A Continuation of A First Look at Early Development: More On Rapid Specification in Snails and Nematodes

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A Continuation of A First Look at Early
Development More On Rapid Specification in
Snails and Nematodes
BIOL 370 Developmental Biology Topic 10

• Lange

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• To understand cleavage, we need additional
  vocabulary
• Vegetal pole the yolk rich region
• Animal pole the yolk devoid region
• Isolecithal roughly equal distribution of yolk
  (such as in sea urchins)
• Holoblastic cleavage complete cleavage
• Meroblastic cleavage partial cleavage where
  only some of the cytoplasm is cleaved (insects,
  fish, reptiles, birds)
• Centrolecithal centrally placed yolk (insects)
• Telolecithal only one area is free of yolk
  (birds and fish)
• Discoidal cleavage cleavage in the telolecithal
  eggs that occurs only in the small disk of
  cytoplasm
• Holoblastic cleavage subtypes
• Radial
• Spiral
• Bilateral
• Rotational

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• Vegetal pole the yolk rich region
• Animal pole the yolk devoid region

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• Isolecithal roughly equal distribution of yolk
  (such as in sea urchins)
• Centrolecithal centrally placed yolk (insects)
• Telolecithal only one area is free of yolk
  (birds and fish)

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• Holoblastic cleavage complete cleavage
• Meroblastic cleavage partial cleavage where
  only some of the cytoplasm is cleaved (insects,
  fish, reptiles, birds)
• Discoidal cleavage cleavage in the telolecithal
  eggs that occurs only in the small disk of
  cytoplasm
• Holoblastic cleavage subtypes
• Radial
• Spiral
• Bilateral
• Rotational

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Summary of the main patterns of cleavage
We will focus on each type of cleavage in greater
detail AGAIN in the next few slides.
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Summary of the main patterns of cleavage (Part 1)
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Summary of the main patterns of cleavage (Part 2)
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The shape of the organism grows much more complex
as you move from the blastula stage to the
gastrula stage.
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Types of cell movements during gastrulation

• In this slide, we are seeing how during
  gastrulation, there is a noticeable shift in
  terms of cell movement. Notice the five types
  shown
• Invagination
• Involution
• Ingression
• Delamination
• Epiboly

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Axes of a bilaterally symmetrical animal
With this increasing complexity due the
in-folding of layers of cells, the 3 dimensional
shape of the organisms will no longer be
symmetrical in every plane.
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Figure 5.6 Cleavage in the sea urchin

• Holoblastic cleavage (complete cleavage)
• Specifically in the urchin
• 1st 2nd cleaveges are meridional ("along a
  meridian" or "in the northsouth direction)
• 3rd cleavage is equitaorial
• 4th cleavage is more specialized the animal
  poles divide meridionally into mesomeres, the
  vegetal poles divides both equatorially but also
  unequally (larger cells called macromeres and the
  smaller called micromeres)

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Cleavage in the sea urchin (Part 1)

• (continued)
• 5th (16 cell stage) cleavage
• mesomeres (animal) equitorially to produce an1
  and an2
• macromeres divide meridionally
• micromeres divide unequally producing four small
  mircomeres and four large micromeres
• 6th division animal hemisphere cells divide
  meridionally and vegetal cells divide
  equatorially.
• 7th division the vegetal cells divide
  meriodionally and the animal cells divide
  equatorially.
• The embryo is now at 120 cells and is called a
  blastula.

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Micrographs of cleavage in live embryos of the
sea urchin Lytechinus variegatus, seen from the
side
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Fate map and cell lineage of the sea urchin
Strongylocentrotus purpuratus (Part 1)
Embryo at 60 cell stage.
Oral towards the mouth Aboral away from the
mouth
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Fate map and cell lineage of the sea urchin
Strongylocentrotus purpuratus (Part 2)
Cell lineage and fate map.
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Ability of micromeres to induce presumptive
ectodermal cells to acquire other fates
In this series we can see how even in the
dissected embryo missing its vegetal portion, the
micromeres can guide and direct differentiation
into a larva.
What does this suggest as the role for micomeres?

