Nematodes or round worms

Dr. Moreno would like to remind you that the assignments are overdue and that today is the last day

Questions asking for specific names & terms.

Name two parasites that on could acquire by eating raw fish.

Which process is used to increase the resilience of trematode egg shells?

What is the name of the stage that hatches from the eggs of trematodes?

Which important human disease is transmitted by flea bite?

Questions that ask for a concept and take a sentence or two

How does infection with Schistosoma cause disease? How do Trematodes and Cestodes amplify the number

of offspring? Why is the pork tape worm more dangerous than the

beef tape worm? What are the most important characteristics of the

immune system? What is the difference between disease prevalence,

incidence and intensity? What is the difference between holo- and hemimetabol

development in insects?

Recognize organisms and their organization

Which organism is this?

How do you call the mouthparts it uses to penetrate the skin?

How do you call the structure it uses to stay in the skin?

Case studies

John is a New Zealand sheep farmer and well known as a breeder of German shepherds. John visits UGA to take a summer class at the Vet School when he gets ill. He comes to your practice with complains about abdominal pain and loss of appetite. You find his liver swollen, and on further X-ray and CT diagnosis you establish a large round space filling lesion.

What might John have contracted? Would you try to demonstrate eggs in the feces to verify your suspicion?

How might John have gotten infected? What can be done to help John?

Parasitic nematodes

(Today) intro into general nematode biology & development (look at two diseases Trichuris and Trichinella)

(Next Tuesday) GI nematodes

(Thursday) Filariasis

There are many, many nematodes - counting species as well as individuals

20,000 nematode species have been described but probably this is only a small fraction of the actual diversity

A square meter of coastal mud yielded 4.4 million nematodes

A single decomposing apple in an orchard contained 90,000

The majority of nematodes are small predators or saprophytes (they eat decomposing organic matter)

Nematodes impact human life in many important ways

Nematodes cause numerous human diseases

Nematodes are abundant pathogens in life-stock and pets

Nematode are important pests of many crops

Nematodes provide powerful genetic models to study the basis of development, aging and many diseases including cancer

The evolution of nematodes

The phylogenetic relationships of nematodes (who is related to who) have been studied using sequence analysis of the small subunit RNA

Few clades are entirely parasitic or entirely free living

Most clades show considerable trophic variation (meaning different members eat different things)

Parasitism of vertebrates has arisen independently at least 4 times

Nematode anatomy

Long worms with often tapered ends

Unsegmented Liquid filled body cavity

(not lined by mesoderm: pseudocoel)

Surrounded by rigid cuticula

Simple organ and neuro-anatomy

Hypodermis organizes worm into quadrants

cuticula

Epicuticle: thin layer of lipids and proteins

Cortex: made from cross-linked cuticilin proteins and collagen fibres

Basal layers are formed by collagen fibers which are put down at angles to generate a lattice

Basal layer is fibrous and closest to hypodermis

cuticula

Nematodes molt 4 times The old cuticle separates from the epidermis and the innermost layer is partially hydrolyzed A new cuticula is secreted by the hypodermis starting with the epicuticle The old cuticle is shed

The hydrostatic skeleton and motility

Nematodes show a characteristic slightly stiff motility using sinuous undulations of their bodies

Nematodes lack circular muscles, the depend entirely on longitudinal muscles

Nematode mobility is generated by the interplay of muscles, pressurized pseudocoel and cuticula

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The hydrostatic skeleton and motility

The musculature is tightly attached to the hypodermis and cuticle

Coordinated muscle contraction in different quadrants (and halves) along the length of the worm results in bending of the cuticula

The stiff cuticula and internal pressure transform some of the bending into movement of the end of the worm

The hydrostatic skeleton and motility

Nematode muscles are organized into quadrants and contract coordinately

Each muscle cell contains striated fiber made up by actin (thin filaments) and myosin (thick filaments) which is responsible for contraction

The cell body of the muscle cell (myocyton) extends a contractile portion towards the hypodermis and cuticula and an ‘arm’ (or enervation process) towards the nerve which runs within the dorsal or ventral epidermal chord to “receive” motor signals

Nervous system The brain consists of a

single ring around the pharynx

A lateral and a ventral nerve extend back through the length of the worm

Labial and aphid nerves extend forward to enervate sensory organs (amphids and various papilla)

Amphids are simple organs of mechano- and chemoreception. The sensory organelles are modified cilia

Nematode digestive system, pumping against the pressure

Nematodes have a complete (but simple) digestive system with a mouth at the very tip of the animal.

The intestine is nonmuscular, and consists of a single layer tall columnar cells which carry microvilli and an underlying basal lamina

Food has to be pumped against the pseudocoel pressure by the muscular esophagus

Food is passed rapidly (minutes)

Reproduction

Most nematodes are dioecious (meaning they have two sexes, male and female), but hermaphrodites occur as well

Males are smaller than females, and often have additional external features at the posterior end of the worm

Partners are attracted to each other by pheromones

Reproduction

The female is held by the male within the bursa

The male inserts two sclerotized copulatory spicules

The sperm duct is muscular an sperm is transferred to the female against the pseudcoel pressure

Shape and size of spicules and bursa are important features to tell different nematodes apart

Male cement glands can close vulva in some species

Reproduction

The testis are usually represented by a single unpaired tubule

Spermatogenesis occurs along the length of the testis

Primary Spermatocytes are a large syncitium and arrested in Meiosis I

Meiosis II yields 4 spermatids which are stored

Upon activation (copulation) the spermatids become mobile spermatozoa

Reproduction

Nematode sperm are highly unusual in relying on amoeboid movement rather than a flagellum

Even more surprisingly, actin (the molecule required for amoeboid movement in most other animals) has no role in sperm motility

Motility depends on Major Sperm Protein (MSP)

Small highly basic protein MSP forms dimers which assemble

into subfilaments, two of these form a helical filament

Cells move by fiber growth through regulated addition of subunits

Reproduction

Most female nematodes have two ovaries

Structure is similar to the linear male system, oogonia are produced at the distal end and mature into oocytes while they move into the growth zone

As the oocytes enter the oviduct they are fertilized by sperm stored in the receptacle

Fertilization triggers the completion of meiosis

Development• Caenorhabdites elegans:

• Simple anatomy• Transparent: individual

cells can be visualized and manipulated

• Small number of cells: 558 in first larval stage

• Cells have stereotyped pattern of development: invariant lineage

• Genome sequenced: has ~19,000 genes

• Great genetics• Development from fertilized

egg to full animal can be watched in ~10 hours

http://www.bio.unc.edu/faculty/goldstein/lab/

Development

Nematodes are eutelic, which means they reach a predefined number of cells which then remains constant (only the germline keeps dividing)

The fate of each cell is predetermined and the embryo follows a very predictable pattern of divisions

Some cells loose nuclear DNA (chromatin diminution) while other proceed to polyploidy (the germline keeps a full diploid set of chromosomes)

Development

The development proceeds through 4 larval stages before reaching adulthood

Molting is required at each step Many nematodes can develop a

specialized resting or Dauer version of the L3 larval stage

In parasitic nematodes this L3 larvae is often the infectious stage for the final host

Transitions between aerobic and anaerobic metabolism

Different stages face very different environments and therefore have to be flexible

One example is the surprisingly flexible energy metabolism:

Larval stages are mostly aerobic. They have a fully active TCA cycle, oxidative phosphorylation and use oxygen as terminal electron acceptor

Adult forms live in the intestine with very little oxygen and have an anaerobic fermentative

metabolism.