Cytostome

A cytostome (from cyto-, cell and stome-, mouth) or cell mouth is a part of a cell specialized for phagocytosis, usually in the form of a microtubule-supported funnel or groove. Food is directed into the cytostome, and sealed into vacuoles. Only certain groups of protozoa, such as the ciliates and excavates, have cytostomes. An example is Balantidium coli, a ciliate. In other protozoa, and in cells from multicellular organisms, phagocytosis takes place at any point on the cell or feeding takes place by absorption.

The cytostome forms and invagination on the cell surface and is typically directed towards the nucleus of the cell. The cytostome is often labeled as the entire invagination, but in fact the cytostome only constitutes the opening of the invagination at the surface of the cell. The rest of the invagination is classified as the cytopharynx. The cytopharynx works in conjunction with the cytostome in order to import macromolecules into the cell. This strong association between the cytostome and cytopharynx is often called the cytostome-cytopharynx complex or the cytopharyngeal apparatus. However, in a small number of cases the cytostome works independently in order to import macromolecules. In these instances, the cytostome imports macromolecules by directly forming vesicles that are imported into the interior of the cell.

The cytostome is associated with microtubules that function in maintaining its form. One set of microtubules is arranged in a triplet formation and is located directly beneath the cytostome membrane. A second set of microtubules is positioned directly beneath the flagellar pocket membrane and forms a quartet.

Equally as important in the function of endocytosis is the structure known as the cytopharynx. The cytopharynx is a long, tube-like structure that forms the invagination associated with the cytostome. While the shape of the cytopharynx is not constant, it is typically directed towards the posterior of the cell, often hooking around a central nucleus. The length of the cytopharynx varies during the cell cycle, however the average length is 8 µm. Much like the cytostome, a set of microtubules form an association with the cytopharynx. Two sets of microtubules follow the path of the cytopharynx in cells. These sets of microtubules form a gutter like structure that surrounds the cytopharynx. One side of the cytopharynx is not associated with these microtubules and is deemed the "naked side". However, vesicles associate with this side of the cytopharynx.

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