Mammary Growth in Holstein Cows During the Dry Period: Quantification of Nucleic Acids and Histology

Abstract

Influence of the dry period on mammary growth was studied using multiparous Holstein cows. Sixty days before expected parturition, 13 cows were dried off, and another 13 cows were milked throughout the prepartum period. Lactating cows and dry cows were slaughtered at 53, 35, 20, and 7 d prepartum. Total mammary parenchymal DNA increased twofold from 53 to 7 d prepartum without influence of lactation status. However, overall rate of [3H]thymidine incorporation by mammary tissue was 80% greater in dry cows than in lactating cows, indicating that replacement of mammary cells was greater in dry cows. Of the mammary cells labeled with [3H]thymidine, the percentage of epithelial cells in dry cows was greater than that in lactating cows (96% vs. 86%). By 7 d prepartum, epithelial calls accounted for a greater percentage of total mammary cells in dry cows than in lactating cows (83% vs. 74%). Tissue area occupied by alveolar or ductular lumina decreased by 25 d into the dry period (35 d prepartum) and then increased to a maximum by 7 d prepartum. None of the mammary epithelial cells in dry cows were classified as secretory at 35 d prepartum, but 98% of the epithelial cells of dry cows were classified as secretory at 7 d prepartum. Results indicated that mammary involution did not occur during a typical dry period of dairy cows. Data suggest that a dry period is important for replacing senescent mammary epithelial cells and increasing the epithelial component of the gland prior to the next lactation.

Effects of continuous lactation and short dry periods on mammary function and animal health

“…The third hypothesis suggested that depressed milk yield in cows with modified DPL is due to reduced mammary epithelial cell (MEC) number. This hypothesis was invalidated in later studies (Swanson et al, 1967;Capuco et al, 1997) that demonstrated no differences in dry fat-free tissue weight, DNA concentration, total DNA content or number of alveoli per tissue section in quarters from cows with 6 week differences in DPL. These authors suggested that reduced milk in CM quarters can be attributed to decreased secretory activity per unit of mammary secretory tissue and physiological factors affecting the cells during lactogenesis, rather than systemic hormonal regulation or MEC numbers.…”

“…Biological basis for dry period requirement Four hypotheses have been proposed to explain the necessity of a dry period between successive lactations to maximize milk production in the subsequent lactation in dairy cows (Swanson, 1965;Smith et al, 1967;Capuco et al, 1997). The first hypothesis was based on productivity and body condition and was proposed by Woodward andDawson in 1926 (cited by Dix Arnold andBecker, 1936).…”

“…These authors suggested that reduced milk in CM quarters can be attributed to decreased secretory activity per unit of mammary secretory tissue and physiological factors affecting the cells during lactogenesis, rather than systemic hormonal regulation or MEC numbers. Using [ 3 H]-thymidine incorporation to evaluate mammary cell proliferation, Capuco et al (1997) demonstrated 80% greater incorporation in mammary tissue from control (60-day dry) cows compared with CM cows. In that study, total mammary DNA content increased twofold from 53 to 7 days prepartum, but was not affected by lactation status.…”

“…Therefore, a fourth hypothesis was proposed, suggesting that a dry period of appropriate length is necessary for promoting cell turnover and replacement of senescent MECs during late gestation. Carryover of a larger population of old or senescent MECs and a smaller population of new MECs could be an explanation for reduced milk yield for these animals in the ensuing lactation (Capuco et al, 1997 and2006). Progenitor cells are responsible for expanding and maintaining the number of mammary secretory cells (Capuco and Akers, 1999).…”

Epigenetics: a possible role in acute and transgenerational regulation of dairy cow milk production

“…In contrast to the rodent mammary gland, involution in cows does not occur to the same extent (Capuco et al, 2001). During the extended process of involution, which occurs in the bovine mammary gland, many alveolar structures are retained (Holst et al, 1987) and Capuco et al (1997) suggest that during involution of the bovine mammary gland senescent and damaged cells are being replaced, rather than extensive remodelling of the mammary epithelial cell compartment. Thus, in the cow, mammary involution may potentially be reversible following extended non-milking periods (Noble and Hurley, 1999).…”