Book - The Frog Its Reproduction and Development 9

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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 9 - A Survey of the Major Developmental Changes in the Early Embryo

Primary Divisions of the Brain

Development of the Eye of the Frog.

The anterior vesicular expansion of the central nervous system becomes constricted at certain levels and the walls begin to differentiate and may be used as identifying landmarks of the embryonic brain. It has just been stated that the brain floor bends ventrally (ventral or cranial flexure) around the tip end of the notochord. This flexure remains as a permanent feature of the brain. The brain floor at this region is known as the tuberculum posterius (posterior tubercle), and is in line with the notochord and the anterior neuropore, and will give rise to the floor of the mesencephalon or midbrain. Slightly anterior and dorsal to the tuberculum posterius the roof of the brain acquires a considerable thickening for a limited distance. This is known as the dorsal thickening and will be identified as the roof of the mesencephalon, later to give rise to the optic lobes. The primary brain very rapidly develops thinnings and thickenings of its walls, invaginations, and evaginations, all of which are part of its differentiation.


It is now possible to de-limit the three primary parts of the embryonic brain. These divisions are present in all vertebrate embryos of a comparable stage of development. They are the prosencephalon, mesencephalon, and rhombencephalon.


Prosencephalon. This is the primary forebrain, consisting of all parts of the brain anterior to a line drawn from the tuberculum posterius to the anterior limit of the dorsal thickening, largely anterior and ventral to the notochord. This portion of the brain develops almost immediately, giving rise to paired evaginations known as the optic vesicles, whose walls will give rise to the various ectodermal parts of the eye, exclusive of the lens and cornea.


Mesencephalon. This is the primary midbrain, or that portion of the primary brain bounded anteriorly and posteriorly by the limits of the dorsal thickening and lines drawn from these limits to the tuberculum posterius. It is located antero-dorsally to the tip of the notochord.


Rhombencephalon. This is the primary hindbrain, or that portion of the primary brain from the posterior limit of the mesencephalon to the spinal cord which, at this stage, is not clearly separated from the brain. The rhombencephalon lies entirely dorsal to the notochord, and in the frog it is never clearly divided further, as it is in higher forms.

Photograph of endocrine anlagen at the 5 mm stage of the frog tadpole.

Derivatives of Forebrain. At this stage of development the forebrain alone has distinguishing derivatives. Ventral to the notochord there develops a vesicular outpocketing of the floor of the forebrain known as the infundibulum. Its cells will contribute later to the formation of the pituitary gland, in conjunction with a cluster of pigmented ectodermal cells seen between the infundibulum and the roof of the pharynx. This cluster of cells is known as the epithelial hypophysis.

Reconstruction of the 5 mm. tadpole in sagittal section.

It is believed that the more anterior infundibulum will form the pars nervosa or posterior pituitary gland. The more posterior hypophysis can be identified as contributing to the anterior pituitary gland (pars distalis, pars intermedia, and pars tuberalis of mammals). At the most ventral aspect of the forebrain, in the floor, is a pronounced thickening known as the optic chiasma, anterior to which is a depression associated with the lateral optic vesicles. The depression is the optic recess (recessus opticus) which is connected with the optic stalk. Anterior to this, also in the floor, is a second thickening known as the torus transversus which becomes the seat of the anterior and other commissures. This structure lies within the more extensive lamina terminalis, which represents the fused anterior neural folds around the temporary anterior neuropore. More anteriorly, but in the roof of the forebrain, and slightly dorsal to a line drawn continuously from the notochord, one finds an evagination of the roof known as the epiphysis. This is the forerunner of the pineal body or gland. Anterior to the epiphysis the roof of the brain eventually becomes nonnervous, vascular, and folded as the anterior choroid plexus. Later the habenular ganglia and commissure develop between the epiphysis and choroid plexus. The only sense organs to develop by this stage (2.5 mm. length) are associated with the forebrain, a further indication of cephalization within the central nervous system. They are the optic vesicles, paired primordia of the eye, which begin to develop as paired lateral vesicular evaginations of the forebrain even before this portion of the brain is closed over dorsally. The presence of these vesicles was described previously as being dorso-lateral to the sense plate, in a facial view of the embryo. The connections of these vesicles with the forebrain, at the point of the optic recess, become partially constricted off into tubes known as the optic stalks.


At this stage the paired olfactory sense organs appear only as olfactory placodes or button-like thickenings of the pigmented surface ectoderm, each slightly ventral and mesial to the corresponding optic protuberances. The auditory placodes may be seen in slightly older stages as similar thickenings of the nervous ectoderm dorso-lateral to the level of the rhombencephalon (hindbrain).

The Enteron or Gut Cavity

Three-dimensional representation of the late neurula stage of the frog, Rana pipiens. (Redrawn and modified after Huettner.)

