Morphological and Histochemical Observations on the Crural Gland-Spur Apparatus of the Echidna <i>(Tachyglossus aculeatus)</i> together with Comparative Observations on the Femoral Gland-Spur Apparatus of the Duckbilled Platypus <i>(Ornithorhyncus anatinus)</i>

Abstract

The echidna and platypus have a crural/femoral gland that is linked by a large duct to a canalized, keratinous spur located on the medial side of the ankle. The echidna crural gland, like the femoral gland of the platypus, exhibits cyclic activity, being prominent in both monotremes when they are sexually active. In the present study, we compared the structure and histochemistry of these glands. During the active phase, the secretory epithelium forming the respective glands of both species increased in height and became packed with secretory granules that differed markedly in structure. Secretory granules of the echidna crural gland were electron dense and characterized by cores or areas of increased electron density. Those of the platypus were initially electron dense, but then became less dense and coalesced into irregular complexes of secretory material. Large cytoplasmic blebs extended from epithelial cell apices and appeared to be shed into the lumen, resulting in an apocrine mode of secretion. Exocytosis was also observed. A similar form of release of secretory product was not observed in the echidna. Secretory granules of both species were periodic acid-Schiff positive and stained for protein, suggesting that much of the secretory product was glycoprotein. Myoepithelial cells enveloped the secretory tubules of the platypus femoral gland, whereas they were not observed surrounding tubules comprising the echidna crural gland. During the quiescent phase, the epithelial cells of both species lost their secretory granules and decreased in height. As a result, the secretory tubules became smaller, intralobular connective tissue increased and the glands decreased in overall size.

Proteomics and Deep Sequencing Comparison of Seasonally Active Venom Glands in the Platypus Reveals Novel Venom Peptides and Distinct Expression Profiles

“…This might point to differences in the abundance of cell populations during the venom cycles. Indeed, secretory epithelial cells have been reported to be lost in the venom gland after the mating season, suggestive of seasonal changes in cell types (2). Care should be taken when interpreting the result of the gene enrichment analyses because of the lack of biological replicates, and the use of human functional annotations to infer relationships.…”

“…Males produce venom in significant quantities only during the breeding season (1). The venom glands increase in size during the breeding period from June to August but regress thereafter, accompanied by a loss of secretory granules and shrinkage of secretory tubules (1,2). Thus, it is believed that platypus venom functions in mate competition and is used against male rivals (1).…”

Tracing Monotreme Venom Evolution in the Genomics Era

“…The venom apparatus is known as the crural system, because it is in the hind limb, and consists of paired crural venom glands connected by venom ducts to an extratarsal spur on each hind leg [16]. The crural glands are derived from modified apocrine sweat glands and migrate during development from the inner surface of each thigh to their final position on the dorsocaudal surface of the pelvis [17,18]. …”

“…The spur is supported by the os calcaris (this bone is absent in females [18]), and is attached to tendon and muscle to allow erection of the spurs prior to envenomation [14,17]. Platypus envenomation is rarely observed; the animals wrap their hind legs around and drive the sharp spurs into the victim.…”

“…Spurs have also been observed in Zaglossus males and some females, with a pus-like exudate emitted from the base of the male spur [17,52]. The spur in Tachyglossus and presumably Zaglossus is connected via a duct to the crural gland below the knee, which is active during the breeding season, and exudes a milky secretion when sliced open postmortem (reviewed in [18,52]). The echidna spur is covered by a protective skin flap from which it cannot be erected or everted.…”