Book - The Frog Its Reproduction and Development 8

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Rugh R. Book - The Frog Its Reproduction and Development. (1951) The Blakiston Company.

Frog Development (1951): Introduction | Rana pipiens | Reproductive System | Fertilization | Cleavage | Blastulation | Gastrulation | Neurulation | Early Embryo Changes | Later Embryo or Larva | Ectodermal Derivatives | Endodermal Derivatives | Mesodermal Derivatives | Summary of Organ Appearance | Glossary | Bibliography | Figures
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Pages where the terms "Historic Textbook" and "Historic Embryology" 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 and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Chapter 8 - Neurulation and Early Organogeny

Formation of the Neural Tube

The axes of the embryo are altered by the development of the archenteron. The antero-posterior axis is made obvious by the development of the neural axis. Either the roof of the archenteron and/or the notochord induces, in the overlying ectoderm, the formation of a thickening, limited to the nervous layer. This becomes the medullary (or neural) plate which extends from the dorsal lip of the blastopore in an anterior direction as a median band of thickened ectoderm which widens anteriorly where the brain will develop.


Shortly the more or less parallel lateral margins of the thickened medullary plate become even more thickened as the lateral folds or ridges, and are continued anteriorly as the transverse neural fold or ridge. A longitudinal neural groove or depression appears in the center of the medullary plate, so that the height of the neural folds appears to be accentuated. This is the very beginning of the formation of the central nervous system, induced by the presence beneath of the archenteric roof and/or the notochord.


The following description covers the period from the time the medullary plate first thickens until about the time the embryo reaches the 2.5 mm. stage. At this time the embryo shows ciliary movement within its jelly capsule. These cilia are lost, except on the tail, by the time the 11 mm. stage is reached.


Early Organogeny

Surface Changes

Gastrulation and early neurulation in the frog; (solid) ectoderm, (cellular) endoderm and yolk, (striated) mesoderm, (circles) presumptive notochord and mesoderm.

The oval-shaped gastrula quickly becomes elongated and the medullary plate provides a slightly elevated (convex) dorsal surface. This soon changes to a flattened and then a concave upper surface, as the development of the central nervous system proceeds. At this time the embryo acquires a distinct head and a body which is ovoid because of the mass of contained yolk.


The appearance of the thickened and elongating medullary plate and the subsequent formation of a neural tube is the main causal factor in the change in shape of the embryo, which follows upon gastrulation. In fact, it may well be the autonomous powers of elongation of the presumptive notochord that are responsible for the general elongation of the entire embryo and its contained archenteron. At this stage, when the primary nervous structures are being formed, the embryo is known as a neurula.


The medullary or neural plate extends from the dorsal lip of the blastopore to the anterior limit of the developing embryo, where it appears somewhat rounded in contour. The elevated neural folds are therefore continuous around the margins of this thickened medullary plate, the anterior junction of the folds being designated as the transverse neural fold to distinguish it from the paired, extensive and more posterior lateral neural folds. This transverse neural fold represents, then, the anterior extremity of the developing brain. The regions of the lateral neural folds represent the posterior parts of the brain and the spinal cord levels.

Development of the frog (Rana pipiens) from the yolk plug stage to the neurula.
Early development of caudal structures. (Top, left) Posterior view of late neurula. (Top, right) Enlargement of illustration at top, left. {Bottom) Sagittal section through the posterior end; (dashes) nervous, {circles) entoblast (presumptive endoderm), {crosses) notochord, {dots) mesenchyme (presumptive mesoderm). (Pasteels, Jean, 1943, Fermeture du blastopore, anus et intestin caudal chez les Amphibiens Anoures, Acta Neerland. morphoL, 5:11.)

