The protonephridium (from the Greek proto, which means "before"; Y nephros, which means “kidney”) are a simple and primitive type of nephridia present in animals such as flatworms, annelids -other types of worms- and some mollusk larvae. They are highly branched blind tubes, which function as an organ of excretion.
They are characterized by having flagellated flagella cells, which are able to beat and undulate their flagella, creating negative pressure and generating a current that pushes the fluids with waste substances, allowing a filtration process..
Protonephridiums can vary depending on the environment in which the organism lives, and, particularly, on its saline concentration..
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The protonephria are composed of a branched tube, the terminal end of which is blind, and have a series of mobile extensions (undolipodia) at the inner end. Embryologically, they come from the outermost germ layer: the ectoderm.
They are typical structures of animals that lack a coelom, but may be present in pseudocoelomed or even coelomed animals.
The tubes are full of perforations where water can enter, as well as small molecules. Proteins and other high molecular weight molecules are left out.
The closed terminal characteristic of protonephridiums obscures the explanation of their possible operation, since a blind capillary is not suitable for filtration. Therefore, it is proposed that cilia play an important role in filtration..
Each animal can have more than two protonephridiums and these can have a significant number of branches in their tubes..
Each tube is organized as follows: one of its ends opens to the outside and the other is branched, ending in flagellate cells. There are different systems responsible for ensuring that these terminal structures do not collapse, such as actin fibers or microtubules.
The most important part of a protonephridium are the flagellar cells. If the cell has a single flagellum it is called a solenocyte, while if it has more than one it is called a flaming cell or flaming cells. From an evolutionary point of view, solenocytes are thought to originate from flaming cells.
Flaming cells bear this name thanks to their characteristic of beating and oscillating their flagella, this peculiar movement is reminiscent of a flaming candle.
The walls of the protonephridium have a series of cilia that direct the liquid to the nephridiopore, the opening that empties into the exterior.
The bulbous cells of the protonephridia are located towards the fluids of the coelom, organized in the walls of the blood vessels. Thanks to this arrangement, the transport of substances contained in body fluids can occur..
Protonephridiums differ from metanephridiums (a more advanced type of nephridium) because the latter are not branched and their ends flow into the lumen of the coelom..
Furthermore, metanephridians do not possess solenocytes; instead they present structures similar to a ciliated funnel called a nephrostoma. In this type of nephridium, both ends are open.
The protonephridiums are flexible structures when it comes to the filtration of fluids that come from different compartments in a channel, while the metanephridiums only filter the fluid from a cavity.
In some worms, such as annelids, the presence of protonephridiums and also metanephridiums can occur.
In all tuberlars, popularly known as planarians, the osmoregulatory and excretory system is of the protonephridial type; It is made up of a set of highly branched tubules. There are numerous protonephridiums in the cestodes.
These branches decrease in diameter until they end at the distal end, where the flame cells are found. These are composed of one end with projections and another tubular end with a tuft of flagella, connected to the tubular cell.
The tubular cell is responsible for connecting the tubule system to the outside through excretory tubes located in the dorsal area of the animal.
The movement of the cilia generates a negative pressure that guarantees the flow of excretions through the system.
The morphology of protonephridium is correlated with the habitat of the individual, depending on whether it is an environment with high or low salt concentrations..
There are certain species of flatworms that are capable of living in both fresh and salt water. It has been found that in brackish water populations they have a more differentiated protonephridium, if we compare them with their counterparts that inhabit the seas. In fact, protonephridia are not present in some marine rotifers..
Rotifers are a Phylum of microscopic pseudocoelomed animals that present an excretion system composed of two protonephridial tubules and, in the place of flamboyant cells, they present flamboyant bulbs..
The flamboyant bulbs have a tuft of flagella and project into the interior of the blood vessels, allowing excretory and osmoregulatory functions..
The tubules open into a vesicle that ends in the cloaca on the ventral side of the animal; also empties into the oviducts and intestines.
Quite long and coiled protonephridiums have been found in rotifer species that live in fresh waters, while species that inhabit the sea lack this structure..
Protonephridia carry out basic functions related to the excretion system of certain invertebrate animals, including ultrafiltration and transport..
Solenocytes or flame cells are closely related to blood vessels, so it has been proposed that blood pressure helps the ultrafiltration process.
The cells in flame are responsible for generating a negative pressure thanks to the movement of their cilia, which causes the filtration of the lymph fluid. This pressure drives the fluids through the tubes.
The protonephridiums will be responsible for removing excess water, connecting it in the tubules and excreting it through the nephridiopores. For example, in planarians, metabolic waste can be increased by a simple diffusion process..
Studies carried out in the pseudocoelomed organism of the genus Asplanchna showed that protonephridiums are involved in osmoregulation and excretion processes, since the rate of urine production decreases proportionally as the salinity of the medium increases..
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