Sponges are the simplest animals in the world: they do not have real tissues and organs, there are only more or less specialized cells or layers of cells that perform certain functions. Their lifestyle matches the structure: being aquatic, mainly sea creatures, the sponges sit still, feeding on organics filtered from the water; only a few are capable of moving a few millimeters a day. (True, sponge larvae are quite mobile, swimming in water and choosing where to descend to become an adult sponge.) Sponges have cells of different specializations, there are integumentary cells sitting on the surface of the sponge, there are those that drive water with nutrient particles through the tubules that penetrate the body of the sponge, there are a number of other cells – however, there are no nerve cells among them. Or, more precisely, they could not be found until recently.
Employees European Molecular Biology Laboratory revised the cellular structure of the sponges in terms of which genes are active in them and which are not; for experiments, we took a freshwater sponge called lake badyaga. Researchers write in Sciencethat in this way they found 18 types of cells, some of them known before. In some of them, the genes necessary for intercellular communication, for signaling in membrane vesicles, worked. The fact that sponges have such cells was known before, but among them very special ones were discovered, which the authors of the work called neuroroid cells.
Neuroid cells clustered next to other cells – choanocytes. These are the very cells that drive water through the channels inside the sponge, providing it with nutrition. Choanocytes have villi on the surface – long cilia that move and create a stream of water. Neuroids that sit next to the choanocytes form membrane protrusions that envelop the villi and cilia of the choanocytes. Some cilia, enveloped in neuroid cells, stopped moving. Neuroid cells release microbubbles with some substances from themselves, and, according to the researchers, these substances were signals that forced the choanocytes to stop the flow of water, or, conversely, to make the water move again. The specific signal depended on how full the sponge was and how much it needed new nutrients.
In general, everything was reminiscent of how real neurons transmit a signal to other cells – as we know, neurons also send signaling neurotransmitter molecules to each other and to other cells. True, the transmission of a signal by neurotransmitters in neurons occurs differently – signal molecules are poured into the gap of the intercellular contact, in addition, the release of a neurotransmitter occurs under the action of an electrochemical impulse. In sponges, neuroid cells do not look like neurons, and communication between them and choanocytes only vaguely resembles the transmission of a signal from a neuron to a receiving cell – but still somewhat resembles. The researchers believe that the new data will help to imagine how the development of the nervous system in animals began in general: after millions of years of evolution, the contact between neuroids and choanocytes has become a complex intercellular synapse.
Source: Автономная некоммерческая организация "Редакция журнала «Наука и жизнь»" by www.nkj.ru.
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