The histomorphology and tensiometry of the fibrous ventral abdominal wall of the baboon (papio anubis) and the goat (capra hircus)
BACKGROUND It is generally known that the fibrous ventral abdominal wall comprises of mainly an admixture of longitudinal, oblique and transverse bundles of collagen and elastic fibres. The formation, orientation and the strength of this fibrous wall would be expected to differ between animals depending on the forces applied to them. The baboon, a terrestrial primate, is a quadruped but occasionally referred to as a facultative biped when feeding or when hunting for food. The goat is a quadruped who is a browser capable of consuming and accommodating large quantities of forages which markedly distends its caudal abdomen. The different modes of locomotion and feeding would influence the structure of the fibrous ventral abdominal wall. Despite these unique differences, there is scanty information regarding the structure and tensile strength of the fibrous ventral abdominal wall in the baboon and the goat. Data on the structure of the fibrous ventral abdominal wall would give us an insight on how these animals cope with the various forces applied on the ventral abdominal wall and the likelihood of predisposition to rectus diastases and/or herniation. OBJECTIVE This study was undertaken to ascertain the histomorphology and morphometry of the fibrous ventral abdominal wall of the baboon and the goat so as to elucidate the elements that are accountable for maintaining the integrity of the wall when it is exposed to forces due to normal functions, posture and movements. The study also undertook to determine the ultimate tensile strength and elastic coefficient of the different regions of the baboon and the goat fibrous ventral abdominal wall and how it relates to structure and function. MATERIALS AND METHODS The fibrous ventral abdominal wall was harvested from seven healthy male baboons aged 12 – 24 months sacrificed at the Institute of Primate Research and six male goats aged 1 - 2 years procured from an animal slaughter market. The baboon experiments were approved by the internal review committee of the Institute of Primate Research. After ensuring that ethics and scientific issues had been addressed, strips of tissue were resected from the mid-epigastric, the umbilical and hypogastric part of the linea alba. Equivalent strips were also obtained from the ventral and dorsal rectus sheath. Some strips of tissues were set aside for mechanical testing, while others were processed for light microscopy and then morphometric measurements. Observations and measurements were coded and tabulated. Data collected were entered into the Statistical Package for Social Sciences software (Version 17.0, Chicago, Illinois) for coding, tabulation and statistical analysis. Since the data was non-parametric, the Kruskas Wallis test was conducted. Graphs, tables, and photographs were used to present the results. RESULTS Observations of this study have revealed that although the fibrous ventral abdominal wall of the baboon and the goat are similar in their gross morphology and hence the formation of the individual laminae, they however, have unique differences in their collagen and elastic ratios, orientation, laminae thickness and tensiometric characteristics. In both animals, the aponeurosis of external and internal oblique abdominis muscle joined to form the ventral rectus sheath while the aponeurosis of transversus abdominis formed the dorsal rectus sheath. Light microscopic observations revealed that the linea alba, ventral and dorsal rectus sheath in the two animals were made up of three, two and one distinct laminae respectively. In both animals the superficial lamina was noted to be lined or at times closely apposed to the deep fascia ventrally. In the baboon, the epigastric linea alba comprised of predominantly obliquely aligned collagen bundles while the umbilical was mainly composed of a fused thick mass of transversely oriented collagen bundles. Virtually all the collagen bundles in the hypogastric linea alba were longitudinally aligned. The rectus sheath however, was found to be more elastic than the linea alba. The baboon ventral rectus sheath was found to be predominantly made up of obliquely aligned collagen fibres with infrequent fine elastic fibres in the deep fascia ventrally and lining the deep lamina dorsally. The oblique collagen bundles in the hypogastric ventral rectus sheath were arranged in a characteristic crisscross manner. The dorsal rectus sheath in contrast, was noted to consist of mainly transversely aligned overlapping collagen bundles which had heavily infiltration of transversely aligned elastic fibres. The deep fascia was indistinct and contained very few elastic fibres. In the goat, the epigastric linea alba was made up of a superficial lamina composed of predominantly obliquely aligned collagen bundles, an intermediate lamina comprising of longitudinally aligned collagen bundles and a deep lamina consisting of transversely oriented collagen fibres. The umbilical and hypogastric linea alba was composed of an intermediate lamina which had discrete and prominent collagen bundles. The ventral rectus sheath was made up of two laminae which comprised of obliquely aligned collagen bundles. The collagen bundles were markedly thinner in the deep lamina of the umbilical and the hypogastric regions of the linea alba and ventral rectus sheath. The goat epigastric dorsal rectus sheath consisted of transversely aligned collagen bundles sandwiched by longitudinally arranged sheets of elastic fibres. These collagen bundles in the umbilical and hypogastric dorsal rectus sheath were noted to be thinner and were surrounded by sparse elastic fibres. The deep fascia (tunica flava abdominis) was intimately apposed to the ventral surface of the superficial lamina in the goat linea alba and the ventral rectus sheath. This fascia was thicker and more prominent in the umbilical and hypogastric regions of the linea alba and the ventral rectus sheath. Results of tensiometry revealed that the goat’s umbilical ventral rectus sheath recorded the highest ultimate tensile strength of 125 N/mm2 in both animals followed by the baboon’s umbilical dorsal rectus sheath (95 N/mm2). This was noted when they were exposed to transversely aligned traction. However, when the umbilical and hypogastric regions ultimate tensile strengths were combined, the goat’s ultimate tensile strength of ventral rectus sheath and the baboon’s ultimate tensile strength of dorsal rectus sheath was noted to have similar values of 201.8 N/mm2 and 195 N/mm2 respectively. The highest elastic coefficient recorded in either animal was seen when the fibrous ventral abdominal wall was exposed to longitudinal loading. Although the baboon umbilical dorsal rectus sheath recorded the highest elastic coefficient of 1000(SD 0.1)N/mm2, it also had the thinnest lamina (0.051mm) recorded. In both animals, however,the rectus sheath recorded a uniformly higher elastic coefficient than the linea alba The findings of this study show that the collagen bundles with the heavy elastic fibre impregnation seen in the baboon dorsal rectus sheath and the unusually elastic rich ventral surface of goat fibrous ventral abdominal wall, constitute a strong fibroelastic structure “band-like” of variable tension” which serve to support the abdominal viscera. This allows for graduated accommodation/ stretch and recoil while at the same time conserving energy by preventing unnecessary muscular use due to increased abdominal pressures/ forces. The findings of an unusually thin baboon DRS which had the highest UTS and also had the highest elastic coefficient, would serve to inform the industry on future development of meshes. The intimate association of the elastic and collagen fibres in the fibrous ventral abdominal wall of the baboon and the goat may allow the strengthening of the wall, like the steel in tyres. Thus, not only would it serve as an energy saving structure, but it would possibly keep the incidences of rectus diastases and ventral hernia formation to minimal levels in these animals.