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Fetal Hematopoiesis

 "process by which cellular elements of blood are formed"

3 anatomical/developmental stages:

1.Mesoblastic (extraembryonic/ yolk sac)

  • Begins by 10-14th day
  • Ceases by 10-12 wk

2.Hepatic (liver)

  • By 6-8wk 1º site of hematopoiesis
  • Predominate site @ 20-24wks

3.Myeloid (bone marrow)

  • Starts increasingly from 2nd trimester
  • Macrophages produced earlier than neutrophils
  • ↑ gestation = ↓ macro : neutro ratio

Fetal granulopoises:

1st neutrophils appear 
  • @ 5th wk around aorta in clusters
Bone marrow space 
  • begins @8wk, 
  • enlarges 8-10wks, 
  • neutrophils in B.M 10.5wk
14wk - term: mc granulocytic cell ➵ Neutrophil
20wk: neutrophil count 0-500/mm3
Macrophage & neutrophil have common progenitor but macrophage appear early from yolk sac, liver, lung, 🧠.

Fetal thrombopoiesis:

BFU-MK (primitive)
  • large multifocal colonies➵50 megakaryocytes /colony
CFU-MK (differentiated)
  • unifocal colonies - 3-50 /colony
Fetal thrombopoiesis is biologically different from adult, 
  • rapid proliferation ➵ cytoplasmic maturation without polyploidization.
  • Significantly smaller
  • Lower ploidy (adult =16N, fetus=2N, Neonate=4N)
  • Fewer platelets are derived
Megakaryocytes ➵ endo reduplication ➵ large cells with polypoid nucleii
TPO: produced predominantly by Liver from fetal to adult life.

Fetal hematopoiesis:

EPO does not cross placenta
BFU-E: more proliferative➵more clusters
CFU-E: 1 or 2 clusters each with 8-100 hb erythroblasts
HIF (hypoxia induced factor)  ➵maintain O2 homeostasis & erythropoiesis by EPO
  • HIF 1α ,2α (regulated by O2 tension)
  • HIF 1β (constitutively expressed)
EPO
  • 1st & 2nd trimester ➵monocytes, macrophages
  • @birth ➵ by kidney (epigenetic switch)

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