Embryology Chapter 4: Early Embryogenesis

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Questions and Answers

What occurs during the cleavage divisions of the zygote?

Rapid mitotic divisions produce blastomeres without significant growth. (correct)

Cell mass increases between divisions.

The zona pellucida is lost, allowing for cell adhesion.

Blastomeres undergo differentiation immediately after division.

Which statement correctly describes the function of the trophoblast?

It plays a critical role in implantation into the uterine wall. (correct)

It has no role in embryonic development.

It undergoes cleavage divisions to produce morula.

It forms the inner cell mass of the blastocyst.

What is the primary purpose of maternal messenger RNA during early zygotic development?

To suppress the activity of the trophoblast.

To facilitate cleavage divisions in the zygote.

To promote the development of the inner cell mass.

To initiate transcription of the embryonic genome. (correct)

In the context of early embryogenesis, what is a morula?

A structure formed during the cleavage stage consisting of a solid ball of cells. Signup and view all the answers

Which of the following is true regarding the lineage potential of blastomeres?

All blastomeres retain totipotency until the formation of a blastocyst. Signup and view all the answers

What primarily determines the cell differentiation in the developing embryo?

The specific gene profile expressed after zygotic genome activation. Signup and view all the answers

What characterizes the blastocyst stage of embryogenesis?

It forms after the morula has differentiated into distinct cell types. Signup and view all the answers

Which concept explains the formation of a chimera during early development?

The fusion of two distinct embryonic cell masses. Signup and view all the answers

What is the primary role of trophoblast cells in early embryonic development?

To contribute to the development of the placenta and extraembryonic membranes Signup and view all the answers

During the transition from morula to blastocyst, which characteristic distinguishes the superficial cells?

They have epithelial characteristics and are linked by tight junctions Signup and view all the answers

What initiates cell determination in the compacted morula stage of embryonic development?

The separation of the inner and outer cell populations Signup and view all the answers

Which of the following statements accurately describes mesenchymal cells in the embryo?

They are loosely arranged and play a role in forming various tissues Signup and view all the answers

What is the significance of gap junctions between the interior cells of the morula?

They enable communication that influences cell differentiation Signup and view all the answers

Which process directly follows the cleavage divisions as the embryo travels to the uterus?

Compaction into a morula structure Signup and view all the answers

How do superficial cells of the morula contribute to the embryo’s development?

They give rise to the trophoblast that forms the placenta Signup and view all the answers

What is the role of zygotic genome activation in embryonic development?

To activate transcription of maternal genes Signup and view all the answers

What is the key function of the inner cell mass (ICM) within the blastocyst?

It develops into all tissues of the body. Signup and view all the answers

Which of the following statements accurately describes the zona pellucida during blastocyst development?

It ruptures during a process referred to as hatching. Signup and view all the answers

In terms of cell potential, how are inner cell mass (ICM) cells classified?

Pluripotent, able to form all body tissues but not all extraembryonic membranes. Signup and view all the answers

During early embryonic development, what happens to the trophoblast cells covering the ICM?

They degenerate and disappear in most species. Signup and view all the answers

What characterizes the shape of the blastocyst as it develops?

It begins as a hollow sphere and can elongate rapidly in some species. Signup and view all the answers

What is a notable feature of the cleavage divisions during early embryonic development?

These divisions occur without an increase in overall size of the embryo. Signup and view all the answers

What is the relationship between cleavage divisions and the morula formation?

Morula is the result of multiple cleavage divisions of the zygote. Signup and view all the answers

What phenomenon allows some species to have delayed implantation after hatching of the blastocyst?

The embryo drifts within the uterus before implanting. Signup and view all the answers

Study Notes

Chapter 4: Early Embryogenesis

Key terms and concepts: Zygote, Histotroph, Blastomere, Cleavage division, Morula, Blastocyst, Blastocoel, Compaction, Inner cell mass (ICM), Trophoblast (aka Trophectoderm), Clone, Chimera, Monozygotic twin, Dizygotic twin, Transgenic animal, Knockout animal, Embryonic stem cell, Induced pluripotent stem (iPS) cell
Learning objectives: Define key terms, describe maternal mRNA in zygote development, describe lineage potential of blastomeres and ICM, identify first lineage determination, identify blastocyst cell types and their later development, describe use of transgenic/knockout animals for studying gene roles, understand relationship between ES cells and ICM
Cleavage and Blastocyst Formation:
- Zygote undergoes cleavage divisions (mitotic divisions) producing blastomeres without growth, keeping total cell mass relatively constant.
- Zona pellucida surrounds dividing cells, preventing adhesion to uterine walls.
- Initial large size of oocyte decreases due to limited growth during cleavage.
- Some species have cytoplasmic substances like yolk concentrated asymmetrically, impacting early cell development.
Morula formation:
- Cluster of about 16 blastomeres.
Compaction:
- Cells of morula huddle together increasing surface contact and forming a compacted cluster at 8-cell, 16-cell, or 32-cell stages (depending on species).
Blastocyst formation:
- Compacted morula develops into a blastocyst forming a blastocoel (fluid-filled space).
- Trophoblast and inner cell mass (ICM) develop.
- Trophoblast forms extra embryonic membranes (placenta).
- ICM forms embryo.
- Zona pellucida usually disappears (hatching).
Chimeras, Twins and Other Interesting Things:
- Early embryo cells are totipotent (capable of forming entire organism).
  - Removal of a cell in a two-celled embryo results in healthy development.
- Blastocyst cells compensate for losses or additions of cells.
- Early Embryo cells can be separated to create genetically identical embryos (twins).
- Monozygotic twins result from splitting of early embryo.
- Dizygotic twins originate from separate egg/sperm interactions.
- Conjoined Twins form from incomplete embryo splitting.
- Chimeras, when cells from different embryos/genotypes fuse, generate a single organism with a mosaic of cell types/genotypes.
- Embryonic Stem (ES) cells can be added to blastocysts, creating chimeras.
Transgenic animals: Foreign genes are intentionally inserted into an animal's genome, typically in fertilized eggs (e.g., creating mice with therapeutic protein in their milk).
Embryonic Stem Cells:
- Derived from inner cell mass (ICM) of mouse blastocysts.
- Pluripotent-capable of differentiating into many cell types.
- Can be used to create chimeric animals.
- Used for research in genetic diseases and development.
- Stem cells are derived from inner cell mass (ICM).

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Test your understanding of early embryogenesis with this quiz focusing on key terms such as zygote, blastomere, and blastocyst. Explore the critical processes including cleavage division and lineage determination of cells. Perfect for students studying developmental biology.