(29) 18.3 Formed Elements in the Blood & 18.3a Hemopoiesis

Hemopoiesis

- Formation and development of blood cells - Formed elements have a relatively short life span; new ones are continually produced by the process of________ - occurs in red bone marrow (myeloid tissue) of certain bones. - The process of _________ starts with hemopoietic stem cells called *hemocytoblasts* - Hemocytoblasts are considered pluripotent cells - meaning that they can differentiate and develop into many different kinds of cells. - Hemocytoblasts produce two different lines for blood cell development: The myeloid line forms (1) erythrocytes, (2) all leukocytes except lymphocytes (this would include granulocytes and monocytes), and (3) megakaryocytes (cells that produce platelets). The lymphoid (lim′foyd) line forms only lymphocytes.

hemocytoblasts

- is the Immature cells that produce all types of formed elements in blood. - The process of hemopoiesis starts with hemopoietic stem cells called __________ - are considered pluripotent cells, meaning that they can differentiate and develop into many different kinds of cells. - produce two different lines for blood cell development: The *myeloid line forms* (1) *erythrocytes*, (2) *all leukocytes except lymphocytes (this would include granulocytes and monocytes)*, and (3) *megakaryocytes (cells that produce platelets).* - The *lymphoid (lim′foyd) line* forms only lymphocytes.

Erythropoiesis

- make up more than 99% of formed elements with a concentration between 4.2 and 6.2 million per cubic millimeter. The process of erythrocyte production is called _______ - Normally, erythrocytes are produced at the rate of about 3 million per second. The hormone erythropoietin (EPO) controls this rate by increasing the rate of erythrocyte formation - The process of ______________ begins with a *myeloid stem cell*, which under the influence of *multi-CSF *forms a progenitor cell. The progenitor cell forms a *proerythroblast,* which is a large, nucleated cell. It then becomes an erythroblast, which is a slightly smaller cell that is producing hemoglobin in its cytoplasm. The next stage, called a *normoblast,* is a still smaller cell with more hemoglobin in the cytoplasm; its nucleus has been ejected. A cell called a *reticulocyte* eventually is formed. The reticulocyte has lost all organelles except some *ribosomes*, but it continues to produce hemoglobin. The transformation from myeloid stem cell to reticulocyte takes about 5 days. - Some reticulocytes finish maturation while circulating in blood vessels (and in normal circumstances, make up 0.5-2.0% of the circulating blood). One to two days after entering the circulation, the organelles in the reticulocyte degenerate, and the reticulocyte becomes a *mature erythrocyte*. Without a nucleus and cellular organelles, the mature *erythrocyte* is essentially a plasma membrane "bag" containing hemoglobin.