Bio Sci 104
XI. What about events controlling differentiation at the tissue level, both in
embryogenesis and in the adult after embryogenesis is complete?
A. Each organ is a mix of different cell types, each contributing its own function.
Where do these different specialized cells originate during embryogenesis?
1. Often, the "ancillary" cells originate outside of the organ itself, and invade it
2. For example: limbs first form primarily of epithelial and mesodermal tissue by a
budding process described in the previous lecture.
however, originate in somites
flanking spinal column, and migrate into
developing limb buds, divide, differentiate, and made appropriate contacts
with developing bones.
Similarly, neuronal axons originating in the spinal
cord invade the developing limbs, and made appropriate contacts with new
At least part of migration regulated by cell-cell and cell-
extracellular matrix communications.
induced to invade limb tissue (angiogenesis) in response to clues released by
Neural crest cells
enter limb to differentiate into efferent nerve
cells, and to form melanocytes (pigment cells).
B. In adult, replicative and relatively undifferentiated stem cells can generate new
differentiated cells to replace those lost through wear and tear; however,
not all populations of cells in the adult do this; some tissues and organs are
formed during embryogenesis and are retained throughout adult life: no
replication occurs and these cells cannot be replaced when damaged.
Examples include nerve cells, heart muscle cells, auditory hair cells, and lens
cells of eye (note that nerve cell axons, if cut, can partially re-grow, but this
depends on neuron body not being damaged).
This does not mean that these
permanent cells do not repair themselves, just that they do not replicate in
Retinal photoreceptor cells
do not replicate in adult, but
continually replace the membrane discs of photosensitive
cells don't replicate in adult, but replace macromolecules as they wear out.
C. In contrast to above, however, many of the differentiated cell populations in the adult
are not permanent, but instead are continually dying and being replaced.
This can occur
through simple duplication (i.e. replication with no change in differentiation state), or by