<i>Cysticercus pisiformis</i>: ultrastructural transformation of the tegument during development from oncosphere to cysticercus :Nahla A Radwan, Mahmoud N El Sefy, Said A Noor El Din, Hewaydah E Abou Shafeey, Samy E Sharaf, Amal I Khalil, Parasitologists United Journal

RESEARCH ARTICLE
Year : 2014 | Volume : 7 | Issue : 1 | Page : 13--26

Cysticercus pisiformis: ultrastructural transformation of the tegument during development from oncosphere to cysticercus

Nahla A Radwan¹, Mahmoud N El Sefy², Said A Noor El Din¹, Hewaydah E Abou Shafeey¹, Samy E Sharaf¹, Amal I Khalil¹,
¹ Department of Zoology, Faculty of Science, University of Tanta, Tanta, Egypt
² Department of Parasitology, Faculty of Veterinary Medicine, University of Kafr-El Sheikh, Kafr-El Sheikh, Egypt

Correspondence Address:
Amal I Khalil
Department of Zoology, Faculty of Science, University of Tanta, Tanta, 31527
Egypt

Abstract

Background Cestode tegument is the barrier that separates the parasite from the host, allowing it to develop and survive the hostile environment of the host. It is the only site for nutrient intake. Objective The present study was conducted to reveal the fine structural transformation of the tegument of Taenia pisiformis cysticerci (Cysticercus pisiformis) during development from egg to cysticercus stages. Materials and methods The present study records the development of C. pisiformis in experimentally infected domestic rabbits, with special emphasis on the ultrastructural variations within different stages of larval development using both scanning (SEM) and transmission (TEM) electron microscopes. Results Three to six days postinfection, the early developed scolex appeared as an invaginated thickening at the anterior end of the developed metacestode. The first development of the rostellar hooks and invagination canal was observed 1 week postinfection (PI), where the hooks appeared as minute conical bodies. Complete development of the invagination canal and hook crown was observed 2 weeks later, synchronizing with the onset of sucker differentiation. Fine structural transformation of the tegument included variations in the structure of microtriches (length, density, and shape); distal cytoplasm and parenchymal vesicles and inclusion bodies (size, shape, distribution, and electron density); and tegumental and parenchymal muscles (thickness, orientation, and distribution). Conclusion The tegument of different developmental stages of T. pisiformis cysticerci has the same basic pattern, with some variations in the subcellular structures, which supports the suggestion that T. pisiformis can be used as an experimental model in cysticerci research.

How to cite this article:
Radwan NA, El Sefy MN, Noor El Din SA, Abou Shafeey HE, Sharaf SE, Khalil AI. Cysticercus pisiformis: ultrastructural transformation of the tegument during development from oncosphere to cysticercus.Parasitol United J 2014;7:13-26

How to cite this URL:
Radwan NA, El Sefy MN, Noor El Din SA, Abou Shafeey HE, Sharaf SE, Khalil AI. Cysticercus pisiformis: ultrastructural transformation of the tegument during development from oncosphere to cysticercus. Parasitol United J [serial online] 2014 [cited 2023 Dec 8 ];7:13-26
Available from: http://www.new.puj.eg.net/text.asp?2014/7/1/13/139686

Full Text

Introduction

Cysticercosis is an important zoonotic disease that causes medical and economic losses in many countries. Taeniid cysticerci have high pathogenic potentials and are capable of surviving for prolonged periods in tissues of their intermediate hosts causing illness to humans (Taenia solium) and losses to breeders (Taenia ovis, Taenia saginata, and Taenia pisiformis). This lies in their ability to exploit the immune system. Studies showed that tegumental vesicles in the bladder wall of taeniid metacestodes appear to transport proteins into the cyst fluid. Some of these vesicles fuse with lysosomes. Cysteine proteinase has been reported in tegumentary lysosome-like vesicles and is probably involved in the modulation of host immune response and the degeneration and utilization of immunoglobulins, which represents a major source of amino acids [1],[2] . Such a mechanism enables the parasites to avoid the host immune response and at the same time to build their own bodies.

