254 | wileyonlinelibrary.com/journal/ahe Anat Histol Embryol. 2018;47:254–267. © 2018 Blackwell Verlag GmbH 1 | INTRODUCTION The histological structure of the gastrointestinal tract of ruminants can be modified by the diet ingested (Ahmed, Martens, & Muelling, 2013; Dieho et al., 2016; Melo et al., 2013). A variety of changes due to a high concentrate provision have been noted in domestic ruminants, including abnormal ruminal papillation, epithelial thick- ness, hyperkeratosis, parakeratosis and inflammatory infiltration of the ruminal mucosa (Ahmed et al., 2013; Berg & Edvi, 1976; Cernik et al., 2011; Steele, Greenwood, Croom, & McBride, 2012). Moreover, the physical form or energy content of otherwise similar diets may further result in changes of ruminal microanatomy (Amaral, Received: 20 August 2017 | Accepted: 17 January 2018 DOI: 10.1111/ahe.12351 ORIGINAL ARTICLE Microanatomy of the digestive tract, hooves and some visceral organs of addax antelope ( Addax nasomaculatus ) following a concentrate or forage feeding regime S. A. Tahas 1 | U. Hetzel 2 | B. Altenbrunner-Martinek 3 | O. Martin Jurado 1 | S. Hammer 4 | A. Arif 4 | J.-M. Hatt 1 | M. Clauss 1 1 Clinic for Zoo Animals, Exotic Pets and Wildlife, Vetsuisse Faculty, University of Zurich, Zurich, Switzerland 2 Institute of Veterinary Pathology, Vetsuisse Faculty, University of Zurich, Zurich, Switzerland 3 University Clinic for Ruminants, University for Veterinary Medicine, Vienna, Austria 4 Al Wabra Wildlife Preservation, Doha, Qatar Correspondence Stamatios A. Tahas, Clinic for Zoo Animals, Exotic Pets and Wildlife, Vetsuisse Faculty, University of Zurich, Zurich, Switzerland. Email: stahas@vetclinics.uzh.ch Present address O. Martin Jurado, Alte Landstr. 133, Küsnacht 8700, Switzerland S. Hammer, Naturschutz-Tierpark Görlitz, Zittauer Str. 43, Görlitz 02826, Germany Summary Subacute ruminal acidosis is a common disease in captive non-domesticated rumi- nants and is mainly diagnosed by rumen fluid pH and rumen histology. Furthermore, differences in ruminant gastrointestinal histology have been hypothesized to cor- relate with the browser–grazer continuum. Twelve surplus addax antelope ( Addax nasomaculatus) were divided into two groups, fed either their usual diet, consisting of a concentrate feed with a limited amount of hay, or a diet of unlimited hay only, for 3 months. After culling, descriptive and morphometric histology and pH meas- urements were compared between groups. Significant variations in cellular sub- populations were noted between groups, with roughage-fed individuals presenting primarily with balloon cells of the Stratum corneum and living layer cell vacuoliza- tion, whereas parakeratosis and intermediate-type cells were more frequent in the concentrate-fed group. Lesions typical of subacute ruminal acidosis were signifi- cantly more pronounced in concentrate-fed individuals. Ruminal pH measurements did not differ significantly, but were more consistent in forage-fed individuals, indi- cating a more stable reticuloruminal environment. The results indicate that ruminal histology may be more appropriate in assessing ruminal health compared to a single post-mortem pH measurement. It is proposed that Stratum corneum balloon cells may indicate cell maturation and not, as previously assumed, hyperfunction. Concentrate-fed individuals scored higher on the presence of inflammatory cells on hoof corium histology. The study further emphasizes the adaptability of ruminant digestive tract microanatomy in adult animals even after a short period of time and the positive effects an increased roughage diet may have in populations of captive grazing ruminants.