Tuesday, July 24, 2007

Origins of brains as sensory organs

Recently (well, not too long ago) I posted (here) a bit about how elements of the modern nervous system of vertebrates can be traced back to some of the most primitive animals – sea sponges.

Now it transpires that the origins of specific brain subsystems can also be found in invertebrates. In this case, the subsystem is the hypothalamus. One of the interesting things about the hypothalamus is that it is not only an important part of the central nervous system, but also part of the endocrine system. As such, it causes production of a number of important hormones itself, such as melatonin, oxytocin, and corticotropin-releasing hormone. It is further tied into the endocrine system via its direct connection to the nearby pituitary gland. Some of these hormones appear to have invertebrate origins the same as the hypothalamus itself:

Modern Brains Have An Ancient Core
Hormones control growth, metabolism, reproduction and many other important biological processes. In humans, and all other vertebrates, the chemical signals are produced by specialised brain centres such as the hypothalamus and secreted into the blood stream that distributes them around the body.

Researchers from the European Molecular Biology Laboratory [EMBL] now reveal that the hypothalamus and its hormones are not purely vertebrate inventions, but have their evolutionary roots in marine, worm-like ancestors. In this week's issue of the journal Cell they report that hormone-secreting brain centres are much older than expected and likely evolved from multifunctional cells of the last common ancestor of vertebrates, flies and worms.

The invertebrate in question here is the marine ragworm Platynereis dumerilii. And if the foregoing findings weren't interesting enough, it seems that not only vertebrates share a common ancestor (Urbilateria) with P. dumerilii, but so too do insects and other worms. Furthermore, P. dumerilii has two types of light-sensing cells – one type of which is found in insects, and the other is found in vertebrates.

Marine Worm Has Insectile and Vertebrate 'Eyes'
Scientists today believe that the eye could evolve from a single light-sensing cell. Scientists disagree over whether it evolved just once, or many times.

It turns out that Nature is both creative and generous with her gifts. Recent research has shown that the tiny marine worm Platynereis dumerilii has two types of light-sensing cells. The eyes of the worm have rhabdomeric photoreceptors, a compound lens formation that is seen almost exclusively in insect eyes. Rhabdomeric photoreceptors are covered in little finger-like protrusions. In its brain, however, it has a different kind of light-sensing cells - ciliary cells that are seen in vertebrate animals. Ciliary cells have hair-like cilia that extend outward and branch out like tiny umbrellas. Two different ways of sensing light in a single organism!

In light of this circumstance, the following, from the first-mentioned research announcement, is especially interesting:
Both of the cell types studied in Platynereis and fish are multifunctional: they secrete hormones and at the same time have sensory properties. The vasotocin-secreting cells contain a light-sensitive pigment, while RF-amide appears to be secreted in response to certain chemicals. The EMBL scientists now assume that such multifunctional sensory neurons are among the most ancient neuron types. Their role was likely to directly convey sensory cues from the ancient marine environment to changes in the animal's body. Over time these autonomous cells might have clustered together and specialised forming complex brain centres like the vertebrate hypothalamus.

"These findings revolutionise the way we see the brain," says Tessmar-Raible. "So far we have always understood it as a processing unit, a bit like a computer that integrates and interprets incoming sensory information. Now we know that the brain is itself a sensory organ and has been so since very ancient times."

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