| There is controversy regarding the development
of the upper lip and jaw. Dursey and His postulated the classical theory
in 1874. They believed that the midface developed by a series of facial
processes: mesodermal projections surfaced with ectoderm, that were surrounded
by clefts. Normally the processes grew until they met one another, then
fusion occurred and with fusion, the ectoderm at points of contact slowly
disappeared producing a solid mesodermal union. Any interruption in growth
of a facial process would cause persistence of the space manifested as
a cleft. Maxillary processes were considered to be free finger like projections
which grow forwards from the superolateral margins of the stomodeum and
fuse with the nasolateral and nasomedial processes. If the fusion of the
maxillary and nasomedial processes should fail, a partial or complete cleft
of the lip, alveolus and primary palate may occur. This theory is still
subscribed to by eminent authorities such as Patten (1971).
Victor Veau was dissatisfied with this
theory and in 1936, Veau and Politzer suggested an alternative one: the
embryo develops five mesenchymal elevations from neural
crest ectomesenchymal migration. This migrating mesenchyme
penetrates between the outer ectoderm and inner endoderm and creates bulges
that 'heap up' into facial processes. The blending of the facial processes
is due to the filling in of the grooves between the processes by more migrating
mesoderm. Inadequate migration results in the persistence of grooves with
subsequent epithelial breakdown and cleft formation. The mesoderm is needed
to bolster the ectoderm and endoderm. A cleft of the lip, alveolus and
primary palate is due to failure of mesodermal penetration in the groove
between the maxillary process and the nasomedial process of one or both
sides. This theory has since been developed by others. |
