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

Abstract

The nasolacrimal duct (NLD) connects the orbital (often associated with the Deep Anterior Orbital gland: DAOG, a.k.a. Harderian gland) and nasal regions in many tetrapods. Adult cetaceans are usually said to lack an NLD, and there is little agreement in the literature concerning the identity of their orbital glands, which may reflect conflicting definitions rather than taxonomic variation. In this study, we examined an embryological series of the pantropical spotted dolphin (Stenella attenuata), and report numerous divergences from other tetrapods. Underdeveloped eyelids and a few ventral orbital glands are present by late Stage (S) 17. By S 19, circumorbital conjunctival glands are present. In S 20, these conjunctival glands have proliferated, eyelids (and scattered palpebral glands) have formed, and a duct similar to the NLD has appeared. Subsequently, both the palpebral glands and the NLD are progressively reduced by S 22, even as the conjunctival glands exhibit regional growth. In most tetrapods examined, the ontogeny of the NLD follows a series of three stages: Inception of NLD, Connection of orbit and nasal cavity by the NLD and Ossification (i.e., formation of the bony canal surrounding the NLD, emerging into the orbit via the lacrimal foramen in the lacrimal bone). In contrast, the dolphin NLD originates at the same time as the lacrimal bone, and a lacrimal foramen fails to develop. The cetacean fossil record shows that a lacrimal foramen was present in the earliest ancestral amphibious, freshwater forms, but was soon lost as the lineage invaded the oceans. Anat Rec, 2017. © 2017 Wiley Periodicals, Inc. Anat Rec, 301:77-87, 2018. © 2017 Wiley Periodicals, Inc.

Whale tear glands in the bowhead and the beluga whales: Source and function

“…There is much disparity in the literature regarding cetacean orbital gland nomenclature and distribution (see Rehorek et al, for review). Some published reports refer to a singular large, circumorbital gland as the orbital (Tarpley & Ridgway, ) or the Harderian (Bodyak & Stepanova, ; Funasaka et al, ; Ortiz et al, ) gland.…”

“…Other authors identify groups of two to three individual glands that include the Harderian, lacrimal, and intervening conjunctival glands (Pütter, ; Rodrigues et al, ; Waller & Harrison, ; Weber, ). Yet, other studies mention glands in the eyelid (Pütter, ; Rehorek et al, ), referring to them as palpebral glands ( Stenella attenuata : Delphinidae: Rehorek et al, ) or “unnamed eyelid glands” ( Hyperoodon rostratus Ziphiidae, Delphinapterus leucas Monodontidae, Phocoena communis Phocoenidae, Delphinus delphis Delphinidae, Balaenoptera ostrata Balaenopteridae: Pütter, ). Thus, when it comes to orbital glandular identity in cetaceans there is much variation.…”

“…Orbital glands, the source of tear fluid, are present in most tetrapod vertebrates. They occur within three orbital regions: anterior (Harderian gland), posterior (lacrimal gland), and within the eyelids (palpebral and rim glands) (Rehorek, Hillenius, Lovano, & Thewissen, ). Up to three orbital glands have been described within Cetacea.…”

“…However, anatomical characterization of these structures is difficult, as identification and characterization of these glands differs between authors. This creates an impression of morphological diversity that may not be accurate, and makes evolutionary comparison to terrestrial mammals daunting (see Rehorek et al, ). For example, in several cetaceans, the anterior and posterior (when present) regions are connected by a series of small intervening (conjunctival) glands, resulting in the formation of a singular orbital gland.…”

“…For example, in several cetaceans, the anterior and posterior (when present) regions are connected by a series of small intervening (conjunctival) glands, resulting in the formation of a singular orbital gland. The palpebral glands, however, are usually described as a singular glandular mass, and are not separated into multiple smaller glands as described in terrestrial mammals (see Rehorek et al, for review). Histochemically, these glands in cetaceans are described as homogeneous and mucoserous (Funasaka, Yoshioka, & Fujise, ; Tarpley & Ridgway, ; Waller & Harrison, ).…”