Because the tegument is the only barrier that separates the parasite from the host, revealing the fine structure of the tegument during development from egg to cysticercus is essential in understanding the mechanisms of metamorphosis and the ability to survive the immune response of the host. Such information may be used to combat the parasite by chemotherapeutic means. A previous comparative ultrastructural study was conducted on the tegument of mature cysticerci of T. pisiformis, T. ovis, and T. saginata, revealing some regional similarities, as well as variations in the subcellular structures of different parts of these cysticerci [3] .

The present study was undertaken to reveal the structural significance and stages of metamorphosis of the subcellular components of the tegument during the stages of development of T. pisiformis from egg to mature cysticercus. In the light of these findings, the study aims to evaluate the possibility of using T. pisiformis as an experimental model in the study of cysticercosis.

Materials and methods

Type of study

This is a comparative ultrastructural study.

The study was conducted in the Central Laboratory, Faculty of Science, Tanta University from August 2009 to September 2010.

Experimental animals

For the experimental life cycle, 36, 3-4-month-old domestic rabbits weighing 900-1500 g were housed in the laboratory and fed standard diet (clover) until used. Three, 30-35-day-old puppies were kept in the laboratory and fed bread and milk. Before experimental infection, puppies were injected with Ivomec (Sigma, Germany) (200 mg/kg body weight) to control possible natural infection with the cysts.

Infection of the dog definitive host

Rabbits collected from markets were killed, dissected, and examined for T. pisiformis cysticerci attached to the upper part of the stomach and the terminal end of the ileum. Cysts with invaginated scoleces were kept in normal saline at 4°C. Puppies were orally infected, each with 15 cysticerci, using a stomach tube.

Infection of the rabbit intermediate host

Gravid segments of T. pisiformis were collected from the stools of experimentally infected dogs 60 days PI. Domestic rabbits were experimentally infected by feeding each with two gravid segments. Infected rabbits were autopsied at different time intervals (15, 30, 60 min, 2 h, 3 days, 1, 3, and 5-6 weeks) to collect cysticerci at different developmental stages. The time schedule was implemented randomly by the authors to detect every possible change in the ultrastructure of the tegument during the development of Cysticercus pisiformis.

Preparation of stained whole mounts of cysticerci

Flattened cysticerci were hydrated in descending grades of ethanol and stained with acetoalum carmine. Specimens were differentiated in acid alcohol, dehydrated in ascending grades of ethanol, and cleared in clove oil. Cleared specimens were permanently mounted in Canada balsam.

Electron microscopy

Cysts were washed in 0.9% saline solution and shaken vigorously to relax and remove any tissue debris. Specimens were fixed overnight in 3% glutaraldehyde in 0.1 mol/l PBS, postfixed in 1% buffered osmium tetraoxide for 1-2 h. For SEM, specimens were dehydrated through graded series of ethanol, critical point dried, coated with gold, and examined with a Joel Electron Microscope (Germany-GmbH) at 15 kV. For TEM, according to the protocol of EM Lab Unit, Alexandria University, cysts were embedded in Epon araldite. Semithin sections (1 μm) were cut using a LKB ultramicrotome (Milpitas, USA) and stained with 1% toluidine blue for light microscopy. For TEM, ultrathin sections (60-90 μm) were mounted on copper grids and stained with uranyl acetate followed by lead citrate. Examination and electron micrographs were made using a Jeol 1200 CX Electron Microscope at 80 kV.

Results

Experimental life cycle study

The developmental stages (from oncospheres to cysticerci) collected from the alimentary tract, liver, and mesenteric membrane of experimentally infected rabbits were examined at different time intervals: 15, 30, 60, 2 h, 3 days, 1, 3, and 5-6 weeks.

