Lamprey Metamorphosis - Metamorphosis in Fish - page 47

the cartilages of the head and branchial regions are much more complex
in adults and new cartilages must appear between these two intervals of
the life cycle.
There have been numerous studies on the nature of this skeleton in
the past three decades. Fine structural studies indicated that the fi brous
component of the extracellular matrix (ECM) of annular, piston, cranial,
and dorsal plate cartilages was not collagen but a network of randomly
arranged and branched fi brils, 15-40 nm in diameter (Wright and Youson,
1983). However, there are some fundamental differences in the ultrastructure
of nasal, branchial and pericardial cartilages compared to trabecular, piston,
and annular cartilages with respect to the composition and extent of the
ECM and the degree of cellular composition (Wright et al . , 1988). Although
all the lamprey cartilage matrices stained with Verhoeff's elastic stain, other
biochemical features and the amino acid composition of the ECM proteins,
suggested that the major protein of the ECM is similar to, but not identical
to elastin and it was called lamprin (Wright et al . , 1983). Subsequently, three
lamprin cDNAs were isolated and their derived protein sequences described
(Robson et al . , 1993). The structure of the lamprin protein variants showed
randomly repeated sequences that are similar to sequences in mammalian
and avian elastin and some invertebrate structural proteins, such as spider
dragline silk (Robson et al . , 1993). Repeated specifi c hydrophobic domains
within the lamprin protein may be responsible for the elastic staining. There
are multiple genes for lamprin in the two species that have been studied
(Robson et al . , 2000). The primary difference between ECMs of the two
groups of cartilages listed above is that nasal, branchial, and pericardial
cartilages also contain elastin-like fibers and elastin-immunoreactive
material in a portion of their ECM (Wright et al . , 1988). The addition of
elastin-like material is consistent with the need for fl exibility and elasticity to
cartilages that surround or support structures that undergo periodic tensile
stresses such as the heart (pericardial) and the gill (branchial). Furthermore,
molecular analyses have shown that the major matrix proteins of branchial
and pericardial cartilages do not contain lamprin, but a related protein
containing hydroxyproline (Robson et al . , 1997).
A unique skeletal structure, called mucocartilage, is present in larval
lampreys (Wright and Youson, 1982) with only the larval neurocranium
and branchial arches composed of cartilage (Fig. 9). The larval cartilages
(except mucocartilage) contain lamprin but the branchial cartilages have
fewer fi brils and more extracellular proteoglycan, supposedly giving the
“springiness” attributed to adult branchial cartilage (Wright et al . , 1988).
The mucocartilage in larva is avascular and consists of a proteoglycan
aggregate, microfi brils, and a few fi broblasts. Mucocartilage does not stain
for elastin or have other biochemical properties shown for lamprin (Wright
and Youson, 1982). However, there seems to be differential expression of the