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Normal sea urchin development, following the fate
of the cellular layers of the blastula

• The blastula continues development into a
  gastrula
• late stage blastula will contain roughly 1,000
  cells
• shape is in the form of a hollow ball
• the blastomeric cells are of different shapes,
  sizes, and properties

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Gastrulation is a phase early in the embryonic
development of most animals, during which the
single-layered blastula is reorganized into a
trilaminar ("three-layered") structure known as
the gastrula. These three germ layers are known
as the ectoderm, mesoderm, and endoderm. Gastrula
tion is followed by organogenesis, when
individual organs develop within the newly formed
germ layers. Each layer gives rise to specific
tissues and organs in the developing embryo.
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Simplified view of the change from blastula to
gastrula.
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The ectoderm gives rise to epidermis and also
tissues that will later form the nervous system.
The mesoderm is found between (meso middle)
the ectoderm and the endoderm and gives rise to
the muscular system, cartilages, the dermis, the
notochord, blood and blood vessels, bone, and
connective tissue. The endoderm gives rise to
the epithelium of the digestive system and
respiratory system, and organs associated with
the digestive system, such as the liver and
pancreas.
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Entire sequence of gastrulation in Lytechinus
variegatus
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Invagination of the vegetal plate
The archenteron is the primitive gut that forms
during gastrulation. It develops into the
digestive tract of an animal.
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Cell rearrangement during extension of the
archenteron in sea urchin embryos
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The imaginal rudiment growing in the left side of
the pluteus larva of a sea urchin
The imaginal rudiment is the precursor tissue for
the developing skeletal structure of the urchin.
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A related line to the skeletal development seen
in the previous slides about the urchin, is the
shell development seen in a variety of snails.
These molluscs that develop conchae (spiral
shaped shells), can develop often develop either
a right handed spiral or a left handed spiral.
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Spiral cleavage of the mollusc Trochus viewed
from the animal pole (A) and from one side (B)
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Looking down on the animal pole of left-coiling
(A) and right-coiling (B) snails
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Fate map of Ilyanassa obsoleta
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The glochidium is a microscopic larval stage seen
in some freshwater mussels, aquatic bivalve
mollusks, and European freshwater pearl
mussels. This larval form has hooks, which
enable it to attach itself to fish (for example
to the gills of a fish host species) for a period
of time prior to when it detaches and falls to
the substrate and takes on the typical form of a
juvenile mussel. Ecologically, since fish are
active and free-swimming, this process helps
distribute the mussel species to potential areas
of habitat that it could not reach via its own
locomotion.
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Formation of a glochidium larva by the
modification of spiral cleavage
Be sure to read the Sidelights Speculations
associated with this work on pp. 167-168.
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Phony fish atop the unionid clam Lampsilis altilis
This fake fish is actually a brood pouch
holding glochidium in this species of clam. It
attracts predatory fish and allows the glochidium
easy access to these fish.
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The two-step process for specifying the marginal
cells of the tunicate embryo
Macho-1 the gene/RNA material FGF fibroblast
growth factor
A fibroblast is a type of cell that synthesizes
the extracellular matrix and collagen, the
structural framework for animal tissues, and
plays a critical role in wound healing.
Fibroblasts are the most common cells of
connective tissue in animals.
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Gastrulation in the tunicate
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Convergent extension of the tunicate notochord
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Caenorhabditis elegans is a free-living,
transparent nematode (roundworm), about 1 mm in
length, which lives in temperate soil
environments. Research into the molecular and
developmental biology of C. elegans was begun in
1974 by Sydney Brenner (Nobel Prize in Medicine
in 2002) and it has since been used extensively
as a model organism.
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The nematode Caenorhabditis elegans (Part 1)
In A, we see the hermaphroditic form In B, we see
how the eggs are forced to pass through sperm
cells as they mature, which in effect forces
fertilization to occur Note that earlier meiotic
divisions actually involve sperm production and
the spermatheca is only for STORAGE.
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The nematode Caenorhabditis elegans (Part 2)
In the embryos formed, the P cell represents
the stem cells.
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The nematode Caenorhabditis elegans (Part 3)
See comparisons with the Development Tree shown
below.
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PAR proteins and the establishment of polarity
Polarity the beginning of orientation of cell
lineages in an organism. In C. elegans, this is
guided by PAR proteins.
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PAR proteins and the establishment of polarity
(Part 2)
Note the green staining in (F) which shows the
PAR proteins fluorescently stained.
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Deficiencies of intestine and pharynx in skn-1
mutants of C. elegans
In the images below, we see the effects of the
SKN-1 mutation in C. Elegans. This mutation
affects most notably the development of
musculature of the pharynx and intestine.
Therefore, this mutation drastically alters the
development of the digestive system in this
species.
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End.