The phylogeny of the fine structure of blood vessels and lymphatics: similarities and differences.

As animals increased in size, various modifications had to come into being to carry nutrients and waste around the body. Different phyla solved the various problems differently; sometimes there was convergent evolution. In invertebrates the endothelial cells are often widely separated from each other; permeability is limited by the pericyte layer; the reverse occurs in vertebrates. In primitive chordates small peripheral vessels often consist only of the basement membrane, and even this may be partly missing; the more centrally one looks, the more the endothelial cells become continuous. Fenestrae appeared first in the agnatha, but only become common in the elasmobranchs. Increased size and activity necessitated still larger blood hydrostatic pressure and increased blood colloidal osmotic pressures to balance this. Since the permeability of the vessels could not be reduced, much more protein (and fluid) had to leak to the tissues. So the lymphatic system had to evolve. This is first seen in the torpedoes and fully evolved in the bony fishes. However, the small venous vessels of the elasmobranchs have openable inter-endothelial junctions and other structures very similar to those of the initial lymphatics. Apart from the absence of fenestrae in lymphatics, or when this system is injected with a tracer, it is not always possible to tell them apart with the electron microscope. There are, however, various differences between them which will help to differentiate them.