The pia mater is the innermost meninges and nestles against the surface of the brain, also reaching the fine interstices of the cerebral convolutions (gyri) and folds (sulci). Together, the three meninges help protect the brain. The permeability of the pia mater is significant for the blood–brain barrier, the exchange of substances between cerebral fluids, and the connection to the lymphatic system.
What is the pia mater?
The pia mater is a delicate layer composed of connective tissue that was anatomically identified 2,300 years ago. According to its location, two sections of this tissue layer can be distinguished in humans: The pia mater encephali is the innermost of the three meninges of the human brain and extends further to the spinal cord as the pia mater spinalis. Above the pia mater encephali lies the spinal meninges (arachnoid) and the hard meninges (dura mater). Because of its fine and thin shape, the pia mater is also known as the delicate meninges. The brain is completely enclosed by the pia mater encephali; the only exceptions are openings at the ventricles and the apertura lateralis and apertura medialis.
Anatomy and structure
On the brain, the pia mater rests directly on the surface of the tissue and penetrates the interstices of the cerebral convolutions (gyri), where it also lines very small folds. To enable it to perform this task, the pia mater is thinner and finer than the other meninges. It is connected internally to the membrana limitans superficialis. This is another layer of tissue, but it does not originate from the meninges, but from the brain itself. The cells of the membrana limitans superficialis arise from spider cells (astrocytes), which belong to the glial cells. Within the pia mater there are only a few cells, but between them there are larger than average interstitial spaces. This extracellular matrix or intercellular substance contains fibers surrounded primarily by proteins and sugar molecules. The fibers include various types of collagen strands as well as elastic fibers, which are composed of fibrillin and elastin and give the connective tissue particular flexibility: The higher the proportion of elastic fibers, the more flexible the structure.
Function and tasks
The meninges protect the underlying tissue and stabilize nerves and blood vessels that supply the organ with signals and nutrients. Because the pia mater can also penetrate the narrow clefts that appear in the brain’s surface, it lends additional support to fine capillaries in particular. In addition, the pia mater contributes to the careful filtering of substances at the blood-brain barrier. This barrier has a protective function and serves not only to separate blood from brain tissue, but also to defend against potentially harmful substances and pathogens that damage the central nervous system. In this context, the blood-brain barrier is selectively permeable to electrolytes, oxygen, glucose and some other substances that ensure the functioning of nerve and glial cells. Without it, the tissue would die. The delicate meninges are also connected to the lymphatic system. Another function of the pia mater is to separate the interstitial fluid from the cerebrospinal fluid; the pia mater also forms a small portion of the cerebrospinal fluid itself, although two-thirds originates from the choroid plexus. Between interstitial fluid and cerebrospinal fluid, the permeability of the pia mater ensures that the concentration of substances in the two fluids can equalize, resulting in equal density. This equalization also helps protect the brain. The cerebrospinal fluid spaces between the skull bones and the brain cushion movement, thereby preventing the brain from hitting the skull wall with even minor impacts and potentially suffering damage as a result. Sensory neurons in the pia mater transmit pain sensations that may indicate damage to innervated areas.
Diseases
Meningitis, or inflammation of the brain, is an infection that can result from infestation with viruses, bacteria, fungi, or parasites. Different types of pathogens may be the cause in each case. Meningitis is potentially fatal; mortality rates vary widely, ranging from 5% for meningococci in general to 80% for the elderly and young children. The symptoms are very diverse.Headache, backache, fever, and severe general malaise are common. Nausea, vomiting and disturbances of consciousness are also possible. Quantitative disorders of consciousness such as drowsiness, unconsciousness and coma may manifest, as well as qualitative disorders of consciousness, which may be accompanied by loss of reality. Affected individuals may have difficulty with spatial and temporal orientation or be unable to accurately identify themselves. Meningitis may also cause neck stiffness, convulsions in general, cramps in the neck and back (opisthotonus), skin changes, barge belly, photophobia, and papilledema. The exact treatment depends on the underlying cause and is done on an inpatient basis, as the condition is very risky and often requires extensive measures to check and stabilize vital signs. Even with otherwise successful treatment of meningitis, permanent damage may remain. In bacterial forms of meningitis, this is the case in about 50% of those affected. Possible sequelae include retrograde amnesia, motor difficulties, and sensory and perceptual disturbances. Severe sequelae include wakefulness or apallic syndrome, which is characterized by inactivity of the cerebrum.
