**Core Concept**
Pericytes are a type of mural cell that plays a crucial role in the regulation of blood flow and immune responses in the microvasculature. They are perivascular cells that wrap around the endothelial cells lining blood vessels, particularly in capillaries, venules, and arterioles.

**Why the Correct Answer is Right**
Pericytes are involved in the regulation of blood flow by contracting and relaxing to change vascular resistance. They also play a role in immune responses by presenting antigens to immune cells and producing pro-inflammatory cytokines. Pericytes are also involved in the regulation of angiogenesis, the formation of new blood vessels from pre-existing ones.

**Why Each Wrong Option is Incorrect**
**Option A:** is incorrect because pericytes are not typically associated with the blood-brain barrier, where astrocytes and endothelial cells play a more significant role.
**Option B:** is incorrect because pericytes are not typically associated with the lymphatic system, where lymphatic endothelial cells and fibroblasts play a more significant role.
**Option C:** is incorrect because pericytes are not typically associated with the myocardium, where cardiac fibroblasts and smooth muscle cells play a more significant role.

**Clinical Pearl / High-Yield Fact**
Pericytes are involved in the pathogenesis of several diseases, including diabetic retinopathy, where they contribute to the breakdown of the blood-retinal barrier and the formation of new blood vessels.

**Correct Answer: D. Microvasculature. Pericytes are seen in the microvasculature.**