A pivotal bone anchor assembly includes a receiver having an axial bore communicating with a bottom surface of the receiver through a bottom opening, and a bone anchor having an attachment portion configured for attachment to the bone of a patient and a capture portion opposite the attachment portion configured for uploading into the axial bore through the bottom opening. The assembly also includes a multi-part retainer comprising at least two separable retainer parts, with each retainer part being positionable within the axial bore prior to the capture portion. The retainer parts are configured to move at least partially away from each other during the uploading of the capture portion into the axial bore, and then to operably engage and pivotably secure the capture portion in spaced relation from an internal engagement surface of the axial bore with the bone anchor extending downwardly through the bottom opening.