Defects in tissue-resident macrophages lead to smaller eardrums and abnormal neurovascular networks with increased middle-ear infection.

The tympanic membrane (TM), or eardrum, is essential for hearing. Macrophages, the primary innate immune cells, are densely distributed in the eardrum after birth, especially near blood vessels and nerve fibers. During postnatal development, neonatal tissue-resident macrophages (TRMs) gradually polarize, and their population declines. These dynamic changes closely parallel the maturation of neurovascular networks. What are the precursors of the early 'wave' of TRMs? Are they critical for postnatal TM development? Using fate mapping, single-cell RNA sequencing, and macrophage depletion, this study reveals for the first time that postnatal eardrum TRMs are heterogeneous, originating mainly from embryonic myeloid lineages, with increasing input from postnatal monocyte progenitor-derived cells. Single-cell RNA sequencing identifies gene signatures vital for vascular and neuronal development. Depleting TRMs results in smaller eardrums, disrupted vascular-neuronal networks, and increased risk of middle-ear infection. This study offers new insight into how the innate immune system supports TM maturation and protects against middle-ear infection during a critical postnatal window.