Characteristics of Early Developmental Stages
The development of primitive chordates, which include amphioxus (Branchiostoma), begins with the fertilization of a specific type of egg cell. It is classified as alecithal (practically devoid of yolk) or primary oligolecithal and isolecithal (containing a small amount of yolk distributed evenly throughout the cytoplasm).
Due to this egg structure, subsequent cleavage of the zygote follows the simplest pattern. It is holoblastic (complete), equal, and synchronous. As a result of this cleavage, an embryo at the blastula stage is formed. In amphioxus, this is represented by a coeloblastula — a regular, single-layered spherical vesicle whose wall is formed by cells of entirely uniform size.
Mechanism of Gastrulation
Gastrulation in amphioxus occurs via invagination. The starting point for this process is the fully formed single-layered coeloblastula.
The core mechanism involves the gradual infolding (invagination) of one hemisphere of the blastula into the other. As a result, the spherical single-layered vesicle radically changes its shape, transforming into a structure traditionally described in embryology as a "double-layered cup." The cells move as an intact epithelial sheet without losing contact with one another.
Morphology of the Early Gastrula
The gastrula formed via invagination (the "double-layered cup") has a clearly differentiated structure consisting of the following elements:
- Primary ectoderm — the outer layer of embryonic cells that remained on the surface following invagination.
- Primary endoderm — the inner layer of cells formed from the hemisphere that folded inward.
- Gastrocoel (archenteron) — the newly formed cavity of the primitive gut, enclosed by the layer of primary endoderm.
- Blastopore (primitive mouth) — the 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 a later stage of gastrulation, the structure of the embryo becomes more complex with the specification of the third germ layer. This process is closely linked to the inner layer of the gastrula.
Cells of the primary endoderm begin to actively proliferate and form two symmetrical "pouches." These evaginations gradually push into the space between the outer (ectoderm) and inner (endoderm) layers. Eventually, these pouches completely separate and form the mesoderm. Thus, the initially double-layered embryo definitively converts into a triploblastic structure, paving the way for subsequent organogenesis.