Book - Developmental Anatomy 1924-14

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Arey LB. Developmental Anatomy. (1924) W.B. Saunders Company, Philadelphia.

   Developmental Anatomy 1924: 1 The Germ Cells and Fertilization | 2 Cleavage and the Origin of the Germ Layers | 3 Implantation and Fetal Membranes | 4 Age, Body Form and Growth Changes | 5 The Digestive System | 6 The Respiratory System | 7 The Mesenteries and Coelom | 8 The Urogenital System | 9 The Vascular System | 10 The Skeletal System | 11 The Muscular System | 12 The Integumentary System | 13 The Central Nervous System | 14 The Peripheral Nervous System | 15 The Sense Organs | C16 The Study of Chick Embryos | 17 The Study of Pig Embryos | Figures Leslie Arey.jpg
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Chapter XIV The Peripheral Nervous System

The peripheral nervous system consists of bundles of myelinated and unmyelinated nerve fibers, and aggregations of nerve , cells, the ganglia. The fibers are of two types: afferent fibers, which carry sensory impulses to the central nervous system, and efferent fibers, which carry motor impulses away from the nervous centers. The peripheral afferent fibers originate from nerve cells located in the ganglion crest (p. 241) outside the neural tube. The peripheral efferent fibers grow from neuroblasts of the basal plate and emerge ventro-laterally out of the neural tube. Fibers of one or both sorts converge into distinct segmental cords called nerves. These belong to two main systems: the cerebro-spinal series and the sympathetic division.

File:Arey1924 fig278.jpg

Fig. 278. Diagrammatic section through the embryonic myelencephalon, showing the arrangement of the functional cell columns and the origin, course and termination of the functional components of the cranial nerves (Ranson).

Functional Classification of Fibers

The early observation that sensory impulses travel in the dorsal root fibers and motor impulses in ventral root fibers (Fig. 281) has been supplemented by a more complete analysis (Fig. 278). All neurons fall within four chief functional groups, which are in turn subdivided as indicated in the accompanying table. No single nerve contains representatives of every fiber type; those components designated - special - are peculiar to the cranial nerves alone.

1. Somatic afferent.

.(а) General (Fibers ending chiefly in the integument).

.(б) Special (Neurons of the eye and ear).

.2. Visceral afferent.

.(а) General (Sympathetic fibers conducting sensory impulses from the viscera).

.(б) Special (Fibers to the olfactory membrane and taste buds).

.3. Somatic efferent. (Fibers ending on skeletal muscle).

.4. Visceral efferent.

.(а) General (Fibers ending about sympathetic ganglion cells, which in turn control smooth and cardiac muscle and glandular tissue).

.(б) Special (Cranial nerve fibers ending directly on striated, branchial-arch musculature) .

The spinal Nerves

The spinal nerves are arranged segmentally, and each is attached to the spinal cord by a dorsal (posterior) root, with which is associated a spinal ganglion, and a ventral (anterior) root (Fig. 246). In embryos of 4 mm., the ventral roots are already developing as outgrowths of neuroblasts in the mantle layer of the spinal cord (Fig. 279). The spinal ganglia are represented as enlargements along the continuous ganglion crest. At the stage of 7 mm., or five weeks, the cells of the spinal ganglia begin to develop centrally directed processes which enter the marginal zone of the cord as dorsal root fibers (Fig. 280). Peripheral processes of the ganglion cells soon join the ventral root fibers in the trunk of the nerve (Fig. 246).

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Fig. 279. The cerebro-spinal nerves of a 4 mm human embryo (Streeter). X 17. Ci-6,

Ventral roots of cervical spinal nerves.

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Fig. 280. The cerebro-spinal nerves of a 7 mm human embryo (Streeter). X i7

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Fig. 281. Transverse section of a 10 mm human embryo, showing a spinal nerve and functional components (Prentiss).

