This animation starts with the formation of the nerve fibres in the peripheral nervous system. First, the formation of the myelinated nerve fibres is illustrated.

Schwann cells and a pioneering axon with a growth cone are depicted in an approximately 3-week-old embryo. Then, the axon is approached by the Schwann cells, cross-sectioned, and observed under high magnification. Once sectioned, the axon and the Schwann cell are oriented toward the spectator. The diagram details the cell membrane, nucleus, and cytoplasm of the Schwann cell as well as the axolemma and axoplasm of the axon in an approximately 4-week-old embryo. Between the 4th week and the beginning of the 4th month, the Schwann cell envelopes the axon, and new structural features develop (mesaxon, periaxonal space). The synthesis of the basal lamina by the Schwann cells is also animated. In an approximately 4-month-old fetus, the tongue process starts to wrap around the axon, thus forming the myelin sheath. In an approximately 5-month-old fetus, the myelin sheath becomes thicker due to successive myelin lamellae. The inner and outer mesaxon then form. Finally, in a fetus at term, the animation shows a completely myelinated nerve fibre and details its structure, including the axon with sheaths (sheath of Schwann and endoneurial sheath).Each sheath is described.

The next sequence shows the formation of the myelin sheath as viewed with a transmission electron microscope. A nerve fibre of a fetus during the course of the 4th month, during which the mesaxon starts to wrap around the axon, is delimited and magnified. The diagram shows the unit membranes of the Schwann cell, axolemma, and mesaxon.

Then, the formation of the tongue process, which wraps around the axon, is animated with the appearance of the first major dense line in an approximately 5-month-old fetus. The subdivision of the mesaxon into the inner and outer mesaxon is also shown. In an approximately 8-month-old fetus, the tongue process further wraps around the axon, and the myelin sheath forms. The diagram details the structure of the myelin sheath.

To show the shape of an unrolled Schwann cell, i.e. of an internodal segment, a Schwann cell is viewed from outside. The diagram details the myelinated nerve fibre, two nodes of Ranvier with microvilli, and the Schwann cell with the outer mesaxon. All structures are enveloped by the basal lamina. In the next sequence, the basal lamina is cut out and removed from the Schwann cell. The microvilli belonging to the adjacent Schwann cells are removed from the Ranvier nodes. In the next sequence, the Schwann cell is opened with two hooks inserted in the outer mesaxon, and the newly created features (myelin sheath, cytoplasmic channels) are viewed from outside. Then, the myelin sheath is spreaded. The final diagram shows the trapezoidal from the tongue process, the cytoplasmic borders, and the cytoplasmic channels.

The next sequence animates the formation of an unmyelinated nerve fibre. The diagram shows four axons with growth cones and a portion of the Schwann cell. The following sequence shows the growth of the axons and the Schwann cell moving among them in an approximately 5-week-old embryo. In an approximately 6-week-old embryo, the Schwann cell is situated among four axons. All structures are cut transversally. Between the 6th week and the 4th month, the cytoplasm of the Schwann cell surrounds the axons, and synthesis of the basal lamina is accomplished. The sequence ends in an approximately 8-month-old fetus with the description of the unmyelinated nerve fibre with its sheath of Schwann cells and the endoneurial sheath.

The chapter concerning the formation of the nerve fibres in the central nervous system comprises the formation of myelinated and unmyelinated nerve fibres (naked axons).The first sequence starts in a fetus at term, showing several myelinated internodal segments, an unmyelinated internodal segment and an oligodendrocyte that has furnished a myelin sheath to two neighboring axons. The diagram also shows naked axons and details the structure of a myelinated nerve fibre within the central nervous system. The next sequence depicts myelination of the unmyelinated internodal segment by a flattened trapezoidal oligodendrocyte sheet. The final diagram details the newly obtained structures a few weeks after birth.

Leukodystrophias are described as malformations but not animated.

(This animation is essential for students of medicine, stomatology, veterinary medicine, and biology; for departments of anatomy, histology, embryology, physiology, and cell biology; and for departments or clinics of neurology, neurosurgery, psychiatry; it is also recommended for brain research institutes, departments of neuroscience, and psychiatric nursing schools).