Why do we have platelets? So we don't bleed to death. But the use of platelets for blood clotting is a trait we only share with other mammals. Let's take a look at why and how mammals developed these useful cells.
Mammals, for the most part, are cute furry animals named for their signature achievement, the lovely mammary gland. Mammals have other common characteristics. They are warm blooded. Most are viviparous, except for the weird, egg-laying monotremes. Most, but not all of the rest, use placentas, a rather strange mammalian birthing custom. But all mammals, and only mammals, have platelets, tiny cells without nuclei that are responsible for plugging the leaks in our blood vessels. Unfortunately, they are also responsible for a lot of heart attacks and strokes.
Other animals use more conventional systems of hemostasis. Some marine invertebrates have a single type of blood cell, responsible for hemostasis, but also for other types of defenses. Non-mammalian vertebrates have specialized nucleated thrombocytes to do the job. So why and how did platelets evolve?
The situation seems even more peculiar when one considers how platelets are formed. The immediate precursor of platelets is the megakaryocyte. This cell undergoes endomitosis; that is it undergoes repeated DNA replication without cell division, at the end of which they have an average of 16 times the normal number of chromosomes. After this DNA replication is complete, the cell enlarges and develops an extensive membrane system. The cell then develops long extensions called proplatelets, which start blebbing off, 5,000-10,000 platelets per cell, until there is little cytoplasm left. The denuded megakaryocyte then undergoes a programmed cell death.
Although small, the platelets are quite complicated, with two sets of physically distinct membranous channels. The platelets are packed with alpha granules and dense bodies, secretory organelles with a complex inventory of factors that influence aggregation and coagulation. The platelets also have an action/myosin contractile system that stabilizes the platelet plug.
So here we have a complete and unusual system for creating platelets without an obvious evolutionary path toward its accomplishment. Where is there an intermediate between the thrombocytes found in reptiles and birds and the platelets in mammals? What selective advantage do platelets provide and what brought the selective pressure to bear to create this complex system? It is a mystery of punctuated evolution. Readers of Scientia are invited to speculate.