The presence of teh cervical and thoracic thymus lobes in marsupials

Abstract

Thymus lobes, in 93 marsupial species studied, were found in two different topographical locations: the thoracic cavity (thoracic thymus), the ventral cervical region, superficial to hyoid, sternomastoid, and depressor neck muscles and just beneath the skin (superficial cervical thymus), or in both places. All polyprotodont species examined (42 examples from Didelphidae, Dasyuridae, Thylacinidae, Notoryctidae, Peramelidae, and Caenolestidae) had two to four lobed thoracic thymus and no superficial cervical thymus. Species examined from the diprotodont Burramyidae, Petauridae, Phalangeridae, Phascolarctidae, and Tarsipedidae (17 species) and Macropodidae (32 species) had both a thoracic thymus with two to four lobes, and a pair of superficial cervical thymus lobes. Both of the two genera in the diprotodont Vombatidae, Lasiovhinus and Vombatus, had a cervical thymus; no thoracic thymus was seen in the latter but the former had specimens with and without it. The presence of superficial cervical thymus lobes in only the superfamily Phalangeroidea is discussed with particular reference to herbivorous diet, small litter size, and immunological development of pouch young.