Some morphological observations on osteoclasts

Osteoclasts of the peripheral portions of the endocranial aspect of young rat parietal and frontal bones were studied by scanning electron microscopy of glutaraldehyde fixed, critical point dried specimens. These studies show osteoclasts to have a much more complicated form than has previously been realised. Extensively branching, elongated, smooth-surfaced cells, which are for the most part elevated above the level of the surrounding bone matrix surface and sometimes above portions of osteoblasts or other osteoclasts, were identified as motile non-resorbing cells. Portions of the former and other entire cells may be embowered in Howship's lacunae, have microvilli on their dorsal surface, and are surrounded by a serrated border of microprojections which have an apparently firm attachment to the matrix surface. Osteoclasts in short term culture show additional free surface ruffles which are not encountered in specimens taken fresh from the animal. No evidence of recruitment of osteoblasts or osteocytes into osteoclasts was found. Disinterred osteocytes retained an ability to migrate from their lacunae on to surrounding bone matrix surface.

Short latency potentials recorded from the neck and scalp following median nerve stimulation in man

Short latency evoked potentials were recorded from sites overlying the cervical and thoracic vertebrae, the clavicles, mastoid processes and cerebral cortex, following percutaneous stimulation of median nerve fibres at the elbow, wrist and fingers in 23 normal subjects. At least four major early components each with simultaneous positive and negative constituents, plus the first component (N20) of the cortical response, were all found to be mediated by sensory afferent fibres with conduction velocity 65--75 m/sec in the forearm of one subject. Study of the distribution of these potentials, using reference electrodes located at Fz or over the lower part of the spine, has led to the proposal of generator sites in the brachial plexus (N9), spinal roots or dorsal columns (N11), spinal grey matter or brain stem (N13), and brain stem or thalamus (N14). Comparison with intrathecal recordings in man lends support to the view that N11 and N13 are generated in or adjacent to the spinal cord. It is hoped the findings may extend the clinical applications of a non-invasive technique for investigating the afferent sensory pathways in man.

The migration of osteoblasts

The endocranial matrix surfaces of parietal bones of 2-week old Albino Wistar rats were partly denuded of osteoblasts and then cultured for various periods up to 24 h, in control or PTE-enriched medium. They were examined by scanning electron microscopy and evidence for cell locomotion was found. Osteoblasts traversed the denuded bone surface and cut edges of bone in either medium, and cells also migrated out from vascular channels. Glass spicules were placed on the otherwise undisturbed osteoblast layer in similar organ cultures for 2, 3 or 5 days. Osteoblasts migrated from the bone to populate the glass, negotiating any angle. The cells in PTE-enriched media were always aligned parallel to one another and elongated, tended to align with the edges of the glass and, in time, formed a substrate of aligned fibrils whose axes were parallel to those of the cells. Osteoblasts in control medium on glass showed variable degrees of alignment and elongation and were less influenced by edges of the glass. Non-locomotory, nearly equidiametrical cells on glass in 5d control cultures had formed a substrate of randomly oriented fibrils. Migrating osteoblasts on bone matrix did not have leading edge ruffles; isolated, migrating ones on glass did.

A study of the arrangement of osteoblasts of rat calvarium cultured in medium with, or without, added parathyroid extract

The osteoblast layers on the inner surfaces of the half-calvaria of neonatal rats of 2 litters were studied by scanning electron microscopy. Some halves were fixed immediately on dissection. Others were carried through incubation in an Eagle's medium, with or without the addition of parathyroid extract (PTE): for 2, 4, 8 or, most importantly, 24 h. In their initial state, presumed as like the o-h material, the osteoblasts presented as a dense tesselation. After 24 h of culture in the medium, osteoblasts were more sparse in the fields examined and although, in a set of 4 preparations, they retained significant alignment, their orientation had become more random. In addition, they were broader and larger in area. By contrast, with PTE added to the medium, osteoblasts almost maintained their number per field, did maintain their cell area, but became longer and narrower and strikingly parallel in their array. The aligning effect of the PTE preceded, and thus was independent of, its elongating effect.

