title: The effect of oestrogen on megakaryocyte differentiation and platelet counts in vivo.
abstract: BACKGROUND
  Oestrogen alters megakaryocyte number in humans and mice. In mice, high-dose oestrogen stimulates an initial increase in megakaryocyte number followed by a decrease. However, the cellular action, effect of physiologically circulating and smaller supra-physiological oestrogen doses and whether changes in megakaryocyte number alter platelet counts have not been studied.
  
  
  METHODS
  To further examine oestrogen's effect on megakaryocytes and platelets we administered intact or ovariectomised mice various doses of oestrogen and measured megakaryocyte and platelet counts. To determine the cellular mechanism by which oestrogen influences megakaryocytopoesis we also examined its effect on markers of megakaryocytic differentiation (CD41, CD61, CD34).
  
  
  RESULTS
  We found that large doses of oestrogen (500 microg/kg) increased mature CD41+ megakaryocyte number within 2 days, and this was associated with an increase in circulating platelets. Smaller supra-physiological doses (100 microg/kg) lacked this anabolic effect, but still suppressed megakaryocyte and platelet number by day 10 in intact and ovariectomised mice. This was preceded by a reduction in the number of CD61+ megakaryoblasts and CD34+ precursors available to form mature megakaryocytes. In contrast, ovariectomy had no effect on megakaryocyte or platelet number, indicating that circulating oestrogen concentrations do not influence megakaryocyte differentiation or activity.
  
  
  CONCLUSIONS
  Our data suggest that in mice at least platelet counts reflect changes in megakaryocyte number, and while both are independent of physiological hormone concentrations, they are sensitive to even small supra-physiological doses of oestrogen. Therefore, to ovoid disrupting platelet homeostasis the dose of oestrogen given should be no more than replacement.
authors.affiliation: ESBRG, School of Biological Science, University of Plymouth, Room A413 Portland Square, Drake Circus, Plymouth PL4 8AA, UK. simon.fox@plymouth.ac.uk
authors.forename: Simon W
authors.initials: SW
authors.lastname: Fox
authors.forename: Timothy J
authors.initials: TJ
authors.lastname: Chambers
mesh_terms: D000818:Animals; D002454:Cell Differentiation / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D004967:Estrogens / Q000494:pharmacology*; D005260:Female; D008533:Megakaryocytes / Q000166:cytology / Q000187:drug effects*; D051379:Mice; D010052:Ovariectomy; D010976:Platelet Count; D040201:Platelet Membrane Glycoprotein IIb / Q000032:analysis
chemical_list: D004967:Estrogens; D040201:Platelet Membrane Glycoprotein IIb