Cell junctions in amphioxus (Cephalochordata): a thin section and freeze-fracture study

Area of Science:

• Cell Biology
• Evolutionary Biology
• Zoology

Background:

• Intercellular junctions are crucial for tissue integrity and function in multicellular organisms.
• Cephalochordates, like Branchiostoma lanceolatum, represent a key evolutionary link between invertebrates and vertebrates.
• Understanding their junctional complexes provides insights into the evolution of vertebrate tissues.

Purpose of the Study:

• To investigate the types and distribution of intercellular junctions in the epidermis, alimentary tract, and notochord of Branchiostoma lanceolatum.
• To compare the junctional characteristics of cephalochordates with those of urochordates, vertebrates, and invertebrates.
• To elucidate the evolutionary significance of junctional diversity in chordate development.

Main Methods:

• Thin section electron microscopy was used to examine tissue ultrastructure.
• Freeze-fracture replica electron microscopy visualized intramembrane particle arrangements.
• Lanthanum tracer infiltration assessed the permeability of intercellular spaces.

Main Results:

• Epithelial cells of the branchial chamber and intestine possess zonulae adhaerentes but lack gap, tight, or septate junctions.
• Basal borders are secured by zonulae adhaerentes and hemidesmosomes; lateral surfaces show random intramembrane particles.
• Notochord membranes lack defined junctions, while epidermis and glial cells exhibit desmosomal-like associations or zonulae adhaerentes.

Conclusions:

• Branchiostoma lanceolatum exhibits a simplified junctional complex, primarily relying on zonulae adhaerentes, which is distinct from vertebrates and invertebrates.
• The absence of tight and septate junctions suggests a unique barrier function and evolutionary trajectory in cephalochordates.
• Ciliary modifications in Branchiostoma lanceolatum resemble those in urochordates, indicating conserved features within tunicates.