Did fish make a placenta from the digestive tract? | Yokohama City University

-Exploring the mechanism of material transport between mother and child in splitfins

The research group of Assistant Professor Atsuo Iida and Professor Eiichi Motomichi of the Graduate School of Life and Agriculture, Nagoya University, Tokai National University Organization, is a joint research with Josai University, the University of Tokyo, Kyoto University, and Yokohama City University. We found that the mechanism of maternal absorption of nutrients found in the “nutrition ribbon” (placement-like structure) of embryonic fish has characteristics similar to the substance absorption mechanism of the digestive tract, which is widely conserved in vertebrates.
This study puts a stone in our understanding of the mechanism of embryonic development in non-mammals and its molecular mechanism. We explored how vertebrates have shaped, altered, adapted to the environment and prospered in the history of life on Earth.
The results of this research were published in the online version of the British scientific journal “Journal of Experimental Biology” dated June 25, 2021.
This research was supported by Sochi Nakatsuji, a general incorporated foundation, and the Daiko Foundation, a public interest incorporated foundation.


  • A group of genes highly expressed in the “nutritional ribbon” of a splitfin embryonic fish was extracted.
  • The molecular mechanism involved in substance absorption was estimated from molecular genes and biochemical analysis.
  • We considered the possibility that the “nutritional ribbon” is a structure derived from the digestive tract.

[Research background and content]

[Viviparity is a mysterious trait]
Viviparity is a breeding method in which a baby is raised in the mother’s stomach and the number of babies is increased by childbirth.Those that are accompanied by nutrition from the mother to the child during pregnancy are “true embryos”*1Many mammals, including humans, are true embryos and provide nutrition between mother and child through the placenta and umbilical cord. On the other hand, for embryos other than mammals (also found in fish, reptiles, and amphibians), Compared to the abundance of morphological reports, there are few reports that mention molecular mechanisms and gene functions, and there are many mysteries.

[Unraveling the mystery with a splitfin fetal fish]
This research group is conducting research using the highland carp (scientific name: Xenotoca eiseni), which is an embryonic teleost fish belonging to the family Splitfins of the order Toothcarps.In Splitfins, the fetal growth in the mother’s body is called the “trophotaenia” pseudoplacenta.

*2It has a structure and is thought to absorb nutrients from the mother. In 2019, the yolk protein vitellogenin as one of the nutrients supplied by the mother*3(Refer to “Research and Education Achievement Information” dated October 9, 2019, “Understanding the nutrients that fish give to hungry children! -Acquiring the fetal mechanism using genes lost by mammals-“) .. On the other hand, the molecular mechanism on the fetal side involved in nutrient absorption was unknown.

Figure 1. Schematic diagram of Splitfins and questions from this study

[This time, I searched for the mechanism on the child’s side]
In this study, we conducted a comprehensive search for expressed genes and conducted a Cubilin / Amnionless receptor.*4Endocytosis involving*5I have noticed that this phenomenon is the driving force for substance absorption in the nutrition ribbon. Furthermore, from molecular genetics and biochemical analysis, it is estimated that intracellular proteolysis by catepsin L, in addition to intracellular substance uptake by the receptor, is the molecular mechanism of maternal nutrient uptake in the nutrient ribbon. did.

Figure 2. Outline of the substance absorption mechanism estimated in this study

[Hypothesis that the mechanism of the digestive tract was diverted to the pseudoplacenta]
Endocytosis involving the Cubilin / Amnionless receptor has been reported to contribute to nutrient absorption in the gastrointestinal tract of some vertebrates.”Some vertebrates” here are stomachless fish
*6It refers to mammals during the lactation period and is thought to supplement the immaturity of digestive function with the intracellular degradation mechanism by endocytosis. In Splitfin fetal fish, it is unlikely that the lumen of the ovary, where the fetus grows, has a digestive function, and it is reasonable that the trophic ribbon has an intracellular degradation function. Based on the above, the research group considered that part of the nutritional ribbon is derived from the digestive tract because of its structural and functional commonalities. This is a hypothesis related to the formation of a unique fetal mechanism, as the nutritional ribbon is a structure peculiar to the splitfins.

[Significance of results]
Vitelogenin, which is supplied to the fetus by a splitfin mother, is lost (no longer needed) at the genetic level in embryonic mammals.In other words, in the acquisition of embryos in the family Splitfins, the maternal trait is that of mammals.Built with different elements..On the other hand, endocytosis on the fetal side is found in vertebrates, including mammals.Consists of a widely conserved molecular mechanism.. From the above, we believe that there is no blueprint for the molecular mechanism common to all species, and that each of them has reached its current form as a result of expansion and renovation with materials that can be used each time. I will.
The search for such species-specific and unique traits leads to the enrichment of information such as pictorial books and textbooks and Wikipedia, and contributes to the accumulation of human knowledge.


* 1 True embryo: A mode in which a child grows by receiving nutrition from the mother in the mother’s body. In contrast, ovoviviparity is one in which the growth of the offspring depends only on its own yolk nutrition.

* 2 Pseudoplacenta: A general term for placenta-like structures involved in substance exchange between mother and child in embryonic animals other than mammals.

* 3 Viterogenin: A protein that constitutes egg yolk nutrition. In oviparous animals, it is produced in the liver and transported through the bloodstream to the ovarian eggs. Female highland carps produce vitellogenin during pregnancy, and the fetus absorbs molecules secreted into the ovarian lumen from the feeding ribbon.

* 4 Cubilin / Amnionless receptor: One of the receptors for the endocytosis pathway that exists in the kidney and intestinal tract in mammals. Although well known for its involvement in vitamin B12 uptake, it is thought to be a “multi-ligand receptor” that also binds to other proteins.

* 5 Endocytosis: One of the processes by which cells take up extracellular substances. It takes up macromolecular compounds and degrades them in intracellular vesicles (lysosomes).

* 6 Stomachless fish: A fish that does not have a stomach. Medaka, zebrafish, and highland carp are classified as stomachless fish. It cannot store what it eats and has low digestive ability.

[Treatise information]

Journal of Experimental Biology
論文タイトル:Cubam receptor-mediated endocytosis in hindgut-derived pseudoplacenta of a viviparous teleost Xenotoca eiseni

Authors: Atsuo Iida (Graduate School of Agriculture, Nagoya University), Kaori Sano (Josai University), Mayu Inoguchi (University of Tokyo), Junpei Nomura (Graduate School of Agriculture, Nagoya University), Takayuki Suzuki (Graduate School of Agriculture, Nagoya University) Department), Mao Kuriki (Kyoto University), Maina Sogabe (Kyoto University), Daichi Susaki (Yokohama City University), Kaoru Tonozaki (Iwate University), Satoshi Kinoshita (Yokohama City University), Eiichi Hondo (Graduate School of Agriculture and Biotechnology, Nagoya University) )
DOI: 10.1242 / jeb.242613


Leave a Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.