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The pharynx is divided into an upper portion, the nasopharynx, and a lower portion consisting of the oropharynx and hypopharynx. The nasopharynx is located above the soft palate and communicates with the nasal passages. The oropharynx extends from the soft palate and the velum palatinum to the epiglottis. The hy-popharynx extends from the tip of the epiglottis to the lower margin of the cricoid cartilage. The term Waldeyer's ring refers to the ring of lymphoid tissues extending throughout the naso- and oropharynx and includes the palatine, pharyngeal, lingual and tubal tonsils.

The earliest embryological stage relevant for the development of the nasopharynx is around the 3rd week of gestation. The embryo has already developed all three germ layers and consists of the notochord with a noto-chordal lumen [134]. At the cranial end, the notochord is still fused with the ecto- and endodermal germ layers, which form the bilaminar oropharyngeal (or buccopharyngeal) membrane. The first process in the formation of the pharynx is the development of a primitive mouth for stomodeum) and a primitive pharynx during the 4th week via rupture of the oropharyngeal membrane. At that time, the primitive nasal cavities and the nasopharynx are still separated from the oral cavity by the primary palate and the oronasal membrane. The oronasal membrane ruptures around week 6, bringing the nasal and oral cavity into continuity and forming the primitive choanae. During the posterior extension of the primary palate and development of the secondary palate, the choanae become located posteriorly and connect the newly formed nasal cavities and nasopharynx. The development of the pituitary gland (or hypophysis) begins around the middle of the 4th week with an upward proliferation of the ectodermal roof of the stomodeum, the Rathke'spouch (or hypophyseal duct), and a downgrowth from the diencephalon called the neurohypophyseal bud. The Rathke's pouch passes through the chondrification centres of the developing sphenoid bones. By the 5th week, both portions of the pituitary gland have come in contact and the Rathke's pouch becomes constricted at its attachment to the oronasal epithelium. The stalk degenerates and the Rathke's pouch involutes to a series of microcysts, which persist throughout adult life in the pituitary gland. Occasionally, they can be recognised mac-roscopically by a zone of colloid cysts. Symptomatic enlargement leads to the formation of a Rathke's cleft cyst (see Sect. 6.2.2.3). Around the same time, during the involution of the notochord, a pharyngeal bursa is formed temporarily at the site of early communication between the notochord and the roof of the pharynx. This ecto-dermally derived pharyngeal bursa normally obliterates during the 6th gestational week. A persistent pharynge al bursa in adults is located at the posterior median wall of the nasopharynx above the superior pharyngeal constrictor muscles at the lower end of the pharyngeal tonsil and is known as Tornwaldt's cyst (see Sect. 6.2.2.2). Remnants of the notochord give rise to cranial chordomas (see Sect. 6.2.7.1).

The oropharynx, mouth and larynx develop from the pharyngeal (or branchial) apparatus during the 4th and 5th weeks of gestation. The growth of the fore-brain and the development of the pharyngeal/bran-chial apparatus produce a prominent elevation of the head with a quite distinct first and second pharynge-al arch around day 24. At the end of the 4th embryonic week, four well-defined and two rudimentary bilateral pairs of pharyngeal arches are separated by the pharyngeal grooves. Each arch consists of a core of embryonic mesenchyme with an artery, a cartilage rod, a nerve and a muscular component, and is covered externally by ectoderm and internally by endoderm. The ectoder-mally derived pharyngeal arches and grooves support the lateral walls of the primitive pharynx. The inner lining consists of endoderm with balloon-like diver-ticuli called pharyngeal pouches, which are also present in four well-defined pairs. The second pharynge-al pouch is the major contributor to the formation of the pharynx and is largely obliterated when the palatine tonsils develop around weeks 12-14 post-conception. A part of the cavity of the second pouch remains as the intratonsillar cleft (or tonsillar fossa) in the palatine tonsils (see Sect. 6.3.1). The neural crest-derived mesenchyme will form most of the skeletal (cartilage and bone) and connective tissue structure of the head and neck, but the original mesenchyme of the second arch forms the blood vessels and skeletal musculature of the pharynx. The nerve supply of the pharynx develops from the 3rd pharyngeal arch (IX glossopharyngeal nerve). The adult vascular pattern of the head and neck depends on a complex transformation of the pha-ryngeal apparatus with involution and obliteration of the early vessels. Incomplete involution, particularly of the first pharyngeal artery, has been postulated to be responsible for the development of nasopharyngeal angiofibromas (see Sect. 6.2.4.1). The auditory (Eustachian) tube is derived from the first pharyngeal arch and pouch. Incomplete regression may be responsible for the occurrence of hairy polyps (see Sect. 6.2.3.2). The tongue also begins to develop at the end of the 4th week. The oral part of the tongue is derived from the first pharyngeal arch; the posterior pharyngeal tongue develops by fusion of the ventromedial parts of the second and the third pharyngeal arch. The tissues derived from the second pharyngeal arch are gradually overgrown by the third pharyngeal arch.

The lymphoid tissues of Waldeyer's ring develop between the 14th and 18th weeks of gestation. The development of the pharyngeal tonsil begins from an anlage consisting of longitudinal folds on the dorsal wall of the nasopharynx around the 12-14th gestational week. The development of the palatine tonsils begins with a proliferation of the endodermal epithelium of the second pha-ryngeal pouch down into the surrounding mesenchyme, forming the epithelium-lined crypts. In the connective tissue, mesenchymal cells of the second pharynge-al pouch form so-called condensation centres. The first primary follicles can be localised around week 14. The parafollicular areas develop into T-cell areas and precursors of interdigitating cells can be identified. Around week 16, the epithelium shows the first signs of cornifi-cation, and the lymphocytic infiltration of the epithelium occurs [59, 60].

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