Cleavage and Specification Flashcards by Lance Andrie Malana

Cleavage Stages

Zygote: Fertilized egg.

Reductive Cleavage: Zygote undergoes rapid mitotic divisions without increasing in overall size. Each division results in smaller daughter cells (blastomeres).

Zygotic Genome Activation: As cleavage continues, the embryo begins transcribing its own genome instead of relying solely on maternal mRNA.

Blastula Formation: Cells continue to divide and rearrange into a hollow structure called the blastula.

Cell Fate Specification: Some cells begin differentiating, setting the stage for gastrulation, where the three germ layers (ectoderm, mesoderm, endoderm) form.

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Cellular Changes (from zygote to blastula)

Size Decrease: The overall embryo does not grow significantly, but the cells become progressively smaller.

Shape Change: Cells reorganize from a single large zygote into a multicellular structure.

Differentiation: Cells begin to specialize, preparing for the next developmental stages.

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Graph of Cell Size Over Development

The graph shows that cell size decreases as cleavage progresses. The large zygote divides into progressively smaller blastomeres, leading to the formation of the blastula.

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How do our cells know when to stop dividing?

when they receive chemical signals from other cells, or when their chromosomes shorten

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How is cell division so tightly regulated?

through a system of “checkpoints” within the cell cycle

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How is the formation of different cells coordinated in space and time?

through morphogen gradients, cell-cell communication, gene regulatory networks, cell cycle control, epigenetic modifications, and mechanical forces, ensuring precise differentiation and tissue organization.

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RESULTS OF FERTILIZATION (Initiation of Cleavage)

❖Completion of meiosis II
❖Restoration of diploidy
❖Chromosomal sex determination
❖Genetic and evolutionary implications
-Paternal and maternal chromosomes
-Further mixing during the next
gametogenesis (crossing over)

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The transition from fertilization to cleavage is caused by the activation of _____,

——– is the cyclin-Cdc complex that was discovered first in frog eggs.

MPF (Maturation/M-Phase Promoting Factor) (Mitosis)

It stimulates the mitotic and meiotic phases of the cell cycle.
MPF promotes the entrance into mitosis (the M phase) from the G2 phase by phosphorylating multiple proteins needed during mitosis

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MPF Composition: MPF consists of _________ and _______ (a cyclin-dependent kinase, CDK1)

Cyclin B; Cdc2

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Activation: During the S phase, cyclin B is synthesized and binds to Cdc2, forming inactive MPF.

Mitosis Entry: As cyclin B accumulates, MPF becomes active, triggering the cell’s entry into mitosis (M phase).

Cyclin Degradation: After mitosis, cyclin B is degraded, leading to MPF inactivation and allowing the cell to transition back into the S phase for another cycle.

This regulatory cycle ensures proper cell division timing by balancing cyclin levels, promoting mitotic progression, and resetting the cycle.

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COMPARE: Cell cycles of early blastomeres and somatic cells

Simplified (Blastomeres) vs. Complete Cell Cycle (Somatic cells)

Regulation by Cyclins and CDKs:
More in Somatic cells

Presence of Gap Phases (G₁ & G₂)
Only in Somatic cells

Cyclin Degradation
More complex in Somatic cells

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Cleavage vs Normal Mitosis

Blastomeres do not grow in size between successive cell divisions. Cleavage occurs very rapidly, and mitosis and cytokinesis in each round of cell division are complete within an hour.
Typical somatic cells divide much more slowly (several hours to days) and even the fastest cancer cells divide much slower than occurs in a zygote during cleavage.

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2 Coordinated Processes during Cleavage

Karyokinesis
Cytokinesis

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Karyokinesis mechanical agent, major protein composition, location, and major disruptive drug

Mechanical agent = Mitotic spindle
Major protein composition = Tubulin microtubules
Location = Central cytoplasm
Major disruptive drug = Colchicine, nocodazole^a

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Cytokinesis mechanical agent, major protein composition, location, and major disruptive drug

Mechanical agent = Contractile ring
Major protein composition = Actin filaments
Location = Cortical cytoplasm
Major disruptive drug = Cytochalasin B

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Because colchicine has been found to independently inhibit several membrane functions, including osmoregulation and the transport of ions and nucleosides, ________ has become the major drug used to inhibit microtubule-mediated processes.

Nocodazole

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Cleavage happens in the uterine tube in __ day/s

1 day

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Morula is found in the uterine tube in ___ day/s

3 days

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Blastocyst is observed in ___ day/s

4 days

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Implantation of egg happens in ___ day/s

6 days

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is the first phase of embryonic development

Cleavage

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❖ Division of cytoplasm w/o increasing its volume
❖ Divisions are not synchronous

Cleavage

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What influences the patterns of cleavage?

