**Core Concept**
Proteoglycans are key components of the extracellular matrix in various connective tissues, including cementum, where they play a crucial role in regulating cell-cell and cell-matrix interactions. This is achieved through their ability to bind growth factors, enzymes, and other molecules, thereby modulating the local microenvironment.

**Why the Correct Answer is Right**
Proteoglycans in cementum interact with cells via specific receptors, such as syndecan-1 and CD44, which are involved in cell adhesion, migration, and differentiation. This interaction is essential for maintaining the integrity and function of the periodontal ligament, which surrounds the tooth and anchors it to the alveolar bone. The presence of proteoglycans also regulates the activity of enzymes involved in cementum formation and remodeling, such as matrix metalloproteinases (MMPs).

**Why Each Wrong Option is Incorrect**
**Option A:** Sharpey's fibers are collagen fibers that extend from the cementum into the dentin, providing mechanical attachment between the two tissues. While they are essential for tooth anchorage, they do not play a primary role in regulating cell-cell and cell-matrix interactions in cementum.

**Option B:** Intrinsic fibers are another type of collagen fiber found in the periodontal ligament, but they are not directly involved in regulating cell-cell and cell-matrix interactions in cementum.

**Option D:** Phosphoproteins are a class of proteins that contain phosphate groups, which can be involved in various cellular processes, including cell signaling and adhesion. However, they are not the primary substances responsible for regulating cell-cell and cell-matrix interactions in cementum.

**Clinical Pearl / High-Yield Fact**
Proteoglycans in cementum are also involved in the regulation of tooth eruption and orthodontic tooth movement. Alterations in proteoglycan composition or function have been implicated in various periodontal diseases, including periodontitis.

**✓ Correct Answer: C. Proteoglycans**