Trks and p75 genes are differentially expressed in the inner ear of human embryos. What may Trks and p75 null mutant mice suggest on human development? Jose Antonio Vega a , Isabel San Jose´ b , Roberto Cabo b , Santiago Rodriguez b , Juan Represa b, c, * a Departamento Morfologia y Biologia Celular, C/Julian Claverı ´a, Universidad de Oviedo, Oviedo, Spain b Instituto de Biologı ´a y Gene´tica Molecular Universidad de Valladolid-C.S.I.C y Departamento de Anatomı ´a, C/Ramo´n y Cajal 7, Valladolid 47005, Spain c Department of Otolaryngology and Neuroscience, Albert Einstein College of Medicine, Bronx, NY 10461, USA Received 19 August 1998; received in revised form 17 July 1999; accepted 17 July 1999 Abstract Recent work has shown the expression of Neurotrophins low (p75) and high affinity (Trk’s A, B, and C) receptors in the developing inner ear sensory neurons of chick and mouse. Likewise the biological significance of such receptor expres- sion was demonstrated by using both Trks and Neurotrophins null mutant mice. The present study was conducted to determine the expression of Trks and p75 proteins in the human inner ear throughout development. Hence to assess the potential role of Neurotrophins in the development of auditory and vestibular specific innervation in man. In other words, we intend to address the issue whether or not what null mutant mice for Trks and p75 have revealed on inner ear development may be relevant for human embryos. Fifty-two inner ears and their cochleovestibular ganglions (CVG) from human embryos and fetuses, ranging from 5 to 24 weeks of pregnancy were analyzed. Both Western blot and immunocytochemistry on frozen sections were used as complementary procedures. Quantitative Western blot studies revealed that Trk-B and C immunoreactivity (IR) appeared by embryonic week 5 in CVG neurons, increased at high levels between embryonic weeks 7 and 12, and later on, in 15 week-old specimens and older began to decrease to minimal levels. Trk-A IR was detected at just moderate levels during 5 and 7 weeks reflecting the presence of NGF high affinity receptors only at these earlier developmental ages. The p75 IR was detected at high degrees in the early stage of the 5th week and at abundant levels in all studied inner ears from the 7th to the 24th pregnancy week. These Western blot observations were corroborated by immunocytochemistry on frozen sections, which also revealed a major distribution of both p75 and Trks on neuronal bodies while p75 appears localized on supporting cells. Our findings reveal a tight correlation between p75 and Trks expression throughout human development and specific inner ear developmental events, such as target-dependent neuronal cell death and afferent hair cells innervation. That kind of association of p75 and Trks temporal pattern with distinctive steps in inner ear developmental schedule, is a feature shared between human embryos and other mammals, such as mouse. Based on the present results and considering them together with the reported phenotype of p75 and Trks null mutant mice, we hypothesize that p75 and Trk receptors, as well as, their binding Neurotrophins may be essential in human inner ear development. Accordingly, they may be required molecules for sensory epitheliums innervation and target-dependent neuronal cell death, during embryogenesis and even early postnatal life, in man. q 1999 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Cochleovestibular ganglion; Inner ear development; Neurotrophins; p75 receptor; Trk proto-oncogene; Human embryos Embryonic development of the human inner ear starts with the formation of the otic placode, which eventually forms the otic vesicle and the cochleovestibular ganglion neurons (CVG). The otic vesicle subsequently differentiates into a complex membranous labyrinth that contains the sensory epithelium of the inner ear and highly specific networks of neuronal connections between the epitheliums and the central nervous system [1]. ‘In vitro’ and ‘in vivo’ Neuroscience Letters 272 (1999) 103–106 0304-3940/99/$ - see front matter q 1999 Elsevier Science Ireland Ltd. All rights reserved. PII: S0304-3940(99)00577-7 * Corresponding author. Tel.: 134-983-423-057; fax: 134-983- 423-057. E-mail address: represa@med.uva.es (J. Represa)