CLEAVAGE Flashcards
(146 cards)
1
Q
- A series of mitotic divisions whereby the enormous volume of egg cytoplasm is divided into numerous smaller, nucleated cells.
- Occurs after fertilization
A
Cleavage
2
Q
Cleavage ia a series of what
A
mitotic division
3
Q
in cleavage, the enormous volume of egg cytoplasm is divided into what
A
numerous smaller, nucleated cells
4
Q
when does cleavage occur
A
after fertilization
5
Q
Characteristic of cleavage
A
- No growth
- No change in shape
- Minimal qualitative changes in chemical composition
- Minimal cytoplasmic displacement
- Increased nucleus-to-cytoplasm ratio
- No major morphological changes
6
Q
division of this is rapid in most embryos
A
Early Cleavage divisions
7
Q
why early cleavage division is rapid?
A
- Cleaving cells have a modified cell cycle, in which Gap phase, G1, and G2 are completely omitted
- the cells cycle rapidly between M and S phases
8
Q
duration of somatic cell cycle
A
hours
9
Q
duration of early cleavage cycle
A
30 mins
10
Q
Chemical Changes During Cleavage
A
- Increase in DNA synthesis (Nuclear Material)
- RNA synthesis
- The Midblastula Transition (MBT)
- Protein Synthesis
11
Q
what happens to the DNA during chemical changes in cleavage
A
- DNA replication occurs before every cell division
- Nuclei double with each cleavage
- No G1/G2 phases → rapid S (DNA synthesis) and M (mitosis)
- Drives increase in nuclear material
- Critical for cell division and future gene expression
12
Q
why DNA replication occurs occurs before every cell division
A
so both daugther cells get complete set of genetic material
13
Q
as fertlized egg divides rapidly into more cells (cleavage) what happens to the numebr of nuclei
A
- doubles– 1 becomes 2, 2 becomes 4, and so on.
- Each new nucleus has the same DNA as the original
14
Q
What is skipped/omitted during early cleavage
A
G1 (growth)
G2 (Preparation for mitosis
15
Q
cells switch rapidly between what during early cleavage
A
between S and M phases, whuch speeeds up the division porcess
16
Q
The total size of the embryo doesn’t grow much yet but the amount of nuclear material increse sharplyduring cleavage. Why is this so?
A
Beacuse DNA is being replicated rapidly and cells are dividing quickly
17
Q
DNA syntehsis is critical for what
A
- cell division
- future gene expression
18
Q
RNA synthesis during Cleavage
A
- Early cleavage uses maternal RNA
- Zygotic transcription begins mid-cleavage
- Start of mRNA and tRNA synthesis
- Enables the embryo to produce its own proteins
19
Q
T OR F: Right after fertilization, the embryo make its own RNA. Explain why
A
F: it relies on RNA nad proteins already present in the egg, ehich were made by the mother before fertilization (called maternal RNAs)
20
Q
what happens in zygotic transcription
A
embryo starts using its own DNA to make RNA
21
Q
What happens when zygotic transcription begin
A
embryo produces proteins (mRNA nd tRNA )
22
Q
Midblastula Transition (MBT) during cleavage
A
- Shift from maternal to zygotic control
- Increased RNA synthesis at MBT
- Confirmed by actinomycin D treatment
- Zygotic genome activation begins
23
Q
chemical that blocks RNA synthesis
A
Actinomycin D
24
Q
What is the major transition at MBT
A
Control shifts to the embryo’s own (‘zygotic) control
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T or F: surge in zygotic transcription occurs -> embryo's DNA is now actively used to make RNA
T: this signals that the zygotic genome is waking up
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T or F: Actinomycin D confirms the RNA synthesis
T: when embryos is treated with actinomycin D, development halts at MBT
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T or F: Zygotmic genome activation is the official start of the embryo's independent genetic activity
True
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Protein Synthesis during cleavage
- Protein synthesis increases throughout cleavage
- In sea urchins, protein synthesis increases drastically; in frogs, it's moderate
- Key proteins:
Nuclear histones
Tubulin
Ribonucleotide reductase
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what animal shows a drastic increase in protein synthesis after fertilization
sea urchin
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what animal shows an moderate increase in protein synthesis after fertilization
frogs
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Types of egss on the basis of AMOUNT of yolk
1. Alecithal
2. Microlecithal
3. Mesolecithal
4.Megalecithal/Macrolecithal/Polylecithal
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yolk is almost absent
alecithal
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example of alecithal egg
eutherian (placental) eggs
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How does embryo gets nourishment if yolk is almostb absent in alecithal eggs
gets nourishment directly from the mother via the placenta, so yolk isn't needed
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- They have very little amount of yolk
- aka Oligolecithal
Microlecithal
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who proposed the term oligolecithal
Romer and Balenski
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Examples of Microlecithal egs
Amphioxus and Tunicates
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- Have moderate amount of yolk
Mesolecithal
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Exmaples of Mesolecithal
Amphibian, Dipnio fishes, Petramizon (lamprey), and annelid worms
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They have large amount of yolk.
