Paper - A Human Embryo of Twenty-five Somites: Difference between revisions
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==Introduction== | |||
THE specimen about to be described was kindly given to me by Dr Jethro Gough, of the Pathological Department, Welsh National School of Medicine. It was obtained at a post-mortem examination of a woman, aged 39, who died of a strangulated hernia. The post-mortem examination was made 47 hours afterdeath; thereis,however, little probability that the material had suffered much from this lapse of time, for the body was kept in an efficient refrigerator, which fact, combined with the season of the year, January, lessens the likelihood of decomposition having set in,and Dr Goughtelsme that he noticed no evidence of this when making the autopsy. | |||
==Technique Employed on Material Received== | |||
The material received from Dr Gough consisted of the uterus, ovaries and tubes. The uterus had been opened in front along the middle line, and there was thus exposed on the anterior wall and near the left cornu a chorionic vesicle, which was unopened. | |||
There was a corpus luteum in the left ovary, which had been opened, and a hydatid cyst was present near the ovarian fimbria of the left side. The whole specimen was placed in 4 per cent formalin for a few days, and then the chorionic vesicle was removed from the uterus and opened, and an apparently normal embryo was exposed. | |||
The embryo, enclosed in the amnion, was lying on its left side; the yolk stalk passed out towards the right and the body stalk towards the left; the tail of the embryo was twisted towards the right. | |||
The amnion was then removed and the embryo, with the body stalk and the adjacent part of the chorion, was embedded in paraffin and cut into sections of 10,um thickness, which were stained on the slides with Ehrlich's haematoxylin and orange G. | |||
Every section was then photographed on to bromide paper at a magnifica- tion of x 100, and many reconstructions, both graphic and of wax, were made of the embryo as a whole and of its various regions. | |||
In order to obtain the correct orientation for the reconstructions the following method was adopted: the sections had been fixed to the slides with a solution of chromatized gelatin, and after the removal of the paraffin with xylol, the sections were stained and it was found that the edges of the paraffin block could be easily identified as they had slightly taken up the haematoxylin stain; provided the paraffin block has been trimmed square, a satisfactory method of orientation is thus easily accessible; in addition to this method use was made of photographs which had been taken of the embryo before, and after, its removal from the chorionic sac. | |||
==Age of the Specimen== | |||
There was no menstrual history, so that in estimating the age comparison must be made with embryos for which more definite data are available. Recent work, in Baltimore, by Streeter (1932, 1933), Hartman and their colleagues, has made itpossible to determine the true ovulation age and rate of development in embryos of the monkey, Macacus rhesus; and while the two genera-man and monkey-differ in form and in rate of differentiation and growth, it has nevertheless been found that such differences do not become appreciable until the end of the second month; so that, for the first 6 weeks, the known ages of Macacus rhesus may be transferred to human embryos of corresponding developmental stages. Streeter has thus estimated the ovulation ages of certain well-known human embryos, and has found that the first somites make their appearance on the 22nd day and that differentiation of the somites occurs rapidly, 1, 5 and 8 somites being found on the same day; a 6.5mm. embryo,with triangular fin-like arm buds and beginning leg buds was obtained on the 27th day. | |||
The present specimen has 25 somites and very early limb buds and may thus be between the 22nd and 27th days; taking into account the rapidity of development at this stage, an ovulation age of 24 days would, I believe, be a fair estimate for this specimen. | |||
From published data on menstrual history it is possible to work out the ovulation age of certain other embryos which resemble, in general, the present specimen; for example, embryos described by Atwell, 17 somites (1930), Watt, 17-19 somites (1915), Davis, 20 somites (1923), Girgis, 22 somites (1926), Johnson, 24 somites (1917), Waterston, 27 somites (1914). Basing the calculation of the ovulation age on the assumption that ovulation has occurred on the 14th day after the commencement of the last menstrual period, I find that this series of embryos covers a range of ages from 19 to 82 days, and of de- velopmental stages from 17 to 27 somites; such ages do not fit in exactly with Streeter's findings, but they are based on menstrual