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

2. 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

3. 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.

4. 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.

5. 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 jpg

6. 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.

7. 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.

8. 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

9. Role of microtubules and microfilaments in cell division
DevBio9e-Fig jpg

10. 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

11. 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 jpg

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

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

14. Types of cell movements during gastrulation
DevBio9e-Fig jpg

15. End.