Megalecithal / Macrolecithal / Polylecithal
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example of Megalecithal / Macrolecithal / Polylecithal
Insect, reptiles, mixine, cartilage fishes and prototherian mammals.
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Types of Eggs on the basis of DISTRIBUTION OF YOLK
1. Isolecithal/homolecithal
2. Telolecithal
3. Centrolecithal
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- even distibution of yolk in the ooplasm
isolecithal/homolecithal
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examples of isolecithal/homolecithal egg
- Alecithal and microlecithal
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- yolk is found on the vegetal pole
- yolk is unequally distributeed in the ooplasm
telolecithal
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what do you call the yolk free pole
animal pole
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example of telolecithal
fishes, amphibians, reptiles and birds
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yolk is present in the center and sorrounded by ooplasm
centrolecithal
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example of centrolecithal
insect and hydrozoa
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refers to the cytoplasm of an egg cell, also known as an ovum
ooplasm
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Types of eggs based on the PRESENCE OR ABSENCE of hard shell
1. Cleidoic eggs
2. Non cleidoic eggs
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- such egss are covered by hard shell for protection which is permeable for gases
- yolk present in sufficient quantity
Cleidoic eggs
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- They are without shell and these develop in aquaic medium and uterus of female
Non cleidoic egg
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example of cleiodoic egg
reptiles and birds
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examples of non cleidoc egg
amphioxis, mammal, frog, and hardmania
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where is non cleidoc egg is developed
aqautic medium and uterus of a female
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Types of eggs on the basis of DEVELOPMENT
1. Determinate or Mosaic eggs
2. Indeterminate or Regulative Eggs
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- types of egg where before fertilization, the different parts of embryo called the mosaic eggs are already determined
- if any part of the egg is removed, the related organs does not originate in the embryo
Determinate or Moasic eggs
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- Different parts determined after third cleavage, these eggs are called regulative eggs.
- If two blastomere are separate after first cleavage both blstomeres developed the complete embryo.
In man twins developed due to this region.
Indeterminate or regulative egg
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2 different patterns of Cleavage
1. Total or Holobalstic Cleavage
2. Meroblastic Cleavage
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- a type of cleavage where the eintire egg undergoes divison
- can be characterized by equal sized or unequal sized balstomeres
Total or Holoblastic Cleavage
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2 types of total or holobalstic cleavage
1. equal holoblastic cleavage
2. unequal holoblastic cleavage
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- In microlecithal and isolecithal eggs, cleavage leads to the formation of blastomeres of equal size.
- Example: Amphioxus and placental mammals
Equal holoblastic cleavage
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Wha formation is prduced when the entire egg divides completely during early embryonic development
formation of individual cells (blastomeres)
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where is equal holoblastic cleavage ocuurs
in microlecithal and isolecithal egg
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- In mesolecithal and telolecithal eggs, cleavage leads to the formation of blastomeres of unequal size.
- Among the blastomeres there are many small sized micromeres and few large sized macromeres.
- Example: Lower Fish and amphibians.
Unequal holobalstic cleavage
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unequal holoblastic cleavage is seen in what type of egg
mesolecithal and telolecithal eggs
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what is formed due to uneven ditribution of yolk (uneven holoblastic cleavage)
many small cells (micromeres) and fewer large cells (macromeres)
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Holobaltic Cleavage can furthere be classifeid into 4 types based on the symmetry of cleavage
1. radial holoblastic
2. spiral holoblastic
3. bilateral holoblastic
4. rotational holoblastic
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- occurs such that the resulting daughter cells are located exactly on top of one another .
- a caharaceristioc of Deuterostomes, and results in indeterminate cells
Radial Cleavage
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Radial cleavage is a key characteristics of what animals
Deuterostomes
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- occurs such that the resulting daughter cells are not located exactly on top of one another; instead, they are located at a slight angle, forming a spiral-like pattern
- a characteristic of protostomes, and results in determinant cells
spiral cleavage
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- has two identical halves when the balstula is cut vertically
- this kind of cleavage establishes a clear left-right symmetry
- occurs dur to unequal holoblastic
Bilateral cleavage
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estbalished by the plane of the first cleavage furrow
- ex: higher mammals, amphibians, and tunicates
plane of bilateral symmetry
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- The first cleavage is a normal meridional division; however, in the second cleavage, one of the two blastomeres divides meridionally and the other divides equatorially
- Characteristics of mammals
Rotational Cleavage
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what happens during the first cleavage in rotational cleavage
it is a meridional division, meaning the egg divides vertically from top to bottom, splitting it into two cells.
