Female murine Egr-1 homozygous negative estrus cycle characterization

Early growth response protein‐1 (Egr‐1) −/− (null) male mice are a powerful model for studying vessel formation in pathological conditions. We characterized the estrus cycle of female Egr‐1 null mice for use in these studies in relation to estrogen levels. Estrus cycle in Egr‐1 null and control C57BL/6 wild‐type (WT) mice was monitored by daily vaginal smear for two weeks and analyzed after staining. WT estrus cycle is 4–5 days with 4 distinct phases: proestrus, estrus, metaestrus, and diestrus, identified by cell population morphology. Egr‐1 null mice have 4 less distinct phases with a persistent leukocyte presence. This is striking in the estrus phase, where WT displays an absence of leukocytes, but Egr‐1 nulls show a strong presence of leukocytes. Egr‐1 null mice are known to have a luteinizing hormone‐β deficiency that impacts estrogen and progesterone regulation, and may not be allowing the mice to fully enter estrus. With this type of cycling, we suggest that the Egr‐1 null female mice (and WT controls) be used during the metaestrus/diestrus phase of the cycle characterized by lowest estrogen levels and longest window of access. Studies using female mice attest that large animal numbers account for variability of female cycling. However, the changes in hormone balance at any given stage for mice with altered cycles may skew results, making a strong case to selectively study them in a specific cycle phase.