Development and distribution of adipose tissue in the human pelvis

Early Human Development, 28 (1992) 79-88 Elsevier Scientific Publishers Ireland Ltd.

79

EHD 01214

Development and distribution of adipose tissue in the human pelvis* Helga Fritsch

and Wolfgang

Kiihnel

Department of Anatomy, Medical University of Liibeck, Liibeck, (F. R.G.) (Received

25 September

1991; revision

received

2 December

1991; accepted

5 December

1991)

Summary The development of the pelvic adipose tissue was studied using 300-600 pm thick sections through the pelves of 9-37-week-old fetuses and newborn children. During fetal life three different anlagens of adipose tissue appear within the pelvic cavity. Development and topography of these anlagens are described, and correlations with the development of the connective tissue compartments within the pelvis were demonstrated. Key words:

human

development;

fetuses;

pelvis; connective

tissue; adipose

tissue

Introduction Although the prenatal morphogenesis of adipose tissue has systematically been described by several authors [3,9,14,16] and the chronology of the appearance of adipose tissue in the human fetus has recently been reported by Poissonnet et al. [15], no attention has so far been paid to the development of adipose tissue within the pelvis. Blechschmidt [1,2] showed that the definitive organization of the adipose and connective tissue in the subcutis results from a strictly arranged developmental process during prenatal life. These findings were corroborated by Koornneef [lo] who referCorrespondence to: H. Fritsch, Department Allee 160, D-2400 Liibeck, F.R.G.

of Anatomy,

Medical

University

of Liibeck,

Ratzeburger

*This work has been approved by the ethical committee of the Medical University of Liibeck. Consent of the parents was legally required for submission of the fetuses to the Department of Anatomy.

0378-3782192/$05.00 0 1992 Elsevier Scientific Printed and Published in Ireland

Publishers

Ireland

Ltd

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red to a similar developmental organization of adipose and connective tissue in the orbit. The pelvic cavitiy is filled by different compartments of connective tissue whose complex topography has not been conclusively clarified. Previously we have described the development of the rectal compartment [5,7] and have shown that the formation of the so-called rectal fascia is closely connected with the development and spread of adipose tissue around the rectal wall. Furthermore, we have found that the strict organization of the rectal connective tissue in young fetuses does not persist in older fetuses but is obliterated by the distribution of adipose tissue. These results raised the question of how the development of adipose tissue correlates with the formation of other compartments within the pelvic cavity. The purpose of this paper is to describe the chronology of the development of adipose tissue within pelvic cavity of human fetuses. Furthermore, it will be clarified how far the formation of the different connective tissue compartments is associated with the development of adipose tissue. Materials and Methods

The development and spread of adipose tissue within the pelvis was studied in three newborn children and 65 fetuses ranging in gestational age from 9 to 37 weeks (pc.). Estimations of fetal age were based upon measurements of crown-rump (CR) length (34-35 1 mm) and were compared with the standard tables of Patten [ 131and Moore [12]. The fetuses were cases of legal abortion or miscarriage with no signs of maceration or macroscopic abnormalities. They had been fixed by immersion and stored in 9% formaldehyde solution for at least 3 months. After preparation, the entire pelvic portion of each fetal specimen was impregnated with the epoxy resin BIODUR@E 12 as previously described in detail [5]. The polymerized epoxy blocks were serially cut with a diamond wire-saw (Well@ ) either in the transverse, in the sagittal or in the coronal plane. According to the CRlength of the fetuses the thickness of the sections ranged from 300-600 pm, being usually thinner in the smaller specimens. After mounting and polishing, the sections were stained with azure II/methylene blue and counterstained with basic fuchsin [6]. In some series only alternate sections were stained. The neighbouring sections remained unstained for subsequent cutting into semithin sections. The stained sections were examined and photographed with a macroscope at magnifications of 4-80 times. For photographic documentation of the semithin sections a light microscope (Zeiss) was used. To examine the pelvic adipose tissue histologically, the tissue was identified on the stained section and the corresponding region on the unstained serial section punched out. A series of 1 pm semithin sections were then cut. These sections were also stained with azure II/methylene blue according to Lascko and LCvai [ 111. To show the three-dimensional extent of the pelvic adipose tissue in human fetuses, computer-assisted reconstructions were made of the serial sections through the pelves of a 24-week-old female and a 26-week-old male fetus. An image analysis system (Ibas 2000, Kontron) was used to produce a contour diagram from digital data input. The x and y coordinates of each contour were stored and then transferred

