Download Sea Urchin (Pluteus) Eggs: Cleavage and Gastrulation

LAB: Sea Urchin Eggs: Cleavage and Gastrulation
Learning Outcomes (Students will be able to …)
a. identify the role of meiosis and mitosis in the production of a zygote and its development through gastrulation
b. utilize the low and high power of the microscope to locate the different stages of cleavage and gastrulation
c. sequence the stages of cleavage and gastrulation with rationales for the sequencing
d. identify of the primary germ layers, describe their formation, and identify their future differentiation into tissues and organs
e. describe the relationship between cell size and number in cleavage and gastrulation
Standards: 4.1d The zygote may divide by mitosis and differentiate to form the specialized cells, tissues, and
organs of multicellular organisms
Claim: The zygote will divide by mitosis to produce a blastula and then a gastrula that have increasingly
smaller cells due to the rapid rate of cell division and differentiate to form the specialized cells, tissues, and
organs of multicellular organisms
Introduction
The pattern of the early stages in the development of the embryo is very much the same in most animals. A
fertilized egg, or zygote, undergoes a series of rapid cell divisions called cleavage. In this process, the onecelled zygote divides into many smaller cells. Together, such cells constitute an embryo.
At first, the embryo appears spherical (round) without special organs or tissues. Later, as internal changes occur,
the shape of the embryo changes gradually. Cleavage is considered to be over when the rate of cell division
slows down and the cells begin to differentiate. That is, the cells, which at first are very similar, change in
structure and form tissues that have different functions.
At fertilization, the egg produces a new outer membrane called the fertilization membrane. Therefore, when
viewed under the microscope a fertilized egg will appear as a single cell with a double membrane (While an
unfertilized egg will possess only a single membrane, the plasma membrane). The major stages in cleavage are
the two-celled, four celled, eight-celled, and sixteen-celled stages followed by the morula (solid ball of cells,
appears knobby like a blackberry), blastula (hollow ball of cells), and the gastrula stage. The start of
gastrulation is marked by the pushing inward of cells that will form a long channel or primitive gut.
Materials
Stained slides of cleavage (starfish or sea urchin) – all stages
Compound Microscope
Lens paper
Procedure Summary
Step Summary
1.
2.
3.
How/Why
Tool Protocols
Modifications
Procedure and Observations
1. Examine a slide of Sea Urchin (or Starfish) embryos under low power and locate a field of view in which
there are several specimens.
2. Moving the slide as needed, locate and draw an unfertilized egg, a zygote, embryo with consisting of two,
four, and eight cells (not all cells can be seen at this point), and a morula (around 32 cell stage).
Assessment: Why does the zygote undergo the process of mitosis and not meiosis?_______________________
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Unfertilized Egg
Zygote
Two-celled Embryo
Four-celled Embryo
Eight-cell Embryo
Morula
a. How does the size of the blastula compare to the size of the morula, how can you explain this?
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b. How come only one sperm cell can fertilize an egg?
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c. Describe internal fertilization in humans.
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Eventually, the mass of cells becomes a hollow ball or blastula. The central, fluid-filled area is the blastocoel.
While the blastula is a single layer of cells, the process of gastrulation produces a gastrula with three cell layers:
an inner endoderm, an outer ectoderm, and a middle mesoderm. Each of these germinal layers are important to
later development. The endoderm, for example, develops into the lining of the gut while the ectoderm becomes
part of the skin and nervous system. The mesoderm develops into muscle, and some internal organs (you will
not be able to differentiate the mesodermal tissue on your slides).
Gastrulation begins when cells on the surface of the blastula begin to “push in” (much like a balloon's skin
follows your finger when you push it into the side of a balloon). The resulting hole in the side of the embryo is
the blastopore and becomes either the animal's anus or mouth
3. Continue moving the slide as needed to locate, draw, and label a blastula and an early (just pushing in)
gastrula and a late (extended with two layers) gastrula.
