The human astrocyte primary cilium resides within a membranous invagination the "cilium-pit" that is involved in endocytosis. Electron micrograph showing a primary cilium extending from the astrocyte cell body into the extracellular matrix. The section was cut obliquely through the basal body (left arrow). The axoneme disappears out of the plane of the section before returning into the focal plane and extends into the extracellular environment. Endocytic vesicles can be seen along the length of the cilium-pit (right arrow, see parallel to shaft of the primary cilium). Scale bar = 250 nm.