Development of Taenia pisiformis from egg to cysticercus

Fifteen minute PI, a small tear was observed on one side of the egg shell in the lumen of the rabbit intestine [Figure 1]a. Few minutes later, a spherical oncosphere was completely liberated in the duodenal cavity of the rabbit. Thirty minute PI, poorly differentiated oncospheres were seen in the mucosa and submucosa of the intestine [Figure 1]b. Sixty minute PI, oncospheres were generally spheroid with three distinct pairs of marginal hooks. Hooks were slender and directed backwards, frequently seen pushed out over the relaxed oncosphere [Figure 1]c and d. Two hours PI, oncospheres were found attached to the duodenal mucosa and submucosa, where they penetrated the intestinal wall of rabbits on their way to reach the parenteral sites and metamorphosed to a cysticercus type metacestode; they usually reach the liver, probably through the blood vessels after 24 h.{Figure 1}

Three days PI, early developed cysticerci appeared as very small undifferentiated immature ovoid cysts attached to the liver surface. The scolex formed an invaginated thickening at the anterior end of the developed metacestode, but hooks and suckers were not differentiated [Figure 2]. An invaginated notch at the tip of the body marked the presence of the invagination canal [Figure 3]a and b. The body was covered with corrugated tegument; no typical microtriches were apparent [Figure 4].{Figure 2}{Figure 3}{Figure 4}

One week PI, the scolex appeared as a tissue mass projecting into the internal cavity of the cysticercus, allowing the invagination canal to increase in depth and length. The scolex was increased in size, and the posterior end of the body developed a bladder [Figure 5]a and b. Rostellar hooks began development as randomly distributed minute conical bodies in the tegument surrounding the basal region of the rostellar cone. At a later stage, the conical bodies formed hooks that accumulated around the rostellar cone. Suckers were not well differentiated at this stage of development [Figure 5]b and [Figure 6]. The tegument was corrugated and microtriches developed into closely packed filamentous structures covering the whole body [Figure 7].{Figure 5}{Figure 6}{Figure 7}

Three weeks PI, apparent development of the cysticercus was shown as illustrated by the presence of suckers and rostellum [Figure 8] and [Figure 9]. The bladder became wider [Figure 10] and microtriches appeared pointed and covered the wrinkled tegumental surface of the body [Figure 11]. Hooks were arranged in two rows, each of 20-21 taenoid hooks at the base of the rostellar cone. The suckers became apparent on the sides of the scolex [Figure 8] and [Figure 9].{Figure 8}{Figure 9}{Figure 10}{Figure 11}

Five to six weeks PI, a fully developed cysticercus was formed, and the scolex was well differentiated [Figure 12]a and b and [Figure 14]a and b. The suckers were formed of thick muscular wall [Figure 13]. Scanning electron microscopy revealed that both the rostellum and the hooks were covered with thin tegumental membrane [Figure 14]a and b. In a fully formed hook, the blade appeared well developed, and the handle and guard were short and continued to increase in length after 5 weeks [Figure 15]. In the completely developed stage, the scolex carried four cup-shaped well-developed suckers and a rostellum armed with two circles, each of 21 taenoid hooks [Figure 16]. The tegument covering the whole body was deeply wrinkled with long slender well-developed microtriches [Figure 17] and [Figure 18].{Figure 12}{Figure 13}{Figure 14}{Figure 15}{Figure 16}{Figure 17}{Figure 18}

Ultrastructural transformation of the tegument of Taenia pisiformis during development from oncosphere to cysticercus

The description of the fine structure of the tegument of different developmental stages is dealt with respect to the three anatomical regions of the cysticercus: the bladder, the scolex, and the invagination canal. The structure of these regions is described in the chronological order.

Tegument covering the bladder

Three days PI, the tegument was poorly differentiated, where the apical plasma membrane was corrugated, dense, and covered with granular material. No microtriches were apparent at this stage of development [Figure 19]. Tegument differentiation started 1 week PI, where the apical plasma membrane of the bladder wall appeared dense, corrugated, and covered with granular material [Figure 20]. Two weeks later, typical microtriches with dense-walled base covered the tegument. Although the shaft was not clear at this stage, the cross and sagittal sections of microtriches appeared to lie on top of apical plasma membrane, revealing that they were oriented parallel to the body surface rather than perpendicular. The fine structure of the tegument revealed the presence of long slender bases, but the typical fibrillar nature of the shaft was not revealed in any of the sections [Figure 22]. However, it was clearly distinguished later in the 5-6 weeks stage, where typical microtriches appeared to be formed of a base, which often enclosed a dense core, a dense pointed shaft, and a base plate in-between [Figure 25].{Figure 19}{Figure 20}

