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BIOL 331 / BIOL331 Module 3 (Latest 2026/2027 Update)
Pathophysiology | Verified Questions & Answers | Grade A |
100% Correct – Portage Learning
Describe the difference between holoblastic cleavage and meroblastic cleavage
- Holoblastic cleavage -> complete cleavage
- Meroblastic cleavage -> incomplete cleavage
Define the difference between totipotency and pluripotency and use examples from sea
urchins to illustrate these concepts (!)
PLURIPOTENCY
- by 60 cell stage, most of the cell fates are specified, but they are not "locked in"; while
blastomeres form a specific cell type or lineage, they are still pluripotent and can give
rise to other cell types;
TOTIPOTENCY
- early cells are totipotent -> have the ability to form the entire embryo when isolated;
not set into a pattern, not determined yet
Outline the early cleavage stages for sea urchin development
- embryo volume stays constant, but cell number increases
- division is radial holoblastic - cleavage furrow extends through the entire egg (low yolk
eggs)
- divides evenly until 8 cell stage; then top half (animal half - less yolk) consists of large
cells called mesomeres and bottom half (vegetal half - more yolk) consists of larger
macromeres and smaller micromeres
- 60 cell embryo - blastomere fates are set along the animal-vegetal axis
- at 120 cell stage hollow sphere called blastula, forms a central cavity - blastocoel (filled
with protein fluid); cells are all the same size (as micromeres slowed division rate);
animal cells secrete a "hatching enzyme" - called hatched blastula
- asymmetric cell division -each daughter cell inherits different proteins/mRNAs
Based on the lab notes and laboratory assignments, identify the general stages of early
cleavage to the blastula stage in sea urchins
- two cell stage -> four cell stage -> eight cell stage -> sixteen cell stage (micromeres
present( -> morula -> blastula
- by 16-cell stage, a small group of micromeres are cleaved at the vegetal pole; these are
primary mesenchyme cells and will be the first to show gastrulation movements; then
blastula stage, then embryo has hatched out of the fertilization envelope and is spinning
around using cilia for locomotion - now hatched blastula; gastrulation begins at the
hatched blastula stage
Identify the same stages in Xenopus embryos
- unequal holoblastic cleavage; yolk impedes cleavage so first division begins in animal
hemisphere and moves into vegetal half -> unequal sizes due to yolk means that the
animal hemisphere becomes packed with many small cells while the vegetal hemisphere
, has smaller number of large yolk filled cells
- 16-64 cells morula (Latin - mulberry)
- 128 cell stage - blastula with a hollow center called the blastocoel
Identify the fates of cells from the animal and vegetal poles of a sea urchin embryo prior
to gastrulation
Design an experiment that tests the ability of the micromeres to conditionally specify
the fate of neighbouring tissues
- later in development, cells are specified; animal cells alone are missing inductive
signal, so become a ciliated ball of cells; micromeres produce a strong inductive signal,
can trigger the isolated animal hemisphere to form a normal embryo;
also can check the midterm answer
Define autonomous specification and the inheritance of maternally deposited factors
fates of different layers determined in 2 different ways:
1)autonomous specification -> the four larger micromeres inherit cues deposited in the
egg that become concentrated in the micromeres at fourth cleavage -> they form
skeletogenic (primary) mesenchyme cells
2)conditional specification -> fates of the neighbouring tissues are induced by the
presence of micromeres, secrete paracrine and juxtracrine factors -> cells above
micromeres induced to become endomesoderm (endoderm and secondary
mesenchyme)
asymmetric cell division -> each daughter cell inherits different proteins/mRNAs
what triggers autonomous specification?
- disheveled is deposited in the vegetal half of the egg and triggers the nuclear
localization of the beta-catenin in the subsequent daughter cells
- sea urchin beta catenin is concentrated in the micromeres and is required for the
specification for the endoderm and mesoderm
- overexpression of proteins that interfere with the nuclear localization and/or function
of the beta catenin lead to exmbryos with the excess ectodermal tissues and lack of
mesenchyme cells and the gut
- conversely, overexpression of the beta catenin leads to embryos deprived of the
ectodermal tissue, consisting mainly of the endodermal and mesodermal derivatives
Explain how you might test for the distribution and inheritance of maternally deposited
factors (!)
check out "basic tools of developmental genetics" in textbook p. 85- 91
Understand the main components of the Wnt signaling pathway and how this pathway
is important to specification of the micromeres
- disheveled, beta-catenin - specifying micromeres: disheveled is physically localized in
the vegetal cortex of the oocyte, specifically, it is localized in the region about to become
micromeres;
- summary pathway: wnt -> frizzled -> LRP 5/6 -> disheveled -] GSK3 -> beta catenin -
> transcription
BIOL 331 / BIOL331 Module 3 (Latest 2026/2027 Update)
Pathophysiology | Verified Questions & Answers | Grade A |
100% Correct – Portage Learning
Describe the difference between holoblastic cleavage and meroblastic cleavage
- Holoblastic cleavage -> complete cleavage
- Meroblastic cleavage -> incomplete cleavage
Define the difference between totipotency and pluripotency and use examples from sea
urchins to illustrate these concepts (!)
