Lecture 8 Flashcards
Where does fertilization occur in mammals?
Fertilization occurs in the AMPULLA
What is completed after sperm entry?
When does the first cleavage happen?
MEIOSIS is completed after sperm entry.
The first cleavage happens a day later.
What is cell cleavage?
Cell cleavage is the period of rapid cell division after fertilization.
In cell cleavage, the egg divides into many smaller cells, but does cell volume or size change?
No, in cell cleavage the egg divides into many smaller cells but cell volume and size DOES NOT change.
What makes cell cleavage in mammals unique?
Mammalia cell cleavage has a unique nature:
- relatively slow (12-24h apart)
- blastomeres organized in unique orientation
- pattern of cleavage within a species doesn’t change (genetic control?)
In most species, what controls cell cleavage?
In most species, MATERNAL FACTORS control cell cleavage
Maternal factors control the following:
6 things
Ribosomes (protein translation)
DNA polymerases (DNA replication)
Lipids (formation of new cell membranes)
RNA (gene expression)
Structural proteins, enzymes, etc
ATP, nucleotides, amino acids, and other building blocks/metabolites
Once maternal factors are exhausted, what takes over?
What is this process called?
As maternal factors are exhausted, ZYGOTIC GENOME starts to take over
This is called MID-BLASTULA TRANSITION (MBT)
In mammals, when is the zygotic genome activated?
during the first cell cycle
True or False: In mammals, cleavage divisions are synchronous.
False
Not synchronous
Following MBT, what occurs?
Maternal to zygotic transition (MZT)
What happens during MZT?
MZT takes place as
the cell cycles lengthen
gap phases are introduced
and the time spent in S phase is increased
What is the subsequent transition that occurs alongside MBT and MZT?
Early gastrula transition (EGT)
What is EGT?
Early gastrula transition (EGT)
When the embryo acquires the ability to perform apoptosis
Describe the somatic cell cycle in general.
Somatic cell cycle:
G1, S, G2 and M phase
G1 - growth and prep for DNA synthesis
S - DNA replication
G2 - growth and prep for mitosis
M - mitosis and cytokinesis
Describe the cell cycle during early cell cleavage. How is it modified?
What is this coordinated by?
In early cell cleavage, the cell cycle is modified so it only have S and M phases
This is coordinated by maternal factors in the egg
MPF = mitosis promoting factor (two subunits: cyclin B and cdc2/cdk1)
What are cell division patterns also called?
cleavage patterns
Cleavage patterns variations are dependent on:
amount and distribution of yolk protein within cytoplasm
distribution of cytoplasmic determinants
type of cell-cell interactions
Many eggs have a polarity based on the presence of _____
yolk
What is the vegetal pole?
yolk-rich part of the zygote that contains cells that divide slowly
What is the animal pole?
the yolk-poor part of the zygote that contains cells that divide rapidly
Where is the zygote nucleus generally displaced? Which pole?
The animale pole
the yolk-poor part of the zygote that has rapidly dividing cells
What are the types of cell cleavage patterns?
Holoblastic (complete) cleavage
Meroblastic (imcomplete) cleavage
What is holoblastic cleavage?
Give an example.
complete cleavage
the entire egg is divided into smaller cells
i.e. amphibians and mammals
animals with low amount of yolk
cleavage furrow extends thru an entire egg
What is meroblastic cleavage?
Give an example.
incomplete cleavage
when only a portion of the cytoplasm is cleaved
i.e. insects, fish, reptiles, birds
animals with large amount of yolk (sufficient to nourish the embryo throughout development)
cell furrow does not go through the yolky part of the egg (membrane formation is inhibited by the yolk)
What are the types of holoblastic cleavage?
isolecithal cleavage
mesolecithal cleavage
telolecithal cleavage
centrolecithal cleavage
What is isolecithal cleavage?
It is a type of holoblastic cleavage
Occurs in embryos with sparse, evenly distributed yolk (Isolecithal = equal yolk)
e.g. mammals, sea urchins
What is mesolecithal cleavage?
It is a type of holoblastic cleavage
Occurs in embryos with an intermediate sized yolk that is concentrated in one hemisphere of the egg (vegetal pole)
e.g. amphibians
Complete cleavage but occurs relatively slowly in the vegetal pole –> asymmetric cell division
What is telolecithal cleavage?
It is a type of holoblastic cleavage
Most of the cell has dense yolk and only one small area at the animal pole that is free of yolk undergoes division
e.g. birds, fish
Discoidal cleavage: Cell division occurs only in a small disc of cytoplasm that is free of yolk
What is centrolecithal cleavage?
