Devo Lect 6 - Cleavage Flashcards
Blastomeres
“sprouting part” - name after zygote divides
Cell divisions and cycle
Stored proteins, mRNA; Cell cycle - M, S, M, S (fast!) - get smaller
Patterns of cleavage
Holoblastic (“entire sprouting” - cell division in whole organism) and Meroblastic (“part sprouting” - incomplete cell division)
Holoblastic cleavage subtypes*
Isolecithal “equal amounts of yolk” (radial - echinoderms; spiral - annelids, molluscs; rotational - humans, nematodes) and Mesolecithal “moderate amounts of yolk” (amphibians)
Meroblastic cleavage subtypes*
Telolecithal “yolk more on one side” (bilateral - cephalopod; discoidal - fish, reptiles, birds) and centrolecithal “yolk in middle” (superficial - insects)
Sea urchin cleavage
isolecithal - synchronous, radial (divides x, y then z); cells get smaller; divides to form Blastula larva: hollow sphere containing water and salts in blastocoel - higher osmotic pressure draws in water - SA/V ratio increases
Blastocoel
Hollow space inside developing sea urchin embryo containing solutes and draws in water; space where organs will develop
Importance and differences of mesolecithal cleavage
It has more yolk, so the frog embryo can divide and grow longer in the egg; still synchronous, radial; blastocoel is smaller; unequal sizes, smaller ones around blastocoel (animal pole)
Formation of gray crescent in amphibians
Caused by cytoplasm movement (microtubules); first cell division bisects gray crescent
Hans Spemann experiment
Took a human hair and put it around an amphibian egg along the axis of the first division and separated them, developed into two embryos; second time split with gray crescent on only one side, one developed normal, the other was a ball. Asymmetry of the fertilized egg!
Spiral holoblastic cleavage
Some worms, most molluscs, snails. Blastomeres divide at angles, cause a spiral. Can be left (sinistral) or right handed (dextral)! No blastocoel = stereoblastula
Nodal
Molecule that determines left/right symmetry in development
Rotational holoblastic cleavage
Mammals; little yolk because it will implant and gain nutrients
Why hard to study human cleavage?
Small eggs; fewer eggs; internal development (but can be done in dish)
Differences between mammalian and sea urchin/amphibian frogs
Very slow cleavage (24h vs 30 mins), still have G1 and G2; first division same, second one different (one in Z plane, one in Y); asynchronous; zygotic transcription happens early; compaction
Compaction
In mammals; Occurs around 8 cell stage; connections change between them so it looks like they squish together (via cell adhesion molecules); communication (gap junctions) increases;
Mammalian cleavage - where
Cleavage occur in oviduct and uterus. Ovary isn’t completely attached to oviduct btw;
Explain fig 11.20! Ovulation to implantation**
Ovulation (FSH); release egg and follicular fluid (Rantes, progesterone); sperm translocation; slow in isthmus (cilia); capacitated; fertilization occurs usually in ampulla; egg moves and divides; becomes morula, then blastocyst; hatching (strypsin); early implantation (collagenases)
Timescale for human cleavage
Day 0 - fertilized; Day 1 - 2 cells; Day 3/4 - at isthmus - Morula stage; Day 5-7 - blastocyst
Compare sea urchin embryo and human embryo **
Sea urchin: all cells divide in X, Y then Z; humans divide in X, then one divides in Y and the other in Z (roational); much slower and asynchronous; genome activated during cleavage; compaction
Morula
a solid ball of cells resulting from division of a fertilized ovum, and from which a blastula is formed
