Publisher Summary This chapter discusses the formal genetics of mouse anemias and their significance for an analysis of patterns of mammalian gene action. It also describes normal hemopoietic development, homeostasis, and hemoglobin polymorphism. Anemias are classified into five categories—(1) anemias of maturation arrest, (2) anemias involving defects in iron utilization, (3) anemias involving greatly reduced erythrocyte life-span, (4) hemoglobinopathies, and (5) anemias almost secondary to genetically induced defects. During the total life of a mouse, including prenatal development, hemopoiesis takes place in four different sites. Although hemopoiesis continues in the liver until birth and for a brief time thereafter, hemopoiesis (largely myelopoiesis) commences in the spleen on fetal day 15 or 16 and in the marrow around the time of birth. Hematologically normal adult mice from many different inbred strains regularly produce more than one kind of hemoglobin. There is a widespread genetic polymorphism at both the Hba locus, determining the α-globin structure and the Hbb locus, determining the β-globin structure, and both the loci involve considerable genetic complexity.

Hereditary Anemias of the Mouse: A Review for Geneticists*

A functional c-myb gene is required for normal murine fetal hepatic hematopoiesis.

Identification: The house mouse (Mus musculus) is a small, slender rodent that has a slightly pointed nose; small, black, somewhat protruding eyes; large, scantily haired ears, and a nearly hairless tail with obvious scale rings. The adult mouse weighs about 2/5 to 4/5 ounces. They are generally grayish-brown with a gray or buff belly. Similar mice include the white-footed mice and jumping mice (which have a white belly), and harvest mice (which have grooved upper incisor teeth.) Native to central Asia, this species arrived in North America along with settlers from Europe and other points of origin. A very adaptable species, the house mouse often lives in close association with humans and therefore is termed one of the "commensal" rodents along with Norway and roof rats. Following their arrival on colonists’ ships, house mice spread across North America and now are found in every state including coastal areas of Alaska, and in the southern parts of Canada.

The house mouse

Although the hormone erythropoietin (Epo) and its receptor (EpoR) are known to play important roles in the regulation of erythropoiesis, several questions remain concerning the developmental role of Epo/EpoR signaling. As the functions of Epo have been defined primarily through studies of definitive erythroid cells, its importance for primitive, embryonic erythropoiesis remains uncertain, as does the significance of EpoR expression in several nonerythroid cell types. To address these questions, mouse embryonic stem cells and embryos lacking a functional EpoR gene were produced by gene targeting. The effects of the mutation were examined in embryos developing in vivo, in chimeric adult mice produced with homozygous mutant embryonic stem cells, and in hemopoietic cells cultured in vitro. No defects were apparent in nonerythroid cell lineages in which the EpoR normally is expressed, including megakaryocytes and endothelial cells. In the mutant yolk sac, primitive erythrocytes were produced in normal numbers, they underwent terminal differentiation, and expressed near normal levels of embryonic globins, although they were reduced in size and their proliferation was severely retarded after E9.5. In contrast, in the fetal liver, definitive erythropoiesis beyond the late progenitor (CFU-E) stage was drastically inhibited by the EpoR mutation, and virtually no definitive erythrocytes were produced in vivo, leading to embryonic death by E13.5. Thus, our results suggest a fundamental difference in the molecular mechanisms stimulating primitive and definitive erythropoiesis. It was also observed that a few mutant definitive erythroid cells could terminally differentiate when cultured with additional cytokines, demonstrating that although Epo/EpoR signaling is important for definitive erythroid cell survival and proliferation, it is not an obligatory step in differentiation.

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Differential effects of an erythropoietin receptor gene disruption on primitive and definitive erythropoiesis.

By seven days of gestation, the yolk sac of the mouse has a sheet of mesoderm adjacent to the basement membrane separating it from the endodermal epithelium. Localized proliferations of this mesoderm produces thickened cellular regions which transform into the angioblastic cords; all of these developmental cells are attached by tight junctions and desmosomes. By eight and onehalf days, lumina appear within the angioblastic cords; the peripheral cells become attenuated and form endothelial cells which will line the primitive vessels while the more central cells become the primitive erythroblasts of the blood island.



The process of vasculogenesis and lumenization occurs between eight and onehalf and nine days of gestation and has been correlated with the reduction of cellular junctions between angioblasts and fixed primitive erythroblasts, a loss of the visceral basement membrane and the formation of wide intercellular channels between endodermal epithelial cells. The primitive erythroblasts comprising the blood islands have abundant polysomes, sparse rough endoplasmic reticulum and possess coated vesicles and ferritin aggregates in their cytoplasm and coated invaginations of their plasma membrane. By nine days of gestation, the primitive erythroblasts lose their attachments and become free in the vitelline vessels. Mitochondria of the primitive and free erythroblasts are slightly enlarged and have lighter matrices than angioblasts and mesodermal cells. By 10 to 11 days of gestation, as differentiation proceeds, coated vesicles and invaginations become more numerous and the developing erythroblasts gradually decrease in both cell and nuclear size. Concomitant with these changes is the decrease in the number and size of the mitochondria, a decrease in polysomal numbers and an increase in hemoglobin and cytoplasmic density.

A phase and electron microscopic study of vasculogenesis and erythropoiesis in the yolk sac of the mouse.

A developmental change in hemoglobins correlated with an embryonic red cell population in the mouse.

Starch gel electrophoretic patterns of hemoglobins from fetal mice of seven inbred strains (two with single, five with diffuse adult hemoglobins), were compared with each other and with electrophoretic patterns of hemoglobins from adults of the same strains. The blood from 15-day-old fetuses of all strains contained four electrophoretically separable heme components. However, there seems to be a difference between strains with single and diffuse adult hemoglobin in the time of emergence of a clear adult pattern.

Marcia L. Craig

Papers

A developmental change in hemoglobins correlated with an embryonic red cell population in the mouse.

Electrophoretic patterns of hemoglobin from fetal mice of different inbred strains.

Some factors associated with a developmental change in hemoglobin components in early fetal mice