Lecture 5: Cleavage Flashcards
Post-fertilization events:
- Zygotes becomes metabolically active.
- Zygote begins to undergo cleavage (mitotic activity).
- Zygote is transported down uterine tube to uterus.
- This journey takes several days.
- Zygote loses zona pellucida prior to implantation.
- Implantation
Cleavage Events
• Zygotes undergoes mitotic division to form an 8-cell embryo.
• Cell division continues and compaction occurs:
• Occurs when outer blastomeres adhere via gap junctions and appear
to lose their individual identity.
• Involves E-cadherins and other calcium-dependent CAMs.
• Note that zona pellucida is still intact.
• Water is transported into the ball of cells (morula – 16 cells): • Occurs 4 days after fertilization
• Results in the formation of a blastocoel as a result of cavitation
• Requires Na and K-ATPase transporters
• Embryo is referred to as a blastocyst at this stage (58 cells).
Features of the Blastocyst
• Surrounded by zona pellucida
• Large, eccentrically placed blastocoel
• Consists of two types of cells: Outer layer of cells = TROPHOBLAST Inner mass of cells = INNER CELL MASS
Because of eccentric placement of blastocoel and inner cell mass, the blastocyst is polarized.
Because of eccentric placement of blastocoel and inner cell mass, the blastocyst is polarized.
- Embryonic pole marks the pole of the blastocyst where the inner cell mass is located.
- Abembryonic pole marks the opposite pole.
Forms extraembryonic structures including placenta.
Trophoblast, outer layer of cells
Forms embryo proper plus some Extraembryonic Structures.
Inner Cell Mass
In invertebrates and non-mammalian vertebrates:
Early control of cleavage is through gene products transcribed from the maternal genome and embryonic gene products often do not appear until after blastulation.
In mammalian embryos:
• Maternal gene products are produced but generally are
degraded by the 2-cell stage of development.
• By the four cell stage, most transcription is via the embryonic genome.
Methylation:
is the addition of methyl groups to specific regions of DNA molecules; it inactivates genes, including enhancers and promoters.
Methylation cycle:
- DNA of mature eggs and sperm is highly methylated.
- Demethylation of maternal and paternal genomes occurs shortly after fertilization until the early morula.
- Remethylation of inner cell mass occurs until late blastocyst state.
- Methylation levels fall after primordial germ cells enter genital ridges.
- Remethylation occurs later during gametogenesis and may lead to maternal/paternal imprinting.
Polarization of blastomeres at 8-16 cell stages
creates recognizable apical and basal surfaces.
Polarization of blastomeres determines
whether cells are destined to become part of the inner cell mass or part of the trophoblastic group of cells.
Two theories of polarization
- Inside-out hypothesis
* Cell polarity model
inside out hypothesis
• Fate of blastomere is determined by its position within the embryo, not from intrinsic properties.
Cell Polarity Model
- Cleavage plane parallel to the outer surface of the embryo: • Outer daughter cell → trophoblast cell (polar):
- Cells pick up a patch of outer cell membrane containing microvilli and ezrin (microfilament-stabilizing protein).
- Inner daughter cell → inner cell mass cell (apolar)
- Cleavage plane perpendicular to outer surface of the embryo:
- Both daughter cells become trophoblast cells.
potency
- Developmental potential = potency.
* Potency is greater than fate.
