Developmental Biology Lecture 8: Cleavage

Cleavage Overview

• Cleavage refers to the initial mitotic divisions of the embryo, dividing the single-celled zygote into smaller cells called blastomeres.
• This process occurs without growth, resulting in progressively smaller cells at each division.
• Cleavage lacks defined G1 and G2 phases, driven by constitutive Cdk1-cyclinB activity, leading to rapid divisions.

Cleavage Characteristics

• Cleavage divisions occur rapidly relative to other cell divisions.
• Cells remain transcriptionally silent; the zygotic genome is inactive, relying on maternal oocyte stores for resources.
• Divisions are typically synchronous, creating a predictable cleavage pattern influenced by yolk distribution.
• Parameters defining cleavage include yolk amount and distribution, the completeness of early cleavages, symmetry of daughter cells, and planes of division.

Cleavage in Frogs

• Frogs exhibit holoblastic cleavage with yolk concentrated in the egg's lower hemisphere.
• The first two divisions are vertical; the third is horizontal, positioned higher in the egg.
• Following initial divisions, upper cells divide more rapidly due to lower yolk content.

Meroblastic Cleavage in Fish and Birds

• Fish and bird embryos show meroblastic cleavage due to thick yolk that prevents complete division.
• Initial cell divisions are vertical and incomplete, with cleavage furrows stopping upon reaching the yolk.
• The sixth division is horizontal, separating the yolk from the upper blastomeres.

Cleavage in Cephalopods

• Cephalopods show variations in early embryonic stages, demonstrating both divergence and convergence in cleavage patterns.

Cleavage in Mammals

• Mammals typically exhibit holoblastic cleavage with evenly distributed yolk (isolecithal).
• Cleavage divisions are complete through the egg, resulting in equally sized blastomeres.
• Mammalian cleavage is slower compared to other species, with early zygotic transcription starting minimally within 24-48 hours.

Rotational Cleavage

• In holoblastic cleavage, the second cleavage occurs at right angles to the first, forming a rotational pattern.
• The orientation of mitotic spindle determines the pattern of cleavage division.

Cleavage in C. elegans

• C. elegans demonstrates holoblastic and asymmetric cleavage.
• Division results in lineage-restricted cell fates; specific cells inherit unique markers such as P granules.
• Ablation of certain cells impacts developmental fate, exemplified by future vulva formation.

Centrosome and Spindle Orientation

• The plane of cell division is influenced by the orientation of mitotic spindles.
• Centrosome positioning is critical and guided by proteins at the cell cortex that interact with centrosomes via astral microtubules.

Meroblastic Cleavage in Fruitflies

• Initial stages include DNA replication and nuclear division prior to actual cell cleavage.
• Nuclei migrate to the cell periphery, setting the stage for subsequent cellular development.

Cleavage Overview

• Cleavage refers to the initial mitotic divisions of an embryo, dividing the zygote into smaller cells known as blastomeres.
• These divisions occur without growth, resulting in increasingly smaller cells, termed altered mitosis.
• Typical cleavage lacks defined G1 and G2 phases due to the presence of constitutive Cdk1-cyclinB (MPF), which is active during mitosis.

Characteristics of Cleavage

• Cleavage divisions are rapid compared to other cell divisions.
• No cell growth occurs; instead, cells become smaller after each division.
• Early cleavage divisions are synchronous and vary in pattern based on yolk distribution and thickness.
• The zygotic genome remains inactive, so embryos rely on maternal oocyte stores for early development.

Cleavage Types Based on Yolk Distribution

• Amount and distribution of yolk significantly influence cleavage patterns across different organisms.
• Cleavage can be categorized as either complete (holoblastic) or incomplete (meroblastic).
• Symmetry of cleavage impacts whether daughter cells are identical or not.

Cleavage in Specific Organisms

• Frogs: Undergo holoblastic cleavage. Yolk is concentrated in the lower half, leading to distinct division patterns; first two divisions are vertical, followed by a horizontal division.
• Fish & Birds: Features meroblastic cleavage due to thick yolk. Initial cell divisions are vertical and incomplete, leading to blastomeres still attached to yolk until later divisions.
• Mammals: Exhibit holoblastic cleavage with minimal yolk distributed evenly. Cleavage is slower with early zygotic transcription starting within 24-48 hours.

Cell Division Orientation

• The orientation of cleavage divisions is determined by the alignment of mitotic spindles, which is influenced by centrosome positioning.
• Centrosomes interact with the cell cortex through proteins, often aided by astral microtubules.

Specific Patterns in Model Organisms

• C.elegans: Display holoblastic and asymmetric cleavage, where specific cell lineages inherit unique fates demonstrated by the distribution of P granules.
• Fruit Flies: Initial stages involve nuclear division and migration before the physical cleavage of cells occurs, signifying meroblastic patterns.

Summary of Key Processes

• Cleavage occurs quickly, resulting in smaller cells without growth, characterized by rapid division and specific egg yolk adaptations.
• The roles of cytoplasmic factors, such as maternal contributions and zygotic transcription, play vital roles in early embryonic development.
• Understanding cleavage patterns provides insight into developmental biology and organismal differences in reproduction.