**Core Concept**
The ectomesenchymal cells surrounding the enamel organ play a crucial role in tooth development, contributing to the formation of various tooth structures. These cells are influenced by the signaling molecules secreted by the enamel organ, leading to their differentiation into specific cell types.

**Why the Correct Answer is Right**
During tooth development, ectomesenchymal cells closer to the outer margin of the enamel organ differentiate into odontoblasts. This differentiation is initiated by the interaction between the enamel organ and the ectomesenchymal cells, which triggers the expression of genes involved in odontoblast differentiation. As a result, odontoblasts begin to produce dentin, a key component of the tooth structure. The signaling molecules, such as BMPs (Bone Morphogenetic Proteins) and WNTs, secreted by the enamel organ play a crucial role in regulating this process.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the differentiation of ectomesenchymal cells into specific cell types during tooth development.
**Option B:** This option is incorrect as the differentiation of ectomesenchymal cells into ameloblasts occurs closer to the inner margin of the enamel organ.
**Option C:** This option is incorrect as the differentiation of ectomesenchymal cells into cementoblasts occurs in a different region of the tooth development process.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the interaction between the enamel organ and ectomesenchymal cells is a critical event in tooth development, leading to the formation of different cell types that contribute to the tooth structure.

**Correct Answer:** D. Odontoblasts