Is the FBS (fetal bovine serum) “ground-up-baby-cow” extract the same as BSA?

Not exactly – BSA stands for bovine serum albumin. Albumins are a family of large, globular proteins that have a variety of functions. In serum, they are best known for their function as a carrier of other proteins – they have lots of sites with weak binding affinity to other proteins. This means that they help other proteins be soluble in blood to be whisked around the body. In the lab, BSA’s property as a “carrier” is used for many purposes including enhancing solubility and blocking off-target binding, FBS contains all the serum contents other than red blood cells, and so it includes BSA.

Can confluency and density be used interchangeably in cell culture?

Confluency is a % area measure. Density is the number of cells per mL or per cm2 of culture surface area. I could have 10 large cells in one field of view for 75% confluency or 10 small cells in one field of view for 25% confluency. No one likes to count cells, so confluency is used as the go-to, day-to-day measurement for working with a well established line. Density is a much more precise measure and is used to communicate unambiguous culture methods. For example, my organoid class uses MEFs to validate custom CRISPR/Cas9 reagents. The manual for the MEFs says, “Cells should be split when they reach confluency….plate cells at approximately 0.8 x 10^4 cells/cm2.” The first several times, I followed their protocol exactly and found that a 1:5-1:6 subcultivation ratio put me right around 0.8 x 10^4 cells/cm2. Now, I don’t bother counting and just split them 1:5 every 3 days.