The enigmatic rodlet cell has been observed in tissues of marine and freshwater teleosts. The origin and function of this cell remain unclear. We describe the association of the rodlet cell with the head-kidney tissue of the southern platyfish, Xiphophorus maculatus. We have observed rodlet cells in various tissues of X. maculatus, e.g., the hemopoietic compartments of the head kidney, gill epithelia, and ovarian tissue, and in melanotic tumors. Recently, we found rodlet cells within the mesothelium and musculature. Intrigued by these observations, we began our current study of the rodlet cell – head kidney association in X. maculatus. We discerned a series of developmental stages of the rodlet cell within the hemopoietic tissue of the head kidney, including early precursor cells through degraded terminal stages. Tight junctions and desmosomes were observed between mature rodlet cells and endothelial cells. Desmosomal junctions were occasionally seen between maturing rodlet cells and neighboring cells within intertubular areas. We never observed junctional complexes between adjacent rodlet cells at any stage. Rodlet sacs were released intact and were seen to interact with other cellular components. We relate our findings to those of others who have described the development and distribution of rodlet cells in teleosts.

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Sperm storage is an important phenomenon occurring in viviparous and oviparous teleosts and contributes to the reproductive life history of these forms. There is a paucity of morphological studies on sperm storage in fishes. The majority of these have been confined to the light level of investigation. In this report, we describe, at the ultrastructural level, sperm storage in the viviparous platyfish, Xiphophorus maculatus. Female platyfish, as is typical of the poeciliids, are capable of storing viable sperm for up to several months within the ovary and gonoduct. We observed that sperm stored within inseminated platyfish became associated with specific epithelial cells (SACs) lining the oviduct. Two forms of association were seen: 1) sperm were found within deep surface pits and pockets, and 2) the spermatozoa were taken up and incorporated within the cytoplasm of the SACs. Junctional complexes in the form of tight junctions (zona occludens) and desmosomes at the apico-lateral surfaces of the SACs were prevalent. The junctions could have contributed to allograft survival of the haploid heterogenetic sperm cells within the female's reproductive tract. Our results shed light on the mechanism of sperm storage in the platyfish and could serve as a model for other poeciliid species and teleosts that are capable of storing sperm.

Rodlet cells occurred in the posterior intestine of embryos and neonates of the swordtail Xiphophorus nigrensis, its hybrids with Xiphophorus pygmaeus and in the platyfish Xiphophorus maculatus. This is the first observation of these enigmatic cells in a viviparous teleost prior to birth. This finding lends support to the endogenous tenet regarding the origin of this cell.

In fishes of the genus Xiphophorus, hybrid offspring of the spotted dorsal female platyfish, X. maculatus, and male swordtails, X. helleri, are genetically programmed to develop melanoma when raised at ambient laboratory temperatures. When these hybrid offspring were raised under hyperthermic conditions, there was no development of melanoma. Electron microscopy revealed degenerative changes in the melanocytes of heat-treated hybrids not seen in hybrids raised at ambient temperatures. The platyfish-swordtail melanoma system represents an appropriate model for the investigation of the relationship between hyperthermia and melanoma formation and treatment in poikilothermic vertebrates.