Imaging hard – inside the skeleton Timothy G. Bromage, Santiago Gomez, Alan Boyde Vertebrate hard tissue biologists study the surface and below-surface microanatomical features and compositional characteristics of bones and teeth of the skeleton (fish scales also deposit calcium phosphate and calcium carbonate into their structure, and we include them among the tissues we study). The variability in bone and tooth histology rivals that of all other organ systems, making it an ideal tissue for understanding the development, function, and physiology of organisms. What is more, in deference to soft tissues, bones and teeth survive as fossils, permitting all that we can know from the skeleton about an organism living today to be extrapolated to animals living millions of years ago. Bromage TG, Gomez S, Boyde A. Imaging hard – inside the skeleton. Royal Microscopy Society infocus 49: 4-31, 2018. 4 ISSUE 49 MARCH 2018 5 By and large, hard tissues are formed by cells that Bright-field microscopy two axes furthest from the maximum transmission air present themselves as superior surface reflection secrete an organic matrix, which then mineralises. Bright-field is the simplest yet most versatile form axes of the two filters. Brightness in images derived instruments for metrology. In addition to this, by But in some cases they may be made from soft tissues of microscopy. Specimens imaged by bright-field from linear polarising microscopes is thus not easily using an immersion objective lens or by placing a such as cartilage or ligament that subsequently microscopy - usually histological sections - are quantifiable nor always fully interpretable because of glass coverslip with a medium onto the specimen mineralise. While it may appear counterintuitive, illuminated by transmitted light from below in an this artefact. when using an objective in air, specular reflections the development of a bone often also requires upright microscope (or from above in an inverted are eliminated and the illuminating beam is allowed The linear polarising problem is eliminated by bone-removing cells to allow the re-sizing and re- microscope). The specimen must provide all of to penetrate deep to the surface. The consequence steering photons into a helical trajectory rather than shaping of a bone during development. These bone- the potential contrast. Contrast mechanisms vary, of these conditions for both laser and white light a plane one, and under this condition, the light is removing cells also assist in the maintenance of bone and those typically used in our laboratories include confocal microscopes begs the most interesting polarised in all 360 degree rotational positions; this throughout life by a process of removal of small manipulating the obliquity of the illuminating light by question; what if we drive the plane of focus of the is called circularly polarised light (CPL) (Boyde et al., packets of tissue followed by the replacement of opening-closing the back aperture, or by employing objective lens below the surface of our opaque 1984; Bromage et al., 2003b). This is accomplished by tissue by forming cells. darkfield, phase, and differential interference contrast material? The result is an image composed of only placing a complementary quarter wave filter between techniques. that light emitted or reflected from the Z region Bones and teeth are hard because of the largely the specimen and each linear polarising filter. Image occupied by the depth of focus of the objective lens. calcium and phosphate mineral deposited into Applying histological stains to sections imaged by brightness under CPL conditions is a true signal them. Regarding this point, the first mistake most bright-field microscopy also provides useful contrast; representative of the orientation of the collagen There is a lot of material in the world we would like hard tissue biologists make is to give their research in our laboratories, Stevenel’s blue and Toluidine in bone, dentine, and cementum. The contrast to image. Most of it is not in the lab, nor is there specimens to a routine histology laboratory that blue are commonly used, which, on bone, produce generated provides insight on the development and any evidence to suggest that it will come to the lab commences to demineralise their specimens and coloured optical density variability relating to function of the tissue. any time soon. Thus, we have to accept that if we make them soft! This permits the laboratory to mineralisation density differences. We do not stain wish to acquire knowledge about it at a microscopic Further contrast enhancing techniques associated make paraffin embedded thin sections using specialist tissues prior to embedding, but rather stain the scale, we have to go where the material is, and we with polarising microscopy can be obtained by knives. There are a handful of us in the world that surfaces of our thick (e.g., 100 µm) sections. These must take our microscopes with us. A big advantage introducing compensator