406
ribbons. These overlap within the lamella, like tiles on a roof. The ribbons are held
together by small quantities of amorphous matrix giving a ‘sea and island’ construction; in which the cross-sectional area of the islands is much greater than that of the
sea. The ribbons are continuous throughout the length of the egg case. Each ribbon
is formed from numerous transversely banded collagen fi brils arranged in a remarkably regular way that varies somewhat in different regions of the egg case.
Each collagen fi bril is practically crystalline, being constructed from a tetragonal
array of collagen molecules. These are regularly kinked approximately midway
between their ends; with the kinked segments running at approximately 20° to the
long axis of the fi bril. The molecules are held together by extensive covalent crosslinking. This contributes to the material’s mechanical and thermal stability, its
extreme insolubility, and its resistance to enzymatic degradation,” (Knight and
Vollrath 2001 ; see also Knight et al. 1996 ).
Glycoxidative, dityrosyl, lysine-derived, and oxidative phenolic covalent
cross- links (Hepworth et al. 1994 ) may contribute to the stability of this biological material. The role of disulphide bonds is also possible. The short length
(approximately 40 nm) of the triple-helical collagen molecules with molecular
weight of 34–35 kDa (Fig. 12.3 ) and an axial ratio (length to width) of about 20
make them ideally suited for nematic liquid crystallization (Knupp et al. 1998 ,
1999 ; Knight and Vollrath 2001 ).
The secretory cells of the D-zone of the nidamental gland of the dogfi sh are
responsible for secretion and storage of the collagen of the egg capsules ( RusaouenInnocent 1990 ). This protein assembles within the Golgi apparatus and migrates
through the cytoplasm within storage granules. It “appears to pass through several
morphologically distinct textures during storage, secretion and fi bril formation
which may represent different lyotropic liquid crystalline phases,” as described in
detail by Feng and Knight ( 1994b ).
Interestingly, the collagen storage granules showed very specifi c structural
behaviour in response to changes in pH for in vitro experiments (Feng and Knight
1994b ). Thus, “from pH 2 to pH 4 most granules appeared completely amorphous;
from pH 5 to pH 7 granules showed the following previously reported liquid crystalline textures: isotropic, lamellar, micellar, hexagonal columnar, transversely banded
twisted nematic, and unhanded twisted nematic. At pH 8 granules showed both the
hexagonal columnar phase (phase IV) and small quantities of the fi nal fi brillar
phase, together with a previously undescribed texture. The latter texture, which we
refer to as phase VII, had a D period (17.5 nm) half that of the lamellar texture
(phase II), and the fi nal egg capsule fi brils (phase VI). From pH 9 to pH 11, only the
fi nal fi brillar texture (phase VI) and small quantities of the new texture (phase VII)
were present,” (Feng and Knight 1994b ).
David Knight suggested the following causal sequence for collagen fi brillogenesis, orientation, storage and secretion, in this system:
“The collagen is maintained under acidic conditions during storage and secretion.
Provided that the isoelectric point is fairly close to neutrality as it is in other collagens, the molecules will carry a net positive charge under these conditions. This will
prevent them from approaching one another close enough to form intermolecular
12 Egg-Capsule Proteins of Selachians
ribbons. These overlap within the lamella, like tiles on a roof. The ribbons are held
together by small quantities of amorphous matrix giving a ‘sea and island’ construction; in which the cross-sectional area of the islands is much greater than that of the
sea. The ribbons are continuous throughout the length of the egg case. Each ribbon
is formed from numerous transversely banded collagen fi brils arranged in a remarkably regular way that varies somewhat in different regions of the egg case.
Each collagen fi bril is practically crystalline, being constructed from a tetragonal
array of collagen molecules. These are regularly kinked approximately midway
between their ends; with the kinked segments running at approximately 20° to the
long axis of the fi bril. The molecules are held together by extensive covalent crosslinking. This contributes to the material’s mechanical and thermal stability, its
extreme insolubility, and its resistance to enzymatic degradation,” (Knight and
Vollrath 2001 ; see also Knight et al. 1996 ).
Glycoxidative, dityrosyl, lysine-derived, and oxidative phenolic covalent
cross- links (Hepworth et al. 1994 ) may contribute to the stability of this biological material. The role of disulphide bonds is also possible. The short length
(approximately 40 nm) of the triple-helical collagen molecules with molecular
weight of 34–35 kDa (Fig. 12.3 ) and an axial ratio (length to width) of about 20
make them ideally suited for nematic liquid crystallization (Knupp et al. 1998 ,
1999 ; Knight and Vollrath 2001 ).
The secretory cells of the D-zone of the nidamental gland of the dogfi sh are
responsible for secretion and storage of the collagen of the egg capsules ( RusaouenInnocent 1990 ). This protein assembles within the Golgi apparatus and migrates
through the cytoplasm within storage granules. It “appears to pass through several
morphologically distinct textures during storage, secretion and fi bril formation
which may represent different lyotropic liquid crystalline phases,” as described in
detail by Feng and Knight ( 1994b ).
Interestingly, the collagen storage granules showed very specifi c structural
behaviour in response to changes in pH for in vitro experiments (Feng and Knight
1994b ). Thus, “from pH 2 to pH 4 most granules appeared completely amorphous;
from pH 5 to pH 7 granules showed the following previously reported liquid crystalline textures: isotropic, lamellar, micellar, hexagonal columnar, transversely banded
twisted nematic, and unhanded twisted nematic. At pH 8 granules showed both the
hexagonal columnar phase (phase IV) and small quantities of the fi nal fi brillar
phase, together with a previously undescribed texture. The latter texture, which we
refer to as phase VII, had a D period (17.5 nm) half that of the lamellar texture
(phase II), and the fi nal egg capsule fi brils (phase VI). From pH 9 to pH 11, only the
fi nal fi brillar texture (phase VI) and small quantities of the new texture (phase VII)
were present,” (Feng and Knight 1994b ).
David Knight suggested the following causal sequence for collagen fi brillogenesis, orientation, storage and secretion, in this system:
“The collagen is maintained under acidic conditions during storage and secretion.
Provided that the isoelectric point is fairly close to neutrality as it is in other collagens, the molecules will carry a net positive charge under these conditions. This will
prevent them from approaching one another close enough to form intermolecular
12 Egg-Capsule Proteins of Selachians