Postnatal differentiation and growth of cat entopeduncular neurons. A transient spiny period associated with branch elongation

Activity-regulated dynamic behavior of early dendritic protrusions: evidence for different types of dendritic filopodia

Dendritic filopodia are long and thin protrusions that occur predominantly during early development of the mammalian CNS. The function of dendritic filopodia is unknown, but they could serve to form early synapses, to generate spines, or to regulate dendritic branching and growth. We used two-photon imaging to characterize the motile behavior of dendritic protrusions during early postnatal development (P2-P12) in pyramidal neurons from acute slices of mouse neocortex. Dendritic protrusions in immature neurons are highly dynamic, and this motility is actin based. Motility and turnover of these early protrusions decreases throughout development, mirroring an increase in their average lifetime and density. Interestingly, density, motility, and length of filopodia are greater in dendritic growth cones than in dendritic shafts. These growth cones disappear after P5. Blocking synaptic transmission globally using TTX or calcium-free solutions led to a 40-120% increase in the density and length of dendritic filopodia in shafts but not in growth cones. Moreover, blocking ionotropic glutamate receptors resulted in an approximately 35% decrease in the density and turnover of shaft filopodia, whereas focal glutamate application led to a 75% increase in the length of shaft filopodia, but neither manipulation affected growth cone filopodia. Our results support the existence of two populations of filopodia, in growth cones and shafts, which are differentially regulated by neuronal activity. We propose that filopodia in dendritic growth cones are involved in dendritic growth and branching in an activity-independent manner, whereas shaft filopodia are responsible for activity-dependent synaptogenesis and, in some cases, may become dendritic spines.

Synaptogenesis via dendritic filopodia in developing hippocampal area CA1

To determine the role of dendritic filopodia in the genesis of excitatory synaptic contacts and dendritic spines in hippocampal area CA1, serial section electron microscopy and three-dimensional analysis of 16 volumes of neuropil from nine male rat pups, aged postnatal day 1 (P1) through P12, were performed. The analysis revealed that numerous dendritic filopodia formed asymmetric synaptic contacts with axons and with filopodia extending from axons, especially during the first postnatal week. At P1, 22 +/- 5.5% of synapses occurred on dendritic filopodia, with 19 +/- 5.9% on filopodia at P4, 20 +/- 8.0% at P6, decreasing to 7.2 +/- 4.7% at P12 (p < 0.02). Synapses were found at the base and along the entire length of filopodia, with many filopodia exhibiting multiple synaptic contacts. In all, 162 completely traceable dendritic filopodia received 255 asymmetric synaptic contacts. These synapses were found at all parts of filopodia with equal frequency, usually occurring on fusiform swellings of the diameter. Most synaptic contacts (53 +/- 11%) occurred directly on dendritic shafts during the first postnatal week. A smaller but still substantial portion (32 +/- 12%) of synapses were on shafts at P12 (p < 0.036). There was a highly significant (p < 0.0002) increase in the proportion of dendritic spine synapses with age, rising from just 4.9 +/- 4.3% at P1 to 37 +/- 14% at P12. The concurrence of primarily shaft and filopodial synapses in the first postnatal week suggests that filopodia recruit shaft synapses that later give rise to spines through a process of outgrowth.

Preferential termination of corticorubral axons on spine-like dendritic protrusions in developing cat

The formation of synaptic contacts is a crucial event during neural development and is thought to be achieved by complex interactions between incoming axons and the neurons in the target. We have focused on spine-like dendritic protrusions (SLDPs), which are transient pleomorphic protrusive structures seen in developing brains. Although the functional significance of SLDPs remains unknown, accumulating in vitro evidence suggests that the SLDP plays an important role in synaptogenetic interactions with axons. As a test of this idea, the present study was performed to examine whether the SLDPs are the preferential sites of synapse formation in vivo. The ultrastructure of biocytin-labeled corticorubral (CR) terminals was examined in serial thin sections during the period of synaptogenesis in newborn cats. We found that a major proportion (86%) of the CR synapses was formed on SLDPs. The presynaptic terminals were often invaginated by fine processes extending from the tips of SLDPs. Synaptic structures presumably of cortical origin were also found on SLDPs of HRP-labeled rubrospinal cells, suggesting that SLDPs postsynaptic to labeled CR terminals originate at least in part from rubrospinal cells. Taken together, these results indicate that SLDPs may represent preferred sites of synapse formation and support the notion that SLDPs play a role in synaptogenic interactions during brain development.

