Macrophages are immune cells that also participate in the development and homeostasis of most tissues. Macrophages can originate during embryonic development, first from the yolk sac and later from the fetal liver. Around birth, the bone marrow also starts producing macrophages that can populate the tissues. Tissue resident macrophages have specific functions, responding to cell interactions and local stimuli within the tissue. Most adult resident macrophages are embryonicderived as these cells are able to self-maintain throughout aging.
Different fat deposits also known as fat pads, exist within the body and may have specific functions. The mammary gland (MG) fat pad is essential for the development of the whole gland, limiting the growth of the ductal network. In mice, epididymal white adipose tissue (eWAT) acts as a cushion for the internal organs, also providing them with energy. Brown adipose tissue (BAT) is a thermogenic organ essential for maintaining the body’s core temperature, especially during the perinatal stage. Adipose tissue resident macrophages have been implicated in the development of different pathologies, such as breast neoplasia and obesity. The study of adipose tissue resident macrophages in a developmental steady state may unravel new targets for treating or preventing these pathologies.
This thesis aims to understand the ontogeny and roles of tissue resident macrophages in the development and function of MG, eWAT, and interscapular (i)BAT.
The results show that adult MG resident macrophages are mainly from an embryonic origin, and are essential for the development of the ductal branches of the gland. In eWAT, the embryonic-derived macrophages maintained steady numbers throughout different dietary challenges, suggesting a function in tissue homeostasis for these macrophages. Alternately, iBAT embryonic-derived macrophages are readily replaced by bone marrow-derived macrophages during the first weeks of life. Nevertheless, these macrophages may have a key role in perinatal thermogenesis. Overall, embryonic-derived macrophages seem to have a role in the normal development of different adipose tissues, having specific functions and kinetics according to the fat pad they populate.