**Core Concept**
The blood-testis barrier is a specialized structure that separates the seminiferous tubules of the testis from the bloodstream, regulating the exchange of substances between the two compartments. This barrier is crucial for the development and maintenance of spermatogenesis.

**Why the Correct Answer is Right**
The blood-testis barrier is primarily formed by tight junctions between Sertoli cells, which line the seminiferous tubules. These tight junctions create a selective permeability barrier, allowing the passage of small molecules essential for spermatogenesis while restricting the movement of larger molecules and immune cells. The presence of tight junctions between Sertoli cells prevents the passage of autoantigens from the testis to the bloodstream, thereby protecting developing sperm from immune destruction.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the blood-cerebrospinal fluid barrier is formed by tight junctions between endothelial cells and astrocytes in the brain, not the blood-testis barrier.
* **Option B:** This option is incorrect because the blood-retinal barrier is formed by tight junctions between retinal pigment epithelial cells, not the blood-testis barrier.
* **Option C:** This option is incorrect because the blood-brain barrier is formed by tight junctions between endothelial cells and astrocytes in the brain, not the blood-testis barrier.

**Clinical Pearl / High-Yield Fact**
The presence of the blood-testis barrier highlights the importance of a separate immune environment within the testis, allowing for the development of self-tolerant immune responses to sperm antigens.

**Correct Answer: C. Sertoli cells. Tight junctions between Sertoli cells form the blood-testis barrier.**