Early Intraembryonic Vasculogenesis
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| Presomite stage (1.5-mm embryo) at approximately 20 days |
PRESOMITE
STAGE
Although not the first organ system
to make its appearance in the embryo, the cardiovascular system reaches a
functional state long before the other systems, and doing so while still in a
relatively primitive state of development. The vascular system grows from a
simple, bilaterally symmetric plexus into an asymmetric, complex system of
arteries, veins, and capillaries a
necessarily dynamic process involving the formation of new vessels and
temporary detours, rerouting of the bloodstream, and the disappearance of
previously dominant channels or even of entire vascular subsystems. The
vascular system needs to enlarge as the embryo grows, adapting to marked
changes in embryonic shape and developmental changes in other organ systems.
While hard at work, the heart also must grow and differentiate from a simple
tube into a complex, four-chambered
organ with sets of valves. Finally, because the very young embryo is tiny
compared to the mass of extraembryonic (placental) tissue, which the young
heart also supplies with blood, this heart is relatively enormous compared with
its relative size in the adult. Describing the development of the
cardiovascular system first requires review of the intraembryonic coelom (“body
cavity”) formed by the confluence of small, initially isolated spaces that
appear in the lateral mesoderm and cardiogenic mesoderm. The spaces fuse
together and form the single, horseshoe-shaped intraembryonic coelom that
extends the length of the embryo in the lateral mesoderm on each side,
communicating across the midline cranially in the cardiogenic mesoderm. Later
in development, a communication develops on each side between the caudal ends
of the intraembryonic coelom and the extraembryonic coelom. The formation of
the coelom separates the lateral mesoderm into two layers: the parietal layer
in contact with the ectoderm and the visceral layer in contact with the
endoderm. The ectoderm with its parietal layer of lateral plate mesoderm is
called somatopleure; endoderm with its visceral mesodermal layer is
called splanchnopleure.