77
what happens during the scond cleavage in rotational cleavage
- one of the two blastomeres continues to divide meridionally but the other blastomere divides equatorially, which is a horizontal division around the middle of the cell.
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In this type the cleavage furrows are restricted to the active cytoplasm found either in the animal pole (macrolecithal egg) or superficially surrounding the egg (centrolecithal egg).
Merobalstic Cleavage
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why egg dos not divide completely in meroblastic cleavage
the egg contains a large amount of yolk, which physically prevents the cleavage furrows from passing all the way through the egg.
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2 types of meoblastic cleavage
1. Discoidal
2. superficial
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- Since the macrolecithal eggs contain plenty of yolk, the cytoplasm is restricted to the narrow region in the animal pole.
- Hence cleavage furrows can be formed only in the disc-like animal pole region.
- Eg: birds and reptiles.
Discoidal Cleavage
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where is the cytoplasm concentrated in discoidal cleavage
in a small disc-shaped region at the animal pole — the top of the egg.
83
where can cleavage furrow fromed in discoidal cleavage
Disc-like animal pole region
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- In centrolecithal eggs, the cleavage is restricted to the peripheral cytoplasm of the egg.
- Eg: insects
Superficial cleavage
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- an embryo that resembles mulberry,
- blastomeres are in early cleavage stage.
Morula
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- forms in meroblastic cleavage
- yolk lies under the disc.
Blastodisc
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- a hollow ball of cells with a fluid-filled cavity, blastocoel, in the center.
- Blastula
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Characteristic of Mammalian Cleavage
1. Holoblastic
2. Slow and Aynchronous
3. Formation of Morula and Blastocytes
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stages of mammalian cleavage
Zygote -> morula -> blastula -> Saggital section of balstula
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- A single cell that results from the fusion of a sperm and an egg during fertilization.
- It represents the earliest stage of embryonic development and contains genetic material from both parents.
fertilization
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an embryo that has reached the 2- to 4-cell stage after fertilization.
day 2 embryo
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an embryo that has reached the 6- to 8-cell stage
day 3 embryo
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a stage of embryonic development that occurs around Day 3-4 after fertilization.
morula
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a stage of embryonic development that occurs around Day 5-6 after fertilization.
blastocyst
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refers to the presence and characteristics of nuclei within blastomeres, the cells of an early-stage embryo.
BLASTOMERE NUCLEATION
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key aspect of blastomere nucleation
1. genetic equivalence
2. functional equivalence
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Each cell or nucleus within an organism or embryo contains the same genetic material, including the same DNA sequence and genes.
genetuc equivalence
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Different cells can perform similar functions or achieve the same outcome, despite potential differences in their underlying structure or mechanisms.
functional equivalence
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refers to the ability of a single cell to develop into a complete organism, having the potential to give rise to every cell type and tissue.
totipotency
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the ability of an embryo or organism to adapt and adjust its development in response to environmental or genetic changes, ensuring normal development.
development flexibility
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a process of generating a genetically identical copy of a cell or an organism
cloning
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most common method used from cloning animals
somatic nuclaer transfer
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process of somaitic cell nucelar transfer
1. Removing of nucleus from an egg
2. insering the nucleus from a somatic cell into enucleated egg
3. stimilating the egg to develop
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development of normal embryos when donors came from where
early and late blastula
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lower successful rate when they are the donor
nuclei from diffrerented cells/ adult
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pros of cloning
- prevent extinction of species
- increase food production
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cons of cloning
- process is not entirely safe and accurate
- regarded as unethical, and the probability of abuse is very high
not yet fully-developed
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Factors Determining the Fate of Blastomeres
1. extrinsic factors
2. intrinsic factors
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- differentiation of cells are not traced to differences in nuclear components of each cell but difference in cytoplasmic components
intrinsic factors
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become differentially inherited during cleavage.
Unevenly distributed RNAs, proteins, and organelles in the egg cytoplasm
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Experimental Evidence Supporting Intrinsic Control
1. Spemann's Constriction Experiment
2. Dentalium Zygotes
3. Germ Plasm Theory
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- Newt zygotes were constricted using a hair loop, separating halves of the egg.
- If both halves received part of the gray crescent (a cytoplasmic region rich in determinants), two normal embryos formed.
- If only one half got the gray crescent, only that half developed properly—showing intrinsic cytoplasmic factors matter.
a. Spemann’s Constriction Experiment
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A specific blastomere (blastomere D) consistently gives rise to mesoderm because it inherits a unique portion of vegetal cytoplasm
dentalium zygotes
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Proposes that specific "germ plasm" is unequally distributed to form germ cells (PGCs), supporting the idea of intrinsic localization.
Germ Plasm Theory (Weismann)
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Before the zygote genome activates, it determines early development.
maternal mRNA/protein
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example of maternal effect genes
Shell coiling in Lymnaea peregra (a snail)
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Key intrinsci factors
1. mRNAs (Maternal mRNAs) - codes for transcription factors or signaling proteins.
