Wrote: “For that matter, how does this whole business of “gene expression” work? I mean, you start off with a little blob of protoplasm, and the cells divide, and somehow one bunch of cells knows it’s going to wind up being the liver, another the eyeball, and another the right pinkie … but how? How does this fabulously complicated creature arise from the genetic information in the chromosomes? I gotta confess to you, friends. It stumps even me.”
Considering that some of this works because gravity affects the cells by the fact that heavier molecules tend to float down. Thus the cell “knows” which way is up and which is down because after division the chemical makeup of the resulting cells are different.
So has there been studies of embryology in free fall?
This has been tested many times in space. Here’s one with fish.
So far no signs that gravity is involved at all. It’s all about genes being switched on or off based on the number of divisions since fertilization. A “ladder” sequence of gene expression is likely. Gene A causes Gene B to be expressed (or suppressed) after the next division, which causes Gene C to be expressed after the next division, etc.
There have been studies of plants, insects, and rodents in freefall conditions on the ISS, and while there are some developmental issues without gravity this doesn’t seem to have any discernible effects at the embryonic stage. This is not surprising because the genome and other material in cells is suspended in intracellular fluid which also acts as a polar solvent and transport medium for nutrients; this material is in constant motion with the active genes in chromosomes being comstantly transcribed and amino acids being sequenced multiple times per second. (Eukaryotes produced protiens at an average rate of about one every two minutes.) So there isn’t really time for molecules to “float down” and settle, and they are in a medium where they would tend to remain suspended regardless.
Gene expression is a complicated process of transcription, splicing, translation, and post-translational modification; controlling or altering expression can be done at any point, as well as controlling access to the genome via methylation and histone modification. Embryonic development is largely controlled by cell signalling and differentiation which is directed by homeotic genes, which is complicated at the system level but really works with a few basic rules. Like building a cabinet, you start with some standardized components like screws and hinges and a bunch of raw lumber and start shaping it by following instructions.