The currently proposed model of the stem cell system in demosponges is described, and the possible applicability of this model to other classes of sponges is discussed. This review introduces what is known about the stem cells in sponges based on histological studies and also on recent molecular biological studies that have started to reveal the molecular and cellular mechanisms of the stem cell system in sponges (mainly in demosponges). Sponges, the sister group of all other animals and probably the earliest branching multicellular lineage of extant animals, are thought to possess totipotent stem cells. Contractile myocyte cells are present around osculum, a large pore for throwing out water from spongocoel.Major questions about stem cell systems include what type(s) of stem cells are involved (unipotent/totipotent/pluripotent/multipotent stem cells) and how the self-renewal and differentiation of stem cells are regulated. Then there are spongocytes which secrete skeletal material. Ectodermal or outer layer of sponge body is made up of pinacocyte cells. There are several other cells in Porifera. The pore through which water flows in is called ostium (singular). Each porocyte allows water to flow inside the spongocoel from surrounding aquatic habitat. Porocytes are unique, elongated, tubular cells. Digestion is also intracellular, taking place within choanocyte. These cells capture microcopic food particles. Choanocyte also has a collar made of microvilli. The flagella help in creating a unidirectional flow of water around the body of sponge. Moreover choanocytes also give rise to germ cells.Ĭhoanocytes are flagellated cells which line the spongocoel and form the so called 'choanoderm': the layer represents endoderm of a diploblastic poriferan organism. Archaeocytes are totipotent cells which can differentiate into other cell types within the body of sponge.
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