Choanocyte

Collar ciliated cells ( Choanozyten ) form the Choanoderm. The Choanoderm is the top layer of cells that lines the interior ( Spongocoel or Gastralraum or scourge Chamber) of sponges ( Porifera ). Here the collar ciliated cells are tightly packed.

The collar scourge cell consists of a rounded cell body ( soma). At one pole of the cell sits a long flagellum. To the scourge sitting around a ring - the eponymous collar - from Stereovilli. Between the individual Stereovilli sits mucus. The result is a Schleimsaum, which is also called plasma collar. Through the scourge blow a stream of water is produced, which supplies water from the sides. The water enters through the Schleimsaum. In the water, floating food particles caught in the mucus. You are then taken up by endocytosis of the collar scourge cell.

In sponges created by the scourge of the impact of all Choanozyten Choanoderms a collective stream of water. The water flows over the sides ( by the Porocyten ) in the Spongocoel, to the collar of the ciliated cells Choanoderms and again by the Ausstromöffnung ( s) ( Osculum / oscula ) addition. The Choanozyten found in the water many particles: particle -rich water flows into the sponge into it and low-particle water addition. The endozytierenden Choanozyten take one hand even the digestion of the captured particles. On the other hand, you also pass on phagosomes amebocyte, another sponge - cell type. The Choanozyten also help in reproduction. Your water flow ensures that the sperm sponge left by the Ausstromöffnungen while foreign sperm are sucked through the Porozyten inside to fertilize the eggs of the Mesohyls.

Collar ciliated cells are found not only in sponges. It is striking, especially their occurrence in the Choanoflagellata. This is a group of unicellular organisms that live as single cells or as small collar scourge cell colonies from collar ciliated cells either. The similarity between the cells of Choanoflagellata and the sponge - Choanozyten is striking. She reaches into fine structural similarities into it.

Occurrence of collar ciliated cells in Animalia

Collar ciliated cells are found only in animals. Most matches are made between the collar ciliated cells of Porifera and Choanoflagellata. However, there are also cells in many other animals, which are also similar to collar ciliated cells stronger. More specifically, it does not resemble the collar ciliated cells of adult sponges, but unfinished cell development stages, which are found in sponge embryos. Such monociliären epithelial cells often meet receptor functions.

• Choanoflagellata: The cells of these unicellular organisms are very similar to the collar ciliated cells of adult Porifera.
• Porifera: The collar ciliated cells located in the wall of the chamber scourge.
• Placozoa: The living today Placozoa do not have cells that look the same at first glance like collar ciliated cells. Instead, they have tönnchenförmige cylinder cells in the high prismatic epithelium of the ventral side of the body. Very much like the collar ciliated cells, these cells carry a central cylinder scourge. In histological thin section preparation is also to recognize that the cell membrane to the Scourge around microvilli everts. The cylindrical cells absorb food particles by endocytosis. On the dorsal side of the body of the animal are also located monociliäre cells. However, they are flat and without microvilli.
• Cnidaria: collar scourge cell -like cells have been found in various Anthozoa.
• Platyhelminthes: In the integument of flatworms are cells that resemble collar ciliated cells. It is unicellular mechanoreceptors.
• Nemertea: collar scourge cell -like cells with up to 12 flagella were identified at a free-floating cord worm larva.
• Annelida: mechanoreceptors, which are strongly reminiscent collar ciliated cells, were also found in Oligochaeta.
• Echinodermata: In the digestive tract of a starfish two different cell types were determined, similar to the collar ciliated cells. In echinoderms monociliäre cells serve as receptor and supporting cells.

Evolutionary significance of the collar scourge cell

The collar ciliated cells of the Porifera and Choanoflagellata are hardly distinguishable from each other. Therefore, it was traditionally assumed that the sponges were the first multicellular animals that had evolved from unicellular or colony forming Choanoflagellata. Recent phylogenomic studies support this hypothesis only partially. Although the most comprehensive work again indicated in the traditional direction. On the basis of this study, the evolutionary events is as follows: During the Neoproterozoic Metazoa emerged from the Choanoflagellata. The first metazoa were creatures that resembled today's Placozoa and Porifera. Today Porifera have the typical collar ciliated cells. Biomarkers from the Cryogenium (middle Neoproterozoic ) indicate the presence of sponges in front of at least 635 million years. From the Placozoa -like creatures the Eumetazoa emerged. The earliest fossils resemble today's Eumetazoa Anthozoa and are about 590 million years old. In Anthozoa Choanozyten -like cells are present today. The first bilateral animals originate from the upper Ediacaran and are 542-560 million years old. Choanozyten -like cells can be found today in different animal phyla. Summary:

• The last common ancestor of all Metazoa possessed Choanozyten as heir to his Choanoflagellata ancestors. This Choanozyten have been preserved in adult sponges.
• The last common ancestor of all Placozoa Eumetazoa had Choanozyten that resembled certain cell stages of development in today's sponge embryos. This animal may have been similar to a modern-day Placozoon.
• The last common ancestor of all Eumetazoa had this Choanozyten -like cells. This animal could have resembled a modern-day Anthozoon.
• The last common ancestor of all Bilateria still had this Choanozyten -like cells.
• The Choanozyten -like cells developed at different Eumetazoa strains in different directions and changes, more or less extensive their shape.
• The Placozoa also evolved further. Therefore, the only living today Placozoon ( Trichoplax adhaerens ) has no longer strong Choanozyten -like cells. His line of development changed this cell type but until after the Eumetazoa had split off.