Lecture 7 - Cleavage Flashcards
characteristics of cleavage
- rapid mitotic division transforming unicellular to multicellular embryo
- no growth in size
what happens to ratio of nucleus to cytoplasm
progressively increases
what are the phases that are not included during cleavage
G1 or G2 phase, no cell growth
what increases during cleavage
nuclear material at expense of cytoplasm
where is the nuclear material from
- ribonucleic acids in cytoplasm
- low molecular weight precursors
what synthesis is limited during cleavage
rRNA and tRNA synthesis
when does rRNA synthesis resume
gastrulation
most important for cleavage
- nuclear histones
- tubulin
- ribonucleotide reductase
types of eggs based on amount of yolk
- macrolecithal
- mesolecithal
- microlecithal
has large amount of yolk
macrolecithal
has moderate amount of yolk
mesolecithal
has small amount of yolk
microlecithal
types of eggs based on the distribution of yolk
- telolecithal
- isolecithal
- centrolecithal
concentrated in one region of the egg
telolecithal
evenly distributed throughout the egg
isolecithal
concentrated in the center of the egg
centrolecithal
patterns of cleavage
- incomplete/meroblastic
- complete/holoblastic
types of cleavage under isolecithal egg
- radial cleavage
- spiral cleavage
- bilateral cleavage
- rotational cleavage
animals with complete radial cleavage
- echinoderms
- amphioxus
animals with complete spiral cleavage
- annelids
- molluscs
- flatworms
animals with complete bilateral cleavage
tunicates
animals with complete roational cleavage
- mammals
- nematodes
types of cleavage under mesolecithal egg
displaced radial cleavage
animals with displaced radial cleavage
amphibians
types of cleavage under telolecithal egg
- bilateral cleavage
- discoidal cleavage
animals with incomplete bilateral cleavage
cephalopod molluscs
animals with discoidal cleavage
- fish
- reptiles
- birds
types of cleavage under centrolecithal egg
superficial cleavage
animals with superficial cleavage
most insects
the entire egg is not completely divided
meroblastic cleavage
- under meroblastic cleavage
- division of the egg is confined to a disc of cytoplasm
- occurs in strongly telolecital eggs like frog, reptiles, and birds
discoidal cleavage
complete division of the egg occurs
holoblastic cleavage
- under holoblastic cleavage
- division of the egg is complete but cells are unequal in size
displaced radial cleavage
- under holoblastic cleavage
- with no obvious polarity
rotational cleavage
blastomeres lie directly on top of each other
radial cleavage
blastomeres lie on the junction between lower blastomeres due to oblique position of spindle
spiral
two types of turn in spiral cleavage
- dextral cleavage
- sinistral cleavage
dextral cleavage
turn spiral in clockwise
sinistral cleavage
turn spiral in counterclockwise
division in centrolecithal egg
only nuclear division, no cytoplasmic
determined to give rise to specific parts of the embryo
definite blastomeres
type of cleavage in definite blastomeres
determinate cleavage
development of determinate cleavage
mosaic development
example of definite blastomeres
Ascaris egg
- embryo resembles mulberry
- blastomeres are in early cleavage stage
morula
- cleavage cells form the blastoderm epithelium enclosing blastocoel
- cells form gap and tight junctions
blastula
what junctions are formed in blastula stage
gap and tight junctions
- forms in merblastic cleavage
- yolk lies under the disc
blastodisc
peculiarities in mammalian cleavage
- holoblastic rotational
- slow division; asynchronous
- embryo undergoes compaction
- early expression of zygotic genes
stage of the egg inside the uterus dueing blastocyst stage
early stage implantation
gamete contains determinants which are apportioned differentially into blastomeres which develop nto different cell types
Weismann’s germ plasm theory
contradicts Weismann’s germ plasm theory
- Spemann’s expt on constricted newt zygote; 2 halves developed into normal embryos
- In dragonfly, one of the first 2 nuclei of fertilized egg was irradiated, but a whole embryo still developed
- cloning expts - first done on frogs
cloning
nuceli transplantation into enucleated and activated egg
had less success in cloning
nuclei from progressively differentiated/ adult donors
Factors determining fate of blastomeres
- intrinsic
- extrinsic
- differentation of cells are not traced to differences in nuclear components of each cell but difference in cytoplasmic components
- cytoplasm dictates
intrinsic
- what the blastomeres become may be dictated by surrounding blastomeres
- environment dictates
extrinsic
Supported the intrisic factors in differentation
- Spemann’s expt of egg constriction
- Dentalium zygote
- Styela partita zygote
- Centrifugation experiments
- germ plasm observation
normal embryos resulted in each half contained grey crescent
Spemann’s expt of egg constriction
only blastomere D with clear cytoplasm from veg pole develop mesoderm
Dentalium zygote
comes from egg with cytoplasm clearly divided into 4, each with different fates
Styela partita
Supported the extrinsic factors in differentiation
- at early stage if future ICM and trophoblast cells are mixed, whatever locates inside becomes embryo and whatever is outside becomes trophoblast
- concept of regulatory/indeterminate cleavage and determinate or mosaic pattern
products or proteins which affect the cells
morphogens
Morphogenetic gradient in egg cytoplasm:
Macromeres
from veg pole develop into ectoderm and endodermal structures
Morphogenetic gradient in egg cytoplasm:
Micromeres
from veg pole into larval skeleton or spicules
Morphogenetic gradient in egg cytoplasm:
Mesomere
from animal pole into most of ectoderm of larva
splitting of blastomeres into vegetal and animal pole cells cause what
- vegetalization or
- animalization of larva
experimented in the vegetalization or animalization of larva
Horstadius
vegetalizing agents
- lithium ions
- sodium axide
- dinitrophenol
animalizing agents
- acidic dyes
- zinc
- mercury
- anionic detergents
- some proteolytic enzymes
vegetalization might involve what
inhibition of oxidative enzymes which produce ATP
animalization might involve what
ability to attack proteins esp. basic proteins
determine the peculiarity of the egg in early development
cells of the maternal body
example of peculiarity of egg that is determined by genotype of mtoher
dextrality or sinistrality of freshwater snail (Lymnea peregra)
- maternal inhertiance in lethal ‘o; gene
- correctiv factor is found in the nucleus of normal egg
axolotl (Amystoma mexicanum)
