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2.7.4 Endocrine
Historically, research on the endocrine regulation of lamprey metamorphosis
has focused largely on the thyroid system and its peripheral regulators, but
the recent identifi cation of hypothalamic-releasing and pituitary hormones
in lampreys has set the stage for the study of central regulatory mechanisms.
Moreover, the importance of lipid accumulation and lipid metabolism prior
to and during metamorphosis in P. marinus , respectively, suggests that
lipolytic and lipogenic hormones such as insulin and somatostatin, and a
leptin-like protein may also be regulators of lamprey metamorphosis.
Biologists have sought to understand the role of TH and the thyroid
system in lamprey metamorphosis since Gudernatsch (Gudernatsch,
1912) first discovered that thyroid gland extracts induce precocious
metamorphosis in anuran amphibians. Most information regarding the
endocrine regulators of lamprey metamorphosis has been obtained from
studies on the thyroid gland, and the thyroid hormones, T4 and the more
biologically active T 3. . Also included are the peripheral regulators of TH
action, such as binding proteins and deiodinase enzymes. It is evident that
TH are important for lamprey development and metamorphosis, however,
their precise role has yet to be fully elucidated and present evidence is
not necessarily consistent with the anuran model. TH modulate, via their
interaction with nuclear receptors, most essential gene expression cascades,
either directly or indirectly, that are required for anuran metamorphosis and
perhaps the metamorphoses of gnathostome fi shes (see other chapters in
this volume). Included among these anuran gene expression cascades are
those associated with cellular proliferation and de novo synthesis of adult
organs (i.e., limbs and lungs), cellular apoptosis and regression of larval
structures (i.e., tail and gills), and remodeling of larval organs into their
adult counterparts (i.e., liver, nervous system, intestine) (reviewed in Shi,
2000; Denver et al . , 2002; Brown and Cai, 2007).
2.7.4.1 Thyroid hormones
The fi rst experiments to examine the role of the thyroid axis in lamprey
metamorphosis failed to trigger metamorphosis using approaches which
had been successful in anurans. Included among these approaches were
immersion in iodine-containing compounds or thyroid extracts (Horton,
1934) and injection of either T 4 (Leach, 1946) or pituitary extracts (Young
and Bellerby, 1935; Knowles, 1941). Furthermore, despite using L. planeri
of presumed pre-metamorphic size and age, the thyrotropic fraction
of ox pituitary extracts Knowles (1941) failed to induce precocious
metamorphosis.
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