Morphologyandlifecyle.Microsporareproduceintracellularlybymeansof
repeated,asexualbinaryormultiplefission(merogony),thenformsporesin
asubsequentphase(sporogony)(andsexualstagesaswellinThelohania).The
developmentalstagesarelocatedfreelyinthehostcellcytoplasm(Enterocy-
tozoon,Nosema)ortheyinhabitaparasitophorousvacuole(Encephalitozoon);
inothergenera(Pleistophora,Trachipleistophora)theintracellularstagesare
separatedfromthecytoplasmbyanamorphouslayer(pansporoblasticmem-
brane).Sporogonybeginswithformationofsporonts,whicharederivedfrom
merogoniccellsandpossessathickercellwall.Thesporontsdividetoform
sporoblasts,followedbymorphologicaldifferentiationintospores.
ThefinestructureofthesesporesistypicalofMicrospora(Fig.9.20a,b).
Thetwo-layeredsporewall(exosporeandendospore)enclosestheuninucle-
ate(rarely:binucleate)infectiveparasitestagecalledasporoplasmor“ame-
boidorganism”andacomplexexpulsionapparatusconsistingofacoiledpo-
lartubuleandthepolaroplast,amembranousanchoringcomponent.Thesize
ofthesporesofMicrosporaspeciesinfectinghumansvariesbetweenabout 1
and 4 lm.Thenumberandpositionofthepolartubulewindingsasseenon
theelectronmicroscopicallevel(Fig.9.20a,b)areofdiagnosticimportance.
Thesporesareeliminatedinfeces,urine,orsputumandcanremainviable
forseveralweeksoutsideofthehost.Followingperoralingestionbyasuit-
ablehost,thepolaroplastswellsup,theinternalpressureinthesporein-
Microspora 539
Microspora
Sporoblast
5 μ
m
Polar tubule
Exospore
Endospore
a b^1 μm c^10 μm
Fig.9. 20 aSpore(modifiedfromBinfordCH,ConnorD,eds.PathologyofTropical
andExtraordinaryDiseases.Vol. 1 .Washington:ArmedForcesInstituteofPathol-
ogy; 1987 :336);bimmaturesporeofEncephalitozoonhellem(TEM);csporesof
Enterocytozoonbieneusiinstoolsmear,chromotropicstainaccordingtoWeber.
9
Kayser, Medical Microbiology © 2005 Thieme