Myelencephalon

Later Development of Embryonic Central Nervous System

In Reference Module in Biomedical Sciences, 2014

Myelencephalon

The myelencephalon , the most caudal subdivision of the rhombencephalon, develops into the medulla oblongata of the adult brain ( Figure 3 ). It is in many respects a transitional structure between the brain and spinal cord, and the parallels between its functional organization and that of the spinal cord are readily apparent ( Figure 4 ). Much of the medulla serves as a conduit for tracts that link the brain with input and output nodes in the spinal cord, but it also contains centers for the regulation of vital functions such as the heartbeat and respiration.

Figure 3. Anatomy of the brain in 9-week (top) and 16-week (bottom) human embryos.

Figure 4. Cross sections through the developing myelencephalon at early (left) and later (right) stages of embryonic development. Motor tracts (from the basal plate) are shown in green; sensory tracts (from the alar plate) are orange.

 Adapted from Sadler, T. (1990). Langman's medical embryology (6th edn.). Baltimore: Williams & Wilkins.

The fundamental arrangement of alar and basal plates with an intervening sulcus limitans is retained almost unchanged in the myelencephalon. The major topographical change from the spinal cord is a pronounced expansion of the roof plate to form the characteristic thin roof overlying the expanded central canal, which in the myelencephalon is called the fourth ventricle (see Figure 4 ).

Special visceral afferent (leading toward the brain) and efferent (leading from the brain) columns of nuclei (aggregations of neuronal cell bodies in the brain) appear in the myelencephalon to accommodate structures derived from the pharyngeal arches.

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Neuroembryology

G.P. Singh , in Essentials of Neuroanesthesia, 2017

Myelencephalon

Myelencephalon gives rise to medulla oblongata. Medulla oblongata has a closed lower part with a central canal and an open upper part forming the caudal area of the fourth ventricle. Like pons, the lateral wall is everted so that the alar plate comes to lie dorsolateral to basal plate and the roof is stretched. The cells of the basal and alar plates give rise to various nuclei. The basal plate forms the hypoglossal nucleus, nucleus ambiguous (which contributes fibers to glossopharyngeal, vagus, and accessory nerves), dorsal nucleus of vagus nerve, and inferior salivatory nucleus of glossopharyngeal nerve. The alar plate contributes to dorsal nucleus of vagus, nucleus of tractus solitarius, spinal nucleus of trigeminal nerve, cochlear and vestibular nuclei. 2 The roof plate of myelencephalon is thin, which is formed by single layer of ependymal cells. Over it lies the piamater derived from vascular mesenchyme. The pia along with ependymal cells forms the telachoroidea into which tuft of capillaries grow. This plexus of capillaries is called choroid plexus that bulges from roof of the fourth ventricle and produces CSF. At three areas the roof the fourth ventricle bulges and finally ruptures forming the foramen of Magendie in the middle and foramen of Luschka on each side. 2

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The Cat

Gerardo De Iuliis PhD , Dino Pulerà MScBMC, CMI , in The Dissection of Vertebrates (Second Edition), 2011

Myelencephalon

The myelencephalon consists of the medulla oblongata, which forms the brain posterior to the metencephalon and connects to the spinal cord (Figures 7.76 and 7.77). The ventral fissure is the median ventral groove of the medulla. To either side are narrow longitudinal bands termed pyramids. Lateral to the anterior part of each pyramid, and just posterior to the pons, lies a trapezoid body.

The remaining cranial nerves are mainly associated with the medulla oblongata, but some of them may be difficult to discern if the meninges have been stripped. Thus, identify the stumps of these nerves before removing the meninges. The nerves to be identified are the abducens, facial, vestibulocochlear, glossopharyngeal, vagus, accessory, and hypoglossal nerves (Figures 7.74, 7.76, and 7.80).

FIGURE 7.80. Cranial nerves and the structures they innervate.

An abducens nerve arises from the anterior part of the medulla, between the trapezoid body and pyramid. The facial nerve arises lateral to the trapezoid body just posterior to the trigeminal nerve, and the vestibulocochlear nerve arises slightly more dorsally, from beneath the flocculonodular lobe. The glossopharyngeal, vagus, and accessory nerves arise in sequence and more posteriorly from the lateral surface of the medulla. The hypoglossal nerve arises farther posteriorly and ventrally. If the meninges have been stripped from the medulla, however, it will be extremely difficult to identify these nerves.

