As far as we know, the human brain is one of the most complex structures in the universe. Capable of astounding feats, our brain has fascinated us for centuries but its function has proved difficult to unravel. Given the ethical issues associated with brain research, the search has been on for the last few decades to find a brain model that could teach us about human brain development, and recent interest has focused on the pig. Jacob Jelsing explains that pig brains are similar to human brains in several respects; they have many of the same morphological features, are quite large and all of the cortical neurons appear to be fully developed at birth. But other aspects of the pig brain are less well characterised. Jelsing, working with Ralf Hemmingsen and Bente Pakkenberg, decided to characterise the pig cortex, the region of brain responsible for processing most of our conscious behaviour, by counting the number of neurons in this fundamental structure (p. 1454).
But rather than looking at just one breed of pig, the team decided to investigate two; a domestic Danish Landrace, Yorkshire crossbreed, and an experimental pig breed, the diminutive Göttingen minipig. Jelsing explains that although the domestic breed is more numerous than the minipig, the minipig's smaller stature and freedom from disruptive pathogens makes them a more attractive breed to work with from the neurobiologists perspective. Aage Olsen and Nanna Grand supplied Jelsing with brains from pigs of both species ready for the team to prepare wafer-thin brain slices before beginning the painstaking task of counting cortex neurons.
Fortunately, the team didn't have to count every single neuron in each cortical sample. Jelsing knew that if he systematically selected brain sections from randomly selected pigs he could calculate the total number of neurons in the cortex, despite having only counted a tiny fraction of the total neurons in the tissue. First Jelsing systematically chose brain slices and then Rune Nielsen counted the number of neurons in a few systematically chosen areas of each section. So long as Jelsing and Nielsen had chosen regions from all of the cortical tissue at random, but then sampled them in a systematic way, they could calculate the total number of neurones in both cortices.
After Nielsen had spent several days peering through a microscope at the delicately stained samples, the team were able to calculate the number of cortical neurons that each breed had at birth: 425 million in the domestic pig and 253 million in the smaller minipig. But when the team calculated the number of neurons in the adults' brains, they were in for a surprise; while the domestic pig's neuron count had hardly changed, the minipig's had increased significantly to 324 million. Unlike the neurons in the human cortex, which do not develop postnatally, the minipig's neurons had continued developing after birth. Jelsing does not know how long it takes the minipig's brain to complete development but it could be anything from weeks to several months. Given the shock finding that the Göttingen minipig's brain continues developing after birth, the team suggest that the domestic pig's brain may be a better model for human brain development than the smaller minipig's.
- © The Company of Biologists Limited 2006