At 10 mm. (Fig. 282), the cellular bridges of the ganglion crest, which hitherto interconnect spinal ganglia, have begun to disappear, and the several parts of a typical spinal nerve are evident (Figs. 246 and 281). The trunk of the nerve, just ventral to the union of the dorsal and ventral roots, gives off laterally the dorsal ramus, the fibers of which supply the dorsal muscles and integument. The ventral ramus, continuing, gives off mesially the ramus communicans to the sympathetic ganglion, and divides into the lateral and ventral terminal rami. The efferent fibers of these rami supply the muscles of the lateral and ventral body wall, and the afferent fibers end in the integument of the same regions.

File:Arey1924 fig282.jpg

Fig. 282. The nervous system of a 10 mm human embryo (Streeter). X 12.

Nerve Plexuses

At the points where the anterior and lateral terminal rami arise, connecting loops may extend from one spinal nerve to another. Thus, in the neck region, superficial and deep cervical plexuses are formed. The deep cervical plexus gives rise to the ansa hypoglossi and the phrenic nerve (Fig. 282).

The nerves supplying the arm and leg also form plexuses that first appear at 7 mm. (Fig. 280) and are clearly indicated in embryos of 10 mm. (Fig. 282). The trunks of the last four cervical nerves and of the first thoracic unite into a flattened plate, the anlage of the brachial plexus. From this plate nervous cords extend into the intermuscular spaces and end in the premuscle masses. The developing skeleton of the shoulder splits the Brachial plexus into dorsal and ventral laminae. From the dorsal lamina arise the musculo-cutaneous, median, and ulna nerves; from the ventral lamina, the axillary and radial nerves.

The lumbar and sacral nerves to the leg unite in a plate-like structure, the anlage of the lumbosacral plexus (Fig. 282). The plate is divided by the skeletal elements of the pelvis and femur into two lateral and two median trunks. Of the cranial pair, the lateral becomes the femoral nerve; the median, the obturator nerve. The caudal pair constitutes the primitive sciatic nerve; the lateral trunk will be the peroneal nerve, the median trunk the tibial.

Twelve pairs of cranial nerves appear at about the end of the first month. They are not arranged segmentally and attempts to interpret them as serial homologues of spinal nerves fail. In addition to the general sensory and motor components of spinal nerves, the cranial group contains special fibers to the major sense organs and to muscles derived from branchial arches. The several sensory and motor nuclei are arranged in definite longitudinal columns within their respective alar and basal plates (Fig. 278). Unlike the spinal series, the cranial nerves vary widely in functional composition. Those of the first two groups in the subjoined table have but a single kind of fiber; on the contrary, the members of the third group are all mixed, as witness the ninth and tenth which contain five different types each. The cranial nerves fall roughly into three functional groups; .

The Cranial Nerves

Special Sensouv .

Somatic Motor .

Visceral Sensory .

AND Motor .

I. Olfactory.

II. Optic.

III. Oculomotor.

V. Trigeminal.

VII. Facial.

IX. Glossopharyngeal.

X. Vagus comple.x (including XI. Spinal Accessory).

VIII. Acoustic.

IV. Trochlear. VI. Abducens.

XII. Hypoglossal.

(A) The Special Sensory Nerves

1. The Olfactory Nerve, though purely sensory, has no ganglion. Its nerve cells lie at first in the epithelium of the nose and are of the bipolar type. From them, peripheral processes develop which end directly at the surface of the olfactory epithelium (Fig. 283). Central processes grow backward during the fifth week and form the strands of the olfactory nerve, around which the cribriform plate later develops. They end in the glomeruli of the olfactory bulb in contact with dendrites of the mitral cells, or olfactory neurons of the second order. Some olfactory cells migrate from the epithelium, with which, however, they retain peripheral connections. Such bipolar elements, found along the entire course of the nerve, resemble ordinary dorsal ganglion cells. The olfactory nerve fibers are peculiar in that they remain unmyelinated. Nerve fibers from the epithelium of the vestigial vomero-nasal organ (of Jacobson) also end in the olfactory bulb.

The ganglionated terminal nerve courses in close association with the olfactory nerve. Its unmyelinated fibers end in the epithelium of the vomero-nasal organ and of the septum.

Although evidently a distinct nerve, its relations and significance are obscure.