Amount and distribution of yolk in the egg
Factors in the egg (molecular cues) that influence the angle of mitotic spindle formation (e.g. patterns of yolk deposition, site of fertilization cone, etc.)
Cell shape
Embryo size

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Animal pole = less yolk, fast division, ectoderm
Vegetal pole = more yolk, slow division, mesoderm and endoderm

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Classification of eggs as to the amount of yolk

1. Isolecithal eggs (iso = equal) have a small amount of yolk that is equally distributed in the cytoplasm (ex. most mammals). 2. Mesolecithal eggs (meso = middle) have a moderate amount of yolk, which is present mainly in the vegetal hemisphere (ex. amphibians). 3. Telolecithal eggs (telo = end) have a large amount of yolk that fills the cytoplasm, except for a small area near the animal pole (ex. fish, reptiles, and birds). 4. Centrolecithal eggs have a lot of yolk that is concentrated within the center of the cell (ex. insects and arthropods)

The individual cells resulting from cleavage are called _______

blastomeres

Types of Embryonic Cleavage

❖ 1. Holoblastic cleavage ❖ 2. Meroblastic cleavage

Type of embryonic cleavage: - Occurs when there is little or evenly distributed yolk. - The entire zygote undergoes cleavage. - Seen in mammals, amphibians, and echinoderms.

Holoblastic

Types of Holoblastic Cleavage:

1. Radial Cleavage – Cells stack directly on top of each other in a symmetrical pattern (e.g., sea urchins, amphibians). 2. Spiral Cleavage – Cells arrange at an angle, forming a spiral pattern (e.g., mollusks, annelids). 3. Bilateral Cleavage – The embryo divides into symmetrical halves (e.g., tunicates). 4. Rotational Cleavage – Characterized by different planes of division within the embryo (e.g., mammals).

Type of embryonic cleavage: - Occurs when the yolk is abundant and prevents complete division. - Only part of the zygote undergoes cleavage. - Seen in birds, reptiles, fish, and insects.

Meroblastic

Types of Meroblastic Cleavage:

1. Discoidal Cleavage – Cleavage occurs at the animal pole, forming a disc of cells (e.g., birds, reptiles). 2. Superficial Cleavage – Nuclei divide within a shared cytoplasm before the cell membranes form (e.g., insects).

Type of meroblastic cleavage: Creates a cellular region above the dense yolk

Discoidal cleavage (ex. fish)

Type of meroblastic cleavage: A large mass of centrally located yolk confines cleavage to the cytoplasmic rim of the egg

Superficial cleavage (ex. Drosophila)

Cleavage pattern in Zebrafish embryos and spindle orientation During the first five cell cycles, spindles (double arrows) remain aligned to a single, horizontal (x–y) plane parallel to the blastodisc plane, alternating 90° every cell cycle. During the sixth cell cycle, spindles tend to align in the vertical orientation, generating two tiers of blastula cells. Spindle orientation becomes random during the seventh cell cycle

Early embryonic cleavage pattern in ascidians (sea squirts):

alternating perpendicular pattern consistent with a dependence on cell shape

Typical Cleavage Patterns (depending on yolk content)

1. Isolecithal egg -> Holoblastic cleavage -> Sea urchin 2. Mesolecithal egg -> Holoblastic cleavage -> Frog 3. Telolecithal egg -> Discoidal cleavage -> Bird 4. Centrolecithal egg -> Superficial cleavage -> Insect

Radial cleavage in sea urchins: simple planes of cleavage* 2 planes

Meridional cleavage planne Equatorial cleavage plane *Cleavage plane is at right angle to the metaphase plate

In sea urchins, the first and second cleavage is _________. The third cleavage in the sea urchin is ________. This creates an animal and vegetal half. The fourth cleavage is _______. Equal cytokinesis occurs in the 4 blastomeres of the animal pole, giving rise to 8 mesomeres (all the same size). Unequal cytokinesis occurs in the vegetal pole. This causes 4 large macromeres and 4 small micromeres. The 5th cleavage is _______. All mesomeres divide equally as do the macromeres. As cleavage progresses, all blastomeres adhere at the outer surface, but attachment is lost at the inner surface. The _________ is a cavity formed due to the unequal adherence of blastomeres.

meridional; equatorial; unique; meridional; blastocoel

_________ cleavage runs from one pole to another (top to bottom), like the meridian on a globe. _________ cleavage encircles the zygote like the equator on the globe.

Meridional; Equatorial

African clawed frog scientific name

Xenopus laevis

Amphibians have mesolecithal eggs and undergo holoblastic cleavage The 1st cleavage is ______, as is the 2nd. The 3rd cleavage is _______. The cleavage is displaced toward the animal pole due to the yolk. This results in 4 small animal blastomeres and 4 large vegetal blastomeres.

meridional; equatorial

Blastocoele is displaced to the (animal, vegetal) pole in amphibians

animal

from the 128 cell stage onward the amphibian embryo is a _____

blastula

Provide strong adhesion between cells through intermediate filaments.

