Thecortical alveolum (pl. cortical alveoli) is acellularorganelle consisting of avesicle located under thecytoplasmic membrane, to which they give support. The term "corticate" comes from an evolutionary hypothesis about the common origin ofkingdomsPlantae andChromista, because both kingdoms have cortical alveoli in at least one phylum. At least threeprotist lineages exhibit these structures:Telonemia,Alveolata andGlaucophyta.

Cortical alveoli have been defined as flattenedmembranous sacs or vesicles that strengthen the cellular cortex through the firm fixation to the underlying membrane andmicrotubules.^[1] They typically form a continuous layer that acts as a flexible film, although they can also constitute a semi-rigid structure or the scales of atheca.^[2]

Cortical alveoli are the defining morphological characteristic of theprotistcladeAlveolata, otherwise united solely bymolecular phylogeny.^[3] The name 'Alveolata' is a reference to this trait,^[4][1] present in all representatives of this group: the parasiticapicomplexans andperkinsozoans,^[5] the predatoryciliates^[2] andcolpodellids,^[6][7] thechromerid algae,^[8][9] and the ecologically diversedinoflagellates,^[10] as well as smallbasalflagellates collectively known ascolponemids.^[11] A smallphylum of eukaryotes,Telonemia, contains flagellated species that exhibit flattened alveoli, much like in Alveolata. Both groups share evolutionary affinity, comprising theTSARsupergroup along withStramenopiles andRhizaria.^[12]

In addition to the TSAR supergroup, theArchaeplastida supergroup contains a lineage of algae with cortical alveoli, known asGlaucophyta. All species of glaucophytes exhibit flattened membranous alveoli lying immediately below theplasma membrane, in every region of the cell except for the flagellar furrow. In addition, these alveoli may contain rigid plates that, similarly to thecate dinoflagellates, define the shape of the otherwise amorphous cell.^[13] The commonmorphological trait with alveolates has been used to propose a common evolutionary origin between the kingdoms Plantae (defined as Archaeplastida) andChromista (containing alveolates).^[14] This is known as the "corticate" hypothesis, postulated by protozoologistThomas Cavalier-Smith,^[15][16] but later rejected due to the non-monophyly of kingdom Chromista.^[17] Instead, both Archaeplastida and the TSAR supergroup are united in a clade known asDiaphoretickes, which contains the remaining chromist groupsHaptista andCryptista.^[17][4]

Although cortical alveoli are very diverse in shape and function among the different groups of protists, they always share the function of supporting the cytoplasmic membrane. In the case of apicomplexan parasites, the alveoli are related both to mobility and to the invasion of host cells and thus have a particular relevance inmedicine. Indinoflagellates, the alveoli containcellulose and compose the scales of their armor. The alveoli ofciliates are part of the cortical complex that supports the extrusomes, the basal bodies of thecilia, and the intricate cortical shell.^[18]

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