Chapter 20 Cestoidea: Form, Function, and Classification of Tapeworms

Chapter 20 Cestoidea: Form, Function, and Classification of Tapeworms

Overview

• Sexually mature tapeworms are found in the intestines of all classes of vertebrates.

• There are two known forms of tapeworms that mature in invertebrates.

• Notably, adult tapeworms do not possess a digestive system.

Adult Body Form

Scolex

• Definition: The scolex is the point of attachment of the tapeworm to the host tissue.

• Contains a variety of holdfast organs including:

  • Acetabula:

  • Description: Cup-shaped structures that help in attachment.

  • Note: Acetabulate worms feature a rostellum, a protruding circle equipped with hooks.

  • Bothridia:

  • Description: Muscular, leafy structures organized in groups of four.

  • Bothria:

  • Description: Grooves that also assist with attachment.

Apical Organs

• Definition: Apical organs are involved in tissue invasion and aid in the development of neck cells.

Function of the Neck

• The neck of the tapeworm contains stem cells responsible for the production of new proglottids (segments).

Strobila

• Definition: Strobila refers to a linear series of maturing and reproductive proglottids, characterized by three stages:

  • Immature: Early developmental stage.

  • Mature: The stage where proglottids can produce eggs.

  • Gravid: Fully developed proglottids filled with eggs.

Proglottid Classification

• Monozoic:

  • Definition: Refers to tapeworms with one proglottid.

• Polyzoic:

  • Definition: Refers to tapeworms with multiple proglottids.

Methods of Fertilization

• Tapeworms use various methods for fertilization, including:

  • Self-fertilization (inward cirus): The cirus invaginates into the vagina for fertilization.

  • Self-fertilization (outward cirus): The cirus evaginates to complete self-fertilization.

  • Mating with another worm: Involves cross-fertilization with another individual.

Egg Release Mechanisms

• Eggs can be released from proglottids in the following ways:

  • Segments exit the body intact.

  • Segments disintegrate after excretion.

  • Apolysis: Segments empty internally, allowing eggs to disintegrate into the environment.

  • Anapolysis: Segments release eggs while remaining attached to the strobila.

  • Hyperapolysis: Immature proglottids break off and mature independently before releasing eggs.

Tegument

Microtriches

• Definition: Microtriches are finger-like projections present on the tegument that significantly increase surface area for metabolic functions.

Functions of the Glycocalyx

• The glycocalyx performs several crucial functions:

  • Provides nutrient transport pathways.

  • Offers resistance to host enzymes as well as host immune cells.

Development of Tapeworms

Larval Stages in the Life Cycle

• The typical life cycle involves various stages:

  1. Egg

  2. Oncosphere (larva within the egg)

  3. Coracidium (may or may not be present)

  4. Larval penetration of the intestine

  5. Metacestode development occurs in a parenteral site.

  6. The definitive host, usually a vertebrate, ingests the larva leading to the adult tapeworm developing in the intestine.

Stimulus for Excystment in the Definitive Host

• Excystment is stimulated by:

  • Conditions within the host's digestive tract, along with enzymes from the stomach, liver, and pancreas.

Factors Influencing Strobilar Development

• The factors influencing the development of strobila are yet to be detailed, but they play a critical role in the maturation and functionality of tapeworms.