These lateral neural folds move toward each other and first make contact at a point slightly anterior to the center of the original medullary plate, a region which will be identified later as the level of the medulla of the brain. From this initial point of contact the neural folds come together and fuse in both an anterior and a posterior direction, thus converting a groove into a closed canal, the medullary (neural) tube or neurocoel. Obviously, fusion will occur last at the extremities, the anterior one being called the anterior neuropore and the posterior one the blastopore. At the posterior end the medullary (neural) folds merge into the sides of the blastopore. As the folds meet they cover over this blastopore and the enteron is therefore no longer opened to the exterior (by way of the blastopore) but into the posterior end of the neurocoel. The original blastopore then becomes a temporary tube-like connection between the gut and the nervous system, known as the neurenteric canal.


As the neural folds fuse and the neurocoel becomes constricted off from the dorsal ectoderm, the latter becomes a continuous sheet of cells above the mid-dorsal line. The enclosed canal, lined with ciliated and pigmented ectoderm, is the neural canal or neurocoel which is found as the much-reduced central canal of the spinal cord and brain of the adult frog.


Slightly ventral to the anterior end of the closing neural folds there appears a semicircular elevated ridge of ectoderm, the two extensions of the elevation merging with the lateral limits of the transverse neural fold. This is called the sense plate which contains the material of the fifth and seventh cranial nerve ganglia. As the neural folds come together, this sense plate remains distinct (i.e., the sides do not become fused as do the neural folds), but its posterior limits merge with the outer margins of the lateral neural folds even after the fusion of the latter structures. These sense plates will give rise to the mandibular (first visceral) arches, lens of the eyes, nasal placodes, and oral suckers. The ridges are formed largely from mesoderm and will give rise to parts of the jaw apparatus. The superficial suckers are paired larval organs that take the shape of an inverted "U." They become glandular and form a mucous secretion which the larva uses to adhere to objects after hatching.

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Organ fields or anlagen of the closed neural tube stage.

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Photograph of the neurenteric canal of the late neurula stage in the frog.

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A face view of the 5 mm. frog tadpole.

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Anlagen of the head region: 5 mm. frog tadpole.

The anterior median level of the sense plate will shortly develop a vertical groove, the stomodeal cleft, which separates the two mandibular ridges (arches) or primordia of the right and left sides of the jaw. The oral suckers appear as V-shaped, pigmented, adhesive, mucus-secreting glands at the ventral end of each sense plate. They reach their maximum development at the 6 mm. stage and then begin to degenerate. The ventral limit of the vertical groove becomes the ectodermal stomodeum when it breaks through to the endodermal pharynx to form the mouth. That part of the mouth derived from the stomodeum will therefore be lined with ectoderm. The dorsal limit of the vertical groove becomes the hypophyseal invagination. Directly dorsal to each oral sucker, above and to either side of the stomodeal cleft, there develop large oval evaginations (bulbous out growths) which will be recognized as the external evidences of the internally enlarging optic vesicles or opticoels.

Visceral Arches

Parallel to the posterior margins of the sense plate there develops another thickened pair of elevations, directed forward. These are the gill plates which merge imperceptibly with the more posterior lateral folds of the sense plate and the closed neural folds. They contain the ninth and tenth cranial nerve ganglia and represent the forerunners of the gill or branchial arches. These are very important in the development of the external gills used for aquatic respiration in the tadpole. These gill plates acquire vertical grooves (furrows) which separate the intermediate bulges into three prominent vertical thickenings known as the visceral or branchial arches. The most anterior of the grooves appears between the mandibular arch of the sense plate and the first thickening of the gill plate and is known as the hyoniandibular groove or furrow. This groove never really opens through from the outside to the pharynx as a cleft. Just posterior to this hyomandibular groove is the first gill thickening, the beginning of the second visceral or hyoid arch which will provide the mesodermal structures to the tongue and operculum. The fifth groove, the most posterior of all, is the next to develop. Then follow the third, fourth, and sixth visceral grooves (the fifth developing later), dividing the gill plate into vertical thickenings. The sixth is rudimentary and posterior to the gill plate. The visceral grooves appear in the sequence of I, II, III, IV, VI, V, counting from the anterior. The intermediate thickenings between the grooves are the visceral arches which remain rather solid, some of which will soon give rise to the external gills. Those which give rise to gills are sometimes referred to as branchial arches, the first of which is the third visceral arch.