and/or bandpass cut-off can honestly say, “we are hard tissue biologists”, and stains penetrate 5-10 µm below the surface, and of white light Nipkow disc confocal microscopes is filters or dichroic mirrors. who have the opinion that to throw away more than when imaged above an opaque white diffusing plate, that they do not require a lot of hardware, such as half of a bone or tooth’s mass is not the best starting present an image formed almost exclusively from the a laser source or power supply. We thought that point for obtaining a complete understanding of the stained thickness layer; this image is equivalent to a Confocal microscopy a portable confocal scanning optical microscope Confocal microscopes belong to the family of nature of the tissue. This is all the more important stained 5-10 µm thick decalcified paraffin-embedded (PCSOM) would satisfy if it was packed into two scanning imaging technologies. A small aperture or in light of microscopy techniques we employ and section. pieces of approved check-in luggage for air travel, if describe below to visualise the histocomposition of slit translated across the field of view transmits both it was robust to whatever it is that happens after hard tissues. However, if we wished to image the Polarising microscopy the illuminating beam and the emitted or reflected check in, and if it was adaptable to “field” conditions collagen fabric of say, a bone, in greater detail, we Birefringent materials - like hydoxy apatite with light returning from the object, which, for reasons of and operating on 110 or 220 V. A number of images may decalcify the embedded mineralised section, negative intrinsic birefringence - or stacked the geometry of the system, returns exclusively from below were taken with this system and we briefly leaving only the collagen for providing morphological structures like collagen molecules, microfibrils and the plane of focus of the objective lens. A single describe it here. contrast. fibrils, with water in between which give positive pinhole scanned across the entire field of view takes We used a PCSOM based on the Nipkow disk form birefringence, generate orientation-dependent a bit of time, and thus the image must be built up. technique, described in detail by Petran and contrast in polarising microscopy. Most polarising However, by using many thousands of pinholes on Microscopy Hadravsky (1966) and first commercialised in the A variety of microscopy technologies and techniques microscopes in the world employ a linear polariser a spinning Nipkow disc, the holes interlace in space early 1980s; applications to hard tissue biology are are employed on either intact unprepared bone and filter on the illumination side of the specimen and and integrate in time to produce a real time confocal available in Boyde (1983), and the history and various tooth tissues, or those infiltrated with and embedded a “crossed” linear analyser filter on the imaging image of the entire field of view when spun at video technical achievements in confocal microscopy are into plastic and cut with any one of a number of side of the specimen. Increasing brightness in the rate. summarised in Boyde (1995). We utilise a “single- precision saws. In our laboratories we employ image is associated with the degree to which the Laser confocal microscopes are used almost sided” Nipkow disk design in which the illumination stereo zoom and compound light microscopy, white birefringent components are in the plane of the exclusively for exciting fluorophores and detecting and detection pinholes are the same (Kino, 1995). We light and laser confocal microscopy, real time 3D section. However, the crossed linear polarising filter fluorescence with high spatial resolution in the Z configured our PCSOM to contend with the variety microscopy, and scanning electron microscopy. arrangement renders an artefactual darkness along axis. White light confocal microscopes imaging in of conditions encountered at museums around the 6 ISSUE 49 MARCH 2018 7 world (Bromage, 2003a). An interesting feature of detectors are used to observe SEs having lower Bone tissues the design of our confocal module was the solution energies than the incoming beam and that are 3D human bone tissue applied to suppress internal non-image-related ejected in large numbers from atoms encountered Figure 1 shows a 3D image of a blood vessel channel (top and centre of image) taken by conventional reflections. Linear polarising light filters and a single by the beam on the surface. Their propagation is transmission compound microscopy, that is contained within layers of human bone tissue called lamellae quarter wave plate are employed for this purpose. substantially determined by topographic variability, and imaged by circularly polarised light. 24 2D images taken 3 microns apart were acquired in a through The unintended benefit of this strategy was that and thus SE-SEM is a superior method for visualising focus series, comprising a digital 24-image data set.