Tetralogy of Fallot (TOF), formerly known as Steno-Fallot tetralogy, is a congenital heart defect characterized by four specific cardiac defects. Classically, the four defects are:
pulmonary stenosis, which is narrowing of the exit from the right ventricle;
a ventricular septal defect, which is a hole allowing blood to flow between the two ventricles;
right ventricular hypertrophy, which is thickening of the right ventricular muscle; and
an overriding aorta, which is where the aorta expands to allow blood from both ventricles to enter.At birth, children may be asymptomatic or present with many severe symptoms. Later in infancy, there are typically episodes of bluish colour to the skin due to a lack of sufficient oxygenation, known as cyanosis. When affected babies cry or have a bowel movement, they may undergo a "tet spell" where they turn cyanotic, have difficulty breathing, become limp, and occasionally lose consciousness. Other symptoms may include a heart murmur, finger clubbing, and easy tiring upon breastfeeding.The cause of tetralogy of Fallot is typically not known. Risk factors include a mother who uses alcohol, has diabetes, is over the age of 40, or gets rubella during pregnancy. : 62  It may also be associated with Down syndrome and other chromosomal defects that cause congenital heart defects.TOF is typically treated by open heart surgery in the first year of life. The timing of surgery depends on the babys symptoms and size. The procedure involves increasing the size of the pulmonary valve and pulmonary arteries and repairing the ventricular septal defect.
Physiology
Role as a model organism
Because Acanthamoeba does not differ greatly at the ultrastructural level from a mammalian cell, it is an attractive model for cell-biology studies; it is important in cellular microbiology, environmental biology, physiology, cellular interactions, molecular biology, biochemistry, and evolutionary studies, due to the organisms versatile roles in the ecosystem and ability to capture prey by phagocytosis, act as vectors and reservoirs for microbial pathogens, and to produce serious human infections. In addition, Acanthamoeba has been used extensively to understand the molecular biology of cell motility and cancer cell dormancy by in-depth exploration of the process of encystation.The recently available Acanthamoeba genome sequence revealed several orthologs of genes employed in meiosis of sexual eukaryotes. These genes included Spo11, Mre11, Rad50, Rad51, Rad52, Mnd1, Dmc1, Msh, and Mlh. This finding suggests that Acanthamoeba is capable of some form of meiosis and may be able to undergo sexual reproduction. Furthermore, since Acanthamoeba diverged early from the eukaryotic family tree, these results suggest that meiosis was present early in eukaryotic evolution. Owing to its ease and economy of cultivation, the Neff strain of A. castellanii, discovered in a pond in Golden Gate Park in the 1960s, has been effectively used as a classic model organism in the field of cell biology. From just 30 L of simple medium inoculated with A. castellanii, about 1 kg of cells can be obtained after several days of aerated culture at room temperature.