Biology Reference
In-Depth Information
There have been only a few studies on the concentration of either
insulin or somatostatin during lamprey metamorphosis (for review see
Youson, 2000). Elliott and Youson (1991) were able to correlate, through
a heterologous radioimmunoassy (RIA), the increased tissue levels of
somatostatin in intestinal-pancreatic extracts with increasing development
of the cranial and caudal principal islets in
P. marinus
. A homologous RIA
was used to show signifi cantly higher insulin concentrations appear over
larval levels by stage 6 of metamorphosis in
P. marinus
(Youson et al., 1994).
These latter data indicate that the developing principal islets release their
newly synthesized hormone into the blood stream. Although the relevance
of these hormones to metamorphosis requires further study, there is some
evidence (see above discussion under
Lipogenic and Lipolytic Hormones
) that
they may be involved in intermediary metabolism in an animal that is still
relying on stored lipid reserves during this non-trophic phase of the life
cycle (Kao et al., 1998, 1999a).
2.8.1.6 Liver
The metamorphic events within the liver are probably among the most
fascinating and complex within the entire developmental process (Youson,
1981c, 1985). The larval liver has bile ducts and an intrahepatic gall bladder
and, in holarctic lamprey species, an extrahepatic bile duct empties into
the anterior intestine at its junction with the oesophagus (Fig. 12). This
entire biliary tree and the bile canaliculi between the liver hepatocytes
completely disappear during metamorphosis in all lamprey species. The
complete process of degeneration of the bile ducts, bile canaliculi and
gall bladder have been described and the consequences of these events
on liver metabolism and bile product excretion reported for
P. marinus
.
The reader is referred to the most recent review on this subject (Youson,
1993). It is noteworthy, and described above under endocrine pancreas,
that detailed autoradiographic, fi ne structural and immunohistochemical
observations have shown that in some species, such as
P. marinus
, the
epithelium of the extrahepatic and intrahepatic common bile duct undergoes
a dedifferentiation to produce cells of the caudal principal islet of the
endocrine pancreas (Elliott and Youson, 1993a, 1993b). There have been
recent studies to show that apoptosis is an important feature of the loss of
bile ducts, lamprey biliary atresia, during metamorphosis (Boomer
et al
.
,
2010; Morii
et al
.
, 2010). This topic is showing renewed interest in recent
years because the lamprey serves as a programmed model system, for its
biliary atresia has features similar to the events of human biliary atresia
that effects human infants (Youson, 1993).
