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Chapter 19
Biology
Sixth Edition
Raven/Johnson
(c) The McGraw-Hill Companies, Inc.
How is development from
a single cell to a highly
organized organism
accomplished?
Although there are
differences among the
development of these four
organisms, it is
surprisingly similar!
Vertebrate Development
Cleavage – the initial
developmental period
of cell division.
Although the number of
cells increase, the overall
size of the cell mass does
not increase.
Mammalian Development
Na+ is pumped out of cells
Cells move
(lamellipodia)
Insect Development
Produce two types of
bodies. The change from a
larva to an adult is called
metamorphosis.
Special nurse cells release
their own mRNA into the
developing embryo – this
orients the development!
Plant Development
Early cell division separate
the embryo from the
suspensor.
Soon, three tissue types
(epidermal, ground, and
vascular) are evident in the
embryo.
Plant cells can’t move – new
tissues are made at
meristems.
Plants grow in response to
their environment.
Consists of 959 somatic cells.
The same 131 cells are programmed to die.
Fate of each cell is the same in every Caenorhabditis
elegans individual.
Basic Mechanisms of Development
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Cell Movement
Induction
Determination
Pattern Formation
Expression of Homeotic Genes
Program Cell Death
Cell Movement
Cells migrate during many stages of animal
development by pulling themselves along using cell
adhesion molecules.
Important molecules involved:
Cadherins span the plasma membrane and attach to
the same kind of cadherins of other cells (over a
dozen known to date; imaginal discs of fruit fly).
Integrins span the plasma membrane and ‘grab’ the
matrix of intracellular collagen or fibronectin.
Induction
Drosphila:
Mammals:
Cells created by initial
cleavage contain different
developmental signals
(determinants)  mosaic
development
Cells created by initial
cleavage contain
equivalent determinants;
body form is determined
by cell-to-cell interactions
 regulative development
Induction – when a cell switches from one
developmental path to another as a result of interactions
with adjacent cells.
Organizers – groups of
cells that secrete a
signaling molecule
(morphogen) that conveys
positional information.
Relative distance from
organizers influences
development.
Determination
Up to the 8-cell stage, cells are totipotent – any one
of them can produce a complete individual if
separated from the others.
Reverse works as well – chimera:
Chimera
Beyond the 8-cell stage each cell’s fate has been
determined.
Determination – the commitment of a particular
cell to a specialized developmental path.
Differentiation – the specialization that occurs at
the end of a developmental path.
Activation of gene regulatory proteins is the
mechanism of determination.
Determination is reversible!
Pattern Formation- free diffusion of morphogens
Positional information is used to determine the basic pattern
of body compartments.
In Drosphila, maternal mRNA (from bicoid gene) is at one end
of egg and translation begins at fertilization – this establishes
an anterior end. Bicoid protein then activates the first
transcription of hunchback genes, which is spread
throughout, but transcription is blocked by nanos.
W/in 2 hours – gap genes
are transcribed – map out
the coarsest subdivision of
the embryo.
W/in 3 hours, a cascade of
segmentation gene activity
produce’s the basic body
plan.
Homeotic genes – code for
proteins that act as transcription
factors – activates a particular
module of the genetic program.
Order of the genes
mirrors the order of the
body parts they control!
Antennapedia
Bithorax
Homeotic genes
typically contain the
homeobox – 180
nucleotides for 60
amino acids.
Ensures that genes
are transcribed at
the appropriate time.
Homeobox genes (Hox) have been found also in mice and
humans.
This indicates that they arose very early in animal history.
In mammals, homeotic genes appear to be lined up in the same
order as the segments they control.
Apoptosis – programmed cell
death – your fingers have to
separate!
The same 131 cells always die
during development of the
nematode worm – under
genetic control.
Same in humans.
Transfer human bcl-2 gene to
nematode with defective ced-9
gene – apoptosis is avoided.
Four-theories on aging:
Accumulated Mutation
Hypothesis
Telomere Depletion
Hypothesis
Wear-and-tear hypothesis
Gene Clock Hypothesis
The End.
Divided into 17 segments,
which are established very
early in development.
Two clusters: Antennapedia
and Bithorax.
Hox genes in mice are very similar to those in
Drosophila and control body part development in mice
It appears that genes that determine organ development in
animals operate in the same manner in plants.