Heterogeneity of lymphatic vessels during embryogenesis is critical for organ-specific lymphatic function. Little is known about lymphatics in the developing kidney, despite their established roles in pathology of the mature organ. We performed three-dimensional imaging to characterize lymphatic vessel formation in the mammalian embryonic kidney at single-cell resolution. In mouse, we visually and quantitatively assessed the development of kidney lymphatic vessels, remodeling from a ring-like anastomosis under the nascent renal pelvis; a site of VEGF-C expression, to form a patent vascular plexus. We identified a heterogenous population of lymphatic endothelial cell clusters in mouse and human embryonic kidneys. Exogenous VEGF-C expanded the lymphatic population in explanted mouse embryonic kidneys. Finally, we characterized complex kidney lymphatic abnormalities in a genetic mouse model of polycystic kidney disease. Our study provides novel insights into the development of kidney lymphatic vasculature; a system which likely has fundamental roles in renal development, physiology and disease.

Within the developing head, tissues undergo cell-fate transitions to shape the forming structures. This starts with the neural crest, which undergoes epithelial-to-mesenchymal transition (EMT) to form, amongst other tissues, many of the skeletal tissues of the head. In the eye and ear, these neural crest cells then transform back into an epithelium, via mesenchymal-to-epithelial transition (MET), highlighting the flexibility of this population. Elsewhere in the head, the epithelium loses its integrity and transforms into mesenchyme. Here, we review these craniofacial transitions, looking at why they happen, the factors that trigger them, and the cell and molecular changes they involve. We also discuss the consequences of aberrant EMT and MET in the head.

High incidence of chronic otitis media is associated with human craniofacial syndromes, suggesting that defects in the formation of the middle ear and associated structures can have a knock-on effect on the susceptibility to middle ear inflammation. Patients with branchio-oto-renal (BOR) syndrome have several defects in the ear leading to both sensorineural and conductive hearing loss, including otitis media. 40% of BOR syndrome cases are due to Eya1 haploinsufficiency, with mouse models affecting Eya1, mimicking many of the defects found in patients. Here, we characterize the onset, consequences, and underlying causes of chronic otitis media in Eya1 heterozygous mice. Cavitation defects were evident in these mice from postnatal day (P)11 onwards, with mesenchyme around the promontory and attic regions of the middle ear space. This mesenchyme was still prominent in adult Eya1 heterozygous mice, while the wild-type littermates had fully aerated ears from P14 onwards. MicroCT analysis highlighted a significantly smaller bulla, confirming the link between bulla size defects and the ability of the mesenchyme to retract successfully. Otitis media was observed from P14, often presenting unilaterally, resulting in hyperplasia of the middle ear mucosa, expansion of secretory cells, defects in the motile cilia, and changes in basal epithelial cell markers. A high incidence of otitis media was identified in older mice but only associated with ears with retained mesenchyme. To understand the impact of the environment, the mouse line was rederived onto a super-clean environment. Cavitation defects were still evident at early stages, but these generally resolved over time, and importantly, no signs of otitis media were observed at 6 weeks. In conclusion, we show that a small bulla size is closely linked to defects in cavitation and the presence of retained mesenchyme. A delay in retraction of the mesenchyme predates the onset of otitis media, making the ears susceptible to its development. Early exposure to OM appears to exacerbate the cavitation defect, with mesenchyme evident in the middle ear throughout the animal’s life. This highlights that permanent damage to the middle ear can arise as a consequence of the early onset of OM.