Morphological Peculiarities of Innervation of Rat Epicardial Adipose Tissue in Early Postnatal Ontogenesis

The study aimed to analyze the structure and innervation of the epicardial adipose tissue (EAT) in the aortic-pulmonary region of the rat heart at early developmental stages (P1, P14) and in adult animals (P90–P120) using histological methods (hematoxylin-eosin and toluidine blue staining), as well as immunohistochemical methods to localize protein gene product 9.5 (PGP9.5), tyrosine hydroxylase and synaptophysin in the neural structures. Histogenesis and differentiation of EAT adipocytes were studied. It was found that the presumptive anlagen of brown and white cardiac adipose fat pads derive from the “epithelial bodies” located at the base of the great vessels in the aortic-pulmonary region of a newborn rat. We believe that it is the epitheliomorphic tissue that is cambial, serving as a source of adipose tissue stem cells. White adipose tissue, represented by ring-shaped adipocytes, is its derivative; it is responsible for the synthesis and accumulation of triglycerides and makes up the bulk of the cardiac fat depot. Histogenesis of brown and white adipose tissue is accompanied by vasculogenesis since the first postnatal day. Simultaneously, terminal plexuses of catecholaminergic and cholinergic varicose axons form in adipose tissue, representing distant en passant synaptic boutons. At early developmental stages (P1–P14), in the period of fat cell differentiation, catecholaminergic postganglionic sympathetic terminal axons predominate. Instead, in mature animals (P90–P120), catecholaminergic fibers decrease in number, yielding to cholinergic (PGP9.5-immunoreactive) nerve fibers.