Age-related Qualitative Histological and Quantitative Stereological Changes in the Equine Pituitary

The aim of this study was to characterize the age-related morphological changes in the equine pituitary and to identify features that allow distinction between pituitary pars intermedia dysfunction (PPID)-associated and non-functional/age-associated pars intermedia (PI) adenoma. Pituitary glands of all horses submitted for necropsy examination at the Institute of Veterinary Pathology, Ludwig Maximilians University Munich, between 2008 and 2012 were examined. The pituitary glands of 124 horses were weighed, cut into ∼2 mm slices and examined histologically. A slightly modified grading scheme (grades 1-5) was applied to evaluate histological alterations of the PI semiquantitatively. The volume fractions and total volumes of the three pituitary lobes, PI, pars distalis (PD) and pars nervosa (PN), as well as the total number and mean size of PI cells (PICs), were determined using state-of-the-art quantitative stereological methods. There were significant associations between histological grade, the appearance of PI adenomas, follicles and cysts in the PI, lipofuscin in the PN (P <0.001) and focal hyperplasia of chromophobes in the PD and age. In contrast, the appearance of follicles and cysts in the PD, invasion of basophil cells into the PN, haemorrhage and necrosis were not age dependent. PI adenomas were observed in 18% (22/124) of the animals, but only four horses were evidently suffering from PPID, therefore clinically overt/PPID-associated PI adenomas were found in 3% (4/124) of all horses. Most PI adenomas were incidental and considered non-functional. Pituitary weight, PI volume, total number and mean volume of PICs increased with age in all horses. The total PI volume and the number of PICs of horses exhibiting PPID-associated PI adenomas were significantly higher, while the mean size of PICs was smaller compared with that of horses showing non-PPID-associated adenoma, which suggests that different growth processes are responsible for adenoma formation. The present study demonstrated various age-associated lesions of the PD and PN and revealed a high frequency of incidental, non-PPID-associated PI adenomas in aged horses. Therefore, post-mortem diagnosis of PPID in horses is possible, by determination of pituitary weight and by demonstration of PIC hyperplasia, using quantitative stereological methods.