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DEVELOPMENTAL STUDY OF THE OLFACTORY BULB IN 

RABBITS Oryctolagus cuniculus 

Ali Faris Reshag1                           Shaker Muhmood Mirhish 

Dept. of Veterinary anatomy, histology and embryology, College of Vet. Med., University of 

Baghdad/ Baghdad-Iraq. 

1Corresponding author: dr0ali1961@gmail.com 

ABSTRACT 

This study was conducted to investigate the development of olfactory bulb 

(OB) in indigenous rabbit. A total of 40 healthy rabbits were divided into two 

groups according to their ages, the first group involved the prenatal stages, while 

the second group involved the post natal ages. All specimens for this study 

processed by the routine paraffin method and stained with H and E stain. The 

light microscopy examination revealed that, At 20 day old fetuses the olfactory 

bulb consisted of two layers. The marginal zone formed of axons of the 

olfactory sensory neurons and the ensheathing progenitors cells surrounded 

amass of cells. At 28 day of gestation and one day postnatal, the olfactory bulb 

consisted of three layers, olfactory nerve layer, mitral cell layer, and granule cell 

layer and at (7-14) day old pups glomeruli appeared as spherical structure 

surrounded by periglomerular cells and the glomerular layer was clearly 

distinguished. The mitral cell layer appeared as cellular zone. 21 days old pups. 

The six layers of olfactory bulb was clearly appeared (olfactory nerve layer, 

glomerular layer, external plexiform layer, mitral cell layer, internal plexiform 

layer, and granule cell layer). At 30 day age pups the histological structure of the 

olfactory bulb was completely developed.  

Key words: Development, olfactory bulb, rabbit pups.  

INTRODUCTION 

The olfactory bulb is laminal structure located at the most rostral region of 

brain protected by cribriform plat. It is part of limbic system, embryologically 

originated from the prosnencephalon. It consists of two types of neurons involve 

the mitral and tufted cells which considered as the principal neurons beside the 

granule cells and short axons cells which considered as intrinsic neurons. The 

olfactory bulb involved in the following function: Detection of odors, 

Discrimination between odors, Filtering out many back ground odors and 

Permitting higher brain areas involved in modify the odor detection and 

discrimination. The olfactory bulb is a path way to transduction of odor to the 

brain via the tract of olfactory bulb (Butter and Hodos, 2005 ; Benignus and 



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Prah, 1982 ; Afifi and Bergman, 1986). The sense of smell in animals is 

depended on the size and function of olfactory bulb, which is developed with 

age (Stephan, 1983 ; Buschhuter et al., 2008 ; Kavio and Jameela, 2011). The 

processing of different odor and cues occur through olfactory bulb to 

hypothalamus, hippocampus and other part of brain. The olfaction controlling 

animals behavior in feeding, mating, maternal relation (Hardy et al., 2005 ; Lin 

et al., 2005 ; 2006 ; Sanchez-Andrad and Kendrick, 2009). 

MATERIALS AND METHODS 

The study involved ten (20, 28) day old, the age of fetuses detected by 

measured of crown-ramp length by digital caliber according to (Mc Laughlin 

and Chiasson, 1999). The group of postnatal ages was divided into six 

subgroups, each group involved five pups, according to their ages as following, 

one day old (P1), one week (P7) two weeks (P14), three weeks (P21), four 

weeks (P30) and eight weeks (P60). The animals of post natal group were 

sacrificed by over dose intra muscular injection of Ketamine 15 mg BW-1, and 5 

mg BW-1 Xylazine. The lower jaw, skin, muscles and the skull bones were cut 

and removed. The olfactory bulb fixed by using 15% formalin solution and 2 

grams of ammonium bromide for at least 48 hours. All specimens processed 

upgrading with ethanol alcohol for paraffin section examination, then sectioned 

serially in frontal plane at 7 µm. The prepared sections were stained with 

Hematoxylin and Eosin according to Luna (1968). 

RESULTS AND DISCUSSION 

The current study revealed that the six layers of olfactory bulb showed 

several changes in their histological structure according to different ages 

included the following: Olfactory nerve layer (ONL): Fetus at 20 day of 

gestation period: This layer consisted of the axons which originated from the 

olfactory sensory neurons, these axons were grew and migrated with 

ensheathing progenitor cells through the mesenchymal tissue (Fig. 1). The axons 

penetrated and inter the developing olfactory bulb and forming marginal zone 

which later developed into the definitive olfactory nerve layer of olfactory bulb, 

Fig. 1. These findings agree with the results of Graziadie et al., (1980) and 

Doneett (1989) in mice and with Treloar et al., (1999). This result suggest that, 

the growth and migration of the axons of olfactory sensory neurons toward the 

developing olfactory bulb were under the effects of many growth factors, these 

factors include the factors release from the ensheathing glial cells, mesenchymal 

tissue and the factors release from olfactory bulb (Runyan and Phelps, 2009 ; 