Experimental study of changes in osteoblastic shape induced by calcitonin and parathyroid extract in an organ culture system

Neonate rat endocranial osteoblasts were cultured on their bone surfaces in control medium (CC) or medium to which either parathyroid extract (PTE) or calcitonin (CT) had been added for 2, 4, 8 or 24 h. Some were cultured for 24 h in CC, then for 2, 4, 8 or 24 h in either CT or PTE medium; or for 24 h in PTE, then for 2, 4, 8 or 24 h in either CC or CT; or 24 h in CT and 2, 4, 8 or 24 h in CC. The dorsal ruffling of the cells in CC was found to be suppressed by later culturing with PTE and the disoriented cells reorganized to form arrays of parallel cells. The effects of PTE were also reversed by CC or CT: the osteoblasts in the second culture (CC) lost elongation and order, and proceeded through a proliferative phase before exhibiting the ruffling form similar to a single CC 24 h culture. PTE-cultured osteoblasts showed an increase in cell overlap and contact so that a more competent barrier was formed separating the bone from the medium. In control or CT medium, however, intercellular gaps were greater than in vivo.

Scanning electron microscopy of bone cells in culture

Embryonic and young rat bone cells have been growing in culture and examined in the scanning electron microscopy (SEM). Compared with cells fixed in situ and taken directly from the animal, the cultured osteoblastic cells were smoother, flatter and more extensive and showed tighter intercellular contacts. Some matrix is formed in culture and undergoes at least partial mineralization as judged by the accumulation of Ca and P measured by energy dispersive x-ray analysis. Findings concerning the morphology of the collagen arrangement were indecisive. Some superficial cells, free of surrounding matrix, resembled osteocytes in normal in vivo bone. This may indicate that a proportion of the extracellular matrix produced by the cultured cells failed to polymerise into recognizable bone matrix, and that osteocytic morphology is not dependent upon the physical characteristics of the bone matrix.

Morphological changes of osteoblasts in vitro

The appearance of neonate rat endocranial osteoblasts exposed in situ and fixed immediately was compared with that of similar osteoblasts organ cultured for short periods of up to 48 hours in control medium and serum alone, or with added parathyroid extract (PTE). Normal osteoblasts showed a range of variation in size and shape, degree of elongation and orientation. Culturing the osteoblasts resulted in an overall loss of elongation and ordering of the cells, and the production of dorsal ruffles which were more complex and larger in the longer culture times. PTE added to the culture medium caused an increase in cell elongation and a striking recordering of the osteoblasts into domains of parallel cells. The swirling patterns made by these domains were similar to those of the underlying bone collagen. Ruffles, where present, were small and more often peripheral than dorsal. The results indicate that fully differentiated osteoblasts are able to adapt very rapidly to survival and function in culture conditions, and that the endocytosis necessary for synthetic activity is suppressed by PTE although some cell movement may continue.

Osteoblasts and collagen orientation

Bone was removed from the calvaria of anaesthetized 70 g rats or freshly killed young monkeys and the fibrous periosteum dissected off the inner, formative surface under 0.15 M cacodylate buffer. The bone and undisturbed osteoblasts were fixed in 3% glutaraldehyde in the same buffer for 24 to 48 hours, critical point dried and coated with evaporated carbon and gold for scanning electron microscopy (SEM). Fields of osteoblasts were photographed and chosen cells dissected off the osteoid using a tungsten needle. The control of the dissection was made possible by the use of a system of real-time stereo tv-speed SEM. The fields were rephotographed and the orientations of the osteoblasts were compared with that of the underlying collagen fibres. 62% of all osteoblasts lay with their long axes within 15 degrees of the collagen fiber orientation below and 80% within 30 degrees. Montages of large areas of osteoblasts were also made, and then compared with ones of the same area after the cells had been stripped off on adhesive tape. In general, the orientation of the collagen tended to be the same as the cell that formed it. Collagen fibers below cells at the periphery of a domain sometimes had the orientation of the cells in the adjacent patch. It is not possible to determine whether the cells controlled the orientation of the collagen, or vice versa, from this experiment, but other SEM evidence suggests that the collagen orientation in hard tissue matrices depends on the freedom of cells to move with respect to matrix surface.

Morphology of calculus formation on the human tooth surface

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