Figure 3.

Proposed model of different cellular events in the mammary gland that occur during the dry period of heat-stressed (upper panel) and cooled (lower panel) cows. The shaded area represents mammary involution in the early dry period (approximately 2 wk after dry off). The black solid line, blue dashed line, and red dashed–dotted line represent the course of mammary cell proliferation, apoptosis, and autophagy during the dry period, respectively. Compared with cooling, heat stress may cause an attenuation of autophagy in the early dry period due to lower placental estrone sulfate production and an extended and enhanced autophagic response later in gestation through decreased nutrient supply by lower mammary gland perfusion. The altered pattern of autophagy driven by heat stress may impede cell elimination in the early dry period and cell proliferation late in pregnancy. Additionally, relative to cooled cows, heat-stressed cows may have reduced mammary cell apoptosis in the early dry period through increased cell heat shock protein expression and blood prolactin, which may negatively affect cell turnover in the mammary gland, ultimately compromising lactational performance.