Mammals possess mammary glands which are composed of two tissue types; secretory parenchyma (PAR), and non-secretory stroma. Amount of PAR is positively correlated with milk production. This work contains results from two separate projects. In the first, estrogen signaling in PAR from dairy heifers was more fully characterized using an ovariectomy (OVX) model. In the second, maternal body condition score (BCS) of gestating ewes was imposed as a treatment and PAR characteristics of female progeny were measured.
Estrogen regulates bovine mammary growth and development and acts mainly through estrogen receptors (ER), specifically ERα, to regulate estrogen-responsive genes. One such gene, CREB1, has recently been identified, but has not been characterized in bovine mammary tissue. The CREB1 gene codes for cAMP response element-binding protein (CREB). OVX of prepubertal heifers inhibits mammary development while causing an increase in ERα-positive cells, and increased abundance of myoepithelial cells which are identifiable by staining for α-smooth muscle actin (SMA). Our objective was to study the effects of OVX on tissue localization of CREB. PAR samples were obtained from 16 prepubertal heifers in a 2 × 2 factorial experiment. The first factor was ovarian status (intact or OVX); the second factor was estrogen treatment (control or estradiol). OVX was performed at ~4.5 mo of age and estrogen treatments began ~5.5 mo. After 54 h of estrogen treatment, heifers were slaughtered, udders removed and PAR sampled. Tissue sections underwent staining for CREB and ERα, or for CREB and SMA. CREB was more abundant in the luminal and embedded layer compared to the basal layer. Neither OVX, estrogen replacement, nor the combination of the two affected the amount of CREB protein. CREB has previously been identified as being estrogen-responsive at the transcript level. Our data indicate that tissue abundance of CREB protein does not show the same pattern. CREB may play a key role in signaling pathways that influence development of the bovine mammary gland because it was constitutively present in all treatment groups examined.
Mammary PAR is formed in utero, so maternal BCS during gestation may affect progeny mammary growth and composition. Pregnant ewes (n = 96; ~80 d of gestation) were grouped based on initial BCS of 2, 3, or 4 (1 to 5 scoring system; 1 = emaciated and 5 = obese). Ewes received gestation diets formulated to maintain initial BCS throughout pregnancy. Post-lambing diet and management were equivalent across treatments. Female progeny (n = 73) were slaughtered at similar BW and age. Udders were removed and mammary tissue subjected to biochemical analyses. Total mammary gland weights did not differ by treatment. However, PAR weight of BCS 2 progeny tended to be greater than that of BCS 3 or BCS 4 progeny. Protein mass within PAR tended to be highest in BCS 2 progeny, as did DNA mass within PAR. Despite detectable differences in PAR due to treatment, no differences in weight or composition of mammary stroma were found. Our observations suggest that BCS during gestation may have important lactation performance implications for female progeny.