Nematode Biology: Structure and Function

• Study notes on Life Sciences II, Zoology and Laboratory Animals
• Study notes on Phylum of Roundworms (Nematoda), and Segmented worms (Annelida)

Phylum: Roundworms (Nematoda)

• Six intestinal nematodes commonly infect humans: Enterobius vermicularis (pinworm), Trichuris trichiura (whipworm), Ascaris lumbricoides (large roundworm), Necator americanus and Ancylostoma duodenale (hookworms), and Strongyloides stercoralis
• Nematodes infect more than 25% of all humans collectively
• Nematode infection can cause embarrassment, discomfort, malnutrition, anemia, and occasionally death

Body Plans of Nematodes

• All intestinal nematodes have cylindrical, tapered bodies covered with an acellular cuticle.
• Layers of muscle, longitudinal nerve trunks, and an excretory system exist between the integument and body cavity
• A tubular alimentary tract runs from the anterior mouth to the posterior anus
• Highly developed reproductive organs fill the body cavity
• Sexes are separate, and males are generally smaller than females

Body Cavities of Nematodes

• Nematodes are triploblastic, display bilateral symmetry, and are unsegmented
• They are pseudocoelomate
• The body cavity, called pseudocoel, derives from the blastocoel formed during embryological development
• The coelom does not form from mesoderm
• Pseudocoel contains vacuolated cells filled with protein-rich fluid, creating high hydrostatic pressure

Nematode Physiology

• Nematodes vary in size, from microscopic to one meter long
• They have a tube-like digestive system (alimentary canal) with two openings
• A fluid-filled space is present between the body wall and alimentary canal, forming a "tube within tube" structure
• Parasitic forms have a simple digestive system
• The excretory system consists of excretory canals running longitudinally and uniting anteriorly into a single canal
• The canal opens via an excretory pore on the ventral surface
• A nerve ring surrounds the pharynx, giving rise to dorsal, ventral, and lateral nerve cords
• Nerves run throughout the body
• Sensory organs are in the form of sensory papillae
• Locomotion uses muscles arranged in four bands (two dorso-lateral, two ventro-lateral).
• They lack circular muscles, allowing only dorso-ventral bending
• Reproduction involves separate sexes
• Female gonads produce eggs
• Male gonads produce sperm
• A larval stage is present during the life cycle.

Species: Ascaris (Ascaris lumbricoides)

• Ascariasis results from parasitic roundworm Ascaris lumbricoides
• Ascaris lumbricoides is a short-lived worm (6-18 months), and the largest and most common intestinal helminth
• It measures 15 to 45 cm in length
• Ascaris stands out amongst other gut roundworms
• Ascaris has a firm, creamy cuticle and more pointed extremities
• The male is smaller than the female
• The male has a curved tail with copulatory spicules
• The female passes 200,000 eggs daily, whether fertilized or not
• Eggs are elliptical, 35 x 55 µm
• Eggs have a rough, mammillated, albuminous coat over chitinous shells
• Ascaris eggs are highly resistant to environmental conditions and can remain viable for up to 6 years in mild climates.
• Females measure 25-49 cm in length and 5 mm in diameter, making them longer and stouter than males
• The female genital aperture, or vulva, is located mid-ventrally at about 1/3 the length from the mouth
• The anus is located a little in front of the tail end
• Egg laying capacity of Ascaris is high, about 200,000 eggs a day
• Males measure 12-23 cm in length and 3-4 mm in diameter
• The tail (posterior end) is curved ventrally, forming a hook with a conical tip
• The ventral side of the tail exhibits a single opening, the cloacal aperture, through which the male and female reproductive systems open
• The cloacal aperture also contains a pair of copulatory spicules of equal length.

Ascaris lumbricoides Life Cycle

• Adult ascarids live high in the small intestine and maintain their position by swimming against the stool stream instead of burrowing
• Eggs deposited into the intestinal lumen are passed in the feces and must embryonate in soil for at least 3 weeks before becoming infectious
• Eggs of ascaris must be ingested for the cycle to continue
• Ascaris larvae hatch and penetrate the intestinal mucosa, then invade the portal venules
• They travel to the liver, squeeze through capillaries, and exit in the hepatic vein
• Next they are carried to the right side of the heart and subsequently pumped to the lung
• Here the larvae increase in size and become too large to pass through pulmonary capillaries to the left side of the heart, and instead rupture into the alveolar spaces
• Larvae are coughed up, then swallowed to regain access to the upper intestine where they complete maturation and mate
• High oxygen tension in the alveoli may provide a growth advantage for larvae