The visceral arches, from the mandibular as the first, may be numbered in sequence in a posterior direction, and may be designated as visceral arches I to VI. However, since the third visceral arch becomes the first branchial arch because it is the most anterior arch to give rise to an external gill, to name the arches "branchial" one must begin with visceral arch III (called branchial arch I) and number them posteriorly as branchial arches I to IV.


The term "visceral arch" is used because of the homology of this structure with similar structures in other vertebrates, even though external gills are formed. In the frog, visceral arches III to VI develop external gills and they therefore can be properly called "branchial arches".

A tabular comparison is given to clarify the distinction:

Original Structure Structure Formed
Visceral arch I Mandibular arch (jaw parts)
Visceral II Hyoid arch (tongue and operculum)
Visceral III Branchial arch I, first external gill
Visceral IV Branchial arch II, second external gill
Visceral V Branchial arch III, third external gill
Visceral VI Branchial arch IV, rudimentary fourth external gill

Three, sometimes four, of the vertical (visceral) furrows will open through to the pharynx as clefts, functioning in gill respiration. They open in the following order: visceral clefts III, IV, II, and V. The presence and order of grooves therefore bears no relation to the break-through order of the clefts.


Following the hyomandibular groove in position, each is numbered in sequence as a branchial cleft, when all are developed. Branchial cleft I, for instance, is the slit from the outside to the pharynx just anterior to branchial arch II and immediately following the hyomandibular groove. Most of the arches are named for the gills to which they give rise. This is true except for the first (mandibular) and the second (hyoid). These do not give rise to gills but to jaw, tongue, and opercular parts. The confusion of the terms visceral and branchial need not be serious if we remember that all arches are visceral and are numbered from the anterior, while the third visceral arch is only the first branchial — i.e., the first to have gills.


Posterior to the dorsal limits of the gill plate will be seen (at the 2.5 mm. stage) a slightly elongated swelling in the direction of the embryonic axis. This is the surface indication of the internal enlargement of mesoderm known as the pronephros or head kidney. More posteriorly and slightly dorsal to this level may be seen the < -shaped surface indications of the internal, mesodermal myotomes, or muscle segments.

Origin of the Proctodeum and Tail

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The 7 mm. frog tadpole: frontal section.

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Development of the respiratory systems of the frog larvae.

At the posterior end of the embryo in the neurula stage the neural folds converge, as do the lateral lips of the blastopore, so that they become confluent. The originally oval blastopore becomes a vertical slit, due to the active merging of the two lateral neural folds.


The lateral lips of the blastopore close together over the posterior end of the neurocoel above and the posterior end of the archenteron below. Internally this provides a temporary (about 1 hour) connection between the central nervous system (neurocoel) and the gut cavity (archenteron) known as the neurenteric canal or chorda-canal. Similar temporary connections of these two major systems are seen in the development of most, if not all, of the higher vertebrates, including man. This region of approximation of the folds is sometimes unfortunately called the "primitive streak" because of certain homologies with similarly developing structures in the chick embryo. The blastopore at the posterior end of the neurocoel is now completely closed, but in the meantime there has developed a new invaginating pit just ventral to the blastopore, known as the ectodermal proctodeum. Occasionally the closing of the dorsal blastopore and the opening of the ventral proctodeum are connected by the aforementioned "primitive streak." The proctodeum is the primordium of the anus, and establishes a new ectodermally lined opening into the hindgut. The extent of the proctodeal ectoderm can be determined in sagittal sections by determining the limit of the invaginated and pigmented ectodermal cells.

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The 3 mm. frog tadpole: sagittal section.

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Formation of the neural groove and neural tube from the neural (medullary) plate.

Formation of the tail bud.


The body of the neurula stage is laterally compressed along the dorsal surface but ventrally the belly region bulges with the large yolk endoderm cells. The tail bud is formed by a backward growth of tissue dorsal to the closed blastopore. As it is elongated it is provided with both a dorsal and a ventral fin, the dorsal fin being developed initially by the posterior growth of myotomes, with accompanying blood vessels and nerves, to form the tail bud.


