Rabbits have been proposed as a model organism for the human lacrimal apparatus (LA), including the nasolacrimal duct (NLD), based principally on comparative studies of adult morphology; however, little is known about its development. The NLD first appears as an incomplete primordium in the subcutaneous region of the primordial eyelid and subsequently elongates to reach the naris. One posterior and three anterior orbital glands are present fetally although one of the anterior glands is soon lost. The NLD follows a tortuous path and passes through a bony canal consisting of lacrimal, maxilla, and maxilloturbinal bones at different regions. Although early developmental similarities exist to haplorhine primates, the narial opening of the NLD resembles strepsirrhines. This distinction, along with the ductal and glandular differences at the orbital end of the NLD, indicates that rabbits may be a poor model for LA drainage in primates, specifically humans.
Identifying appropriate organisms for experimental models of human craniofacial growth, development, and dysmorphology is challenging. Suitable animal models need not hinge entirely on phylogenetic affinity; in some cases, region-specific factors may determine when a more distantly related animal is appropriate [
The orbital and nasal regions of most tetrapod vertebrates are connected by means of the nasolacrimal duct [
For most mammals, the route of the nasolacrimal duct is well documented [
The route and composition of the NLD in nonprimate mammals was recently reviewed by Frame and Burkat [
Ten fetal specimens of
After sectioning and staining, one of the 21-day and one of the 27-day specimens were selected for three-dimensional reconstruction of the nasolacrimal duct using Scion Inage software (release 4.02, National Institute of Health). To acquire sections for reconstruction, every 10th section of each nasal chamber was digitally photographed using an AX-70 microscope and digital capturing program. Images were transferred to Adobe Photoshop 7.0 and saved as bitmap files. These files were aligned in sequence for three-dimensional reconstruction and were used to isolate and depict the NLD following the method of Rehorek and Smith [
Three dry adult skulls of
The terminology for the planes of orientation follows different conventions for adult primates [
The nasolacrimal duct (NLD) begins in the orbital region with an opening in the lower eyelid, it then traverses through the tissues that form the wall of the rostrum and extends the length of the nasal cavity to end in the lateral wall of the naris. In rabbits, there is only a single punctum in the lower eyelid near the anterior canthus.
Up to four separate orbital glands are observed in rabbits (Figure
Dorsal rendition of the architecture of the orbital glands of a fetal rabbit. Orientation: anterior is top, posterior is bottom, lateral is left, and medial is right. Abbreviations: arrow = route of nasolacrimal duct, DAOG = deep anterior orbital (Harderian) gland, E = eye, L = primordial eyelid, LAOG = lateral anterior orbital gland, NC = nictitating cartilage, NM = nictitating membrane, and SAOG = superficial anterior orbital gland.
Results of the fetal observations are summarized in Tables
Measurement of NLD in the bony canal, showing the extent and type of bones contributing to the canal at different fetal ages. “+” indicates missing sections at the inferior limits.
Age of fetus (days) | Total NLD length in bony canal (mm) | Length of NLD in maxilla canal only (mm) | Length of NLD under lacrimal (mm) |
---|---|---|---|
19 | — | — | — |
20 | — | — | — |
21 | 1 | 1 | — |
23 | 1.18+ | 0.51+ | 0.67 |
25 | 1.605+ | 0.865+ | 0.74 |
27 | 2.8 | 1.2 | 1.6 |
Presence of orbital glands. Abbreviations: DAOG = deep anterior orbital (Harderian) gland, LG = lacrimal gland, LAOG = lateral anterior orbital gland, SAOG = superficial anterior orbital (nictitans) gland. + = present, − = absent.
