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mucosal folds (Youson and Connelly, 1978). The features of degeneration,
differentiation, and proliferation in the transformation of the epithelium
and submucosal tissues over seven stages of metamorphosis are provided
in a light and electron microscopic study (Youson and Horbert, 1982). The
intestinal epithelium of adult lampreys develops from surviving cells of the
larval primary epithelium; this is in contrast to the situation in amphibians
where the adult epithelium (secondary epithelium) develops from nests of
undifferentiated cells in the larval epithelium.
2.8.1.5 Endocrine pancreas
The larval lamprey has insulin-containing cells arranged as follicles
within the submucosal tissue near the junction of the oesophagus and the
intestine. (Fig. 12). There is some variability of the distribution of these
follicles between northern and southern hemisphere lampreys relative to
their intestinal diverticula (Youson and Al Mahrouki, 1999; Youson, 2000,
2007). As noted above, the extrahepatic common bile duct in larva drains
the liver of bile products and empties into the intestine at its junction with
the oesophagus in northern hemisphere species but into the cephalic end
of the left diverticulum in southern hemisphere species (Fig. 13). Adults
of the southern hemisphere species have a cranial principal islet near the
cardiac region that is believe to develop from larval follicles that migrate
and proliferate during metamorphosis (Hilliard et al . , 1985). In contrast,
northern hemisphere species have both a cranial and caudal principal
islet. Although some larval follicles may contribute to the development
Figure 13. Diagrammatic representation of the distribution of pancreatic tissue in islets (P)
of a larva (a) and an adult (c) of G. australis and a larva (b) and an adult (d) of P. marinus .
Note the difference in position of the bile duct (BD) and the size of diverticula (D) in larva of
the two species and that G. australis adult has only a single, large cranial pincipal islet (CR).
The P. marinus adult has both a CR and a caudal principal islet (CD) connected by a stand of
intermediate (I) pancreatic islets. The CD arose during metamorphosis from the epithelium
of the BD, but no such contribution occurred during metamorphosis in G. australis (Modifi ed
from Youson and Elliott, 1989).
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