The adult human endolymphatic sac (ES) has been described as a complex network of interconnected tubules. Embryologic examination describes the human ES as a single-lumen, pouch-like structure. Transition from saccular shape to tubules during the entire fetal period has not been previously reported. Tubular ES structure is thought to be unique to humans. Animal investigations describe similar saccular appearance, but without tubules in mature sacs. The authors examined 45 human fetal temporal bones to trace ES development and reviewed six types of animal sacs. Results in humans reveal tubular structure as early as 26 weeks' gestation. Maturation variably occurred in the fetal period and postnatally. For the first time, the tubular system is noted in the animal, the rhesus monkey. These findings suggest that the tubular system may represent more advanced specialized function.
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