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The SA has been reported to contain a variety of substances that may be important for spine and synapse function. These include Ca2+, actin, MAP2 protein kinase, Ca2+-ATPase, calsequestrin and inositol 1,4,5-triphospate 3-kinase. We have also found ribosome-like particles in the inner dense plates. The function of the SA is not clear. It has long been supposed that the SA plays an important role in buffering (sequestering and releasing) Ca2+ in the dendritic spine compartment. The SA may also be a source of synaptic proteins (i.e. neurotransmitter receptors or other components of the PSD). This synthesis and/or storage function may be important for synaptic plasticity.
Fig. 4: (A) A dendritic segment with a mushroom-shaped spine possessing a perforated synapse (red) and with two thin spines. (B) The mushroom-shaped spine contains the spine apparatus (arrow) with the inner dense plates (red). Note an intimate association between the smooth endoplasmic reticulum (purple) and mitochondrion (gray). (C) Another spine apparatus with inner dense plates at higher magnification.
Some authors incorrectly refer to profiles of smooth endoplasmic reticulum in spines of cerebellar Purkinje cell dendritic branchlets as SA. Although an occasional parallel configuration of tubules or sacs of endoplasmic reticulum can resemble the spine apparatus, any signs of higher differentiation and dense plates characteristic of the spine apparatus associated with perforated synapses are lacking in Purkinje cell spines.
An organelle identical in appearance to the SA is also found in the axon initial segment (Fig. 5). This organelle is often referred to simply as a cisternal organelle. Its function is completely unknown. Perhaps a comparative analysis of this organelle together with the SA will help to uncover its functional significance. Another striking structural similarity exists between the outer dense plates and axial strips described in a subsynaptic cytoplasm of neuromuscular synapses.
| Fig. 5: A cisternal organelle located as an extraspinous variety of the spine apparatus (yellow) in an axon initial segment. Two axo-axonal synapses are placed on the axonal surface (red) and a bunch of linked microtubules, characterising the axon initial segment is present in its cytoplasm (green). |
Related literature:
Spacek J (1985a) Three-dimensional analysis of dendritic spines. II. Spine apparatus and other cytoplasmic components. Anat. Embryol. 171:235-243.
Spacek J (1985b) Relationships between synaptic junctions, puncta adhaerentia and the spine apparatus at neocortical axo-spinous synapses. Anat. Embryol. 173:129-135.
Spacek J, Harris KM (1997) Three-dimensional organization of smooth endoplasmic reticulum in hippocampal CA1 dendrites and dendritic spines of the immature and mature rat. J.Neurosci. 17:190-203.
Spacek J, Harris KM (1998) Three-dimensional organization of cell adhesion junctions at synapses and dendritic spines in area CA1 of the rat hippocampus. J.Comp.Neurol. 393:58-68.
Couteaux R, Spacek J. (1988) Specializations of subsynaptic cytoplasms. Comparison of axospinous synapses and neuromuscular junctions. In: Cellular and Molecular Basis of Synaptic Transmission. Zimmerman H (Editor), NATO A50 Series, Springer Verlag, Heidelberg. pp. 25-50.
| Last Updated: 12/2/99 |