In utero exposure to thermal stress has long-term effects on mammary gland microstructure and function in dairy cattle

Abstract

Earth’s rising temperature has substantial repercussions for food-producing animals by increasing morbidity and mortality, diminishing reproductive potential, and reducing productivity. In the dairy industry this equates to massive losses in milk yield, which occur when cows are exposed to heat stress during lactation or during the non-lactating period between lactations (i.e. dry period). Furthermore, milk yield is significantly lower in first-lactation heifers that experienced fetal heat stress. The mechanisms underlying intrauterine effects of heat stress on the offspring’s future lactation have yet to be fully elucidated. We hypothesize that heat stress experienced through the intrauterine environment will alter the mammary gland microstructure and cellular processes involved in cell turnover during the cow’s first lactation. Mammary biopsies were collected from first-lactation heifers that were exposed to heat stress or cooling conditions while developing in utero (IUHT and IUCL; respectively, n = 9–10). IUHT heifers produced less milk compared to IUCL. The mammary glands of IUHT heifers differed morphologically from IUCL, with the IUHT heifers having smaller alveoli and a greater proportion of connective tissue relative to their IUCL herdmates. However, intrauterine heat stress had little impact on the proliferation and apoptosis of mammary cells during lactation. Our results indicate that fetal exposure to heat stress impairs milk production in the first lactation, in part, by inducing aberrant mammary morphology. This may result from alterations in the developmental trajectory of the fetal mammary gland that persist through the first lactation rather than to alterations in the cellular processes controlling mammary cell turnover during lactation.

Dry period heat stress induces microstructural changes in the lactating mammary gland

“…The Ki67 assay was performed to detect the number of proliferating cells (stained brown; positive) and non-proliferating cells (stained blue; negative) in a section of mammary tissue. The protocol for Ki67 staining was previously described in a companion study [24] using the UltraVision One Detection System: HRP Polymer/DAB Plus Chromogen kit (Thermo Fisher Scientific, Cat# TL-015-HDJ). Briefly, sections were deparaffinized and rehydrated.…”

“…The TUNEL assay was conducted to quantify the number of cells undergoing apoptosis (stained brown; positive) and those not undergoing apoptosis (stained blue; negative) in a section of mammary tissue. The procedure followed was previously described in a companion paper [24] Briefly, we used the ApopTag Plus Peroxidase in situ Apoptosis Kit (Millipore, Cat# S7101) according to manufacturer’s instructions. Tissues were deparaffinized and rehydrated, and antigen retrieval was performed by incubating slides in 20 μg/ml proteinase K (Invitrogen, Cat# AM2542) for 8 min.…”

“…In addition, we have shown that heat stress experienced by the developing fetus in-utero (i.e. during late gestation, coinciding with the dry period) leads to reduced mammary alveolar area and MEC proliferation during these animals’ first lactation [24]. Dry period heat stress has been shown to impair MEC proliferation, thus potentially impairing the ability of the hyperthermic dry cow to successfully undergo the redevelopment phase in preparation for the next lactation [9,25].…”

The Impact of Heat Load on Cattle

“…Some consideration must also be extended to the impact of epigenetic mechanisms that regulate thermotolerance as well as understanding of transgenerational effects [183,184]. Recently, there have been studies attempting to quantify epigenetic change in cattle populations [184,185,186,187,188].…”

Food intake dependent and independent effects of heat stress on lactation and mammary gland development

“…Depressed fetal development may negatively impact subsequent performance of the offspring. In fact, in utero heat stress has been shown to depress milk production and mammary gland structure in offspring’s first lactation (Fabris et al, 2019; Monteiro et al, 2016; Skibiel et al, 2018).…”