2. The Optic Nerve is formed by fibers which grow from neuroblasts in the nervous layer of the retina. Since the retina differentiates from the evaginated wall of the fore-brain (Fig. 264), the optic nerve is not a true peripheral nerve, but belongs to the central system of tracts. The neuroblasts from which the optic nerve fibers develop constitute the ganglion cell layer of the retina (Fig. 301). During the sixth and seventh weeks these cells give rise to central processes which form a nerve fiber layer on the inner side of the retina. The optic fibers converge to the optic stalk and grow through its wall back to the brain (Fig. 284 A). The cells of the optic stalk are converted into a neuroglia framework and its cavity is obliterated {B). In the floor of the fore-brain, at the boundar}" between telencephalon and diencephalon, the fibers from the median half of each retina at about the end of the second month cross to the opposite side, and this decussation constitutes the optic cluasma. The crossed and uncrossed fibers constitute the optic tract (Fig. 271).

Efferent fibers, terminating in the inner reticular layer of the retina, are present also. In certain fishes, where their function has been studied, these fibers resemble visceral efferent components (Arey, 1916).

File:Arey1924 fig283.jpg

Fig. 283. Diagram of the relations of the fibers in the olfactory nerve.

8. The Acoustic Nerve is composed of fibers which grow from the acoustic ganglion. Its cells arise directly from the brain wall of 2 mm. embryos (Bartelmez, 1922) and soon lie just cranial to the otic vesicle (Fig. 305). The cells become bipolar, central processes uniting the ganglion to the tuberculum acusticum of the myelencephalon and peripheral fibers connecting it with the wall of the otocyst.

The acoustic ganglion is differentiated into vestibula?- and spiral ganglia (Fig. 285). The original ganglion elongates and is subdivided into superior and inferior portions in 7 mm. embryos. The superior part supplies fibers to the utriculus and to the ampullae of the anterior and lateral semicircular ducts. Part of the inferior portion innervates the sacculus and the ampulla of the posterior semicircular duct, and this portion, together with the entire pars superior, constitutes the vestibular ganglion. Most of the pars inferior, however, differentiates into the spiral ganglion, the peripheral fibers of which innervate the hair cells of the spiral organ (of Corti) in the cochlea. The spiral ganglion appears in 9 mm. embryos and conforms to the spiral turns of the cochlea, hence its name. Its central nerve fibers form the cochlear division of the acoustic nerve. This is distinctly separated from the central fibers of the vestibular ganglion which constitute the vestibular division of the acoustic nerve, the fibers of which are equilibratory in function. The pars inferior of the vestibular ganglion becomes closely connected with the n. cochlearis, and thus in the adult it appears as though the sacculus and posterior ampulla were supplied by the cochlear nerve.

File:Arey1924 fig284.jpg

Fig. 284. Transverse sections through the human optic stalk during its transformation into the optic nerve (redrawn after Bach and Seefelder). /I, 14.5 mm.; B, 19 mm.

File:Arey1924 fig285.jpg

Fig. 285. The development of the acoustic ganglion and nerve (Streeter). The vestibular ganglion is finely stippled, the spiral ganglion coarsely stippled.

(B) The Somatic Motor Nerves

This group, consisting of the three nerves to the eye muscles and the n. hypoglossus, is purely motor, the fibers originating from neuroblasts of the basal plate of the brain stem, near the midplane. They are regarded as homologues of the ventral motor roots of the spinal cord, but they have lost their segmental arrangement and are otherwise modified. The nuclei of origin of these nerves are colored red in Fig. 287.

3. The Oculomotor Nerve develops from neuroblasts in the basal plate of the mesencephalon (Fig. 260 B). The fibers emerge as small fascicles on the ventral surface of the mid-brain, in the concavity due to the cephalic flexure (Figs. 282 and 287). The fascicles converge, form the trunk of the nerve, and end in the premuscle masses of the eye. The nerve eventually supplies all of the extrinsic muscles of the eye, save the superior oblique and external rectus.

