Department of Histology and Embryology, University School of Medicine, Wroclaw, Poland Histological Studies of Growing and Mature Antlers of Red Deer Stags (Cervus elaphus) M. Cegielski 1 *, I. Izykowska 1 , M. Podhorska-Okolow 1 , B. Gworys 2 , M. Zabel 1,3 and P. Dziegiel 1 Addresses of authors: 1 Department of Histology and Embryology, 2 Department of Anatomy, University School of Medicine, Chalubinskiego 6a, 50-368 Wroclaw; 3 Department of Histology and Embryology, University of Medical Sciences, Swiecickiego 6, 61-781 Poznan, Poland; *Corresponding author: Tel.: +48 717841355; fax: + 48 717840082; e-mail: marcel@hist.am.wroc.pl With 11 figures Received May 2008; accepted for publication August 2008 Summary This study aims at presenting histology of growing and mature antlers in red deer stag (Cervus elaphus). Growing antlers constitute a model organ for examining regeneration processes of tissues because they are the only mammalian appendages capable of regeneration. Histological study revealed that the tip of a growing antler consists of hairy skin, perichondrium, mesenchyme and chondroprogenitors area. By performing immunochistochemistry, we found that cell expressing Ki-67 and PCNA antigens were localized in basal layer of epidermis, skin glands and beneath their secretory sections, mesenchyme as well as within and in the vicinity of central blood vessels. Ultrastructurally, cells from chondroprogenitors zone have chondroblast-like morphology and take part in producing of collagen fibres followed by the process of cartilage minerali- zation. However, mature antlers also consist of lamellar osse- ous tissue. Introduction Great development and progress of regenerative medicine in the recent years constitute inspiration for further studies not only aiming at explaining mechanisms and special involvement of stem cells in regenerative processes but also applying this knowledge in fulfilling human needs and hopes for finding new, more efficient therapeutic methods. Every year in red deer stag (Cervus elaphus) the old set of antlers is shed and replaced with a new one. The periodicity of these processes makes antlers a unique material for research underlying the basic regeneration of mammal tissues. Antlers are the only appendages capable of regen- eration in mammals. Besides, they are one of the fastest growing organs, because during most intense growth period the daily growth is 2 cm. The initiation of antler growth takes place at the bone appendix (pedicle), starting on the frontal bone covered with antlerogenic periosteum (Price and Allen, 2004; Li et al., 2005). During growth period of the pedicle, some cells from the antlerogenic periosteum are transferred to the apical region of pedicle where they participate in creating antler growth centres (Li and Suttie, 1998). A similar process takes place during regeneration and rebuilding of antlers, when some antlerogenic cells are transferred from growth centres to the apex of an antler (Cegielski et al., 2006). It is believed that antlerogenic periosteum cells as well as antlerogenic pedicle cells retain the nature of embryonic cells for they contain large amounts of glycogen granules (Li and Suttie, 1998, 2001; Cegielski et al., 2006). What is more, among the mentioned types of cells there are also stem cells which can differentiate into cartilage, bone and adipose cells (Kierdorf et al., 2003; Li et al., 2005; Cegielski et al., 2006; Berg et al., 2007). Grow- ing antlers are built from a mixed chondro-osseous tissue and elongate as a result of a modified endochondral ossification, while at the same time around the antler shaft intramembranous ossification takes place. The apical region of antlers consists of several layers: from outside – hairy skin, perichondrium, mesenchyme, transient layer and cartilage (Li et al., 2002). During the reproduction period, the antlers cease to grow which manifests itself in its total ossification and atrophy of the overlying skin (velvet), which is removed from its surface. This process coincides with the parallel growth of testosterone levels. Antlers formed in this manner serve maintaining hierarchy within the herd and constitute fight tools during mating season to be finally shed with the coming spring (Price et al., 2005). This study aims at presenting histology of growing and mature antlers in red deer stag (Cervus elaphus). Materials and Methods Obtaining the material The studies were performed on terminal, lateral fragments of growing antlers in red deer stag (Cervus elaphus) during the period of their most intense growth. Additionally, mature antlers were harvested after the previous set had been cast. The experiment was carried out in the course of routine procedures of nursing care in the Wroclaw Zoological Garden. The animal protocol was reviewed and approved as set out in Decision No. 41/2006 of the I Local Ethics Committee for Animal Research in Wroclaw. In both examples, we obtained specimens 0.5 cm thick and of 1–2 cm 2 surface (about 2–3 g) in sterile manner and under anaesthesia, through the remote injection system. The obtained tissue was destined for light microscopy and electron microscopy. The material from growing antler was later fixed in 4% buffered formalin solution, dehydrated and embedded in paraffin blocks. Sections for micro- scopic examination were stained with haematoxylin and eosin. The specimens were assessed under Olympus BX41 microscope (Olympus Corporation, Tokyo, Japan). Ó 2009 The Authors. Journal compilation Ó 2009 Blackwell Verlag GmbH Anat. Histol. Embryol. 38, 184–188 (2009) doi: 10.1111/j.1439-0264.2008.00906.x ISSN 0340–2096