Embryonic Cleavage and Mitosis

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Questions and Answers

What is the main function of Hox genes in vertebrate development?

Regulating myogenesis

Controlling segmentation and patterning (correct)

Facilitating blood cell differentiation

Inducing muscle growth

Which transcription factor family is associated with the development of sense organs and the nervous system?

Pax genes (correct)

T box genes

Sox genes

Zinc finger proteins

What is the role of Sox genes in cellular processes?

Regulating cell proliferation in limb development

Controlling segmentation patterns

Binding to DNA and causing conformational changes (correct)

Inducing mesoderm layer formation

Which family of transcription factors is primarily involved in myogenesis?

Basic helix-loop-helix proteins Signup and view all the answers

What characterizes the Zinc finger protein family?

Binding of DNA through a finger-like conformation Signup and view all the answers

Which transcription factor absence causes headless mammalian embryos?

Lim proteins Signup and view all the answers

Which cytokine family is known for its role in mesodermal induction and myoblast proliferation?

TGF-β family Signup and view all the answers

FGF signaling plays essential roles in all of the following except:

Muscle differentiation Signup and view all the answers

What is the primary condition associated with the activin molecule within the TGF-β family?

Granulosa cell proliferation Signup and view all the answers

What major role do Msx genes play during prenatal and postnatal development?

Maintaining tissue proliferative capacity Signup and view all the answers

What is the main outcome of cleavage during early embryogenesis?

Division of egg cytoplasm into smaller cells Signup and view all the answers

Which of the following correctly identifies the subunits of Mitosis Promoting Factor (MPF)?

Cyclin B and cdk2 Signup and view all the answers

What factor influences embryonic cell specialization from intrinsic sources?

Maternal lineage information Signup and view all the answers

What type of cleavage pattern is exhibited by amphibian eggs?

Holoblastic unequal Signup and view all the answers

During the midblastula transition, what significant change occurs?

Zygotic gene transcription activation Signup and view all the answers

Which cleavage type allows for incomplete cleavage during early embryogenesis?

Superficial meroblastic Signup and view all the answers

What is the characteristic of indeterminate cleavage?

Cells can develop into a full organism if separated Signup and view all the answers

What role does the trophoblast have after implantation in mammals?

Provides nutrients and develops into the placenta Signup and view all the answers

What describes the main function of transcription factors in early embryo development?

Regulate mRNA synthesis and gene products Signup and view all the answers

Which type of cleavage occurs in bird eggs?

Discoidal meroblastic Signup and view all the answers

What is the capacity of cyclin B in relation to the cell cycle?

Control entry into M phase through synthesis and degradation Signup and view all the answers

How do integrins in trophoblast cells influence the implantation process?

Attach to maternal tissues for nutrient exchange Signup and view all the answers

What is the role of cytoplasmic determinants in embryonic development?

Provide positional information for cell specialization Signup and view all the answers

At what stage do human embryos typically reach the blastocyst stage?

4 days after fertilization Signup and view all the answers

Study Notes

Cleavage

A rapid series of mitotic divisions that partitions the egg cytoplasm into smaller, nucleated cells called blastomeres
Process is called blastulation
Involves two coordinated processes: karyokinesis (nuclear division) and cytokinesis (cytoplasmic division)

Mitosis Promoting Factor (MPF)

Triggers the cell to enter M phase (mitosis).
MPF activation triggers events like:
- Chromosome condensation.
- Breakdown of the nuclear envelope.
- RNA polymerase inhibition, shutting down transcription.
- Myosin regulatory subunit phosphorylation, inhibiting cytokinesis.
Plays a crucial role in the transition from fertilization to cleavage.
Responsible for meiotic cell division in the egg cell.
Contains two subunits: Cyclin B and cyclin-dependent kinase (cdk2).

