DUHS medulla

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					                  Development of mid and hind brain
                                Learning objectives


At the end of the lecture, students should be able to understand ;

   •   General overview of nervous system development

   •   Derivatives of primary and secondary brain vesicles

   •   Developmental stages of The Hind-brain or Rhombencephalon.

   •   Developmental stages of The mid-brain or mesencephalon.




                                    Lecture outline
General overview of Nervous system development
   •   Nervous system develops from an area of embryonic ectoderm called the neural plate which
       appears during week 3

   •   The neural tube gives rise to the central nervous system (brain and spinal cord)

   •   The neural crest gives rise to the peripheral nervous system (cranial, peripheral, autonomic
       ganglia and nerves) and Schwann cells, pigment cells, odontoblasts, meninges, and bones and
       muscles of the head
Development of Primary and secondary vesicles and their derivatives




  •   Neuroblasts of the brainstem develop in a manner similar to the spinal cord.

  •   From the medulla through the midbrain, alar and basal plates form motor and sensory columns
      of cells that supply cranial nerves.

  •   However, the organization of alar and basal plates differ from of the spinal cord in that,

      1) in the medulla and pons the alar plate lies lateral to the basal plate, not dorsal to it, since the

         4th ventricle is “open”

      2) there are migrations of neuroblasts of both plates from the ventricular floor to other

        locations,
         3) “special” sensory and motor structures of the head require new/different cell groups for
innervation.

   •

   The lateral walls of the brain-tube, like those of the medulla spinalis, are divided by internal furrows
   into alar or dorsal and basal or ventral laminae
                               Development of Hind brain




•   The Hind-brain or Rhombencephalon.—The cavity of the hind-brain becomes the fourth
    ventricle.

•   At the time when the ventral cephalic flexure makes its appearance, the length of the hind-brain
    exceeds the combined lengths of the other two vesicles.

•    Immediately behind the mid-brain it exhibits a marked constriction, the isthmus
    rhombencephali

•   From isthmus the anterior medullary velum and the superior peduncle of the cerebellum are
    formed.

•   It is customary to divide the rest of the hind-brain into two parts, viz., an upper, called the
    metencephalon, and a lower, the myelencephalon.

•   The cerebellum is developed by a thickening of the roof, and the pons by a thickening in the
    floor and lateral walls of the metencephalon.
                         Development Of Medulla oblongata


•   The development of the medulla oblongata resembles that of the medulla spinalis with slight
    modifications

•   The floor and lateral walls of the myelencephalon are thickened to form the medulla oblongata;

•   Its roof remains thin, and, retaining to a great extent its epithelial nature, is expanded in a lateral
    direction.

•   Later, the roof is folded inward toward the cavity of the fourth ventricle; it assists in completing
    the dorsal wall of this cavity, and is also invaginated to form the ependymal covering of its
    choroid plexuses.

•   Above it is continuous with the posterior medullary velum; below, with the obex and ligulæ.

•   Medullary pyramids consist of fibers from the cerebral cortex and develop on the ventral surface
    near the midline. On transverse section the myelencephalon at an early stage is seen to consist
    of two lateral walls, connected across the middle line by floor- and roof-plates




•
•   Each lateral wall consists of an alar and a basal lamina, separated by an internal furrow, the
    remains of which are represented in the adult brain by the sulcus limitans on the rhomboid
    fossa.

•   Pear-shaped neuroblasts are developed in the alar and basal laminæ. and their narrow stalks are
    elongated to form the axis-cylinders of the nerve fibers.

•

•
•

•   Opposite the furrow or boundary between the alar and basal laminæ a bundle of nerve fibers
    attaches itself to the outer surface of the alar lamina. This is named the tractus solitarius and is
    formed by the sensory fibers of the glossopharyngeal and vagus nerves.

•   It is the homologue of the oval bundle seen in the medulla spinalis, and, like it, is developed by
    an ingrowth of fibers from the ganglia of the neural crest.

•   By the fifth week the dorsal part of the alar lamina bends in a lateral direction along its entire
    length, to form what is termed the rhombic lip




    Exterior of brain of human embryo of four and a half weeks




•
                           Development Of Pons


The pons is developed from the ventro-lateral wall of the metencephalon by a process similar to
that which has been described for the medulla oblongata.
                                Development Of Cerebellum
•   The cerebellum is developed in the roof of the anterior part of the hind-brain

•    The alar laminae of this region become thickened to form two lateral plates which soon fuse in
    the middle line and produce a thick lamina which roofs in the upper part of the cavity of the
    hind-brain vesicle; this constitutes the rudiment of the cerebellum,

•   The fissures of the cerebellum appear first in the vermis and floccular region, and traces of them
    are found during the third month; the fissures on the cerebellar hemispheres do not appear until
    the fifth month.

•   The primitive fissures are not developed in the order of their relative size in the adult—thus the
    horizontal sulcus in the fifth month is merely a shallow groove. The best marked of the early
    fissures are: (a) the fissura prima between the developing culmen and declive, and (b) the
    fissura secunda between the future pyramid and uvula.




•   The flocculus and nodule are developed from the rhombic lip

•   The groove produced by the bending over of the rhombic lip is here known as the floccular
    fissure; when the two lateral walls fuse, the right and left floccular fissures join in the middle line
    and their central part becomes the post-nodular fissure.
•

•

•   On the ventricular surface of the cerebellar lamina a transverse furrow, the incisura fastigii,
    appears, and deepens to form the tent-like recess of the roof of the fourth ventricle.
             Development of the Mid-brain or Mesencephalon
•   The mid-brain exists for a time as a thin-walled cavity of and is separated from the isthmus
    rhombencephali behind, and from the fore-brain in front, by slight constrictions.

•   Its cavity becomes relatively reduced in diameter, and forms the cerebral aqueduct of the adult
    brain.

•    Its basal laminæ increase in thickness to form the cerebral peduncles, which are at first of small
    size, but rapidly enlarge after the fourth month.

•   The neuroblasts of these laminæ are grouped in relation to the sides and floor of the cerebral
    aqueduct, and constitute the nuclei of the oculomotor and trochlear nerves, and of the
    mesencephalic root of the trigeminal nerve.




•   By a similar thickening process its alar laminæ are developed into the quadrigeminal lamina.

•   The dorsal part of the wall for a time undergoes expansion, and presents an internal median
    furrow and a corresponding external ridge; these, however, disappear, and the latter is replaced
    by a groove.

     Subsequently two oblique furrows extend medially and backward, and thickened lamina is thus
    subdivided into the superior and inferior colliculi.
                               Summary :Rhombencephalon
•   Myelenchephalon gives rise to the medulla oblongata.

•   Metencephalon give rise to cerebellum and pons.

•   All cranial nerves arise except I-III arise from the rhombencephalon

    The figure shows the brain vesicles in an 8 week old embryo with the roof of the
    rhombencephalon removed.




                              Summary ;Mesencephalon
•   The basal plates consist of two groups of motor nuclei (III/IV and n. of Edinger-Westphal) (B).

•   The marginal layers of the basal plate enlarge and form the crus cereebri.

•   A transverse groove forms along the alar plates to delineate the superior and inferior colliculi.

				
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posted:8/30/2011
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