This study examines the normal postnatal (p.n.) development of the Mon
golian gerbil's vagina from birth to the age when consistent cyclic ch
anges of the estrus occur. At birth, the Mullerian vagina has a lumen
and is lined by a pseudostratified or 2- to 3-layered epithelium. The
cells contain large amounts of glycogen. The sinus vagina forms a soli
d plate and contains hardly any glycogen. During the first few p.n. da
ys the cell layers separate, and the newly formed lumen becomes contin
uous with the Mullerian vagina. Only a vaginal closure membrane persis
ts. Between p.n. days 10 and 15 the epithelium increases in height rea
ching 2-5 cell layers. The amount of glycogen diminishes, but in the l
uminal cells the first mucous granules develop. Leukocyte diapedesis s
tarts, and immature Langerhans cells become discernible. Between p.n.
days 18 and 30 the apical cells become mucified, and some shedding of
these cells begins. Between p.n. days 36 and 45 bundles of keratin fil
aments appear in many of the luminal mucous cells intermingled with th
e mucous granules. In the middle layers keratin filaments, keratohyali
n granules and membrane-coating granules indicate the beginning of the
keratinization process. Diapedesis of leukocytes may lead to cell agg
regations below top layers. By day 50 the shedding of the mucous cell
layers reveals the underlying keratinized cell layers which then becom
e exfoliated. The first steps of the estrous cycle show a heterogeneou
s pattern. At about p.n. day 60 the cycle becomes established.