Patterns of Embryonic Cleavage

Cell specialization is influenced by:
- Intrinsic factors (lineage): Information inherited from the mother cell during cell division.
- Extrinsic factors (positional): Received from the surrounding environment or neighboring cells.
Determining the body axes:
- Cytoplasmic determinants: Maternal mRNAs or proteins present in the egg before fertilization, influencing cell fate.
- Yolk polarity: Found in eggs with abundant yolk, with the animal pole anterior and vegetal pole posterior.
- Induction: Cell-to-cell communication that leads to different cell fates among initially identical cells.
The type of cleavage is categorized based on the:
- Nature of cleavage furrow: Holoblastic (complete) or Meroblastic (incomplete).
- Fate of germ layer: Determinate (cells with predetermined fates) vs. Indeterminate (cells capable of becoming a full organism if separated).
- Planar division/arrangement of cells: Radial, Spiral, Bilateral, or Rotational.

Holoblastic Cleavage

Complete cleavage.
Can be:
- Holoblastic Equal: Blastomeres of equal size, found in microlecithal and isolecithal eggs.
- Holoblastic Unequal: Unequal blastomeres (micromeres and macromeres) formed in mesolecithal eggs.

Meroblastic Cleavage

Incomplete cleavage.
Types:
- Superficial meroblastic: Cleavage restricted to the peripheral cytoplasm, found in centrolecithal eggs.
- Discoidal meroblastic: Cleavage furrows form only in a disc-like animal pole region, found in macrolecithal eggs.

Planar Division

Radial Cleavage: Division planes at 90 degrees, blastomeres directly above or beside each other.
Spiral Cleavage: Division planes not at 90 degrees.
Bilateral Cleavage: Equal cleavage activity on both sides, creating left and right halves.
Rotational Cleavage: First cleavage is meridional, but in the second cleavage, one blastomere divides meridionally and the other equatorially.

Fate of Germ Layer

Determinate Cleavage: Cells with predetermined fates at an early stage.
Indeterminate Cleavage: Cells that can each develop into a complete organism if separated.

Midblastula Transition

Activation of Zygotic Gene Transcription: The zygote begins producing its own mRNAs from its DNA, replacing maternal mRNA.
Cell Cycle Changes: The cell cycle slows down, adding G1 and G2 phases, and cell division becomes asynchronous.
Cell Migration: A central process in development and maintenance of multicellular organisms.

Cleavage in Fish Eggs

Discoidal and meroblastic cleavage occurs in the blastodisc.
Calcium ions play a role in the construction of the actin cytoskeleton.
Division time is approximately 15 minutes per division.
Midblastula transition occurs with gene transcription and slower cell division.
Distinct cell populations:
- YSL (yolk syncytial layer): Directs cell movements during gastrulation.
- EVL (enveloping layer): The most superficial layer, becoming periderm.
- Deep cells: Located between EVL and YSL, giving rise to the embryo proper.

Amphibian Cleavage

Radially symmetrical and holoblastic unequal.
Animal pole and vegetal pole establish polarity.
Formation of a morula (16-64 cell stage) and blastocoel (128-cell stage).

Cleavage in Birds Eggs

Discoidal meroblastic, occurs in the blastodisc.
First cleavage furrow forms centrally.
Equatorial and vertical cleavages further divide the blastoderm.
Subgerminal cavity: Space between the blastoderm and yolk.
Area pellucida: Region of the blastoderm forming most of the embryo.
Area opaca: Peripheral ring of the blastoderm that has not shed its deep cells.
Marginal zone: The area between the area opaca and area pellucida.

Cleavage in Mammals

Meridional and equatorial (rotational cleavage), with asynchronous cell division.
Blastomeres do not divide at the same time.
Smallest and slowest cleavage compared to other vertebrates (human egg: 100µm).
Blastomeres do not increase exponentially.
Compaction: Blastomeres maximize contact, forming a compact ball of cells.
Morula: Contains outer cells and inner cells.
Cavitation: Formation of the internal cavity of the morula, fluid secreted by the trophoblast.

