cell differentiation

Meaning: Differentiation is how a cell becomes specialized. Signals such as morphogens, cell-cell contact and transcription factors, together with epigenetic changes like DNA methylation and histone modification, switch particular genes on or off. Cells usually lose potency as they specialize. A totipotent cell (zygote) can form the embryo and placenta. A pluripotent cell (embryonic stem cell) can form all three germ layers. A multipotent cell (adult stem cell, such as a hematopoietic stem cell) gives a limited set of related types.

On the MCAT: Passages describe stem cells, embryonic induction, or a transcription factor that is added or knocked out, and ask which cell type results or how potency changes. Know the germ layer fates: ectoderm gives epidermis and nervous system, mesoderm gives muscle, bone, blood and kidney, and endoderm gives the gut lining, liver, pancreas and lungs. Nuclear transfer experiments and induced pluripotent stem cells show that a differentiated nucleus can be reprogrammed.

Common confusion: Students think a specialized cell has lost the genes it does not use. In most cells the genome matches the zygote's; what differs is which genes are expressed, so mRNA and protein content differ. Exceptions exist, such as mature red blood cells losing their nucleus and B and T cells rearranging receptor genes.