Characterization of mammary stromal remodeling during the dry period

Abstract

During the dry period between successive lactations, the mammary gland of dairy cows undergoes extensive remodeling that is marked by phases of involution and mammogenesis. Changes in the mammary epithelium during the dry period have been well characterized; however, few studies have examined the changes that occur in stromal tissue. The objective of this study was to characterize changes that occur in mammary stroma during the dry period. Mammary biopsies were taken from 9 multigravid Holstein cows in late lactation, at 1 wk after dry-off, 3 wk before expected calving date, and 1 wk before expected calving date. Tissue was fixed in formalin, embedded in paraffin, and cut into 5-mum sections. Sections were stained with hematoxylin and eosin or with immunohistochemistry for expression of smooth muscle alpha actin (SMA), fibronectin, stromelysin-1 (MMP-3), transforming growth factor-beta1 (TGF-beta1), and TGF-beta receptor 2 (TGF-betaR2). Images of tissues were captured with light microscopy, and imaging software was used to measure intralobular stromal area, number of activated fibroblasts, as identified by expression of SMA, and percentage of intralobular stromal area expressing fibronectin, MMP3, TGF-beta1, and TGF-betaR2. Analyses of variance were conducted and statistical differences were based on the least squares means of biopsy stage. Number of activated fibroblasts was greater at 1 wk dry than at 1 wk before calving (2,720 vs. 1,800 cells/mm(2)), percentage intralobular stromal area was greater at 1 wk dry (32%) and 3 wk before calving (37%) than at 1 wk before calving (25%), and TGF-beta1 expression decreased 15% from late lactation to the dry period. The percentages of stromal area expressing fibronectin, MMP-3, and TGF-betaR2 and the percentage of myofibroblasts were not different across biopsy stages. These results support the concept that stromal expression of transforming growth factor-beta1 and fibroblast proliferation may be important for remodeling during the dry period.

Characterization of Primary Cilia Distribution and Morphology During Lactation, Stasis, and Involution in the Bovine Mammary Gland

“…Primary cilia sensitivity to cyclic tensile loading has also been reported in rat tail tenocytes (Gardner et al, ). If a similar form of mechanotransduction occurs in mammary stromal fibroblast primary cilia, it may allow sensation and transduction of acute changes in alveolar volume and pressure, potentially stimulating increased secretion of fibrous collagenous components during early involution (De Vries et al, ).…”

“…Transforming growth factor‐β (TGF‐β) is a key hormonal regulator in mammary stromal tissue. It has an inhibitory effect on epithelial cell proliferation, but is stimulatory to stromal fibroblasts and their secretion of ECM components (Musters et al, ; De Vries et al, ). In human lung fibroblasts, a connection between TGF‐β activity and components of the primary cilium Hedgehog pathway has been reported (Cigna et al, ).…”

“…At the cessation of milk removal, the greatest degree of cell loss is observed (Wilde et al, ; Capuco et al, ), and the mammary gland initiates the process of involution, which is considered a de‐differentiative phase, where the lactation capacity is progressively lost (Wiens et al, ; Wilde et al, ). Complete involution requires a sustained decline in SEC number and substantial fibrous remodeling of the surrounding stroma (Akers et al, ; Capuco and Akers, ; De Vries et al, ), although, the time frame varies between mammary lobes and between animals (Noble and Hurley, ).…”

Bovine mammary stem cells: new perspective for dairy science

“…This process is crucial to ensure the production of large quantities of milk in later lactation. Even though no major change is evident during the dry period, small variations in the stromal compartment of the mammary gland have been reported (De Vries et al 2010). The total number of fibroblasts shows an increase during late lactation and immediately after dry-off and then moderately decreases while approaching parturition.…”

“…A similar trend is described for the total amount of intralobular stromal tissue. De Vries et al (2010) suggest that paracrine factors (such as TGF-b1) produced in the stromal compartment may play an important role in determining a moderate remodelling of the stromal compartment during the dry period which in turn might influence the epithelial cells turnover. It is unclear how this massive proliferation should be supported by a population of primitive cells that are 'activated' by a specific hormonal framework that ensures the necessary cellular catabolism.…”