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They start to secrete renin and substances that trigger other changes: New nerves grow like weeds; immature smooth muscle cells build up; scars form around the tiny blood vessels, called arterioles; and inflammatory cells infiltrate.

The marker also proved capable of showing information about the microenvironment, such as whether blood cell production was occurring within sinusoids or arterioles, two distinct types of blood channels found within bone marrow.

They also reduced heart muscle growth and scaring while prompting arteriole formation and growth.

Small arteries and arterioles in the limbs also relax over the course of pregnancy, enhancing blood flow and cooling.

The answer often lies in the smaller branches and twigs of the vascular system—arterioles and capillaries that deliver oxygen and nutrients to heart muscle.