One method of exposing the nerves is to make a midventral cut through the meninges using fine scissors. Begin posteriorly and work your way forward. When you reach the level of the abducens nerves, carefully reflect the meninges. As you peel the meninges back, look for a series of fine rootlets arising from the ventrolateral surface of the posterior part of the medulla. These rootlets merge to form the hypoglossal nerve. Separate it from the meninges and continue peeling the latter. Soon you will note the glossopharyngeal, vagus, and accessory nerves. The accessory will probably be the most prominent at first, because it forms a longitudinal nerve adjacent to the surface of the medulla. Follow it forward, and you will find the stumps of the three nerves arising very close to each other. The glossopharyngeal and vagus arise by a series of small rootlets, so they may not be readily discernable. The accessory nerve arises as a series of very fine rootlets along the posterior part of the medulla. Posterior to it, your specimen may preserve the first spinal nerve. Its origin appears similar to that of the hypoglossal nerve.

Examine the dorsal surface of the medulla. A good part of its roof is covered by a tela choroidea. Remove it to expose the fourth ventricle, the cavity within the medulla that continues forward under the cerebellum. This part of the roof of the fourth ventricle is covered by a separate membranous structure, the medullary velum.

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Development and topography of the nervous system

J.L. Wilkinson OBE, MD, FRCS , in Neuroanatomy for Medical Students (Second Edition), 1992

The hindbrain (rhombencephalon)

The caudal myelencephalon has a central canal and becomes the closed part of the medulla. Rostrally this canal widens into the fourth ventricle: its floor, derived from myelencephalon (medulla) and metencephalon (pons), has a longitudinal sulcus limitans on each side, separating the alar and basal laminae. Cranial nerves with nuclear origins in these laminae differ from spinal nerves in the number and type of their components. In addition to four general components, there are special sensory nuclei concerned with taste (gustatory), hearing (cochlear) and equilibration (vestibular), and special motor nuclei innervating muscle of branchial origin. Some cranial nerves have only one component, either sensory or motor, while others have more, for example the vagus nerve has five. Concerned with input to the developing cerebellum, numerous pontine nuclei and the medullary inferior olivary nucleus migrate ventrally from the alar laminae. Long ascending and descending fibres ultimately traverse the ventral region.

The attenuated roof of the developing fourth ventricle is a single layer of ependymal cells with a thin covering of pia mater; this is a tela choroidea, a vascularized membrane in which a choroid plexus of blood vessels forms ( Fig. 1.5). Cerebrospinal fluid formed within the ventricular system leaves the fourth ventricle through three apertures, one median and two lateral (see p. 88).

The cerebellum grows from rhombic lips, which are bilateral dorsal extensions of the alar plates of the metencephalon ( Fig. 1.5). These meet and fuse over the fourth ventricle's roof, developing there and folding the tela choroidea and its plexuses inwards towards the ventricular cavity. The neocerebellum, phylogenetically recent and forming much of the cerebellar hemispheres, grows rapidly to overlie the more primitive archecerebellum ( Fig. 1.4).

The otocyst, from which the membranous labyrinth of the internal ear develops, is an invagination of a small area of thickened surface ectoderm (otic placode) on each side of the hindbrain; it becomes isolated from the surface.

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The Medulla Oblongata

D.E. Haines , G.A. Mihailoff , in Fundamental Neuroscience for Basic and Clinical Applications (Fifth Edition), 2018

Abstract

The medulla (myelencephalon) is caudally continuous with the C1 level of the spinal cord and rostrally with the caudal pons at the pons-medulla junction. Characteristic structures of the medulla include the pyramid (containing corticospinal fibers), olivary eminence, pre- and post-olivary sulci, restiform body, and tubercles formed by the gracile and cuneate nuclei and fibers of the spinal trigeminal tract. Medullary nuclei derived from the basal plate include the hypoglossal, dorsal motor vagal, and nucleus ambiguus; the vestibular, cochlear, and solitary nuclei, and the spinal trigeminal tract and nucleus arise from the alar plate. The level of the motor decussation is at caudal medullary levels and followed immediately rostral by the sensory decussation (crossing of posterior column-medial lemniscus fibers). At midolivary levels the principal olivary nucleus, pre- and post-olivary sulci, and restiform body, and reticular formation are especially obvious. The hypoglossal, vestibular, and solitary nuclei, and the spinal trigeminal nucleus and tract are also clear at this level. The cochlear nuclei are specifically characteristic of the medulla-pons junction while the spinal trigeminal tract and nucleus extends throughout the length of the medulla. Lesions of the medulla may damage nuclei and/or exiting roots of cranial nerves located therein as well as long tracts traversing the medulla. The blood supply to the medulla is primarily via anterior spinal, vertebral, and posterior inferior cerebellar arteries.

