The“Tetralogy of Fallot”is the most common defect of the conus
arteriosus/truncus arteriosus and is due to unequal division of the conus
due to anterior displacement of the conotruncal septum. The mnemomic
IHOPis useful to remember the four cardiovascular alterations which com-
prise the Tetralogy: 1) interventricular septal defect, 2) hypotrophy of the
right ventricle, 3) overriding aorta, and 4) pulmonary stenosis.
Vasculature
Vasculogenesis versus Angiogenesis
The endothelial lining of most blood vessels forms by coalescence of vascular
endothelial progenitors (angioblasts)of mesodermal origin. The endothelial
cells proliferate, migrate, differentiate, and organize into tubular structures
with subsequent vacuolization to form a lumen. Subsequently, periendothe-
lialcells form from local mesoderm and differentiate into muscle and connec-
tive tissue elements (i.e., smooth muscle, fibroblasts, and pericytes). That
process is known as vasculogenesisand occurs in both embryonic and adult
tissues. Vasculogenesis is the de novoformation of blood vessels and differs
from angiogenesis, initiated in a pre-existing vessel. Both of those processes are
regulated in part by vascular endothelial growth factor (VEGF),which
induces chemotactic (migratory) and proliferative responses in endothelial
cells. Uterine angiogenesis occurs in adult women during each menstrual
cycle. Angiogenesis also is a prominent characteristic of inflammation, pathol-
ogy such as diabetic retinopathy,wound repair, placental development dur-
ing embryogenesis, and tumor formation. Molecular triggers for angiogenesis
include thecytokines,small, extracellular signal proteins or peptides that
function as local mediators in cell-cell communication. For example, during
inflammation or hypoxia, cytokines induce endothelial cell proliferation and
differentiation and stimulate matrix metalloproteinasesthat digest type IV
collagen in the basement membrane creating a new branch point in the vessel.
Tumor angiogenesismimics the process observed during inflamma-
tion. Tumor angiogenesis has become a potential target in cancer treatment.
Tumors produce antiangiogenic factorssuch as endostatinandangio-
statin,which are derived from type XVIII collagen and plasminogen
respectively. Pharmaceutical agents modeled after these anti-angiogenic
peptides are being developed to inhibit tumor growth.
Development of the Vasculature
The paired doral aortae and the five aortic arches form an early symmetric
arterial system. Regression of portions of these vessels later results in the
asymmetrical adult arterial system.
8 Anatomy, Histology, and Cell Biology