Development of the nasolacrimal apparatus in the Mongolian gerbil (Meriones unguiculatus), with notes on network topology and function

Abstract

The nasolacrimal apparatus (NLA) is a multicomponent functional system comprised of multiple orbital glands (up to four larger multicellular exocrine structures), a nasal chemosensory structure (vomeronasal organ: VNO), and a connecting duct (nasolacrimal duct: NLD). Although this system has been described in all tetrapod vertebrate lineages, albeit not always with all three main components present, considerably less is known about its ontogeny. The Mongolian gerbil (Meriones unguiculatus) is a common lab rodent in which the individual components of the adult NLA have been well studied, but as yet nothing is known about the ontogeny of the NLA. In this study, serial sections of 15 fetal and three adult Mongolian gerbil heads show that the development of the NLA falls into three fetal stages: inception (origin of all features), elongation (lengthening of all features), and expansion (widening of all features). No postnatal or juvenile specimens were observed in this study, but considerable growth evidently occurs before the final adult condition is reached. The development of the orbital glands and the VNO in the Mongolian gerbil is largely consistent with those in other mammals, despite a slight nomenclatural conundrum for the anterior orbital glands. However, the Mongolian gerbil NLD follows a more circuitous route than in other tetrapods, due mainly to the convoluted arrangement of the narial cartilages, the development of a pair of enlarged incisors as well as an enlarged infraorbital foramen. The impact of these associated features on the ontogeny and phylogeny of the NLA could be examined through the approach of network science. This approach allows for the incorporation of adaptations to specific lifestyles as potential explanations for the variation observed in the NLA across different tetrapod clades.

Ontogeny of the Orbital Glands and Their Environs in the Pantropical Spotted Dolphin (Stenella attenuata: Delphinidae)

“…No terminal bud was observed by stage 22, though the ducts take on their final stratified structure. Thus the formation of the terminal bud must occur during the later fetal stages of development, as it does in other mammals (e.g., Rehorek et al, ).…”

“…In typical mammals, the nasolacrimal duct (NLD) drains the orbital fluid into the nasal region. It connects the orbital region (via a pair of lacrimal puncta in the upper and lower eyelids) to the nasal region (typically at the level of the naris: Rehorek et al, ). From the orbital region, via the two lacrimal puncta in the palpebrae, the NLD passes through one to two perforations in the lacrimal bone (lacrimal foramen if only one, or lacrimal canaliculi if there are two).…”

“…). (1) Inception: the NLD develops from the lateral nasal wall or the vicinity of the orbit (Kaufman and Bard, ; La Cuadra‐Blanco et al, ; Rehorek et al, ). (2) Connection: the NLD connects the orbit to the nasal cavity, passing through undifferentiated mesenchymal tissue (Rehorek et al, ).…”

“…(1) Inception: the NLD develops from the lateral nasal wall or the vicinity of the orbit (Kaufman and Bard, ; La Cuadra‐Blanco et al, ; Rehorek et al, ). (2) Connection: the NLD connects the orbit to the nasal cavity, passing through undifferentiated mesenchymal tissue (Rehorek et al, ). (3) Ossification: bones develop to either completely envelop (lacrimal) or partially cover (maxilla, maxilloturbinate, and premaxilla) the NLD tissue (Rehorek et al, ).…”

“…The very limited literature on orbital gland ontogeny in cetaceans is restricted to a few fetal specimens (Pütter, ). There are, however, several descriptions of orbital gland ontogeny in other mammals (Rehorek et al, ) that show a distinct relationship between the orbital glands and the NLD, in terms of timing of development. The purpose of this study was to examine the ontogeny of these features in an embryonic and fetal series of the pantropical spotted dolphin ( Stenella attenuata : Delphinidae: Ondontoceti).…”