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Development of the Nervous System

W.A. Grow , in Fundamental Neuroscience for Basic and Clinical Applications (Fifth Edition), 2018

Brainstem

The brainstem consists of the myelencephalon (medulla oblongata), the pons (a part of the metencephalon), and the mesencephalon (midbrain). Although developmentally the cerebellum is a part of the metencephalon, it is considered a suprasegmental structure and not a part of the brainstem.

Comparing the rostral part of the spinal cord to the caudal part of the brainstem (medulla oblongata), two features are notable. First, the appearance of the cerebellum and the flaring open of the central canal into the fourth ventricle force the posterior portion of the neural tube (alar plate) to rotate posterolaterally (Fig. 5.14). This rotation results in a lateral to medial orientation of sensory (alar plate) versus motor (basal plate) areas of the developing brainstem, in contrast to their posterior-anterior relationship in the spinal cord. Second, the sulcus limitans, which disappears in the spinal cord during development, is retained as an important landmark in the floor of the fourth ventricle (Fig. 5.14).

The basal plate in the brainstem gives rise to motor cranial nerve nuclei, and the alar plate gives rise to sensory cranial nerve nuclei. As in the spinal cord, these portions of the basal and alar plates differentiate into rostrocaudally oriented cell columns that are associated with functional components.

Derivatives of the basal plate ultimately give rise to cell columns that form the motor nuclei of the brainstem (see Chapter 10 for details). Muscles that arise from paraxial mesoderm, either directly or through the pharyngeal arches, are innervated by somatic efferent (SE) motor neurons located in the brainstem. Within the medulla, these SE nuclei are the hypoglossal nucleus (XII, tongue musculature) and the nucleus ambiguus (IX and X, laryngeal and pharyngeal musculature) (Fig. 5.14). The SE motor nuclei of the pons include the abducens nucleus (VI, lateral rectus muscle); the facial nucleus, commonly called the motor nucleus of the facial nerve (VII, muscles of facial expression); and the trigeminal motor nucleus (V, muscles of mastication). The SE motor nuclei of the midbrain include the oculomotor nucleus (III, extraocular muscles) and trochlear nucleus (IV, superior oblique muscle).

In addition to SE motor cells that innervate skeletal muscle (striated muscle), the basal plate also gives rise to visceral efferent (VE) motor neurons. These neurons provide preganglionic parasympathetic innervation to peripheral ganglia that serve visceral structures, structures composed of smooth muscle, cardiac muscle, or glandular epithelium or combinations thereof. The VE motor neurons of the medulla are found in the dorsal motor vagal nucleus (X) and project to the intermural ganglia of thoracic and abdominal viscera (to the splenic flexure) and the inferior salivatory nucleus (IX), which projects to the otic ganglion. In the caudal pons, the VE motor neurons are found in the superior salivatory nucleus (VII), exit via the facial nerve, and project to the pterygopalatine and submandibular ganglia. The VE cells in the midbrain are located in the Edinger - Westphal preganglionic cell group (EWpg); these cells specifically project to the ciliary ganglion.

Neurons relaying sensory information conveyed on cranial nerves of the brainstem originate from the alar plate. Sensory afferent (SA) input from the vestibular labyrinth (SA proprioception, vestibular ganglion) and from the cochlea (SA exteroception, spiral ganglion ) is conveyed into the vestibular and cochlear nuclei of the caudal pons and medulla. SA information originating from receptors in the skin of the face and portions of the scalp, lining of the oral cavity, nasal membranes, teeth, anterior two thirds of tongue, cornea and conjunctiva, membranes of the maxillary and frontal sinuses, masticatory muscles, periodontal ligament, and dura of anterior and middle fossae centrally terminate in the various portions of the trigeminal nuclei. The spinal trigeminal nucleus receives exteroceptive input, predominantly pain and thermal sense, conveyed on the trigeminal (V) and, to a much lesser degree, on the facial, glossopharyngeal, and vagus nerves. The cell bodies of these primary sensory fibers are in the sensory ganglia on these respective nerves. The fibers conveying SA discriminative touch terminate in the principal sensory nucleus, and those conveying SA proprioception enter the brain and form the mesencephalic tract and have their cell bodies in the mesencephalic nucleus.

