137. General Development of The Central Nervous System

  1. 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
    1. THE FIRST SOMITE appears behind the cephalic tip of the notochord
    2. SUCCESSIVE SOMITES appear in a craniocaudal sequence, and approximately 41 to 44 pairs of somites are present by day 31 of human development
    3. MOST OF THE AXIAL SKELETON AND MUSCULATURE develop from the somites (see the units on skeletal and muscular development)
  2. 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
    1. THE PROSENCEPHALON OR FOREBRAIN is the most anterior vesicle
    2. THE MESENCEPHALON OR MIDBRAIN is the central vesicle
    3. THE RHOMBENCEPHALON OR HINDBRAIN is the most posterior vesicle
  3. Unequal growth rates and cell migration result in flexures, constrictions, thickenings, invaginations, and evaginations
    1. 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
      1. A cervical flexure: at the junction of the spinal cord and hindbrain
      2. A cephalic flexure: in the midbrain
      3. Later, between the above 2 major flexures, unequal growth in the hindbrain produces the pontine flexure, in the opposite direction
  4. Five components in the developing brain at week 5
    1. THE PROSENCEPHALON OR FOREBRAIN now consists of 2 parts
      1. An anterior telencephalon or endbrain which will give rise to 2 anterolateral expansions called the primitive cerebral hemispheres
      2. An intermediate brain or diencephalon, characterized by the outgrowth of the optic vesicles
        1. Dorsal and ventral evaginations from the diencephalon become the primordia of the pineal gland and the posterior hypophysis, respectively
    2. THE MESENCEPHALON OR MIDBRAIN undergoes little change by this age
    3. THE RHOMBENCEPHALON consists of 2 parts
      1. The anterior metencephalon, which later forms the pons and the cerebellum
      2. The posterior myelencephalon which later forms the medulla oblongata
      3. The boundary between the metencephalon and myelencephalon is marked by the third or pontine flexure, compensatory to the cervical and cephalic flexures
  5. 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
    1. THE CAVITY OF THE RHOMBENCEPHALON is the fourth ventricle
    2. THE CAVITY OF THE DIENCEPHALON is the third ventricle
    3. THE CAVITIES OF THE CEREBRAL HEMISPHERES are the lateral ventricles
    4. THE LUMEN BETWEEN THE THIRD AND FOURTH VENTRICLES is the narrow aqueduct of Sylvius
    5. THE LATERAL VENTRICLES communicate with the third ventricle via the foramina of Monro
    6. THE FOURTH VENTRICLE opens into the subarachnoid space via the foramina of Luschka (2) and Magendie (1)

general development of  the central nervous system: image #1