Chapter 5 Flashcards
The egg cytoplasm plays a major role in determining
patterns of cleavage, gastrulation, and cell specification (by interacting with the nuclear genome established at fertilization).
During cleavage and gastrulation, the major axes of the embryo are determined and the embryonic cells begin to acquire their respective fates. Three body axes:
- Anterior-posterior (head-tail)
- Dorsal-ventral (back-belly)
- Left-right
Different species specify these aces at different times, using different mechanisms.
Metazoans Sponges – develop completely different from any other animal group.
- Has three major types of somatic cells, one of which, the archeocyte, can differentiate into all the other cell types in the body.
- Individual cells can reaggregate to form new sponges, in some instances, species-specific (thought that the motile archeocyte collects its own species).
- Contain no mesoderm => no true organ systems, no digestive tube, circulatory system, nerves, or muscles.
- Do undergo gastrulation and pass through an embryonic and larval stage however.
- Share many features of development with all the pther metazoan phyla; including gene regulatory proteins and signalling cascades.
Metazoans Diploblasts – animals that have two germ layers, ecto and endoderm. Includes jellyfish etc.
- Most have no mesoderm and radial symmetry.
- Some however, have mesoderm like derived tissues, but it seems to have evolved independently from traditional mesoderm (evolutionary convergence)
Metazoans Triploblasts – animals with three germ layers, bilateral symmetry (anterior-posterior)
- Protostomes – (Greek, “mouth first”), includes molluscs and arthropods, mouth is formed first, anus later at a different location.
- Deuterostomes – (Greek, “mouth second”), includes chordates (includes vertebrates, chord = notochord) and echinoderms, mouth is formed second, anus first
Blastomere
cell derived from cleavage in an early embryo
Blastula
embryonic stage composed of blastomere (mammalian = blastocyst)
Blastocoel
fluid filled cavity within the blastula
Stereoblastula
blastula that lacks blastocoel
Blastopore
invagination where gastrulation begins
Cleavage is?
a series of mitotic divisions after fertilization, whereby the enourmous volume of egg cytoplasm is divided into numerous smaller, nucleated cells.
In most species (mammals chief exclusion), both the initial rate of cell division and the placement of the blastomeres with respect to one another are under the control of the proteins and mRNAs stored in the oocyte. Only later does these come under the control of the newly formed genome.
Cleavage and cytoplasmic volume
Initially, cytoplasmic volume does not increase instead the zygote cytoplasm is divided into increasingly smaller cells.
Cleavage is so rapid in beginning in vertebrates probably to quickly restore the somatic ratio of nuclear volume to cytoplasmic volume, often accomplished by abolishing gap periods of the cell cycle (G1 + G2). Fx. Frog egg – 37k cells in 43 hrs
Blastomere cell cycle
generally biphasic – M (mitosis) and S (DNA synthesis).
- MPF highest during M, undetectable during S.
- Shift between driven solely by the gain and loss of MPF activity.
Mitosis-promoting factor (MPF)
regulates cell cycle of early blastomeres.
- Triggers entry into M phase (nuclear envelope breaks down, chromatin condenses into chromosomes), lasts about an hour, then it’s degraded and chromosomes return to S phase.
- Consists of two subunits: larger = cyclin B, smaller = cyclin-dependent kinase (CDK) (fx cdc2)
- Most regulators of cyclin B (and thus MPF) is stored in egg cytoplasm => cell cycle independent of nuclear genome in beginning, until used up
Mid-blastula transition (MBT)
When MPF is no longer controlled by regulators present in egg cytoplasm, but nucleus begins to synthesize them by itself.
- Gap stages (G1 + G2) are added to biphasic cell cycle; xenopus at 12th cleavage, drosophila at 14th +17th respectively.
- Synchronicity of cell cycle lost, pga different cells synthesize different regulators of MPF.
- New mRNAs are transcribed, many encode proteins necessary for gastrulation
Cytoskeletal mechanisms of mitosis - Mitotic cleavage is the result of two coordinated processes:
- Karyokinesis: mitotic division of cell nucleus, mechanical agent is mitotic spindle, with its microtubules composed of tubulin
- Cytokinesis: the division of the cell itself, mechanical agent is contractile ring of microfilaments made of actin.
Division plane is controlled by
the placement of the centrioles, pga controls orientation of mitotic spindle.
Depending on placement, daughter cells can separate along ventral/dorsal axis, anterior/posterior or left/right axis, and symmetrical / asymmetrical.
Embryonic cleavage patterns determined by:
- The amount and distribution of yolk protein within the cytoplasm – determines where cleavage can occur and the relative size of the blastomere. In general, yolk inhibits cleavage.
- Factors in the egg cytoplasm that influence the angle of the mitotic spindle and timing of its formation
Embryonic cleavage patterns
- Holoblastic – (Greek holos, “complete”), cleavage furrow extends through the entire egg, little yolk = other ways to obtain food (most through voracious larval form, mammals through placenta)
- Meroblastic – (Greek meros, “part”), only a portion of the cytoplasm is cleaved, cleavage furrow does not penetrate the yolky portion, which serves as sufficient food to nourish these animals throughout embryonic development
Embryonic cleavage patterns - Holoblastic (Greek holos, “complete”), cleavage furrow extends through the entire egg, little yolk = other ways to obtain food (most through voracious larval form, mammals through placenta)
- Isolecithal – (Greek, “equal yolk”), eggs with sparse, equally distributed yolk
- Mesolecithal – moderate vegetal yolk disposition