4. The Trochlear Nerve fibers arise from neuroblasts of the basal plate, located just caudal to the nucleus of origin of the oculomotor nerve (Fig. 287). They are directed dorsad, curve around the cerebral aqueduct, and, crossing in its roof, emerge at the isthmus (Fig. 260 A). From this superficial origin, each passes ventrad as a slender nerve which connects with the anlage of the superior oblique muscle of the eye (Fig. 282).

6. The Abducens Nerve takes origin from a nucleus of cells in the basal plate of the myelencephalon, located directly beneath the fourth neuromere (Figs. 282 and 287). The converging fibers emerge ventrally at a point caudal to the future pons, and, as a single trunk, course cranially, mesial to the semilunar ganglion, finally ending in the anlage of the external rectus muscle of the eye. Vestigial rootlets of the abducens and hypoglossal nerve tend to fill in the gap between these two nerves.

12. The Hypoglossal Nerve results from the fusion of the ventral root fibers of three to five precervical nerves. Its fibers originate from neuroblasts of the basal plate and emerge from the ventral wall of the myelencephalon in several groups (Figs. 279 and 287). In embryos of 7 mm., the fibers have converged ventrally to form the trunk of the nerve (Fig. 289). Later, they grow cranially, lateral to the ganglion nodosum, and eventually end in the muscle fibers of the tongue (Fig. 282).^ The nerve in its development unites with the first three cervical nerves to form the ansa hypoglossi.

That the hypoglossus is a composite nerve, homologous with the ventral roots of the spinal nerves, is shown: (1) by the segmental origin of its fibers; (2) from the fact that its nucleus of origin is a cranial continuation of the ventral gray column, or nucleus of origin for the ventral spinal roots; (3) from the fact that in mammalian embryos (pig; sheep; cat; etc.) rudimentary dorsal ganglia are developed, one of which at least (Froriep - s ganglion) sends a dorsal root to the hypoglossus. In human embryos, Froriep - s ganglion may be present as a rudimentary structure (Figs. 282 and 286), or it may be absent and the ganglion of the first cervical nerve may also degenerate and disappear. In pig embryos there are one to four accessory ganglia (including Froriep - s) from which dorsal roots extend to the root fascicles of the hypoglossal nerve (Fig. 391).

(C) The Visceral Mixed Nerves

The motor roots of this group arise in a lateral series, distinct from the dorsal and ventral roots already described (Figs. 256 and 278). The trigeminal nerve contains not only visceral fibers but numerous somatic sensory neurons which supply the integument of the head and face. The facial, glossopharyngeal, and vagus nerves are essentially visceral in function. Their sensory fibers innervate the sense organs of the branchial arches and viscera. A few somatic sensory fibers, having the same origin and course in the myelencephalon, pass to the adjacent integument.

5. The Trigeminal Nerve is chiefly sensory. Its large semilunar ganglion, a derivative of the ganglion crest, arises at the very beginning of the hind brain (Fig. 280). Centrally directed processes form the large sensory root that enters the wall of the hind-brain at the level of the pontine flexure (Fig. 282). These fibers fork, and then course cranially and caudally in the alar plate of the myelencephalon; the caudal fibers constitute the descending spinal tract of the trigeminal nerve (Fig. 287). The peripheral processes separate into three large divisions, the ophthalmic, maxillary, and mandibular nerves, and supply the integument of the head and face and the epithelium of the mouth and tongue.

The motor fibers of the trigeminal nerve arise largely from a dorsal motor nucleus that lies opjiosite the point at which the sensory fibers enter the brain wall (Fig. 287). In the embryo, these fibers emerge as a separate motor root, course along the mesial side of the semilunar ganglion, and, as a distinct trunk, supply the premuscle masses which later form the muscles of mastication. From the chief motor nucleus, a line of cells, extending cranially into the mesencephalon, constitutes a second source of origin for motor fibers. In the adult, the motor fibers form a part of the mandibular division of the nerve.

File:Arey1924 fig287.jpg

Fig. 287. Reconstruction of the nuclei of origin and termination of the cranial nerves in a 10 mm. human embryo (Streeter). X 30. The somatic motor nuclei are colored red.