The distal cytoplasm of 3-day-old and 1-week-old cysticerci appeared as an opaque granular matrix containing fibrillar elements and inclusion bodies and vesicles [Figure 19] and [Figure 20]. Differentiation of the distal cytoplasm started 3 weeks PI, where vesicles with dense material and dense discoid bodies were formed [Figure 22] and [Figure 23] and persisted in the later stage (5-6 weeks) of development [Figure 24] and [Figure 25]. Fibrillar elements observed separately or in bundles in 3-day-old cysticerci [Figure 19] developed into 1-week-old cysticerci to form thick bundles of tegumental muscles underneath the tegument [Figure 21], [Figure 29], that may contain dense deposits [Figure 29]. Bundles of circular and longitudinal muscles appeared irregularly scattered underneath the basement lamella of cysticerci as early as 3 weeks PI and became well oriented in 5-week-old cysticerci [Figure 26].{Figure 21}{Figure 22}{Figure 23}{Figure 24}{Figure 25}{Figure 26}

Three days PI, both tegument and parenchymal cells were not yet differentiated [Figure 27]. In 1-week-old cysticerci, dense ovoid tegumental cells with large nuclei containing dense chromatin bodies and perinuclear cytoplasm enclosing glycogen granules and lucent vacuoles were distributed irregularly deep in the bladder wall [Figure 28]. Lucent vesicles, large vacuoles, and dense bodies were seen deep in this layer [Figure 28]. Tegumental cells with perinuclear cytoplasm rich in elements of endoplasmic reticulum, ribosomes, glycogen granules, and dense bodies appeared in 5-6-week-old cysticerci [Figure 30].

Three days PI, the inside of the cysticercus appeared filled with lucent material that contained membrane-bound lacunar structures [Figure 27]. The parenchymal matrix of 3-week-old cysticerci contained fibrillar elements enclosing large lucent and opaque bodies and irregular parenchymal cells of different shapes and sizes [Figure 31]. Fibrillar and membranous structures, large lucent vacuoles, and opaque and dense bodies were observed in the parenchymal matrix in the 5-6-week stage [Figure 32].{Figure 27}{Figure 28}{Figure 29}{Figure 30}{Figure 31}{Figure 32}

Tegument covering the invagination canal

Tegument covering the invagination canal of 1-week-old cysticerci was corrugated and carried undifferentiated small projections; typical microtriches were not apparent [Figure 33]. Two weeks later, the tegument appeared corrugated with the apical plasma membrane thrown into numerous microtriches with long dense-walled base and a dense shaft that varied in length in different sites [Figure 34] and [Figure 35]. This stage was the first to express the basic pattern of microtriches being formed of a base and a shaft. In 5-6-week-old stage, the outer plasma membrane was thrown into numerous filamentous microtriches [Figure 36] and [Figure 37].{Figure 33}{Figure 34}{Figure 35}{Figure 36}{Figure 37}

In the distal cytoplasm of 1-week-old stage, only lucent vesicles appeared [Figure 33]. In 3-week-old cysticerci, the distal cytoplasm became thick and rich in inclusion bodies and electron-lucent vesicles of different shapes and sizes, which sometimes appeared merged with the apical plasma membrane [Figure 34] and [Figure 35]. The electron-lucent basal lamella appeared as an undulated ribbon underneath the distal cytoplasm [Figure 34]. Considerable variation appeared in the 5-6-week stage, where deep notches and plateau-like areas were detected on the surface [Figure 36]. Electron-lucent vesicles of different shapes and sizes and electron-dense discoid bodies were scattered irregularly in the homogenous syncytium [Figure 37].

