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Of the principal sensory systems (vision, olfaction, taste, hearing, and balance), olfaction is one of the oldest. This ubiquitous system has both peripheral and central subdivisions. The peripheral subdivision is comprised of the olfactory epithelium and nerve fascicles, whereas the central subdivision is made up of the olfactory bulb and its central connections. Humans lack the "accessory olfactory system" of many other mammals, exhibiting only a nonfunctioning vestige of its peripheral element, the vomeronasal organ. Compared to most mammals, major elements of the human olfactory system are reduced; for example, humans have fewer turbinates than many mammals, and their olfactory epithelia are found only on one or two of these structures and their adjacent surfaces. Nonetheless, humans retain a full complement of functional cellular elements including a regenerating population of olfactory sensory neurons. These neurons extend long ciliary processes into the mucus that form a mat of cilia on which the odorant receptors are located. The olfactory sensory neurons send their axons directly to synapse within the olfactory bulb. Mitral and tufted cells then relay impulses from the bulb to other brain regions. This chapter describes the general anatomy and microanatomy of the olfactory system., (Copyright © 2019 Elsevier B.V. All rights reserved.)

The terms "microsmatic" and "macrosmatic" refer to species with lesser or greater levels, respectively, of olfactory function. Historically, primates are considered microsmats (olfactory sense reduced) with a concomitant increased emphasis on vision. The olfactory bulbs (forebrain centers that receive peripheral olfactory input) are proportionately smaller in primates compared to most other mammals. Similarly, the regions of the nasal cavity that are covered with olfactory epithelium (containing receptor cells) have proportionately less surface area in primates than other mammals. Thus, the generalization that primates are microsmatic is most frequently stated in terms of the proportional rather than absolute size of olfactory structures. Yet the importance of scaling to body size is unclear in regard to the chemical senses such as the olfactory or vomeronasal systems-do chemosensory structures such as olfactory bulbs and olfactory epithelium exhibit the same neural relationship to body mass that is seen for neural tissues that supply innervation to musculature or the skin? Previous studies examining neuronal density, volume, and/or surface area of the olfactory epithelium illustrate that different conclusions may be supported based on the parameter used. Plots of olfactory bulb volume versus body mass that generated for large-scale taxonomic studies or growth studies benefit from body mass (or total brain volume) with a comparative perspective. However, our examination of proportional versus absolute measurements implies that in comparisons within taxa, body size adjustments needlessly distort the data. As a final consideration, another embryonic derivative of the nasal placode, the vomeronasal organ, may warrant consideration regarding a definition of microsomia versus macrosomia.

The accessory olfactory bulb (AOB) is the first neural integrative center for the olfactory-like vomeronasal sensory system. In this article, we first briefly present an overview of vomeronasal system organization and review the history of the discovery of mammalian AOB. Next, we briefly review the evolution of the vomeronasal system in vertebrates, in particular the reptiles. Following these introductory aspects, the structure of the rodent AOB, as typical of the well-developed mammalian AOB, is presented, detailing laminar organization and cell types as well as aspects of the homology with the main olfactory bulb. Then, the evolutionary origin and diversity of the AOB in mammalian orders and species is discussed, describing structural, phylogenetic, and species-specific variation in the AOB location, shape, and size and morphologic differentiation and development. The AOB is believed to be absent in fishes but present in terrestrial tetrapods including amphibians; among the reptiles AOB is absent in crocodiles, present in turtles, snakes, and some lizards where it may be as large or larger than the main bulb. The AOB is absent in bird and in the aquatic mammals (whales, porpoises, manatees). Among other mammals, AOB is present in the monotremes and marsupials, edentates, and in the majority of the placental mammals like carnivores, herbivores, as well as rodents and lagomorphs. Most bat species do not have an AOB and among those where one is found, it shows marked variation in size and morphologic development. Among insectivores and primates, AOB shows marked variation in occurrence, size, and morphologic development. It is small in shrews and moles, large in hedgehogs and prosimians; AOB continues to persist in New World monkeys but is not found in the adults of the higher primates such as the Old World monkeys, apes, and humans. In many species where AOB is absent in the adult, it often develops in the embryo and fetus but regresses in later stages of development. Finally, new areas in vomeronasal system research such as the diversity of receptor molecules and the regional variation in receptor neuron type as well as in the output neurons of the AOB and their projection pathways are briefly discussed. In view of the pronounced diversity of size, morphologic differentiation, and phylogenetic development, the need to explore new functions for the vomeronasal system in areas other than sexual and reproductive behaviors is emphasized.

