Abstract
The wall of the egg case of the dogfish,Scyliorhinus canicula, contains a network-forming collagen assembled into a regular three-dimensional (3D) structure. It accomplishes supportive, protective and filtering functions for the embryo contained within it. The collagen molecules in the egg case are organized into a body-centred unit cell of dimensions (mean ± s.d.) (11.6 ± 1.0) nm X (11.6 ± 1.0) nm X (81.6 ± 3.2) nm, which belongs to the I422 space group. At a higher hierarchical level, the collagen molecules assemble into parallel arrays of fibrils, ca. 100 nm in diameter, which aggregate to form laminae ca. 0.5 μm thick. These laminae are organized into a plywood-like structure and account for 98% of the thickness of the wall of the egg case. X-ray diffraction patterns of the wall of the egg case were taken along mutually perpendicular directions, one being perpendicular to the surface of the egg case. Three different kinds of diffraction pattern were observed. One of the patterns was characteristic of an X-ray direction perpendicular to the laminae in the egg case (along the x-direction). The two other patterns were obtained with the X-rays directed parallel to the plane of the laminae, either along the capsule long axis (z) or perpendicular to this (y). These two patterns were observed interchangeably in either of the x- or y-directions depending on the specimen. The diffraction patterns were analysed and interpreted taking into consideration the 3D electron microscope data of the egg case. The results confirm and extend previous findings from transmission electron microscopy and low-angle X-ray diffraction and they suggest that there is only one major type of ordered collagen arrangement in the wall of the egg case.
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Selected References
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