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<p>As vertebrate embryos grow and develop into adults, their organs must acquire mass
and mature tissue architecture to maintain proper homeostasis. While juvenile growth
encompasses a significant portion of life, relatively little is known about how individual
cells proliferate, with respect to one another, to orchestrate this final maturation.
For its simplicity and ease of genetic manipulations, the teleost zebrafish (Danio
rerio) was used to understand how the proliferative outputs of individual cells generate
an organ from embryogenesis into adulthood. </p><p>To define the proliferative outputs
of individual cells, a multicolor clonal labeling approach was taken that visualized
a large number of cardiomyocyte clones within the zebrafish heart. This Brainbow technique
utilizes Cre-loxP mediated recombination to assign cells upwards of ~90 unique genetic
tags. These tags are comprised of the differential expression of 3 fluorescent proteins,
which combine to give rise to spectrally distinct colors that represent these genetic
tags. Tagging of individual cardiomyocytes was induced early in development, when
the wall of the cardiac ventricle is a single myocyte thick. Single cell cardiomyocyte
clones within this layer expanded laterally in a developmentally plastic manner into
patches of variable shapes and sizes as animals grew into juveniles. As maturation
continued into adulthood, a new lineage of cortical muscle appeared at the base of
the ventricle and enveloped the ventricle in a wave of proliferation that fortified
the wall to make it several myocytes thick. This outer cortical layer was formed from
a small number (~8) of dominant cortical myocyte clones that originated from trabecular
myocytes. These trabecular myocytes were found to gain access to the ventricular surface
through rare breaches within the single cell thick ventricular wall, before proliferating
over the surface of the ventricle.</p><p> </p><p>These results demonstrated an unappreciated
dynamic juvenile remodeling event that generated the adult ventricular wall. During
adult zebrafish heart regeneration, the primary source of regenerating cardiomyocytes
stems from this outer wall of muscle. Regenerating cardiomyocytes within this outer
layer of muscle are specifically marked by the cardiac transcription factor gene gata4,
which they continue to express as they proliferate into the wound area.</p><p>Using
heart regeneration to guide investigation of juvenile cortical layer formation, we
found that both processes shared similar molecular and tissue specific responses including
expression and requirement of gata4. Additional markers suggested that juvenile hearts
were under stress and that this stress could play a role to initiate cortical morphogenesis.
Indeed, experimental injury or a physiologic increase in stress to juvenile hearts
caused the ectopic appearance of cortical muscle, demonstrating that injury could
trigger premature morphogenesis.</p><p>These studies detail the cardiomyocyte proliferative
events that shape the heart and identify molecular parallels that exist between regeneration
and cortical layer formation. They show that adult zebrafish heart regeneration utilizes
an injury/stress responsive program that was first used to remodel the heart during
juvenile growth.</p>
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