Timing of human sperm chromosome replication following fertilization of hamster eggs in vitro

The visualization of human male pronuclear chromosomes is possible by utilizing a technique in which human sperm fertilize zona‐free Golden hamster (Mesocricetus auratus) ova in vitro. R banding of these chromosomes can be achieved by adding 20 μg/ml 5‐bromode‐oxyuridine (BrdU), a thymidine analogue, to the culture medium during mid‐to‐late S phase and subsequently staining the chromosomes with 0.1 mg/ml acridine orange. BrdU was added 3.0–10.0 hr postinsemination (hr pi). R bands were obtained when fertilized eggs were transferred into medium containing BrdU between 5.0 and 6.5 hr pi. This indicates that the human (male) pronuclear chromosomes were in mid‐to‐late S phase at these times.