Trcloar et al., 2010). Fetus at 28 day of gestation–One day old pup: The 



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olfactory nerve layer appeared as regular mass of axons and glial cells which 

surrounded the cellular population of developing olfactory bulb which consisted 

mainly of an mature mitral and tufted cells (Fig. 2). At 7 days old pup: This 

layer appeared more regular and their axons were clearly identified and 

presented many of glial cells (Fig. 3). The present result compatible with the 

results of Qin-guo et al., (2008). The olfactory nerve layer play a role in 

transduction the impulses which detected by the olfactory sensory neurons to the 

olfactory bulb (Benignus and Prah, 1982 ; Afifi and Bergman, 1986). 

Glomerular Layer (GL). Fetus at 20 day of gestation – one day old pup day: the 

glomerular layer was not distinguished and there were no individual glomeruli 

(Fig. 1). This observation was similar with result of Greer et al., (1982) in rat 

pups during sucklines period, and with result of Schneider et al., (2009) in the 

fetuse and neonate of tammar wallaby. At this age the axons of the olfactory 

sensory neurons made up the synapses with dendrites of mitral and tufted cells 

without forming the typical individual glomeruli (Treloar et al., 1999). At 7 day 

old pup: The individual glomerulus appeared as small spherical densely stained 

structure and wasn’t completely surrounded by the periglomerular and glial cells 

(Fig. 3). At 14 day pup: The individual glomerulus was clearly distinguished, 

and appeared completely surrounded by round periglomerular cells of dark 

nucleus, and glial cells (Fig. 3). These results were corresponding with the 

results of Price and Powell (1970); Jeune and Jourdan (1991) and with result of 

Meisami and Sendero (1993); Willey (2004). At 21-30 days pup: The 

glomerular layer was very clearly distinguished and individual glomeruli were 

closely surrounded by the periglomerular and glial cells (Fig. 5 "a and b"). The 

present result shows that at P 30 days the glomerular layer appeared similar to 

that of adult. These results parallel result of Greer (1982); McLLean and Shipley 

(1987); Kathleen and Christina (2002). The olfaction information can be 

converged the olfactory impulse and conducted it to reach the mitral – tufted 

cells and periglomerular cells. The axons of the olfactory sensory neurons 

synapses with the dendrites of mitral and tufted cells and the periglomerular 

cells and short axons cells forming the individual glomeruli (Kosaka and 

Kosaka, 2005). The glomeruli with the olfactory nerve axons and the mitral–

tufted dendrites forming functional unit, which distributed to form the odor map. 

Each individual glomeruli respond to specific odor molecule detected by 

specific olfactory sensory neurons, these specific glomeruli of olfactory bulb 

helping the brain to understand the odor stimulance, these opinion agree with 

results of Mori et al., (2006) and Lin et al., (2006). External plexiform layer and 



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the internal plexiform layers (EPL) (IPL). Fetus at 20 day of gestation: The 

external and internal plexiform layers were absent at this age (Fig. 1). Fetus at 

28 day of gestation-one day old pup: The development of two layers could not 

be distinguished because the mitral cells was diffused (Fig. 2). These 

observation supported with the results of Green et al., (1982) in rat, Schneider et 

al., (2009) in tamor Wallaby and Yokosuka et al., (2011) in rat and Crow.  

At 7 day old pup: The layers began to be noticed (Fig. 3). At 14 day old 

pup: The two layers were clearly distinguished where the mitral cell layer 

separated between them. The external plexiform layer consisted of few granule 

cells which appeared small rounded with dark stained nucleus and fusiform 

mitral cells of different sizes and pale stained nucleus (Fig. 4). At 21 day old 

pup: The two layers were taken the form of adult pattern (Fig. 5 a). These results 

confirmed to the finding of Creer et al., (1982) in rat and Qin-guo et al., (2008) 

in dog, and Young and Less (1999). Mitral cell layer (MCL), Fetus at 20 day of 

gestation: The development of mitral and tufted cells was not organized into 

layers, and appeared as mass of non-differentiated immature cells (Fig. 1). This 

observation confirmed the results of Hinds (1968); Hinds and Ruffett (1973); 

Baye (1983); Bergmann et al., (1993) and Winpenny et al., (2011). The 

development and maintenance of the mitral cells and tufted cells affected by the 

arrival of olfactory nerve axons and it's contact with developing olfactory bulb 

(Couperleo and Brunjes, 2003). Fetus 28 day of gestation-one day old pup: The 

mitral and tufted cells appeared diffused forming cellular band consisted of 

many cell layers. The mitral and tufted cells (large cells) in developing olfactory 

bulb originated and differentiated earlier before the interneuron (small cells) 

