Difference between revisions of "Book - The Frog Its Reproduction and Development 6"

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(Created page with "=Chapter 6 - Blastulation = Internally the 8 -cell stage of the frog shows the beginning of a cavity where the well-formed animal hemisphere cells have completed their 6-s...")
 
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{{Rugh1951 header}}
 
=Chapter 6 - Blastulation =
 
=Chapter 6 - Blastulation =
  
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Internally the 8 -cell stage of the frog shows the beginning of a cavity where the well-formed animal hemisphere cells have completed their 6-sided cell membrane. Once a blastomere is formed it tends to
 +
acquire a certain degree of rigidity and will maintain its spherical cell boundaries independently of its neighbors. Cortical and vitelline membrane pressure may flatten cells slightly against each other. When segmentation of the egg occurs, dividing it into smaller and smaller cellular units, there naturally follows the appearance of an internal cavity known as the segmentation cavity or blastocoel. The aggregate becomes about 20 per cent larger than the total volume of the cells. The earlier cleavages (described above) tend to be perpendicular to the egg surface. However, after the 32-cell stage there appear division planes more or less parallel to the egg surface, cutting off surface protoplasm from inner protoplasm and yolk. With increased cell rigidity, following each division, these surface cells push against each
 +
other until they are lifted up and away from the underlying cells. A crude simile would be to hold four or more marbles tightly together in the hand, and no matter how much pressure is exerted there will always be a space in the center of the group of marbles (i.e., cells). In the frog's egg the formation of complete cells is rather slow, and the blastocoel is small and not readily apparent until about the 32-cell stage.
  
Internally the 8 -cell stage of the frog shows the beginning of a
 
cavity where the well-formed animal hemisphere cells have completed
 
their 6-sided cell membrane. Once a blastomere is formed it tends to
 
acquire a certain degree of rigidity and will maintain its spherical cell
 
boundaries independently of its neighbors. Cortical and vitelline membrane pressure may flatten cells slightly against each other. When segmentation of the egg occurs, dividing it into smaller and smaller
 
cellular units, there naturally follows the appearance of an internal
 
cavity known as the segmentation cavity or blastocoel. The aggregate
 
becomes about 20 per cent larger than the total volume of the cells.
 
The earlier cleavages (described above) tend to be perpendicular to
 
the egg surface. However, after the 32-cell stage there appear division
 
planes more or less parallel to the egg surface, cutting off surface
 
protoplasm from inner protoplasm and yolk. With increased cell
 
rigidity, following each division, these surface cells push against each
 
other until they are lifted up and away from the underlying cells. A
 
crude simile would be to hold four or more marbles tightly together in
 
the hand, and no matter how much pressure is exerted there will always
 
be a space in the center of the group of marbles (i.e., cells). In the
 
frog's egg the formation of complete cells is rather slow, and the
 
blastocoel is small and not readily apparent until about the 32-cell
 
stage.
 
  
The blastocoel is therefore a cavity, and this stage of development  
+
The blastocoel is therefore a cavity, and this stage of development is known as the blastula, overlapping the stages of cleavage. The cavity is enlarged with each of the early cleavages and it is filled with an  
is known as the blastula, overlapping the stages of cleavage. The cavity  
+
albuminous fluid, arising from the surrounding cells. Even though the frog's egg is telolecithal, cleavage has been described as holoblastic (opposed to meroblastic) and the presence of this cavity distinguishes  
is enlarged with each of the early cleavages and it is filled with an  
+
it as a coeloblastula (as opposed to stereoblastula which has no cavity). Since the horizontal cleavages appear toward the animal hemisphere, this newly forming blastocoelic cavity will appear in an  
albuminous fluid, arising from the surrounding cells. Even though the  
+
eccentric position above the level of the equator, and slightly toward the gray crescent side of the cleaving egg. It will remain in this position, beneath the animal hemisphere, until it is later displaced by the development of other cavities. The size of the blastocoel increases with the formation of smaller and smaller surrounding cells. The late blastula of the frog, by virtue of this blastocoel, has a volume displacement about 20 per cent greater than that of the fertilized egg.
frog's egg is telolecithal, cleavage has been described as holoblastic  
 
(opposed to meroblastic) and the presence of this cavity distinguishes  
 
it as a coeloblastula (as opposed to stereoblastula which has no  
 
cavity). Since the horizontal cleavages appear toward the animal  
 
hemisphere, this newly forming blastocoelic cavity will appear in an  
 
eccentric position above the level of the equator, and slightly toward  
 
  
  
Line 38: Line 16:
  
  
 
+
Vegetal hemisphere  
 
 
 
 
~ Vegetal hemisphere  
 
 
 
 
 
  
 
Hemisected blastula  
 
Hemisected blastula  
 
 
  
 
Segmentotion covity (blastocoel)  
 
Segmentotion covity (blastocoel)  
 
 
  
  
 
Lote biostula  
 
Lote biostula  
 
 
  
 
Hemisected late blaslulo  
 
Hemisected late blaslulo  
 
 
  
 
Blastulation in the frog. (Redrawn and modified after Huettner.)  
 
