SIDMAN 291There was a lot for us to learn, some with help from other scientists
at the NIH and some on our own. I contacted Clifford Grobstein, the
outstanding developmental biologist of his era and a tower of strength
at that time in the National Cancer Institute, because of our mutual inter
est in organ culture. He taught me about the existence of inbred strains
of mice, and showed me how to breed mice and how to recognize the
first day of gestation so that the pregnancies could be timed.^9
In addition to all that we learned from others, we also taught each
other from descriptions of methods on journal pages. One example was
mastering how to work in complete darkness to make autoradiograms
by dipping microscope slides into liquid photographic emulsion and
then hanging them with clothespins onto a wire suspended above the
laboratory bench to dry, a technique based on the newly published
method of the distinguished Canadian histologist, Charles P. LeBlond.^10
A bit of luck always helped, and we were fortunate in choosing mice
at the eleventh day of gestation for the first trial injections of tritiated
thymidine. Younger embryos, as we learned later, do not receive enough
of the radioactive compound after its injection into the mother because
the placental circulation connecting mother and embryo is not yet well
enough developed. The patterns of radioactive cells in older fetuses might
have been too complex for us to analyze and understand at that initial
phase of our venture into uncharted territory. No one before us had
used tritiated thymidine to look at the nervous system or indeed, at any
tissues in mammalian embryos.
We killed the first four injected pregnant mice at 1, 6, 24, and
48 hours after injection. The embryos were fixed for histological and
autoradiographic workup. Beta rays from the tritium produce a latent
image in the photographic emulsion layer just as light does with the film
in a camera. The difference from the camera is that for autoradiography,
exposure time of the film to tritium is measured in weeks or months, not
in fractions of a second. However, at the end of the exposure time, the
slides with the emulsion are developed in the darkroom with the stand
ard chemicals used for photographic development, and the cells with
sufficient radioactivity in their nuclei are then seen to be overlaid with
reduced (black) silver grains in the emulsion.