histories and on the as- sumption that menstruation has been regular and that ovulation has occurred on the 14th day after the commencement of the last period. Whilst some observers would fix a more or less hard and fast date for ovulation, others would consider the date to be more variable; for example, Hain (1934) has shown in an individual in whom intermenstrual bleeding was recorded for a period of over 2 years that,if such intermenstrual bleeding may beconsidered indicative of ovulation, then ovulation may occur as late as, and after, the 20th day from the beginning of the previous period, and that there may be more than one ovulation in a cycle; there may thus be great variation in ovulation ages as estimated from menstrual histories, and this emphasizes the importance of Streeter's and Hartman's work. | |||
==The Uterus and Embryonic Membranes== | |||
The uterus, after fixation, measured 96 mm. from fundus to external os, and 67 mm. from side to side at the widest part of the body; the walls were 12mm. thick. | |||
The chorionic vesicle was removed intact from the uterus and was covered completely with vili; itwas rather flattened and measured 18 x 15 mm. | |||
The part of the chorion, at the attachment of the body stalk, that was cut out and sectioned with the embryo was carefully examined; the general ap- pearance is very like that shown in Grosser's Fig. 107, p. 133, in Keibel & Mall (1910), of a section through the chorion of an aborted ovum of 1 month. Each villus and its branches are usually covered with two layers of cells: an outer layer in which cel boundaries are not well defined, and in which the nuclei are flattened, loosely spaced and often faintly stained; and an inner layer in which celboundaries are defined, and in which the nuclei are round, closely packed and deeply stained; these two layers of the chorionic epithelium are the syncytiotrophoblast and cytotrophoblast respectively. Here and there it is quite impossible to make out any cellular structure whatever in the syncytiotrophoblast, whilst in some places the nuclei of this layer can be faintly discerned. | |||
When the final ramifications of a villus are reached it is found that the syncytiotrophoblast fades away or disintegrates, and the cytotrophoblast becomes spread out in a sheet so that the cells of which it is composed are seen in surface view instead of in section; the cels, too, appear to increase in size and their boundaries become every sharp,and the sheet of cells forms a beautiful mosaic. When thecytotrophoblastoftwo adjacent vili thus becomes spread out into sheets the syncytiotrophoblast dips in between the sheets and gives the appearance of being an intrusion(PI.I,fig.4);similarlyapartoftheinter- villous space comes to be included in the sheet and contains maternal blood and sometimes some fibrinous material. These intrusions into the sheet are composed of masses of protoplasm without any cell boundaries and with small, numerous and deeply stained nuclei which stand out very prominently front the lightly stained and well-defined cells of the trophoblast sheet; these sheets are the"cell islands"of Grosser. But there are to be found in the sheets and in the intervillous spaces other protoplasmic masses of an appearance quite different from that of the intrusions mentioned above;thesearesmallmassesof protoplasm, of any shape, homogeneous in appearance, rather opalescent and stained slightly pink, with crowds of brightly stained nuclei arranged usually in a circle to rinarosette (PI.I,fig.4); these are the"giant cells" of Grosser, and they are either protrusions or actually detached portions of syncytiotro- phoblast, and they seem to be very similar to the "proliferation nodes" to | |||
which also Grosser refers. | |||
The mesodermal core of the vili has undergone some shrinkage from the overlying trophoblast; it contains the subdivisions of the umbilical vessels, and is composed of a loose tissue, the cells of which are of an irregular polygonal or stellate shape with round, oval or spindle-shaped nuclei; there are a few ofthe large celsofHofbauer, but they are not at all numerous. | |||
