Comparison of mammalian collagen and elasmobranch elastoidin fiber structures, based on electron density profiles

Involvement of the sonic hedgehog, patched 1 and bmp2 genes in patterning of the zebrafish dermal fin rays

The signaling molecule encoded by Sonic hedgehog (shh) participates in the patterning of several embryonic structures including limbs. During early fin development in zebrafish, a subset of cells in the posterior margin of pectoral fin buds express shh. We have shown that regulation of shh in pectoral fin buds is consistent with a role in mediating the activity of a structure analogous to the zone of polarizing activity (ZPA) (Akimenko and Ekker (1995) Dev. Biol. 170, 243–247). During growth of the bony rays of both paired and unpaired fins, and during fin regeneration, there does not seem to be a region equivalent to the ZPA and one would predict that shh would play a different role, if any, during these processes specific to fish fins. We have examined the expression of shh in the developing fins of 4-week old larvae and in regenerating fins of adults. A subset of cells in the basal layer of the epidermis in close proximity to the newly formed dermal bone structures of the fin rays, the lepidotrichia, express shh, and ptc1 which is thought to encode the receptor of the SHH signal. The expression domain of ptc1 is broader than that of shh and adjacent blastemal cells releasing the dermal bone matrix also express ptc1. Further observations indicate that the bmp2 gene, in addition to being expressed in the same cells of the basal layer of the epidermis as shh, is also expressed in a subset of the ptc1-expressing cells of the blastema. Amputations of caudal fins immediately after the first branching point of the lepidotrichia, and global administration of all-trans-retinoic acid, two procedures known to cause fusion of adjacent rays, result in a transient decrease in the expression of shh, ptc1 and bmp2. The effects of retinoic acid on shh expression occur within minutes after the onset of treatment suggesting direct regulation of shh by retinoic acid. These observations suggest a role for shh, ptc1 and bmp2 in patterning of the dermoskeleton of developing and regenerating teleost fins.

Elastoidin turn-over during tail fin regeneration in teleosts. A morphometric and radioautographic study

During teleostean fin regeneration the actinotrichia, rods of a collagen-like protein, the elastoidin, are immersed in the blastema, maintaining their apical position. In this epimorphic event the latter fact might be achieved by either a cellular carriage or a continuous turn-over of these hyperpolimerized fibrils. By means of a 3H-proline pulse and radioautographic chase experiment of the isolated actinotrichia we have found a turn-over of collagen within the structure. From these and additional morphometric results, we present in this work an operational hypothesis of how gradually differentiating blastemal cells and an appropriately shaped basal lamina, can control the number and distribution of actinotrichia which might be under the balanced control of their synthesis and degradation.

Axial electron density of human scleral collagen. Location of proteoglycans by x-ray diffraction

The low angle meridional x-ray diffraction pattern from fresh human sclera was analyzed to ascertain if collagen-bound proteoglycans affect the axially-projected electron density distribution to the same extent as appears to occur in the cornea. The results showed that, unlike cornea, the electron density of the sclera is similar to that seen in rat tail tendon collagen. The proteoglycans were specifically stained using either Cuprolinic blue or Cupromeronic blue, both under critical electrolyte conditions. The tissue was then examined by electron microscopy and by low angle x-ray diffraction. The electron-optical observations suggested that proteoglycans associate with collagen near the d/e staining bands in the gap zone. A difference Fourier analysis from the x-ray results confirmed that these observations were not e.m. preparative artefacts and allowed a quantitative estimate to be made of the axial extent of the proteglycans in the wet tissue.

Glutaraldehyde-induced changes in the axially projected fine structure of collagen fibrils

The fine structure of the collagen fibril, as seen in axial projection, is changed by treatment with glutaraldehyde. The changes are detectable in electron-optical staining patterns and in the intensities of the low-angle meridional X-ray diffraction maxima. Current knowledge of the amino acid sequence of collagen and of the axial arrangement of molecules in fibrils permits interpretation in terms of specific alterations to the axial distribution of electron density along the fibril. Analysis of fibril staining patterns from glutaraldehyde-treated calf skin collagen shows that uptake of staining ions in positive staining patterns is inhibited at residues known to interact with glutaraldehyde (lysyl, hydroxylysyl and probably histidyl side-chains) and on other charged residues in the immediate neighbourhood of the glutaraldehyde-reactive residues. This can be seen as a "stain-exclusion effect" due to the presence of bulky polymeric complexes of glutaraldehyde molecules at cross-linking sites. Such stain exclusion accounts for the drastic changes in the negative staining pattern following treatment with glutaraldehyde. The intensity changes observed in the low-angle meridional X-ray reflections from rat tail tendon, similarly treated, also can be explained by the presence of these bulky complexes. Existing data have been used to predict a model of the altered electron density profile indicating the axial distribution of glutaraldehyde along a D-period of moist tendon collagen.

