BIOL 370 – Developmental Biology Topic #6 A First Look at Early Development: Rapid Specification in Snails and Nematodes Lange

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

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.

A typical rendition of the cell cycle. G1 Growth S Growth and DNA synthesis G2 Growth and final preparations for division M G2 checkpoint G1 checkpoint (restriction point) A typical rendition of the cell cycle.

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. DevBio9e-Fig-05-01-0.jpg

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.

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.

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

Role of microtubules and microfilaments in cell division DevBio9e-Fig-05-02-0.jpg

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

Summary of the main patterns of cleavage We will focus on each type of cleavage in greater detail in the next few slides. DevBio9e-Fig-05-03-0.jpg

Summary of the main patterns of cleavage (Part 1) DevBio9e-Fig-05-03-1R.jpg

Summary of the main patterns of cleavage (Part 2) DevBio9e-Fig-05-03-2R.jpg

Types of cell movements during gastrulation DevBio9e-Fig-05-04-0.jpg

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