The process of neurulation, or the formation of the central nervous system of the frog embryo, is too complicated to understand by means of verbal instructions alone. By means of time-lapse photography these developmental changes can be telescoped into a few minutes and understood more clearly. About the time the neural folds close, there are numerous surface evaginations and invaginations which indicate correlating changes within the embryo. The neurula develops surface cilia which tend to rotate the embryo within the fertilization membrane and its jelly (albuminous) coverings.

Internal Changes

Formation of the neural groove and neural tube from the neural (medullary) plate.

The lining of the neural canal consists of the original pigmented outer epidermal layer of the blastula which, by the time the canal is formed, is ciliated, as is the entire outer ectoderm of the embryo. The central canal of the nervous system, when it is formed, is therefore lined with ciliated and pigmented cells, later to be identified as the ependymal layer. The bulk of the nervous system, however, is derived from the inner layers of polyhedral cells from the original nervous layer of ectoderm of the blastular roof. This properly named "nervous ectoderm" gives rise to the neuroblasts of the central nervous system.

The roof and the floor of this neural tube are relatively thin, but due to this nervous layer the lateral walls are very thick. The roof is simply the region of the junction of the two neural folds. Some of this nervous layer, at the level of dorsal fusion, is pinched off on either side dorso-laterally to the neurocoel as the neural crests. The neural crests are actually part of the original ectodermal neural folds which do not form an integral part of the neural tube. The paired neural crests extend the full length of the central nervous system, lie dorsolateral to that system, and will give rise to various ganglia of the central and sympathetic nervous systems and to chromatophores. Covering the central nervous system dorsally is the reconstituted ectoderm, derived by the fusion of ectoderm lateral to each of the neural folds which are brought together at the time of closure of the neurocoel, along the mid-dorsal line.


The continuous and paired neural crests, lodged between the neural folds and the overlying dorsal ectoderm, become metamerically subdivided by the developing somites. These crests are therefore neural from the beginning, but are extra-neural in position in that they are left outside the axial nervous system. These crests retain cellular connections with the dorso-lateral wall of the developing spinal cord and will give rise chiefly to the dorsal root ganglia of the spinal nerves. At the level of the brain they give rise to the ganglia and to the fifth and seventh to tenth roots inclusive. They may also give rise to the visceral and cranial cartilages. At the body level they give rise not only to the paired spinal ganglia but also to the sympathetic nervous system, to the chromatophores of the body, and to the medulla of the adrenal gland.


The medullary plate at its anterior extremity is the last region of the central nervous system to be closed off from the exterior. The opening from the presumptive brain region to the exterior is the anterior neuropore, which has a homologue in the development of all vertebrates. Due to the original spherical condition of the gastrula this anterior region of the central nervous system curves ventrally at about the level of the future midbrain, and this ventral curvature of the brain persists and is characteristic of all vertebrates. The notochord, which functions as an axial skeleton for the embryo, is ventral to the nervous system, and terminates just at this ventral curvature of the brain (i.e., at the cranial flexure). The anterior neuropore is then found at the anterior extremity of the embryo in the sagittal plane directly in line with the terminated notochord, but in the roof of the brain. Occasionally a sagittal section of a 2 mm. embryo will show a knot of cells in this region which represents the puckered closure of the anterior neuropore.


Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
Pages where the terms "Historic Textbook" and "Historic Embryology" 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 and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)
Frog Development (1951): Introduction | Rana pipiens | Reproductive System | Fertilization | Cleavage | Blastulation | Gastrulation | Neurulation | Early Embryo Changes | Later Embryo or Larva | Ectodermal Derivatives | Endodermal Derivatives | Mesodermal Derivatives | Summary of Organ Appearance | Glossary | Bibliography | Figures

Reference

Rugh R. Book - The Frog Its Reproduction and Development. (1951) The Blakiston Company.


Cite this page: Hill, M.A. 2017 Embryology Book - The Frog Its Reproduction and Development 8. Retrieved November 23, 2017, from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_The_Frog_Its_Reproduction_and_Development_8

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