...gives to the nose is essential for the functional development of the reproductive organs and hence fertility. Recently much progress has been made in the understanding of placode development, at both a molecular and embryological level. This is important as abnormal development of placodes occurs in a number of human syndromes. Furthermore, knowledge of placode development will give insight into therapeutic strategies to prevent degenerative change such as deafness. This review highlights the current knowledge of placode development and the future challenges in unravelling the cascades of signalling interactions that control development of these unique structures.

Keywords: sensory organs

Introduction

The sensory organs of the vertebrate arise at a very early stage of development from discrete ectodermal thickenings, the placodes, which in the case of the developing ear and nose undergo complex morphogenesis and differentiation. They not only contribute to our sensory perception--sight, hearing and smell--but have other essential functions. For example, the inner ear functions in the detection of balance and movement whilst the olfactory (nasal) placode gives rise to the gonadotrophin-releasing hormone neurons, which are essential for the functional development of the reproductive system. This review focuses on the development of the placodes responsible for perception of sight, smell and sound, the lens, olfactory and otic placodes, respectively. The other placodes which also contribute to sensory detection and form part of the peripheral nervous system will be mentioned briefly. They are reviewed in detail by Baker and Bronner-Fraser (1).

Defects in sensory placode development are associated with a number of human syndromes, and the molecular basis for these syndromes is now beginning to be identified. For example, Branchio-Oto-Renal and Hypoparathyroidism (HPR) syndromes, characterised in part by deafness, are due to mutations in the EYES ABSENT(EYA1) and GATA3 genes, respectively. Blindness or anirida, due to defects in lens development, can occur following mutations in the PAX6, PITX3 or EYA1 genes (1). Kallmann syndrome, due to defective olfactory development, is not only characterised by anosmia--an inability to smell--but also hypogonadotropic hypogonadism (abnormal development of the reproductive organs) and loss of libido due to deficiency in the gonadotrophin releasing hormones (GnRH) (also known as luteinizing hormone releasing hormone). The anosmia is due to the loss or hypoplasia of the olfactory tracts and bulbs. The X-linked form of the syndrome can be due to mutations in the KAL] gene (also called KALIGi and ADMLX). In this synd rome, the neuronal derivatives of the olfactory placode fall to elongate and migrate into the forebrain (1). KAL1 encodes an extracellular protein, anosmin-1, which is associated with the cell surface, and has been speculated to modulate cell adhesion, act as a chemoattractant and/or have protease activity. Consistent with the proposed migratory function, studies in vitro have shown that anosmin-1 can mediate cell adhesion, although this does not appear to be integrin mediated, and that it can modulate neurite outgrowth in a cell-specific manner (2). Furthermore, anosmin-1 is expressed in the target tissues, the olfactory bulb and developing forebrain, in chicks and humans, and its expression is temporally coincident with neuron migration (1). Kallmann syndrome also can be associated with deafness, neurological defects, cleft lip/palate and unilateral renal agenesis.

Placodes are induced by surrounding tissues. Induction is defined as the ability of one tissue/signal to specify/direct the cell fate of another. However, placodes are not just passive recipients of inducing signals, and once specified, they are essential for normal development of surrounding structures. As with many early developmental processes, this is achieved by reciprocal signalling interactions, which coordinate growth and differentiation of the two tissues or structures. Indeed, for the placode derivatives to function, they must develop and connect to the appropriate neural structure. The lens is essential for the normal development of the adjacent structures, the retina, iris, ciliary body and overlying cornea. (1,3-6) Ablation studies in Xenopus also have shown that the olfactory placode is essential for normal forebrain development and this is due, at least in part, to a proliferative effect of the olfactory nerve on the developing forebrain. (1) In addition, the olfactory and otic epithelia induce chondrogenesis in the surrounding mesenchyme, providing a protective and strong structural framework for these sensory organs and contributing to the structural scaffold of the head.

In addition, there are other placodes--the trigeminal (ophthalmic and maxillomandibular) and epibranchial (geniculate, petrosal and nodose)--which contribute to the sensory ganglia. The trigeminal placodes form the cutaneous sensory neurons that innervate the face and jaw and transmit information about touch, pain and temperature. The nodose visceral sensory neurons innervate the heart, lungs and gut, and they convey information about heart rate, blood pressure, bronchial irritation and visceral distension, whilst the petrosal and geniculate neurons innervate the taste buds. In anamniotes, an additional placodal structure has been identified, the unique lateral line system which conveys information on mechanoreception and electroreception. (1)

The sensory placodes and neural crest have been proposed to be key evolutionary features of vertebrates, resulting in active predation as opposed to passive filter feeding. Indeed, development of the placodes and neural crest shows many parallels. (7) In both cases, cells are derived from the ectoderm, are capable of undergoing an epithelial-mesenchymal transformation and can migrate and differentiate into a variety of cell types. However, recent studies in ascidians, which possess a simple brain structure and are closely related to vertebrates, has challenged this idea of neural crest/placode co-evolution and the uniqueness of placodes to vertebrates. Analysis of the expression of pax258, a homologue of the vertebrate Pax2, -5 and -8 genes, has shown that it is expressed in rudiments called the atrial primordia. These arise from a thickened region of ectoderm and will ultimately form a sensory structure consisting of ciliated sensory cells in cupular organs. (8) The atrial primordia have previously been prop osed to be the homologue of the vertebrate ear, which also expresses Pax2. (9) The expression of pax258 is consistent with this possibility, suggesting that placode-like structures existed before the evolution of neural crest. However, without doubt, the evolution of neural crest has been essential for the development and function of vertebrate sensory structures and the peripheral nervous system. For example, the neural crest derived mesenchyme is essential for olfactory placode induction and/or differentiation. Furthermore, the ganglia of the trigeminal, otic and epibranchial complexes are formed from both neural crest and placodal derivatives. The significance of this dual origin of neurons, which have distinct biochemical properties, is unclear. However, one proposal is that the placodally derived neurons, which are formed earlier than neural-crest derived neurons, act as a scaffold to guide the neural crest-derived neurons (10). Indeed, in the absence of placodal neurons the cranial neural crest sensory neurons do not project peripherally, and the motor neuron populations do not migrate properly (10).

Anatomical development

The nose

The olfactory placodes in mammals give rise to four types of...

NOTE: All illustrations and photos have been removed from this article.



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