Teacher (1924), in his account of the embryo T.B. 2, describes a condition which is just like what I find in this specimen; he writes, p. 172, that the syn- cytium "ends alitlebeyond the tip of the villus leaving the cytotrophoblast uncovered, and the latterfuses with that ofneighbouring viliforming a com- plex structure tunnelled here and there by passages for the maternal blood". Coventry(1923),in an account of the placenta of the Guinea baboon at the 4th month, writes of trophodermic cel islands being completely covered with a layer of syncytium continuous with that of the villus to which they are attached; comparing this with the condition in the human subject, he states that the trophoblastic masses in man are not covered with a continuous layer of syncytium, but that it is usual to find syncytial masses and buds scattered over the surface of the masses, and he suggests that the syncytium is at first stretched out over the developing cel mass and then, its elasticity failing, it becomes broken up into patches; this is just the impression that I get from a study of this specimen, and it seems to correspond with the condition described by Johnson (1917) in an embryo of 24 somites. The line of attachment of the amnion to the body wall follows closely the ventral borders of the two umbilical veins, and where these enter the heart the lines of reflection join just caudal to the heart and cranial to the yolk stalk, with the result that the heart is included within the amniotic cavity and the yolk stalk is not; the condition is very similar to that shown by Politzer & Sternberg (1930) in an embryo of 2-7mm. in length and with 25 somites.The amnion covers the caudal aspect of the body stalk and is just beginning to encroach on the ventral aspect; it is most closely applied to the body of the embryo in the forebrain and tail regions, whereas it stands out at some distance from the rest of the brain and from the dorsal surface of the body, though it does follow to a slight extent the dorsal concavity of the body. | |||
Filmy strands passed from the outer surface of the amnion and of the yolk sac to the inner surface of the chorion, but they could be removed from both structures without causing any damage. I am unable to describe the structure of these strands, but they seem to be a fairly common feature in young chorionic vesicles and are, presumably, remains of the extra-embryonic mesoderm which have persisted after the formation of the extra-embryonic coelom. | |||
The amnion is a very thin bilaminar structure and, since most of it was removed before the embryo was sectioned, it can be studied microscopically only at the cut edges of the lines of reflection; at such places the two layers of which it is composed can be seen well; near the line of reflection between heart and yolk stalk the amnion is a little thicker than elsewhere, as was described by Johnson (1917), and here it is possible to recognize a small amount of mesenchyme between the true mesodermal and ectodermal layers. | |||
==The Embryo== | |||
External form. The embryo was cut into 297 sections, each 10 ,u thick, and since the sections are almost horizontal, the embryo measures nearly 3mm. inlength. | |||
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Revision as of 21:34, 24 January 2012
Cecil M. West
University College, Cardiff
<pubmed>17104635</pubmed>| PMC1252340
Historic Embryology
This is a slightly edited version of the original 1935 paper published in Journal of Anatomy and Physiology. The full paper is still available as a PDF document through PubMed Central.
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- Links: Embryology History
Introduction
THE specimen about to be described was kindly given to me by Dr Jethro Gough, of the Pathological Department, Welsh National School of Medicine. It was obtained at a post-mortem examination of a woman, aged 39, who died of a strangulated hernia. The post-mortem examination was made 47 hours afterdeath; thereis,however, little probability that the material had suffered much from this lapse of time, for the body was kept in an efficient refrigerator, which fact, combined with the season of the year, January, lessens the likelihood of decomposition having set in,and Dr Goughtelsme that he noticed no evidence of this when making the autopsy.
Technique Employed on Material Received
The material received from Dr Gough consisted of the uterus, ovaries and tubes. The uterus had been opened in front along the middle line, and there was thus exposed on the anterior wall and near the left cornu a chorionic vesicle, which was unopened.
There was a corpus luteum in the left ovary, which had been opened, and a hydatid cyst was present near the ovarian fimbria of the left side. The whole specimen was placed in 4 per cent formalin for a few days, and then the chorionic vesicle was removed from the uterus and opened, and an apparently normal embryo was exposed.
The embryo, enclosed in the amnion, was lying on its left side; the yolk stalk passed out towards the right and the body stalk towards the left; the tail of the embryo was twisted towards the right.
The amnion was then removed and the embryo, with the body stalk and the adjacent part of the chorion, was embedded in paraffin and cut into sections of 10,um thickness, which were stained on the slides with Ehrlich's haematoxylin and orange G.
Every section was then photographed on to bromide paper at a magnifica- tion of x 100, and many reconstructions, both graphic and of wax, were made of the embryo as a whole and of its various regions.
