Initially during embryologic development, as the neural tube is developing, three rudimentary brain regions develop: the prosencephalon, the mesencephalon, and the rhombencephalon. As development proceeds, the prosencephalon forms two distinct regions: the telencephalon and diencephalon, while the rhombencephalon develops into the metencephalon and the myelencephalon. These divisions are important to understand in the greater anatomic organization of the brain. The telencephalon is composed of the two cerebral hemispheres and associated lateral ventricles, while the diencephalon includes the thalamus, hypothalamus, and third ventricle as it passes both dorsally and ventrally through the thalamus. The mesencephalon is traversed dorso-centrally by the mesencephalic aqueduct which carries cerebrospinal fluid from the lateral ventricles to the fourth ventricles and eventually to the lateral aperture. The metencephalon gives rise to the pons along the ventral portion and the cerebellum dorsally. Lastly, the myelencephalon becomes the medulla oblongata. The spinal cord develops from a spherical tube to its mature form characterized by a prominent dorsal medial sulcus and ventral median fissure with laterally arranged white matter and a central, “butterfly” of gray matter. The central canal runs the middle of the spinal cord and carries cerebrospinal fluid. A basic understanding of the development of the brain is especially helpful as it relates to malformations commonly encountered in the central nervous system. Nervous tissue is a dynamic structure, studded with numerous cell types. While a full discussion of the functions of these cell types is outside the scope of this text, a brief review is provided here for context. Neurons populate the gray matter of the brain and spinal cord, and their axons traverse both the gray and white matter where they are surrounded by myelin. In the central nervous system, myelin is produced by oligodendrocytes which are found in both the gray and white matter, in the latter forming chains of cells. Astrocytes provide a myriad of functions in the brain including helping form the blood–brain barrier as well as various physiologic roles. Microglia are the resident macrophages of the central nervous system and have phagocytic behavior as well as a variety of other immunologic roles. Other macrophage populations, including meningeal and perivascular, also contribute to inflammation and phagocytosis. Blood vessels are scattered throughout the parenchyma. Ependymal cells line the ventricles and aid in production and flow of cerebrospinal fluid. Epithelial cells line the choroid plexus with an additional role in the production of cerebrospinal fluid. As the neural tube is forming, a population of cells that has its origin in neuroepithelium coalesces dorsal to the developing tube. This collection of cells, referred to as the neural crest, is the origin of the neurons and the Schwann cells of the peripheral nervous system. These neural crest cells migrate away from the dorsal midline and form ganglia of the sympathetic trunk, dorsal root, enteric plexi, and parasympathetic ganglia. Once at these locations, these cells provide hubs for synaptic transmission throughout the body. The peripheral nervous system is an arborizing network of nerves throughout the body. They are endowed with axons and myelin, the latter of which is produced by Schwann cells. On cross-section, nerves are encapsulated by a loose matrix of fibrovascular tissue (epineurium) with individual nerve fascicles surrounded by epineurium. Single nerve fibers are bordered by a fine layer of endoneurium that surrounds the myelin sheath of each nerve fiber. Macrophages are present throughout nerves performing immunosurveillance and phagocytic roles. Anatomically, brain differentiation is similar across widely disparate species. The pattern of folds (gyri and sulci) in the cerebral cortex varies from species to species, and these folds are completely lacking in some species, including rabbits, and rodents (Figure 11.1). In larger animals, like horses and cattle, the choroid plexi can be remarkably large and prominent. Figure 11.1 Comparison of mammalian brains. The equine brain is fixed in formalin, and the others are fresh. Access to the brain and spinal cord can be difficult without the proper tools. To remove the brain and spinal cord, a knife/scalpel, bone-cutting forceps, and/or a saw (a vibrating saw works best; however, a standard bone saw is also acceptable) is required. Remove the head as described in Chapter 3. Once the head is removed from the cervical spine, the foramen magnum can be evaluated for coning of the cerebellum (indicating increased intracranial pressure) or malformative abnormalities of the occipital bone (Figure 11.2). To facilitate removal of the brain from the calvaria, it is important to remove as much muscle and soft tissue as possible including the large temporalis muscle and the soft tissue attachments around the caudal occipital bone. Figure 11.2 Canine skull, caudal aspect. The occipital bone is malformed with a wedge-shaped section of occipital bone absent exposing the subjacent cerebellar vermis. The approach to brain removal depends on the size of the animal and the presence of obstacles, such as horns. In smaller animals, such as rabbits, ferrets, cats, and small dogs, bone-cutting forceps can be used to cut small pieces of the calvaria starting at the level of the foramen magnum until the entire brain is visualized (Figure 11.3). This will minimize inadvertent cutting of the brain with the saw and is a safe method of brain removal in a rabies suspect. In fetuses or neonatal animals, the skull bones may not be fused, and bone-cutting forceps allow removal of the bone and exposure of the brain. In small rodents, the entire head can be removed, skinned, fixed, and decalcified prior to sectioning for histopathology. Figure 11.3 Bone-cutting forceps can be used to cut the calvaria for brain removal in smaller animals. The cuts are as described in Chapter 3 (see Figure 3.31). For brain removal with a saw, follow the procedure outlined in Chapter 3 (see Figure 3.31). As the top of the calvaria is removed, the dura will pull away from the brain, and the tentorium cerebelli will also be removed. If the tentorium remains in situ, it can be carefully dissected away from the brain. Depending on the force with which the dura pulls away from the brain, it may bring along with it the pineal gland (normally nestled between the occipital lobes and dorsal to the mesencephalon) and the choroid plexus from the lateral ventricle. Neither of these structures should be confused with a neoplasm. For animals with horns, the horns can be cut off prior to cutting the skull or the skull cuts can be modified slightly to avoid the horns. For example, instead of one transverse cut at the level of the zygomatic process, make two angled cuts, adjacent to each of the horns, that intersect rostrally at midline (Figure 11.4). Another option for brain removal in large animals is to use a hand saw or bandsaw to split the head (and brain) longitudinally, removing the two halves of the brain separately. This approach is not recommended in animals with suspected pituitary or hypothalamic disease, as these structures will be damaged by the midline cut.
Chapter 11
The Nervous System
11.1 Anatomy Review and Species Differences
11.2 Organ Examination and Sampling
11.2.1 Brain
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