Tissue Development, Repair, and Remodeling - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 677

and α 8 β 1 -integrin are synthesized in the ureteric bud epithelium and metanephric

mesenchyme, respectively.

11.2 Cell Differentiation

During cellular differentiation, an immature cell becomes a mature specialized
cell. Cell differentiation occurs not only during embryogenesis, but also in adults.
In particular, adult stem cells generate differentiated daughter cells for tissue
repair and during cell turnover. Cell differentiation modifies cells size, shape,
polarity, metabolic activity, and responsiveness to signals due to modifications in
gene expression. Dedifferentiation allows differentiated cell to revert to an earlier
developmental stage.
Three basic cell classes include germ, somatic, and stem cells. Germ line
cells give rise to gametes (eggs and sperm). Totipotent cells (zygote and early
embryonic cells) can differentiate into all cell lineages. Pluripotent stem cells (e.g.,
hematopoietic and mesenchymal stem cells) are able to differentiate into many cell
types. Multipotent stem cells and unipotent progenitors give rise to functional cells.
Each differentiated cell type expresses a gene subset of the genome due to a
peculiar pattern of gene expression. Growth factors control the switch from one
gene expression pattern to another. Distinct cell types differentially retrieve the
genetic information encoded in the genome. Transcription factors and microRNAs
form the largest families of gene regulatory factors [ 1391 ]. Sets of combinatorially
expressed transcription factors and microRNAs delineate cell types. Transcription
factors and microRNAs share many similar features although they have their
specific properties that determine specialized regulatory niches (Table 11.3 ). Some
microRNAs are expressed in a cell- or tissue-specific manner and contribute to cell
identity. MicroRNAs can control the expression of transcriptional regulators. They
also regulate alternative splicing during tissue development.

11.3 Branching Morphogenesis

The vasculature is characterized by its branched architecture. In addition, to their
own perfusion vasculature, organs can contain branched structures in a limited body
region, such as the tracheobronchial tree, collecting parts of renal nephons, and
glandular ducts.
Branched conduit networks connect body's environment to organs and/or con-
versely. Branching augments the cummulated functional surface area (e.g., trumpet
shape of the cummulated surface area of the tracheobronchial tree from large
bronchi to alveolar ducts).
The respiratory tract links atmospheric air to gas exchange membrane of
pulmonary alveoli. This alveolocapillary membrane is crossed by blood gas. Blood