Comparison of methods for the estimation of the thickness of ultrathin tissue sections

Structural synaptic correlate of long-term potentiation: formation of axospinous synapses with multiple, completely partitioned transmission zones

Synapses were analyzed in the middle molecular layer (MML) and inner molecular layer (IML) of the rat dentate gyrus following the induction of long-term potentiation (LTP) by high-frequency stimulation of the medial perforant path carried out on each of 4 consecutive days. Potentiated animals were sacrificed 1 hour after the fourth high frequency stimulation. Stimulated but not potentiated and implanted but not stimulated animals served as controls. Using the stereological disector technique, unbiased estimates of the number of synapses per postsynaptic neuron were differentially obtained for various subtypes of axospinous junctions: For atypical (giant) nonperforated synapses with a continuous postsynaptic density (PSD), and for perforated ones distinguished by (1) a fenestrated PSD and focal spine partition, (2) a horseshoe-shaped PSD and sectional spine partition, (3) a segmented PSD and complete spine partition(s), and (4) a fenestrated, (5) horseshoe-shaped, or (6) segmented PSD without a spine partition. The major finding of this study is that the induction of LTP in the rat dentate gyrus is followed by a significant and marked increase in the number of only those perforated axospinous synapses that have multiple, completely partitioned transmission zones. No other synaptic subtype exhibits such a change as a result of LTP induction. Moreover, this structural alteration is limited to the terminal synaptic field of activated axons (MML) and does not involve an immediately adjacent one (IML) that was not directly activated by potentiating stimulation. The observed highly selective modification of synaptic connectivity involving only one particular synaptic subtype in the potentiated synaptic field may represent a structural substrate of the long-lasting enhancement of synaptic responses that characterizes LTP.

Perforated axospinous synapses with multiple, completely partitioned transmission zones: probable structural intermediates in synaptic plasticity

Analysis of axospinous synapses in the rat dentate gyrus, using three-dimensional reconstructions from electron micrographs of serial sections, revealed a novel synaptic subtype. Synapses of this subtype exhibit partitions that emanate from the postsynaptic spine head and invaginate the presynaptic axon terminal, dividing its portion contracted by the spine into distinct protrusions. Such complete spine partitions provide barriers between two to four discrete transmission zones, each one being formed by a separate axon terminal protrusion and delineated by a separate segment of the postsynaptic density (PSD). Spine partitions that differ from the complete ones were found in two other synaptic subtypes. One of these is characterized by a sectional partition the base of which is placed between the arms of a horseshoe-shaped PSD. Synapses of another subtype exhibit a focal partition the base of which is restricted to a perforation in a fenestrated PSD. Although both sectional and focal partitions invaginate a presynaptic axon terminal, they do not divide into separate protrusions and do not split a single transmission zone into disjointed entities. All three subtypes of partitioned synapses have nonpartitioned counterparts exhibiting segmented, horseshoe-shaped, or fenestrated PSDs. These observations suggest a model of structural modifications underlying synaptic plasticity. According to this model, synapses with multiple, completely partitioned transmission zones that appear to be designed as elements of an unusually high strength, represent pivotal structural intermediates in synaptic plasticity. The formation of such synapses from those that belong to other subtypes is postulated to result in a sustained increase in the efficacy of synaptic transmission. Conversely, a disassembly of complete partitions with the transformation of multiple transmission zones into a single one is proposed to lead to a persistent depression of synaptic responses.

Pre- and postnatal development of the primary visual cortex of the common marmoset. II. Formation, remodelling, and elimination of synapses as overlapping processes

During ontogenesis changes in the numerical density of synapses are usually assumed to depend essentially on variations in the formation of synapses. Only the final adjustment to adult synapse densities is thought to include the elimination of synapses in some brain regions of certain species. Here, we focus attention on quantitative aspects of synapse elimination throughout development of area 17 of marmoset monkeys (Callithrix jacchus). Mature synapses, various precursor forms, and indicators of lysosomal degradation of synapses were quantitatively analysed by electron microscopy and morphometric methods. A total number of about 135 x 10(9) synapses was calculated for area 17 in each adult hemisphere corresponding to a volume density of 600 x 10(6) synapses/mm3. At 3 months of age, the respective values were 508 x 10(9)/area and 1,159 x 10(6)/mm3, while at birth these values were 69 x 10(9)/area and 328 x 10(6)/mm3. Consequently, at least three out of four synapses are eliminated between 3 months and adulthood. However, the real number of synapses being eliminated during development is probably much larger if the time course of lysosomal degradation is additionally taken into account. The frequency of lysosomes in presynaptic endings is highest before net-elimination of synapses occurs, i.e., between 1 and 3 months. This suggests that lysosomal degradation is not directly responsible for the majority of synapses removed during ontogenesis but apparently represents a second mechanism for synapse remodelling and elimination. Thus, it appears from this study that remodelling and elimination of synapses are quantitatively as important as their formation, and accompany synaptogenesis from its very onset onwards.

A review of recent advances in stereology for quantifying neural structure

The science of stereology has undergone a revolution over the past decade with the introduction of design-based (assumption- or model-free) methods which are highly efficient and generally unbiased. No other morphometric approach currently offers these twin benefits. Stereology is ideal for extrapolating 3-D structural quantities (real volumes, surface areas, lengths and numbers) from simple counts made on 2-D slice images. The images may take various forms (e.g. physical or optical sections, MRI slices, CT scans) but they must be sampled so as to be random in orientation and/or position if valid estimates are to be made. All the recent developments in stereology are applicable to problems in neuromorphometry. This review provides an account of major developments and the state of the art, emphasizes the importance of properly randomized sampling and identifies some applications to neural structure at different levels of organization. These include the counting and sizing of synapses, neurites, cells and whole brains.