In order to obtain the correct orientation for the reconstructions the following method was adopted: the sections had been fixed to the slides with a solution of chromatized gelatin, and after the removal of the paraffin with xylol, the sections were stained and it was found that the edges of the paraffin block could be easily identified as they had slightly taken up the haematoxylin stain; provided the paraffin block has been trimmed square, a satisfactory method of orientation is thus easily accessible; in addition to this method use was made of photographs which had been taken of the embryo before, and after, its removal from the chorionic sac.
Age of the Specimen
There was no menstrual history, so that in estimating the age comparison must be made with embryos for which more definite data are available. Recent work, in Baltimore, by Streeter (1932, 1933), Hartman and their colleagues, has made itpossible to determine the true ovulation age and rate of development in embryos of the monkey, Macacus rhesus; and while the two genera-man and monkey-differ in form and in rate of differentiation and growth, it has nevertheless been found that such differences do not become appreciable until the end of the second month; so that, for the first 6 weeks, the known ages of Macacus rhesus may be transferred to human embryos of corresponding developmental stages. Streeter has thus estimated the ovulation ages of certain well-known human embryos, and has found that the first somites make their appearance on the 22nd day and that differentiation of the somites occurs rapidly, 1, 5 and 8 somites being found on the same day; a 6.5mm. embryo,with triangular fin-like arm buds and beginning leg buds was obtained on the 27th day.
The present specimen has 25 somites and very early limb buds and may thus be between the 22nd and 27th days; taking into account the rapidity of development at this stage, an ovulation age of 24 days would, I believe, be a fair estimate for this specimen.
From published data on menstrual history it is possible to work out the ovulation age of certain other embryos which resemble, in general, the present specimen; for example, embryos described by Atwell, 17 somites (1930), Watt, 17-19 somites (1915), Davis, 20 somites (1923), Girgis, 22 somites (1926), Johnson, 24 somites (1917), Waterston, 27 somites (1914). Basing the calculation of the ovulation age on the assumption that ovulation has occurred on the 14th day after the commencement of the last menstrual period, I find that this series of embryos covers a range of ages from 19 to 82 days, and of de- velopmental stages from 17 to 27 somites; such ages do not fit in exactly with Streeter's findings, but they are based on menstrual histories and on the as- sumption that menstruation has been regular and that ovulation has occurred on the 14th day after the commencement of the last period. Whilst some observers would fix a more or less hard and fast date for ovulation, others would consider the date to be more variable; for example, Hain (1934) has shown in an individual in whom intermenstrual bleeding was recorded for a period of over 2 years that,if such intermenstrual bleeding may beconsidered indicative of ovulation, then ovulation may occur as late as, and after, the 20th day from the beginning of the previous period, and that there may be more than one ovulation in a cycle; there may thus be great variation in ovulation ages as estimated from menstrual histories, and this emphasizes the importance of Streeter's and Hartman's work.
The Uterus and Embryonic Membranes
The uterus, after fixation, measured 96 mm. from fundus to external os, and 67 mm. from side to side at the widest part of the body; the walls were 12mm. thick.
The chorionic vesicle was removed intact from the uterus and was covered completely with vili; itwas rather flattened and measured 18 x 15 mm.
The part of the chorion, at the attachment of the body stalk, that was cut out and sectioned with the embryo was carefully examined; the general ap- pearance is very like that shown in Grosser's Fig. 107, p. 133, in Keibel & Mall (1910), of a section through the chorion of an aborted ovum of 1 month. Each villus and its branches are usually covered with two layers of cells: an outer layer in which cel boundaries are not well defined, and in which the nuclei are flattened, loosely spaced and often faintly stained; and an inner layer in which celboundaries are defined, and in which the nuclei are round, closely packed and deeply stained; these two layers of the chorionic epithelium are the syncytiotrophoblast and cytotrophoblast respectively. Here and there it is quite impossible to make out any cellular structure whatever in the syncytiotrophoblast, whilst in some places the nuclei of this layer can be faintly discerned.
