The mammalian ovary shows extensive variation mainly in relation to the interstitial tissue of the ovary, the so-called interstitial gland, and the degree of gonad regionalisation, which implies the existence of a cortex and a medulla. Three mammalian species, mouse, human, and mole, have been reviewed here as representative animal models for ovarian variability. Whereas the human ovary may be considered to have a conventional pattern of development, the mouse and the mole represent the two extremes of the variation range. The mouse is exceptional among mammals because ovarian regionalisation is much less relevant than in other species. Contrarily, the mole ovary is very similar to that of humans regarding the cortical region, but shows a testis-like pattern of development in the medullary region. Thus, the mole ovary is in fact an ovotestis, a phenomenon also described in other mammals. Accordingly, current studies on the development of the mouse ovary are not sufficient to understand the process in a more general context, because ovarian organogenesis is exceptionally simple in the mouse. From an evolutionary perspective, mammals show a tendency to eliminate or reduce gonad regionalisation, in contrast with the situation in other vertebrates, where this trait has been preserved. Since developmental variants may not be associated with particular taxonomic groups, their origin seems to be adaptive rather than phylogenetic. The demonstration that gonad development is a rather plastic process in mammals helps to explain how some mammals could have evolved towards more primitive gonad developmental models in response to selective pressure.

Keywords:
Development, Gonad regionalisation, Interstitial gland, Mammals, Organogenesis, Ovarian evolution, Ovary
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