"Spindle-shaped" means tapered at both ends. The term comes from the shape of thread-holding "spindles" used on weaving and sewing machines (the large industrial kind). As the name implies, these things are made to rotate at high speed and to provide a smooth flow of thread off the end. They are the same shape at both ends so that orientation on the machine isn't a concern. It happens that this double tapered shape occurs quite commonly in biological structures, as it's admirably adapted to many other applications, such as lying in between the large myofibers of an anatomic muscle.