Biomedical Engineering Reference
In-Depth Information
Animalia (Chapters 25
32)
Fungi (Chapters 33
37)
Class Metamonda is a class of anaerobic eukaryotes that lack mitochondria.
As discussed in Chapter 15 (Overview of Class Eukaryota), the first eukary-
otes possessed three defining anatomic structures: a nucleus, mitochondria
(one or more), and flagellum (one or more). Evolution can be an intolerant
process. Structures that serve no important biological purpose cannot justify
the resources required to maintain their continued existence. In particular,
mitochondria may be a great way for deriving energy from oxygen, but these
complex organelles may have little value when conditions are anaerobic.
As we will see again and again throughout this topic, phylogenetic traits
are seldom lost, without leaving some trace of their heritage. The anaerobic
members of Class Metamonada maintain a relict organelle, derived from an
ancestral mitochondrium. The relict is usually referred to as a mitosome,
though a specific term, hydrogenosome, is used to refer to mitochondrial
relicts that use iron-sulfide proteins to yield molecular hydrogen and ATP.
Various so-called amitochondriate eukaryotic classes that have lost classic
mitochodria have retained mitosomes or hydrogenosomes that form molecu-
lar hydrogen: Class Metamonada (Chapter 16) [65], Class Amoebozoa
(Chapter 22) [66], and Class Microsporidia (Chapter 37) [67].
Metamonada
Trichozoa
Parabasalidea
Trichomonadida
Trichomonadidae
*Trichomonas
Monocercomonadidae
*Dientamoeba
Members of Class Trichomonadida live as commensals with a single mor-
phologic form: trophozoites. Cysts are not formed. The absence of a cyst
form is significant because cyst forms resist adverse environmental condi-
tions, permitting the organism to live outside the host for varying lengths of
time, and to infect organisms without direct contact with the infected host.
Without a cyst form, members of Class Trhichomonadida depend on direct
transmission between host organisms.
The two human pathogens in Class Trichomonadida are: Trichomonas
vaginalis and Dientamoeba fragilis.
Trichomonas vaginalis is the most common sexually transmitted disease,
with about 8 million new cases occurring annually, in North American
[32,70]. Its reservoir is humans. Its role as a causative agent of urethritis,
vaginitis, and cervicitis, in women, is well known. Fewer people seem to be
aware that both men and women are commonly infected. Though many
