Lecture 4

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Questions and Answers

What is the significance of exponential growth in embryonic development?

It is irrelevant to embryonic development

It slows down the process of cleavage division

It generates a large number of cells (correct)

It only occurs in the later stages of development

What is the primary function of cleavage division?

To generate a large number of cells from a single fertilized zygote (correct)

To distribute the zygotic genome unevenly among cells

To halt embryonic development

To generate a small number of specialized cells

What is the significance of each cleavage cell having a copy of the zygotic genome?

It restricts their ability to differentiate

It allows each cell to express a subset of the genome (correct)

It is unnecessary for embryonic development

It prevents further cell division

What is the outcome of ordinary mitotic division during cleavage?

Genetically equal cells Signup and view all the answers

What is the analogy used to describe the process of cleavage?

Building a house using a single big boulder of stone Signup and view all the answers

What is the significance of embryonic cells being able to move relative to their position?

It is essential for building specific structures Signup and view all the answers

What is the term for the cells resulting from cleavage division?

Blastomeres Signup and view all the answers

What is the purpose of generating many copies of the zygotic genome during cleavage?

To allow each cell to express a subset of the genome Signup and view all the answers

What is the process that phosphorylates target proteins inside the blastomere or cleavage cells?

Kinase Signup and view all the answers

What type of proteins are phosphorylated by the Cyclin B-CDC2 complex?

Histone proteins Signup and view all the answers

What is the source of MPF in the early embryo?

Maternal RNAs Signup and view all the answers

What is the result of the depletion of maternally-loaded Cyclin B and other cell cycle factors?

Zygotic genome activation Signup and view all the answers

What is the characteristic of the late embryonic cell cycle?

Asynchronous and slow Signup and view all the answers

What is the result of the transition from maternal to zygotic genome transcription?

All of the above Signup and view all the answers

What is the role of the Cyclin B-CDC2 complex in mitosis?

To phosphorylate target proteins Signup and view all the answers

What is the consequence of the degradation of the nuclear membrane?

Breakdown of the nuclear membrane Signup and view all the answers

What is the significance of the transition from maternal to zygotic genome transcription?

It leads to the loss of synchronous division Signup and view all the answers

What is the role of the regulators of Cyclin B in the early embryo?

They reside in the cytoplasm of the egg Signup and view all the answers

What is the primary function of maternal cytoplasmic factors in an oocyte?

To segregate cytoplasmic components into different blastomeres Signup and view all the answers

What is the result of an unequal distribution of cytoplasmic components in the progeny of cells?

Different combinations of cytoplasmic components in the progeny of cells Signup and view all the answers

What is the significance of the ratio of nuclear volume to cytoplasmic volume during cell division?

It is a universal aspect of cell division Signup and view all the answers

What is the role of MPF (mitosis promoting factor) in the transition from the zygote stage to the cleavage stage?

To fuel the transition from the zygote stage to the cleavage stage Signup and view all the answers

What are the two subunits of MPF?

Cyclin B and cdc2 Signup and view all the answers

What is the fate of MPF during mitosis?

It is degraded during mitosis Signup and view all the answers

What happens to Cyclin B during mitosis?

It is degraded during mitosis Signup and view all the answers

What is the consequence of Cyclin B breakdown during mitosis?

MPF is inactivated Signup and view all the answers

What is the result of the re-synthesis of Cyclin B during the S phase?

MPF is activated Signup and view all the answers

What is the significance of the set point of nuclear volume to cytoplasmic volume ratio?

It is species-specific Signup and view all the answers

What type of cell cycle is characterized by the absence of gap phases and a biphasic pattern?

Cell cycle of early cleavage divisions Signup and view all the answers

Which factor promotes the transition from G2 to M phase in normal cell cycles?

Cyclin-cdc2 complex Signup and view all the answers

What is the characteristic of the first cleavage furrow in early cleavage divisions?

It occurs in a plane perpendicular to the axis of the mitotic spindle Signup and view all the answers

Which type of egg is characterized by a dense amount of yolk concentrated at one pole?

Telolecithal Signup and view all the answers

What is the characteristic of holoblastic radial cleavage in amphibians?

It is complete and involves the whole oocyte Signup and view all the answers

What is the role of maternal gene products in cleavage pattern?

They determine the orientation of the mitotic spindle Signup and view all the answers

Which type of cleavage is characteristic of birds, reptiles, and fish?

Meroblastic discoidal cleavage Signup and view all the answers

What is the effect of yolk on cleavage divisions in slightly telolecithal eggs?

It slows down the cleavage divisions Signup and view all the answers

What is the characteristic of the third plane of cleavage in amphibians?

It is horizontal and cuts the oocyte more displaced towards the animal pole Signup and view all the answers

Which type of cleavage is characteristic of insects?

Meroblastic superficial cleavage Signup and view all the answers

What is the primary function of the cells that form outside the blastocyst during embryonic development?

To form the trophectoderm and facilitate implantation Signup and view all the answers

What is the characteristic of the cleavage process in avian embryos?

