Early Development: Rapid Specification in Snails and Nematodes Part A

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Early Development Rapid Specification in Snails
and NematodesPart A

• BIOL 370 Developmental Biology
• Chapter 5a
• Lange

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• Metazoans - a group (Metazoa) that comprises all
  animals that are multicellular and eukaryotic
  with bodies composed of cells differentiated into
  tissues and organs.
• There are 35 metazoan phyla taxonomically.
• The four MAJOR branches of metazoans that
  comprise the 35 phyla are
• Sponges
• Diploblasts
• Protostomes
• Deuterostomes

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• Following fertilization
• The cell begins significant protein synthesis
• DNA synthesis
• The cell cycle begins
• The next steps in development involve the process
  of cleavage - the division of cells in the early
  embryo. The different cells derived from cleavage
  are called blastomeres and form a compact mass
  called the morula. Cleavage ends with the
  formation of the blastula.

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A typical rendition of the cell cycle.
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Figure 5.2 Cell cycles of somatic cells and
early blastomeres
The amphibian blastomere uses cyclin B to
regulate its two stage cell cycle. Think about
how this will promote division without growth.
The typical somatic cell also uses cyclin B, but
other cyclins as well. Go represents a variation
in the growth (aka gap) phase specific to
differentiating cells.
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• In embryology, cleavage is the division of cells
  in the early embryo.
• The zygotes of many species undergo rapid cell
  cycles with no significant growth, producing a
  cluster of cells the same size as the original
  zygote.
• The different cells derived from cleavage are
  called blastomeres and form a compact mass called
  the morula.
• Cleavage ends with the formation of the blastula.

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• Steps associated with induction of cleavage
• MPF mitosis promoting factor induces the
  stages of mitosis (prophase, metaphase, anaphase,
  telophase).
• MPF cyclicity is guided by cyclin B.
• The initial activation of MPF leads to
  alternating M and S phases with no gap phases.
• As the cytoplasm components for M S are used
  up, the nucleus will then begin to synthesize
  these components. This is when the MBT
  (mid-blastula transition) phase begins. This is
  when the growth gap phases arise.

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• Steps related to cleavage
• Karyokinesis mitotic division of the nucleus,
  driven by the mitotic spindle
• Cytokinesis the division of the cell itself,
  involving the contractile ring of actin
  microfilaments

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Figure 5.2 Role of microtubules and
microfilaments in cell division
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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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Figure 5.3 Summary of the main patterns of
cleavage
We will focus on each type of cleavage in greater
detail in the next few slides.
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Figure 5.3 Summary of the main patterns of
cleavage (Part 1)
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Figure 5.3 Summary of the main patterns of
cleavage (Part 2)
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Figure 5.4 Types of cell movements during
gastrulation