The Hypothalamic Supraoptic and Paraventricular Nuclei of the Echidna and Platypus

Abstract

The monotremes are an intriguing group of mammals that have major differences in their reproductive physiology and lactation from therian mammals. Monotreme young hatch from leathery skinned eggs and are nourished by milk secreted onto areolae rather than through nipples. Parturition and lactation are in part controlled through the paraventricular and supraoptic nuclei of the hypothalamus. We have used Nissl staining, enzyme histochemistry, immunohistochemistry for tyrosine hydroxylase, calbindin, oxytocin, neurophysin and non-phosphorylated neurofilament protein, and carbocyanine dye tracing techniques to examine the supraoptic and paraventricular nuclei and the course of the hypothalamo-neurohypophysial tract in two monotremes: the short-beaked echidna (Tachyglossus aculeatus) and the platypus (Ornithorhynchus anatinus). In both monotremes, the supraoptic nucleus consisted of loosely packed neurons, mainly in the retrochiasmatic position. In the echidna, the paraventricular nucleus was quite small, but had similar chemoarchitectural features to therians. In the platypus, the paraventricular nucleus was larger and appeared to be part of a stream of magnocellular neurons extending from the paraventricular nucleus to the retrochiasmatic supraoptic nucleus. Immunohistochemistry for non-phosphorylated neurofilament protein and carbocyanine dye tracing suggested that hypothalamo-neurohypophysial tract neurons in the echidna lie mainly in the retrochiasmatic supraoptic and lateral hypothalamic regions, but most neurophysin and oxytocin immunoreactive neurons in the echidna were found in the paraventricular, lateral hypothalamus and supraoptic nuclei and most oxytocinergic neurons in the platypus were distributed in a band from the paraventricular nucleus to the retrochiasmatic supraoptic nucleus. The small size of the supraoptic nucleus in the two monotremes might reflect functional aspects of monotreme lactation.

Evolution of oxytocin pathways in the brain of vertebrates

“…As for the SON, it was speculated that neurons located in the ventral PON migrate in ventro-lateral direction to the place of the later SON (Herget et al, 2013), leaving remaining cells of further AN. It is interesting that in one of the most primitive modern mammals—monotreme platypus Ornithorhynchus anatinus , most of the magnocellular OT neurons reside in the stream between the PVN and the retrochiasmastic part of the SON and never form the main nuclei found in other mammals (Ashwell et al, 2006). …”

Development of the hypothalamus and pituitary in platypus (Ornithorhynchus anatinus) and short‐beaked echidna (Tachyglossus aculeatus)

“…The hypothalamus of the adult monotreme is structurally and chemoarchitecturally similar to that of therians, although the supraoptic nucleus is small (Ashwell et al. 2006).…”