Interstitial cells can describe any cells lying between or among other layers of cells, which does not particularly point to a specific anatomical structure. However, seminiferous tubules are ducts specific to the testes (the male gonads), serving as the location where sperm cells (the male gametes) are produced.
The testes are considered an endocrine gland because, along with sperm, they produce the hormone and anabolic steroid testosterone, colloquially attributed as assigning male characteristics to an individual like their body hair, deeper voices, their libido, their increased bone mass, stronger muscles, and their fertility.