Pia Mater

PIA MATER (Latin: soft membrane; syn.: meninx vasculosa, tunica propria), a thin, vascular-rich membrane which, by adhering tightly to the surface of the brain, lines not only all its sulci but also penetrates into the cerebral ventricles, forming vascular coverings (telae chorioideae) there. The pia mater of the spinal cord (pia mater spinalis) adheres just as tightly to the spinal cord. In the ventral longitudinal sulcus, this membrane penetrates deeply right up to the white commissure itself; in the dorsal sulcus, it either does not enter at all or enters only in the form of a short wedge. Both the exiting nerves and the filum terminale receive their sheaths from the pia mater. From the outer surface of the spinal pia mater, on both sides, the denticulate ligament (ligamentum denticulatum) extends to the dura mater. Numerous connective tissue filaments are stretched between the pia mater and the arachnoid membrane; there are especially many such filaments on the posterior surface of the spinal cord. The spinal pia mater consists of an outer and an inner layer, separated from each other by a capillary lymphatic space. The outer layer consists of a layer of endothelial cells, under which lies a thin network of elastic fibers, and beneath that run longitudinal connective tissue fibers. The composition of the inner layer (intima piae) includes 1) endothelial cells, 2) a network of very thin elastic fibers, 3) a layer of circular soft connective tissue fibers, 4) a second network of elastic fibers, and 5) an endothelial covering. Pigment cells are encountered here and there in the tissue of the pia mater. The cerebral pia mater, like the spinal pia mater, provides sheaths for the nerve roots exiting the brain. It is much thinner than the spinal pia mater and consists of an endothelial covering, under which lies a network of connective tissue fibers. In this network, among the stationary connective tissue cells, one finds wandering round formations—lymphocytes and histiocytes; in addition, mast cells are also encountered here. On the basal surface of the pia mater, mainly around the infundibulum, cells with elongated processes containing dark green pigment (chromatophores) are found. The cerebral and spinal pia mater contains a great many vessels that nourish both the membrane itself and the underlying brain substance. These vessels, upon their penetration into the brain, receive sheaths from the pia mater. The nerves of the pia mater belong to the sympathetic system. There are many more of them at the base of the brain than on its convex surface. The nerve fibers, which constitute the continuation of the processes of uni- or multipolar cells scattered in abundance throughout the pia mater, form numerous nerve networks in the pia mater. Among the pathological-anatomical changes in the area of the pia mater (pia + arachnoidea), in addition to hyperemia, hemorrhage, and adhesions with the underlying brain substance, various inflammatory processes—meningitis (see)—are observed. Many tumors of the brain and spinal cord originate from the pia mater. Besides tumors, cysticerci cysts are sometimes found in the pia mater. Very rarely, a racemose arterial angioma (angioma arteriale racemosum) is encountered in the area of the pia mater, which sometimes causes fatal hemorrhage into the brain. Literature—see literature to the articles Brain and Meninges.