Tissue Development, Repair, and Remodeling - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 673

female egg male sperm

fertilization

zygote

totipotent stem cells blastomeres

morula

pluripotent stem cells blastocyst

multipotent stem cells gastrula

unipotent stem cells

organogenesis

Fig. 11.1 Different types of stem cells. Fusion between female and male gametes (fertilization)
leads to the zygote and embryogenesis. After fertilization, a set of fast mitoses occurs (cleavage)
with formation of blastomeres building the blastula. In mammals, the blastula is called the
blastocyst. The morula corresponds to an organized cell set with external and internal cells. After
the mitosis rate has slowed down, the blastomeres move (gastrulation), forming the gastrula with
3 germ layers: ecto-, endo-, and mesoderm.

11.1.2 Precursor Cells

During the early stages of embryogenesis, the fertilized egg starts to divide into
blastomeres. Embryonic stem cells (ESC), derived from the totipotent cells 1

of

preimplantation embryos are pluripotent cells (Fig. 11.1 ).
Pluripotent stem cells either self-renew continuously or form the various types of
mature, functional cells. Pluripotent embryonic stem cells derive at various stages
of embryo development according to their type, embryonic germ cells occurring at
an earlier phase than others. 2

1 A single totipotent cell leads to the development of several hundred cell types.
2 Human embryonic stem cells can be removed either from the blastocyst or morula, or from
later-stage embryos, using various isolation procedures. The optimal method consists in deriving
pluripotent embryonic stem cells from the inner cell core of blastocysts, before implantation in
uterus (preimplantation blastocyst), i.e., 5 to 6 days after in vitro fertilization. The inner cell
mass is then large enough (outer trophoblasts differentiate into the placenta). A single blastomere
can be extracted from the 8-cell (3-day) embryo for preimplantation genetic diagnosis. Cultured
blastomeres prior to genetic analysis can divide, so the genetic check-up can be associated with
the production of human embryonic stem cells without reducing embryo developmental potential.