Pituitary
The anterior portion of the pituitary is derived from oral ectoderm arising
from the roof of the oral cavity (Rathke’s pouch) anterior to the buccopha-
ryngeal membrane and migrating through the sphenoid anlagen to unite
with a downgrowth of neuroectoderm (posterior pituitary).
Eye
The eye is derived from three different germ layers:
Neuroectoderm:Vesicular outgrowths of the forebrain differentiate into
retina and optic nerve.
Surface ectoderm:Contributes to thelens, cornea,and epithelial cover-
ings of the lacrimal glands, eyelids, and conjunctiva.
Mesoderm:Thesclera and choroidare derived from lateral plate
mesoderm.
Theextraocular musclesare derived from myoblasts of the cranial
somitomeres.
Structures of the outer and middle earare derived from the first and
second pharyngeal archesand thefirst pharyngeal cleft.
Structures of the inner earare derived from the ectodermal otic pla-
code,not neuroectoderm.
Maternal rubellacan cause defects in both eye (fourth to sixth weeks
of gestation) and ear (seventh to eight weeks).
FORMATION OF THE CARDIOVASCULAR SYSTEMAll components of the cardiovascular system, including the epithelia, are
derived from splanchnic lateral plate mesoderm.
The heart tubes forming on either side of the endodermal tube are
brought together by lateral body folding.
Looping of the heart tubeoccurs while the tube is being divided into
left and right portions by the interatrial and interventricular septa.
In the interatrial septum, the septum primumandseptum secun-
dumdo not close off the foramen ovaleuntil birth.
Failure of the atrioventricular endocardial cushionsto fuse can
result in septal and valve defects.
Neural crest cellscontribute to septation of the truncus arteriosus and
the formation of the aortic and pulmonary outflows, as well as the aortic
arches.
High-Yield Facts 7