Download File - Mr. Schmitt Biology 12 AP

Notes to Instructors
Chapter 47 Animal Development
What is the focus of this activity?
Chapter 21 provided a review of how genes act to control development. Chapter 47
reviews some of the major morphological changes that occur in the development of many
animals. Similarities in early development are often used as evidence for evolutionary
relationships among organisms.
What is this particular activity designed to do?
Activity 47.1 What common events occur in the early development of animals?
This activity provides a brief overview of major events that occur in the early
development of vertebrates and many other animals.
Answers
Activity 47.1 What common events occur in the early
development of animals?
The early stages in the development of all vertebrates (and many other animals) include
zygote formation, cleavage, blastula formation, gastrula formation, and organogenesis
(for example, neurulation). Among the major aspects of development are cell adhesion/
recognition, cell growth (in number and/or size), cell induction, and cell determination.
1. What key events occur at each stage of development?
Developmental
stage
What occurs during this
stage?
What is the influence or effect on
the subsequent development of the
embryo?
a. Cleavage
In vertebrates, cell division
is radial and indeterminate.
The first mitotic cell
divisions that follow
fertilization occur rapidly
and are not accompanied by
cell growth. The
In many animals (mammals are an
exception), the zygote has a definite
polarity. As a result, the cleavage
divisions create cells that differ in
their cytoplasmic contents (e.g.,
cytoplasmic determinants: mRNAs
and proteins present in the egg cell).
(Continued on next page)
310
Notes to Instructors
Copyright © Pearson Education, Inc., Publishing as Benjamin Cummings
chromosomes duplicate and
are partitioned to daughter
cells, but the cytoplasm is
simply partitioned among
the daughter cells. As a
result, these divisions are
called cleavage.
These differences can affect the
subsequent development of the
daughter cells.
b. Blastula
formation
As cleavage continues, the
blastula forms. The blastula
is a hollow ball of cells
surrounding a fluid-filled
cavity, or blastocoel.
Cells on the outside of the blastula
are in direct contact with the external
environment. Those on the inside are
in contact with the fluid-filled
blastocoel. Those that lie between
are only in contact with other cells.
As a result, the cells experience
differences in chemical and physical
factors that can affect how each
develops.
c. Gastrula
formation
Blastula cells (near what
will become the blastopore)
begin to divide rapidly.
These cells undergo
changes in their shape,
motility, and adhesion
properties. Overall, there is
a general migration of cells
into the blastocoel to form
the archenteron (primitive
gut). Ultimately,
gastrulation results in the
formation of the germ
layers: ectoderm, endoderm,
and mesoderm.
These cellular movements and
rearrangements produce new
gradients in chemical signals and new
types of cell-cell interactions between
ectoderm and mesoderm and between
mesoderm and endoderm.
Extracellular glycoproteins, such as
fibronectin, have been shown to play
a role in directing the cell
movements of gastrulation. By the
late gastrula stage, the fates of many
of the cells have been determined. In
general, ectodermal cells develop into
the nervous system and the
epidermis. The endodermal cells
become the digestive tract and
associated organs (liver and
pancreas).Mesodermal cells give rise
to the muscles, the circulatory
system, the kidneys, and the dermis.
In deuterostomes (echinoderms and
chordates), the anus forms near the
old blastopore and the mouth forms
from a new opening (opposite the site
of the blastopore).
(Continued on next page)
Activity 47.1
Copyright © Pearson Education, Inc., Publishing as Benjamin Cummings
311
Developmental
stage
What occurs during this
stage?
What is the influence or effect on
the subsequent development of
the embryo?
d. Organogenesis
(for example,
neural tube
formation)
Organogenesis is basically
the formation of the organs
from the germ layers set up
in the gastrula. The
notochord and neural tube
(nervous system) are among
the first organ systems to
develop in vertebrates.
The development of one structure is
often triggered by or dependent on
signals (chemical or physical)
produced by other structures. For
example, in early development, the
cells of the dorsal lip of the
blastopore appear to initiate the
development of the neural tube and a
number of other organs.
2. Many animal species share these similarities in early development. Yet, the stage at
which the individual cells of the embryo lose their totipotency can vary considerably
among these species.
a. What does it mean to say that a cell is totipotent?
A cell that is totipotent is undifferentiated and has the potential to differentiate
into any of a number of different types of cell.
b. What factors can affect the point at which a cell loses its totipotency—that is,
when its fate becomes determined?
Among the factors that affect when a cell loses it totipotency are:
• the distribution of cytoplasmic determinants in the cells,
• the presence of “organizing centers or regions” that appear to trigger the
development of surrounding cells,
• an unequal distribution of chemical signals, and
• specific cell-cell interactions encountered by the cell.
Each of these factors appears to have some effect on which genes are activated in
the cells during development and, as a result, how the cells differentiate.
312
Activity 47.1
Copyright © Pearson Education, Inc., Publishing as Benjamin Cummings
47.1 Test Your Understanding
1 and 2.
Choose the graph that best fits each situation.
A
B
C
D
E
time
time
time
time
time
F
time
1. Which graph best describes the change in size of individual cells of a vertebrate
embryo from the time of zygote formation to the end of cleavage? Explain your
answer.
D—During cleavage cells divide rapidly, but there is little or no intervening cell
growth between mitoses. As a result, the cells become smaller and smaller with each
division.
2. Which graph best describes the change in the number of cells in the embryo from
fertilization to gastrulation? Explain your answer.
B—The overall number of cells in the embryo increases continuously from
fertilization to gastrulation.
3 to 6. While the mechanics of gastrulation may differ among various organisms, the
overall objectives and problems are the same. Which of the following are
accomplished by the end of gastrulation?
T/F 3. formation of the three germ layers—ectoderm, mesoderm, and endoderm
True
T/F 4. establishment of the embryonic axis (anterior to posterior)
True
T/F 5. determination of the fates of the individual cells of the gastrula
True
T/F 6. formation of the neural tube
False—This occurs as part of organogenesis
Activity 47.1
Copyright © Pearson Education, Inc., Publishing as Benjamin Cummings
313
7. In a now classic experiment, Spemann and Mangold took cells from the dorsal lip of
the blastopore from one frog embryo (the donor) and transplanted them to an area
opposite the dorsal lip in another frog embryo (the host). As a result, the host
embryo developed two heads, one at the site of the “new” or transplanted dorsal lip
and the other at the site of the host’s original dorsal lip. A thorough examination
indicated that the second head was formed from host cells.
Which of the following developmental cues or mechanisms was most likely the
trigger (or cause) for the generation of the second head. Explain your answer.
a. cytoplasmic determinants that are unequally distributed in the host embryo
b. gradients set up in the egg that gave rise to the host embryo
c. hox genes (or homeobox genes) present in the embryo receiving the transplant
d. cell-cell signalling between the transplanted dorsal lip and the host embryo
e. all of the above
d—If the donor cells had not been transplanted to the host, the host would have
developed normally. As a result, the altered development must have been triggered
by some chemical or cell-cell signal from the transplanted or donor tissue.
314
Activity 47.1
Copyright © Pearson Education, Inc., Publishing as Benjamin Cummings