1. Institute of Neurobiology, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
2. Department of Anatomy, Faculty of Medicine, Trakia University, 6003 Stara Zagora, Bulgaria
3. Institute of Biology and Immunology of Reproduction, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
4. Department of Anatomy and Histology, Medical University of Sofia, 1431 Sofia, Bulgaria