CHAPTER 25 



THE HUMAN PINEAL ORGAN 

 Development and Histogenesis 



The stages in development of the derivatives of the primary ependymal 

 elements of the brain and spinal cord, which we have described in 

 Chapter 22, are of great importance in connection with the study of 

 the structure of the fully developed pineal body. In the early phases of 

 development it was shown by Cajal that the ependymal cells extend 

 through the whole thickness of the wall of the neural tube, as is seen in 

 Golgi preparations of the chick embryo at the third day of incubation. 

 At a later stage, when the width of the neural tube has increased, there is a 

 tendency for the central part or body of the cell, which contains the 

 nucleus, to separate from its attachment to either the internal or the 

 external limiting membrane, or it may lose its connection with both of 

 these membranes. In the latter case the cell is said to be liberated, and 

 it may form a branched neuroglial cell of the astrocyte or oligodendric 

 type. In those cases in which the attachment of the inner end of the cell 

 element to the internal limiting membrane is retained, the cell may 

 develop into a definitive ependymal cell, lining either the central canal 

 of the spinal cord or a ventricle of the brain (Fig. 261, a), whereas if the 

 internal connection is lost (Fig. 261, c) and the cell body remains anchored 

 to the external limiting membrane or to the glial membrane covering the 

 pial sheath of vessels penetrating the substance of the brain or spinal 

 cord, the cell may differentiate into a subpial astrocyte, e.g. the cells or 

 fibres of Bergmann, in the molecular layer of the cerebellum, or into a 

 fixed " vascular " astrocyte, as contrasted with the free unattached type, 

 which appears to be connected by its processes with those of neighbouring 

 astrocytes. Besides these three principal types, intermediate forms are 

 frequent : thus, one large process of an astrocyte provided with an 

 expanded foot-plate may preserve its attachment to the sheath of a vessel, 

 while the other small branched processes appear in Golgi preparations 

 to end in free extremities. 



There is, however, a considerable amount of doubt as to whether the 

 apparent continuity of the neuroglial cell-elements is a true uninterrupted 

 connection of one glial cell with another, since many of the modern 

 neurologists hold the view that both the glial cells and the nerve cells 



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