Tegumental cells of 1-week-old cysticerci were numerous, arranged at different depths underneath the muscle layer [Figure 33]. As development proceeded (5-6-week-old cysticerci), the tegumental cells increased in number, differentiated in shape and size, possessed large nuclei, and connected to the distal cytoplasm by distinct cytoplasmic extensions crossing the muscular layer [Figure 30] and [Figure 38].{Figure 38}

In 1-week-old cysticerci, muscle fibers were arranged mainly in a circular direction with few vertically arranged bundles [Figure 33]. Small but numerous bundles of longitudinal and circular muscle fibers appeared in the 3-week-old stage [Figure 34]. Numerous longitudinally and circularly oriented muscle bundles supported the invagination canal of 5-6-week-old cysticerci [Figure 36], [Figure 37], [Figure 38].

In the parenchyma matrix of 1-week-old stage, lucent vesicles of variable size and electron-dense bodies were frequently seen [Figure 33]. Homogenous opaque bodies that seem like precursors of calcareous corpuscles were seen deep in the parenchyma matrix of 1-week-old cysticerci [Figure 39]. The parenchyma matrix of 5-6-week-old cysticerci held fibrillar and membranous structures, large lucent bodies, and glycogen granules [Figure 40].{Figure 39}{Figure 40}

Tegument covering the scolex

In 1-week-old cysticerci, the scolex was not observed in all examined sections.

Tegument covering the sucker

The apical plasma membrane of 3-week-old and 5-6-week-old cysticerci carried microtriches with electron-dense core, each formed of a long cylindrical base and a short pointed shaft [Figure 41] and [Figure 42]. The distal cytoplasm of 3-week stage appeared rich in electron-lucent vesicles, dense bodies, and ovoid mitochondria [Figure 41]. In 5-6-week-old cysticerci, the distal cytoplasm was filled with electron-lucent round and ovoid vesicles of various sizes; some enclosed electron-dense material. Large lucent vacuoles were seen interrupting this layer [Figure 42]. In 3-week-old stage, the thin fibrous basement lamella appeared underlined by small bundles of muscle fibers arranged in different directions and separated by fibrillar matrix rich in vesicular bodies [Figure 41]. Secretory bodies were commonly found in between the muscle bundles [Figure 41]. In 5-6-week-old stage, thick dense fibrillar connective tissue basal lamina was observed underneath the moderately thick distal cytoplasm [Figure 42], and the suckers differentiated to form outer and inner edges. A very thick layer of muscle bundles was detected, arranged in vertical, oblique, circular, and longitudinal planes. In-between these muscle bundles, packets of electron-dense and others of electron-lucent bodies (most probably parts of secretory cells) and large mitochondria were observed [Figure 43] and [Figure 44]. In 3-week-old stage, the parenchyma was formed of fine reticular fibrillar extracellular matrix that contained parenchymal cells with large nucleus and dense perinuclear cytoplasm that contained glycogen and vesicular bodies. The fibrillar matrix enclosed homogenous opaque bodies with dense walls [Figure 45]. In 5-6-week-old stage, the parenchyma enclosed calcareous corpuscles, lucent vesicles, bundles of fibrillar structure, and parenchymal cells [Figure 46].{Figure 41}{Figure 42}{Figure 43}{Figure 44}{Figure 45}{Figure 46}

Tegument covering the area between suckers

Microtriches were seen in 3-week-old cysticerci; however, they increased in number and became densely packed in 5-6-week old stage [Figure 49]. The distal cytoplasm of 3-week-old and 5-6-week-old cysticerci was vesicular and contained electron-dense bodies [Figure 48]. As early as 3 weeks the tegument muscles appeared to be arranged in circular and longitudinal bundles and enclosed dense inclusion bodies and lucent vesicles [Figure 47] and [Figure 48]. Two types of tegumental cells were detected in 3- and 5-6-week-old cysticerci [Figure 47] and [Figure 50]. The first type of tegumental cells was characterized by granular cytoplasm, large nucleus, and mitochondria, whereas the second possessed lucent cytoplasm, dense nucleus, and perinuclear cytoplasm rich in ribosomes. Lucent vacuoles and mitochondria were distinctly observed in 5-6-week-old stage. Calcareous corpuscles with successive concentric lamella and granular matrix were observed as early as 3 weeks PI in the parenchymal matrix [Figure 51].{Figure 47}{Figure 48}{Figure 49}{Figure 50}{Figure 51}

