Lecture 5- Vertebrate Ontogeny Flashcards
How do eggs of most organisms protect the embryo from starvation?
> by accumulating yolk within the cell cytoplasm
True or False: yolk does take part in metabolic activities in the embryo?
> false
> yolk does not take part in metabolic activities in the embryo
How is yolk stored?
> either in granules or flattened platelets
True or false: yolk is identical in various species?
> false
> yolk is similar but not identical in various species
What is yolk composed of?
> High energy compounds such as proteins, phospholipids, and neutral fats
Term for yolk formation?
> vitellogenesis
Where is most yolk synthesized in vertebrates?
> in vertebrates, most yolk is synthesized in the liver, none in the ovary
Where are the insoluble and soluble yolks found?
> yolk is transferred in soluble form via the blood stream to the ovary
> in the ovary, yolk is converted into an insoluble form
Egg classification is based on what?
> the amount of yolk present in the egg
> the distribution of the yolk in the egg
What are Microlecithal eggs?
> eggs containing a very small amount of yolk
> examples: mammalian eggs (except for the egg-laying mammals)
What are Mesolecithal eggs?
> eggs containing a moderate amount of yolk (=70% of egg volume)
> examples: amphibian eggs
What are Microlecithal eggs?
> eggs containing a large amount of yolk (= 99% of egg volume)
> examples: bird and reptilian eggs
What are isolecithal eggs?
> eggs containing a very small amount of yolk which is more-or-less evenly distributed throughout the egg
> examples: mammalian eggs (except for egg-laying mammals)
> NOTE that these would also include Microlecithal eggs
What are telolecithal eggs?
> eggs containing a moderate (Mesolecithal) to very large (macrolecithal) amount of yolk that is concentrated toward one pole of the egg
> examples: amphibian, reptilian, and again eggs
> the distribution of the yolk results in a definite polarity of the egg
» the end of the egg with the concentrated yolk is the vegetal pole
> > > the opposite end of the egg with the yolk-free cytoplasm is the animal pole
True or false: for most species, for eggs are laid, fertilized and develop externally in an aquatic environment?
> true
Why is amphibian development easily observed?
> because development is external and because the developing embryos are not obscured by membranes
What is the first stage of development following fertilization?
> cleavage
> > > cleavage is a mitotic activity not accompanied by growth or differentiation
> > > cleavage results in a multicellular mass
> > > subsequent differentiation will result in the formation of embryonic germ layers and the beginning of the formation of specialized tissues and organs
What type(s) of eggs does holoblastic cleavage occur?
> in Microlecithal and Mesolecithal eggs
> cleavage planes can pass entirely through the egg, creating separate cells
What type(s) of eggs does meroblastic cleavage occur?
> in macrolecithal eggs
> the cleavage plane does not penetrate the yolk
Through where does the first cleavage plane pass in amphibian development?
> the first cleavage plane passes through the animal-vegetal axis of the egg
> > > the first cleavage produces two identical daughter cells (blastomeres)
> > > note that the first cleavage plane begins in the animal pole and is slowed down when it encounter the yolk in the vegetal pole
> > > the second cleavage will begin before the first cleavage plane has been completed
Through where does the second cleavage plane pass in amphibian development?
> the second cleavage plane passes through the animal-vegetal axis of the egg perpendicular to the first cleavage plane
> > > at the end of the second cleavage, there will be four daughter cells equal in size with the same amount of yolk each
Through where does the third cleavage plane pass in amphibian development?
> the third cleavage plane passes perpendicular to the first two
> > > the third cleavage is not equal; it is pushed above the equator of the embryo into the animal pole
> > > the third cleavage will result in eight cells
> > > four cells will be smaller and relatively yolk-free (micrometers)
> > > four cells will be larger and densely pack with yolk (macro eyes)
After the third cleavage occurs, what happens next in amphibian development?
> cleavage continues, but is slowed down in the vegetal pole
> > > results in a solid ball of cells (32-64 cells) consisting of more and smaller cells at the animal pole and fewer and larger yolk-filled cells at the vegetal pole
