Grey matter refers to the part of the brain that is responsible for the processing of information. It integrates the information which we acquire through our senses and transforms them into actions, which are then executed by our bodies. In general we can distinguish between two scales at which this development takes place, the microscopic scale (development of cells) and the macroscopic scale, defined by the development of valleys (sulci) and hills (gyri) in a process called cortical folding.
One of the most important time frames during microscopic grey matter development is the period between 24 and 40 weeks. The major events which take place in this period are called neurogenesis, neural migration and neural apoptosis.
Neurogenesis refers to the creation of neurons. It mainly occurs during the second trimester. In most parts of the brain no new neurons are created after birth. One important exception is the hippocampus, where the neurons are linked with memory formation. However, from 24 weeks of gestation until around 4 weeks after birth, rapid cell death occurs as well, reducing the total number of neurons by half.
Neurons do not stay at the place where they are created. After they are produced, they migrate towards the developing neocortex. They do so by using a scaffolding (basal processes) to which they attach themselves. In particular, the first migrating neurons form a structure known as preplate. This structure splits then into two layers (marginal zone and subplate), between which the cortical plate forms.
At around 3 weeks the neural tube, the ancestor of the brain, forms. From there three principal enlargements form, the forebrain (proencephalon), the midbrain (mesencephalon) and the hindbrain (rhombencephalon).
Over the course of pregnancy, the hindbrain develops into the cerebellum, pons and medulla oblongata, whereas the proencephalon first splits into the diencephalon and the telencephalon. The diencephalon continues to develop into the optic vesicles (the retina of eyes), the thalamus (responsible e.g. for distributing information) and the hypothalamus (responsible e.g. for regulating temperature or hunger). The telencephalon, on the other hand, forms the largest part of the human brain, the cortex.
The cortex starts to form sulci and gyri in a process called cortical folding from approximately 15 weeks of gestation onwards. Sulci ("valleys") and gyri ("hills") form the major landmarks of the human brain. The primary sulci are already present by 28 weeks, however smaller, secondary and tertiary sulci form afterwards and continue to form after birth.