The Questions of Developmental Biology

*It is critical to note the difference between a chimera and a hybrid. A hybrid results from the union of two different
genomes within the same cell: the offspring of an AA genotype parent and an aa genotype parent is an Aa hybrid. A
chimera results when cells of different genetic constitution appear in the same organism. The term is apt: it refers to a
mythical beast with a lion's head, a goat's body, and a serpent's tail.
Human Somatic and Germ Line Gene Therapy
Embryonic Stem Cells In 1998, two laboratories (Gearhart 1998; Thomson et al. 1998)
announced that they had derived human embryonic stem cells. In some instances, these cells were
derived from inner cell masses of embryos that were not implanted into infertility patients. In
other cases, they were generated from germ cells derived from spontaneously aborted fetuses. In
both instances, the embryonic stem cells were pluripotent, since they were able to differentiate in
culture to form more restricted stem cells that produced neurons and blood when injected into
immunodeficient mice. The hope is that human ES cells can be used to produce new neurons for
patients with degenerative brain disorders or spinal cord injuries, and to produce new blood cells
for people with anemias (Figure 4.22A).
Such therapy has already worked in
mice, where ES cells were cultured in conditions
causing them to form glial stem cells. These
glial stem cells were transplanted into mice that
had a genetic deficiency of glial function, and
cured the defect (Figure 4.22B; Brüstle et al.
1999). Similarly, neural stem cells derived from
mouse ES cells were able to divide and
differentiate into functional neurons when
injected into a damaged rodent nervous system
(McDonald et al. 1999). But laboratory mice are
inbred, and humans are not. To get around the
problem of host rejection, human ES cells might
have to be modified, or doctors might be able to
clone an early embryo from a patient's somatic
cells and then generate his or her own ES cells.