Age of fetus (days) | LAOG | SAOG | DAOG | LG |
---|---|---|---|---|
19 | − | − | + | − |
20 | − | − | + | + |
21 | − | + | + | + |
23 | + | + | + | + |
25 | + | + | + | + |
27 | + | − | + | + |
In the 19-day fetus, the skin covering the eye is still intact (primordial eyelid), and separate eyelids have not yet formed. There is a partial, semipatent NLD, which extends from the dermis of the anterior portion of the primordial eyelid. Here, the NLD begins as a nonpatent cord superior to the solitary, semipatent duct of the DAOG (Figure
Light micrograph of a transverse section through the head of a 19 day rabbit fetus. Orientation: anterior is right, posterior is left, lateral is top and medial is bottom. Abbreviations: DAOG = deep anterior orbital gland, E = eye, NLD = nasolacrimal duct (in the dermis of the lower eyelid), NM = nictitating membrane. Scale bar = 110
In the 20-day fetus, the maxillary bone makes its first appearance, as a roughly rod-like structure at the maxillary tooth row. Along its superior side, there is a thin bony projection (the future frontal process) that forms the inferior part of the lateral (bony) nasal wall and part of the bony orbit. The NLD is still only semipatent, but it is now virtually complete, stretching from the lower part of the primordial eyelid to the naris. The orbital opening of the NLD is in the conjunctival aspect of the primordial eyelid, about 40
At day 20, the DAOG is not larger than that of the 19-day fetus and still only consists of a single semipatent duct. No other anterior orbital glands are present. However, there is a small LG superodorsal in the orbit. At this stage, the LG is longer than the DAOG and is tubular with 1-2 small branches.
In the 21-day fetus, the NLD is complete but follows a tortuous path (Figure
Route of nasolacrimal duct (NLD) in the 21-day rabbit fetus. (a) Lateral view of the 3D reconstruction of the NLD, from the orbit (O) to the naris (N). Nonbony nasal refers to the section of the NL canal that is not enveloped by the maxilla. Abbreviations: I = inferior, S = superior. (b) Light micrograph of transverse section indicated by horizontal line indicated in (a). The plane is perpendicular to that viewed in (a) (orientation: anterior is right, posterior is left, medial is top, and lateral is bottom). Abbreviations: M = maxilla, PM = premaxilla, and VNO = vomeronasal organ. Scale bar = 350
By day 21, the DAOG is a simple, tubular structure, consisting of a single duct, with 3-4 smaller branches. It is 2-3 times larger than that that of day 20 fetus. In addition to the DAOG, there are 1-2 small glands in the medial side of the nictitating membrane. These are the superficial anterior orbital glands (SAOG), which at this stage are small, solid, spherical structures in the nictitating membrane. There are no lateral anterior orbital glands (LAOG). The LG is larger than that of the 20 day fetus and appears to have 2-3 small, inferior branches extending down. At this stage a slit begins to form in the primordial eyelid, separating a large inferior eyelid (where the NLD, DAOG, LAOG and SAOG reside) and a small posterosuperior eyelid (which at this stage houses only the opening of the LG).
The 23-day fetus shows only two differences in NLD from that of the 21-day fetus: (1) the progressive enclosure of the NLD by the maxillary bone and (2) the appearance of the lacrimal bone. As the NLD passes along the groove on the medial aspect of the maxilla, a set of bony flanges have developed, intermittently, along the top and bottom margins of the groove. These flanges extend medially from the maxilla and represent the incipient roof and floor of the osseous NL canal. At this developmental stage, the superior and inferior flanges of the maxilla do not yet meet medially; the intervening space between the flanges (which lies medial to the NLD but lateral to the cartilaginous nasal capsule) consists of connective tissue. Only along the proximal portion of the bony passage is the NLD covered medially by bone: here, an extension of the newly developed lacrimal bone complements the maxillary groove to help form the osseous canal for the NLD. Unfortunately, the full extent of the bony canal could not be observed due to damaged sections in this specimen.
