovary.