Tissue Development, Repair, and Remodeling - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 674

The morula (morus: mulberry; 12-32 blastomeres within the zona pellucida)
corresponds to an early stage of embryonic development produced by the division
of the zygote. After reaching the 16-cell stage, cells of the morula differentiate.
The central, fluid-filled blastocyst cavity (a.k.ablastocele and cleavage or seg-
mentation cavity) appears and the morula becomes a hollow spheric blastocyst
(blastulation). The inner and outer blastomeres form the inner cell mass and tro-
phoblast, respectively. The blastula (blastosphere) that follows the morula consists
of a spherical layer of about 128 cells surrounding the blastocele. At the blastula
stage, the cell cluster contains nearly identical cells.
During the next stage, the gastrula is characterized by structured tissues. Gas-
trulation is the first stage in vertebrate development when different progenitor types
sort out and assemble into 3 germ layers: ecto-, endo-, and mesoderm. The germ
layers in tissue development involve 3 processes: folds, splits, and condensation.
After gastrulation, pluripotent embryonic stem cells give birth to multipotent
stem cells that are irreversibly programmed for a given tissue. They form: (1) meso-
derm, from which are derived connective tissue, muscles, and circulatory apparatus;
(2) ectoderm (nervous system, skin, etc.); and (3) endoderm (respiratory apparatus,
digestive tract, and endocrine glands).
Tissue and organ development includes several stages under the control of the
genetic code: (1) acquisition of cell identity and transient activation of specific
regulatory genes; (2) stabilization of the regulation and exclusion of alternative
regulatory states by feedback; (3) reception of signals required for development
of embryonic cell lineages with respect to neighboring lineages and lineage-specific
activation of organogenic genes.

11.1.3 Organogenesis, Vascularization, and Innervation

Organ development requires the differentiation and coordinated development of
nerves and blood vessels that contain many cell types. The peripheral parasympa-
thetic nervous system innervates many organs during embryogenesis.

Human embryonic stem cells are self-renewing pluripotent cells, that provide a source for
cell replacement therapies in certain pathologies (e.g., infarction sites). These cells are able to
differentiate into sufficient quantities of a specific, transplantable, functional cell type. They
behave normally within a blastocyst but can induce tumors when they are ectopically transplanted.
Cultured human embryonic stem cells have the following main time-invariant characteristics: (1) in
vitro proliferation on appropriate substrata; (2) maintenance of a normal karyotype throughout
long culture durations; (3) expression of standard molecular markers of stem cells (stable
expression of surface markers and transcription factors associated with an undifferentiated state);
and (4) differentiation into cell lines of all 3 germ layers with an epigenetic status (autosomal
imprinting and X-chromosome inactivation) that differently respond according to environmental
stimuli [ 1384 ]. Signaling primed by FGF is strongly involved in the maintenance of a proliferative,
undifferentiated state in cultured human embryonic stem cells.