Bio Sci 104
Development I: Early Development and Cell Determination
A. Overall process mediated by fairly simple set of activities:
1. Cells grow, divide, or die
2. Form mechanical attachments and generate forces for movement
3. Generate signals to influence adjacent cells, and respond to adjacent signals
4. Differentiate by turning on or off expression of specific sets of genes
5. This process is mediated and controlled by the genome, which although identical in all cells,
can define the rules by which these different cell activities are called into play.
fertilized egg generates the highly spatially ordered adult.
B. Will first discuss these aspects generally for all organisms.
C. Three major phases in development:
1. Phase 1: major steps include cleavage of fertilized egg to generate many smaller cells,
to generate gut cavity and general
ectoderm, endoderm, mesoderm
, which creates the
By end of phase one, basic body plan is
2. Phase 2
, in which the basic structure of the major body organs is formed
(eyes, heart, limbs, etc).
3. Phase 3: in which the final, detailed structures that make up the neonate are generated.
4. Much of phase 3 is continuing throughout adult life: e.g. constant generation of newly
differentiated skin and red blood cells to replace those that wear out.
II. Cleavage and establishment of head/tail axis
A. Best understood for amphibians (e.g. Xenopus)
1. egg is large, single cell with yolk concentrated toward lower end (
) relative to
Has actin-rich cortex and a more central cytoplasmic core.
2. Vegetal and animal poles have different sets of
, and will have different
will primarily form
such as the gut, and the
contribute primarily to
outer structures, such as skin
. This theme will continue, in that part of
the determination of the final body plan often reflects an asymmetry pre-existing in the egg,
and established by the mother.
3. entry of sperm initiates many important events: