Progesterone induces side-branching of the ductal epithelium in the mammary glands of peripubertal mice

Abstract

Development of the functional secretory epithelium in the mammary gland of the female mouse requires the elongation of the anlage through the mammary fat pad to form the primary/secondary ductal network from which tertiary ductal side-branches and lobuloalveoli develop. In this study we examined the hormonal requirements for the spatial development of the primary/secondary epithelial network and tertiary side-branches by quantifying ductal growth and epithelial cell proliferation in normal and hormone-treated BALB/c mice between 21 and 39 days of age. In normal mice, an allometric increase in ductal length commenced at 31 days of age and resulted in completion of the primary/secondary ductal network by 39 days of age. Concurrent with this allometric growth was a significant increase in cellular proliferation in the terminal end-buds (TEBs) of the ductal epithelium from 29 days of age, as determined by 5-bromo-2 -deoxyuridine (BrdU) incorporation. A level of cellular proliferation similar to that in the TEBs of 33-day-old control mice could be induced in the TEBs of 25-day-old mice following treatment for 1 day with estrogen (E), or progesterone (P) or both (E/P), indicating that both E and P were mitogenic for epithelial cells of the peripubertal TEBs. However, the period of allometric ductal growth in untreated mice did not correspond to an increase in serum E or P (which might have been expected during the estrous cycle). In addition, epithelial growth was not observed in mammary glands from 24-day-old mice that were cultured in vitro with E, P or E/P. In contrast to treatment with E, treatment with P promoted a dramatic increase, relative to control mice, in the number of tertiary branch points upon the primary/secondary ductal network. BrdU labeling of mammary glands from 24-33-day-old mice pelleted with cholesterol (C), E, P or E/P confirmed the greater mitogenicity of P on the epithelial cells of the secondary/tertiary ducts as compared with C or E. Concurrent with these changes, localized progesterone receptor (PR) expression in clusters of cells in the ductal epithelium was associated with structures that histologically resembled early branch points from ductules. In conclusion, our results suggest that additional endocrine growth factor(s) other than E and P contribute to the development of the primary/secondary ductal network, and that P is responsible for the formation of tertiary side-branches in the mammary glands of mice during puberty.

In utero and lactational exposure to vinclozolin and genistein induces genomic changes in the rat mammary gland

“…In the developing mammary gland of cycling animals, short 'tertiary' side branches of the ductal epithelium form in response to progesterone, and lobuloalveolar structures develop at the end of the tertiary branches (Atwood et al 2000, Haslam et al 2008, El Sheikh Saad et al 2011. We found striking differences in gene expression in the mammary gland between PND35 and PDN50 following exposure to V or GV, consistent with the major histological modifications observed in postpubertal mammary glands exposed to V and GV (El Sheikh Saad et al 2011).…”

“…Estradiol (E 2 ) and GH, and their interaction with the corresponding receptors (ERa and GHR), are required for normal duct growth and morphogenesis (Daniel et al 1987, Korach et al 1996. Progesterone induces side branching of the ductal epithelium (Lydon et al 1995, Atwood et al 2000. E 2 and progesterone are required for the development of alveolar structures at each estrous cycle.…”

Progesterone enhances branching morphogenesis in the mouse mammary gland by increased expression of Msx2

“…As development of the mammary gland is tightly regulated by hormones (Hovey et al, 2002), where its branching morphogenesis is regulated by P (Atwood et al, 2000), we investigated the effect of P on Msx2 gene expression. Both estrogen (E) and P treatment increased Msx2 expression in mammary glands from ovary-intact mice (Figure 2a).…”

“…Previously, we showed that progesterone (P) induces side-branching of the ductal epithelium in the peripubertal mouse mammary gland (Atwood et al, 2000). P receptor (PR) is critical for branching morphogenesis (Fernandez-Valdivia et al, 2005) and localizes to ductal branch points during mouse mammary development (Ismail et al, 2002).…”

The double-stranded RNA-binding protein, PACT, is required for postnatal anterior pituitary proliferation

“…When whole mount mammary glands were examined, there was defective tertiary branching and very few alveolar buds in the Pact Ϫ/Ϫ animals ( Fig. 2B), likely due to low serum progesterone and/or prolactin levels (22,23). In rodents, prolactin is the major luteotrophic hormone, released by the pituitary, that maintains the structural and functional integrity of the corpus luteum for several days after mating (24).…”