Embryology

Question 1

List the seven (7) sites of haematopoiesis (all cellular components of blood) in the developing embryo and fetus (3½) (NB)

Answer 1

Wall of the yolk sac, Liver, Spleen, Kidneys, Thymus, Lymph nodes, Bone marrow

Question 2

Indicate the sequence of organs and tissues as sites of haematopoiesis from the embryonic to fetal to postnatal periods. May use well labelled diagram (6)

Answer 2

Yolk sac at start
Liver takes over
Then spleen
Then bone marrow and lymph nodes

Question 3

Outline the erythropoietic organs for each trimester of the intrauterine period of human development. May use diagram (6)

Answer 3

Extraembryonic phase of erythropoiesis (2): Stem cells in the blood islands of the umbilical vesicle/ wall of yolk sac give rise to the first erythrocytes, which contain nuclei. The centrally lying cells of these blood islands join up to form nucleus-containing, large erythroblasts, while the peripherally located cells become the endothelial cells.

Intra-embryonic phase of erythropoiesis

Hepatolienal phase (2): Dominates the second trimester. This extraembryonal erythropoiesis is gradually replaced by the intra-embryonic erythropoiesis, which occurs in the liver. The stem cells in this phase have their origin in the aorto-gonado-mesonephros region. The erythrocytes that arise in the liver are non-nucleated in contrast to those formed outside the embryo. A small portion is also formed in the spleen.

Myeloid phase (2) of the erythropoiesis occur in the last trimester in which the bone marrow is the most important hematopoietic organ.