Table 1.

Special terminology for invaginating projections, used in this review

Appendages	This general term is used by some authors to describe dendritic projections that vary in shape from knob-like to  filiform, emanating from unipolar brush cells and other related cell types of the cerebellum and cochlear nuclei  (“Small Postsynaptic Invaginating Projections: Spinules” section). Those “appendages” that invaginate into  mossy terminals look similar to some of the larger spinules described in this review for other neurons and  synapses
Boutons	French term (means button, bud, spot, knob, pimple) used to describe the short invaginating projections from  neuron-like cells in sponges (Pavans de Ceccatty 1966; “Synapse/Neuronal Invaginating Projections in the  Simplest Animals” section); commonly also used to describe presynaptic axon terminals in other animals
Capitate projections	Specialized glial spinules invaginating into the presynaptic terminals of photoreceptor/retinular cells of fly eyes;  they have a unique intercellular substance or “third membrane” in the spinule head (“Glia-Derived Invaginating  Projections” section)
Filopodia	Finger-like, actin-filled processes, typically>100 to 400 nm in diameter and of varying length from 1 to>200 μm
Gnarls	Specialized short, glial projections, sometimes with a stalk and head region, associated with α and β fibers of fly  eyes (“Glia-Derived Invaginating Projections” section)
Invaginating projections	An arbitrary term used in this review to define all kinds of these processes
Microvilli	Similar to filopodia in structure but tend to form more regular, parallel bundles on a cell surface. Here, they are  described from dendrites in nematode peripheral sense organs including amphids (“Large Postsynaptic  Invaginating Projections: Filopodia and Spines” section) and from the apical end of the developing R8 retinal  cell in Drosophila (“Function of Invaginating Projections in Cell Signaling” section)
Probosci	Thin spinules arising from parallel fiber axons and invaginating into Purkinje cell dendrites in the cerebellum of  the early postnatal rat (Altman and Bayer 1997; “Presynaptic Terminal/Axon-Derived Invaginating  Projections” section). These authors also refer to this structure as growing “like a tongue”
Pseudopodial  interdigitations (PSIs)	“PSIs” are large invaginating projections between abutting presynaptic terminals in the torpedo ray electrical  organ (Boyne and McLeod 1979) and mammalian limbic system (Boyne and Tarrant 1982). They can be short  or long, finger-like or highly irregular, and simple or compound (in which PSIs may interdigitate with each  other), and they usually contain some synaptic vesicles
Spine protrusions	Synonym for spinule, used by Erisir and Dreusicke (2005) for invaginating projections in the ferret visual cortex  (“Small Postsynaptic Invaginating Projections: Spinules” section)
Spinules	Typically slender processes, mainly <100 nm in diameter, although often with an enlarged head region, and  usually less than 1 μm long
Tongues	Small spinules from the presynaptic terminal membrane invaginating into the postsynaptic process (here  described by Waxman et al. 1980, in a lizard and gymnotid fish; “Presynaptic Terminal/Axon-Derived  Invaginating Projections” section). The analogy to a tongue has been used for other projections, including the  protrusions of membranes of vesicles at synapses in the simple, flatworm-like animal, Xenoturbella westbladi  (Raikova et al. 2000; “Synapse/Neuronal Invaginating Projections in the Simplest Animals” section), and for  projections of parallel fibers in the developing cerebellum (“Presynaptic Terminal/Axon-Derived Invaginating  Projections” section; Altman and Bayer 1997; see definition of proboscis)
Trophic prolongations	Long processes with enlarged heads containing a desmosome attachment; parenchymal cells in an ectocommensal  flatworm invaginate these into adhesive secretion gland cells; the end lies very close to the gland cell nucleus  (Williams 1994; “Synapse/Neuronal Invaginating Projections in the Simplest Animals” section)
Trophospongium	Elongate glial processes of varying lengths and shapes that invaginate into neuron cell bodies or axons, and are  believed to have a trophic function (“Glia-Derived Invaginating Projections” section)
Tunnel fibers	Fibers formed from the trunks of small neurons called microneurons, and that invaginate, often in groups of three  or more, into presynaptic terminals in the optic lobe of octopi (these terminals also have invaginating  postsynaptic spines; Dilly et al. 1963; Case et al. 1972; “Large Postsynaptic Invaginating Projections: Filopodia  and Spines” section)
Varicosities	Generally used to describe highly irregular and often swollen projections. The parallel fiber terminals that enwrap  Purkinje spines of the developing cerebellum also are described by Altman and Bayer (1997) as varicosities  (“Large Postsynaptic Invaginating Projections: Filopodia and Spines” section)