Assessment: How does the female prepare for the implantation of the blastula? _________________________
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Blastula
Early Gastrula
Late Gastrula
a. How does a blastula differ from a gastrula?
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b. How does the size of the blastula compare to the early and late gastrula, how can you explain this?
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c. As the cells of the gastrula push in they form a channel (called the archenteron), what do you think this
channel will become, and why?
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Conclusion and Regents Questions
A. Describe the processes of fertilization, cleavage, and differentiation in humans.
B. Describe the formation of an organisms body systems from a zygote.
C. Describe how mitosis and meiosis are involved in embryonic development
D. Sexual reproduction involves the processes listed below.
Processes
A. Differentiation C. Gamete production
B. Fertilization
D. Mitosis
Which sequence represents the order in which these
processes occur?
(1) A B C D
(2) B A C D
(3) C B D A
(4) D B C A
F. The development of an embryo is represented in the
diagram below.
These changes in the form of the embryo are a direct result of
(1) uncontrolled cell division and mutations
(2) differentiation and growth
(3) antibodies and antigens inherited from the father
(4) meiosis and fertilization
E. The diagram below represents a process that occurs
during human reproduction.
The process represented by the arrow will ensure that the
(1) zygote contains a complete set of genetic information
(2) gametes contain a complete set of genetic information
(3) zygote contains half of the genetic information
(4) gametes contain half of the genetic information
G. The diagram below represents stages in the processes of
reproduction and development in an animal.
Cells containing only half of the genetic information
characteristic of this species are found at
(1) A
(2) B
(3) C
(4) D
Reflection Question
1. How well does your lab report demonstrate your understanding of the Learning Outcome (c) rationale for sequencing the steps
of cleavage and gastrulation?
2. What are the limitations of the evidence in terms of supporting your claim? And what would you do differently or next to
reduce the lab’s limitations?
3. How have you improved your microscope skills from prior labs, and what is still challenging for you?
Additional Informational Text
Cleavage, Gastrulation, and Induction
As soon as the sperm enters the egg, the cell membrane of the egg undergoes changes that prevent the entrance of additional
sperm. Meanwhile, the chromosomes from each parent come together and, within a few hours, the first cell division begins. The egg
degrades the cytoplasm and organelles of the sperm; only the chromosomes of the sperm contribute to the fertilized egg.
The early cell divisions of the fertilized egg are called cleavage. The fertilized egg divides into two daughter cells called blastomeres.
These two blastomeres divide into four blastomeres, the four blastomeres divide into eight, and so on. During cleavage, the total
number of cells increases, but the size of each cell decreases. The reason for this strange situation is that cell division occurs so
rapidly that there is not enough time for the individual cells to grow bigger. The constant doubling of cells during cleavage results in
a multicellular embryo very quickly. In a short period, the embryo has over one hundred cells arranged as a solid ball of blastomeres
called a morula. The cells of the morula rearrange themselves into a single layer of cells surrounding a fluid-filled central cavity; the
embryo at this stage is called a blastula.
The next step in development is the formation of the gastrula by invagination, the folding in of the cells of the blastula at a point
called the blastopore. The resulting gastrula is a double-layer cup of cells. The outer layer of cells is termed the ectoderm and the
inner layer of cells is termed the endoderm. The inner endodermal layer surrounds a new cavity, the primitive gut. A third layer of
cells, the mesoderm, develops between the ectoderm and endoderm in most animals. Ectoderm, mesoderm and endoderm are the
three germ layers from which all cells, tissues and organs develop. Cells of the ectoderm differentiate into the epidermis, hair, nails,
claws, sweat glands, tooth enamel, brain, and spinal cord. Mesoderm differentiates into muscles, blood, blood vessels, heart, spleen,
reproductive organs, and kidneys. Endoderm differentiates into the cells lining the digestive and respiratory systems, the liver,
gallbladder, and pancreas.