2. Proteins - often localized to specific regions
3. Transcription Factors - activate or repress specific genes in the blastomere that inherits them.
4. Localized Cytoplasmic Determinants - physically partitioned into certain cell.
5. Organizing Centers - act as embryonic organizers
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- Factors outside the cell that influence its fate.
extrinsic factors
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- Blastomeres communicate via paracrine factors.
- Regulated by environment rather than pre-determined.
Inductive signaling
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Experimental support of inductive signaling
1. determinate development (mosaic cleavage)
2. indeteminate developmet (regulative cleavage)
3. Horstadius’ Experiments in Sea Urchins
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. Horstadius’ Experiments in Sea Urchins
He separated blastomeres from different poles:
- Micromeres (vegetal pole) → form larval skeleton
- Macromeres → form endoderm and ectoderm
- Mesomeres (animal pole) → form ectoderm
Micromeres transplanted to animal pole could induce nearby cells to form endoderm—clear evidence of extrinsic inductive power.
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- The egg cytoplasm contains gradients of morphogens.
- These gradients provide positional information,
Morphogenetic Gradients
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experiments that supports morphogenetic gradient
1. Spemann and Mangold Organizer - Induction of second body axis.
2. Xenopus Animal Cap Assay - Fate changes with environment.
3. Notch-Delta Signaling - Lateral inhibition in C. elegans neurogenesis
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Early cleavage stages are often driven by
intrinsic cues
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activatred after MBT
zygote genome
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what happens when zygotic genome become activated
cells bcome responsive to extrinsic cues
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Svem Horstadius experiment
Focused on purple sea urchin, Paracentrotus lividus
exhibits a pigmented vegetal line
Investigate the roles of different regions in development, particularly during blastulation and gastrulation
dissected early embryos (typically at the 8, 16, 32, or 64-cell stages) into their constituent blastomeres along and perpendicular to the vegetal axis
127
effects of manipulating different vegetal layers of the embryo
1) Normal embryo (top and bottom rows): All vegetal tiers are intact, leading to the formation of a normal larva with proper endoderm and mesoderm development.
2) Complete animalization (second row): The vegetal part (especially Veg2 and micromeres) has been removed. Only animal pole cells are left, so the embryo becomes a hollow ball of ectodermal cells — no gut, no mesoderm, just ectoderm (epidermis).
3) Incomplete animalization (third row): Removal of deeper vegetal cells (e.g., Veg1), but some Veg2 cells are left. This causes partial development, often resulting in abnormal structures or incomplete guts.
4) Recognizable larva (fourth row): Micromeres are removed, but Veg2 is retained. The embryo still develops into a somewhat normal larva, suggesting that Veg2 can give rise to mesoderm even in the absence of micromeres
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investigated the double gradient hypothesis by John Runnstrom
recombination of micromeres
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were added to layers of cell
micromeres
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isolated cells without micromeres
useless ciliated cells
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led to “better” development
gradual addition of micromeres
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- promote the formation of ectoderm
- could be inhibited by lithium ions, sodium azide, dinitrophenol
- might involve inhibition of oxidative enzymes which produce ATP
Vegetalizing agents
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- promote the formation of endoderm and mesoderm
- could be inhibited by acidic dyes, zinc, mercury, anionic detergents, some proteolytic enzymes
- animalization may involve ability to attack proteins especially basic proteins
Animalizing agents
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the mother’s genetic makeup influences the early developmental processes of the embryo.
organisms that exhibit a maternal effect
135
where is rhe products of the mother’s genes (usually mRNA or proteins) are deposited into
the egg cytoplasm before fertilization.
136
Maternal effect trait: shell coiling direction of Lymnaea peregra (Freshwater Snail)
dextral = right-handed
sinistral = left-handed
137
The shell coiling direction is established early in the
first few cleavage divisions (when the embryo divides into multiple cells
138
orientation of shell coiling is driven by
the cytoplasmic environment of the egg, which reflects the mother’s genotype.
139
A fascinating amphibian, famous for its neoteny (retaining juvenile features into adulthood) and regenerative abilities.
Ambystoma mexicananum — the axolotl
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maternal effect lethal mutation
o gene
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What happens when an axolotl inherits two copies of the 'o' gene (homozygous oo)
it normally dies at an early stage of development.
142
what happens to the embryo if the egg originated from a mother carrying the normal allele
the embryo can develop normally — even if the embryo’s own genotype is oo
143
provides some corrective factor (likely a protein or RNA product) from the nucleus, which can rescue the development of an otherwise genetically defective embryo.
mother’s egg
144
If the egg lacks this , the embryo will die, regardless of its own genetic makeup.
maternal corrective factor (from an 'o' gene mother)
145
