Dendritic Spines and Development: Towards a Unifying Model of Spinogenesis—A Present Day Review of Cajal's Histological Slides and Drawings

Figure 6

(a) Illustrates a Cajal scientific drawing [17]. The English translation of the original legend of this drawing is in the endnote 3 of the text. We would like to remark filopodia in (D) motor cell, spinal cord, one-month-old cat, and the presence of dendritic appendages with mixed characteristics between filopodia and spines (protospines) in (B) visual cortex, two-month-old child. (b) Synapses at sites of contact between filopodium-like dendritic protrusions and axons [18]. Dendritic appendages of great motility (arrows), a FAST DiO-labeled dendrite (green) and FM4–64-labeled presynaptic buttons (red) in an in vitro preparation of 14 days. (c) Differential interference contrast image of the same field. The parallel axon (arrowheads) 5–7 mm below the labeled dendrite (dashed outline) has formed synapses (red) with several dendritic protrusions. (d) New PSD95 clusters emerge in dendritic filopodia that transform into protospines and spines [19]. In a confocal time-lapse sequence of a neuron expressing PSD95-GFP, transient filopodia, devoid of clusters, repeatedly protrude and withdraw at a site (1, 0 h) overlying a shaft cluster. Later, a new filopodium emerges (arrowhead, 3.3 h) and transforms into a cluster-containing protospine (arrowhead, 5.5 h). At another site (2, 1 h), a filopodium extends, persists and stabilizes coincident with de novo formation of a cluster (arrow, 1.8–3.3 h), scale bar, 3 μm. (e) synaptic interaction between axonal (af) and dendritic filopodia (df), postnatal day 4, Hippocampus, CA1, three-dimensional reconstruction of electron microscopy images, scale bar 1 μm [20].