It is a type of holoblastic cleavage
Yolk in the center of the egg
division occurs only in the rim of cytoplasm around the cell periphery (superficial cleavage)
e.g. insects
At the end of the cleavage, what is the embryo referred to as?
blastula
At the end of the cleavage, the embryo is referred to as the blastula. What are the individual cells of the blastula called?
blastomeres
Blastomeres enclose a fluid-filled cavity called _____
blastocoel
insulates cells from signalling molecules and provides space for gastrulation
Blastomeres enclose a fluid-filled cavity called the blastocoel.
What is the role of blastocoel?
insulates cells from signalling molecules and provides space for gastrulation
Many mammalian embryos undergo __________ cleavage.
rotational cleavage
What is rotational cleavage?
first cleavage is a normal meridional cleavage
but in second cleavage, one blastomere divides meridionally and the other equatorially
Mammalian blastomeres can contain an odd number of cells. Why?
Because mammalian blastomeres do not divide synchronously (at the same time)
Cell number does not increase exponentially (i.e. 2, 4, 8, 16)
What is the unique feature in mammalian cleavage?
E-cadherin becomes expressed (8 cell embryo)
changes adhesion properties
blastomeres become tightly attached and form a compact ball of cells
Mammalian cleavage - compaction
Compacted 8-cell embryo divides to form _________
16-cell morula
Compacted 8-cell embryo
What are the properties of the 16-cell morula ?
16-cell morula
contains a group of outer cells (trophoblast) that surround a group of inner cells (inner cell mass – ICM)
Trophoblasts secrete fluid into morula (cavitation) to form blastocoel
ICM is positioned on one side of the ring of trophoblast cells - forms blastocyst
i.e. mammalian blastula
What is the ICM and what does it give rise to?
Inner cell mass (ICM) are pluripotent internal cells that give rise to
- The embryo
- Yolk sac: enables nutrient uptake and development of circulatory system
- Allantois: develops on posterior end of the embryo, stores waste products
- Amnion: “Water sac” – allows the embryo to float in a fluid environment and prevents shock and desiccation
What are trophoblast cells (trophectoderm)?
Trophoblast cells (trophectoderm) are external cells that form extraembryonic tissues
Chorion: contains blood vessels that exchange gasses with outside environment and is used for nutrition
- Secretes hormones that cause the mother’s uterus to retain the fetus
- Produces immune system regulators –prevents mother’s body from rejecting the fetus
Embryonic portion of placenta
What activates genes that specify the outer cells to become trophoblasts?
Transcription factor Tead4 promotes transcription of Cdx2
As the embryo moves through oviduct towards uterus, where does the blastocyst expand?
blastocyst expands within the zona pellucida
What prevents the expanding blastocyst from adhering to the oviduct walls?
zona pellucida
ZP prevents it from adhering to the oviduct walls
What is an ectopic pregnancy?
- embryo adheres to oviduct walls
- medically critical condition
Embryo needs to ‘hatch’ from ZP once it reaches the uterus
Mediated by trophoblasts —> secrete proteases –> digest a small hole in ZP
How does the blastocyst hatch?
Blastocyst ‘hatches’ by digesting a small hole in ZP and squeezing through it as blastocyst expands
Once outside, the blastocyst makes direct contact with the uterus
Implantation in the uterus
What alters the epithelial lining of the uterus?
estrogen and progesterone alter the epithelial lining of the uterus
After initial binding, several adhesion systems come into play to hold the embryo to the uterine lining
What is the epithelial lining of the uterus called?
endometrium
extensive extracellular matrix that is ready to ‘catch’ the blastocyst
Trophoblasts synthesize integrins that bind to _____, _____, and _____
Trophoblasts synthesize integrins that bind to collagen, fibronectin and laminin
P-cadherins on trophoblasts and endometrium help with adhesion
Wnt pathways get activated -trophoblasts secrete a set of proteases (e.g. collagenase)
Proteases digest the ECM
Blastocyst ‘buries’ itself in uterine wall
Implantation in the uterus
There is an ‘implantation window’ for the blastocyst to adhere to the endometrium
There is an ‘implantation window’ for the blastocyst to adhere to the endometrium e.g. LIF expression goes up and pinopodes (apical protrusions of endometrium epithelium) appear blastocyst has LIF receptors
How long does fertilization to implantation take?
About 7 days
