Article Figures & Data
Figures
- Fig. 1.
Representative CA1 neuropil from postnatal day 1 (R53) has large amounts of extracellular space (ECS). A dendrite (D) with well organized microtubules gives rise to a dendritic filopodium (df), which continues on adjacent serial sections. An asymmetric synapse (s) on a stubby dendritic protrusion is evident at the top of the figure. Also in evidence are nonsynaptic surface specializations (solid square arrows). The tip of an axonal filopodium (af), identified by tracing it back to its axonal origin, has a surface specialization or possibly a nascent synapse (open arrow) where it contacts the tip of a dendritic filopodium (f), also identified through series. Scale bar, 1 μm.
- Fig. 2.
Representative CA1 neuropil from postnatal day 4 (R48a). A dendritic filopodium (df) emerges from the middle of a dendrite (D). The entire filopodium is reconstructed in Figure 5. A synapse on an apparently stubby profile is actually at the base (B) of a filopodium that could be traced on adjacent serial sections. A shaft synapse (sh) has synaptic vesicles in a docked position, suggesting the synapse is functional. The profile in the upper left corner (f) makes synaptic contact with an axon containing docked vesicles. This synapse occurs in the middle of a dendritic filopodium that could be followed for 70 sections to its tip, at which there was no synaptic contact. Scale bar, 1 μm.
- Fig. 3.
Neuropil from postnatal day 6 (R43b) showing varicosities (V) with interposed thin regions that give some dendrites a beaded appearance. Some varicosities had a watery cytoplasm (W) with organelles compressed to the periphery, suggesting that they might have undergone swelling. A synapse (s) located on a dendritic shaft has the appearance of a symmetric synapse with equally thin densities on both membranes. Asymmetric synaptic contacts on two dendritic filopodia profiles (f) were identified through serial sections. Note that these profiles might be mistaken for dendritic spines if they were viewed only on this section. Scale bar, 1 μm.
- Fig. 4.
The neuropil from postnatal day 12 (R45a) is much denser than in the first postnatal week, with many more profiles of small processes. Asymmetric synapses are seen on thin dendritic spines (t), as well as on stubby spines (s) and dendritic shafts (sh). Many dendritic spines at this age do not have the typical characteristics of adult CA1 spines. An atypical thin spine (a) receives synaptic contacts from two different axons. Multiple synapses are rarely identifiable on single sections. The atypical stubby (as) in the lower left has only one of its two synapses visible on this section, whereas two synapses (arrows) are apparent on the atypical stubby (as) in the inset. One postsynaptic process (?) could not be traced to its origin and, therefore, could not be unequivocally identified. Scale bar, 1 μm.
- Fig. 5.
Three-dimensional reconstruction of the dendritic filopodium whose origin is shown in Figure 2. The filopodium is ∼11 μm long from origin to tip. Filopodia of this length were rarely captured in series of only 100 ultrathin sections. At its narrowest point (solid arrow) the filopodium has a diameter of ∼0.1 μm, whereas the maximum diameter is ∼1 μm. The filopodium is enfolded in stubby protrusions from the axon, one of which (star) appears to make a synaptic contact with the filopodium (F) (inset, star arrow). Scale bar in inset, 0.5 μm; 3D scale bar, 1 μm.
- Fig. 6.
A montage of four sections from postnatal day 6 (R44a) showing a filopodium (f) emerging from a dendrite (D), and terminating in a bulbous head (h) that has two synapses from different axons. The filopodium is similar in shape to an atypical spine (see Materials and Methods), but its unusual length (∼3 μm) is more characteristic of a filopodium. In addition, the “neck” has a variable diameter, a dark cytoplasm suggesting a dense accumulation of actin, and exhibits a surface specialization at one point along its length where it contacts another dendritic process (arrow). Scale bar, 1 μm.
- Fig. 7.
A montage of two sections from P1 (R55) showing a vesicle-filled process (P) emerging from a bend in a dendrite (D), all of which is outlined inwhite. This process shares more characteristics with growth processes than most dendritic filopodia (compare with Figs. 1and 2), suggesting that it might represent a budding branch of the dendrite. Note the mitochondrian (arrow) partly invades the base of the process. Mitochondria were never observed in filopodia. Scale bar, 1 μm.
- Fig. 8.