Along with the posterolateral-anteromedial division of the brainstem into alar and basal plates, there is a rostral-caudal segmentation of the developing rhombencephalon into rhombomeres. These are clusters of immature neurons separated from each other by thin, transversely oriented bands of neuroepithelial cells. Cells in one rhombomere give rise to a specific motor nucleus (or nuclei) but will not migrate into adjacent rhombomeres. In general, the cell clusters forming the rhombomeres represent the rostral continuation of the alar and basal plates of the developing spinal cord. Indeed, motor nuclei originate from specific rhombomeres, and input from the corresponding sensory ganglia enters the corresponding rhombomere.

Rhombomeres are also sites of homeobox gene expression. These genes (abbreviated Hox) are "master switches" that control the formation of large blocks of tissue. One example of a congenital defect affecting developmental sequences within the rhombomeres is the Möbius syndrome. This syndrome is characterized by a variety of developmental defects, including (1) aplasia (poor development of, or absence of) of the abducens and facial motor nuclei, with the correlated motor deficits of facial and eye movement; (2) weakness of the muscles innervated by the oculomotor and trochlear nerves; (3) atrophy or weakness of the tongue and weakness of the masticatory muscles; and (4) a variety of skeletal defects affecting the face and body. A rare disorder, pontocerebellar hypoplasia, is a congenital but progressive atrophy of the pons and cerebellum that causes hypotonia, poor feeding, growth retardation, and profound mental retardation.

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Vertebrate Nervous System, Development of the

R.D. Fields , in Encyclopedia of the Neurological Sciences (Second Edition), 2014

Hindbrain Development

The rhombencephalon divides into the metencephalon and myelencephalon. The metencephalon grows into the cerebellum and pons of the adult brain, and the myelencephalon will become the medulla of the adult brain, containing a large fourth ventricle and posterior choroid plexus in the thin roof. The hindbrain is a conduit for information passing between the forebrain and the spinal cord. Input from the pons relays information from the cerebral cortex to the cerebellum to control goal-directed movement. Axons of the auditory nerve synapse first on cells in the cochlear nuclei of the medulla before projecting to the tectum of the midbrain (inferior colliculus). Touch and taste are controlled by neurons in the medulla that relay sensory information from the spinal cord to the thalamus.

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Insights on nervous system biology and anatomy

Madalena Esteves , ... Hugo Leite-Almeida , in Handbook of Innovations in Central Nervous System Regenerative Medicine, 2020

1.3.1 Spinal cord

The spinal cord develops from the neural tube caudal to the myelencephalon. The lumen of the neural tube develops into the central canal, which is continuous with the ventricular system ( Section 1.3.3.2). The spinal cord is segmentally organized in five levels—cervical (C), thoracic (T), lumbar (L), sacral (S), and coccygeal (Co)—each further divided in segments that vary across species: human: C1–C8, T1–T12, L1–L5, S1–S5, and Co1; rat/mice: C1–C8, T1–T13, L1–L6, S1–S4, and Co1–Co3. Each segment contains a bilateral pair of nerves (31 pairs in the human) with an anterior/ventral and a posterior/dorsal root with motor and sensitive functions, respectively, as initially described Magendie and Bell (see Introduction). The gray matter occupies the core of the spinal cord with an "H" shaped form and has in its center the central canal [42]. The posterior horns of the "H," contain neurons that receive peripheral inputs from the dorsal roots, thta is, sensory information, and whose cell bodies are located in nearby ganglia [the dorsal root ganglia (DRG)]. These peripheral neurons develop from the neural crest (Section 1.2.3) and are the primary afferents to the CNS conveying information related to mechanical and chemical stimuli, temperature, light touch, and pain. Motor neurons occupy the ventral horns and project through the ventral root to enervate skeletal muscles.

An additional group of neurons, the intermediolateral cell columns, present at T1–L3 levels (human), provide sympathetic enervation. The axons of these cells also project through the ventral roots branching soon after to form the white ramus, which then enters in the corresponding ganglia from the sympathetic chain ganglia. There, these preganglionic neurons will synapse on postganglionic neurons that project via the white ramus, then reaching the dorsal and ventral nerves from the somatic division on the spinal nerve and providing sympathetic enervation on the target organs. The preganglionic neurons of the parasympathetic division, on the other hand, are located in the brainstem and sacral levels S2–S4. Contrary to the sympathetic division, the postganglionic neurons are located in the walls of the target organs.

