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Bidder's organ is a spherical, brownish organ in any member of the family Bufonidae (true toads). The organ is located just in front of the kidney, or, mesonephros. It is formed at the cranial tip of the male and female gonad during the larval stage. Normally it is inactive and contains miniature follicles which have the capability to mature (becoming active). Zoologists have experimented with the physiology of the organ by castrating male toads (removing the testes). In doing so, the Bidder's organ enlarges and produces viable oocytes, that is, egg cells, and then it produces gonadotropins, which stimulate the growth of the Müllerian ducts to form uterus and oviducts. It therefore becomes a fully sexually functional female, which leads some zoologists to conclude that toads are actually paedomorphic.
However, the Bidder's organ only develops into an ovary when testes are removed experimentally or not functioning properly (e.g. due to exposure to endocrine disrupting chemicals). Importantly, females can have Bidder's organs in addition to true ovaries which suggests that the Bidder's organ might not be a rudimentary ovary. The true function of the Bidder's organ remains a mystery.
It is named in honour of Friedrich Bidder.
The organ's internal anatomy consists of two parts, the central portion, consisting of connective tissue and rich in blood vessels, and its periphery consists of the cortex, which contains follicles in various stages of development. Molecules of various proteins are present in the outer layer of the follicles, homologous to the zona pellucida of the ovum. The physiology of the Bidder's organ is unique, having no connection with the sexual changing capabilities of amniotes (i.e., reptiles, avians, and mammals).
Location in a dissected toad
The ideal specimen for dissection is Bufo marinus, or Giant Cane Toad. In the male toad, lift the intestine leftward. In the mesentery of the intestine between the two mesonephros is the Bidder's organ. It is a conspicuous organ, being colored red, and at the cranial poles of the large testes.
- Brown, Federico D., Eugenia M. Del Pino, and Georg Krohne. "Bidder's organ in the toad Bufo marinus: Effects of orchidectomy on the morphology and expression of lamina-associated polypeptide 2." Development, Growth & Differentiation. Wiley Online Library. Volume 44, Issue 6, pages 527–535, December 2002. First posted online on 28 January 2003. doi:10.1046/j.1440-169X.2002.00665.x.