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to an IBM PC-At. With the help of a GW-Basic@ programme the z coordinate was completed by putting corresponding contours one upon the other. Finally the data were transferred to a CAD-system (Auto-CAD@ 10.0 and Autoshade@) for conversion to a three-dimensional display. Results Fetal specimens chosen for description are those which show significant developmental changes of the pelvic adipose tissue and the developmental characteristics of the pelvic connective tissue. In the early fetal period (9- 1 l-week-old fetuses) the different pelvic compartments are filled with mesenchyme. From the 12th week onwards the latter is replaced by loose connective tissue containing an abundance of thin fibres. In 15 16-week-old fetuses (1 11- 128 mm CR-length) a proliferation of primitive vessels (stage 2 of Poissonnet et al. [14]) is the first indication of adipose tissue appearance within the pelvic cavity (Fig. 1). The proliferating vessels are situated within loose connective tissue in the paravisceral compartment, i.e. at the medial side

Fig. 1. Enlargement of the paravisceral compartment in a transverse section (300 pm) through the pelvis of a female fetus of 128 mm CR-length. Proliferation of primitive vessels at the medial aspect of the obturator internus muscle (asterisk) as indicator of developing adipose tissue in the pelvis (magnification x60).

Fig. 2. Paravisceral region in a transverse section (600 pm) through the pelvis of a male fetus of 191 mm CR-length. A continuous aggregation of primitive organs (Wassermann) constitute the primary parietal anlage of pelvic adipose tissue. (a) Survey (magnification x 11). (b) Enlargement (magnification x25). The ventral edge of the primary parietal anlage is covered by the tendinous arch of the pelvic fascia (asterisk). On its medial side the primary parietal anlage is accompanied by the nerves of the pelvic plexus (arrowheads). R, rectum; U, urethra; 01, obturator internus.

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Fig. 3. Paravisceral region in a transverse section (500 pm) through the pelvis of a female fetus of 225 mm CR-length. The fat lobules of the primary parietal anlage cover the lateral part of the bladder. 1‘hey penetrate its dense connective tissue sheath, which is separated into a thicker medial (asterisk) and a t hinner lateral (arrowhead) lamella (magnification x25). V, vagina; B, bladder; 01, obturator internus

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of the obturator internus opposite the uterus in the female and opposite the prostate in the male fetus. In 18-20-week-old fetuses (160-185 mm CR-length) a remarkable condensation of collagenous tibres has occurred within the pelvic connective tissue. Areas of dense connective tissue are situated around the rectal wall and at the ventral aspect of bladder. Several primitive organs of Wassermann [ 161 or so-called mesenchymal lobules (stage 3 of Poissonnet et al. [14]) (Fig. 2a) can be found as precursors of adipose tissue within the paravisceral compartment. They constitute a continuous aggregation and, therefore, will be considered as primary parietal anlage of adipose tissue within the pelvis. As the storage of lipid droplets already starts in this age-group

Fig. 4. A three-dimensional reconstruction of the pelvic organs, the obturator intemus, the levator ani and the primary parietal anlage (dark) of a female fetus of 225 mm CR-length. R, rectum; RP, rectouterine pouch; U, uterus; B, bladder; 01, obturator intemus; LA, levator ani.

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(Fig. 2b), these mesenchymal lobules change to primitive fat lobules (stage 4 of Poissonnet et al. [14]). Each of them is covered by a thin capsule of connective tissue (Fig. 2b). The lobular organization of the adipose tissue is already visible. The whole primary parietal anlage is covered by a thick lamella of dense connective tissue, particularly at its medial border, thus being distinctly separated from the neighbouring structures (Fig. 2a). The ventral edge of the primary parietal anlage is covered by a cap of fibrous connective tissue which is the same as the tendinous arch of the pelvic fascia [8] (Fig. 2a,b). In fetuses aged between 20 and 26 weeks (175-240 mm CR-length) the primary parietal anlage increases in size. New primitive fat lobules have joined the primary parietal anlage especially in the retropubic region (Fig. 3) where it participates in the covering of the caudal and lateral part of the bladder and the cranial and caudal portion of the urethra. Ventrally these organs are ensheathed by dense connective tissue. Between the 20th and 26th week of fetal development the primitive fat lobules

