TY  - CHAP
M1  - Book, Section
TI  - Hematology of the Fetus and Newborn
A1  - Palis, James
A1  - Segel, George B.
A2  - Kaushansky, Kenneth
A2  - Lichtman, Marshall A.
A2  - Prchal, Josef T.
A2  - Levi, Marcel M.
A2  - Press, Oliver W.
A2  - Burns, Linda J.
A2  - Caligiuri, Michael
PY  - 2015
T2  - Williams Hematology, 9e
AB  - SUMMARYDuring  embryogenesis,  hematopoiesis  occurs  in  spatially  and  temporally  distinct  sites,  including  the  extraembryonic  yolk  sac,  the  fetal  liver,  and  the  preterm  marrow.  The  development  of  primitive  erythroblasts  in  the  yolk  sac  is  critical  for  embryonic  survival.  Primitive  erythroblasts  differentiate  within  the  vascular  network  rather  than  in  the  extravascular  space  and  circulate  as  nucleated  cells.  Although  it  is  widely  assumed  that  primitive  red  cells  remain  nucleated  throughout  their  life  span,  primitive  erythroblasts  ultimately  enucleate  upon  terminal  differentiation.  After  7  weeks  of  gestation,  hematopoietic  progenitors  are  no  longer  detected  in  the  yolk  sac.  Hematopoietic  stem  cells  emerge  from  major  arterial  vessels  at  5  weeks  of  gestation.  The  liver  serves  as  the  primary  source  of  red  cells  from  the  9th  to  the  24th  week  of  gestation.  Like  primitive  erythropoiesis  in  the  yolk  sac,  definitive  erythropoiesis  in  the  fetal  liver  is  necessary  for  continued  survival  of  the  embryo.  In  contrast  to  the  yolk  sac,  where  hematopoiesis  is  restricted  to  maturing  primitive  erythroid,  macrophage,  and  megakaryocytic  cells,  hematopoiesis  in  the  fetal  liver  consists  of  definitive  erythroid,  megakaryocyte,  and  multiple  myeloid,  as  well  as  lymphoid  lineages.  Hematopoietic  cells  are  first  seen  in  the  marrow  of  the  10-  to  11-week  embryo,  and  they  remain  confined  to  the  diaphyseal  regions  of  long  bones  until  15  weeks  of  gestation.  Lymphopoiesis  is  present  in  the  lymph  plexuses  and  the  thymus  beginning  at  9  weeks  of  gestation.  Yolk  sac  stem  cells  were  first  thought  to  seed  the  liver  and  eventually  the  marrow.  However,  later  experiments  in  avian  and  amphibian  embryos  indicate  that  the  hematopoietic  stem  cells  that  seed  the  marrow  arise  within  the  body  of  the  embryo  proper  rather  than  from  the  yolk  sac.  The  aorta-gonad-mesonephros  (AGM)  region  generates  hematopoietic  stem  cells  that  seed  the  liver  and  the  marrow  to  provide  lifelong  hematopoiesis.  Hemoglobin  (Hgb)  Gower-1  (ζ2ε2)  is  the  major  hemoglobin  in  embryos  younger  than  5  weeks.  Hgb  F  (α2γ2)  is  the  major  hemoglobin  of  fetal  life.  The  fetal  hemoglobin  concentration  in  blood  decreases  after  birth  by  approximately  3  percent  per  week  and  is  generally  less  than  2  to  3  percent  of  the  total  hemoglobin  by  6  months  of  age.  The  mean  hemoglobin  level  in  cord  blood  at  term  is  16.8  g/dL,  with  95  percent  of  the  values  falling  between  13.7  and  20.1  g/dL.  The  red  cells  of  the  newborn  are  macrocytic,  with  a  mean  cell  volume  in  excess  of  110  fL/cell.  The  red  cell,  hemoglobin,  and  hematocrit  values  decrease  only  slightly  during  the  first  week  after  birth,  but  decline  more  rapidly  in  the  following  5  to  8  weeks,  producing  the  physiologic  anemia  of  the  newborn.  The  absolute  number  of  neutrophils  in  the  blood  of  term  and  premature  infants  is  usually  greater  than  that  found  in  older  children.  Segmented  neutrophils  are  the  predominant  leukocytes  in  the  first  few  days  after  birth.  As  their  number  decreases,  the  lymphocyte  becomes  the  most  numerous  cell  type  and  remains  so  during  the  first  4  years  of  life.  Phagocytosis  of  bacteria  by  neutrophils  from  premature  and  term  infants  is  normal.  Bactericidal  activity  varies  according  to  the  conditions  of  ...
SN  - 
PB  - McGraw-Hill Education
CY  - New York, NY
Y2  - 2024/04/19
UR  - hemonc.mhmedical.com/content.aspx?aid=1121088931
ER  -