Tegument covering the rostellum

The first differentiation of the tegument covering the rostellum could be detected in the 5-6-week-old cysticerci, being characterized by thick dense distal cytoplasm with outer plasma membrane carrying relatively long microtriches with cylindrical base and dense tapered posterior-directed shaft. In comparison with those of the sucker, the rostellar microtriches were more densely distributed [Figure 52].{Figure 52}

Discussion

T. pisiformis develops as adults in the intestine of dogs, whereas its metacestode, known as C. pisiformis, develops in rabbits and becomes lodged in the body cavity to form infective cysts in the mesenteries. T. solium and T. saginata are of such close taxonomic relationship that each has long been regarded as a model for the other. In these species, the only available final hosts are humans. This imposed ethical and economic restrictions on the extent of the experimental work that could be attempted. In the present study, T. pisiformis was chosen as an experimental model because of its developmental stages that closely resemble that of T. saginata and T. solium, and the hosts - dogs and rabbits - were chosen because they are easily maintained. In a previous study, Khalil et al. [3] revealed the fine structural variations of the tegument among cysticerci of T. pisiformis, T. solium, and T. saginata. The present study involves the tegumental development and metamorphoses of cysticerci.

The rostellar development of T. pisiformis cysticerci in the present study resembles that described by several studies [4],[5],[6],[7] but varies from that of Jennifer et al. [8] who detected differentiation of the scolex tissues in C. pisiformis as early as 6 days PI. In the present study, invagination of the tegument of the scolex anlage was detected in 1-week-old cysticerci, and proliferation of the tegument cells at the anterior end was observed in 10-15-day-old cysticerci that had prominent rostellar anlage, suckers, rostellar pad, and hooks, but the development was not complete. Regarding the development of rostellar hooks, the present study revealed that it involves elongation and flattening of the lateral surface and the appearance of a striated core. Secondary thickening of the hook blade occurred by an increase in the thickness of the anterior and posterior blade margins together with vacuolation of the blade core. Development of the handle commenced to deposit hook material, which was associated with a frayed appearance in the fully formed hook base. The blade was well developed and all trace of active deposition disappeared. However, the handle and guard appeared short and continued to increase in length after 35 days. During development of the rostellar hooks, evagination occurred with increase in size to reach that of hooks of the adult worm. Movese-Yan [9] found that development of rostellar hooks in T. pisiformis took place from 32 to 46 days; the double crown was first differentiated into large and small hooks in 30-day-old cysticerci, and the growth of hooks was completed in 60-day-old cysticerci, although they became infective to dogs by day 35, and that growth of hooks continued in sexually mature forms. Several studies [10],[11],[12] reported that hooklets arise from specialized tegumental microtriches and increase in size by the internal deposition of hook material; secondary blade thickening and formation of the hook handle and guard occur subsequently by secretion from the hypertrophied tegument surrounding the blade or from the tegument at the base of the hook.

In cestodes, a gut is lacking and nutritional uptake has to be mediated completely by the tegument surface. Moreover, the tegument has to resist the attack of digestive enzymes and the immune response of the host; therefore, the study of the tegument in all developmental stages is of fundamental importance. In 1-week-old cysticerci, the apical plasma membrane, which was dense, corrugated, covered with granular material, and provided with few microtriches, is similar to that of Taenia taeniaformis in rat at 12 days as described by Engelkirk and Williams [13] .

