Atlas of the rostral nervous system of a 7.4 mm GL Echidna Stage U (M153)
Introduction
The short-beaked echidna specimen depicted here is probably Tachyglossus aculeatus aculeatus, given that the most likely point of collection is eastern New South Wales near Sydney. An echidna in ovo of 7.4 mm greatest length (GL; see calculation below) is probably 7 days before hatching (Ashwell, 2013), where hatching occurs at about 14 to 16 mm GL, and the specimen would be stage 16 (Werneburg and Sánchez-Villagra, 2011).
Details on short-beaked echidna biology can be found at the Short-beaked Echidna page of this website.
Methods
The incubation (in ovo) stage U (Hill) specimen illustrated here is M153 of the Hill collection stored at the Museum für Naturkunde (MfN) in Berlin. The specimen had been collected in the late 19th century by JP Hill, embedded in paraffin wax and sectioned horizontally at 8 µm thickness (total of 922 sections). Estimated post-dehydration GL is 922 x 0.008 mm = 7.376 mm. The stain was not identified on the specimen card but appears to be haematoxylin and eosin. Head width is approximately 1.0 mm after dehydration.
Images previously taken by KK Sulik and stored under open access within the MfN collection have been used for labelling (please see the MfN for access). Those images were placed in Adobe Illustrator 2023 and delineated.
Notes on the specimen
Special sense organs
The olfactory epithelium is still at the nasal placode stage (nplac in Plates 1 to 3).
Optic vesicles (Optic in Plates 2, 3) are present, but the developing lens is still at the placode stage (lplac in Plates 2, 3). The otic vesicle (OticV in Plates 5, 6; otocyst or embryonic inner ear) has separated from the surface of the embryo but is still spherical (i.e. no semicircular ducts or separation of utricle and saccule), with thickening of the vesicular wall at the ventral side. The first pharyngeal cleft (PC1 in Plates 4 to 6; future external acoustic meatus) and the first pharyngeal pouch (PP1 in Plates 4 to 6; future middle ear) are both present, but primitive.
Brain
The neural tube is only at the neuroepithelial stage, with no postmitotic neurons apparent. Three primary brain vesicles (prosencephalon, mesencephalon, rhombencephalon in Plates 1 to 3) are readily identified, but it is also possible to deduce the positions of both secondary brain vesicles (telencephalon and diencephalon) and the constituent neuromeres (prosomeres 1 to 3 of diencephalon in Plates 1 and 2) and subzones within the hypothalamus (hy2, hyat in Plates 2 and 3). Rathke’s pouch (Rathke in Plate 3) is in contact with the infundibular recess of the hypothalamus (InfRe).
The mesencephalon has two segments (mes 1 and mes 2 in Plates 1 and 2) and a central aqueduct (Aq). The rhombencephalon (Plates 3 to 8) has a very thin roof and notional positions of rhombomeres (r2 to r11) have been indicated. Alar and basal plates (for generation of sensory and motor neurons, respectively) can be seen (ap, bp in Plates 2 to 8).
Cranial nerves and ganglia
Without special stains the obvious cranial nerves are confined to the trigeminal nerve (5n in Plate 3) and its ganglion (5Gn in Plates 3, 4), and the facioacoustic ganglion primordium (fag in Plate 5; future geniculate, cochlear and vestibular ganglia).
Spinal cord
The spinal cord is divided into alar and basal plates (ap and bp in Plates 9 to 12) separated by a sulcus limitans. The spinal cord has roof and floor plate regions (rfp and fp in Plates 9 to 12).
Nerves are growing out from the brachial region of the spinal cord to the forelimb bud.
Vasculature
The major vessels associated with the embryonic brain include the proto-internal carotid artery (ictd in Plates 2 and 3) and the anterior cardinal vein (acardv in Plates 3 to 10). Two pharyngeal arch arteries can be seen (arch 1 artery or aa1 in Plates 3 to 6; and arch 2 artery or aa2 in Plates 5 and 6).
Acknowledgements
I would like to thank Dr Peter Giere of the MfN, Berlin Germany, for access to the collection and for all his kind help during the work.
References
Ashwell KW (2013) Embryology and post-hatching development of the monotremes. In KWS Ashwell (Ed.), Neurobiology of Monotremes: Brain Evolution in Our Distant Mammalian Cousins (1st ed., pp. 31-46). CSIRO.
Werneburg I and Sánchez-Villagra MR (2011) The early development of the echidna, Tachyglossus aculeatus(Mammalia: Monotremata), and patterns of mammalian development. Acta Zoologica (Stockholm) 92, 75–88.
