Fetal Hematopoiesis

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 "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)