Characteristics of early developmental stages
The development of primitive chordates, which include amphioxus, begins with the fertilization of a specific egg cell. It is classified as alecithal (virtually devoid of yolk) or primarily oligolecithal and isolecithal (containing a small amount of yolk evenly distributed throughout the cytoplasm).
Due to this structure of the egg, subsequent zygote cleavage follows the simplest pattern. It is holoblastic (total), equal, and synchronous. As a result of this cleavage, an embryo at the blastula stage is formed. In amphioxus, it is represented by a coeloblastula — a regular single-layered spherical vesicle whose wall is formed by cells of absolutely identical size.
Gastrulation mechanism
Gastrulation in amphioxus occurs via invagination. The initial stage for the start of this process is the formed single-layered coeloblastula.
The essence of the mechanism lies in the gradual in-folding (invagination) of one half of the blastula into its second half. As a result, the spherical single-layered vesicle drastically changes its shape, transforming into a structure traditionally described in embryology as a "two-layered cup". The cells move as an entire sheet without losing contact with each other.
Morphology of the early gastrula
The gastrula formed via invagination (the "two-layered cup") has a clearly differentiated structure and consists of the following elements:
- Primary ectoderm — the outer layer of embryonic cells that remained on the surface after invagination.
- Primary entoderm — the inner layer of cells formed from the half of the coeloblastula that was folded inward.
- Gastrocoel — the newly formed cavity of the primitive gut, bounded by the layer of primary entoderm.
- Blastopore (primitive mouth) — a special opening that connects the gastrocoel cavity with the external environment.
Special attention is paid to the structure of the blastopore margins. The opening is bounded by lips, of which amphioxus has exactly four: dorsal, ventral, and two lateral.
Mesoderm formation (late stage)
At the late stage of gastrulation, the structure of the embryo becomes more complex — the third germ layer is established. This process is closely associated with the inner layer of the gastrula.
Cells of the primary entoderm begin to proliferate actively and form two symmetrical "pouches". These evaginations gradually invade the space between the outer (ectoderm) and inner (entoderm) layers. Subsequently, these very pouches completely detach and form the mesoderm. Thus, the initially two-layered embryo finally transforms into a three-layered one, paving the way for further organogenesis.
Briefly: answers to topical questions
What is the embryological term for the method of mesoderm formation characteristic of amphioxus?
The method of mesoderm formation characteristic of amphioxus is called enterocoelous. In this mechanism, mesodermal cells segregate in the form of "pouches" of the archenteron.
In amphioxus, at the late stage of gastrulation, the cells of the primary entoderm form two "pouches" that invade the space between the ectoderm and entoderm. Multiplying, these structures form the third, middle germ layer — the mesoderm, resulting in the embryo becoming three-layered.
To remember the type of gastrulation in amphioxus, imagine a soft rubber ball (the coeloblastula). If you press your finger into one half of it and push it inside the other, the ball turns into a two-layered cup — this is invagination.
Class and exam questions
How many lips does the blastopore form in amphioxus?
The margins of the blastopore (primitive mouth) in amphioxus consist of four lips: one dorsal, one ventral, and two lateral.
Which germ layer gives rise to the mesoderm in amphioxus?
The mesoderm is formed at a late stage from primary entoderm cells, which form two pouches that invade the space between the ectoderm and entoderm.
What else to study on this topic
- Structural features of alecithal and oligolecithal eggs
- Synchronous holoblastic equal cleavage: process characteristics
- Detailed structure and functions of the gastrocoel
- Significance and fate of the four blastopore lips in further development
- Mechanism of formation of primary entoderm pouches
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