Human Cleavage

Measured in days:
- 2-cell stage: 1 day.
- 4-cell stage: 2 days.
- 16-cell stage: 3 days.
- Blastocyst: 4 days.
- Blastocyst with trophoblast and ICM: 5 days.
Human eggs have limited stored ribosomes and RNA during oogenesis, relying on gene products.
Oct4 gene is important in early development.
Maternal mRNA is limited, and transcription products from maternal and paternal chromosomes guide early development.

Blastocyst Attachment

Zona pellucida disintegrates (using trypsin), exposing the blastocyst to the uterine wall.
Integrins in the trophoblast cells attach to collagen, laminin, and fibronectin in the endometrium.
Protein-digesting enzymes enable the blastocyst to embed in the uterine wall.

Tissue Formation of Early Mammalian Embryo

Trophoblast forms the tissue of the chorion and contributes to the placenta.
Decidua: Uterine lining that forms the maternal part of the placenta (influenced by progesterone).

Tissue Formation of Early Mammalian Embryo (cont.)

Cytotrophoblast (layers of Langerhans): Inner layer.
Syncytiotrophoblast: Epithelial covering of embryonic placental villi, invades the uterine wall to establish nutrient circulation between the embryo and the mother.
Mesodermal tissue originates from the embryo, yolk sac, and primitive streak-derived cells.
Connecting stalk of extraembryonic mesoderm connects the embryo to the trophoblast and forms the vessels of the umbilical cord.

Molecular Basis for Embryonic Development

Transcription factors: Proteins with domains that bind to DNA promoters or enhancers, regulating gene expression.
Homeobox: Nucleotide sequences encoding for homeodomain, coding for transcription factors that regulate gene expression.
Hox genes: Homoebox gene complex, with 39 homologous homeobox genes, involved in cranial-to-caudal patterning.
Pax genes: Crucial for sense organ and nervous system development.
Sox genes: Transcription factors with a common HMG (high mobility group) domain, influencing gene expression.
Basic helix-loop-helix proteins: Transcription factors with alpha helices separated by a loop, regulating myogenesis.
Zinc finger protein (ZnF): Transcription factors with cysteine and histidine bound to zinc ions, enabling binding to DNA, RNA, and proteins.
Lim proteins: Bind to DNA in the nucleus, essential for head development.
T box genes: Induces mesoderm layer and specifies forelimb and hindlimb development.
Dlx gene: Patters outgrowth structures, including appendage development and morphogenesis of jaws and inner ear.
Msx gene: Involved in epitheliomesenchymal interactions for limb and facial development, inhibits cell differentiation prenatally, and maintains tissue proliferative capacity postnatally.

Signaling Molecules

Also known as cytokines, influencing neighboring or distinct cells.
Part of the growth factor family.
Bind to receptor molecules as ligands.

TGF-β family

Comprises 30 molecules.
Influences events such as mesodermal induction, myoblast proliferation, granulosa cell proliferation (activin), limb development (decapentaplegic), body symmetry determination (left), and gene expression of target cell (Shh).
Transforming growth factor beta (TGF-β) is a multifunctional cytokine, produced by all white blood cell lineages.

Fibroblast Growth Factors (FGFs)

Broad-spectrum mitogens regulating cellular functions like migration, proliferation, differentiation, and survival.
Play critical roles in development, metabolism, and tissue homeostasis.

FGF family

FGF-1: Keratinocyte proliferation and liver induction.
FGF-2: Hair growth and renal tubule induction.
FGF-3: Inner ear formation.
FGF-4: Trophoblast mitotic activity.
FGF-5: Ectodermal placode formation.
FGF-8: Midbrain patterning, limb outgrowth, teeth induction, and filiform papillae induction.
FGF-10: Limb induction and prostate gland morphogenesis.

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Description

Explore the processes of cleavage, mitosis promoting factor (MPF), and patterns of embryonic cleavage. This quiz covers the intricate details of mitotic divisions, the transition from fertilization to cleavage, and the factors influencing cell specialization in embryonic development.