The second approach used to understand the partitioning of dextran was to examine the underlying dynamics and interactions of the three vesicle populations. The endo-lysosomal vesicles undergo repeated splitting and fusion events with all three types of vesicles splitting off and trafficking through the cell. We found that the majority of Rab7- and LAMP1-vesicles could be traced back to an origin at a Rab7/LAMP1-vesicle. The Rab7- and LAMP1-vesicles are LY2157299 highly mobile with transport speeds indicative of active transport carried out by motor proteins moving on the cytoskeleton. We followed the trajectory of individual Rab7- and LAMP1-vesicles and noted their fusion partner. Interactions that led to the formation or maintenance of Rab7/ LAMP1-vesicles represent the majority of interactions. The fusion of LAMP1-vesicles with other LAMP1-vesicles was also observed, but the fusion of Rab7-vesicles with other Rab7- vesicles was rarely observed. The presence of Rab7 and LAMP1 on a single vesicle invites comparison to hybrid vesicles that have properties of both late endosomes and lysosomes. Hybrid vesicles have been observed directly using electron microscopy, density gradient ultracentrifugation of cell-free endosomes and lysosomes, and functional assays probing the enzyme-mediated degradation of low-density lipoprotein and ovalbumin within late endosomes. The Rab7/LAMP1-vesicles described in these experiments are hybrids in the sense that they are positive for both conventional late endosomal and lysosomal proteins. However they function as terminal vesicles rather than intermediates in the transport between late endosomes and lysosomes. This research may help to explain previous results, especially functional assays that have reported lysosomal activity associated with Rab7- positive vesicles, as the majority of these vesicles are likely Rab7/LAMP1-vesicles. In conclusion, we have found that the endo-lysosomal pathway is composed of at least three distinct populations of vesicles: Rab7-, LAMP1-, and Rab7/LAMP1-vesicles. The majority of endolysosomal vesicles are positive for both Rab7 and LAMP1. Measuring the location of dextran, a fluid phase cargo, as it moves through the endo-lysosomal pathway allows us to examine the role of Rab7/LAMP1-vesicles in the endo-lysosomal pathway. Dextran is present in Rab7-, LAMP1-, and Rab7/LAMP1-vesicles at early times, but shifts to primarily LAMP1- and Rab7/LAMP1-vesicles at later times demonstrating that LAMP1- and Rab7/LAMP1- vesicles are terminal vesicles in the endo-lysosomal pathway. Totipotency in human embryos persists until 4–8 cell stage. Subsequently, genome activation initiates differentiation, with certain blastomeres forming the outer, polar, trophectoderm while others retain their pluripotent potential and generate the non-polar inner cell mass that will give rise to the future organism. These two morphologically distinct cell populations continue to divide as they form the blastocyst, and can easily be physically separated. Human embryonic stem cells are typically derived from the pluripotent inner cell mass cells of the blastocyst.