There are two distinct eyelids, each with a specific set of glands, as observed in the 21-day fetus. The DAOG is at least twice the size of that in the 21-day fetus, and consists of a single duct with three distinctive lobes that infiltrate the surrounding mesenchyme. The two SAOGs are larger than of the day 21 fetus, and consist of tubular structures that open superior to the opening of the DAOG. Additionally, 3-4 tubular ducts, belonging to the LAOG are observed on the lateral aspect of the nictitating membrane. These individually penetrate the nictitating membrane to open superior to the ducts of the SAOG. The LG expands inferiorly and now consists of two glandular masses, one in the suborbital region and the other near the opening in the eyelid. These two clumps of glandular material share a single duct, which open in the superior aspect of the posterior eyelid.
The 25-day fetus is not very different than the 23-day fetus. NLD groove on the medial aspect of the maxilla is deeper, and the superior and inferior flanges extend further medially than the in 23 day fetus. Superiorly, the bony canal completely encircles the NLD, with the lacrimal bone forming the posteromedial wall. However, anterior to the lacrimal bone, the medial aspect of the canal still consists of connective tissue, not bone.
The DAOG has proliferated and now consists of a single duct with at least 5 distinct lobes. It does not take up more space than it did in the 23 day fetus, but the lobes consist of many more convoluted branches, and there is a reduced amount of mesenchyme between the acini. The SAOG consists of at least 5 distinct tubular glands, and the LAOG consists of 2-3 branched, tubular glands (Figure
Light micrograph of a transverse section through the head of a 25-day rabbit fetus. Orientation: anterior is right, posterior is left, lateral is top and medial is bottom. Abbreviations: DAOG = deep anterior orbital gland, E = eye, L = lower eyelid, LAOG = lateral anterior orbital gland, NLD = nasolacrimal duct, NM = nictitating membrane, and SAOG = superficial anterior orbital gland. Scale bar = 180
By day 27 (Figure
The nasal cavity of the 27-day fetus is elongate and the bony canal of NLD is substantially longer than that of the previous fetuses (Table
The DAOG has once again almost doubled in size compared to the previous stage (i.e., day 25) although there is no distinct connective tissue capsule, the extent of the gland is clearly seen. There is still some mesenchyme between the acini, but much less than in the younger fetuses. There are 3 to 4 distinct lobes clearly identifiable, with numerous lobules therein. The LAOG expands to form longer, tubular structures lateral to the nictitating cartilage. There is, however, no SOAG. The LG is an elongate, flattened structure that fills the posterior orbital region. The distinct inferior and superior lobes are no longer as distinctly observed as in previous stages.
Route of nasolacrimal duct (NLD) in the 27 day rabbit fetus. (a) Lateral view of the 3D reconstruction of the NLD, from the orbit (O) to the naris (N). Nonbony nasal refers to the section of the NL canal that is not enveloped by the maxilla. Abbreviations: I = inferior, S = superior. (b) Light micrograph of a transverse section indicated by horizontal line indicated in (a). The plane is perpendicular to the view in (a) (orientation: anterior is right, posterior is left, medial is top, and lateral is bottom). Abbreviations: E = eye, M = maxilla, and PM = premaxilla. Scale bar = 750
A rectangular wedge of two facial bones that house the nasolacrimal duct (NLD) is shown, drawn after a three-dimensional reconstruction that was generated from a 27 day old rabbit fetus. For reference, the approximate location of these bones is shown at upper left, using an illustration of a juvenile rabbit skull. Note the maxilla (Max, in yellow) is show developing flanges around the NLD at the anteromedial section of the NLD. The proximal end of the NLD includes a sharp angular transition between the maxilla and the lacrimal (La, in blue). The maxilloturbinal bone is removed. Abbreviations: A = anterior, L = lateral, M = medial, and P = posterior. (Original illustration by T. D. Smith).
The route and location of the flexures of the adult rabbit NLD has already been adequately described elsewhere in the literature [
Medial view of midsagittal cut through the adult rabbit skull, with the nasal septum and nasal turbinates (including the full extent of the maxilloturbinate) removed. Abbreviations: BC = maxillary bone canal, NC = nonmaxillary bone canal, SO = subepithelial orbit. Bones are colored as follows: frontal (pink), lacrimal (light blue), maxilla (yellow), nasal (salmon), palatine (indigo/dark blue), and premaxilla (green). (Original illustration by J. L. Johnson).