As stated earlier, the spinal cord develops from the caudal portion of the neural tube. There, neuroepithelial cells proliferate in the ventricular layer, that is, adjacent to the lumen of the central canal. These neurons will migrate peripherally, forming the mantle layer, and then their axons will grow even more peripherally, forming the marginal layer. The mantle and marginal layers correspond to the gray and white matter of the mature spinal cord. Neurons organize in dorsal and ventral plates/columns, which will develop into sensory and motor neurons, respectively. This functional diversity in spinal cord emerges early in development, and is strongly conditioned by the dorsal-to-ventral position of the developing neurons and the resulting exposure to a number of morphogenes, notably Sonic Hedgehog (Shh) produced by nearby organizers (floor plate, notochord; Section 1.2.2) [18]. At the periphery, surrounding the gray matter core, a number of ascending and descending tracts convey information into/from other spinal levels and into/from the brain (consult [43,44] for an overview of the human spinal cord anatomy).

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The Brain and Spinal Cord

Robert Lewis Maynard , Noel Downes , in Anatomy and Histology of the Laboratory Rat in Toxicology and Biomedical Research, 2019

The Hindbrain or Rhombencephalon

Development of the rhombencephalon leads to formation of the medulla oblongata (myelencephalon: the surface is white), the pons and cerebellum (metencephalon). The word pons means a bridge, and it does indeed provide a bridge of nerve fibres connecting the two halves of the cerebellum via the middle cerebellar peduncles. The medulla, pons and mesencephalon make up the brainstem. The medulla is sometimes referred to as the bulb because it appears as an anterior expansion of the spinal cord.

The central canal widens during early development. The roof of the fourth ventricle becomes very thin and rises at its centre like a rather flat tent. The cerebellum grows from the dorsal edges of the rhombencephalon and forms two hemispheres that fuse in the midline over the roof of the fourth ventricle. The pons, like the cerebellum, is much less well developed in the rat than in man. The cerebellum is also connected on each side to the brainstem by superior and inferior peduncles. The superior mainly carries fibres leaving the cerebellum; the inferior carries fibres running to the cerebellum from the spinal cord and nuclei of the medulla.

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Development of the Nervous System

Alexander de Lahunta DVM, PhD, DACVIM, DACVP , Eric Glass MS, DVM, DACVIM (Neurology) , in Veterinary Neuroanatomy and Clinical Neurology (Third Edition), 2009

METENCEPHALON: CEREBELLUM AND PONS (See Figs. 2-9 and 2-10)

The initial development of the metencephalon is comparable to that of the myelencephalon. Cranial nerve V, the trigeminal nerve, is associated with this segment of the brainstem ( Fig. 3-11). Its motor neurons arise in the basal plate mantle layer, migrate a short way ventrolaterally into the parenchyma of the pons, and form a small, well-defined motor nucleus. These neurons function in the general somatic efferent system and innervate the muscles of mastication derived from the somitomeres in branchial arch 1. The sensory neurons in cranial nerve V are derived primarily from neural crest cells and form the trigeminal ganglion. Most of these neurons are GSA and their dendritic zones are widely spread over the entire surface of the head and to the inner surface of the upper respiratory and digestive systems. A smaller component is composed of general proprioceptive neurons for the muscles and joints in the head region. These sensory neurons greatly outnumber the motor neurons. Therefore, when these sensory axons enter the alar plate region of the metencephalon, they spread out for a short distance rostrally and for a long distance caudally, forming the spinal tract of trigeminal nerve in the pons and medulla. These axons terminate in telodendria at synapses in the alar plate neurons, which form the sensory pontine nucleus of the trigeminal nerve in the pons and the nucleus of the spinal tract of the trigeminal nerve in the medulla. This spinal tract and nucleus extends the full length of the medulla, where caudally they meet the comparable functional neurons developing in the first cervical spinal nerves and spinal cord segment (see Fig. 3-11).

The cerebellum, or dorsal metencephalon, is formed primarily from the proliferation of the germinal epithelial cells of the alar plate, forming the rhombic lip (see Figs. 3-11 and 3-12). This growth dorsolaterally from each side overgrows the roof plate of the fourth ventricle so that the cerebellum forms part of the dorsal boundary of the fourth ventricle in the metencephalon. The development of the cerebellar cortex and nuclei are described in Chapter 13. The ventral metencephalon is the pons. A ventral migration of alar plate mantle layer neurons forms the pontine nucleus (see Fig. 3-12). The axons of these neurons cross the midline and course dorsally into the cerebellum. This forms the transverse fibers of the pons, which demarcate the ventral surface of the pons and the middle cerebellar peduncle.

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