One of the more fascinating aspects of development is the determination of body form, pattern, and differentiation. Put simply, how
does a cell know what it is supposed to grow up to be? How do cells of the endoderm know they are supposed to form the digestive
and respiratory systems? Induction is the process during which individual cells are "told" what they are supposed to become. Hans
Spemann (1869–1941) received the Nobel Prize in 1935 for over twenty years of research on development in amphibians. In a series
of elegant and delicate "baby hair loop" experiments, he demonstrated that when cells invaginate during gastrulation, they are
induced to form specific cells and organs and that the primary inducer is a specific region of the blastopore. Spemann tied a strand
of baby's hair around a fertilized newt egg so that the nucleus and some cytoplasm were on one side of the ligature while the other
side contained only cytoplasm. After several cell divisions, Spemann loosened the ligature and allowed a nucleus to pass over into
the other side. When cell divisions began on the side with the transported nucleus, the ligature was again tightened to separate the
two masses of cells. The result was the production of two newt larvae, one a bit older than the other. These experiments
demonstrated that all the nuclei of an early embryo are capable of producing embryos. This ability is the basis for much current
practice of cloning agriculture or lab animals.
ARGUMENTATIVE ESSAY
Using the evidence you obtained from the lab, information provided in the lab, your classwork, and the Additional
Informational Text write an argument to support the lab’s claim: The zygote will divide by mitosis to produce a blastula
and then a gastrula that have increasingly smaller cells due to the rapid rate of cell division and differentiate to form the
specialized cells, tissues, and organs of multicellular organisms. Then using the attached rubric you should self-evaluate
your essay – and indicate your next steps.
Please note: in Science Argumentative Essays we only present the evidence (from multiple source – and connected
together) to support our claim. And in general, it is most common to cite 3-5 sources of evidence to support a claim.
Science Argumentative Essay Rubric
4
Claim
The text introduces a compelling claim that is clearly
arguable and takes a purposeful position on an issue.
The text has a structure and organization that is carefully
crafted to support the claim.
Content and
Development
Content is accurate, focused, and consistent; exhibits
control in development of ideas; unified with a fresh
insight.
The text provides convincing and relevant data and
evidence to back up the claim and skillfully addresses
counterclaims.
The conclusion
effectively strengthens the claim and evidence
Use of Evidence
Develops the topic thoroughly by selecting the most
significant and relevant facts, concrete details, quotations,
or other information and examples from the text(s)
Skillfully integrates information into the text selectively
to maintain the flow of ideas and advance the thesis
Skillfully assesses the strengths and limitations of each
source
3
The text introduces a precise claim that
is clearly arguable and takes an
identifiable position on an issue.
The text has an effective structure and
organization that is aligned with the
claim.
Content is somewhat accurate and
fairly clear; offers solid but less
accurate reasoning; contains some
appropriate details and/or examples.
The text provides sufficient and
relevant data and evidence to back up
the claim and fairly addresses
counterclaims.
The conclusion effectively reinforces
the claim and evidence
Develops the topic by selecting
significant and relevant facts, concrete
details, quotations, or other information
and examples from the text(s) (W2)
Integrates information into the text
selectively to maintain the flow of
ideas and advance the thesis(W8)
Assesses the strengths and limitations
of each source (W8)
Organization
The introduction is inviting, states the main topic, and
provides an overview of the paper.
The introduction states the main topic
and provides an overview of the paper.
Information is relevant and presented in a logical order.
A conclusion is included.
2
The text introduces a claim that is
arguable and takes a position.
The text has a structure and
organization that is aligned with the
claim.
The text attempts a structure and
organization to support the position.
Content is somewhat vague OR only
loosely related to the writing task; at
times may be off topic OR too broad
with limited support.
Content unclear; lapses in coherence
OR no relation to writing task; offers
simplistic, undeveloped support for
ideas.