Rostral to the point where the NLD emerges from the maxillary bony canal, the medial wall is largely comprised by the maxilloturbinal bone (formed through endochondral ossification of the cartilaginous nasal capsule). At the interface of the maxillary and maxilloturbinal bones, the maxillary canal is medially incomplete (the flanges are not fused together) (Figures
Light micrographs of frontal sections through the nasal cavity of the adult rabbit. These show the exit point of the NL (asterisk) from the bony canal (a, b) and the anterior passage towards the naris (c, d), as the NL passes through the inferior (ventral) meatus. Note the changing relationship between the maxilla (M) and maxilloturbinal (MT) bones. Abbreviation: S = nasal septum. Scale bars: (a, c) = 500
The focus of this study is to describe the development of the NLD and the organogenesis of the orbital glands in rabbits. These are then compared to other amniotes, with special reference to humans (and primates in general), to evaluate Frame and Burkat’s [
There is comparatively little published literature that details the development of the NLD. The available descriptions indicate that there are marked differences in NLD development among the few amniotes studied. Nevertheless, despite its varied development origins, in each case, the mature NLD connects the orbit to the nasal cavity and serves to convey orbital fluids to the nasal region. In all cases, it develops relatively early in fetal growth (Table
Summary of inception points for the NLD in amniotes.
Rabbit (this study) | Mice [ | Reptile (snake) [ | Reptile (alligator) [ | |
---|---|---|---|---|
Location of origin | Subcutaneous | Nasolacrimal groove | Subcutaneous | Subcutaneous |
Inception point | Day 19 (out of 30–32) | Mice: Day 11 | Stage 28 (out of 37) | Before Stage 16 (out of 28) |
Growth | Originates in the lower eyelid and grows towards the nose. Opening partway in the lateral aspect of the narial wall [ | A complete nasolacrimal groove connect nasal and orbital regions at a very early stage. Sinks into the underlying dermis and canalizes. | Originates from the lateral side of the duct for the VNO and grows towards the orbit. | Originates in the lower eyelid and grows towards the nose. Opening partway in the nasal cavity proper. |
With respect to the ontogenetic origin of the NLD, there appear to be patterns between mice, reptiles, and rabbits (Table
The developmental association between the maxilla and the NLD has received scant attention. There is no published literature on the development of the bony canal in nonprimates.
In humans [
In adult rabbits, the NLD has the same three regions as described in other mammals (orbital lacrimal canaliculi and sac, bony NL canal, and nonbony NL canal in nasal cavity). However, there appears to be quite some variation within the other animals thus far examined (see Table
Summary of posterior part of the NLD.
Rabbit (this study + [ | Primate [ | Other mammals [ | Reptiles and basal amniotes [ | |
---|---|---|---|---|
Number of canaliculi | 1 | 1 (Cynomolgus monkey) | 2 | 2 |
Associated bones | Maxilla, maxilloturbinal and lacrimal enclose the NLD | Maxilla and lacrimal enclose the NLD. Added inferior concha in humans covers the ventromedial aspect of the NLD when it opens into the nasal cavity proper. | Lacrimal only encloses the posterior aspect of the NLD. The maxilla forms only the lateral wall. No mention of maxilloturbinal. | Lacrimal encloses the NLD, the maxilla only forms the lateral wall. In basal amniotes, the septomaxilla in basal amniotes encloses the anterior portion of the NLD. Maxilloturbinals are absent. |
The route of the NLD shows variation amongst mammals (see Table
Summary of route of NLD in mammals.
