title: Pregnancy in the mature adult mouse does not alter the proportion of mammary epithelial stem/progenitor cells.
abstract: BACKGROUND
  In humans, an early full-term pregnancy reduces lifetime breast cancer risk by up to 50% whereas a later pregnancy (>35 years old) can increase lifetime risk. Several mechanisms have been suggested, including changes in levels of circulating hormones, changes in the way the breast responds to these hormones, changes in gene expression programmes which may alter susceptibility to transformation and changes to mammary stem cell numbers or behaviour. Previous studies have shown that the mammary tissue isolated from both virgin and parous mice has the ability to repopulate a cleared mammary fat pad in transplant experiments. Limited dilution transplant assays have demonstrated that early pregnancy (at 5 weeks of age) reduces stem/progenitor cell numbers in the mouse mammary epithelium by twofold. However, the effects on stem/progenitor cell numbers in the mammary epithelium of a pregnancy in older animals have not yet been tested.
  
  
  METHODS
  Mice were put through a full-term pregnancy at 9 weeks of age, when the mammary epithelium is mature. The total mammary epithelium was purified from parous 7-week post-lactation and age-matched virgin mice and analysed by flow cytometry and limiting dilution cleared fat pad transplants.
  
  
  RESULTS
  There were no significant differences in the proportions of different mammary epithelial cell populations or numbers of CD24(+/Low) Sca-1- CD49f(High) cells (stem cell enriched basal mammary epithelial compartment). There was no significant difference in stem/progenitor cell frequency based on limiting dilution transplants between the parous and age-matched virgin epithelium.
  
  
  CONCLUSIONS
  Although differences between parous and virgin mammary epithelium at later time points post lactation or following multiple pregnancies cannot be ruled out, there are no differences in stem/progenitor cell numbers between mammary epithelium isolated from parous animals which were mated at 9 weeks old and virgin animals. However, a recent report has suggested that animals that were mated at 5 weeks old have a twofold reduction in stem/progenitor cell numbers. This is of interest given the association between early, but not late, pregnancy and breast cancer risk reduction in humans. However, a mechanistic connection between stem cell numbers and breast cancer risk remains to be established.
authors.affiliation: Breakthrough Breast Cancer Research Centre, The Institute of Cancer Research, London, UK. Kara.Britt@med.monash.edu.au
authors.forename: Kara L
authors.initials: KL
authors.lastname: Britt
authors.forename: Howard
authors.initials: H
authors.lastname: Kendrick
authors.forename: Joseph L
authors.initials: JL
authors.lastname: Regan
authors.forename: Gemma
authors.initials: G
authors.lastname: Molyneux
authors.forename: Fiona-Ann
authors.initials: FA
authors.lastname: Magnay
authors.forename: Alan
authors.initials: A
authors.lastname: Ashworth
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Smalley
mesh_terms: D000818:Animals; D002478:Cells, Cultured; D004847:Epithelial Cells / Q000166:cytology*; D005260:Female; D005434:Flow Cytometry; D005455:Fluorescent Antibody Technique; D007774:Lactation; D008321:Mammary Glands, Animal / Q000166:cytology*; D051379:Mice; D010298:Parity; D011247:Pregnancy; D011270:Pregnancy, Animal / Q000502:physiology*; D013234:Stem Cells / Q000166:cytology*