Biomedical Engineering Reference
In-Depth Information
*Sarcocystis
*Toxoplasma
Ciliophora (ciliates) (Chapter 20)
Heterokontophyta (Chapter 21)
Unikonta (1-flagellum)
Amoebozoa (Chapter 22)
Opisthokonta
Choanozoa (Chapter 23)
Animalia (Chapters 25
32)
Fungi (Chapters 33
37)
Apicomplexa and Ciliophora (Chapter 20) are the two classes within Class
Alveolata that contain organisms that produce human disease. Members of
Class Alveolata are single-celled organisms with distinctive sacs underlying
the plasma membrane (alveoli). They all have characteristic mitochondria,
containing tubular cristae.
Members of Class Apicomplexa are all spore-forming parasites that live
in animals. They all have a characteristic structure on the apex of their cells
that helps them invade into animal cells; hence, the name Apicomplexa,
referring to the apical complex. The lives of the apicomplexans are complex
and individualized, but all members share several basic properties: the ability
of apicomplexan cells to leave host cells, to travel to other host cells, to
invade host cells, to feed and grow within host cells, and to divide into many
small cells through a process called schizogony.
Schizogony is a biological feature that characterizes the apicomplexans,
accounting for most of the histopathologic findings in apicomplexan infections,
and inspiring a set of arcane terms that apply specifically to apicomplexans.
When you understand schizogony, you understand apicomplexan pathology.
When human cells divide, they undergo a long process wherein the cell
produces all of the material required for two full-sized cells, before it splits:
two sets DNA, double the volume of cytoplasm, double the nuclear proteins,
double the number of mitochondria, and a plasma membrane sufficient to
cover two full-sized cells. In humans, cell division requires much energy and
time. The apicomplexans have discovered a short-cut that greatly reduces the
time and energy required for cell division. The shortcut is called schizogony,
and consists of rapid nuclear replication without synchronous cytoplasmic
synthesis. Basically, the apicomplexan cell can divide into a large number of
much smaller cells, and these smaller cells serve a specific purpose in the
apicomplexan cell cycle. The two apicomplexan cell types that are capable
of schizogony are the trophozoite and the oocyst.
The full-sized, vegetative apicomplexan cell is the trophozoite. It lives
inside a host cell (such an erythrocyte or a hepatocyte, or an intestinal epi-
thelial cell), feeding off the host. After a time, the trophozoite undergoes
schizogony, producing a
collection of
small,
infective
cells
called
