150. The Brainstem: Metencephalon (fourth Vesicle) – The Cerebellum

  1. The roof plate is derived from the dorsal part of the alar plate and thickens to form the cerebellum, which is developed increasingly as one ascends the phylogenetic scale of vertebrates. It begins development between days 40 and 45
    1. THE RHOMBIC LIPS, in the caudal part of the metencephalon, are widely separated, but just below the mesencephalon, they approach each other in the midline to form a transverse thickening which extends to the thin roof plate. With further deepening of the pontine flexure, the rhombic lips become compressed in a cephalocaudal direction to form the cerebellar plate
      1. In the 12-week embryo, the plate shows a small midline portion, the vermis, and 2 bulging lateral masses, the lateral lobes
      2. A transverse fissure soon separates the nodule (arising from the vermis) and the flocculus (derived from the lateral lobes) to form the flocculonodular lobe, which is the most primitive part of the cerebellum, maintains connections with the vestibular system, and is concerned with subconsciously controlled equilibrium
  2. Other transverse fissures: many appear with development, giving the cerebellum its characteristic adult appearance
    1. THE CEREBELLUM is divided into 2 fundamental parts: the flocculonodular lobe and the corpus cerebelli, separated by the posterolateral fissure. The corpus cerebelli is further subdivided into an anterior and middle lobe by the primary fissure
      1. The anterior lobe consists of the lingula, the central lobule, the culmen, the alae of the central lobules, and the quadrangular lobules
      2. The middle lobe consists of the declive, folium, tuber, pyramid, uvula, lobus simplex, biventral lobule, semilunar lobules, and the tonsils
    2. THE ARCHICEREBELLUM consists of the flocculonodular lobe (vestibular) and the lingula (has spinocerebellar and vestibular connections)
    3. THE PALEOCEREBELLUM consists of the anterior lobe (minus the lingula) with the pyramid and uvula of the middle lobe. Phylogenetically, it appears after the archicerebellum and is of spinocerebellar function (for sensory limb information)
    4. THE NEOCEREBELLUM consists of the middle lobe (minus the pyramid and uvula) and is mostly corticopontocerebellar in function (of selective limb control). It is well developed in mammals, especially in humans, and parallels cerebral neocortical development. The neocerebellum matures slowly after birth, whereas the paleocerebellum is completed before term
  3. Histogenesis of the cerebellum
    1. THE CEREBELLAR PLATE initially consists of a neuroepithelial, a mantle, and a marginal layer. With development, cells of the neuroepithelium migrate through the mantle layer to the marginal layer to form the external granular or superficial cortical layer. Simultaneously, some neuroepithelial cells, in a second wave, migrate into the marginal layer to form the Purkinje cells
    2. DURING MONTH 6, THE EXTERNAL GRANULAR LAYER releases various cell types, which migrate inward toward the differentiating Purkinje cells and form the granule cells, the basket cells, the stellate cells, and the Golgi cells. The layer eventually becomes the definitive cerebellar cortex, with all these cells. Production and migration of the cells continue postnatally for about 1 1/2 years
    3. THE DEEP PARAVENTRICULAR CEREBELLAR NUCLEI, i.e., the dentate, are formed by nonmigrating neuroblasts in the mantle layer and reach their definitive position long before birth. Their axons constitute the major part of the superior cerebellar peduncles containing the cerebellovestibular and cerebellorubrothalamic tracts
    4. THE GREATER PART OFTHE ORIGINAL ROOF PLATE of the fourth ventricle forms the pia mater on the cerebellar surface. However, the part in front of and behind the cerebellum specializes to form the anterior and posterior medullary velum, respectively

the brainstem: metencephalon  (fourth vesicle) – the cerebellum: image #1