Comparison of platelet production in two strains of mice with different modal megakaryocyte DNA ploidies after exposure to hypoxia.
Thrombocytopenia develops with prolonged exposure to hypoxia. Although decreases in megakaryocyte numbers due to hypoxia have been well documented, the effects of hypoxia on megakaryocyte DNA content have not been reported. In this study, megakaryocytopoiesis and platelet production were compared in both C3H mice (whose megakaryocyte modal ploidy class is 32N) and C57/BL mice (whose modal ploidy class is 16N), by enclosure in cages covered with silicone-rubber membranes. After equilibration, O2 levels inside the cages were 6%-7%. Hematocrits, platelet counts, platelet sizes, percent 35S incorporation into platelets, megakaryocyte size and number, and megakaryocyte DNA content of mice were measured before and at various days after hypoxia. Although hematocritis increased and platelet counts decreased in both strains of mice with time in hypoxic chambers, megakaryocyte and platelet responses of C3H mice differed from those of C57/BL mice in several respects; hematocrits of C3H mice were higher and platelet counts were lower than those in C57/BL mice. C3H mice produced larger platelets than C57/BL mice in response to hypoxia. Total circulating platelet counts (TCPC) and total circulating platelet masses (TCPM) of both mouse strains showed similar biphasic responses, that is, elevated TCPC and TCPM on days 2-4 and decreased values after 6-14 days of hypoxia. However, hypoxic C3H mice had lower TCPC on days 4-14 and lower TCPM on days 10-14 of hypoxia than C57/BL mice. Both C3H and C57/BL mice had decreased megakaryocyte numbers at 6-10 days of hypoxia, but only C3H mice had decreased numbers of megakaryocytes at day 14. Elevated megakaryocyte size was observed in both mouse strains at day 14 of hypoxia. However, after hypoxia, C3H mice showed a greater depression in megakaryocyte number and a larger increase in megakaryocyte sizes than did C57/BL mice. C3H mice maintained 32N as the modal megakaryocyte DNA content through day 10 of hypoxia, but 64N was the modal megakaryocyte DNA content at day 14; 16N remained the modal megakaryocyte DNA content in hypoxic C57/BL mice. Hypoxic C3H mice had an increase in 16N megakaryocytes after 6 days of hypoxia, followed by an increase in the proportion of 64N cells at 14 days compared to values of untreated C3H control mice. Hypoxic C57/BL mice had an increased proportion of 16N cells at 6 days but a decreased proportion of 32N cells at 14 days. These studies demonstrate that the decreased platelet production resulting from prolonged exposure to hypoxia is primarily the result of decreased differentiation of hematopoietic precursors into the megakaryocyte lineage rather than decreased megakaryocyte DNA content, because higher ploidy classes actually increase as thrombocytopenia becomes more severe. Stem cell competition could explain the findings of reduced platelet production and increased red blood cell production in both strains of mice after exposure to hypoxia.
McDonald
T P
TP
Department of Animal Science, College of Veterinary Medicine, University of Tennessee, Knoxville 37901-1071.
Cottrell
M B
MB
Steward
S A
SA
Clift
R E
RE
Swearingen
C J
CJ
Jackson
C W
CW
D000818:Animals; D001792:Blood Platelets / Q000166:cytology*; D001853:Bone Marrow / Q000378:metabolism; D002454:Cell Differentiation / Q000502:physiology; D002455:Cell Division / Q000502:physiology; D004247:DNA / Q000032:analysis* / Q000235:genetics; D006400:Hematocrit; D006410:Hematopoiesis / Q000502:physiology; D000860:Hypoxia / Q000503:physiopathology*; D008297:Male; D008533:Megakaryocytes / Q000737:chemistry*; D051379:Mice; D008809:Mice, Inbred C3H; D008810:Mice, Inbred C57BL; D010100:Oxygen / Q000494:pharmacology; D010976:Platelet Count; D011003:Ploidies; D013462:Sulfur Radioisotopes
D013462:Sulfur Radioisotopes; D004247:DNA; D010100:Oxygen