The Olfactory Bulb in Companion Animals-Anatomy, Physiology, and Clinical Importance

Abstract

The Olfactory Bulb is a component of the Olfactory System, in which it plays an essential role as an interface between the peripheral components and the cerebral cortex responsible for olfactory interpretation and discrimination. It is in this element that the first selective integration of olfactory stimuli occurs through a complex cell interaction that forwards the received olfactory information to higher cortical centers. Considering its position in the organizational hierarchy of the olfactory system, it is now known that changes in the Olfactory Bulb can lead to olfactory abnormalities. Through imaging techniques, it was possible to establish relationships between the occurrence of changes secondary to brain aging and senility, neurodegenerative diseases, head trauma, and infectious diseases with a decrease in the size of the Olfactory Bulb and in olfactory acuity. In companion animals, this relationship has also been identified, with observations of relations between the cranial conformation, the disposition, size, and shape of the Olfactory Bulb, and the occurrence of structural alterations associated with diseases with different etiologies. However, greater difficulty in quantitatively assessing olfactory acuity in animals and a manifestly smaller number of studies dedicated to this topic maintain a lack of concrete and unequivocal results in this field of veterinary sciences. The aim of this work is to revisit the Olfactory Bulb in companion animals in all its dimensions, review its anatomy and histological characteristics, physiological integration in the olfactory system, importance as a potential early indicator of the establishment of specific pathologies, as well as techniques of imaging evaluation for its in vivo clinical exploration.

Keywords: Olfactory Bulb; companion animals; imagiology; olfaction; olfactory anatomophysiology; olfactory system.

Figure 1

Simplified schematic representation of the histological layers of the OB, the interactions between the different bulbar cells, and the basic neural circuits in the OB. All the axons of the olfactory sensory cells project to a certain olfactory glomerulus depending on the type of receptor they have; PG—Periglomerular Cells; T—Tufted Cells; M—Mitral Cells; G—Granule cells.

Figure 2

Histological overview of the dog OB: (a) Histological organization of the seven constituent layers of the olfactory OB: 1—olfactory nerves layer; 2—glomerular layer; 3—external plexiform layer (between black arrows); 4—mitral cell layer; 5—internal plexiform layer (between gray arrows); 6—granular cell layer; and 7—subependymal layer. (b) Structural details of the glomerular, external plexiform, mitral, internal plexiform, and granular layers. Olfactory glomerulus highlighted by blue arrow; (c) Structural details of the glomerular, external plexiform, mitral, internal plexiform, and granular layers. Olfactory glomeruli evidenced by blue arrows; (d) Structural details of the ependymal layer of the OB surrounding the olfactory ventricle, highlighted by a white arrow. Luxol fast blue and cresyl violet.

Figure 3

Simplified schematic representation of the olfactory pathways and their components.

Figure 4

CT skull of a 3-year-old Jack Russell Terrier, acquired as a volume and reconstructed in a high frequency reconstruction algorithm, displayed in a bone window. (a) Sagittal/slightly parasagittal reconstruction to line through with the OB; (b) Transverse plane at the level of the OB; and (c) Dorsal plane reconstruction at the level of the OBs. White arrows on each image highlight the thin curved lamina cribrosa (cribriform plate) that bounds the rostral aspect of the OBs. Multiple tiny defects through which the olfactory nerve bundles pass are visible in all planes, although most distinct on the transverse plane compared to the remaining ones.

Figure 5

MRI brain of two dogs with different conformation, showing the location of the OBs (white arrows show the rostral extent, bounded by the lamina cribrosa (cribriform plate)). (a,d) T2W sagittal/slightly parasagittal to line through with the OB, with white arrowheads highlighting the olfactory fissure which separates the OB from the frontal lobe; (b,e) T2W transverse at the level of the OB; and (c,f) T1W transverse equivalent. (a–c) are of a brachycephalic breed (6-year-old French Bulldog) showing that the OBs are located on the rostroventral aspect of the brain; therefore, are seen ventral to the frontal lobes on the transverse view. (d–f) is a mesocephalic breed (12-year-old Golden retriever) with the OBs protruding from the rostral aspect of the brain. Transverse slices from patients with this conformation therefore show only the OBs and small segments of the surrounding brain.

Figure 6

MRI brain of two cats, showing the location and appearance of the OBs (white arrows). (a,d) T2W sagittal/slightly parasagittal to line through with the OB; (b,e) T2W transverse at the level of the OB; and (c,f) T1W transverse equivalent. (a–c) is of a domestic short hair cat (8-year-old) showing that the OBs are located protruding from the rostral aspect of the brain, extending slightly more rostrally than in dogs. On the transverse view, the OBs are noted between the globes. (d–f) is of a 16-year-old cat with a common variation of anatomy, showing a distension of the olfactory recess of the lateral ventricles, leading to a T2W hyperintense and T1W hypointense central ovoid structure within the OB, which connects to the lateral ventricle via a thin stalk (white arrowheads).