A First Look at Early Development: Rapid Specification in Snails and Nematodes
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A First Look at Early Development:
Rapid Specification in Snails and Nematodes
Lange
BIOL 370 – Developmental Biology
Topic #6
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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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G1
Growth
SGrowth and DNA
synthesis G2
Growth and finalpreparations for
divisionM
G2 checkpoint
G1 checkpoint(restriction point)
A typical rendition of the cell cycle.
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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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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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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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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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Types of cell movements during gastrulation
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End.