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Flagellates : Definition, Classification, Habitat, Morphology, Life Cycle, & Movement

Flagellates : Definition, Classification, Examples, Characteristics, Facts, Habitat, Morphology, Life Cycle, Movement

Definition

  • Flagellates are parasitic protozoan which bears whip-like flagella as their organs of locomotion.

Classification

  • Phylum-Sarcomastigophora
  • Subphylum-Mastigophora
  • Class- Zoomastigophorea

Based on their habitat, they are considered as following:

1. Intestinal (Lumen-dwelling) flagellates: Present in the alimentary and urogenital tracts.
2. Haemoflagellates: Present in blood and tissues.

Intestinal (Lumen-dwelling) flagellates

  • Found in the alimentary and urogenital tracts.
  • Most of are nonpathogenic commensals except: Giardia lamblia (Diarrhoea),  Trichomonas vaginalis (Vaginitis and urethritis).

Intestinal flagellates found in humans are as following:

  • Giardia lamblia (Duodenum, jejunum)
  • Trichomonas vaginalis (Vagina, urethra)
  • Trichomonas tenax (Mouth)
  • Trichomonas hominis (Caecum)
  • Chilomastix mesnili (Caecum)
  • Enteromonas hominis (Colon)
  • Retortamonas intestinalis (Colon)
  • Dientamoeba fragilis

Haemoflagellates

  • Found in blood and tissues.
  • Medically important haemoflagellates need two hosts to complete their life cycle (Digenetic or Heteroxenous).
  • They live in the blood and tissues of vertebrate hosts (e.g. human), and in the gut of insect vectors.
  • Multiplication: Binary fission. No sexual cycle is known.

Morphological stages

Haemoflagellates exist in two or more of four morphological stages.

New names are formed by the suffix ‘mastigote’ combined with various prefixes referring to the origin, course and arrangement of the flagellum in relation to the position of the nucleus, and its point of emergence from the cell.

1. Amastigote (Leishmanial) stage

  • Rounded or ovoid without any external flagellum.
  • Nucleus, kinetoplast and axial filament can be seen.
  • This is the stage in which T. cruzi and Leishmania are found intracellularly in vertebrate hosts.

2. Promastigote (Leptomonad) stage

  • Lanceolate.
  • Kinetoplast is anterior to the nucleus (antenuclear kinetoplast), near the anterior end of the cell, from which emerges the flagellum.
  • No undulating membrane.
  • Infective stage of Leishmania found in the midgut and proboscis of the insect vector. This is also the form in which Leishmania occur in cultures in vitro.

3. Epimastigote (Crithidial) stage

  • More elongated
  • Kinetoplast placed more posteriorly though close to and in front of the nucleus (juxtanuclear kinetoplast).
  • Flagellum runs alongside the body as a short undulating membrane before emerging from the anterior end.
  • This is the stage in which T.gambiense and T. rhodesiense occur in the salivary glands of the vector testes fly, and T.cruzi in the midgut of the vector reduviid bug.
  • Absent in leishmania.

4. Trypomastigote (Trypanosomal) stage

  • Elongated, Spindle shaped
  • Central nucleus
  • Kinetoplast posterior to the nucleus (postnuclear kinetoplast) situated at the posterior end of the body.
  • Flagellum runs alongside the entire length of the cell to form a long undulating membrane before emerging from the anterior end.
  • Infective stage of trypanosomes found in the vector arthropod and the stage found in the blood of the infected vertebrate.
  • Absent in Leishmania.

Some transitional stages have been recognised.

  • Sphaeromastigote, a motile round form with free flagellum, which is a transitional stage from amastigote to promastigote, seen in the genus Trypanosoma
  • Paramastigote, a transitional form leading to the infective promastigote in leishmania.

Trypanosomatidae (Genera Trypanosoma & Leishmania): Family of haemoflagellates infecting human.

Morphology:  A nucleus, a kinetoplast and a single flagellum.

  • Kinetoplast (micronucleus) consists of a parabasal body and a adjacent dot-like blepharoplast. The blepharoplast and parabasal body are connected by one or more delicate fibrils.
  • Flagellum arises from the blepharoplast. The portion of the flagellum which is inside the body of the parasite is called the axoneme or axial filament.
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