Species: Pinworm (Enterobius vermicularis)

• Pinworm infection, known as enterobiasis, results from the pinworm
• Adult female pinworms are 10 mm long, cream-colored, with a sharply pointed tail
• Running longitudinally on both sides of the body are small ridges that widen anteriorly to fin-like alae
• Males are smaller (3 mm) and possess a ventrally curved tail and copulatory spicule
• Clear, thin-shelled, ovoid eggs are flattened on one side and measure 25 by 50 μm

Enterobius Life Cycle

• Enterobius has the simplest life cycle of intestinal nematodes
• Adult worms attach to the mucosa of the cecum, where the male inseminates the female
• The female migrates down the colon and deposits around 20,000 sticky eggs on host perianal skin, bedclothes, and linens
• Eggs are near maturity at the time of deposition
• Handling bedclothes or scratching the perianal area results in adhesion of the eggs to fingers and ingestion during eating or thumb sucking
• Eggs may be shaken into the air, inhaled, and swallowed
• Eggs hatch in the upper intestine and the larvae migrate to the cecum, where they mature and mate
• The entire adult-to-adult cycle completes in 2 weeks

Species: Trichina (Trichinella spiralis)

• Trichinosis (trichinellosis) is caused worldwide by Trichinella spiralis and related Trichinella species
• The disease spreads through ingestion of undercooked meat, most commonly pork in areas where pigs feed on garbage
• When infected raw meat is ingested, Trichinella larvae free themselves from cyst walls using gastric acid and pass into the small intestine
• Larvae then invade intestinal epithelial cells, where they develop into adults
• The adults release infective larvae
• Females are 2.2 mm long, and males are 1.2 mm
• The life span in the small bowel is approximately 4 weeks
• After a week, the females release larvae, which migrate to striated muscles and encyst
• Parasites travel to skeletal muscle via the bloodstream.
• They invade muscle cells, enlarge, and form cysts
• Larvae may be viable for years
• Pigs and other animals become infected by eating infected, uncooked food scraps or other animals, such as rats

Phylum: Segmented Worms (Annelida)

• Body is metamerically segmented
• Symmetry is Bilateral
• The body wall has outer circular and inner longitudinal muscle layers
• Chitinous Setae are often, but not always, present
• A well-developed coelom is divided by septa, except in leeches
• Coelomic fluid supplies turgidity and functions as a hydrostatic skeleton

Annelida Closed Systems and Gas Exchange

• Blood system is closed and segmentally arranged
• Hemoglobin, hemerythrin or chlorocruorin are often present
• Amoebocytes are found in blood plasma
• The digestive system is complete and not metamerically arranged
• Respiratory gas exchange occurs through the skin

Annelida Nervous System

• These organisms have a nervous system with double ventral nerve cords and a pair of ganglia with lateral nerves in each metamere
• They also have a brain of dorsal cerebral ganglia with connectives to cord

Annelida Reproduction

• Earthworms are hermaphrodites, where each individual contains both male and female sex organs
• The male and female sex organs can respectively produce both sperm and egg in each earthworm
• Although earthworms are hermaphrodites, most still need a mate to reproduce
• The two worms line up inverted from each other so sperm can be exchanged
• Earthworms each have two male openings and two sperm receptacles, which take in the sperm from another mate
• Earthworms have a pair of ovaries that produce eggs
• The slittelum will form a slime tube around it, and fill with albuminous fluid
• The earthworm will move forward out of the slime tube
• As the earthworm passes through the slime tube, the tube will pass over the female pore picking up eggs
• The tube will continue to move down the earthworm and pass over the male pore called the spermatheca, which contains stored sperm or spermatozoa
• The eggs will fertilize and the slime tube will close off as the worm moves completely out of the tube
• The slime tube will form an "egg cocoon" in the soil
• The fertilized eggs will develop and become young worms
• Reproduction is hermaphroditic or asexual
• Larvae, if present, are a trochophore type
• Asexual reproduction can occur via budding
• Spiral cleavage and mosaic development happens

Explore the distinctive structural features of nematodes and their impact on their biology. This includes their alimentary canal as a vulnerable target for drugs, their hydrostatic pressure, and the body plan adaptations that facilitate their parasitic lifestyles. Understand their success in diverse environments, and their reproductive strategies.