In the present study, long microtriches typically formed of dense pointed shaft and elongated dense flattened base were presented as early as 2 weeks in the scolex of cysticerci of T. pisiformis and in the invagination canal of 3-week-old larval stages. A similar type of microtriches was reported in Hymenolepis spp. [14],[15] , on the bothria of the trypanorhyncha Nybelinia edwinlintoni [16] , and on T. pisiformis cysticerci [3] . In the 5-6-week-old metacestode, tegument of the invagination canal showed deep notches with plateau-like areas and microtriches formed of elongated base and needle-like cylindrical shaft. The density and size of microtriches of the scolex of 5-6-week-old T. pisiformis cysticerci are similar to those covering the scolex of T. ovis [3] and T. crassiceps [17] cysticerci. The presence of various types of microtriches during larval development of T. pisiformis cysticercus reflects the multifunctional role of such structures, including absorption, transportation, protection, and anchorage [3],[14],[18],[19],[20],[21] .

The present study revealed the common features of cestode distal cytoplasm as having various types of dense inclusion bodies and lucent vesicles in all regions of different developmental stages, including scolex (suckers, rostellum, and tegument between suckers), invagination canal, and bladder. These vesicles may have dense walls as seen in the tegument between the suckers in different larval stages, and some of them were seen emerging in the invagination canal of 3-week-old and 5-6-week-old cysticerci. Large lucent vacuoles were seen interrupting the distal cytoplasm in the tegument of the area between suckers from 2- to 5-6-week-old cysticerci.

In two studies [22],[23] , it was suggested that the rod-shaped bodies in the distal cytoplasm may be secretory in function. Flisser [24] and Abo laban [25] suggested that the electron-lucent vesicles, which sometimes enclose particulate material and that were sometimes seen disrupting the apical plasma membrane, may be secretory, excretory, or pinocytotic. The basic structure of the tegumental cells was similar in different larval stages of development and resembles to a great extent to those in other adult cestodes. However, variations between different regions in different stages were recorded in the shape and size of nuclei, the lucent vesicles in the perinuclear cytoplasm, and in the presence or absence of ribosomes, vacuoles, and glycogen granules. The presence of free ribosomes, mitochondria, and ribosome-rich cisternae of the endoplasmic reticulum indicates the involvement of these cells in different metabolic processes, including protein and carbohydrate synthesis [26],[27] , as well as energy production and transport mechanisms [20],[21] , suggesting that structural and enzymatic proteins may be synthesized in the tegument cells and transported into the distal cytoplasm.

Spindle-shaped tegument cells filled with electron-dense bodies that were found in-between the scolex musculature, invagination canal, and in the suckers of all larval stages of development resemble to a great extent to secretory cells described by several investigators [28],[29],[30],[31],[32] . Thompson et al. [29] and Smyth and McManus [32] suggested that these cells are neurosecretory in nature and their occurrence in the scolex adds a new physiological function to this organ. Besides its function as organ of attachment, it may have placental function and absorb nutrients directly from the host. This function is limited in cysticerci but becomes active as the cysticerci evaginate and mature. Variations in the structure of muscles in different stages of development result from the fact that these stages of development inhabit different environments such as liver surface, peritoneal cavity, and mesenteries of the intermediate host (rabbit), and hence are subjected to different types and degrees of pressure and different environmental conditions in the final host. The parenchyma with its numerous inclusions has storage function. Muscular contraction is probably responsible for the movement of fluids within the interstices between parenchymal tissues.

Conclusion

The present study revealed that the tegument of different developmental stages of T. pisiformis cysticerci has the same basic pattern, with some variations in the subcellular structures. These variations may result from the fact that the tegument metamorphoses during development to survive different habitats. This allows mature cysticerci fitness to survive the hostile environment of the intermediate host. These variations also provide it with the appropriate structural mechanisms that support its ability to infect the final host and live in different habitats. Comparison between the present study and previous reports supports the suggestion that T. pisiformis can be used as an experimental model in cysticerci research; however, further studies on the biochemical characteristics and antigenicity of this parasite in comparison with other taeniid species are needed.

Author contribution

N.A. Radwan contributed in performing TEM and data analysis and in writing and editing of the manuscript, M.N. El Sefy and S.A. Noor El Din supervised data analysis, H.E. Abou Shafeey contributed in editing the manuscript, S.E. Sharaf collected the material and performed the experiments, A.I. Khalil suggested the study point, supervised data analysis, and performed the proof reading.

Acknowledgement

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