The anterior limit of the original archenteron expands both ventrally and laterally beneath the notochord and the infundibulum of the brain. This expanded cavity will give rise to the foregut and all of its derivatives. The midgut, at this stage, is simply the tubular archenteron dorsal to the mass of yolk endoderm. The hindgut is that portion of the archenteron found in the vicinity of the temporary neurenteric canal.


The foregut has a prominent median antero-ventral evagination of its endoderm known as the oral evagination. This will make contact with the head ectoderm just postero-ventral to the level of the hypophysis. There is an opposed stomodeal invagination of head ectoderm which will meet this endodermal evagination, later to break through as the mouth. When these two germ layers make contact, prior to the rupture, they constitute the oral plate.


The large cavity of the foregut is the pharynx, which expands laterally to form endodermally lined visceral pouches on either side of the developing vertical arches. These pouches are therefore vertically elongated endodermally lined sacs, at all times continuous with the pharynx. The most anterior pouch is called the hyomandibular because it comes to lie between the mandibular and the hyoid arches. Following this will be the first, second, and third branchial (gill) pouches, otherwise known as the second, third, and fourth visceral pouches. These all expand outwardly toward the opposed invaginations of the ectoderm which form external visceral grooves.


A medio-ventral and posteriorly directed pocket develops from the foregut, extending beneath the yolk a short distance. This is the liver diverticulum, the forerunner of the bile duct, the gallbladder, and the liver. This is the extent of gut development by the 2.5 mm stage.

The Axial Skeleton

Three-dimensional representation of the tail bud stage of the frog embryo, Rana pipiens. (Redrawn and modified after Huettner.)

The notochord was derived from cells indistinguishable from the mesoderm at the region of the dorsal lip. These cells very quickly expand and become vacuolated, and take on the appearance which they will manifest throughout development until they are displaced by bone of the vertebral column. Even the intercellular material of the notochord becomes vacuolated. The entire group of cells becomes enclosed in an outer elastic sheath and an inner fibrous sheath, both of which are classified as connective tissue.


The Mesoderm and Its Derivatives

Embryonic, loosely dispersed presumptive mesoderm is known as mesenchyme. At the end of gastrulation most of the frog mesoderm is in the form of sheets of such loose cells extending in all directions from the lips of the blastopore. However, the most anterior mesoderm, that found in the head and pharyngeal regions, is in the form of mesenchyme.


Origin of the Arches

Lateral to the pharynx are developed vertical concentrations of mesoderm known as the arches. Subsequently most of these will contain blood vessels and nerves but at this stage they are merely condensations of mesenchyme. The most anterior arch is anterior to the first endodermal pouch and ectodermal groove, and is known as the mandibular arch associated with the development of the jaw muscles. This was first seen ventral to the sense plate on either side of the stomodeal and hypophyseal cleft. Posterior to this mandibular arch is a parallel hyoid arch, and between these arches is developed the rudimentary hyomandibular groove (ectodermal) and pouch (endodermal) which never break through to form the cleft. Posterior to the hyoid arch is the first endodermal branchial pouch followed by the first mesodermal branchial arch; the second branchial pouch followed by the second branchial arch; and so on as the more posterior derivatives develop. The second arch at this stage may not as yet have its mesenchyme clearly marked off from the succeeding arches.


Origin of the Somites

The mesoderm posterior to the pharynx and lateral to the notochord (as seen in transverse section) takes an inverted horseshoe shape around the archenteron and the yolk. The uppermost level of this mesoderm, lateral to the nerve and notochord, is termed the segmental or vertebral plate and the more ventral portion of the same sheet of mesoderm is called the lateral plate mesoderm. This lateral plate extends continuously in a ventral direction and surrounds the yolk, just within the belly ectoderm. Shortly, all of this mesoderm will be separated off into a dorsal somite mass

Rugh 098.jpg

Frontal ( horizontal) section of the 7 mm frog larva at the level of the developing heart.

(Redrawn and modified after Huettner.)

Rugh 099.jpg

The 7 mm. frog tadpole: frontal section.

(epimere), an intermediate cell mass (mesomere), and a lateral plate (hypomere).

The epimere (segmental or vertebral plates) posterior to the pharynx becomes divided into sections known as the metameric somites. These are developed in sequence from the anterior to the posterior so that at any time in the development of an early embryo the anterior somites show the greatest differentiation. About four pairs will be seen in the 2,5 mm. embryo. The somites sever their connections with the intermediate cell mass and the more ventral lateral plate mesoderm. They become blocks of cells within each of which there develops a cavity, the myocoel. This myocoel is eccentric in position, appearing displaced toward the lateral margin of the somite. As a result of this, the outer layer of somite cells is the thinner. It is known as the dermatome, or cutis plate, having to do with the derivatives of the dermis and of the appendage musculature. The inner layer of somite cells is thicker and will give rise to the skeletal muscles of the back and body. This portion is known as the myotome. A few scattered cells may be seen between the myotome and the notochord, proliferating off from the somite. These are mesenchymal cells known as the sclerotome. They will give rise to the vertebral skeleton.