7. The Facial Nerve is composed for the most part of efferent fibers that supply the facial muscles of expression. In 10 mm. embryos these   fibers arise from a cluster of neuroblasts in the basal plate of the myelen- - 7 cephalon, located beneath the third neuromere (Fig. 287). The fibers-pass laterally, and emerge just mesial to the acoustic ganglion. The motor trunk then continues caudally and is lost in the tissue of the hyoid branchial arch, tissue which later gives rise to the muscles of expression (Fig. 282). The sensory fibers of the facial nerve arise from the eells of the geniculate ganglion, which Bartelmez (1922) asserts is a derivative of the brain wall rather than of the ganglion crest. This ganglion is present in 7 mm. embryos (Fig. 280), located cranial to the acoustic ganglion. The centrally directed processes of the geniculate ganglion enter the alar plate and form part of the solitary tract. Some peripheral fibers course with motor fibers in the chorda tympani, join the mandibular branch of the trigeminal nerve, and end in the sense organs of the tongue. Other sensory fibers form later the great superficial petrosal nerve, which extends to the spheno-palatine ganglion.

The motor fibers of the facialis at first grow straight laterad, passing cranial to the nucleus of the abducens. The nuclei of the two nerves later shift their positions, that of the facial nerve moving caudad and laterad, while the nucleus of the abducens shifts cephalad. x\s a result, the motor root of the facial nerve bends around the nucleus of the abducens, producing the genu, or knee, of the former. Together, they produce the rounded eminence in the floor of the fourth ventricle known as the facial colliculus.

9. The Glossopharyngeal Nerve takes its superficial origin just caudal to the otic vesicle (Figs. 280, 286 and 288). Its few motor fibers arise from neuroblasts in the basal plate beneath the fifth neuromeric groove. These neuroblasts form part of the nucleus amhiguus, a nucleus of origin which the glossopharyngeal shares with the vagus (Fig. 287). The motor fibers course laterally beneath the spinal tract of the trigeminal nerve and emerge to form the trunk of the nerve. These fibers later supply the muscles of the pharynx derived from the third branchial arch.

The sensory fibers of the glossopharyngeal nerve arise from two ganglia, the su perior, or root ganglion, and the petrosal, or trunk ganglion (Figs. 282 and 288). These fibers constitute the greater part of the nerve, and divide peripherally to form the tympanic and lingual rami to the second and third branchial arches. Centrally, the sensory fibers enter the alar plate of the myelencephalon and join similar fibers of the facial nerve coursing caudally in the solitary tract.

10. u. The Vagus and Spinal Accessory. - The vagus, like the hypoglossus, is composite. It represents the union of several nerves which supply the branchial arches of aquatic vertebrates (Figs. 282 and 288). The more caudal fascicles of motor fibers take their origin in the lateral gray column of the cervical cord, as far back as the fourth cervical segment. These fibers emerge laterally, and, as the spinal accessory trunk (in anatomy a distinct nerve), course cephalad along the line of the neural crest (Figs. 280, 282 and 288). Other motor fibers originate from the neuroblasts of the nucleus ambiguus of the myelencephalon (Fig. 287). Still others arise from a dorsal motor nucleus that lies median in position. The fibers from these two sources emerge laterally as separate fascicles and join the fibers of the spinal accessory in the trunk of the vagus nerve. The accessory fibers soon leave the trunk of the vagus and are distributed laterally and caudally to the vdsceral premuscle masses which later form the sterno-mastoid and trapezius muscles of the shoulder (Fig. 282). Other motor fibers of the vagus supply muscle fibers of the pharynx and laryn.

X As the vagus is a composite nerve, it has several root ganglia which arise as enlargements along the course of the ganglion crest (Figs. 282 and 288). The more cranial of these is the jugular ganglion. The others, termed accessory ganglia, are vestigial structures and not segmentally arranged. In addition to the root ganglia of the vagus, there is the nodose ganglion of the trunk (Fig. 288). The trunk ganglia of both the vagus and glossopharyngeal nerves are believed to be derivatives of the ganglion crest, their cells migrating ventrad in early stages. The central processes from the neuroblasts of the vagus ganglia enter the wall of the myelencephalon, turn caudad, and, with the sensory fibers of the facial and glossopharyngeal nerves, complete the solitary tract. The peripheral processes of the ganglion cells form the greater part of the vagus trunks after the separation from it of the spinal accessory fibers.

