Late-gestation heat stress in Holstein dams programs in utero development of daughter's germline, triggering skin and hair morphology adaptations of granddaughters.

Homeostasis and thermoregulation are influenced by the interplay of hair coat and skin characteristics. Our previous work indicated that hair and skin adaptations, triggered by in utero heat stress, affect thermoregulation in postnatal life. Herein, we investigate multigenerational carry-over effects of late-gestation heat stress on hair and skin characteristics beyond the first generation. Pregnant Holstein dams (F, grand-dams) were heat stressed (HT, shade, n = 41) or provided active cooling (CL, shade, fans, and water soakers, n = 41) for the last 56 d of gestation during summer (temperature-humidity index ≥68). The first generation of heifers (F daughters) who were in utero heat stressed (HT, n = 36) or not (CL, n = 37) were born and raised as a cohort from birth to first calving. Thirty second-generation heifers (F, granddaughters), born to HT (HT, n = 12) and CL (CL, n = 18), were raised as a cohort from birth to 70 d of age. Hair samples and skin biopsies from HT and CL were collected on postnatal d 70 (n = 6/group). Hair samples were subdivided into topcoat and undercoat, and skin tissue was fixed for ~18 h in 10% formalin, dehydrated, paraffin-embedded, sectioned, and stained with hematoxylin and eosin to visualize morphology. Variables analyzed included hair length and diameter; stratum corneum cross-sectional area and thickness; epidermis thickness; sweat gland depth, number, cross-sectional area, and average size; and sebaceous gland number, cross-sectional area, and average size. Measurements were performed using the ImageJ software and analyzed using PROC MIXED in SAS (SAS Institute Inc.). Hair length and diameter tended to be shorter and thicker in HT, compared with CL. The HT skin had smaller stratum corneum cross-sectional area and tended to a thinner epidermis. to CL, HT skin had more but smaller sebaceous glands, whereas no differences in sweat glands were observed. In summary, we report phenotypic alterations in hair and skin characteristics of granddaughters. Whether these adaptations grant improved postnatal thermoregulatory ability for the granddaughters remains to be investigated.