Sequences

The term sequence implies that a series of events occurs after a
single initiating abnormality, which may be a malformation,
a deformation or a disruption. The features of Potter sequence
are classed as a malformation sequence because the initial
abnormality is renal agenesis, which gives rise to
oligohydramnios and secondary deformation and pulmonary
hypoplasia. Other examples are the holoprosencephaly
sequence and the sirenomelia sequence. In holoprosencephaly
the primary developmental defect is in the forebrain, leading
to microcephaly, absent olfactory and optic nerves, and midline
defects in facial development, including hypotelorism or
cyclopia, midline cleft lip and abnormal development of the
nose. In sirenomelia the primary defect affects the caudal axis
of the fetus, from which the lower limbs, bladder, genitalia,
kidneys, hindgut and sacrum develop. Abnormalities of all
these structures occur in the sirenomelia sequence.