Fig. 5. Semithin section through the adipose mm CR-length (magnification x 600).

tissue of the primary

parietal

anlage of a male fetus of 320

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of the primary parietal anlage penetrate the lamellae of this connective tissue sheath and separate it into a thicker medial and a thinner lateral part (Fig. 3) [8]. As can be seen from a three-dimensional reconstruction (Fig. 4), the crania-caudal extent of the primary parietal anlage is almost comparable with that of the obturator internus within the pelvic cavity. Thus the primary parietal anlage is bordered by the obturator fascia (Fig. 2a) laterally. Medially, the dorsal part of the primary parietal anlage is bordered by the branches of the internal iliac vessels. The mid-most part of the primary parietal anlage is accompanied by the nerves of the pelvic plexus (Fig. 2a) medially. The most ventral tip of the primary parietal anlage is situated between the pubic bone and the cranial portion of the urethra (Fig. 4). Caudally the primary parietal anlage abuts on the cranial border of the levator ani muscle. The storage of lipid droplets within the cells of the primary parietal anlage increases enormously in fetuses aged between 20 and 26 weeks. As seen from a semithin section (Fig. 5), some adipocytes still contain several lipid droplets. These droplets coalesce into a single large droplet as already seen in other cells. In 27-28-week-old fetuses (245-263 mm CR-length) little primitive fat lobules occur prevertebrally, where no signs of fat formation have been found in earlier stages of fetal development. They are situated in front of the anterior sacral foramina and accompany the nerves and vessels when leaving the pelvic cavity through the greater sciatic foramen (Fig. 6). In 33-week-old fetuses (326 mm CR-length) new fat lobules also occur within the presacral space (Fig. 6). Together with those lobules situated in front of the sacral formina they can be considered as secondary parietal anlage.

Fig. 6. Prevertebral region in a transverse section (500 pm) through the pelvis of a male fetus of 326 mm CR-length. Fat lobules (arrowheads) are situated in front of the anterior sacral foramina and within the presacral space (magnification x IO). S, sacral vertebra; R, rectum.

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In the newborn the primary parietal anlage has enormously increased in size due to the storage of lipid droplets within the adipocytes. Thus the paravisceral region of the pelvis is now mainly filled with adipose tissue. New primitive fat lobules begin spreading between the dense connective tissue of the adventitia recti [7], which thus develops into a fat body covered by a thick connective tissue lamella laterally. The fat lobules around the rectal wall can be regarded as the only visceral anlage of adipose tissue within the pelvis. As far as it is possible to classifiy adipose tissue by means of light microscopy, it can be stated that the adipocytes of the described fat lobules within the pelvis belong to one cell type. During fetal development the cells pass through a multi- or plurilocular stage (Fig. 5) before attaining their definite unilocular stage. No obvious differences are observed regarding the appearance and extent of the described fat lobules in the pelvis of male and female fetuses. Discussion The organization of the pelvic connective tissue in the human adult is difficult to comprehend because adipose tissue is abundant and therefore the borders between different compartments are poorly demarcated. Previously [8] we have shown that the pelvic connective tissue is well defined in young fetuses (9-20 weeks). This organization changes when adipose tissue appears. This study shows that within the pelvic cavity adipose tissue develops at different times. In 1S-16-week-old fetuses the first sign of fat formation is found in the paravisceral region where a large aggregation of fat lobules develops which is called the primary parietal anlage. The time of its occurrence correlates with the chronology of fat formation stated by Poissonnet et al. [15], who showed that the development of adipose tissue starts in the head and neck in 14.5-week-old fetuses and in the trunk in 15-16-week-old fetuses. Within the other compartments of the pelvic cavity fat formation starts much later. Pre- and paravertebral fat lobules occur in 27-33-week-old fetuses. These accompany nerves and vessels and constitute the secondary parietal anlage. In the newborn child other fat lobules are finally found around the rectal wall. They constitute the only visceral anlage of pelvic adipose tissue. As the primary parietal anlage covers parts of the urethra and bladder, it serves as a cushion of fat for the displacement of these organs. The primary parietal anlage is divided into lobules by connective tissue septa which develop parallel with the adipose tissue. In contrast to this, the fat lobules of the visceral anlage around the rectal wall develop between connective tissue lamellae which have already existed [7]. Thus the surrounding connective and adipose tissue of bladder and rectum develop in different ways. To what extent the arrangements of connective and adipose tissues are adapted to the functional needs of bladder and rectum can only be clarified by means of additional studies in the adult, because both organs are not yet completely functional in fetuses. The time of occurrence is not the only difference concerning the anlagen of adipose tissue within the pelvis. The development of the primary parietal analge follows the stages described by Wassermann [16], or the more detailed stages of Poissonnet et al. [14]. No precursors, however, have been found for the secondary parietal

