The blind begin to walk without sticks. And without cones

The eye can see without rods and conesAlexey Tymoshenko, GZT.RU

Biologists have found that the loss of the main photosensitive cells does not deprive rodents of vision. Left without sticks and cones, the mice continued to see.

A study by scientists from Johns Hopkins University (Jennifer L. Ecker et al., Melanopsin-Expressing Retinal Ganglion-Cell Photoreceptors: Cellular Diversity and Role in Pattern Vision) is published in the journal Neuron. To find out how mice can see without the main types of photosensitive cells, and even just to reveal this fact itself, specialists had to use a combination of the most modern and sophisticated research methods.

Blind Mutants

First of all, scientists needed to get a mouse that would not just be blind, but which would not work with sticks and cones.

Visual information processing begins with photons hitting one or more of the 125 million photosensitive retinal cells. Sticks use the pigment rhodopsin to register dim light, and cones use a complex of pigments to distinguish colors.  This inner layer of cells converts the received signals into electrical impulses and transmits them to the cells of the middle layer, which, in turn, send them to the ganglion cells, the processes of which form the optic nerve.

Mice whose blindness would have been due to other reasons were not suitable – and the only way to get the required animals was genetic engineering. The removal of the Gnat1 and Gnat3 genes encoding special enzymes associated with the work of rhodopsin gave the desired effect – and this was only a small part of the work.

Another retinal diagram.
It's not just a light-sensitive surface –
on top of the sticks and cones (the ratio between them in the figure is distorted,
there are ten times fewer cones that really distinguish colors)
there are several layers of nerve cells.
Source: Electronic Biology Textbook

Checking, checking and checking again

But the mice, whose sticks and cones were disabled by genetic engineering methods, still saw something. The question immediately arises: where is the guarantee that genetic engineering specialists have done their work exactly as stated? Or maybe the mice were completely confused, some other animals were brought to the laboratory? Or maybe molecular biologists had misconceptions about what is necessary for the work of rods and cones?

To answer all these questions (and most of the original article is devoted to them), biologists traced the connections between nerve cells and retinal cells.

To understand how difficult this task was, a comparison will help. Imagine a tangle of tens of thousands of threads, buttons tied to the end of each of which, and, in addition, the tangle itself is filled with glue so that it cannot be untangled. And yet, to determine which pairs of buttons are connected by one thread is a simpler task than the one that biologists had to solve, because buttons and threads are at least visible to the naked eye!

The multicolored "bushes" at the bottom of the picture are mossy cells, more precisely, their appendages.
A stunning series of microphotographs of the brain
it was obtained back in 2007 by biologists from Harvard and Cambridge Universities,
who managed to create a transgenic mouse, each neuron of which was colored in a randomly selected color
due to a combination of three fluorescent proteins. Source: Nature

Genetic engineering came to the rescue again. The researchers specially ordered several lines of laboratory mice at once, including Brainbow mice – each nerve cell of which synthesized its own combination of fluorescent proteins and thereby colored in its own color. This made it possible to trace the neural connections between the retina and the brain... however, again with a number of additional tricks.

Genetic Designer

So, scientists had "theoretically blind" mice with inoperable sticks and cones. Biologists in a series of behavioral tests (for example, in simple Y-shaped mazes) made sure that these mice still see, and at the next stage of research it was necessary to show what exactly the mice see. To do this, it was necessary to trace the neural connections.

Scientists have at their disposal a microscope and another series of mutant animals, each brain cell of which (as well as retinal cells) themselves are painted in the right color. What to do? To extract the brain with the retina and optic nerve, put it under a microscope and trace individual nerves? This approach will not work: the analogy with a taped skein of thread is not accidental, it will not work to see something in the dense structure of the nervous tissue.

The structure of the protein necessary for the work of rods and cones. Three-dimensional reconstruction.
Source: Emw

To solve the described puzzle, genetic engineering was again useful. Combining the properties of several lines of transgenic animals, it was possible to create a mouse whose cells are selectively colored in the desired color – that is, not all neurons in a row are colored, but only those that are located in the retina and, moreover, really react to light. A hodgepodge of tens of thousands of fibers turns into a much simpler structure, and that's what you can study under a microscope.

Confirmation of the guess

Having described the technical problems facing scientists and the methods of solving them, we can proceed to the presentation of the solution – how exactly mice without sticks and cones saw the figures painted on the walls of the maze.

As it was possible to establish after painstaking study of the connections between cells (and at the same time electrophysiological verification, the researchers were able to catch the electrical impulses of cells), the role of rods and cones is partly assumed by photosensitive cells of a different type.

The ability of cells of the ipRGC type, they are also retinal ganglion cells, to perceive light was discovered in the 1990s, but it was not associated with vision at that time, deciding that their number was too small for full participation in the formation of the image. The role of these cells was reduced to a general reaction to light at the level of sleep and wakefulness cycles – and, as it turned out now, completely in vain.

A detailed study of several subtypes of these cells is quite convincing evidence in favor of the fact that they can form, albeit of very poor quality, but still an image. So, if the rods with cones do not work, then this does not mean blindness.

Justified cruelty

At the end of the story, it is worth mentioning one detail that is usually not advertised by scientists, and some people far from science cause extremely ambiguous emotions. In the section "Methods", which is in every scientific article, it is mentioned that at a certain stage, when it was necessary to show that the system of cell labels chosen by scientists really worked, the mice were blinded completely. Under general anesthesia, the rodents had their eyes removed, after which the animals lived blindly for another three weeks, and after three weeks of blinding, their neurons no longer contained any labels.

Cruel? According to the accepted norms – yes, no one will allow such experiments to be done just like that, they require a special permit, issued only when justifying the manipulations being performed. And the rationale in this case was very strong: a certain number of blinded mice allowed us to prove that all the complex and painstaking work was done correctly, its results deserve attention. Including the attention of doctors looking for ways to overcome blindness.

According to Samer Hattar, one of the scientists who conducted a series of experiments, there is now hope that at least a small part of practically blind people will be able to develop the ability to use their ipRGC cells. For the simplest actions, where high visual acuity is not required, this could be enough.

Portal "Eternal youth" http://vechnayamolodost.ru16.07.2010