48 Localization of Tight Junctions between Tanycyte-Like Cells of the Sulcus Medianus Organum in Rat Brain Fadhil H. Ahmed 1 BVM&S, Muthanna A. Al-Kaabi 1 PhD, Sarmad E. Al-Marsoummi 1 MSc, Hayder A. Al-Aubaidy 2 PhD 1 Dept. of Human Anatomy, College of Medicine, Al-Nahrain University, Baghdad, Iraq, School of Medicine, Faculty of Health, University of Tasmania, Australia Abstract Background Tanycytes are highly specialized ependymal cells (ECs) that line the regions of circumventricular organs (CVOs) where the blood-brain barrier is absent. These cells show tight junctions (TJs) that form a blood–cerebrospinal fluid barrier at the regions of these organs, thus modulating the transport mechanisms between the cerebrospinal fluid and the brain capillaries. Objective To investigate the presence of TJs between ECs of the sulcus medianus organ (SMO) in the floor of the 4 th ventricle in comparison to the region of the median eminence (ME) of the hypothalamus, a known CVO in the brain. Methods Ten adult male rats (Rattus Norvegicus Albinus) aged 3-6 months were used to study the general histological morphology of ECs at the SMO and ME regions by H. and E. stain, and to explore the immunofluorescence labelling of these cells for TJs using anti-ZO1 FITC. Results The ECs were arranged in 2-3 layers in the depth of the median sulcus (MS) at the SMO region as seen with H. and E. stain. Immunofluorescent labelling with anti-ZO1 FITC revealed the presence of TJs between ECs of the SMO in a way similar to that reported between tanycytes of the ME. Conclusion The SMO contained tanycytes or tanycyte-like cells that exhibited TJ complexes. This unique cell population added a strong evidence for the presence of a CVO in the rostral part of the MS to link CVOs on both sides of the cerebral aqueduct. Keywords Tanycytes, circumventricular organs, sulcus medianus organum, tight junctions Citation Fadhil H. Ahmed, Muthanna A. Al-Kaabi, Sarmad E. Al-Marsoummi, Hayder A. Al-Aubaidy. Localization of tight junctions between tanycyte-like cells of the sulcus medianus organum in rat brain. Iraqi JMS. 2017; Vol. 15(1): 48-63. doi: 10.22578/IJMS.15.1.7 List of abbreviation: AP = Area postrema, BBB = Blood-brain barrier, BCSFB = Blood–CSF barrier, CNS = Central nervous system, CSF = Cerebrospinal fluid, CVOs = Circumventricular organs, ECs = Ependymal cells, ME = Median eminence, MS = Median sulcus, NH = Neurohypophysis, OVLT = Vascular organ of the lamina terminalis, Pi = Pineal gland, SCO = Subcommissural organ, SFO = Subfornical organ, SMO = Sulcus medianus organum, TJs = Tight junctions, ZO1 = Zonula occludens-1 Introduction n the rat brain, neuroepithelial cells form ependymal cells (ECs) before day 10 of embryonic development and start to proliferate after the neural plate formation (1) . Altman and Bayer (2,3) observed that ependyma was first formed on day 16 of embryonic development but reported that it was not completed in some regions until the second postnatal week. In addition, the bulk of common ECs within the cerebral ventricles form on embryonic days 16-18, however, the specialized tanycytic epithelium is generated mainly postnatally. Horstmann (4) was the first to describe that the elongated bipolar ependymal cells lining the infundibular recess of the 3 rd ventricle had their proximal pole in the ventricular wall and I Iraqi JMS Published by Al-Nahrain College of Medicine P-ISSN 1681-6579 E-ISSN 2224-4719 Email: iraqijms@colmed-alnahrain.edu.iq http://www.colmed-alnahrain.edu.iq http://www.iraqijms.net