Table I.

Quantitative Analysis of Tf-HRP and BSA-Gold Uptake in 6H5.DM Cells

6H5DM		Type 1		Type 2		Type 3		Type 4		Type 5		Type 6
3 min		73		22		5		−		−		−
Tf-HRP
10 min		79		18		3		−		−		−
Tf-HRP
3 min		47		24		29		−		−		−
BSA-gold
10 min		35		32		33		−		−		−
BSA-gold
10 + 20 min		10		10		40		37		3		−
BSA-gold
10 + 50 min		2		2		7		49		31		9
BSA-gold

6H5.DM cells were incubated at 37°C with Tf-HRP for 3 and 10 min or pulse–chased with 5-nm BSA-gold particles for the indicated time periods. Tf-HRP was visualized on ultrathin cryosections with an HRP antibody and 10-nm gold particles. For each incubation, 20 cell profiles were randomly selected, and gold particles labeling Tf-HRP were assigned to a compartment when within 20 nm of a membrane. Only BSA-gold particles within the limiting membrane of a compartment were counted. Percentages of gold particles present in each compartment are shown. Six types of endocytic compartments could be distinguished, each with a characteristic morphology, as shown in the table. The highest percentage of Tf-HRP is present in type 1 compartments, both after 3 and 10 min of incubation. Tf-HRP has not been detected in types 4, 5, and 6 to a significant extent. The quantitation of BSA-gold particles present in each of the compartments during the pulse–chase shows that at 3- and 10-min pulse only types 1, 2, and 3 are reached, whereas type 4, 5, and 6 compartments are sequentially reached by this tracer after 20- and 50-min chase.