Hemopoiesis
The formation of formed elements of blood is hemopoiesis. There are three types of formed elements of blood-Erythrocytes[RBC], leucocytes[WBC], and Thrombocytes [platelets].
Types of hemopoiesis
So, hemopoiesis is also of three types –
Erythropoiesis-formation of erythrocytes [RBC].
Leucopoiesis -formation of leucocytes [WBC],
Megakaryocytopoiesis – formation of thrombocytes [platelets].
Site of erythropoiesis
Erythropoiesis is the process of erythrocyte formation; it is a continuous process. However, at different ages, the site of erythropoiesis differs. The sites :
It may be extravascular– hepatic and myeloid stages.
It may be intravascular – mesoblastic stage.
The mesoblastic stage
The mesoblastic stage of erythropoiesisstartsin theearly embryo and lasts up to three months of fetal life. RBC is formed from the ‘area vasculosa’-mesoderm of ‘yolk sac.’ The mesoderm of the yolk sac consists of a nucleated mass of protoplasm that creates a network of capillary vessels lined by endothelium and filled with plasma. Endothelial cells proliferate and form masses of nucleated cells. These nucleated cells contain hemoglobin. And detach from capillaries. Later, these cells lose their nuclei and become non-nucleated red blood cells.
The hepatic stage
The hepatic stage of erythropoiesisstarts afterthree months of fetal life and lasts until six months of fetal life in the liver and spleen. Then, RBCs are formed from the mesenchyme between the tissue cells and the blood vessels.
The myeloid stages
Myeloid stages of erythropoiesis start in the bone marrow after six months of fetal life. If the bone marrow fails to form RBCs, the liver and spleen will start erythropoiesis.
At a young age, erythropoiesis occurs in red marrow and is present in almost all bones.
In old age, erythropoiesis occurs only in the red marrow of flat bones like the sternum, iliac crest, and vertebrae.
In myeloid erythropoiesis, red blood cell formation occurs from committed stem cells and takes about seven days to mature.
Pluripotent stem cells of the bone marrow develop into Lymphoid stem cells and myeloid stem cells.
Now myeloid stem cells will form ‘colony forming unit EM’ [Erythrocyte and megakaryocyte] and ‘colony forming unit GM ‘[Granulocyte myelocyte].
The ‘colony-forming unit EM’ will form colony-forming units CFU-E’ and CFU-M.
CFU E will form proerthroblast, which [blast is rapidly growing
cells], proliferating to form erythroblast. A proerthroblast is a large cell of 15 -20 microns with a large nucleus and 3-4 nucleoli. Chromatin is open-eukaryotic. It is a highly basophilic and rapidly dividing cell.
The size of the erythroblast is less than the proerythroblast [15-18 microns] and is less basophilic cell. The nucleus also decreases in size and loses nucleoli. Chromatin condenses. It also divides. This erythroblast is known as basophilic erythroblast or early normoblast.
The polychromatic erythroblast is 10-15 microns in size. The nucleus decreases in size, and chromatin condenses. Hemoglobin appears in the cytoplasm, giving it a red color. The ribosome is still present in the cytoplasm, showing a blue color. So, this cell is known as a polychromatic erythroblast and intermediate normoblast.
The size of the orthochromatic erythroblast is 8 to 10 microns. This is also known as the late normoblast. The orthochromatic erythroblast is red due to the cytoplasm’s large amount of hemoglobin. Some amount of ribosomes is also present. The nucleus becomes very small -looks like a cartwheel.
[These stages of erythrocyte development occur in the bone marrow and take about seven days in normal conditions.]
A reticulocyte is a small cell [7 to 8 microns]without a nucleus. Some RNA presently gives a reticular appearance to the cell. Only 1-2% of reticulocytes are present in the circulation. However, in rapid erythropoiesis, reticulocyte percentage increases in circulation. Reticulocyte is also known as ‘immature red blood cell.’
.After about two days, reticulocytes become erythrocyte-mature red blood cells.
Erythrocyte is also a tiny cell [7-8 microns] without a nucleus and RNA remnants. It is red.
Hemoglobin appears in the intermediate normoblast stage.
Mitosis stops in the late normoblast stage.
The nucleus and all organelles, such as mitochondria, Golgi apparatus, ribosomes, and endoplasmic reticulum, disappear in the later part of the late normoblast stage.
Reticulocyte maturation continues even after it loses its nucleus and other organelles. The reticulocyte loses 20 to 30 % of the cell surface and eliminates membrane-bound cytosolic organelles through an autophagy /exosome-combined pathway.
Stages of erythropoeisis are:-
Stages Size Other feature
1. Proerythroblast 15-20 µm large nucleus
2. Early normoblast 14-18 µm hemoglobin appears
3. Intermediate normoblast– 10-14 µm more hemoglobin appears
4. Late normoblast – 8-10 µm nucleus disappears
5. Reticulocyte – 7.2 µm reticular structure in the cytoplasm
6.Erythrocyte – 7.2 µm
Stages 1 to 5 are present in the bone marrow. One to two percent of reticulocytes are currently in circulation. In rapid erythropoiesis, the number of reticulocytes increases in circulation. Stage 6, that is, erythrocytes are present only in circulation.
Main features In erythropoiesis –
The size of the cell reduces progressively.
The nucleus decreases in size, and nucleoli disappear as maturation occurs. Open chromatin in the nucleus condenses and finally disintegrates. The nucleus and nucleoli degenerate.
Hemoglobin appears in the intermediate normoblast stage and increases progressively.
Mitosis takes place up to the intermediate normoblast.
Image created by Author with Canva.
learn-and-fly.co.in 