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anlage as well as for the visceral anlage around the rectal wall. In these compartments fat lobules are very small. They develop with intimate contact to neighbouring vessels [7]. It therefore seems obvious that accumulation of lipid droplets occurs very quickly within the adipocytes of these fat lobules so that the changing pattern of the cells could not be established with the methods we applied. By means of light microscopy we were able to show the different appearance of the adipocytes from the multilocular to an unilocular stage [4] within the primary parietal anlage. The definite cell type of the secondary parietal and of the visceral anlage is unilocular. Thus it can be stated that all pelvic fat lobules consist of white adipose tissue. Acknowledgements

The technical assistance of Mrs. M.-L. Leppin and Mr. H. Manfeldt is gratefully acknowledged. This work has been supported by grants of the Deutsche Forschungsgemeinschaft (Fr829/1-1). References Blechschmidt, E. (1931): Zur Anatomie des Subkutangewebes. Z. Zellforsch. Mikroskop. Anat., 12, 284-293. 2 Blechschmidt, E. (1934): Die Architektur des Fersenpolsters. Gegenbaurs Morphol. Jahrb., 73, 20-68. 3 Dabelow, A. (1957): Die Entwicklung der Fettorgane (Wassermann) im subcutanen Fettgewebe menschlicher Feten (nach Untersuchungen an dicken Schnitten mit Gefaflinjektion). Anat. Anz., Suppl. 104, 83-96. 4 Fawcett, D.W. (1986): Adipose tissue. In: A Textbook of Histology, pp. 174-187. Editor: B. Fawcett. W.B. Saunders Book Company, Philadelphia. 5 Fritsch, H. (1988): Developmental changes in the retrorectal region of the human fetus. Anat. Embryol., 177, 513-522. 6 Fritsch, H. (1989): Staining of different tissues in thick epoxy resin impregnated sections of human fetuses. Stain Technol., 64, 75-79. 7 Fritsch, H. (1990): Development of the rectal fascia. Anat. Anz., 170, 273-280. 8 Fritsch, H. (1991): Entwicklung des Beckenbindegewebes. Habilitationsschrift, Liibeck. 9 Hoffmann, A. (1950): Die Entwicklung des Fettgewebes beim Menschen. Anat. Arm., 97,242-250. 10 Koornneef, L. (1976): The development of the connective tissue in the human orbit. Acta Morphol. Neerl.-Stand., 14, 263-290. Lasckd, J. and L&ai, G. (1975): A simple differential staining method for semithin sections of ossifying cartilage and bone tissues embedded in epoxy resin. Mikroskopie, 31, l-4. Moore, K.L. (1988): The fetal period. In: The Developing Human, pp. 87- 103. W.B. Saunders Book Company, Philadelphia. Patten, B.M. (1968): Age, growth, and changes in external form of body. In: Human Embryology, pp. 139-156. MC Graw Hill Book Company, New York. Poissonnet, C.M., Burdi, A.R. and Bookstein, F.L. (1983): Growth and development of human adipose tissue during early gestation. Early Hum. Dev., 8, l-l 1. 15 Poissonnet, C.M., Burdi, A.R. and Garn, St. M. (1984): The chronology of adipose tissue appearance and distribution in the human fetus. Early Hum. Dev., 10, l-l 1. 16 Wassermann, F. (1926): Die Fettorgane des Menschen. Z. Zellforsch. Mikrosk. Anat., 3, 233-328. 1