A specialization of the endomembrane system found in some classes of dendritic spines consisting of two or more closely apposed lamellae with interspersed electron dense material. The endomembrane component is continuous with the smooth endoplasmic reticulum. [ http://www.neurolex.org/wiki/sao725931194 http://www.ncbi.nlm.nih.gov/pubmed/20400711 http://www.ncbi.nlm.nih.gov/pubmed/8987748 ]