The aims of this study on the human olfactory bulb were two. First morphometry of the bulbs revealed marked declines during aging in the numbers of mitral cells and glomeruli, the bulb's principal integrative and relay elements. Numbers of glomeruli and mitral cells in each bulb of the young adult human were found to be approximately 8,000 and 40,000, respectively; these numbers declined steadily with age at an approximate rate of 10% per decade, so that in the ninth and tenth decades less than 30% of these elements remain in place. Such a marked decline with aging is suggested to underlie in part the decline in olfactory abilities (odor detection and identification) of humans with aging. In a separate study a systematic search for presence of an accessory olfactory bulb in the adult and aging bulbs was undertaken. No positive evidence for such an organized formation was found in the various regions of the adult bulbs of different age groups. The implications of these negative findings for the recent theories on human vomeronasal function and pheromonal perception are discussed.

The present study documents the morphological changes in the aging human olfactory bulb. Eight bulb pairs from white females between the ages of 25 and 102 years were used. The number of mitral cells in layers IV and III was determined for each bulb and corrected for split cell error. Counts were made on 10-micron thick Nissl-stained sections at 250-micron intervals. The mean number of mitral cells per olfactory bulb at age 25 was estimated from linear regression to be 50,935; at age 60, 32,718; and at age 95, 14,501. The average loss over the time interval studied was 520 mitral cells per year. The volume of each bulb layer, except layer IV, was determined. The difference in the volume of each layer within individuals with age and the total volume with age was not significant (P less than 0.11). The estimated mean bulb volume was found to be 50.02, 43.35, and 36.68 mm3 at ages 25, 60, and 95 years, respectively. The estimated reduction in bulb volume per year increase of age was 0.19 mm3. The ratio of mitral cells to bulb volume for layer III decreased by 19.4 units for every year increase in age. No significant difference was found between the left and the right bulbs in regard to the number of mitral cells and the bulb volume. Histologically, the glomerular layer thickness as well as the mitral cell size and concentration per unit area decreased with age. The intrabulbar anterior olfactory nucleus was discontinuous, highly variable in size, and presumably variable even in neuronal numbers.

Volumes of the main olfactory bulb (MOB) in 76 species of Insectivora, Scandentia, Primates and Macroscelidea, and some of the laminar components in 34 species were measured. No statistically significant differences were found (1) between the two sides in the 162 individuals and (2) between males and females in the 19 species in which both sexes were examined. In interspecific comparisons the relative size (expressed by size indices) shows a definite tendency to decrease from Insectivora through prosimians and simians to man. The average indices were 100-64-10-3.6, respectively. Scandentia and Macroscelidea have the highest average MOB indices (135 and 166). The relative MOB size is discussed in relation to feeding and social behaviour. It was shown that MOB development is largely linked to dietary adaptations but that its importance in feeding behaviour is paralleled by a similar importance in social behaviour. Within each dietary type, MOB development is associated with the different characteristics of the occupied niche. Among Insectivora, ground-dwelling species occupy the upper, and semiaquatic species the lower positions in the size scale for the MOB. Among Primates, in closely related species, the nocturnal species have in general better developed MOBs than the diurnal species. The composition of the MOB is relatively stable, i.e. the laminar components (layers 1 + 2, 3, and 4-6) show no clear change in their percentage size from well developed to strongly reduced MOBs. Only in the diurnal simians are layers 4-6 relatively small. This corresponds with the general observation (obtained from light-microscopy) that the granular layer (layer 6) is reduced and decomposed in higher Primates, and especially in man.

