a. Thymus
b. Tonsils
c. Bone marrow
d. Spleen
e. Blood islands
The answer is a. The thymic parenchyma (epithelial cells) develops from endoderm of the third pharyngeal (branchial) pouches. The thymic rudiment is invaded by bone marrow–derived lymphocyte precursors early in the third month of development. The tonsils (answer b) develop as partially encapsulated lymph nodules. Their parenchymal framework is derived from pharyngeal mesoderm. Bones, of course, whether formed by intramembranous or endochondral ossification, are derived from mesoderm. Their forming marrow cavities are populated by hematopoietic stem cells (answer c) beginning in the second month of fetal life. The connective tissue capsule and skeletal framework of the spleen develop from splanchnic lateral plate mesoderm during the fifth week and are quickly invaded by hematopoietic cells of the myeloid lineage (answer d). It remains an active hematopoietic organ until at least the seventh month in utero. Blood islands develop by differentiation of mesodermal cells in the extraembryonic mesoderm lining the yolk sac during the third week of fetal development (answer e). They give rise to vitelline vessels and are the major site of red blood cell formation in the early embryo.
b. Tonsils
c. Bone marrow
d. Spleen
e. Blood islands
The answer is a. The thymic parenchyma (epithelial cells) develops from endoderm of the third pharyngeal (branchial) pouches. The thymic rudiment is invaded by bone marrow–derived lymphocyte precursors early in the third month of development. The tonsils (answer b) develop as partially encapsulated lymph nodules. Their parenchymal framework is derived from pharyngeal mesoderm. Bones, of course, whether formed by intramembranous or endochondral ossification, are derived from mesoderm. Their forming marrow cavities are populated by hematopoietic stem cells (answer c) beginning in the second month of fetal life. The connective tissue capsule and skeletal framework of the spleen develop from splanchnic lateral plate mesoderm during the fifth week and are quickly invaded by hematopoietic cells of the myeloid lineage (answer d). It remains an active hematopoietic organ until at least the seventh month in utero. Blood islands develop by differentiation of mesodermal cells in the extraembryonic mesoderm lining the yolk sac during the third week of fetal development (answer e). They give rise to vitelline vessels and are the major site of red blood cell formation in the early embryo.
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