A new at­las il­lus­trates how the hu­man ret­ina is de­vel­op­ing.

De­tail of a cross-​section of a ret­inal or­ganoid. Dif­fer­ent tis­sue struc­tures are made vis­ible with dif­fer­ent colors.
Pho­to­ Credit: Wahle et al. Nature Bi­o­tech­no­logy 2023

What cell types are found in which hu­man tis­sue, and where? Which genes are act­ive in the in­di­vidual cells, and which pro­teins are found there? An­swers to these ques­tions and more are to be provided by a specialized at­las – in par­tic­u­lar how the dif­fer­ent tis­sues form dur­ing em­bryonic de­vel­op­ment and what causes dis­eases. In cre­at­ing this at­las, re­search­ers aim to map not only tis­sue dir­ectly isol­ated from hu­mans, but also struc­tures called or­ganoids. These are three-​dimensional clumps of tis­sue that are cul­tiv­ated in the labor­at­ory and de­velop in a way sim­ilar to hu­man or­gans, but on a small scale.

“The ad­vant­age of or­ganoids is that we can in­ter­vene in their de­vel­op­ment and test act­ive sub­stances on them, which al­lows us to learn more about healthy tis­sue as well as dis­eases,” ex­plains Bar­bara Treut­lein, Pro­fessor of Quant­it­at­ive De­vel­op­mental Bio­logy at the De­part­ment of Biosys­tems Sci­ence and En­gin­eer­ing at ETH Zurich in Basel.

To help pro­duce such an at­las, Treut­lein, to­gether with re­search­ers from the Uni­ver­sit­ies of Zurich and Basel, has now de­veloped an ap­proach to gather and com­pile a great deal of in­form­a­tion about or­ganoids and their de­vel­op­ment. The re­search team ap­plied this ap­proach to the or­ganoids of the hu­man ret­ina, which they de­rived from stem cells.

4i im­age of the cross-​section of a ret­inal or­ganoid, which is about one mil­li­metre in length. A sec­tion of it is shown above.
Pho­to­ Credit: Wahle et al. Nature Bi­o­tech­no­logy 2023

Many pro­teins vis­ible sim­ul­tan­eously

At the heart of the meth­ods the sci­ent­ists used for their ap­proach was the 4i tech­no­logy: it­er­at­ive in­dir­ect im­mun­o­fluor­es­cence ima­ging. This new ima­ging tech­nique can visualize sev­eral dozen pro­teins in a thin tis­sue sec­tion at high res­ol­u­tion us­ing fluor­es­cence mi­cro­scopy. The 4i tech­no­logy was de­veloped a few years ago by Lu­cas Pelk­mans, a pro­fessor at the Uni­ver­sity of Zurich and coau­thor of the study that has just been pub­lished in the sci­entific journal Nature Bi­o­tech­no­logy. It is in this study that the re­search­ers ap­plied this method to or­ganoids for the first time.

Typ­ic­ally, re­search­ers use fluor­es­cence mi­cro­scopy to high­light three pro­teins in a tis­sue, each with a dif­fer­ent fluor­es­cent dye. For tech­nical reas­ons, it is not pos­sible to stain more than five pro­teins at a time. In 4i tech­no­logy, three dyes are used, but these are washed from the tis­sue sample after meas­ure­ments have been taken, and three new pro­teins are stained. This step was per­formed 18 times, by a ro­bot, and the pro­cess took a total of 18 days. Lastly, a com­puter merges the in­di­vidual im­ages into a single mi­cro­scopy im­age on which 53 dif­fer­ent pro­teins are vis­ible. They provide in­form­a­tion on the func­tion of the in­di­vidual cell types that make up the ret­ina; for ex­ample, rods, cones, and gan­glion cells.

The re­search­ers have sup­ple­men­ted this visual in­form­a­tion of ret­inal pro­teins with in­form­a­tion on which genes are read in the in­di­vidual cells.

High spa­tial and tem­poral res­ol­u­tion

The sci­ent­ists per­formed all these ana­lyses on or­ganoids that were of dif­fer­ent ages and thus at dif­fer­ent stages of de­vel­op­ment. In this way, they were able to cre­ate a time series of im­ages and ge­netic in­form­a­tion that de­scribes the en­tire 39-​week de­vel­op­ment of ret­inal or­ganoids. “We can use this time series to show how the or­ganoid tis­sue slowly builds up, where which cell types pro­lif­er­ate and when, and where the syn­apses are loc­ated. The pro­cesses are com­par­able to those of ret­inal form­a­tion dur­ing em­bryonic de­vel­op­ment,” says Gray Camp, a pro­fessor at the Uni­ver­sity of Basel and a senior au­thor of this study.

Fur­ther tis­sue types planned

So far, the sci­ent­ists have been study­ing how a healthy ret­ina de­vel­ops, but in the fu­ture, they hope to de­lib­er­ately dis­rupt de­vel­op­ment in ret­inal or­ganoids with drugs or ge­netic modi­fic­a­tions. “This will give us new in­sights into dis­eases such as ret­in­itis pig­mentosa, a hered­it­ary con­di­tion that causes the ret­ina’s light-​sensitive re­cept­ors to gradu­ally de­gen­er­ate and ul­ti­mately leads to blind­ness,” Camp says. The re­search­ers want to find out when this pro­cess be­gins and how it can be stopped.

Treut­lein and her col­leagues are also work­ing on ap­ply­ing the new de­tailed map­ping ap­proach to other tis­sue types, such as dif­fer­ent sec­tions of the hu­man brain and to vari­ous tumor tis­sues. Step by step, this will cre­ate an at­las that provides in­form­a­tion on the de­vel­op­ment of hu­man or­ganoids and tis­sues.

Published in journal: Nature Bi­o­tech­no­logy

Research Material: The re­search­ers pub­lished their im­age in­form­a­tion and more find­ings on ret­inal de­vel­op­ment on a pub­licly ac­cess­ible web­site: EyeSee4is.

Source/Credit: Eidgenössische Technische Hochschule Zürich | Fa­bio Ber­ga­min

Reference Number: bio05082301

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