**Core Concept**
Tight junctions, also known as occluding junctions, are specialized intercellular connections that play a crucial role in maintaining the integrity and function of epithelial and endothelial barriers. They are composed of transmembrane proteins, including claudins and occludins, which seal the intercellular space and regulate the passage of ions and molecules.

**Why the Correct Answer is Right**
Tight junctions are essential for creating an impermeable barrier in various tissues, such as the blood-brain barrier, intestinal epithelium, and skin. They prevent the free movement of solutes, ions, and water across the epithelial layer, thereby maintaining cellular homeostasis and regulating the passage of nutrients and waste products. The integrity of tight junctions is critical in preventing the entry of pathogens and toxins into the body.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because tight junctions are indeed a characteristic feature of epithelial cells, which are involved in the formation of various barriers, including the intestinal epithelium.
**Option B:** This option is incorrect because tight junctions are also present in endothelial cells, which line blood vessels and play a crucial role in maintaining vascular integrity and regulating blood flow.
**Option C:** This option is incorrect because tight junctions are not typically associated with muscle cells, which are primarily involved in contraction and relaxation.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the integrity of tight junctions is compromised in various pathological conditions, such as inflammatory bowel disease, where the increased permeability of the intestinal epithelium allows toxins and pathogens to enter the body.

**Correct Answer:** D.