RESUMEN
Intercellularly aligned microtubule arrays are present in cell layers associated with the growth and secretion of scales in the zebra fish Brachydanio rerio and the neon tetra fish Hyphessobrycon innesi. The layers in question are: the osteoblast layer that covers the ossified outer surface of a scale, and the layer of fibroblasts that is situated immediately underneath the inner collagenous surface of a scale's fibrillary plate. In certain osteoblasts, the proximal portions of microtubules (with respect to centrosomes) run closely alongside the anterior margin of each cell where it flanks one of a scale's ridge-shaped circuli. These osteoblasts and microtubule portions are arranged in aligned rows that are parallel to circuli. However, the distal portions of the microtubules curve into an orientation that is approximately at right angles to circuli and they are aligned with each other and similar microtubule portions in adjacent osteoblasts. Such microtubule alignments only occur in osteoblasts that are associated with circuli. In Hyphessobrycon osteoblasts situated elsewhere on a scale's surface, microtubules radiate from cell centres but their distal portions curve into alignment with each other and are oriented alongside cell margins. The proximal portions of fibroblast microtubules radiate from centrally positioned centrosomes but the distal portions curve into alignment with each other and distal microtubule portions in neighbouring fibroblasts. The overall pattern of microtubule alignment is similar to that of collagen fibres, which these fibroblasts are secreting onto the fibrillary plate. The immunofluorescence protocol that was used to demonstrate the microtubule alignments described above did not reveal such alignments in the osteoblast and fibroblast layers associated with scales of the brown trout Salmo trutta fario. These findings are assessed in terms of intra-and inter-cellular control of microtubule alignment, and decentralized reorientation of microtubules at distances of several micrometres from centrosomal microtubule-organizing centres. The functional significance of the relationships between microtubule alignment and supracellular patterns of alignment that take place as collagen deposition and ossification proceed during scale formation is also considered.
Asunto(s)
Cipriniformes/anatomía & histología , Fibroblastos/ultraestructura , Microtúbulos/ultraestructura , Osteoblastos/ultraestructura , Animales , Matriz Extracelular/ultraestructura , Técnica del Anticuerpo Fluorescente , Microscopía Fluorescente , Trucha/anatomía & histología , Pez Cebra/anatomía & histologíaRESUMEN
Distinct changes in epidermal cell shaping largely define the overall pattern of growth and form during generation of the ectodermal ridge and early stages of fin fold morphogenesis. The epidermal portion of the ridge and early fin fold are formed from a strip of epidermal cells that is only six to nine cells wide. There is apparently no increase in the number of these cells during initial formation of the ridge and its subsequent conversion into a fin fold which contains extracellular matrix fibres. Epidermal cells adopt a wedge-shaped morphology during ridge production. Distinct changes in the shaping and contact relationships between basal portions of these cells generate intercellular spaces at several discrete loci within the ridge. These spaces become continuous with each other to form a subepidermal space. Hence, the subepidermal space is not produced by straight-forward folding of an epidermal sheet. Cells flanking the sides of the ridge start to flatten as it is converted into a fin fold. A continuous row of distinctive cells is positioned along the apex of the developing fold. The term 'cleft cells' is suggested for these apical cells. Each cleft cell retains a wedge-shaped form during fold formation and develops a basal cleft-shaped invagination. Invaginations are aligned in neighbouring cleft cells so that these cells cap the distal boundary of the subepidermal space where collagenous extracellular fibres called actinotrichia run anteroposteriorly along the length of the fin fold. This orientation is in direct contrast to the proximodistal orientation of actinotrichia within the remainder of the subepidermal space. During early stages of fold production a temporary set of previously unreported extracellular cross fibres spans the subepidermal space at right angles to actinotrichia. These configurations of extracellular fibres could be advantageous for maintaining the structural integrity of the early fin fold.