Rugh 100.jpg

The 7 mm frog tadpole: transverse sections. Through the mid-body level.

Rugh 101.jpg

Early organogeny. The 5 mm. frog tadpole at mid-body level. Photograph of cross section.


The earliest complete, closed blood vascular system of the frog embryo (found at the 4 mm. stage). Schematized drawing.

Origin of the Excretory System

Frontal (horizontal) section of the 7 mm frog larva at the level of the pharynx. ( Redrawn and modified from Huettner.)

The mesomere and hypomere are still connected. The lateral plate (i.e., mesomere), separate from the epimere (somite), now develops along its dorsal border a continuous, antero-posterior band of mesoderm known as the nephrotome. This will give rise to parts of the larval and the adult excretory systems. The nephrotomal band enlarges in a lateral direction but maintains cellular connections with the remaining dorsal limit of the lateral plate mesoderm. The influence of the segmentation of the vertebral plate extends to the separated nephrotomal band, dividing it also into a series of metameric nephrotomes. This nephrotomal segmentation is transitory in the frog, but persists in the embryos of some vertebrates. At about the level of the second to the fourth somites, the center of each nephrotome becomes evacuated to develop a nephrocoel. This is the very beginning of the embryonic head kidney or pronephros. The effect of the expansion of the intermediate mass of mesoderm, due to the development of the nephrocoel, was seen on the surface of the earlier embryo, lying just dorsal and posterior to the gill plates.

Rugh 103.jpg

The earliest complete, closed blood vascular system of the frog embryo (found at the 4 mm stage). Schematized drawing.

Rugh 104.jpg

Frontal section through the level of the heart of the 7 mm tadpole.

Origin of the Mesodermal Epithelium, Coelom, and Its Derivatives

In the more ventrally placed hypomere (lateral plate mesoderm) we find a continuous split which separates the mesoderm into an outer, parietal or somatic layer and an inner, visceral or splanchnic layer of mesoderm. The outer somatic mesoderm, in conjunction with the adjacent body ectoderm, is called the somatopleure, and gives rise to the skin with its blood and connective tissue. The inner splanchnic mesoderm, in conjunction with the gut endoderm. with which it later becomes intimately associated, is called the splanchnopleure and gives rise to the lining epithelium, muscles, and blood vessels of the entire mid- and hindgut.

Rugh 105.jpg

The 7 mm frog tadpole transverse section through the level of the thyroid gland.

Rugh 106.jpg

The 7 mm frog tadpole transverse section through the level of the Heart.

In between these two sheets of lateral plate mesoderm the cavity is known as the primary body cavity or coelom. Eventually the coelomic slit becomes continuous ventrally, from one side of the embryo to the other, forming a single visceral or coelomic cavity. Dorsally the junction of the lateral plates is interrupted by the notochord and the sub-notochordal rod. This latter structure, also known as the hypochordal rod, is a small rod of pigmented cells between the gut roof and the notochord. Presumably this represents a vestige of the connection between the two at the time of their simultaneous origin at the vicinity of the dorsal lip of the blastopore. It appears first at the 2.5 mm. stage.


Origin of the Heart

The lateral plate mesoderm extends into the head, ventral to the pharynx, as mesenchyme. This mesenchyme becomes organized into sheets, coextensive with the more posterior lateral plate sheets of somatic and splanchnic mesoderm. These will give rise to parts of the heart. As the coelomic split occurs at the body level, there is an extension of this split into the forming heart mesoderm, which will give rise to the pericardial cavity. The outer layer of mesoderm, corresponding to the body somatic layer, will become the pericardial membrane. The inner layer of mesoderm, corresponding to the body splanchnic layer, will become the myocardium or heart muscle. As in the body region, the mesoderm from the two sides grows together ventrally to fuse below the foregut. Both the coelom and the continuous and related pericardial cavity arise as bilateral cavities only to fuse and form single cavities around particular organs.

The endocardium or lining of the heart arises from scattered cells of mesodermal origin found beneath the pharynx. These cells become organized into a sheet of epithelium as they are enclosed by the bilateral folds of myocardium as they come together.

Rugh 107.jpg
Development of the heart of the frog embryo.
Rugh 108.jpg
Representative transverse sections of an 8 mm frog larva.



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 9. Retrieved September 26, 2017, from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_The_Frog_Its_Reproduction_and_Development_9

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