The somites: alongside the developing neural tube are strips of mesoderm which begin to show segmentation
into somites near the end of week 3 of development
THE FIRST SOMITE appears behind the cephalic tip of the notochord
SUCCESSIVE SOMITES appear in a craniocaudal sequence, and approximately 41 to 44 pairs of somites are
present by day 31 of human development
MOST OF THE AXIAL SKELETON AND MUSCULATURE develop from the somites (see the units on skeletal and muscular
development)
Divisions of the neural tube: the cephalic (head) end of the neural tube is larger than its caudal (tail)
end, and even before the neural tube is closed cephalically, the cephalic end shows 3 distinct dilatations,
the primary brain vesicles
THE PROSENCEPHALON OR FOREBRAIN is the most anterior vesicle
THE MESENCEPHALON OR MIDBRAIN is the central vesicle
THE RHOMBENCEPHALON OR HINDBRAIN is the most posterior vesicle
Unequal growth rates and cell migration result in flexures, constrictions, thickenings, invaginations,
and evaginations
NEURULATION contributes to the cephalocaudal flexion of the embryo. The extensive proliferation of the
nervous tissue causes curvature of the embryo on its long axis. Progressive dorsal flexion results in
raising and isolating the embryo from its membranes. With the appearance of the vesicles (about a 5
mm embryo stage), the neural tube bends ventrally to form 2 flexures
A cervical flexure: at the junction of the spinal cord and hindbrain
A cephalic flexure: in the midbrain
Later, between the above 2 major flexures, unequal growth in the hindbrain produces the pontine flexure,
in the opposite direction
Five components in the developing brain at week 5
THE PROSENCEPHALON OR FOREBRAIN now consists of 2 parts
An anterior telencephalon or endbrain which will give rise to 2 anterolateral expansions called
the primitive cerebral hemispheres
An intermediate brain or diencephalon, characterized by the outgrowth of the optic vesicles
Dorsal and ventral evaginations from the diencephalon become the primordia of the pineal gland and the
posterior hypophysis, respectively
THE MESENCEPHALON OR MIDBRAIN undergoes little change by this age
THE RHOMBENCEPHALON consists of 2 parts
The anterior metencephalon, which later forms the pons and the cerebellum
The posterior myelencephalon which later forms the medulla oblongata
The boundary between the metencephalon and myelencephalon is marked by the third or pontine flexure,
compensatory to the cervical and cephalic flexures
The lumen of the spinal cord, the central canal is continuous with the brain vesicles, permitting cerebrospinal
fluid to circulate freely between the brain and cord
THE CAVITY OF THE RHOMBENCEPHALON is the fourth ventricle
THE CAVITY OF THE DIENCEPHALON is the third ventricle
THE CAVITIES OF THE CEREBRAL HEMISPHERES are the lateral ventricles
THE LUMEN BETWEEN THE THIRD AND FOURTH VENTRICLES is the narrow aqueduct of Sylvius
THE LATERAL VENTRICLES communicate with the third ventricle via the foramina of Monro
THE FOURTH VENTRICLE opens into the subarachnoid space via the foramina of Luschka (2) and Magendie
(1)