Campbell and Reece, Chapter 21
Embryology has been fundamental in Biology since the 1800's. Most universities
have (or had) courses in (animal) embryology (ours is BL
A344) taught by Dr.
Schreiweis, Director of Pre-Health
Profession, or Dr. Medoff).
Traditional animal embryology is essential in comparative biology in the
classification of animals, and my lecture
outline for Bio 106 (second semester) on this material can be found
on the web. Also, there is substantial treatment
second semester (after reproduction) on embryology. Many Universities have
either replaced or supplemented their traditional embryology courses with
"developmental biology" courses, material more akin to this chapter,
and our course, offered only occasionally, is BLA460. Development is also
predominant in biology research; at SLU, Dr.
Coulter's field is developmental genetics.
The 1995 Nobel prize
in Physiology and Medicine was awarded jointly to: EDWARD B. LEWIS, CHRISTIANE
NÜSSLEIN-VOLHARD and ERIC F. WIESCHAUS for their discoveries concerning
the genetic control of early embryonic development. This, of course, added
a degree of prestige to this field which was already quite firmly established.
Their work on Drosophila is covered in this chapter (p. 397 onward) and
in this lecture.
There are advantages of certain organisms, Drosophila (fruit fly), mouse,
Caenorhabditis elegans (round worm), zebrafish and Arabidopsis (plant)
PICTURE stages in
fly life (egg, 3 larval instars, pupa, adult)
TRANSPARENCY (Fig. 21.11) This diagram summarizes some of the stages in
Drosophila development, as well as presenting the additional information:
Nucleus divides to make multinucleated embryo
Then cells vs yolk are laid down in blastula
Then there are 3 larval instars
Then comes complete metamorphosis in the pupa (like the butterfly chrysalis
or moth cocoon)
Finally, there is the adult (imago)
TRANSPARENCY (this figure was in earlier editions of your book, but it is
not there any more) One reason Drosophila are so interesting is that, like
many insects (the holometabolous insects), they undergo a complete metamorphosis.
This diagram shows that there are certain tissues in the larva called imaginal
disks that are determined to become adult structures but that do not differentiate
until the pupa.
TRANSPARENCY (Fig. 21.12) Nurse cells in the "mother" put mRNA
for a protein called bicoid in one side of the egg (hence "maternal
effect genes"). When that mRNA is translated a little later in the
embryo, a gradient of bicoid protein defines anterior to posterior.
As you will learn second semester (classification of animals), one fundamental
aspect of the morphology of many phyla is segmentation. Just as segmentation
is fundamental in comparative anatomy, the processes leading to segmentation
are fundamental in comparative embryology. After the anterior-posterior
axis gets laid down several types of genes lay down the segmented pattern
of the larva.
TRANSPARENCY (Fig. 21.13) (work for which Nusslein-Volhard and Wieschaus
won the Nobel Prize) three types of genes, gap genes, pair-rule genes and
segment-polarity genes function in sequence to define segments.
TRANSPARENCY (from another book, but like Fig. 21.14) (now here is what
you would call a mutant!) - legs where antenna should be. Such mutants are
called homeotic mutants, and EBLewis's Nobel Prize contributions pertain
to homeotic mutants.
(back to previous TRANSPARENCY) Homeotic genes act after segmentation genes.
TRANSPARENCY (Fig. 21.15) After homeotic mutants and their respective genes
were elaborated in Drosophila, they were found in numerous organisms, and
the mouse is shown here. Amazingly, genes are ordered along the chromosomes
in the same direction as the anterior to posterior organization in the animal.
(Not cloning as in cloning a gene, covered previously, but cloning an organism)
TRANSPARENCY (Fig. 21.6) 1950's work on amphibians - Since all nuclei should
have all the genes, any nucleus should work to make whole organism, here
taken out of an intestine cell. But not all cells work, so put it in an
egg where it is certain that the nucleus already there has been destroyed.
TRANSPARENCY (Fig. 21.7) 1997 work to make sheep Dolly. Nuclear transfer
by removal followed by cell fusion. Need surogate mother.
Cloning has been extremely controversial, and human cloning is banned. Some
scientists hoping to advance medical treatments would like to distinguish
"theraputic cloning" from cloning to produce a person genetically
identical with the donor. (Also, the word "clone" has several
meanings.) Some think the issue would be simplified by use of the term "nuclear
TRANSPARENCY (Fig. 21.17) Cells are often told what they should become developmentally
by signalling molecules and signal transduction cascades originating from
cells that are nearby. In this example, the anchor cell is organizing the
vulva (egg laying pore) in the nematode.
TRANSPARENCY (Fig. 21.18) Apoptosis -
There has been a lot of research, especially in the last 10 years, on apoptosis
(programmed cell death) in which a signalling mechanism mediates the "deliberate"
developmental loss of cells.
Two types of cell death:
1. necrosis - from injury, cells burst, there is inflammation
2. programmed cell death = apoptosis
In apoptosis, nuclei condense & fragment, cell is phagocytosed (by macrophages
for instance), there is no inflammation, and DNA gets chopped into 50-300
bp lengths, hence "ladder" seen in gel.
examples: lymphocytes eliminated, tadpole tail lost, get rid of webs between
TRANSPARENCY (Fig. 21.4) C elegans usually has clearly specified cell development
In C. elegans, mutants ced-3 and -4 (cell death abnormal) have 131 cells
which should die but survive
ced-9, if normal, represses death
protein Ced-3 is a protease,
There is a histological technique called TUNEL (Tdt-mediated deoxyuridine
triphosphate nick end labeling, of course) which stains cells which are
dying of apoptosis
TRANSPARENCY (Fig. 21.9) Determination
Consider what has been stated repeatedly this semester (with various wordings),
that all the cells have the same genes but express a different subset of
these genes. That implies that an initially generic cell becomes determined
to become a particular type of cell, then eventually differentiates into
a particular cell type. What distinguishes the terminally differentiated
muscle cell (the example in Fig. 21.9) from, say a visual receptor is that
the muscle cell expresses muscle proteins (like actin and myosin) (while
the visual receptor expresses proteins of the visual cascade like rhodopsin,
the visual pigment). This figure shows that transcription factors (here
MyoD) regulate what genes are expressed.
(1) Before cells are determined and differentiated (referred to above as
"generic," these cells can become anything, hence they are called
(2) Even though determined and differentiated cells have all the genes other
cells have, they permanently lose their pluripotency.
TRANSPARENCY (Fig. 21.8)
Stem cell research
Because cells lose their pluripotency, researchers have focussed on their
discovery that embryonic stem cells are better at differentiating into cells
that can repair cell damaged areas such as in the case of spinal cord injury;
the issue is very controversial because it may encourage practitioners to
create and destroy human embryos for no other purpose than to harvest stem
cells. For this reason, for humans, only the use of some 60 cell lines that
are already in culture is permitted.
Several colleagues and I are collaborating
to cure blindness in a mouse mutant with cells that started as embryonic
and were induced to become precursors of nerve cells; identified by green
fluorescent protein, here
is a cell that has been put into the retina and is beginning to show a neuron-like
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