When the final ramifications of a villus are reached it is found that the syncytiotrophoblast fades away or disintegrates, and the cytotrophoblast becomes spread out in a sheet so that the cells of which it is composed are seen in surface view instead of in section; the cels, too, appear to increase in size and their boundaries become every sharp,and the sheet of cells forms a beautiful mosaic. When thecytotrophoblastoftwo adjacent vili thus becomes spread out into sheets the syncytiotrophoblast dips in between the sheets and gives the appearance of being an intrusion(PI.I,fig.4);similarlyapartoftheinter- villous space comes to be included in the sheet and contains maternal blood and sometimes some fibrinous material. These intrusions into the sheet are composed of masses of protoplasm without any cell boundaries and with small, numerous and deeply stained nuclei which stand out very prominently front the lightly stained and well-defined cells of the trophoblast sheet; these sheets are the"cell islands"of Grosser. But there are to be found in the sheets and in the intervillous spaces other protoplasmic masses of an appearance quite different from that of the intrusions mentioned above;thesearesmallmassesof protoplasm, of any shape, homogeneous in appearance, rather opalescent and stained slightly pink, with crowds of brightly stained nuclei arranged usually in a circle to rinarosette (PI.I,fig.4); these are the"giant cells" of Grosser, and they are either protrusions or actually detached portions of syncytiotro- phoblast, and they seem to be very similar to the "proliferation nodes" to which also Grosser refers.
The mesodermal core of the vili has undergone some shrinkage from the overlying trophoblast; it contains the subdivisions of the umbilical vessels, and is composed of a loose tissue, the cells of which are of an irregular polygonal or stellate shape with round, oval or spindle-shaped nuclei; there are a few ofthe large celsofHofbauer, but they are not at all numerous.
Teacher (1924), in his account of the embryo T.B. 2, describes a condition which is just like what I find in this specimen; he writes, p. 172, that the syn- cytium "ends alitlebeyond the tip of the villus leaving the cytotrophoblast uncovered, and the latterfuses with that ofneighbouring viliforming a com- plex structure tunnelled here and there by passages for the maternal blood". Coventry(1923),in an account of the placenta of the Guinea baboon at the 4th month, writes of trophodermic cel islands being completely covered with a layer of syncytium continuous with that of the villus to which they are attached; comparing this with the condition in the human subject, he states that the trophoblastic masses in man are not covered with a continuous layer of syncytium, but that it is usual to find syncytial masses and buds scattered over the surface of the masses, and he suggests that the syncytium is at first stretched out over the developing cel mass and then, its elasticity failing, it becomes broken up into patches; this is just the impression that I get from a study of this specimen, and it seems to correspond with the condition described by Johnson (1917) in an embryo of 24 somites. The line of attachment of the amnion to the body wall follows closely the ventral borders of the two umbilical veins, and where these enter the heart the lines of reflection join just caudal to the heart and cranial to the yolk stalk, with the result that the heart is included within the amniotic cavity and the yolk stalk is not; the condition is very similar to that shown by Politzer & Sternberg (1930) in an embryo of 2-7mm. in length and with 25 somites.The amnion covers the caudal aspect of the body stalk and is just beginning to encroach on the ventral aspect; it is most closely applied to the body of the embryo in the forebrain and tail regions, whereas it stands out at some distance from the rest of the brain and from the dorsal surface of the body, though it does follow to a slight extent the dorsal concavity of the body.
Filmy strands passed from the outer surface of the amnion and of the yolk sac to the inner surface of the chorion, but they could be removed from both structures without causing any damage. I am unable to describe the structure of these strands, but they seem to be a fairly common feature in young chorionic vesicles and are, presumably, remains of the extra-embryonic mesoderm which have persisted after the formation of the extra-embryonic coelom.
The amnion is a very thin bilaminar structure and, since most of it was removed before the embryo was sectioned, it can be studied microscopically only at the cut edges of the lines of reflection; at such places the two layers of which it is composed can be seen well; near the line of reflection between heart and yolk stalk the amnion is a little thicker than elsewhere, as was described by Johnson (1917), and here it is possible to recognize a small amount of mesenchyme between the true mesodermal and ectodermal layers.
The Embryo
External form. The embryo was cut into 297 sections, each 10 ,u thick, and since the sections are almost horizontal, the embryo measures nearly 3mm. inlength.
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Cite this page: Hill, M.A. (2024, May 1) Embryology Paper - A Human Embryo of Twenty-five Somites. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_A_Human_Embryo_of_Twenty-five_Somites
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