Download Chapter 44 – How Do Organisms Become Complex

Chapter 40 – Animal Development
Stages of development
1. gamete formation
2. fertilization
3. cleavage-the division of the zygote into many smaller cells by mitosis.
Division of the zygote is necessary because the fertilized egg has a very low surface to
volume ratio.
Early protein synthesis, which is responsible for cleavage, is directed by maternal mRNA
that is already present in the cytoplasm of the egg when it is fertilized. Thus, the nucleus
does not play a role in this process, it is only passively replicated.
Cleavage differs between species due to the amount of yolk present.
Sea urchins have very no yolk, so they exhibit complete cleavage with all cells being
approximately the same size. Frogs have a some yolk, so their cells exhibit complete
cleavage, but the cells at the vegetal pole (with yolk) are larger than those at the animal
pole (without yolk).
In humans, the cells divide and form a blastocyst, which is a hollow ball of cells with
another mass of cells inside it. The outer sphere of cells is called the trophoblast, which
will become the placental attachment to the mother. The mass on the inside is called,
surprise surprise, the inner cell mass. The inner cell mass will develop into the embryo.
4. Gastrulation (formation of three germ layers).
This is the formation of the three layers of precursor cells.
a) The outer layer is the ectoderm, which will give rise to the nervous system, skin, and
sensory organs.
b) The middle layer is the mesoderm, which will give rise to the connective tissues
including muscles, bones, and blood cells.
c) The inner layer is the endoderm, which will give rise to the GI tract and associated
organs as well and the respiratory system.
Sea Urchin
Sea urchins gastrulate by an invagination in the hollow ball of cells. This infolding is
done by actin filaments. The archenteron is the primitive gut, the opening of which is
called the blastopore. In sea urchins the blastopore will become the mouth.
Frogs
Frogs’ gastrulation is similar to a sea urchin’s in that it involves an invagination. In frogs
this invagination occurs at the grey cresent (the border between the cells and the yolk).
This initial site of invagination is called the dorsal lip of the blastopore, and it serves as
the “primary organizer” because it seems to be directing what happens to the other cells.
(If you remove the dorsal lip from one embryo and implant it into another, two
blastopores, and ultimately siamese frogs, will form.) The invagination occurs as the cells
from the animal pole begin to spread out and push the cells from the vegetal pole inward
at the dorsal lip. These inner cells will form the mesoderm and the endoderm. (see figure
40.11)
Humans
Humans gastrulate in a manner similar to birds and reptiles, even though we have
comparatively little yolk. First, two layers of cells form: the hypoblast and the epiblast.
The hypoblast will become the extraembyronic membranes and does not contribute cells
to the developing embryo. The cells of the epiblast will invaginate at the primitive streak
and these migrating cells will form the endoderm and the mesoderm. The remaining cells
of the epiblast become the ectoderm. A three layer structure is formed called the
embryonic disk.
5. Organ formation – the movement and specialization of cells to form functioning
organs. One highly studied organ development is the process of neurulation.
Humans
Cells from the mesoderm form the notochord, which induces the overlying ectoderm to
change structurally to form the neural plate. Mesodermal cells also form two somites on
either side of the notochord, which will become muscles and bones. The notochord
induces the neural plate to invaginate, forming the neural groove. As the neural groove
pinches off to form the neural tube, some cells break off and become the neural crest
(forms the peripheral nerves and glia).
Q.
What structures will the neural tube form in an adult?
6. Growth
7. Metamorphosis
8. Aging
Chapter 15 – Cell fate
As the zygote develops, the fate of the cells becomes increasingly specified.
1. Totipotent – cell has the potential to form an entire organism. No selective
transcription is taking place.
2. Determined – the fate of the cell has been decided. For example, it will become a
muscle cell. But, it has not yet become that muscle cell. Stem cells, which are
present throughout life, are determined cells that have not yet differentiated.
3. Differentiated – the cell has become what it was fated to be.
The question then becomes why developmental potential gradually decreases. Originally
it was thought that cells lost genes, which is what determined what type of cell they
would be. Now we know that cells do not lose genes. If they lost genes how would they
still retain the ability to direct the development of an entire organism? Cells develop
characteristics of a particular tissue through differential gene expression. Some genes are
expressed in muscle cells that are not expressed in neurons and vice versa. Tissues also
direct the development of their neighboring cells by secreting inducers, called
morphogens, that reflect positional information and direct proper development.
Pattern formation can also be directed by cytoplasmic determinants (elements in the
cytoplasm that tells cells what to do). These determinants are transcription factors
present in the egg that become partitioned in specific cells. In the case of the bicoid
protein in drosophila, the transcription factor directs the development on the head and
thorax structures. If this factor is lacking, the embryo will develop without a head.
Q.
Q.
If you draw some cytoplasm from the future head end of a drosophila into a syringe
and then inject it into the future rear end, what would you expect to happen?
How is Dolly proof that cells do not lose genes?