Reprod. Fertil. Dev., 1991, 3, 303-12 Transillumination Patterns of Seminiferous Tubules in Two Species of Australian Rodents, Pseudomys australis and Notomys alexis P. ~ollanen* and W. G. reed^ A Department of Anatomy, Institute of Biomedicine, University of Turku, SF-20520 Turku, Finland. Department of Anatomy, University of Adelaide, Adelaide, SA 5000, Australia. Abstract The transillumination-assisted isolation of segments of seminiferous tubules in defined stages of the wave of the seminiferous epithelium was investigated in Pseudomys australis and Notomys alexis. In P. australis, three different transillumination patterns (pale, spotty and dark) of the seminiferous tubules could be isolated. They corresponded to histological stages 1-2, 3-5 and 6-8 of the wave of the seminiferous epithelium. The dark pattern made up about 40% of the wave, the pale pattern about 31%, and the spotty pattern about 24%. In N. alexis, no such isolation of segments with particular cell associations was possible. In a comparison of P. australis with the laboratory rat, the dark, pale and spotty transillumination patterns of seminiferous tubule segments corresponded to the same cell associations in both species, but the length of the spotty segments in the laboratory rat made it possible to subdivide the spotty region into pale spotty and dark spotty areas and thus to isolate four different transillumination patterns. Further separation into other subgroups to isolate additional types of segments could not be performed repeatedly in any of the studied species because of the gradual change of one subgroup of transillumination pattern into another and the short length of some of the subgroups. Extra keyword: spermatogenesis. Introduction The plains rat (Pseudomys australis) and the hopping mouse (Notomys alexis) are two species of Australian hydromyine rodents that probably separated from a common ancestor with the laboratory rat (Rattus norvegicus) and the mouse (Mus domesticus) more than 10 million years ago (Baverstock et al. 1983). In spite of all members of the Pseudomys and Notomys genera having a reciprocal translocation between chromosomes 1 and 2, thus presumably sharing a common ancestor in which this rearrangement took place (Baverstock et al. 1983), they differ significantly from each other in many aspects of their reproductive anatomy and cytology (Breed 1986). Whereas the testes of the plains rat make up about 3% of the body mass and anatomically resemble those of the laboratory rat, the testes of the hopping mouse are extremely small, consist only of 5-8 seminiferous tubules, and constitute only about 0.15% of the body mass (Breed 1982). In the present study, the seminiferous tubules of plains rat and hopping mouse testes were investigated in order to determine whether stages of the wave of the seminiferous epithelium could be ascertained from observed transillumination patterns as previously shown for the laboratory rat (Parvinen and Ruokonen 1982). Segments of tubules in certain stages of the wave of the seminiferous epithelium could then be isolated for biochemical analysis in order to compare the regulation of spermatogenesis in the Australian rodents with that in the laboratory rat. 1031-3613/91/030303$05.00