Physiologie Neuronaler Systeme

Up to present day only three mammalian groups
developed retinas dominated by cones:

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Evolution of Mammalian Photoreceptors

A) Sciurids

including: ground squirrels, prairie dogs and marmots.

On the left you see a Ziesel
(European Ground Squirrel, Citellus citellus).

these rat sized sciurids establish burows in the dry soils in the warm dry regions of Central and Southern Europe

In Austria modern agriculture has reduced them to a few colonies in protected areas and some vineyards.

This stained semithin section shows the photoreceptor mosaic. A few rods are present but not well distinguishable at this level of sectioning.
In the ground squirrel retinas (also the Alpine marmot for example) they are a small minority (5%).

Otherwise, apparently there are two types of cones.

We know now that the
subpopulation of larger darker staining cells
are the short wavelength sensitive
(Blue or S-) cones. They establish a regular sub-mosaic of ca. 10%.

The other cells are mostly (>80% )
long wavelength sensitive or "Green" cones.

As we saw above in the case of ground squirrels, the presence of morphological criteria have been helpful to detect cone subpopulations. This however is an exceptional hint of nature not found in most other mammals. Furthermore morphological differences per se do not directly indicate the presence and positions of different spectral sensitivities.

One way to characterize the identy of the photoreceptors is "Stimulus-related histochemistry"

Here you see a NBT-incubated squirrel retina. It is a similar preparation as above for cat retina but with an important difference:

Conditions were chosen to elicit differences WITHIN the dominant cone population.

The retina had been exposed before
yellow light
pr4sumably stimulating mainly
the (smaller) L-cones.

This is in a way similar to photochemical paper:
instead of silver "grains" here the cone cells
are more or less induced to induce precipitation,
depending on whether the wavelength of light
is appropriate
for switching their metabolic state.

The result is a bluish precipitation in the majority cones - it is the the small subtype.
The sparse mosaic pattern of presumptive S-cones (compare to semithin section !)
shows NO precipitates and thus remains bright in transmittent light.
So instead of the basic Rod / Cone-Dualism (as seen in the cat above) we see here
a histochemical correlate
for the differential spectral sensitivity of cone cell subtypes

As is the case with most animals with good vision the blue sensitive cones constitute a minority of the mosaic
They thus provide a coarser sampling from the image projected on it, while high acuity originates
from paathways linked with the L-cones.

B) Tree shrews (Tupaia)

(Click here for a nice Acrobat.pdf File from the German Center of Primate Research.)


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Evolution of Mammalian Photoreceptors

Tree shrews are interesting social animals.
They may look squirrel-like (in fact "Tupai" means "squirrel" in Malaysian).
Still zoologists discuss a close relationship to primates or insectivores.
Presently a special order has been assigned for them within the mammals
ranking them at the same level just as primates or rodents.


The tree shrews and ground squirrels
were previously thought to have
"All Cone - Retinas".

Now better techniques are available and a second look on this Tupaia section

will certainly reveal to you
3 small-diameter rod cross sections -

where ?

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Evolution of Mammalian Photoreceptors

This is a semithin section through tree shrew photoreceptor inner segments. The retinal area is covered
almost completely by large diameter cones. This homogeneity leads to the formation of
almost crystalline arrays, analogous to the human fovea, squirrels or many fish retinas.