PLURIPOTENCY
- by 60 cell stage, most of the cell fates are specified, but they are not "locked in"; while
blastomeres form a specific cell type or lineage, they are still pluripotent and can give
rise to other cell types;
TOTIPOTENCY
- early cells are totipotent -> have the ability to form the entire embryo when isolated;
not set into a pattern, not determined yet
Outline the early cleavage stages for sea urchin development
- embryo volume stays constant, but cell number increases
- division is radial holoblastic - cleavage furrow extends through the entire egg (low yolk
eggs)
- divides evenly until 8 cell stage; then top half (animal half - less yolk) consists of large
cells called mesomeres and bottom half (vegetal half - more yolk) consists of larger
macromeres and smaller micromeres
- 60 cell embryo - blastomere fates are set along the animal-vegetal axis
- at 120 cell stage hollow sphere called blastula, forms a central cavity - blastocoel (filled
with protein fluid); cells are all the same size (as micromeres slowed division rate);
animal cells secrete a "hatching enzyme" - called hatched blastula
- asymmetric cell division -each daughter cell inherits different proteins/mRNAs
Based on the lab notes and laboratory assignments, identify the general stages of early
cleavage to the blastula stage in sea urchins
- two cell stage -> four cell stage -> eight cell stage -> sixteen cell stage (micromeres
present( -> morula -> blastula
- by 16-cell stage, a small group of micromeres are cleaved at the vegetal pole; these are
primary mesenchyme cells and will be the first to show gastrulation movements; then
blastula stage, then embryo has hatched out of the fertilization envelope and is spinning
around using cilia for locomotion - now hatched blastula; gastrulation begins at the
hatched blastula stage
Identify the same stages in Xenopus embryos
- unequal holoblastic cleavage; yolk impedes cleavage so first division begins in animal
hemisphere and moves into vegetal half -> unequal sizes due to yolk means that the
animal hemisphere becomes packed with many small cells while the vegetal hemisphere
, has smaller number of large yolk filled cells
- 16-64 cells morula (Latin - mulberry)
- 128 cell stage - blastula with a hollow center called the blastocoel
Identify the fates of cells from the animal and vegetal poles of a sea urchin embryo prior
to gastrulation
Design an experiment that tests the ability of the micromeres to conditionally specify
the fate of neighbouring tissues
- later in development, cells are specified; animal cells alone are missing inductive
signal, so become a ciliated ball of cells; micromeres produce a strong inductive signal,
can trigger the isolated animal hemisphere to form a normal embryo;
also can check the midterm answer
Define autonomous specification and the inheritance of maternally deposited factors
fates of different layers determined in 2 different ways:
1)autonomous specification -> the four larger micromeres inherit cues deposited in the
egg that become concentrated in the micromeres at fourth cleavage -> they form
skeletogenic (primary) mesenchyme cells
2)conditional specification -> fates of the neighbouring tissues are induced by the
presence of micromeres, secrete paracrine and juxtracrine factors -> cells above
micromeres induced to become endomesoderm (endoderm and secondary
mesenchyme)
asymmetric cell division -> each daughter cell inherits different proteins/mRNAs
what triggers autonomous specification?
- disheveled is deposited in the vegetal half of the egg and triggers the nuclear
localization of the beta-catenin in the subsequent daughter cells
- sea urchin beta catenin is concentrated in the micromeres and is required for the
specification for the endoderm and mesoderm
- overexpression of proteins that interfere with the nuclear localization and/or function
of the beta catenin lead to exmbryos with the excess ectodermal tissues and lack of
mesenchyme cells and the gut
- conversely, overexpression of the beta catenin leads to embryos deprived of the
ectodermal tissue, consisting mainly of the endodermal and mesodermal derivatives
Explain how you might test for the distribution and inheritance of maternally deposited
factors (!)
check out "basic tools of developmental genetics" in textbook p. 85- 91
Understand the main components of the Wnt signaling pathway and how this pathway
is important to specification of the micromeres
- disheveled, beta-catenin - specifying micromeres: disheveled is physically localized in
the vegetal cortex of the oocyte, specifically, it is localized in the region about to become
micromeres;
- summary pathway: wnt -> frizzled -> LRP 5/6 -> disheveled -] GSK3 -> beta catenin -
> transcription
