title: Fate of the mesenchyme in the process of pneumatization.
abstract: OBJECTIVE
  This study's aim was to find histologic data that would indicate the mode of disappearance of the embryonal mesenchyme.
  
  
  BACKGROUND
  The basic studies made during the first half of the 20th century concluded that mesenchyme disappears by regression and resorption. Recently, it was suggested that mesenchyme disappears by receding, spreading, and thinning to match the enlarging bony spaces.
  
  
  METHODS
  We studied 11 serially sectioned temporal bones from newborns to adults and describe detailed findings in a 9-day-old newborn and in a 1.5-year-old infant. The temporal bones were sectioned to 20 mum and stained by hematoxylin and eosin.
  
  
  RESULTS
  Histologic evidence of regression was found in the form of degenerating mesenchymal cells and fibers, in areas free of cells, and with empty spaces of varying size between the fibers. Vacuoles differing much in size appeared, and phagocytic cells were frequent. A rich capillary network allowed resorption of hemopoietic cells dispersed from the marrow spaces into the mesenchyme. From the lower lateral attic, from Prussak's space, and from the mastoid air cells, mesenchyme can disappear only by regression-there is no space where it could recede.
  
  
  CONCLUSIONS
  Pneumatization of the middle ear spaces occurs by regression and resorption with an individual speed under genetic guidance. The osteoclastic activity of the periosteum, intertwined with the nearest mesenchyme, is decisive in the mastoid air cell formation. Dispersion and reabsorption of hemopoietic cells is a normal phenomenon in this process. Underpressure in the middle ear spaces, caused either by a meconium-related foreign body otitis media in infancy or by chronic otitis media in childhood, are factors that may lead to a partial or full arrest of pneumatization.
authors.affiliation: Department of Otolaryngology, Helsinki University Central Hospital, Helsinki, Finland.
authors.forename: Tauno
authors.initials: T
authors.lastname: Palva
authors.forename: Hans
authors.initials: H
authors.lastname: Ramsay
mesh_terms: D000368:Aged; D000388:Air; D002648:Child; D005260:Female; D005547:Foreign Bodies / Q000473:pathology*; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D008297:Male; D008648:Mesoderm / Q000473:pathology*; D010579:Petrous Bone / Q000473:pathology; D012075:Remission, Spontaneous; D013701:Temporal Bone / Q000473:pathology*