Biology Reference
In-Depth Information
down the lamprin-containing ECM. This process has been described in detail
in the transformation of mucocartilage in the ventro-medial longitudinal bar
(VMLB) of larva
P. marinus
into the piston cartilage of adults (Armstrong
et al
.
, 1987). Dedifferentiation of mucocartilage occurs during stages 1 and
2 of metamorphosis and is typical vertebrate mesenchyme by the end of
the latter stage. The process involves migration of unspecialized peripheral
cells into the presumptive site of cartilage production and degradation
of mucocartilage ECM by these cells. Stage 3 is characterized by the
development of a blastema of precartilage cells in the core of the VMLB
with much mitosis and staining of the ECM suggesting secretion of sulfated
proteoglycan. Secretion of the lamprin component into the ECM seems to
coincide with the completion of differentiation of chondroblasts in stage
4 as well as both increased cellular degeneration and proliferation. As the
piston cartilage matures over the fi nal three stages of metamorphosis, there
is a decline in cell proliferation and degeneration with the production of
an extremely cellular cartilage surrounded by an increasingly dense ECM.
The mechanism of development of the piston cartilage during lamprey
metamorphosis follows a similar pattern to that seen in hyaline cartilage of
higher vertebrates. Since the tools for specifi c identifi cation of expression
of the various isoforms of lamprin are now available (Robson et al
.
, 1993;
Robson et al
.
, 2000), it would be of value to study this system of development
at the molecular level.
2.8.1.2 Excretory system
Past reviews have provided a detailed description of the many changes
that occur in the kidneys during metamorphosis (Youson, 1980; Youson,
1981b; Youson, 1985). In summary, the paired larval opisthonephroi that
are located in the anterior region of the coelomic cavity undergo a complete
regression and are replaced by more posterior adult kidneys that develop
from undifferentiated nephrogenic tissue. A cord of nephrogenic tissue
appeared during embryogenesis and consists of two parallel rows of cell
condensations attached to the peritoneum at their proximal end (Fig. 10);
the cord runs parallel to the archinephric duct towards the cloaca (Ooi
and Youson, 1976,1977; Youson and Ooi, 1979). This tissue, an anlage, has
waited for the stimuli of metamorphosis to begin differentation. Since the
above descriptions, ultrastructural studies have provided the morphological
details of the development and differentiation of the renal tubules from the
rudimentary nephron units through the seven stages of metamorphosis in
P. marinus
(Youson, 1984). This model for renal tubulogenesis provides an
excellent opportunity to study features of the morphogenesis of microvilli,
the endocytotic apparatus, mitochondria, and a smooth tubular network, but
is particularly valuable for studies of basal body replication and ciliogenesis
