An ultrastructural investigation, performed on embryos, neonates, subadult and adult females, demonstrated that in Torpedo marmorata oogenesis occurs very early in life and continues, in its proliferative phase, also after birth. Clusters of early meiotic cells were already evident in the ovarian cortex of 6-cm-long embryos, as well as in the ovary of newborns and three-month-old young. Conversely, in the ovaries of subadult and adult females, all the germ cells present were organized into follicles, and no clusters of oogonia and early meiotic cells were generally found in the cortex, except for one adult female where clusters of germ cells not organized in follicles were found in the cortex. These data demonstrated that, in Torpedo marmorata, oogenesis is immediate, and, as oogonia persist after birth, more similar to that of mouse, monkey, rabbit, and ferret (Mauleon Arch Anat Microsc, 1967; 56:125-150; Byskov and Hoyer 1994) than to that of human, rat, pig, and guinea pig (Byskov and Hoyer 1994). Such a pattern is in agreement with the reproductive strategy of Torpedo, a scantly prolific species with low uterine fecundity. The presence of meiotic cells that are not organized in follicles in one adult female might be consistent with the large individual variability characterizing cartilaginous fishes. The possibility that such a character is typical of mature females should be rejected as oogonia and early meiotic cells were not found inside the totally sectioned gonads of subadult and adult females.
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