In thecerebral andcerebellarcortices,gray matter(cell bodies and
immediately adjacent processes) is located peripherally and white matter
centrally; this pattern is reversed in the spinal cord.Cerebellar cortex con-
sists of molecular, Purkinje,andgranularlayers[mnemonic: miles per
gallon (MPG)]with extensive arborization of the neuronal processes. The
cerebral cortexconsists of a homogenous layer I with multiple deeper lay-
ers of large pyramidaland other types of neurons.The number of layers
varies, depending on the cortical region. Neuronal cell bodies (perikarya)
are also localized in gangliain the peripheral nervous system and auto-
nomic nervous system (ANS).
CARDIOVASCULAR SYSTEM, BLOOD,
AND BONE MARROWBlood vessels are comprised of three layers or tunics: intima, media, and
adventitia. In large arteries close to the heart, the tunica mediacontains high
amounts of elastin to buffer the heart’s pulsatile output. Smaller muscular
arteries distribute blood to organs and capillary beds; their contractions are
mediated by both the sympathetic nervous system (SNS) and by humoral
factors.Endothelial cellslining the vascular lumen secrete vasoactive sub-
stancesthat regulate relaxation and contraction of the underlying smooth
muscle. For example, endothelin 1 is apotent vasoconstrictor; nitric
oxideissynthesizedfromL-arginineand induces relaxation of the smooth
muscle through a cGMP-dependent mechanism. Prostacyclin,also synthe-
sized by endothelial cells, is a smooth muscle relaxant that functions
through a cAMP-dependent mechanism. Prostacyclin inhibits platelet
adhesionandprevents intravascular clot formation.Endothelial cells
produce molecules that regulate fibrinolysis and thrombogenesis. Endothe-
lial cell-derived factors are stored in intracellular granules and released into
the blood stream upon stimulation. There are two contrasting factors, fibri-
nolytic tissue-type plasminogen activator (tPA)andvon Willebrand fac-
tor (vWF).In contrast to tPA, vWF induces coagulation and thrombus
formation.Endothelial cells also produce tissue factor,the only non-
plasma protein in the clotting cascade, which initiates the common blood
clotting pathway. E-selectinexpression on endothelial cells modulates
extravasationofmonocytesandneutrophils. Chemokines (chemoat-
tractant cytokines)induce expression of E-selectins on the endothelium
under normal conditions and following inflammation. Smooth muscle
cellsundergo hyperplasiaandhypertrophyin hypertension. The heart
24 Anatomy, Histology, and Cell Biology