The mechanism of bruising

Over the weekend, whilst I was being pelted by a firing squad of my friends with the dangerous, high velocity weapon that is a football, I asked myself 2 things. One, why do I play as a goalkeeper? Two, what causes the myriad bruises that bloom across my arms and legs in the aftermath of my exploits defending a string netting and metal frame from a rubber ball? Well, I discovered that the discolouration under the skin is caused by cells bursting open and spilling their contents; the organelles and everything else in the cytoplasm. This happens due to trauma induced premature autolysis- basically the force of the ball hitting my limbs, causes cells to digest their membranes with their own hydrolytic enzymes, in a process known as necrosis. Who knew such an everyday thing like bruising could be so interesting? However, I also discovered a second type of cell death, known as apoptosis. Apoptosis is altogether more natural and controlled carefully by the body, also known as programmed cell death (PCD). Apoptitic proteins target the mitochondria of cells, and can effect them in different ways, for example by the formation of membrane pores, or an increase in the permeability of the mitochondrial membrane, which in turn causes apoptotic effectors to leak out. Either way the mitochondria is destroyed, and the cell cannot respire aerobically, so it dies. There is also a pretty complicated process involving direct initiation of apoptotic mechanisms, and there are seemingly 2 rival theories: the Tumour Necrosis Factor model, and the First Apoptosis Signal model. Anyway, unlike necrosis, apoptosis produces cell fragments called apoptotic bodies that phagocytic white blood cells are able to engulf and quickly remove before the contents of the cell can spill out onto surrounding cells and cause damage. Pretty neat huh?  And in an average human adult, around 60 billion cells die each day due to PCD, which seems like a lot until you consider there are around 37 trillion cells in our body. How all these cells organise themselves into complex structures, and work together in near perfect unity, is part of the reason I never ceased to be amazed by the field of Biology.