The text provides data and evidence to
back up the claim and addresses
counterclaims.
The text provides data and evidence
that attempt to back up the claim and
unclearly addresses counterclaims or
lacks counterclaims.
The conclusion ties to the claim and
evidence.
Attempts to develop the topic using
facts and other information, but
evidence is inaccurate, irrelevant,
and/or insufficient
Attempts to integrate information into
the text selectively to maintain the flow
of ideas and advance the thesis, but
information is insufficient or irrelevant
Attempts to assess the strengths and
limitations of each source, but
misinterprets information
The introduction states the main topic.
A conclusion is included.
The conclusion is strong.
1
The text contains an unclear or
emerging claim that suggests a vague
position.
The conclusion merely restates the
position.
Does not develop the topic by selecting
information and examples from the
text(s)
Does not integrate information from
the text
Does not assess the strengths and
limitations of each source
There is no clear introduction,
structure, or conclusion
Voice
The author’s purpose of writing is very clear, and
there is strong evidence of attention to audience.
The author’s extensive knowledge and/or experience
with the topic is/are evident.
Use of Analysis
Skillfully draws evidence from informational texts to
support analysis and thesis/claim
Skillfully delineates and evaluates the argument and
specific claims in cited texts, assessing whether the
reasoning is valid and the evidence is relevant and
sufficient
Skillfully identifies false statements and fallacious
reasoning
The author’s purpose of writing is
somewhat clear, and there is some
evidence of attention to audience.
The author’s purpose of writing is
somewhat clear, and there is
evidence of attention to audience.
The author’s knowledge and/or
experience with the topic is/are
evident.
The author’s knowledge and/or
experience with the topic is/are
limited.
Draws evidence from informational
texts to support analysis and
thesis/claim (W9)
Attempts to draw evidence from
informational texts to support analysis
and thesis/claim but evidence is
insufficient and/or irrelevant
Delineates and evaluates the argument
and specific claims in cited texts,
assessing whether the reasoning is
valid and the evidence is relevant and
sufficient (R8)
Identifies false statements and
fallacious reasoning.(R8)
Attempts to delineate and evaluate the
argument and specific claims in cited
texts, assessing whether the reasoning
is valid and the evidence is relevant
and sufficient, but analysis is
insufficient
Diagrams
Diagrams are scientifically accurate and thorough, neatly
drawn; fully labeled; clearly illustrates the scientific
concepts and principles involved
Diagrams are scientifically accurate,
have some roughness; not all labels
present; illustrates the scientific
concepts and principles involved
Attempts to identify false statements
and fallacious reasoning, but argument
is incomplete or insufficient
Diagrams are mostly accurate,
somewhat messy; few or no labels;
scientific concepts and principles not
clearly conveyed.
Terminology &
Conventions
Use of scientific terms and concepts reveals a deep
understanding of underlying scientific ideas.
Scientific terms and concepts are
used correctly.
Scientific terms and concepts are
mostly used correctly.
Grammar and usage are correct and contribute to
clarity.
Very few mistakes in grammar and
usage.
There are several mistakes in
grammar, usage, spelling, and
punctuation.
Punctuation is correct and paragraphing adds to
organizational structure.
Paragraphing may create minor
problems.
Spelling and punctuation are almost
entirely correct.
Paraphrasing is uneven in its use
and not always clear.
The author’s purpose of writing is
unclear.
Does not use evidence from the
informational texts to support analysis
and/or thesis/claim
Does not delineate or evaluate claims
in text
Does not identify false claims or
fallacious reasoning
Diagrams are inaccurate or incomplete
with limited ability to convey scientific
information to the reader.
Scientific terms and concepts are
used incorrectly, or topic is
discussed in completely nonscientific terms.
Errors in grammar and usage affect
meaning.
Paragraphing is missing or does not
relate to organization of text.
Frequent spelling errors.
Punctuation is often missing or
incorrect