A cytophotometric study of the nuclear DNA content of bat cerebellar and olfactory bulbar neurons was performed with particular attention to corrections for distributional error and non-specific light loss and to selection of appropriate control nuclei for the establishment of reliable haploid and diploid DNA values. Feulgen stained sections were measured with an integrating, scanning microdensitometer for correction of distributional error. The values thus obtained were further corrected in 3 different ways to subtract the contribution of background absorbance at 546 nm. Bat haploid (1c) DNA values were derived from spermatozoa, and diploid (2c) values from pancreatic acinar cells, hepatocytes and non-hepatocyte liver cells. Microglia of olfactory bulb also had 2c values. After correction, all neurons measured, except Purkinje cells, had DNA values more closely approximating the tetraploid (4c) than the diploid (2c) level. Purkinje neurons had mean DNA values closer to 2c than to 4c, but inherent technical difficulties in measuring these very large, light staining nuclei has created less confidence in the values obtained than for those of the other neurons. This uncertainty, plus the apparent existence of two populations of Purkinje neurons, one 2c and the other hyperdiploid, suggested by the DNA distribution curves, make it difficult to eliminate the possibility of polyploidization of Purkinje neurons. A critical analysis of various cytophotometric, radioautographic and biochemical approaches to the problem of CNS neuron polyploidization has revealed potentially serious flaws in many of them, rendering virtually impossible interpretation of the numerous contradictory results in the literature. Standardization of the cytophotometric technique and improvement of the radioautographic approach seem to be prerequisite to the resolution of the existing dilemma.

The nasal epithelia of two species of bats were quantified with respect to relative surface areas and olfactory epithelial volumes. In the macrosmatic Aribeus jamaicensis 55.9% of the nasal cavity surface was covered by olfactory epithelium (232.4 mm2), in contrast to only 28.9% in the microsmatic Myotis lucifugus (36.4 mm2). The roles of the various nasal epithelia have been discussed as they may relate to olfaction, respiration and echolocation. In the olfactory bulbs of both species, the estimated concentration of mitral cells approximated at 2,500/mm2 compared to an olfactory nerve concentration of 5/mm2. In Artibeus, calculated total volume of olfactory epithelium was on the order of 16 times greater than in Myotis, and Artibeus' olfactory bulb diameter was twice as great. These findings, together with previously published surface, volume and physiological relationships, suggest a developmental design mechanism for an olfactory bulb in which the number of olfactory receptors increases some 450-fold above an initially established ratio of 2:1 between receptors and mitral cells. Key governing factors could be requisite mechanical rigidity of the cribriform plate of the ethmoid bone and response thresholds of higher brain centers.

Bouin-perfused brains of 148 bats (76 species, 48 genera, 8 families) were examined in serial sections for the presence of an accessory olfactory bulb. A moderate to well developed AOB was identified in 26 species. However, absence of an AOB in a particular species does not preclude its presence in some other species of that genus. Descriptions and measurements of the AOBs of each species are reported. The unmyelinated vomeronasal nerve enters the bulb medially and posteriorly. The glomeruli, variable in diameter, appear better circumscribed than previously described. Mitral cells often form thick layers, up to five cells deep, which sometimes reach the dorsolateral surface of the bulb formation. Both external and internal plexiform layers are thin. The latter, however is seen only in a few species. The internal granular layer, reaching the ventricular ependyma in some species, is a prominent component of the bulb. The pars dorsalis of the lateral olfactory tract usually courses between the mitral and internal granular layers. The chiropteran AOB does not differ in significant detail from that of insectivores, primates and other mammals. The occurrence of a functional vomeronasal system in the frugivorous, nectarivorous, and sanguivorous Phyllosotomatidae points to a primary functional role of this system in feeding strategy, at least in bats.