title: Evidence of specialized tissue in human interatrial septum: histological, immunohistochemical and ultrastructural findings.
abstract: BACKGROUND
  There is a paucity of information on structural organization of muscular bundles in the interatrial septum (IAS). The aim was to investigate histologic and ultrastructural organization of muscular bundles in human IAS, including fossa ovalis (FO) and flap valve.
  
  
  METHODS
  Macroscopic and light microscopy evaluations of IAS were performed from postmortem studies of 40 patients. Twenty three IAS specimens underwent serial transverse sectioning, and 17--longitudinal sectioning. The transverse sections from 10 patients were immunolabeled for HCN4, Caveolin3 and Connexin43. IAS specimens from 6 other patients underwent electron microscopy.
  
  
  RESULTS
  In all IAS specimens sections the FO, its rims and the flap valve had muscle fibers consisting of working cardiac myocytes. Besides the typical cardiomyocytes there were unusual cells: tortuous and horseshoe-shaped intertangled myocytes, small and large rounded myocytes with pale cytoplasm. The cells were aggregated in a definite structure in 38 (95%) cases, which was surrounded by fibro-fatty tissue. The height of the structure on transverse sections positively correlated with age (P = 0.03) and AF history (P = 0.045). Immunohistochemistry showed positive staining of the cells for HCN4 and Caveolin3. Electron microscopy identified cells with characteristics similar to electrical conduction cells.
  
  
  CONCLUSIONS
  Specialized conduction cells in human IAS have been identified, specifically in the FO and its flap valve. The cells are aggregated in a structure, which is surrounded by fibrous and fatty tissue. Further investigations are warranted to explore electrophysiological characteristics of this structure.
authors.affiliation: Department of Pathology, Federal Almazov Medical Research Centre, Saint-Petersburg, Russia.
authors.forename: Lubov B
authors.initials: LB
authors.lastname: Mitrofanova
authors.affiliation: Department of Pathology, Federal Almazov Medical Research Centre, Saint-Petersburg, Russia; Laboratory of cell morphology, Institute of cytology of the Russian Academy of Sciences, Saint-Petersburg, Russia; Laboratory of structural and functional proteomics, Research Institute of Influenza, Saint-Petersburg, Russia.
authors.forename: Andrey N
authors.initials: AN
authors.lastname: Gorshkov
authors.affiliation: Department of Arrythmology, Federal Almazov Medical Research Centre, Saint-Petersburg, Russia.
authors.forename: Dmitry S
authors.initials: DS
authors.lastname: Lebedev
authors.affiliation: Department of Arrythmology, Federal Almazov Medical Research Centre, Saint-Petersburg, Russia; Neuromodulation unit, Federal Almazov Medical Research Centre, Saint-Petersburg, Russia.
authors.forename: Evgeny N
authors.initials: EN
authors.lastname: Mikhaylov
mesh_terms: D000328:Adult; D000368:Aged; D054087:Atrial Septum / Q000378:metabolism / Q000473:pathology* / Q000648:ultrastructure; D051244:Caveolin 3 / Q000276:immunology / Q000378:metabolism; D018031:Connexin 43 / Q000276:immunology / Q000378:metabolism; D005260:Female; D006351:Heart Valves / Q000378:metabolism / Q000473:pathology / Q000648:ultrastructure; D006801:Humans; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels / Q000276:immunology / Q000378:metabolism; D007150:Immunohistochemistry; D008137:Longitudinal Studies; D008297:Male; D008854:Microscopy, Electron; D008875:Middle Aged; D009124:Muscle Proteins / Q000276:immunology / Q000378:metabolism; D015221:Potassium Channels / Q000276:immunology / Q000378:metabolism
chemical_list: D051244:Caveolin 3; D018031:Connexin 43; C119587:HCN4 protein, human; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D009124:Muscle Proteins; D015221:Potassium Channels