File:Arey1924 fig288.jpg

Fig. 288. The peripheral nerves in the occipital region of an 18 mm. human embryo (Streeter).X 17


In aquatic vertebrates, special somatic sensory fibers from the lateral line organs join the facial, glossopharyngeal, and vagus nerves, and their ganglion cells form parts of the geniculate, petrosal, and nodose ganglia. In human embryos, the organs of the lateral line are represented by ectodermal thickenings, or placodes, which occur temporarily over these ganglia. The placode of the hyoid arch is said to contribute cells to the substance of the geniculate ganglion, and possibly the ganglia of the ninth and tenth nerves receive similar additions (Bartelmez, 1922).

Neurobiotaxis. - The positions of the motor nuclei vary widely in the several vertebrate groups. This is because the cell bodies of motor neurons migrate during development toward the centers from which their principal afferent impulses proceed. Such a reSponse to some unknown attractive force is called neurobiotaxis.

The Sympathetic Nervous System

The sympathetic nervous system is composed of a series of ganglia and peripheral nerves, the fibers of which supply gland cells and the cardiac- and smooth muscle fibers of the viscera and blood vessels. The nerve cells are of the multipolar ganglion type and their axons remain unmyelinated.

Sympathetic ganglia arise from cells of the ganglion crest (and the neural tube), which, at 10 mm., migrate distally along the nerve roots and accumulate in masses dorso-lateral to the aorta (Figs. 281 and 406). In the region of the trunk, these paired, segmental clusters unite from segment to segment to form longitudinal cords, which, at 10 mm., are converted into nerve fibers that thereafter link the ganglia in a commissural manner (Fig. 289). The resultant ganglionated cords are the sympathetic trunks.

Root fibers from the cerebro-spinal nerves pass into the adjacent ganglia of the sympathetic trunks (Figs. 289 and 409). Some are efferent and terminate about the ganglion cells, whence their impulses are relayed by unmyelinated sympathetic neurons to their destination (Fig. 281). Others are afferent, bringing visceral sensory impulses directly from the viscera to the spinal ganglia and central nervous system. Both fiber types acquire myelin sheaths and so constitute the white communicating rami. Unmyelinated sympathetic fibers also grow back into the spinal nerves by separate gray communicating rami. These are efferent in function and are distributed with the spinal nerves.

In addition to the primary ganglia of the paired sympathetic trunks, there are other more peripheral ones, known as collateral ganglia, belonging to the great prevertebral plexuses, such as the cardiac, coeliac, and hypogastric (Fig. 289). Still further distad, are the terminal ganglia, located near or even within the structures they innervate; this group includes the ciliary and cardiac ganglia and the small ganglion masses of the myenteric and submucous plexuses. Each cell in these several types of ganglion is in direct relation to the axon of a cerebro-spinal cell, so that every sympathe.

File:Arey1924 fig289.jpg

Fig. 289. The sympathetic system of a 16 mm. human embryo (Streeter). X 7. The ganglionated trunk is heavily shaded, cil., Ciliary ganglion; coe., cceliac artery and plexus; lit., heart and cardiac plexus; ot., otic ganglion; pet., petrosal ganglion; s-vi., submaxillary ganglion; sph-p., spheno-jmlatine ganglion.

The neuron forms a terminal link in a chain whose first link is a neuron belonging to the central nervous system.

The ganglion cells of the prevertebral plexuses originate, in embryos of 7 mm., like those of the sympathetic trunks, and differ only in migrating greater distances. The terminal ganglia related to the cardiac, pulmonary, and upper enteric plexuses arise at about the same time from cells of cerebro-spinal origin which advance peripherally along the vagus nerves.