Top. Interaction between axonal and dendritic filopodia at postnatal day 4 (R48a). A, An axon has a broad protrusion with a grainy cytoplasm indicative of actin. This actin-filled region is devoid of microtubules and ends in a narrow process (af) identical in appearance to dendritic filopodia except for the presence of synaptic vesicles (black arrows). There are two synapses at the base of this axonal filopodium, one (blue arrow) with a dendrite shaft and another (red arrow) with the tip of a dendritic filopodium (df). B, Three-dimensional reconstruction of segments of the same axon and dendrite with the interacting filopodia that are shown in A. Scale bars, 1 μm.
- Fig. 9.
Bottom. A cluster of filopodia emerging from a varicosity in a beaded dendrite at P6 (R43a). Reconstruction of a segment of the dendrite shows the location of six synapses (blue). The filopodia (copper) receive synaptic contacts near the tips (1, 6) and at the base (3, 4, 5). An additional shaft synapse (2) can be seen on the surface of the dendrite. Two other shaft synapses and one spine synapse (data not shown) also occurred on this dendritic segment. Electron micrographs (insets) show the details of each synapse. Scale bars in insets, 0.5 μm; 3D scale bar, 1 μm.
- Fig. 10.
Distribution of synapse locations in CA1 during the first postnatal week. Bars represent the mean percentages of synapses in each class. Light shading indicates mean percentages for atypical spines and stubbies (see Materials and Methods for details). Error bars show the minimum and maximum values for combined means measured in 12 series from seven animals aged P1, P4, and P6.
- Fig. 11.
The location of synapses on 162 complete filopodia as a percent of the total number (n) of synapses on these filopodia at each age. There were more tip locations at P12 (F(3,12) = 6.21;p < 0.009), but apparent trends in mid and base locations were not statistically significant (p < 0.07 and p < 0.26, respectively).
- Fig. 12.
Distribution of synapse locations by age as revealed by the mean percentage of synapses at each location.Light regions depict percentages with atypical characteristics, (see Materials and Methods for details). Error bars indicate SDs.
- Fig. 13.
Possible mechanisms by which filopodia might induce shaft synapses. A, Synapses form on the receptive surface of filopodia, but only those near the base of filopodia are stabilized into shaft synapses. B, Filopodia retract, pulling the presynaptic partner to the dendrite. C, Dendritic filopodia guide axonal filopodia to the dendrite where synapses are stabilized by subsequent maturation of the axonal filopodium into an axon branch. D, Dendritic filopodia act as conduits along which synapses move to the dendrite shaft.
- Fig. 14.
Genesis of most excitatory synapses may involve four stages. 1, Initial induction of a synapse by a filopodium. In early stages of development, atypical or “protospines” are apparent, but their ultimate fate is unclear.2, Retraction of the filopodia and stabilization of the synapse on the dendrite shaft. 3, Emergence of a stubby protrusion beneath the synapse. 4, Outgrowth of protrusion into a mature spine.
Tables
- Table 1.
Location, volume, and number of synapses for each set of serial section electron micrographs. R numbers are individual rat numbers for a total of nine rats; letters a, b, and c are the specific series identifiers
Series Age Total width of apical arbor (micrometers) Series location (micrometers from cell body layer) Volume analyzed (cubic micrometers) Total number of synapses Number of filopodia synapses Number of shaft synapses Number of stubby synapses Number of spine synapses R52a PN1 165 100 346.22 64 9 41 5 4 R52b PN1 165 130 1066.41 116 25 76 6 2 R53 PN1 160 20 781.38 97 24 42 22 1 R55 PN1 140 125 391.04 50 13 22 6 5 R48a PN4 200 100 482.17 80 18 46 5 3 R48b PN4 200 100 332.81 51 8 31 5 2 R60a PN4 250 115 638.12 90 11 45 15 7 R60b PN4 250 115 573.78 109 27 58 16 2 R43a PN6 300 265 270.92 120 33 54 8 17 R43b PN6 300 265 200.96 103 10 58 5 17 R44a PN6 275 40 553.87 118 17 53 14 17 R44b PN6 275 115 610.48 114 27 31 25 27 R45a PN12 300 266 77.06 107 12 25 16 45 R45b PN12 300 266 122.27 99 10 18 12 48 R47b PN12 400 180 85.33 105 3 47 18 33 R47c PN12 400 180 60.54 60 2 23 20 12 The total number of synapses includes synapses that could not be identified through serial sections. Only complete, identifiable synapses were used for analysis, yielding a total of 1417 for all ages.
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