Desmosome

A fluid-filled cavity inside the blastula, which provides space for future cell movements during gastrulation.

Blastocoel

Chick scientific name

Gallus gallus

In eutherians, cleavage is completely holoblastic, and the trophectoderm (TE) forms, which later contributes to the placenta.

In mammals, the inner cell mass (ICM) forms within the blastocyst, giving rise to the epiblast and primitive endoderm (PrE).

In birds and reptiles, all embryonic tissues and extraembryonic mesoderm arise from the epiblast. In mammals, the trophectoderm contributes to extraembryonic structures like the placenta, while the epiblast forms the embryo.

discoidal cleavage occurs in the ______ -1st division is central and does not extend in the cytoplasm, then other cleavages follow

blastodisc

Epiblast Hypoblast Area pellucida Area opaca Marginal zone Subgerminal space Yolk

Formation of the two-layered blastoderm of the chick embryo

(A,B): Primary hypoblast cells delaminate individually to form islands of cells beneath the epiblast (C) Secondary hypoblast cells from the posterior margin (Koller's sickle and the posterior marginal cells behind it) migrate beneath the epiblast and incorporate the polyinvagination islands (D) Sagittal section of an embryo near the posterior margin shows an upper layer consisting of a central epiblast that trails into the cells of Koller's sickle (ks) and the posterior marginal zone (mz).

In birds and reptiles, the 1st cleavage is meridional. It starts at the animal pole but does not progress far. The 2nd and 3rd cleavages are also meridional. The 4th cleavage is equatorial, and it creates a layer of small cells on top of the huge uncleaved area below (yolk).

Chick embryonic development begins inside the oviduct of the hen before the egg is laid. The stages are divided into: Eyal-Giladi and Kochav (EGK) Stages - Occur before egg laying (intrauterine development). - Focus on cleavage, cellularization, and area pellucida formation. Hamburger and Hamilton (HH) Stages - Occur after egg laying and continue through gastrulation and organogenesis. - Start at HH1 and progress through streak formation and elongation.

1. EGK-I (Early Cleavage) The zygote undergoes meroblastic cleavage, meaning division occurs only in the blastodisc (the embryonic region), while the yolk remains undivided. Key processes: - Cellularization: Formation of separate blastomeres. - Maternal determinants: Maternal mRNAs and proteins regulate early development. - Zygotic Genome Activation (ZGA): The embryo begins to control its own gene expression. - Mid-Blastula Transition (MBT): Cell divisions slow down, and differentiation begins. 2. EGK-VI (Late Cleavage) - More cell layers are added, increasing the complexity of the blastodisc. - Cells organize into the epiblast and hypoblast, setting up the future body plan. 3. EGK-X (Area Pellucida Formation) The area pellucida (transparent central region of the blastodisc) forms. Key processes: - The anterior-posterior (A-P) polarity is established, marking the first signs of body orientation. - Layer reduction: Some cells die, creating a distinct epiblast layer. - Lineage specification: Cells start committing to specific fates. - Yolk syncytium formation: A multinucleated yolk mass supports further development.

Developmental Timeline Fertilization → Cleavage (~5 hrs) → Blastodisc formation (EGK-I to EGK-VI, ~10 hrs) → Area pellucida formation (EGK-X, ~10 hrs more) → Egg laying (~6 hrs after EGK-X) → Primitive streak formation (EGK-XIV to HH stages, post-laying).

Type of holoblastic cleavage in mammals

Rotational

Cleavage in Mammals ❖ Zona pellucida… retained until implantation ❖ At 8-cell stage: Compaction (expression of E-Cadherin)/Cavitation ❖ Polarity…an Inner cell mass and trophoblast (trophoectoderm) formed later ❖ Mammalian genome is activated during early cleavage (newly formed nuclei produce the proteins necessary for cleavage and development)

Cleavage pattern in the early mammalian embryo

Meridional -> Meridional or Equatorial -> Meridional-Meridional or Meridional-Equatorial or Equatorial-Meridional or Equatorial-Equatorial

Cavitation or formation of blastocoel in mouse happens in day _____, day ______ in humans, and day ______ in bovine

3 to 4; 5 to 6; 7 to 8

A blastocoel develops as cleavage proceeds to the ____ cell stage

32-64 cell stage 16 to 32 cell stage in humans

is the process of establishing cell-to-cell adhesion and anatomical differences between cells.

compaction

is the process of forming a fluid-filled cavity in the embryo.