An X-ray diffraction analysis of rat tail tendons treated with Cupromeronic Blue

Cupromeronic Blue was used to stain selectively the proteoglycans in rat tail tendons under 'critical electrolyte' conditions. Earlier electron microscopical observations indicated that at least one type of proteoglycan filament is associated with tendon collagen fibrils at the positive staining band 'd'. To ensure that this was not an artefact caused by specimen preparation or the subsequent positive staining of the collagen fibrils, we have analysed low angle meridional diffraction patterns from stained but not dehydrated, embedded or counterstained tissues. Axial electron density profiles of Cupromeronic Blue-stained compared with unstained rat tail tendons revealed the axial locations and relative amounts of dye in both mature and young wet specimens. In mature tendons, the difference electron density profile contained a broad peak centred near residue 180 along the 234-residue D-period. This corresponds to the electron-optical staining band 'd'. In young tendons a similar distribution of stain was observed although in this case there was evidence of a doublet of peaks, one centred near residue 182 (band 'd') and the other near residue 165 (midway between bands d and e1). The wet proteoglycan--Cupromeronic Blue complexes distribute over about 30 nm along the collagen fibril axis. Comparison with the images of filaments seen in the electron microscope suggests that the dye complexes collapse significantly on dehydration and embedding.

Fine structural comparative peculiarities of the developing dipnoan dermal skeleton in the fins of Neoceratodus larvae

This study shows that actinotrichia are present in the growing fins of the dipnoan larvae Neoceratodus . They develop in advance of the camptotrichia , which will finally sustain the adult fin. Actinotrichia in dipnoan fins are similar to those already described in other osteichthyans . This feature strongly suggests the plesiomorphic character of the actinotrichia in the fish fins. In dipnoan , unlike other osteichthyan , actinotrichia are integrated in the growing camptotrichia and thus disappear in the adult fin. Consequently, only in dipnoan are actinotrichia a transitory dermal skeleton.

Structure and packing of dry elastoidin: a collagen phase change

Wide angle X-ray patterns of wet shark fin elastoidin always show a very intense meridional streak arising from the small angle diffraction pattern. When the fin ray spicules (ceratotrichia) are permitted to air dry, the pattern departs from the meridian. Studies show that the dry spicules are, in fact, radially anisotropic. Diffraction patterns taken with the X-ray beam perpendicular to the edge of the ribbon-shaped spicules have two distinct arms (with an interarm angle of as much as 70 degrees), while the small angle streak taken with the beam perpendicular to the ribbon face is generally either on the meridian, or is so weak that it cannot be resolved from the background. An analysis of the intensities of the small angle diffraction pattern of the dry form indicate that it has essentially a normal dry collagen structure. Addition of methanol serves only to expand the dry structure, without reversal of the phase transition. These data can be interpreted by postulating that as the wet structure collapses during drying, the collagen molecules tilt in a direct perpendicular to the face of the ribbon, forming domains of molecules with similar axial orientation. The implication is that the original interactions are lost, and the new structure is formed by overlaps between formerly non-interacting molecules.

The fine structure of the developing pelvic fin dermal skeleton in the trout Salmo gairdneri

The morphogenetic and ultrastructural features of the dermal skeleton in the pelvic fin bud of a teleost, the rainbow trout Salmo gairdneri, have been examined by light and electron microscopy. The principal structural components observed are lepidotrichia and actinotrichia. Lepidotrichia consist of two parallel and symmetrical bony demirays that form jointed segments within the fin. The demirays calcify in a proximodistal direction within the extracellular collagen network of the basal lamella belonging to the epidermal-dermal interface of the fin. Needle- and plate-like particles of a solid mineral phase appear to be associated with the collagen fibrils and with a fine, granular, interfibrillar material central to the demirays. Cellular processes and membrane-bound vesicles are absent from the regions of calcification. During fin growth, the bony, acellular lepidotrichia are separated from the epidermal-dermal interface by infiltrating mesenchymal cells in proximal fin regions; in distal areas, the lepidotrichia remain within the basal lamella. The actinotrichia are extensive unmineralized rods of elastoidin that occupy the distal margin of the fin and precede the differentiation of lepidotrichia. Once the lepidotrichia form, actinotrichia lie preferentially between their demirays. In some instances, structural interactions are suggested between actinotrichia and lepidotrichia. Considerations of embryologic and structural features of fin components fail to support the hypothesis that individual segments of lepidotrichia are modified scales in all fish.

The axial electron density in collagen fibrils from human corneal stroma

Low angle synchrotron X-ray diffraction patterns were obtained from the demembranated human corneal stroma. These patterns showed differences in the relative intensities of the meridional reflections compared to those obtained using bovine corneas. In particular, the first order reflection, conspicuously absent in the bovine pattern, is present in the human, although it is still only one third as intense as the first order in the pattern from rat tail tendon. Using phases deduced from the electron-optical images of negatively stained corneal collagens, a difference electron density map between bovine and human corneal collagen was drawn. The features in this map are explained in terms of a different inter-relationship between collagen and proteoglycans in the two species. The function of these species differences is unknown.

Elastoidin actinotrichia in Coelacanth fins: a comparison with teleosts

In this work, we present the first ultrastructural evidences of actinotrichia in the Coelacanth Latimeria. We describe its actinotrichia with the electron microscope (SEM and TEM) and compare their structure to Teleost actinotrichia. Both elements present similar fine structure, i.e. a periodic cross-striation of 60-65 nm; the plesiomorphic character of actinotrichia is discussed in Osteichthyes.