Quantitative morphology of the nervous system: expanding horizons

In this review, we show how some of the recent developments in quantitative morphology (QM) are creating exciting new opportunities for studying the structure of the nervous system. We begin with a brief overview of QM, focusing on the problems neurobiologists are likely to encounter when collecting and interpreting data from tissue sections. Many of these problems, which range from selecting a sampling method to learning the latest methods, are being solved by creating a new generation of research tools. We describe several of these new tools and show how they can be used to assemble new quantitative methods for in situ hybridization, immunocytochemistry, and camera lucida drawings. The review includes examples of how QM is being used to study the brain and concludes with a brief discussion of diagnostic pathology and its need for new quantitative approaches.

A protocol for 3D image reconstruction from a single image of an oblique section

Oblique section 3D reconstruction can produce a 3D image of a sectioned crystal from a single electron micrograph. We describe here in detail a reconstruction protocol applicable to an electron micrograph of an oblique section through a 3D crystal. The protocol is described in six steps: (1) selection criteria for images, (2) preprocessing steps to correct for image defects, (3) determination of unit cell coordinates, (4) interpolation of strip images with correction for image distortions and crystal disorder, (5) production of a crystallographic serial section reconstruction, (6) correction for skewed sampling to produce an oblique section reconstruction. In addition, we explore Wiener filter deconvolution of the section thickness. We describe a method for determining the section thickness by comparing data from projections of the oblique section reconstruction with corresponding data from a thick longitudinal section. Several schemes for Wiener filter deconvolution are described that differ in the way information on the signal-to-noise ratio is used in the filter.

x-y-recording in transmission electron microscopy. A versatile and inexpensive interface to personal computers with application to stereology

An interface for IBM XT/AT-compatible computers is described which has been designed to read the actual specimen stage position of electron microscopes. The complete system consists of (i) optical incremental encoders attached to the x- and y-stage drivers of the microscope, (ii) two keypads for operator input, (iii) an interface card fitted to the bus of the personal computer, (iv) a standard configuration IBM XT (or compatible) personal computer optionally equipped with a (v) HP Graphic Language controllable colour plotter. The small size of the encoders and their connection to the stage drivers by simple ribbed belts allows an easy adaptation of the system to most electron microscopes. Operation of the interface card itself is supported by any high-level language available for personal computers. By the modular concept of these languages, the system can be customized to various applications, and no computer expertise is needed for actual operation. The present configuration offers an inexpensive attachment, which covers a wide range of applications from a simple notebook to high-resolution (200-nm) mapping of tissue. Since section coordinates can be processed in real-time, stereological estimations can be derived directly "on microscope". This is exemplified by an application in which particle numbers were determined by the disector method.

An unbiased estimation of the total number of synapses in the superior cervical ganglion of adult rats established by the disector method. Lack of change after long-lasting sodium bromide administration

Previous physiological and morphological studies suggested that sodium bromide promotes synaptogenesis of implanted cholinergic nerves in the superior cervical ganglion of adult rats. To check whether sodium bromide also modifies synaptic numbers in the intact ganglion, quantitative electron microscopy was used to determine the total number of synaptic junctions in the superior cervical ganglion of adult rats. Untreated controls were compared with animals which drank water containing 280 mg ml-1 sodium bromide for 7 days. The disector method, an unbiased estimator of volume density of certain particles, has been adapted to this particular case. To accomplish the task, an on-line counting procedure was developed, which permitted the efficient adaptation of the disector method for the superior cervical ganglion, in which the synapses are known to be distributed sparsely. Three pairs of (control and treated) ganglia have been completely processed by three independent examiners. The estimated number of synapses in the ganglia ranged from 4 to 8 million while the volumes of the ganglia varied from 0.65 to 0.90 mm3. Evaluation of the results showed that variations in the total number of synapses were in each case proportional to differences in ganglionic volumes. This suggests that: (1) sodium bromide does not lead to changes in density of intrinsic synapses; and (2) the morphogenetic action of sodium bromide on principal ganglion cells previously described is essentially postsynaptic and requires additional presynaptic elements to increase the number of synapses.

The brain's record of experience: kindling-induced enlargement of the active zone in hippocampal perforated synapses

Kindling is a consequence of intermittent electrical stimulation of a local forebrain area leading to a durable augmentation of synaptic responsiveness in the stimulated circuit. The basis for this functional change is unknown, but there is evidence suggesting that it entails a structural modification of synapses. The present report demonstrates that hippocampal kindling induces a selective enlargement of active zones in perforated axospinous synapses formed by stimulated axons. Since the active zone is the site of intracellular transmission, its enlargement involving only a certain subpopulation of synapses provides a likely structural substrate of synaptic plasticity associated with kindling.

Serial sectioning, electron microscopy, and three-dimensional reconstruction of fine nerve fibres and other extended objects

Two different techniques for three-dimensional reconstructions of nerve fibres or other extended fine tissue structures were developed to study the afferent innervation of the cat's knee-joint capsule. The re-embedding technique starts with series of semithin sections for light microscopic reconstructions. In a second step, selected semithin sections can be re-embedded for ultramicrotomy to examine ultrastructural details. This method offers the possibility to investigate fine structures over a distance of several hundred micrometres without any loss of ultrastructural information. The serial section-ESI technique is based on the electron spectroscopic imaging of semithin sections. Small tissue blocks are cut into series of either semithin or alternate semi- and ultrathin sections which can be directly used for a complete ultrastructural investigation. Finally, true-to-scale three-dimensional reconstructions are performed by graphical techniques or computer-aided methods.