(Fig. 2). At 7 day old pup: The mitral and tufted cells formed an identitive band 

between the external and internal plexiform layers (Fig. 3). At 14 day old pup: 

the mitral cells layer consisted mainly of large pyramidal cells contained large 

and round pale nucleus, and few oval or fusiform tufted cells which were 

smaller than mitral cell. Small round with dark nucleus granule cells were 

noticed in the margin of the layer (Fig. 6). These finding agree with Greer et al., 

(1982), Schneider et al., (2009) and Yokoswka et al., (2011). At 21 day old pup: 

The mitral cells body arranged into 1-2 cells layers. The granule cells 

aggregated at the margin of the mitral layer and intermingling with its cells. The 

tufted cells were clearly present. (Rosselli- Austin and Altman, 1979 ; Royet et 

al., 1998). At 30 day old pup: The Mitral cells were similar to that of adult and 

no important changes were noticed. This result agree with results of Greer et al., 

(1982) and Qin – guo et al., (2008). The relationship between mitral-tufted cells 



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and the periglomerular cells is dendrodendritic synapses formed between this 

opinion is explained by Kasaka and Kosaka (2005). The secondary lateral 

dendrites of mitral and tufted cells form dendrodendritic synapses with granule 

cells in the external plexiform layer this agree with result which mentioned by 

Price and Pwell (1970). The axons of the mitral and tufted cells extended 

through the lateral olfactory tract to reach different regions in the brain this 

mentioned by many authors like the Afifi and Borgmeans (1986) ; Seveg (1999); 

Mast and Griff (2005) ; Nagayama et al., (2010). The Mitral and tufted cells 

different in their sizes, but both have similar function (Christie et al., 2001). The 

mitral and tufted cells when stimulated by axon of olfactory sensory neurons 

which release glutamate that act as neurotransmitters causes stimulation to the 

periglomerular and granule cells, Ayluin et al., (2005) and  Eyre et al., (2009). 

Granule cell layer (GCL) Fetus at 20 day of gestation: the granule cell layer was 

absent (Fig. 42). This finding agree with the result of Baye (1983) and 

Bergmann et al., (1993) and Winpenny (2011), they revealed that the 

interneuron are three cells (periglomerular cells, short axons cells and granule 

cells), these proregenerated at late prenatal and postnatal from the sub-

ventricular zone of proencephalon (Wang et al., 2005). The interneuron 

continuous in neurogenesis even in adult life of animal, the same results 

observed by Baye (1983) ; Mayoshi et al., (2009). Fetus at 28 day of gestation– 

one day old pup: This layer was occupied by granule cells which scattered and 

diffused within layer spaces (Fig. 2). At 7 day old pup: the granule cells began 

to cluster with each other (Fig. 3) Its width was 517 µm. At 14-21 day old pups: 

The granule cells clusters to form nest of cells arranged centrifugally (Fig. 4 and 

5 a). At 30-60 day old pups. There were no important histological changes (Fig. 

8). The development sequence of the granule cells layer was proved the finding 

of Greer et al., (1982) and Kathleen and Christina (2002) in rat and Qin-guo et 

al., (2008) in dog, and Schneider et al., (2009) in Tammar Wallaby. The 

clustering of the granule cells was very important for cells, performance and 

response to the same impulses and participates in inhabitation and odor 

discrimination. The results go with the results of Reyha et al., (1991); Gheusi et 

al., (2000). The present result suggests that, the mechanism of neurogenesis of 

new granule cells due to the effects of new odor stimulants and its migration to 

the olfactory bulb by mechanism similar to that of lymphocyte by the lymphatic 

system when new irritant or antigen. This opinion supported by result of Dong et 

al., 2007 ; Heinbockel et al., 2007 ; Laaris et al., 2007). The results of this study 

showed that the width of olfactory bulb layers and the diameters of glomeruli of 



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glomerular layer were increased with age (Rossell-Asustin and Altman, 1979; 

Geune and Gordan, 1991 ; Mesami and Sendera, 1993 ; Qin-gou, 2009 ; Kilkash 

et al., 2010). The structural organization of olfactory bulb and the mature 

lamination reach the adult pattern at P 30 (Graziadei and Graziadei, 1980 ; 

McLen and Shipley, 1987 ; Mesami and Sendera, 1993 ; Gordan, 1991 ; Youn 

and Lee, 1994 and Kathleen and Christina, 2002). 

 

 

 
 

 

 

 

 

 

 

 

  

 

 

 

 

 

 

 

 

 

 

Figure 2. Histological section of 

Olfactory bulb (E28-P1) day shows: 

(A) olfactory nerve layer. (B) Non 

differentiated G L. Arrows show 

diffused cells. ( H and E x 40 ) 

 

 

Figure 3. Histological section of OB. (P7) 

shows: (A) ONL (B) Glomeruli of GL. 

