Syncytial Specification

Table of Contents

Introduction to Syncytial Specification

Syncytial specification is a strategy of cell specification that involves elements of both autonomous and conditional specification.
A cytoplasm with many nuclei is called a syncytium, and the specification of cells within such a syncytium is called syncytial specification.
A well-known example of a syncytial specification is found in the fruit fly, Drosophila melanogaster.

During the early cleavage stages of the Drosophila embryo, nuclei divide through 13 cycles without cytoplasmic cleavage.
This creates an embryo of many nuclei contained within one shared cytoplasm surrounded by one common plasma membrane, called the syncytial blastoderm.
In the syncytial blastoderm, the identity of future cells is established simultaneously across the entire embryo along the anterior-to-posterior axis.
The identity is established without any membranes separating nuclei into individual cells. Membranes eventually form around each nucleus through cellularization, which occurs just before gastrulation.

The cell fates (head, thorax, abdomen, and tail) are specified before cellularization.
Nuclei in the anterior part of the blastoderm are exposed to cytoplasmic determination factors not present in the posterior part, and vice versa.
The interaction between nuclei and differing amounts of determination factors specifies cell fate.
These gradients of determination factors are established during maturation of the egg prior to fertilization.
After fertilization, as nuclei undergo synchronous waves of division, each nucleus becomes positioned at specific coordinates along the anterior-to-posterior axis and experiences unique concentrations of determination factors.

Nuclei maintain their position within the syncytial blastoderm through their own cytoskeletal machinery (centrosome, microtubules, actin filaments, and interacting proteins).
In interphase, each nucleus radiates dynamic microtubule extensions organized by their centrosome to establish an “orbit” and exert force on neighboring nuclei.
This radial microtubule array is reestablished every time the nuclei divide, ensuring regular spacing of nuclei across the syncytial blastoderm.
Maintaining the positional relationships between nuclei across the early embryo is essential for successful syncytial specification.

The concentration of transcription factors in the cytoplasm dictates a nucleus’s position and its identity.
The anteriormost portion of the Drosophila embryo produces a transcription factor called Bicoid, with a highest concentration in the anterior region of the egg.
The posteriormost portion of the egg forms a posterior-to-anterior gradient of the transcription factor Caudal.
Bicoid and Caudal form opposing gradients across the long axis of the Drosophila egg and are considered morphogens because they determine the identity of cells based on their position.