The adult cervical sympathetic ganglia represent fusions between the primitive ganglionic masses of this region (Figs. 288 and 289). The sympathetic ganglia related to the brain are from the first unsegmental. They are derived chiefly from the primitive semilunar ganglia, although the brain wall and the geniculate and petrosal ganglia also contribute (Fig. 289).

The Chromaffin Bodies and Suprarenal Gland

Certain cells of the sympathetic ganglia are transformed into peculiar glands, rather than into neurones. The internal secretion formed by these elements causes them to stain brown when treated with chrome salts - hence the designation, chromaffin cells. Cells of this type, derived from the ganglionated cord of the sympathetic system, give rise to structures known as chromaffin bodies. Chromaffin derivatives of the coeliac plexus, together with mesenchymal tissue, also form the suprarenal gland.

File:Arey1924 fig290.jpg

Fig. 290. Section through a chromaffin body in a human fetus of ten weeks (after Kohn). .X 450.

The chromaffin bodies of the ganglionated cords are rounded, cellular masses partly embedded in the dorsal surfaces of the ganglia, and so termed paraganglia (Fig. 290). They appear during the third month, and, at birth, may attain a diameter of i to 1.5 mm. In number they vary from one to several for each ganglion. Similar chromaffin bodies may occur in all the larger sympathetic plexuses. The largest, found in the abdominal sympathetic plexuses, are the aortic chromaffin bodies{ of Zuckerkandl). These occur in embryos of seven weeks, on either side of the inferior mesenteric artery, ventral to the aorta and mesial to the metanejihros. At birth, they attain a length of about i cm. and are composed of cords of chromaffin cells intermingled with strands of connective tissue, the whole being surrounded by a connective-tissue capsule. After birth the chromaffin bodies degenerate, but do not disappear entirely. Associated with the intercarotid sympathetic plexus is a highly vascular chromaffin organ known as the carotid body. Its analge has been first observed in emliryos of seven weeks.

The Suprarenal Gland has a double origin. The cortex is derived from mesoderm, the medulla from chromaffin tissue. In an embryo of 6 mm., the anlage of the cortex begins to form from ingrowing buds of the peritoneal mesothelium; this proliferation occurs on each side of the mesentery, near its root. At about 9 mm. the paired glands are definite organs and their vascular structure is evident (Fig. 114). The anlages of the suprarenals early project from the dorsal wall of the coelom, between the mesonephros and mesentery; here they become relatively huge organs (Figs. 145, 154 and 155). The differentiation of the cortex into its three characteristic layers is not completed until between the second and third years. The inner reticular zone is formed first, the fasciculate zone next, and finally the glomerular zone appears during the third month.

File:Arey1924 fig291.jpg

Fig. 291. Section through the right suprarenal gland of a 16 mm. human embryo (Bryce). *, Invading groups of chromaffin cells.

The chromaffin cells of the medulla are derived from the coeliac plexus of the sympathetic system. In embryos of seven weeks, whien the cortex is already prominent, masses of these cells begin to migrate from the median side of the suprarenal anlage to a central position (Fig. 291). Such penetration probably continues until after birth. The primitive chromaffin cells are small and stain intensely.


Portions of the suprarenal anlage separate frequently from the parent gland and form accessory suprarcnals. As a rule, such accessory glands are composed only of cortical substance; they may migrate some distance from their original position, accompanying the genital glands. In fishes, the cortex and medulla occur normally as separate organs.

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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)
   Developmental Anatomy 1924: 1 The Germ Cells and Fertilization | 2 Cleavage and the Origin of the Germ Layers | 3 Implantation and Fetal Membranes | 4 Age, Body Form and Growth Changes | 5 The Digestive System | 6 The Respiratory System | 7 The Mesenteries and Coelom | 8 The Urogenital System | 9 The Vascular System | 10 The Skeletal System | 11 The Muscular System | 12 The Integumentary System | 13 The Central Nervous System | 14 The Peripheral Nervous System | 15 The Sense Organs | C16 The Study of Chick Embryos | 17 The Study of Pig Embryos | Figures Leslie Arey.jpg


Arey LB. Developmental Anatomy. (1924) W.B. Saunders Company, Philadelphia.

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