Stack of cell layers with a subgerminal cavity Signup and view all the answers

What is the significance of the pluripotency gene markers in the inner cell mass (ICM)?

They indicate the presence of pluripotent stem cells Signup and view all the answers

What is the outcome of the 4th plane of cleavage in avian embryos?

Formation of centrally located and peripherally located cells Signup and view all the answers

What is the term for the cells that form the hypoblast layer in avian embryos?

Hypoblast Signup and view all the answers

What is the characteristic of the area pellucida in avian embryos?

It appears light or translucent under the microscope Signup and view all the answers

What is the significance of regulative development in embryonic cells?

It enables the development of embryonic cells based on cell-cell interaction Signup and view all the answers

What is the characteristic of the end of cleavage in mammalian embryos?

Formation of 2 distinct cell populations Signup and view all the answers

What is the characteristic of the cleavage division in the animal hemisphere?

It cleaves the animal hemisphere faster Signup and view all the answers

What is the term for the smaller blastomeres at the top portion of the embryo?

Micromeres Signup and view all the answers

What is the significance of the gray crescent in a fertilized oocyte?

It is a sign of fertilization Signup and view all the answers

What is the stage of embryonic development at which the internal cavity called blastocoel is formed?

Blastula stage Signup and view all the answers

What is the characteristic of the cleavage in mammalian cleavage?

It is holoblastic and rotational Signup and view all the answers

What is the significance of the slow cleavage in mammals?

It is an evolutionary adaptation to secure the embryo in the uterus Signup and view all the answers

What is the event that occurs at the 8-cell stage in mammalian cleavage?

Compaction Signup and view all the answers

What is the result of polarization at the 16-cell stage in mammalian cleavage?

The formation of the inner cell mass Signup and view all the answers

What is the term for the formation of the blastocoel in mammalian cleavage?

Cavitation Signup and view all the answers

What is the characteristic of the cell division in mammalian cleavage?

It is asynchronous and slow Signup and view all the answers

Study Notes

Cleavage to Blastulation

Functions of Cleavage Division

Generates a large number of cells:
- From a single fertilized zygote to a multicellular embryonic phase
- Embryonic cells capable of moving relative to each other
- Analogy: breaking a large boulder into smaller stones to build a house
Generates many copies of the zygotic genome:
- Each cell in the cleaving embryo has its own copy of the zygotic genome
- Cell division is ordinary mitotic division, making all cells genetically equal
- Each cell has the freedom to express its own subset of the genome, guiding its developmental pathway
Segregates cytoplasmic components into different blastomeres:
- Maternal cytoplasmic factors (preformed guidelines) are distributed into different cells
- In an oocyte, these components are in the form of maternal mRNAs (silenced) ready to be translated into proteins once activated by fertilization

MPF (Mitosis Promoting Factor)

Translator of maternal mRNAs, fuelling the transition from the zygote phase to the cleavage phase
Composed of two subunits: Cyclin B and cdc2 (cyclin-dependent kinase 1)
MPF is periodically active, used by mitosis, and gets degraded, then re-synthesized to fuel mitosis

Cycling of Cells

Synthesis and degradation of Cyclin B
Activation and deactivation of cdc2 kinases
The cyclin B-cdc2 complex is active, phosphorylating target proteins inside the blastomere or cleavage cells

Source of MPF in the Early Embryo

Cyclin B is translated from maternal RNAs
All necessary components for cell cycling are also maternally derived

Depletion of Maternally-Loaded Cyclin B and Other Cell Cycle Factors

Zygotic transcription must begin, marking the transition from maternal to zygotic genome transcription
New phenomena occur: addition of gap phases, loss of synchronous division, transcription of new mRNAs, and synthesis of different regulators

Cell Cycle Control

MPF controls the cell cycle
In a typical somatic cell, the cell cycle includes M, S, G1, and G2 phases
In early cleavage divisions, there are no gap phases, and the cell cycle is biphasic (M and S phases)

Factors Influencing Cleavage Pattern

Maternal cytoplasmic factors
Amount and distribution of yolk
Types of egg yolk content: isolecithal, mesolecithal, telolecithal, and centrolecithal

Patterns of Cleavage and Cleavage Symmetry

Holoblastic (complete) cleavage: isolecithal and mesolecithal oocytes
Meroblastic (incomplete) cleavage: telolecithal oocytes
Cleavage patterns: radial, rotational, spiral, and bilateral

Cleavage to Blastula

Amphibian cleavage: holoblastic radial, with three distinct regions recognized
Mammalian cleavage: holoblastic rotational, slow, and asynchronous, with a transient window of time for formation of an odd-numbered cell
Avian cleavage: involves only the blastodisc, with a subgerminal cavity forming, and eventually, the epiblast and hypoblast

Developmental Properties of Cleaving Embryos

Regulative development: development of embryonic cells based on cell-cell interaction
Totipotent: embryonic cells have the ability to develop into any cell type

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Description

Learn about the process of cleavage division, its functions, and how it leads to the next phase of development in fertilized oocytes.