Introduction

African tree pangolin M. tricuspis is an insectivorous mammal found in the southwest region of Nigeria (Ofusori et al., 2007a). The scientific classification is shown in table 1. It feeds primarily on ants and termites, using their front claws to open insect nests and long sticky tongues to capture their prey (Redford, 1983 & Redford and Dorea, 1984). They are largely terrestrial, spend most of the day in underground burrows, and curl into a ball when attacked, protected by thick scales that cover most of their bodies (Fig. 1). They walk mainly on their hind legs, although they keep their body horizontal to the ground, dragging their tail behind them. They are also proficient climbers and can swim. They use their claws to break open ant or termite nests and capture the occupants with their long sticky tongue (Griffiths 1990). They can take up to 200,000 ants each night. Its natural predators are leopards, hyenas, and pythons, but it is also hunted by humans. Humans use the meat, which is considered a delicacy. The meat of the Cape pangolin is especially popular with local people (Schlitter, 2005). Pangolin hide is used for boots and other leathery goods. The scales are thought to have medicinal value as an antiseptic, fever combatant, skin disease defense, and aphrodisiac. The scales are used as is or ground into powder for potions. These uses have led to reduced pangolin population even though there are bans on commercial trade of all pangolin species. The seven species of pangolin are now all becoming extremely rare as a result of this (Pangolin Specialist Group, 1996).

Few researches have been conducted on some aspect of this animal. Ofusori et al. (2007a) in their work on the morphometric study of the stomach of African pangolin reported that the corpus of pangolin’s stomach is metabolically more active than the other parts of the stomach. Also, Adenowo and Caxton-Martins (2000) found from their investigation that the pineal gland of pangolin is markedly different from the human bovine pineal glands histologically.

In M. tricuspis, two birth records indicate that a litter was found in September and another in October. In other pangolins single young are born in a burrow and after several weeks are carried on their mother's tail, gradually learning to forage for themselves after several months (Schlitter, 2005).

One ovary or the other produces one egg per month during reproductive life in human. The ovary's spongy medulla is a mass of vessels in a loose connective tissue stroma. It fades into the cortex, where the eggs are. Cortical fibroblasts are arranged in dense cellular whorls, and are called “ovarian stroma”. After ovulation, the granulosa cells and the theca interna become the corpus luteum, a new endocrine organ which makes estrogen and progesterone last until the end of the cycle, and even longer if pregnancy takes place. The wall of the follicle collapses into folds which are mirrored into the structure. Blood and lymph vessels invade at once. The granulosa cells expand greatly (“luteinize”) and become steroid-secreters. At the center is the blood from ovulation, which organizes into scar just like in wound healing. The scar will remain as a wiggly corpus albicans . The corpus luteum is maintained by LH from the pituitary and (if pregnancy occurs) chorionic gonadotropin (hCG) from the trophoblast.

A thorough search into the literatures revealed that only little is known about the reproductive biology of this species. Hence, the need to comparatively investigate the morphostructural differences in the left and right ovaries of M. tricuspis. This will add to the known biology of this mammal.

Figure 1

Table 1: Scientific classification of African tree pangolin (Rafinesque, 1821)

Figure 2

Figure 1: Photograph of African tree pangolin (Ofusori and Caxton-Martins, 2005)

Materials And Methods

Care of the animals

Excision of the ovary

Histological Analysis

Morphometric Analysis

Statistical Analysis

Results

The left and right ovaries were observed to be spherical in shape and encapsulated by tunica albuginea. Further macroscopic examination of the two ovaries revealed that the average weight of the left is somewhat higher than the right. This was evident when the relative weights were compared (Table 2). A significant difference was again recorded when the diameters were compared (Table 2). The length of the left ovary 1.45±0.05 was observed to be significantly different (p<0.05) from the right ovary 0.89 ± 0.08. The volume of the two ovaries differ significantly (p<0.05) when compared (Table 2). The mean value of the thicknesses of the right and left ovaries also present a significant difference as shown in Table 2.

The results of the microstructural investigation of the right and left ovaries revealed some microanatomical differences in the two ovaries. The left ovary which is the larger of the two was observed to conform to the basic microstructural organization having both the cortex and medulla (Fig 3a). Few capillaries, fibroblast and smooth muscle cells were observed in its cortical region with follicles in various sizes and states of development (Fig 2a & 3a). This is however different from the right which has its entire stroma dominated by connective tissue fibers as well as smooth muscle cells without any clear demarcation into cortical and medullary regions (Fig 2b and 3b).

Figure 3

Table 2: Showing various measured parameters in the ovary of African pangolin (M ± SEM)

Figure 4

Figure 2: Photomicrograph of the ovaries of (a) left. Note the corpora albicantes (CA) and developing follicle (arrow) in the cortical region (b) right (H&E Mag. x40)

Figure 5

Figure 3: Photomicrograph of the ovaries of (a) left. Note the corpora albicantes (CA) (b) right (H&E Mag. X100)

Discussion

Our morphostructural study in this investigation demonstrates morphometric and microanatomical differences between the left and right ovaries of the African tree pangolin M. tricuspis. The results from the morphometric assessment of the left and right ovaries (Table 2) suggest a bilateral variation and a preponderance of the left ovary over the right. All the parameters of the two ovary showed significant (p<0.05) variation, indicative of better sequence of growth phase, development and differentiation in the left ovary as compared to the right. Although little is known about the reproductive biology of M. tricuspis, evidence from the histological analysis in this investigation suggests that the left ovary might be the principal organ of reproduction. A super-tough fibrous capsule- tunica albuginea was seen around the cortex in the two ovaries. Also, peritoneal membrane was attached and folded over most of the surface of the ovary. Histologically, two layers: theca interna and theca externa are recognized externally. The function of these layers may not be too different from that in human due to similar way of organization. For instance, theca interna consist of rounded cells which are believed to secrets estrogen, progesterone and androstenedione while theca externa are predominated by flattened stroma cells. The medullary part of the left ovary was observed to be highly vascularized and dominated by loose connective tissue (Fig 2a) while the cortical part was dominated by fibroblasts developing follicles corpora albicantes and few smooth muscle cells. This smooth muscle cells may be the termination of sympathetic fibers thus, playing some part in follicular maturation and ovulation (Heat et al., 1999). Developing follicles and the presence of corpora albicantes (Fig. 2a & 3a), makes it synonymous to what is obtainable in human as researched by Heath et al. (1999). This however placed the M. tricuspis ahead of some other mammals’ vis-à-vis the development and differentiation of the reproductive organ. In a similar work, Ofusori and Caxton-Martins (2005) revealed that the gastrointestinal tract in M. tricuspis was well developed and organized than what is obtainable in Rattus norvegicus and Eidolon helvum. This presupposes that the pangolin has some advanced (mammalian) features when compared with some other mammals.

The right ovary on the contrary has a uniform stroma without any clear demarcation in to cortical and medullary parts (Fig 2b & 3b). The entire stroma which is dominated by connective tissue fibers as well as smooth muscle cells makes it more or less rudimentary in contrast to the left.

In conclusion, the comparative morphometric and histological study reveals some differences between the left and right ovaries of M. tricuspis, suggesting the left to be the principal sex organ. Additional studies are needed to clarify if there are other bilateral differences in the remaining parts of the reproductive system of African tree pangolin M. tricuspis.

Acknowledgements

The authors are very grateful for the technical assistance of Kola Oyewole, Oluwaseun Timothy and Iniola Gbela of the Obafemi Awolowo University, Ile-Ife, Osun state, Nigeria.