Vol.:(0123456789) 1 3 Anatomical Science International https://doi.org/10.1007/s12565-020-00565-9 ORIGINAL ARTICLE Development of the smooth muscle layer in the ileum of mouse embryos Esrat Jahan 1  · Ashiq Mahmood Rafq 2  · Akihiro Matsumoto 1  · Nusrat Jahan 1  · Hiroki Otani 1 Received: 24 April 2020 / Accepted: 17 August 2020 © Japanese Association of Anatomists 2020 Abstract The smooth muscle layer (SML) comprises a signifcant portion of the intestines and other tubular organs. Whereas epi- thelial development has recently been extensively studied, SML development has drawn relatively less attention. Previous morphological reports revealed that the inner circular layer (IC) diferentiates earlier than the outer longitudinal layer (OL), but detailed development of the SML, including chronological changes in the cell layer number, precise cell orientation, and regional diferences in relation to the mesentery, has not been reported. We here observed the development of the SML in the C57BL/6J mouse ileum near the ileocecal junction at embryonic day (E) 13.5, 15.5, and 17.5. By histo-morphometric analyses, in IC, smooth muscle cells (SMCs) were oval-shaped and irregularly arranged in 3–4 layers at E13.5, then adopted an elongated spindle shape and decreased to two cell layers at E15.5 and E17.5. The IC SMC nuclear angle was not vertical, but oriented at 60–80° against the mid-axis of the intestinal lumen. The single SMC layer in OL was observed at E17.5, and the SMC nuclear angle was parallel to the luminal mid-axis. No clear regional diference against the mesentery was observed. Collectively, the fndings suggest that development and diferentiation of the ileal SML is not simple but regulated in a complex manner and possibly related to the macroscopic organogenesis. Keywords Smooth muscle cell · Inner circular layer · Ileum · Mouse · Embryo Introduction The intestinal wall consists of the luminal epithelium, con- nective tissue, smooth muscle layer (SML), and covering serosa. Through the organogenetic and histogenetic peri- ods, the luminal epithelium and glands develop from the endoderm, whereas the covering mesoderm develops into the connective tissue, blood vessels, smooth muscle cells (SMCs) of the lamina muscularis mucosae, and SML, together with the myenteric ganglia (for review, see Young 2008). Since the intestine not only develops radial layers but also elongates and increases its luminal surface area in a manner disproportionate to the increase in body size over these periods, there must be mechanisms to ensure that these events occur in each tissue part. Further, the disproportion- ately elongated intestine must be folded and arranged in a basically reproducible fashion in the limited space of the abdominal cavity. Although we are still far from a complete picture of how these events occur synchronously and in har- mony, more recent fndings are accumulating and deepening our understanding (for review, see Walton et al. 2016a). We previously reported that a convergent extension mechanism (not intercalation of layered cells but change in/rearrange- ment of the nuclear position in the pseudostratifed mono- layer epithelium) is involved in the midgut epithelial elonga- tion (humans: Matsumoto et al. 2002; mice, via the Wnt5a/ Ror2 signaling pathway: Yamada et al. 2010). Recent studies have suggested the mechanical tension-driven elongational growth of the embryonic gut in chicks (Chevalier et al. 2018) as well as a mathematical model of the generation of a repro- ducible looped pattern of the gut tube connected with the mesentery in multiple species (Savin et al. 2011). Esrat Jahan and Ashiq Mahmood Rafq equal frst authors. * Hiroki Otani hotani@med.shimane-u.ac.jp 1 Department of Developmental Biology, Faculty of Medicine, Shimane University, 89-1 Enya-cho, Izumo, Shimane 693-8501, Japan 2 Center for the Promotion of Project Research, Organization for Research and Academic Information, Shimane University, 1060 Nishikawatsu-cho, Matsue, Shimane 690-8504, Japan