Chapter 150. The Brainstem: Metencephalon (fourth Vesicle) – The Cerebellum - Review of Medical Embryology Book

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

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

In the 12-week embryo, the plate shows a small midline portion, the vermis, and 2 bulging lateral masses, the lateral lobes
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

Other transverse fissures: many appear with development, giving the cerebellum its characteristic adult appearance

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

The anterior lobe consists of the lingula, the central lobule, the culmen, the alae of the central lobules, and the quadrangular lobules
The middle lobe consists of the declive, folium, tuber, pyramid, uvula, lobus simplex, biventral lobule, semilunar lobules, and the tonsils

THE ARCHICEREBELLUM consists of the flocculonodular lobe (vestibular) and the lingula (has spinocerebellar and vestibular connections)
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)
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

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
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
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
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