Cyclical phenomena occurring during the maturation of the enamel of rat incisor teeth. Their manifestation during drying

A pattern of obliquely oriented bands has been demonstrated at the surface of the maturation zone enamel of freshly dissected rat incisor teeth as they dry. This pattern, of which there is no evidence in the fresh, wet, or completely dry teeth, consists of up to 4 or 5 pale grey, translucent lines separated by wider, whiter, more opaque bands and has been shown to correlate directly with a similar pattern seen on the same teeth after staining with toluidine blue and previously described as the maturation cycle banding pattern (Boyde and Reith 1982). A second pattern comprised of much more closely spaced bands is also described. In this pattern, which again correlates with a similar pattern seen after toluidine blue staining, translucent and opaque bands cross the maturation zone more transversely and have a width of about 200 microns, approximating to 8 h tooth growth. It is postulated that these banding patterns reflect alternately different drying rates of the maturation zone enamel and that they may correspond to cyclical changes in the hydrophobicity and hydrophilicity of enamel matrix both on a daily basis and on a larger time scale.

Simple collectors for cathodoluminescence in the SEM made from aluminium foil

Inexpensive cathodoluminescence collectors for scanning electron microscopes can easily be made from aluminium foil fashioned as tubes which abut against the window of the photomultiplier at one end and shroud the specimen at the other. Their use in the study of fluorescent labelled mineralized tissues is illustrated.

A new method of scanning electron microscopy for imaging biological tissues

The scanning electron microscope (SEM) has proved of little value in the examination of material prepared for light microscopic histology. One of the chief reasons for this is that the secondary electron signal used for image formation in routine scanning microscopy derives from the surface of the specimen. In the case of histological material this surface is one which has been severely distorted by processing and cutting procedures. Light microscopy sections can be usefully studied in te SEM if the signal used to form the image derives from a considerable portion of the thickness of the section. Thus the backscattered electron (BSE) image has been successfully used in studying the distribution of dense material or densely staining components several micrometres deep to the surface of dried sections. Such sections are, however, usually mounted on low density (poorly BSE reflecting) non-transparent substrates such as beryllium or carbon, so that matching light microscopy of the same samples is not possible. We report here a method by which histological sections mounted on glass slides can be imaged in the SEM at a resolution higher than that obtained using conventional light microscopy. The method exploits the facts that the ordinary, cheap light microscope slide is strongly cathodoluminescent, yet the standard histological (7 micrometers) section is of such a mass thickness that it absorbs a significant proportion of electrons which energies (5-20 keV) usually used in biological SEM. Thus the measure of the glass cathodoluminescence signal is the measure of the electron flux passing through the specimen.

Presynaptic inhibition, EPSP amplitude, and motor-unit type in triceps surae motoneurons in the cat

1. Composite group Ia excitatory postsynaptic potentials (EPSPs) produced by heteronymous nerve stimulation were recorded from triceps surae motoneurons of barbiturate-anesthetized cats. Motoneuron rheobase, input resistance, and axonal conduction velocity were measured, and motor units were classified on the basis of the mechanical responses of their muscle units. 2. The amplitude of EPSPs recorded from 33 medial gastrocnemius (MG) motoneurons ranged from 0.6 to 4.3 mV. The mean EPSP amplitude differed among the major MG motor-unit types, increasing in the order fast twitch, fast fatiguing (FF); fast twitch, fatigue resistant (FR); slow twitch, fatigue resistant (S) (FF less than FR less than S). The amplitude of EPSPs recorded from 15 soleus motoneurons ranged from 0.3 to 3.4 mV, with a mean of 1.4 mV. 3. Presynaptic inhibition of EPSPs was produced by trains of conditioning volleys in the posterior biceps-semitendinosus (PBST) nerve. In 33 MG cells PBST conditioning stimulation reduced the amplitude of EPSPs by 11-50%, with a mean inhibition of 27%. The amplitude of EPSPs in 15 soleus motoneurons was decreased by 5-84%, with a mean inhibition of 37%. 4. When the magnitude of presynaptic inhibition was expressed as percent inhibition, there was no relation between presynaptic inhibition and either motor-unit type or the amplitude of the EPSP. However, when presynaptic inhibition was expressed as the absolute amount of inhibition in millivolts, the magnitude of inhibition was highly correlated with EPSP amplitude both across the entire triceps surae population (MG, lateral gastrocnemius, soleus) as well as within each muscle population. This correlation was also significant within the MG FF and FR motor-unit populations. 5. We conclude that EPSP amplitude and not motor-unit type is the major determinant of the magnitude of presynaptic inhibition. However, because of the effect of motor-unit type on EPSP amplitude, the net effect is that presynaptic inhibition increases in the order FF less than FR less than S.

A new method of image formation using cathodoluminescence in the SEM: CL absorption by superficial stains

The distribution of superficial histological stains on tooth samples can be demonstrated in the scanning electron microscope (SEM) due to the absorption by the stain of the auto-cathodoluminescence (light) arising from the underlying substrate. Practical procedures by which this has been achieved are described.

Tetracycline cathodoluminescence in bone, dentine and enamel

The emission of light under electron bombardment (cathodoluminescence-CL) in an SEM has been demonstrated from tetracycline and tetracycline labelled areas of bone, dentine and secretory and maturation zone enamel. CL images could be compared with backscattered (BSE) and secondary (SE) electron images in (a) fresh, air-dried or freeze-dried samples taken without fixation at a short term after labelling: (b) polished block faces of fixed, plastic embedded tissue and (c) cut section surfaces cut through fixed and unfixed tissue. These results indicate a considerable future potential for the tetracycline-CL-SEM mode in experimental studies on teeth and histopathological and growth studies of bone.

Acute FeCl3-induced epileptogenic foci in cats: electrophysiological analyses

Fifteen cats were rendered acutely epileptic via transpial iontophoresis of ferric ions. Extracellular electrical activity of individual cortical neurons was temporally correlated with ECoG epileptic spikes. Each unit's activity was characterized as excited, inhibited, or unaffected. The spatial coordinates of each neuron were logged, thereby generating a 3-dimensional activity map. Discharge patterns were further classified as bursting or non-bursting. A significantly increased population of inhibited cells was detected at radial distances greater than 3 mm from the iontophoretic site. This increase was at the expense of "unaffected-bursting" cells. Pathological discharge patterns, including units which fired in long-first-interval bursts, were common in the focus. A system was designed to electronically calculate single-unit firing probability before, during and after surface epileptiform spikes. An 8-12 Hz oscillation in firing probability level, time-locked to the epileptic spike, was observed in most excited-bursting units. Many of these findings in the ferric-induced epileptic focus accurately model the electrophysiological characteristics previously reported for human epileptic foci examined at neurosurgery.

Collagen synthesis in the muscle of developing chick embryos

Radioactive protein was prepared from the leg muscle of chick embryos, 11, 14, 16 and 17 days old, each injected with radioactive proline and incubated for 30, 60 or 90 min afterwards. The radioactive protein was incubated with collagenase purified by chromatography on a Sephadex G-100 column. Under this condition, only collagen is digested into products soluble in trichloroacetic acid. The relative rate of collagen synthesis was determined by comparing the amount of radioactivity released into the supernatant fraction and that in the residue, by the method of Diegelmann & Peterkofsky [(1972) Dev. Biol. 28, 443--453]. The results show that the rate of collagen synthesis remains at approx. 10% of the rate of synthesis of other non-collagenous proteins during the development of chick embryonic muscle from 11 to 17 days. This suggests that the synthesis of collagen and that of other proteins are co-ordinately regulated at these stages of development.