Question

In the embryonic stage, erythrocytes develop from _______.
(a) Bone marrow
(b) Liver and kidney
(c) Spleen and kidney
(d) Liver and spleen

Answer 1

Blood islands are specific structures in the developing embryo that lead to many different parts of the circulatory system.

Complete answer:
Blood cell formation is also known as hematopoiesis. Through this continuous process, the cellular constituents of blood are replenished as needed. Blood cells are generally divided into three main categories namely: red blood cells or erythrocytes, white blood cells or leukocytes, and the blood platelets or thrombocytes.
The first site of blood formation in the human embryo is the yolk sac but later in embryonic life, the liver becomes the most important red blood cell-forming organ and is soon succeeded by the bone marrow. Bone marrow in adult life is the only source of both red blood cells. It is observed that in developing embryos, blood formation occurs in aggregates of blood cells in the yolk sac called blood islands but as the development progresses, blood formation occurs in the spleen, liver, and lymph nodes. Also, when bone marrow gets fully developed, it assumes the task of forming most of the body’s blood cells.

Additional information: Through a series of complex, gradual, and successive transformations from primitive stem cells both the red and white blood cells are produced. These stem cells also have the ability to form any of the precursors of a blood cell. Precursor cells (a cell, from which another substance, cell, or cellular component is formed) are stem cells that have developed to the stage where they are committed to forming a particular kind of new blood cell.
About half a liter (almost one pint) of blood is produced by the bone marrow every week in a normal adult. Nearly 1 percent of the body’s red cells are generated each day, and the balance between red cell production and the removal of aging red cells from the circulation is precisely maintained
So, the correct answer is ‘(d) Liver and spleen’.

Note: Blood islands are basically cell clusters in the early stage of embryo, which first appear in the human yolk sac at approximately day 13-15 of gestation. These give rise to vascular endothelium and blood cells. It is basically composed of a loose inner mass of embryonic hematopoietic precursor cells and an outer luminal layer of angioblasts.