Blastulation in the frog. (Redrawn and modified after Huettner.)  
 
 
  
  
Line 75: Line 38:
  
  
 
+
Following the 32-cell stage the egg loses all semblance of rhythmic or synchronized cleavage and there is developed a gradient of cleavage with the most active region being at the animal pole and the least  
the gray crescent side of the cleaving egg. It will remain in this position,
+
active being at the vegetal pole. In addition to this there are some characteristic changes in the bias tula of the frog embryo. First, the smaller pigmented animal pole cells tend to spread their activity in a  
beneath the animal hemisphere, until it is later displaced by the development of other cavities. The size of the blastocoel increases with
+
downward direction toward the vegetal pole, by a sort of contagion, so that there is an actual migration of pigment-covered cells toward the vegetal pole. This, and the horizontal cleavages, result in increasing the cell layers but thinning of the roof of the blastocoel. Second, the horizontal cleavages in the very small animal pole cells give the blastula a double or multi-layered roof. The single outer layer of cells  
the formation of smaller and smaller surrounding cells. The late blastula of the frog, by virtue of this blastocoel, has a volume displacement about 20 per cent greater than that of the fertilized egg.
+
contains most of the superficial pigment and is now recognized as the epidermal layer which will give rise to epithelium, either of the integument or lining the nervous system. The inner tiers of cells of the  
 
+
blastular roof are less pigmented and are known collectively as the nervous layer because they will give rise largely to the neuroblasts of the nervous system.  
Following the 32-cell stage the egg loses all semblance of rhythmic  
 
or synchronized cleavage and there is developed a gradient of cleavage  
 
with the most active region being at the animal pole and the least  
 
active being at the vegetal pole. In addition to this there are some  
 
characteristic changes in the bias tula of the frog embryo. First, the  
 
smaller pigmented animal pole cells tend to spread their activity in a  
 
downward direction toward the vegetal pole, by a sort of contagion,  
 
so that there is an actual migration of pigment-covered cells toward  
 
the vegetal pole. This, and the horizontal cleavages, result in increasing the cell layers but thinning of the roof of the blastocoel. Second,  
 
the horizontal cleavages in the very small animal pole cells give the  
 
blastula a double or multi-layered roof. The single outer layer of cells  
 
contains most of the superficial pigment and is now recognized as the  
 
epidermal layer which will give rise to epithelium, either of the integument or lining the nervous system. The inner tiers of cells of the  
 
blastular roof are less pigmented and are known collectively as the  
 
nervous layer because they will give rise largely to the neuroblasts of  
 
the nervous system.  
 
  
  
Either on the surface of the whole blastula or in sections of the  
+
Either on the surface of the whole blastula or in sections of the blastula, one can determine the margins of the downward-moving pigmented coat and active animal hemisphere cells and the consequent slight thickening of the equatorial band. This margin is known  
blastula, one can determine the margins of the downward-moving  
 
pigmented coat and active animal hemisphere cells and the consequent slight thickening of the equatorial band. This margin is known  
 
 
as the marginal zone, marginal belt, or germ ring. Such a marginal  
 
as the marginal zone, marginal belt, or germ ring. Such a marginal  
 
region of activity, where yolk is most actively being transformed into  
 
region of activity, where yolk is most actively being transformed into  
Line 106: Line 51:
 
vertebrate embryos. It will have much to do with the subsequent  
 
vertebrate embryos. It will have much to do with the subsequent  
 
formation of the lips of the future blastopore.  
 
formation of the lips of the future blastopore.  
 +
  
 
The marginal zone is approximately equatorial in the blastula  
 
The marginal zone is approximately equatorial in the blastula  

Revision as of 10:12, 6 April 2013

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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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Chapter 6 - Blastulation

Internally the 8 -cell stage of the frog shows the beginning of a cavity where the well-formed animal hemisphere cells have completed their 6-sided cell membrane. Once a blastomere is formed it tends to acquire a certain degree of rigidity and will maintain its spherical cell boundaries independently of its neighbors. Cortical and vitelline membrane pressure may flatten cells slightly against each other. When segmentation of the egg occurs, dividing it into smaller and smaller cellular units, there naturally follows the appearance of an internal cavity known as the segmentation cavity or blastocoel. The aggregate becomes about 20 per cent larger than the total volume of the cells. The earlier cleavages (described above) tend to be perpendicular to the egg surface. However, after the 32-cell stage there appear division planes more or less parallel to the egg surface, cutting off surface protoplasm from inner protoplasm and yolk. With increased cell rigidity, following each division, these surface cells push against each other until they are lifted up and away from the underlying cells. A crude simile would be to hold four or more marbles tightly together in the hand, and no matter how much pressure is exerted there will always be a space in the center of the group of marbles (i.e., cells). In the frog's egg the formation of complete cells is rather slow, and the blastocoel is small and not readily apparent until about the 32-cell stage.


