Retinal Organoid

Biologists have grown a human retina "in vitro"

Sergey Vasiliev, Naked Science

Color "daytime" vision is provided to our eye by photosensitive cones. The retina contains six to seven million cones, divided by sensitivity range. S-type receptors detect the blue-violet part of the spectrum; M-type – green-yellow; L – red. This specialization occurs in two stages: at the first, cones of the S-type are distinguished, at the second there is a separation of L- and M-types. Robert Johnston and his colleagues from Johns Hopkins University decided to find out exactly how the fate of the future photoreceptor is chosen at the first, key step. The results of their work are presented in an article published in the journal Science (Eldred et al., Thyroid hormone signaling specifications cone subtypes in human retinal organoids).

In order to better understand how cones of different types develop, being distributed in the manner of a mosaic, the authors obtained an organoid of the retina. Creating such model fragments of living tissue in artificial conditions requires a lot of effort, and scientists spent about nine months growing them. However, the resulting tiny organoid was able to normally perceive color and transmit information to nerve cells, and its development was monitored and observed in detail.

Previous studies on other animals, including fish and mice, have shown that thyroid hormones play an important role in the early stages of eye development. It turned out that the same thing happens with us: the addition of triiodothyronine to the nutrient medium of maturing organoids triggered the specialization of cones, the appearance of S-type receptors and the release of the M/L line. And vice versa: the suppression of cellular receptors capable of receiving the signal of this hormone blocked this process, leading to the fact that all cones matured into S-type.

Triiodothyronine is an active form of thyroid hormone that circulates through the body mainly in the form of thyroxine. The authors showed that maturing retinal cells show different sensitivity to them at different stages of their development. At the very first stages, they produce an enzyme that quickly neutralizes both triiodothyronine and thyroxine, so that they almost do not react to their presence. However, then the synthesis of another enzyme begins, which converts the "transport" thyroxine into "active" triiodothyronine, which eventually leads to the separation of different lines of photosensitive cones.

A drawing from an article in Science – VM.

The authors note that this mechanism may fail with some visual development disorders. So, it is known that prematurely born babies are more prone to color vision defects – perhaps precisely because they left the mother's body too early, which constantly supplied them with large amounts of thyroid hormone.

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