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Questions and Answers
What type of cleavage occurs when the whole of the egg becomes subdivided into blastomeres?
What type of cleavage occurs when the whole of the egg becomes subdivided into blastomeres?
- Meroblastic Cleavage
- Holoblastic Cleavage (correct)
- Partial Cleavage
- Complete Cleavage
What is the term for the daughter cells that arise during the early embryonic cleavage?
What is the term for the daughter cells that arise during the early embryonic cleavage?
blastomeres
Which type of cleavage refers to the subdivision of only a part of the egg into blastomeres?
Which type of cleavage refers to the subdivision of only a part of the egg into blastomeres?
- Total Cleavage
- Meroblastic Cleavage (correct)
- Holoblastic Cleavage
- Complete Cleavage
An ovum with a very small amount of yolk is called __________.
An ovum with a very small amount of yolk is called __________.
What term is used for an ovum with a moderate amount of yolk?
What term is used for an ovum with a moderate amount of yolk?
Which type of egg has yolk concentrated in the interior, with cytoplasm forming a thin coat externally?
Which type of egg has yolk concentrated in the interior, with cytoplasm forming a thin coat externally?
What is a morula in embryology?
What is a morula in embryology?
Which type of cleavage would you expect in mammals?
Which type of cleavage would you expect in mammals?
The inner cell mass is also known as the trophoblast.
The inner cell mass is also known as the trophoblast.
What is the blastocoele?
What is the blastocoele?
What is the role of the trophoblast in embryonic development?
What is the role of the trophoblast in embryonic development?
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Study Notes
Early Embryonic Development
- Series of mitotic divisions occur in the early embryo, within the zona pellucida, producing daughter cells known as blastomeres.
- Each cleavage division results in progressively smaller daughter cells, transforming the embryo into a morula, a small ball of cells.
Types of Cleavage
- Holoblastic Cleavage: Complete cleavage where the entire egg is divided into blastomeres.
- Meroblastic Cleavage: Incomplete cleavage where only part of the egg is subdivided into blastomeres, typically due to a large yolk content.
Yolk Classification
- Oligolecithal (Miolecythal): Ovum with very small yolk amounts.
- Mesolecithal (Medialecithal): Ovum with moderate yolk amounts.
- Polylecithal (Megaleicthal): Ovum with large yolk quantities.
- Isolecithal: Ovum with evenly distributed yolk.
- Centrolecithal: Yolk concentrated in the egg's center with cytoplasm as a thin layer on the outside.
- Telolecithal: Yolk displaces embryo-forming cytoplasm to a small area at the animal pole.
Cleavage Patterns and Associated Animals
- Isolecithal/Oligolecithal: Holoblastic cleavage observed in mammals and mollusks.
- Mesolecithal: Holoblastic cleavage in amphibians and some fishes.
- Telolecithal/Polylecithal: Meroblastic cleavage in birds, reptiles, and most fishes.
- Centrolecithal: Meroblastic cleavage seen in insects and arthropods.
Morula and Blastocyst Formation
- "Morula," derived from the Latin for mulberry, is a cellular mass that fills the perivitelline space.
- Inner and outer blastomeres differentiate; outer ones form tight junctions for trophoblast development, while inner ones form gap junctions leading to the inner cell mass (ICM).
- The embryo's mass occupies 60-70% of the perivitelline space.
Blastocyst Structure
- Inner Cell Mass (Embryoblast): Located at one pole; initiates embryonic development.
- Trophoblasts: Surface cells of the blastocyst.
- Blastocoele: Fluid-filled cavity within the blastocyst.
- Polar and mural trophoectoderm label different sections of the blastocyst enveloping the ICM.
Implantation and Development
- The diameter of the embryo significantly increases while the zona pellucida thins.
- Enzymatic digestion of the zona pellucida allows the blastocyst to adhere to the uterine mucosa for implantation.
- Expansion of the blastocyst varies by species; horses, dogs, and cats exhibit round to oval expansion, while cattle, sheep, and pigs show thread-like expansion.
- In primates and rodents, limited expansion occurs as the blastocyst invades the endometrium.
Differentiation of Inner Cell Mass
- ICM divides into two populations:
- Cells facing the blastocyst cavity become flattened, forming the hypoblast.
- Remaining cells develop into the multilayered epiblast, critical for future embryonic development.
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