Lecture 16- Development I Flashcards
Are sperm motile? Are eggs motile?
Sperm are motile (flagellated), while eggs (ovum) are nonmotile.
What are the major steps in fertilization? (How does sperm get into egg, how do we ensure same species, how do we prevent other sperm from attaching)
- Sperm dissolves protective layers and binds to surface receptors (Zona pellucida)
- Zona pellucida ensures same species, because it has specific receptors that can only be bound by sperm of the same species.
- Once the sperm has bound to surface receptors, it changes the surface of the egg, preventing polyspermy (so now, no other sperm can bind to the surface receptors.
What are the egg coverings in order (starting from inside egg)?
Plasma membrane
Zona pellucida
Follicle cell
Explain the steps of fertilization. (ALL)
Start with secondary oocyte (arrested in Metaphase II).
Sperm pushes through follicle cells to bind zona pellucida receptors (can only bind if same species). Sperm bound to zona pellucida-> Acrosomal Reaction occurs. Acrosome bursts and releases enzymes that break down the ZP3, allowing sperm to pass through glycoprotein layer-> plasmogamy.
Wave of Ca2+ in egg causes 3 results
1. Cortical Reaction: Cortical granules-> exocytosis. Fuse with plasma membrane. Enzymes trigger slow block-> ZP3 hardens, prevents polyspermy (other sperm attaching to ZP3).
2. Egg Activation
Wave of calcium in egg causes this. Series of metabolic reactions and start of embryonic development. Increases respiration, activates maternal proteins and enzymes, increases protein synthesis.
3. Finish meiosis/do cytokinesis/karyogamy.
Secondary oocyte arrested in Metaphase II must complete meiosis. Then it must complete cytokinesis to separate from second polar body. Then karyogamy occurs-> microtubules guide sperm nucleus to egg. Results in diploid zygote.
ALL HAPPEN AT THE SAME TIME.
What are the consequences of fertilization?
- Determines sex
- Restores diploid state
- Egg Activation
Totipotent meaning
Potential to give rise to all cell types (Diploid zygote has this ability)
What is cleavage?
2nd step in embryonic development. Process by which zygote forms into a multicellular embryo. Rapid divisions without growth.
What is the purpose of yolk? How does yolk affect patterns of division?
Yolk consists of proteins, phospholipids, and fats. Provides food for the developing embryo. Amount and distribution varies between animal groups.
Little yolk= faster, equal division
Lots of yolk= slower, unequal division
Stages of Cleavage
Zygote- 1 cell
Embryo- 2+ cells
Blastula: Hollow embryo (with blastocoel)
Equal Holoblastic
Complete division
Cells are equal in size
Central blastocoel
Echinoderms, mammals, annelids
Unequal Holoblastic
Asymmetric cleavage- due to yolk distribution
Yolk slows down cytokinesis
Amphibians
Meroblastic
Only regions without yolk divide completely
Birds, reptiles, many fish, insects
What function does the outer membrane/covering of an egg have?
Prevention of polyspermy (gametic isolation)
In organisms with shelled eggs, the vegetal pole becomes a(n) _____.
Yolk
What is the first step of fertilization?
Acrosomal reaction
What is the zygote considered to be after the first cleavage?
An embryo (blastula)
What type of cleavage pattern do embryos with a large yolk undergo?
Meroblastic
The _____ is a process by which sperm releases enzymes to degrade the glycoprotein zona pellucida.
Acrosomal reaction
_____ is the ability of a single cell to divide and produce all of the differentiated cells in an organism.
Totipotency
The _____ reaction prevents polyspermy during fertilization.
Cortical
A blastula is composed of approximately 128 _____.
Blastomeres
The concentration of yolk in the vegetal pole of amphibian embryos causes what type of cleavage pattern?
Unequal holoblastic cleavage (smaller and more cells in animal hemisphere / fewer and larger cells in the vegetal hemisphere)
Which event marks the end of fertilization?
First cleavage of the zygote, making it an embryo
The first cleavage of a zygote occurs at the ____ of the ____.
Animal pole / Animal hemisphere
The _____ is an organelle that develops over the anterior half of the head in spermatozoa and contains degradative enzymes.
Acrosome
True or False: Following fertilization, the embryo undergoes rapid growth.
False; the embryo undergoes rapid cleavage (mitosis) but does NOT grow in size or synthesize proteins
_____ are regulatory secretory organelles found within oocytes that prevent polyspermy after fertilization.
Cortical granules
In the absence of a large concentration of yolk, zygotes and blastomeres are completely divided during cleavage, so the number of blastomeres doubles with each cleavage. This process is known as _____.
Equal holoblastic cleavage
True or False: Once the zygote undergoes cleavage it is no longer totipotent.
TRUE
True or False: Only the vegetal hemisphere undergoes cleavage in the meroblastic pattern.
False; ONLY the ANIMAL hemisphere undergoes cleavage in the meroblastic pattern
How are cortical granules activated during the cortical reaction?
Fusion of sperm and egg triggers smooth ER to release calcium into egg cytoplasm which activates cortical granules
