Thy-1 immunoreactivity in the larval sea lamprey (Petromyzon marinus L.), a vertebrate without a definitive thymus

Abstract

Agnathans are the only vertebrates without a thymus, yet lampreys possess putative lymphocytes whose responses parallel those of T cells in gnathostomes. The phylogenetically conserved Thy-1 antigen is often associated with the thymus, thymocytes, and T cells. An immunohistochemical study, using commercial anti-rat brain Thy-1.1 antiserum and an immunoperoxidase procedure (peroxidase anti-peroxidase) was conducted to identify any Thy-1 antigenicity in various tissues of larval sea lampreys (Petromyzon marinus L.) lightly fixed in a chilled aldehyde-based solution. The primary focus of the experiment was to discover if Thy-1 immunoreactivity was associated with haemopoietic sites and (or) leukocytes. This technique permitted conventional histological sectioning, preserved tissue architecture, and retained Thy-1 antigenicity. In the haemopoietic typhlosole, the peripheral stroma and many lymphocyte-like cells stained intensely, as did similar cells in the opisthonephros, intestinal venous sinus, and liver. Immunoreactivity in the pharynx was evidenced only by weakly staining stroma in small labyrinthine subdermal foci dorsal and ventral to some external gill openings. It is proposed that if functions analogous to thymic activities exist in the larval lamprey, the typhlosole is the most likely site for their discovery.