Mesenchyme
for formation of the head region is derived from paraxial mesoderm and lateral
plate mesoderm, neural creast and thickened region of ectoderm known as
ectodermal placodes.
The
most typical feature in development of the hea 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.Initialy they consist of bars of mesenchymal tissue separated by
deep cleafts known as pharyngeal ( branchial ) cleft.Simultaneously ,with
development of the arches and clefts ,a number of outpocketing ,the pharyngeal
gut ,the pharyngeal pouch,appears along the lateral walls of the pharyngeal
gut,the most cranial part of the foregut.
The pouch penetrate the
surrounding the mesenchyme, but do not establish an open communication with the
external clefts. Therefore the term pharyngeal (arch, cleft, and pouch) has
been adopted for the human embryo.
The
human embryo has five pairs of pharyngeal pouches . The last one of these is atypical
and considered as a part of the fourth. Since the epithelial endoderm lining of
the pouch gives rise to a number of important organs. the fate of each pouch is
given bellow :
Structures developed from
pharyngeal pouch
Pouch
|
Derivatives
|
1st pouch
|
1. Middle
ear cavity(primitive tympanic)
2. Auditory
tube ( from tuubotympanic recess)
3. Endodermal
lining of tympanic membrane
|
2nd pouch
|
1. Palatine
tonsil
|
3rd pouch
|
1. Inferior
parathyroid gland
2. Thymus
|
4th pouch
|
1. Superior
parathyroid gland
2. Lateral
thyroid element
|
5th pouch
|
1. Ultimobranchial
body which later forms the parafollicular cells or C cell of thyroid gland
|
*Data are collected from Langman's Medical Embryology
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