Please cite this article in press as: Demircan, T., et al., A histological atlas of the tissues and organs of neotenic and metamorphosed axolotl. Acta Histochemica (2016), http://dx.doi.org/10.1016/j.acthis.2016.07.006 ARTICLE IN PRESS G Model ACTHIS-51108; No. of Pages 14 Acta Histochemica xxx (2016) xxx–xxx Contents lists available at ScienceDirect Acta Histochemica journal homepage: www.elsevier.de/acthis A histological atlas of the tissues and organs of neotenic and metamorphosed axolotl Turan Demircan (PhD) a,d,∗ , Ays ¸ e Elif ˙ Ilhan d , Nilüfer Aytürk b,d , Berna Yıldırım d , Gürkan Öztürk c,d , ˙ Ilknur Keskin (PhD) (MD) b,d,∗ a Department of Medical Biology, International School of Medicine, ˙ Istanbul Medipol University, Istanbul, Turkey b Department of Histology and Embryology, School of Medicine, Istanbul Medipol University, Istanbul, Turkey c Department of Physiology, International School of Medicine, ˙ Istanbul Medipol University, Istanbul, Turkey d Regenerative and Restorative Medicine Research Center, REMER, Istanbul Medipol University, Istanbul, Turkey a r t i c l e i n f o Article history: Received 19 April 2016 Received in revised form 7 June 2016 Accepted 11 July 2016 Available online xxx Keywords: Axolotl Neoteny Metamorphosis Histological map Thyroid hormones a b s t r a c t Axolotl (Ambystoma Mexicanum) has been emerging as a promising model in stem cell and regeneration researches due to its exceptional regenerative capacity. Although it represents lifelong lasting neoteny, induction to metamorphosis with thyroid hormones (THs) treatment advances the utilization of Axolotl in various studies. It has been reported that amphibians undergo anatomical and histological remod- eling during metamorphosis and this transformation is crucial for adaptation to terrestrial conditions. However, there is no comprehensive histological investigation regarding the morphological alterations of Axolotl organs and tissues throughout the metamorphosis. Here, we reveal the histological differences or resemblances between the neotenic and metamorphic axolotl tissues. In order to examine structural features and cellular organization of Axolotl organs, we performed Hematoxylin & Eosin, Luxol-Fast blue, Masson’s trichrome, Alcian blue, Orcein and Weigart’s staining. Stained samples from brain, gallbladder, heart, intestine, liver, lung, muscle, skin, spleen, stomach, tail, tongue and vessel were analyzed under the light microscope. Our findings contribute to the validation of the link between newly acquired functions and structural changes of tissues and organs as observed in tail, skin, gallbladder and spleen. We believe that this descriptive work provides new insights for a better histological understanding of both neotenic and metamorphic Axolotl tissues. © 2016 Elsevier GmbH. All rights reserved. 1. Introduction Metamorphosis term is used to define the innate process of amphibian transition from larval stage to adult form (Shi, 2000). This transformation provides an excellent model system to under- stand vertebrate organogenesis and remodeling of the organs. During and following this transformation, commonly observed phenotypical changes are anatomical and histological reconstitu- tion of the organs as well as appendages to function properly in terrestrial life conditions. Regression, disappearing and/or remod- eling of the existing organs as well as formation of new organs are the observed adjustments of metamorphosis (reviewed in (Brown and Cai, 2007)). For the description of changes at organ and sys- ∗ Corresponding authors at: Regenerative and Restorative Medicine Research Cen- ter, REMER, Istanbul Medipol University, Istanbul, Turkey. E-mail addresses: tdemircan@medipol.edu.tr (T. Demircan), ilknurkeskin@medipol.edu.tr ( ˙ I. Keskin). tem level, Xenopus leavis is the widely used organism among the amphibians (Burggren and Warburton, 2007; Colombo et al., 2015). Previous studies have demonstrated that from tadpole to adult frog transformation, most of the organs undergo remodeling such as skin (Yoshizato, 1996), lung (Dodd and Dodd, 1976) and liver (Atkinson et al., 1998). The external gills of the tadpoles, which are the primary site for respiration in aquatic environment, disappear at the end of the metamorphosis (Ishizuya-Oka et al., 2010). Bone marrow, functional limbs and glands in skin and stomach are the examples of newly formed cells, tissues and structures with meta- morphosis. Timing and rate of this complex process is regulated by hormonal activity and several external factors such as temperature (Hayes et al., 1993), density of population (Semlitsch and Caldwell, 1982), threat of predator presence and food levels (Kupferberg et al., 1994). In terms of hormonal regulation, this remodeling cascade starts with production and secretion of thyroid hormones (THs). It has been found that, THs levels in amphibians are low at early larval stage and peak at metamorphic climax (Mondou and Kaltenbach, 1979). http://dx.doi.org/10.1016/j.acthis.2016.07.006 0065-1281/© 2016 Elsevier GmbH. All rights reserved.