Choanocytes (also known as "collar cells") are

cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...

s that line the interior of

asconoid Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through th ...

, syconoid and leuconoid body types of

sponges Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through ...

that contain a central

flagellum A flagellum (; ) is a hairlike appendage that protrudes from certain plant and animal sperm cells, and from a wide range of microorganisms to provide motility. Many protists with flagella are termed as flagellates. A microorganism may have f ...

, or ''cilium,'' surrounded by a collar of microvilli which are connected by a thin membrane.

They make up the

choanoderm The choanoderm is a type of cell layer composed of flagellated collar cells, or choanocytes, found in sponges. The sponge body is mostly a connective tissue; the mesohyl, over which are applied epithelioid monolayers of cells, the outer pinacoder ...

, a type of cell layer found in

. The cell has the closest resemblance to the

choanoflagellates The choanoflagellates are a group of free-living unicellular and colonial flagellate eukaryotes considered to be the closest living relatives of the animals. Choanoflagellates are collared flagellates, having a funnel shaped collar of interconn ...

which are the closest related single celled protists to the animal kingdom (metazoans). The flagellae beat regularly, creating a water flow across the microvilli which can then

filter Filter, filtering or filters may refer to: Science and technology Computing * Filter (higher-order function), in functional programming * Filter (software), a computer program to process a data stream * Filter (video), a software component tha ...

nutrients from the water taken from the collar of the sponge. Food particles are then

phagocytosed Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is c ...

by the cell. Anderson, D. (2001) ''Invertebrate Zoology'' Oxford University Press

Location

Choanocytes are found dotting the surface of the spongocoel in

sponges and the radial canals in syconoid sponges, but they comprise entirely the chambers in leuconoid sponges.

Function

By cooperatively moving their flagella, choanocytes filter particles out of the water and into the

spongocoel A spongocoel (), also called paragaster (or paragastric cavity), is the large, central cavity of sponges. Water enters the spongocoel through hundreds of tiny pores ( ostia) and exits through the larger opening (osculum). Depending on the body pla ...

, and out through the

osculum The osculum (plural "oscula") is an excretory structure in the living sponge, a large opening to the outside through which the current of water exits after passing through the spongocoel. Wastes diffuse into the water and the water is pumped th ...

. This improves both respiratory and digestive functions for the sponge, pulling in oxygen and nutrients and allowing a rapid expulsion of carbon dioxide and other waste products. Although all cells in a sponge are capable of living on their own, choanocytes carry out most of the sponge's ingestion, passing digested materials to the

amoebocytes An amebocyte or amoebocyte () is a mobile cell (moving like an amoeba) in the body of invertebrates including cnidaria, echinoderms, molluscs, tunicates, sponges and some chelicerates. They move by pseudopodia. Similarly to some of the white bloo ...

for delivery to other cells. Choanocytes can also turn into

spermatocyte Spermatocytes are a type of male gametocyte in animals. They derive from immature germ cells called spermatogonia. They are found in the testis, in a structure known as the seminiferous tubules. There are two types of spermatocytes, primary and ...

s when needed for sexual reproduction, due to the lack of reproductive organs in sponges (amoebocytes become the

oocyte An oocyte (, ), oöcyte, or ovocyte is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in a female fetus in the ovary during female gametogenesis. The female ...

s).

Evolutionary Significance

Choanocytes bear a superficial resemblance to

Choanoflagellates The choanoflagellates are a group of free-living unicellular and colonial flagellate eukaryotes considered to be the closest living relatives of the animals. Choanoflagellates are collared flagellates, having a funnel shaped collar of interconn ...

. Molecular phylogenies indicate that choanoflagellates and

metazoans Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...

are sister groups. One can see some modern choanoflagellates living in small colonies. The evolutionary relationship between the two cell types is debated. Jasmine L. Mah, Karen K. Christensen‐Dalsgaard, Sally P. Leys "Choanoflagellate and choanocyte collar‐flagellar systems and the assumption of homology", 2014, https://doi.org/10.1111/ede.12060

References

{{reflist Histology Animal cells Sponge anatomy

Location

Function

Evolutionary Significance

See also

References