Rabbit (this study) [ | Buffalo, Llama, Goat, Camel [ | Sheep, cats* and dogs* [ | Horse and Strepsirrhine primates [ | Cats# and primates [ | |
---|---|---|---|---|---|
Number and type of flexures | 2 pronounced flexures: one at the upper NLD and one near the incisor tooth row | 2 flexures, in same place a rabbit, but less pronounced | 1 dorsal flexure (described as an arch) only | No flexures—straight NLD | No flexures—straight NLD |
Descent of (maxillary) bony NLD canal | Diagonal | Diagonal | Diagonal | Diagonal | Vertical |
Nasal opening of the NLD | Naris | Naris | Naris | Naris | Nasal cavity |
*Refers to both dolichocephalic (elongate faced) and mesocephalic (normal faced).
#Refers to brachycephalic (short faced).
In both brachycephalic (short-faced) cats and anthropoid primates, the descent of the NLD is nearly vertical from the lacrimal foramen to the nasal cavity [
The NLD opens in the ventral or lateral aspect of the naris in many mammals [
In the 25-day rabbit fetus, there were three sets of anatomically distinct anterior orbital glands, but by the 27th fetal day, only 2 distinct anterior orbital glands remained. The nomenclature of these anterior orbital glands is problematic, as there are multiple definitions and criteria that are used (cf. [
The presence of two anterior superficial glands associated with the nictitating membrane (SAOG and LAOG) poses an additional nomenclatural problem. Historically, any gland in the superficial aspect to the nictitating membrane was considered to be the “nictitans” gland [
The rabbit has a series of glands that surround the nictitating membrane. One set of these, the SAOG, is absent by day 27. Only two other studies have detailed the fetal development of the SAOG: one showed the growth of a large SAOG in two species of deer [
Developmentally, the inception of the DAOG is barely preceded by that of the NLD (see [
The DAOG in the rabbit, as a lobular structure, is well documented [
A lacrimal gland was also observed posteriorly in the orbit of the rabbit. This gland is common to most tetrapods, and thus, the fetal rabbit condition is not unique. In the adult rabbit, however, there appear to be either two [
From this study, it can be concluded that the rabbit NL system has features in common with humans, other amniotes and certain other features are unique. Like haplorhine primates, the rabbit NLD is enclosed by the maxilla (human) and has only one lacrimal punctum (cynomolgus monkey). The manner in which the maxilla comes to surround the NLD in humans is currently unknown. Unlike haplorhine primates, but akin to strepsirrhine primates and other non-mammals, the rabbit NLD has a narial opening, much of its distal path is bordered by the maxilloturbinal, and the bony canal through the maxilla is diagonal. The rabbit NLD has two very distinct flexures, which bears some similarity to other mammals, but may be a unique feature associated with the ventral deflection of the snout. However, the rabbit NLD appears to have a subcutaneous origin, which is unlike the few mammals thus far examined, and similar to the condition in reptiles. Thus, the NLD itself is appears to be a mixture of mammalian and nonmammalian features.
With respect to the orbital glands, rabbits are much like deer and pigs in that the DAOG is large and bilobed but also like humans in that the SAOG is lost in the late fetal stages. The multiple LGs are apparently unique to rabbits.
The purpose of this study was to describe the organogenesis of the rabbit nasolacrimal system and to determine whether this system could be used as a model for the human condition. Based on the anatomical (both fetal and adult) observations of this study, it appears that the answer is not clear-cut. Certain aspects that dictate the nature of NL drainage in rabbits differ from humans. These distinctions, regardless of their phylogenetic significance, indicate a markedly different NL drainage in rabbits compared to humans. Curiously, there are numerous similarities between rabbits and humans despite extreme differences in midfacial form. Thus the rabbit NLD system as a “model” for the human NL system may have some utility. However, a stronger case for this could be made if strepsirrhine primates were to be examined, as their nasal anatomy (elongated snout) is more reminiscent of the rabbit. In most respects, however, the rabbit apparatus is more clearly similar to the strepsirrhines, which may indicate severe limitations in the rabbit model. Although the systematic implications of these similarities have not been investigated, many of these shared characteristics may be plesiomorphic.
The authors would like to thank Ms. Kait Brumbaugh for technical assistance. The project was funded in part by a 2007 Slippery Rock University CHES research grant.