Cavitation

In morulla Tight junctions between outside cells Gap junctions between inside cells

is the hallmark of early embryonic development in mammals

Blastocyst

A thin layer of cells that helps a developing embryo attach to the wall of the uterus, protects the embryo, and forms a part of the placenta

Trophoblast

How many cells are in morulla

16 cells

Mediated by the activity of the Hippo pathway, Cdx2 expression blocks the expression of Oct4 and Nanog on cells destined to become trophoblasts, whereas blocking Cdx2 results in the expression of Oct4, Sox2, and Nanog in cells of the inner cell mass.

POLARITY DEPENDENT CELL POSITIONING OF ICM AND TE Hippo signaling pathway - OFF = TE Hippo signaling pathway - ON = ICM Polarity vs Contractility More polar = re-polarization = retention of an outer position More contractile = internalization = retention of an intracellular organization

Developmental fates of cleavage cells

Specification Determination Differentiation

The fate of a cell or a tissue is said to be _________ when it is capable of differentiating autonomously when placed in a neutral environment such as a petri dish or test tube

specified

A cell or tissue is said to be _________ when it is capable of differentiating autonomously even when placed into another region of the embryo.

determined (committed)

Human Blastocyst Embryo Inner Cell Mass is made up of ...

Epiblast (Epi) and Primitive endoderm (PE)

Describe a zygote and a 2-cell in terms of their potential to develop into different types of cells

Totipotent

Describe a late morula and an early blastocyst in terms of their potential to develop into different types of cells

Plenipotent

Describe a late blastocyst in terms of their potential to develop into different types of cells

Pluripotent

Growth factors

EGF - Ectodermal TGF - Transforming VEGF - Vascular Endothelial

Potency in decreasing order Totipotent (ex. Zygote, morula) Pluripotent (ex. Late blastocyst) Multipotent (ex. Hematopoeitic stem cells, Mesenchymal stem cells) Unipotent (ex. Erythrocytes, Leukocytes, Thrombocytes)

Cell Specification (3)

I. Autonomous (mosaic) - cells develop only according to early fate e.g. Tunicates II. Conditional (regulative) - fate depends on context e.g. vertebrates III. Syncytial - cell fate dependent on exposure to cytoplasmic determinants in a syncytium e.g. insects

Cell specification: Morphogenetic determinants in the egg cytoplasm specify the cell type

Autonomous Specification

Cell specification: (A) Cell fate is determined by interactions with neighboring cells. (B) If cells are removed from the embryo the remaining cells can regulate and compensate for the missing part.

Conditional Specification

Cell specification: Interactions occur not between cells, but between parts of one cell. Nuclei divide within the egg cytoplasm. (There is a gradient)

Syncytial Specification

After fixed number of divisions, cleavage ends in a controlled manner ! A change in the cell cycle in early embryos where specialized synchronous rapid cycles slow down and become asynchronous.

Mid-Blastula Transition *Mammals lack MBT due to the initially slow cycles upon fertilization.

Mid-Blastula Transition in Drosophila Early embryonic cell cycle - M and S Entering of transcriptional input - M, S, and G2 Entering of transcriptional input - M, G1, S, and G2 (Normal cell cycle)

MBT Events

❖Interphase occurs in the cell cycle ❖Activation of transcription and lengthening of Cell cycle ❖Embryo starts to synthesize its own RNA (often maternal RNA is actively destroyed at this time) ❖Cell cycle transits from maternal to zygotic controls ❖Changes in metabolism ❖The cells become motile

Embryonic pole Abembryonic pole Polar trophectoderm Mural trophectoderm

a structure in plant cells that helps form a new cell wall between daughter cells

Phragmoplast *The phragmoplast and cell plate are initiated in the center of the cell, and then expand laterally to complete formation of a new cell wall while microtubules are removed from the center of the phragmoplast.

Cytokinesis in higher plants requires __________ to generate the cell plate, while animal cell cytokinesis requires contraction of an acto-myosin ring

Vesicle fusion

cell wall contains building blocks made of sugar polymers, such as _________

cellulose, hemicellulose, pectin, and proteins

The _______marks the division zone, so that the cell knows where to build the new wall.

preprophase band

Vesicles carry cell wall building materials and the protein “workers” along tracks called microtubules to the division zone. Vesicles fuse together in the center of the division zone and form the cell plate, which is the beginning of the cell wall. As materials are added, the cell plate expands toward the existing wall. When the new cell wall is completed, one big cell becomes two small cells

Arabidopsis embryo development Zygote 1-cell 2-cell 4-cell Octant Dermatogen Globular Transition Heart Seedling

space b/w blastoderm and yolk

subgerminal cavity

when cleavage has progressed such that there are many blastomeres in the animal pole, it is a _______

blastoderm

Early development and cleavage in humans 2-cell embryo = 1.5 days 16-cell morula = 3 days 58-cell blastocyst = 4 days Hatched blastocyst = 4.5 days