Paper - The area of the chorionic villi in the full-term placenta (1922)
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Dodds GS. The area of the chorionic villi in the full-term placenta. (1922) Anat. Rec. 24(5): -294.
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The Area of the Chorionic Villi in the Full-term Placenta
G. S. Dodds
West Virginia University
When studying the placenta with classes in embryology, the question has often arisen as to the extent of the surface presented by the chorionic villi to the maternal blood in the intervillous blood space — that is, the surface through which exchange of materials between maternal and fetal blood takes place. If such computations have previously been made, I am not aware of them. No figures of such areas are given in the text -books with which I am familiar, and it is with, the hope that the facts may be as new to many other embryologists as to myself that without extensive search of the literatm-e I venture to publish the result of my computations.
As is well known, the placenta consists of the chorion frondosum (fetal part of the placenta) and the decidua basalis (maternal part), a portion of the uterine mucosa. The chorion bears numerous villi which branch extensively in the inter\allous blood space which hes between the chorion and the uterine wall. The trunks of some of the attaching or anchoring villi are surrounded by or adherent to the decidua, but by far the greater number, including thousands of small branches, float freely in the blood space, bathed continually with the maternal blood. Through the surface of these villi exchanges of materials take place between the maternal blood in the sinus and the fetal blood which circulates through the villi in the branches of the umbilical arteries and veins.
The following calculations are based upon the study of a single human afterbirth of about average size, one of those in the collection used for demonstration to my classes. On account of various sources of error to be mentioned later, the figures given are but an approximation, though a very useful one, and I believe no advantage would be gained by the study of more specimens.
The placenta in question (fig. 1) had an outline approximately circular, with a diameter of 16 cm. and an average thickness of about 2.5 cm. The placenta had undoubtedly undergone some shrinkage during the formalin perservation and so is somewhat less in volume than when fresh, but this difference will not so much affect the final result as to greatly lessen its value. The volume, as near as could be computed, was 502 cc, though irregularities of form made such methods rather inaccurate. A more accurate figure, 510 c.c. was obtained by displacement of water, and has been used the calculations to follow. Of this volume a small part is taken up by the chorion frondosum on one surface and the decidua basalis on the other, with a combined thickness of about 1 mm. (4 per cent of the thickness of the placenta). Making this deduction, about 490 cc. remains as the volume of the blood sinus in which the villi are contained.
Fig. 1 Placenta cut vertically near its greatest diameter. Position of section from which measurements were made is shown by unshaded band.
The measurements and calculations are made from microscopic sections as follows: First the parts of villi in a section 10 [>. thick, extending across the placenta from chorion to decidual surface, were measured and the total surface area of the portion of villi included in this section computed (fig. 1, a). From this was determined the surface area of the vilU in 1 cu. mm. of similar tissue, and from this their surface area in the whole placenta.
Fig. 2 Part of drawing on which villi were measured, 1 sq.mm. of section included. The rounded stippled areas represent villi cut directly across, the elongated ones those cut diagonally at various angles. The two villi marked partly with line shading are those covered with 'canalized fibrin' and not included in the computed area.
The details of procedure are as follows: The whole section including 2,294 sections or shces of villi, was drawn to a known scale by the use of a microprojection lantern. The slices of villi are of various sizes and shapes, as in figure 2 which is a drawing of 1 sq. mm. of the section. The shaded areas represent slices of villi which are of somewhat different sizes, run in all directions, are crooked, and branch richly. The sUces of villi in the section would appear, if seen in perspective as shown in Figure 3, a, b, c, which represent sections of c}"linders of the same diameter cut at different angles, and when seen from directly above, as under the microscope hke a', b', c'. The size and shape of the slice depend not only on the size of the villus, but also on the direction in which it is cut. To compute the surface area of a single slice of a villus is easy, provided it is cut directlj' across as in figure 3, a, where the circumference or perimeter of the slice multiplied by the thickness of the section gives the surface area (the area of the surface shaded with lines in fig. 3, a). But when the villus is cut obliquely as in 3, 6 and c, the product of the perimeter by the thickness of the section is somewhat less than the actual area, because of the slanting surface at the end of the elliptical sections. In practice, however, this would probably not be a serious error, because in thin sections, such as actually used, the slanting end shown by the lined surface in figure 3, b' and c', is small, and in making the drawing would usually be covered by the width of the line, and so would be within the limit of error of the method employed. We may conclude that the error is not great if we assume that to measure any sUce of villus in the section we have but to take the product of the perimeter of the slice by its thickness. This method gives a pretty good approximation of the correct figure, and any constant error is of a negative sign, i.e., the computed area is somewhat less than the actual.
Fig. 3 Diagram to show cylinders of same diameter cut at various angles. a, b, c, showing sections in perspective, a', b', c', projections of these sections, such as the sections of villi as seen in the microscopic section. The part shaded with lines represents the area to be measured, and roughly equals the product of the perimeter of the section by its thickness.
In practice it is not necessary to make separate calculation of the surface area of each villus in the section, but merely to ascertain their combined perimeter and multiply this sum by the thickness of the section and thus obtain the total surface area of all the slices in the section.
The measurements of the drawn slices of villi were made by using a map measurer, an instrument having a small wheel with which to trace the Une to be measured, geared to a hand which registers on a dial the distance. Considerable shrinkage takes place during the paraffin imbedding, as shown by measurement before and after. Allowance is made for this in all the linear measurements, and the figures given below are those after this correction has been made.
