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NORMAL MICROSCOPIC ANATOMY AND NEUROHISTOLOGY
Sections of normal brain are labelled NormalNP001 through NormalNP007. The labeled macroscopic sections of various regions of the brain precede the microscopic description of the same area. Some abnormalities are nearly always present and vary with age, so a thumbnail clinical history for the sections is provided. The goal in studying these normal sections is not to have you become a neuroanatomist, but to help you recollect your brief experiences in neurohistology in terms of the histologic methods employed by the pathologist, then use them to study the diseases represented in the sections that follow. Questions along the way may help you to focus your attention on features that change a good deal in disease, and remind you of some gross-microscopic relations.
NormalNP001. [WebScope] [ImageScope] CEREBRAL CORTEX, BASAL FRONTAL REGION (Luxol). From a 70 year-old man with little cerebral arteriosclerosis who died of lung cancer (NormalNP001 through NormalNP005 and NormalNP007 are from this case). Cortex gray matter, where nerve cell bodies are concentrated, forms an eighth-inch-thick pink layer on the surface; white matter is blue. White matter is composed of axons from many of these cell bodies as well as axons coming into this cortex from elsewhere. Olfactory bulb stalk, on the orbital surface of the brain, is at the right margin of the section. Higher power magnification shows that nerve cell (neuron) bodies tend to be aligned at right angles to the tangent of the cortical surface. Myelinated fibers (axons) tend to parallel this vertical orientation in the cortex. Nerve cell nuclei with their nucleolus look like bull’s eye targets, glial and other nuclei have more diffuse chromatin. Oligodendroglia that make and maintain myelin are represented by round small nuclei along the myelinated fibers. They are also in the cortex gray matter as satellite cells for neurons. Astrocyte nuclei are usually larger, but you sometimes need a special silver stain to be sure which glial cell is which. The pink staining background in the section is the fibrillary matrix or feltwork of the cortex which is made up of the myriad of interwoven cytoplasmic processes of the cells of the brain: dendrites of neurons, processes of astrocytes, oligodendroglia, and microglia. A useful term for this matrix is neuropil (neuro + pil = hair, felt).
NormalNP002. [WebScope] [ImageScope] BASAL GANGLIA (Luxol). This section includes the corpus callosum, cingulate gyrus, caudate nucleus, putamen, internal capsule, the most anterior portion of the globus pallidus which is abutting on the ventral-lateral portion of the internal capsule, the anterior portion of the lateral ventricle, the basal portion of the frontal lobe, and, lastly, the septum pellucidum. Can you locate all of these structures? Compare this with section NormalNP003.
NormalNP003. [WebScope] [ImageScope] STRIATUM (Luxol) and adjoining insula cortex, internal capsule, and anterior commissure region, and anterior thalamus. The fornix is most medial in the section. Also note the optic chiasm, the external and extreme capsules, and the claustrum. Can you identify the putamen and globus pallidus? Notice that most of the neurons in the basal ganglia are uniformly smaller that those in the cortex. There are a few larger ones. The calcification of the walls of arteries in the globus pallidus is not normal, but often seems to develop independently of atheromatous arteriosclerosis in many older people. There are also calcific “beads” in the adventitia of some small vessels.
NormalNP004. [WebScope] [ImageScope] HIPPOCAMPAL REGION (Luxol). This coronal section includes the hippocampus (hippocampus = sea horse) and adjacent temporal lobe gyrus (entorhinal cortex). Above the temporal (ventral or inferior) horn of the lateral ventricle the lateral geniculate nucleus is present. Lateral to this structure is the tail of the caudate. The medial surface of the section is the posterior portion of the thalamus and a small portion of the cerebral peduncle. Within the ventricle is the choroid plexus. Can you identify all of these structures? The pyramidal cell layer (large cells) in the hippocampus is most vulnerable to damage in severe circulatory failure and by anoxia of persistent severe seizures (especially Sommer sector - where is it?). In the dentate gyrus portion of the hippocampus, granule cells (densely packed) are the characteristic neuron type. There are small beady calcific bodies in part of the hippocampus. Calcific bodies are present in the choroid plexus, another common site of accumulation as the years pass. In earlier years some degree of calcification in the choroid plexus may be classified as disease, but the beads (and calcification mentioned in the globus pallidus – NormalNP003) are usually regarded as “disease” because it is assumed that impaired blood supply is an accompaniment.
NormalNP005. [WebScope] [ImageScope] MIDBRAIN (Luxol). This coronal section is at the level of the exiting third nerve. Cerebral peduncle (basis pedunculus), substantia nigra, superior colliculus, medial lemniscus, aqueduct, and medial geniculate are landmarks telling you that you are in the midbrain. Large, somewhat round neurons in the ventral lateral margin of the central gray are from the mesencephalic nucleus of the trigeminal nerve. The big central zone of myelinated fibers is the superior cerebellar peduncles about to decussate caudal to the red nucleus. The broad ventral part is the basis pedunculus. The substantia nigra, just above it, is partly made up of pigmented (melanin) neurons in adults. The medial longitudinal fasciculus runs in the center of the tegmentum close to nuclei that run the eye muscles. Identify the aqueduct of Sylvius and the periaqueductal gray matter.
NormalNP006. [WebScope] [ImageScope] PONS (Luxol). From a 37-year-old man with chronic rheumatic heart disease and a fatal thrombotic embolus to his femoral artery. The corticospinal (pyramidal) tract is present here as a concentration of large longitudinal fiber bundles in the center of the base of the pons on each side. What effect would an extensive infarct involving the motor-sensory cortex or middle of the internal capsule, have on this tract? Identify the pontine nuclei and the crossing or transverse pontine fibers. Can you identify the locus coeruleus? It should be easy. Why?
NormalNP007. [WebScope] [ImageScope] MEDULLA, CEREBELLUM (Luxol). In this coronal section can you identify corticospinal (pyramidal) tracts (pyramids), medial lemniscus, 12th nerve nucleus and the inferior olivary nuclei? Purkinje cells in the cerebellum are actually flat cells, their dendrites extended in sagittal plane. Identify the three layers of the cerebellum, molecular, Purkinje cell, and granule cell.