The blastocoel is therefore a cavity, and this stage of development is known as the blastula, overlapping the stages of cleavage. The cavity is enlarged with each of the early cleavages and it is filled with an albuminous fluid, arising from the surrounding cells. Even though the frog's egg is telolecithal, cleavage has been described as holoblastic (opposed to meroblastic) and the presence of this cavity distinguishes it as a coeloblastula (as opposed to stereoblastula which has no cavity). Since the horizontal cleavages appear toward the animal hemisphere, this newly forming blastocoelic cavity will appear in an eccentric position above the level of the equator, and slightly toward the gray crescent side of the cleaving egg. It will remain in this position, beneath the animal hemisphere, until it is later displaced by the development of other cavities. The size of the blastocoel increases with the formation of smaller and smaller surrounding cells. The late blastula of the frog, by virtue of this blastocoel, has a volume displacement about 20 per cent greater than that of the fertilized egg.


Animal hemispheres^ Segmentotion cavity (blastocoel)


Vegetal hemisphere

Hemisected blastula

Segmentotion covity (blastocoel)


Lote biostula

Hemisected late blaslulo

Blastulation in the frog. (Redrawn and modified after Huettner.)


Vegetal hemisphere

Frog blastula: sagittal section.


Progressive stages in blastulation.


Following the 32-cell stage the egg loses all semblance of rhythmic or synchronized cleavage and there is developed a gradient of cleavage with the most active region being at the animal pole and the least active being at the vegetal pole. In addition to this there are some characteristic changes in the bias tula of the frog embryo. First, the smaller pigmented animal pole cells tend to spread their activity in a downward direction toward the vegetal pole, by a sort of contagion, so that there is an actual migration of pigment-covered cells toward the vegetal pole. This, and the horizontal cleavages, result in increasing the cell layers but thinning of the roof of the blastocoel. Second, the horizontal cleavages in the very small animal pole cells give the blastula a double or multi-layered roof. The single outer layer of cells contains most of the superficial pigment and is now recognized as the epidermal layer which will give rise to epithelium, either of the integument or lining the nervous system. The inner tiers of cells of the blastular roof are less pigmented and are known collectively as the nervous layer because they will give rise largely to the neuroblasts of the nervous system.


Either on the surface of the whole blastula or in sections of the blastula, one can determine the margins of the downward-moving pigmented coat and active animal hemisphere cells and the consequent slight thickening of the equatorial band. This margin is known as the marginal zone, marginal belt, or germ ring. Such a marginal region of activity, where yolk is most actively being transformed into cytoplasm, is found in Amphioxus and chick, and probably in all vertebrate embryos. It will have much to do with the subsequent formation of the lips of the future blastopore.


The marginal zone is approximately equatorial in the blastula stage, except for the slight reduction in pigmented cells at the side where the original gray crescent was located. Opposite to this gray crescent region the wall of the blastula appears to contain relatively more layers of cells and is therefore somewhat thicker. These changes occur in anticipation of the next process in embryonic development, namely gastrulation.

There have been numerous attempts to explain blastocoel formation and the movements anticipating gastrulation. These have been based largely on purely physical phenomena. An attempt has been made recently to summarize these concepts, as follows (Holtfreter, 1947: Jour. Morph., 80:42):

If present in large numbers, the cells at the periphery of the body tend to establish a semipermeable layer, while the cells of the interior become separated from each other by secretion fluid. Thus the aggregation may be transformed into the configuration of a blastula or, if the amount of internal fluid increases further, into an epithelial vesicle. The occurrence of cylindrical stretchings and of migrations of the peripheral cells into the interior of the vesicle has been interpreted as possibly being caused by a surface tension lowering action of the inner fluid. Even in the absence of a gradient of surface tension, however, isolated embryonic cells tend to elongate and to migrate in one direction, with their original proximal pole leading the way, and possibly invagination movements may be partly due to this inherent dynamic tendency of the individual cell. Other factors influencing shape and movements of the cells in aggregate, or in a normal embryo, appear to be cell-specific differences of adhesiveness arising in the course of differentiation and making for a sorting out, grouping and aggregation of the various ceil types into tissues and organs. With the appearance of fibrous and skeletal structures, of local differences in mechanical stress and hydrostatic pressure, of differential growth and a variety of metabolic processes, so many new factors are introduced that the principle of interfacial tension loses more and more of its original primacy as a morphogenetic agent.