(C) EPL. (D) Band of mitral cells. (E) 

IPL. (F) Projected nerve from olfactory 

epithelium toward olfactory bulb. (G) 

Cribriform plate. Arrow shows olfactory 

epithelium (H and E x 100) 

 

 

Figure 4. Histological section of 

Olfactory bulb (P14) days shows:  

(A) ONL. (B) Glomeruli of GL. (C) 

EPL. (D) Mitral cells layer (E) IPL. 

(F) GCL. aggregation of granule 

cells (Arrows head) (H and E x 200) 

 

 
Figure 1. Histological section of OB. 

(E. 20) shows: (A) Neuroblast. (B) 

Nerve fibers. (C) Hyaline cartilage. 

(Arrows show penetrations of nerve 

fiber to olfactory N L. (H and E x 

100) 

 

 

 



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Figure 5 a. Histological section of OB 

(21-30) day shows: (A) ONL. (B) 

Glomeruli of GL. (C) EPL (D) MCL 

(E) IPL (F) GCL(Black arrow shows 

tufted cells, granule cells (yellow 

arrow), periglomerular (red arrow)  

(H and E x 200) 

 

 

Figure 5 b. Histological section of 

GL (30-60) day shows: (A) 

Individual glomerulus. (B) ONL (C) 

EPL. (Arrows show periglomerular 

cells. (H and E x 400) 

 

Figure 6. Histological section of mitral 

cells layer (14 day): (A) EPL (B) 

MCL. (C) IPL. (1)  Mitral cells. (2) 

Tufted cells. (3) Granule cells.  GCL 

(Arrows showed). (H and E x 400) 

 

Figure 7. Histological section of 

MCL (30-60) day old pup shows: 

(A) EPL (B) MCL. (C) IPL. (1)  

Mitral cells. (2) Tufted cells. (3) 

Granule cells.  (D) Granule cells 

layer. (H and E x 400) 

 



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  دراسة تطورية للبصلة الشمية في االرانب

Oryctolagus Cuniculus 

 شاكر محمود مرهش                       1علي فارس رشك

 .العراق –بغداد  –جامعة بغداد  ،كلية الطب البيطري –فرع التشريح واالنسجة واالجنة 

 dr0ali1961@gmail.comالمسؤول عن النشر: 1

 مستخلصال

قسمت الى  ا  سليم ا  ارنب 04تم استخدام  فقد ،بحث تطورالبصلة الشمية لالرنبأجريت هذه الدراسة ل

المجموعة الثانية ضمت اعمار ماقبل الوالدة و المجموعة االولىضمت  ،مجموعتين اعتمادا على العمر

 تماستخدتم تمريرها بطريقة البرافين العادية و )البصلة الشمية( جميع العيناتاعمار ما بعد الوالدة. 

 هالبصلة الشمية في جنين عمراظهر الفحص بالمجهر الضوئي ان  .صبغة الهيماتوكسلين ايوسين للتصبيغ

تتألف من محاور الخاليا الشمية الحسية للظهارة الشمية والطبقة الخارجية  ،من طبقتينتتكون  ا  يوم 04

تتكون فبقة الداخلية اما الط ،المبطنة للتجويف االنفي وتحيط بهذه المحاور الخاليا المولدة للخاليا الغمدية

تتكون البصلة فوعمر يوم واحد بعد الوالدة  ا  يوم 02في الجنين بعمر أما  ،خاليا غير متمايزةمن كتلة من 

الطبقة الثانية تتكون من الخاليا و ،الطبقة الخارجية تمثل طبقة العصب الشمي ،الشمية من ثالث طبقات

اخلية فتمثل الخاليا الحبيبية. تبدأ الكبيبات والطبقة العميقة او الد ،المترالية التي تكون منتشرة التوزيع

بعد الوالدة بالظهور على شكل تراكيب كروية داكنة الصبغة ومحاطة  ا  يوم 10 -7بالظهور عند عمر

بشكل غير كامل بالخاليا الدبقية لتكون طبقة رابعة )الطبقة الكبيبية( وبهذا العمر ايضا تبدأ طبقة الخاليا 

كل حزام خلوي مما يؤدي الى بداية ظهور الطبقات التشابكية الداخلية المترالية بالظهور على ش

تالحظ الطبقات الست النموذجية المكونة للبصلة الشمية كاملة.  ا  يوم 04 -01وعند عمر ،والخارجية

 استنتجت الدراسة ان البصلة الشمية لالرنب تتغير نسجيا بتقدم العمرويكتمل نموها بعمر شهر من الوالدة.

 التطور، البصلة الشمية، جراء األرنب.ات المفتاحية: الكلم