Basic Biology IV Human embryonic stem cells (hESCs) are able to
divide indefinitely and under the proper conditions, can
essentially become any cell in the human body. They are derived
from the developing human embryo and carry great promise for
regenerative medicine. However, these cells demonstrate an
instability surrounding the state of the X chromosome. Male (XY)
cells and female (XX) cells use a mechanism called X chromosome
inactivation (XCI) to achieve equal expression of genes on the X
chromosomes. XCI happens specifically in female cells and shuts
down one of the two X chromosomes. The silencing of the X
chromosome is a life-or-death decision that is made early in
development, but when exactly remains unclear. Female cells from
humans always possess one inactive and one active X chromosome.
hESCs with the same X inactivation pattern as adult cells and
those that have not undergone XCI have been derived, yet the
cause for this heterogeneity is unknown. Given that lines can
change their XCI state over