Ectoderm Derivatives
Formation of the primitive central nervous system is induced in the ecto-
derm layer by cells forming the notochordin the underlying mesoderm.
The neural plate ectoderm (neuroectoderm)forms two lateral folds
that meet and fuse in the midline to form the neural tube (neurulation).
Cells from the tips of the folds (neural crest)migrate throughout the
body to form many derivatives including the peripheral nervous system.
FORMATION OF THE HEAD REGIONNeural crest contributes significantly to formation of connective tissue ele-
ments in the head.
The bony skeleton of the head is comprised of the viscerocraniumand
theneurocranium.
The neurocranium (cranial vault) is composed of a base formed by endo-
chondral ossification (chondrocranium) and sides and roof bones
formed by intramembranous ossification.
The chondrocranium is derived from both somitic mesoderm(occipital)
and neural crest.
The viscerocranium (face) is derived from the first two pharyngeal
(branchial) arches(neural crest in origin).
LIMB FORMATIONThe limbs form as ventrolateral buds under the mutual induction of ecto-
derm[apical ectodermal ridge (AER)]and underlying mesoderm begin-
ning in the fifth week. The AER influences proximal-distal development.
Somatic lateral plate mesoderm (somatopleure) forms the bony and
connective tissue elements of the limbs and limb girdles while skeletal
muscle of the appendages is derived fromsomites.
Cranio-caudal polarityis determined by specialized mesoderm cells
[zone of polarizing activity (ZPA)]that release inducing signals such as
retinoic acid.
Homeobox genesare the targets of induction signals. They are named
after their homeodomaincalled the homeobox which is a DNA-binding
motif. Homeobox genes encode trancription factors that regulate processes
such as segmentation and axis formation.
Rotation of the limb buds establishes the position of the joints, the loca-
tion of muscle groups, and the pattern of sensory innervation (dermatome
map).
4 Anatomy, Histology, and Cell Biology