In making the measurements, care was taken to include only those villi which seemed to have a surface suitable for absorption, that is, those covered with thin, syncytial epithelium, and to exclude those covered -nith ' canalized fibrin,' these latter mostl}of large size, evidently the main trunks from which the smaller ones arise. These smaller ones we will call 'absorbing' villi.
Measurements of placenta
Diameter 160 mm.
Average thickness 25 mm.
Volume b}' calculation .502,000 cu.mm.
Volume by water displacement 510,000 cu.mm.
Volume of blood sinus (510,000 less 4 per cent) 490,000 cu.mm.
Area of section drawn and measured 60.14 sq. mm.
Thickness of section drawn and measured, 10 ti, or 0.01 mm.
Measurements of drawing of section
Number of absorbing villi in section 2,294
Total perimeter of absorbing villi in drawing 124,400 mm.
Magnification of drawing 155 diam.
Actual perimeter of villi 124,400 802.5 mm.
From these figures one may compute the total surface area of villi in the whole placenta as follows:
Area of absorbing villi in measured section :
802.5 X .01 mm. = 8.025 sq.mm.
Volume of measured section :
60.14 sq.mm. X .01 mm. = .6014 cu.mm.
Area of absorbing villi per cubic millimeter of material like that composing the section:
= 13.3438 sq.mm.
Total area of all absorbing villi in placenta:
13.3438 X 490,000 = 6,538,462 sq.mm. 6.5 sq. meters 69.94 sq.ft.
This area, as previously pointed out, does not include the total surface exposed within the placenta, but only that of the small villi which, on account of their size and the nature of their surface, seem suited to function as absorbing structures. There is omitted from the above area the surface of those villi, mostly large ones, covered with fibrous or hyaline material, the surface of the decidual septa, and the inner surfaces of decidua basalis and chorion frondosum. From measurements made in the same way as for the smaller villi, it appears that these surfaces include nearly villi, of the total internal surface of this placenta, which thus is about 7 square meters.
The functional surface of the absorbing villi is doubtless reduced somewhat by the numerous thickenings in their epithelium in the form of masses of nuclei, variously known as 'proliferation islands', 'nuclear groups,' etc., and by the crowding of villi, so that sui-faces of adjacent villi are in contact and thus not freely exposed to the maternal blood. Thus it appears that the effective, absorbing surface uithin the placenta includes not the total of 7 square meters, nor the measured area of the absorbing villi, 6.5 square meters, but a figure somewhat less than this and hard to estimate.
Inasmuch as the above calculations are based upon measurements of but one section with an area of 60.14 sq.mm., sections from other parts of the placenta were studied, the number of villi per sq.mm. counted and measures of the size of these villi taken. From these observations it seems that the section which forms the basis of these figures is representative, and no great difference would appear if several sections were measured. That is, in any section of this size, no matter from what part of the placenta it is cut, we find about the same number of villi per sq.mm. and the average size of the villi cut is remarkably constant.
Concerning the method used, it is to be noted that though the actual microscopic section was cut 10 micron thick, the result would have been the same had it been 5 or 15 micron or any other thickness, because the slices of villi in the section are assumed to be cross-sections of various shaped bodies extending in a direction perpendicular to the plane of the section not only within the section, but indefinitely outside the section to the border of the placenta. Not only this, but all the villi of the placenta are assumed to be straight, parallel bodies of the same average size as those measured. The element of truth in this assumption is that in any section cut. there appear as many villi and of the same size, as in the one measured, though not the same vHh. The element of error is the obUque direction taken by many of the vilh in the section, which makes their actual area less than the computed area.
The result of 6.5 square meters (70 sq.ft.) is so large as at first to seem xmreasonable. In order to try to surprise myself into inconsistencies, I tried several modifications of the method and always anived at the same result. Certain figures from such calculations may be of interest : the average number of villi cut per sq. mm . was 38, in some areas running to a munber above 100, in others much fewer on account of less dense arrangement and presence of large trunks of villi. In a complete vertical section across the placenta there would be cut 133,760 villi, and in a complete horizontal section 763,648. The average perimeter of the sections of absorbing villi measured is 0.357 mm. — a figure from which there is a surprisingly small deviation when any area as large as 1 sq.mm. is measured. A villus of this average size and with a surface area of 6.5 square meters would have a length of a little more than 18 kilometers (Hi miles). Lest this measiire may seem excessive, we note that a cylinder of this length and area will have a diameter of 0.113 mm., and occupies a volume of 174,000 cu.mm. (174 cc), about 37 per cent of the volume of the blood sinus in which the villi actually float, leaving 63 per cent of the space for the circulation of the maternal blood — a relation which, from a general inspection of the sections, seems not uiu-easonable.
As a matter of fact, the diameter of this 'average' villus (0.113 mm.) is larger than the actual free terminal branches of the absorbing villi, because many of these sections are oblique, and also a few of the larger tnmks are included in the measured sections. The diameters of these free villi, as actually measiu-ed both on an enlarged drawing and on the sections, fall mostly between 0.03 and 0.06 mm., so that the combined length of the villi must be considerably in excess, possibly nearly double the figiu"e, 18 kilometers, given above.
The total surface area of the chorionic villi exposed within the placenta described above measured about 7 square meters, of which 6.5 square meters was included in those numerous small branches which by nature of their surface seem suited to act as organs of absorption. The efficiency of these is doubtless further reduced by the numerous thickenings of their epithelium in the form of the masses of nuclei so abundant on the surface of these villi.
Cite this page: Hill, M.A. (2020, October 22) Embryology Paper - The area of the chorionic villi in the full-term placenta (1922). Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_The_area_of_the_chorionic_villi_in_the_full-term_placenta_(1922)
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