Postnatal development of cat hind limb motoneurons. II: In vivo morphology of dendritic growth cones and the maturation of dendrite morphology

The maturation of dendrite morphology was studied by light and electron microscopy in cat spinal alpha-motoneurons intracellularly labeled with horseradish peroxidase. Alpha-motoneurons supplying the triceps surae (TS) and the intrinsic foot sole (SP) muscles were investigated in kittens from birth to 44-46 days of postnatal (d.p.n.) age. At birth, a large number of dendritic branches displayed growth cones, filopodia, and fusiform processes. The growth cones were of lamellipodial and filopodial types, but intermediate forms also occurred. The growth cones shared several morphological features with the neuritic growth cones studied in vitro. It was suggested that the occurrence of different types of growth cones--even in the same dendrite--may reflect their transformation from one type to the other and the level of growth activity could be inferred from the number and form of the growth cones. About 50-70% of the terminal branches in the dendrites of newborn kittens possessed growth cones, filopodia, and/or fusiform processes. The corresponding figure for preterminal branches was 20-30%, with a clear decrease in incidence when approaching the soma. During the period under study, most of these growth-associated processes disappeared from the dendrites so that at 44-46 d.p.n. of age only about 10% of the terminal and less than 1% of the preterminal branches had growth-associated processes. Analysis of the three-dimensional distribution of dendritic branches with such processes disclosed that they were relatively more frequent in the medial, rostral, and caudal dendritic territories. It was concluded that the pattern of distribution and disappearance of growth cones, filopodia, and fusiform processes coincided with postnatal longitudinal dendritic growth and the development of the adult dendritic territories described in a preceding paper (Ulfhake et al., '88). Dendritic growth, with respect to length and caliber, also occurred in the absence of growth cones and filopodia. It is suggested that the important role of these processes may be to act as a steering device in establishing the adult distribution and synaptology of the dendrites. Comparison of TS and SP alpha-motoneuron dendrite morphology at birth and at 22-24 d.p.n. age showed that the SP neurons lagged in the maturation process. Light and electron microscopic observations indicated that postnatally direct contacts might exist between dendrites and fine blood vessels in the neuropil without any interposing glial sheath. The number of such suspected contacts diminished during the period under study, indicating that the glial ensheathment of the blood vessel takes place, in part, postnatally.

Dendritic development and preferential growth into synaptogenic fields: a quantitative study of Golgi-impregnated spinal motor neurons

Branching patterns of dendrites may be modulated by the way in which dendritic growth cone filopodia come into initial synaptic relationships with afferent axons. This synaptotropic hypothesis of dendritic branching predicts that dendritic growth will be directed preferentially into regions containing numerous prospective presynaptic elements. The developing mouse spinal cord provides a natural experiment to test this prediction, because synapses are found exclusively within the marginal zones bordering the motor columns during the early (E11-14) period of synaptogenesis. During this time, therefore, most motor dendritic growth would be expected to be directed laterally or ventrally into the marginal zones, whereas internally directed growth should become more prevalent later, when synaptogenesis begins to take place within the intermediate zone, i.e., the motor columns proper. A computer-assisted three dimensional reconstruction system has been used to test these expectations in Golgi preparations of developing mouse (C57BL/6J) spinal cords ranging in age from E13 through P1. Mean dendritic lengths and branch densities are significantly greater for marginal zone dendrites than for intermediate zone dendrites at early ages (E13-14), but there are no significant differences in these measures at later stages of development (P0,1). These findings are interpreted as meaning that motor dendritic growth is initially biased into the marginal zone by synaptogenic afferents and that this preferential distribution is progressively lost as synapses develop within the intermediate zone to attract or to stabilize internally directed dendritic growth. Thus the findings of this study are consistent with predictions of the synaptotropic hypothesis of dendritic branching.

Three-dimensional reconstruction of tissue using computer-generated images

In the study of brain ventricles for both teaching and research, it is often of considerable advantage to graphically display the reconstructed shape of the specimen. This paper describes the making of two-dimensional serial cross-sections and their storage for subsequent manipulation and display on a personal computer; the three-dimensional (3D) reconstructions may have hidden lines removed and various parts coloured for definition of areas of interest, for example the density and position of pyknotic (dead) nuclei. Current research involved the investigation of the effects of maternal hyperthermia on early embryonic brains. The 3D reconstructions were found ideal for understanding the temporal changes occurring in embryonic brains subjected to defined maternal heat stresses. The method involved 4 X 4 matrices using homogenous coordinate theory, being the most ideal and allowing a constant mechanism for all transformations. Total time from examination of sections to obtaining an accurate printed 3D reconstruction is approximately 1 h if 22 sections are used.

The entopeduncular nucleus: Golgi morphometrics of serially reconstructed neurons in adult cats

Computer-assisted morphometrics were used to characterize mature somatodendritic architecture in Golgi-stained neurons of the entopeduncular nucleus (EN) of the adult cat. Only one form of adult EN neuron was apparent and characterized by common features including: relatively large conical somata, long aspiny and moderately branched dendrites and discoid to spherical dendritic fields oriented randomly within the EN. These results indicate that feline EN neurons have some properties in common with large neurons of the primate medial pallidal segment.