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第21 章 颜面、颈和四肢的发生
CHAPTER 21 DEVELOPMENT OF THE FACE,NECK AND LIMBS
KEY POINTS
Derivatives of the pharyngeal pouches
Development of the tongue
Development of the thyroid
Formation of the face
Development of the palate
Development of the teeth
Congenital malformations of the face, neck, and limbs.
OUTLINE
The most typical feature in development of the head and neck is formed by the
pharyngeal or branchial arches.
These arches appear in the fourth and fifth weeks of development and contribute
to the characteristic external appearance of the embryo. Initially they consist of bars
of mesenchymal tissue separated by deep clefts known as pharyngeal (branchial)
clefts. Simultaneously with development of the arches and clefts, a number of
outpocketings, the pharyngeal pouches appear along the lateral wall of the pharyngeal
gut, the most cranial part of the foregut.
Pharygeal arches not only contribute to formation of the neck but also play an
important role in formation of the face. At the end of the fourth week the center of the
face is formed by the stomodeum, surrounded by the first pair of pharyngeal arches.
At the end of the fourth week, five mesenchymal prominences can be recognized: the
mandibular prominences, caudal to the stomodeum; the maxillary prominences, lateral
to the stomodeum; and the frontonasal prominence, a slightly rounded elevation
cranial to the stomodeum. On both sides of the frontonasal prominence, local
thickenings of the surface ectoderm, the nasal placodes, originate under inductive
influence of the ventral portion of the forebrain.
During the fifth week, the nasal placodes invaginate to form nasal pits. In so
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