**Core Concept**
Fibroblast proliferation and differentiation are crucial steps in wound healing, particularly in the proliferative phase. Fibroblasts play a pivotal role in synthesizing the extracellular matrix, which provides structural support and facilitates tissue regeneration. The origin of fibroblasts in wound healing is a topic of interest in tissue engineering and regenerative medicine.

**Why the Correct Answer is Right**
Fibroblasts in healing wounds are primarily derived from the proliferation and differentiation of **fibrocytes** (also known as circulating fibroblasts or colony-forming units-fibroblasts). Fibrocytes are a subpopulation of mononuclear cells that circulate in the bloodstream and can migrate to the wound site, where they differentiate into fibroblasts. These fibroblasts then produce collagen and other extracellular matrix components to facilitate wound closure and tissue repair. The fibrocytes may originate from the bone marrow or peripheral blood mononuclear cells.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because fibroblasts are not directly derived from keratinocytes, which are a type of epithelial cell responsible for skin barrier function.
* **Option B:** This option is incorrect because endothelial cells, which line blood vessels, do not directly differentiate into fibroblasts in wound healing.
* **Option C:** This option is incorrect because myofibroblasts, which are involved in wound contraction, are not the primary source of fibroblasts in wound healing.

**Clinical Pearl / High-Yield Fact**
Fibrocytes play a key role in wound healing, and their dysfunction or imbalance can lead to impaired wound closure and tissue repair. Understanding the origin and function of fibrocytes is crucial in developing new therapeutic strategies for wound healing and tissue engineering.

**Correct Answer: D. Fibrocytes**