Brain and ganglion development from two genotypic classes of cells in allophenic mice.

Several strain-specific markers were found to be histochemically visualizable in parts of the central nervous system in allophenic mice. These markers therefore provide a new basis for mapping the normal developmental lineages of major parts of the nervous system, and for identifying the focus of mutant gene action in some neurological mutations. Cell strains in mosaic animals were visualized on the basis of a quantitative difference in β-galactosidase activity (Bgs-locus), in the Purkinje zone of the cerebellum, and in the hippocampal pyramidal zone of the cerebrum. The differential between strains was increased if the beige ($\text{bg}{}^{\mathrm{J}}\text{bg}{}^{\mathrm{J}}$) mutation was included in the high-activity strain. (β-galactosidase is lysosomal, and enhanced visualization in beige results from its enlarged and aggregated lysosomes.) Purkinje cell-strain visualization was also obtained by an indirect fluorescent antibody technique, in sections treated with antisera containing antibodies against strain-type histocompatibility alloantigens, including H-2. The above markers reveal considerable interspersion of cells from separate lineages in short sequences of each genotype. Purkinje and pyramidal cells of the same brain sometimes differ appreciably in genotypic composition. The enzyme glucosephosphate isomerase was found histochemically to be localized in nerve fibers rather than cell bodies in the brain. However, it was prominent in the cell bodies of the spinal ganglia, so that biochemical determination of ganglion strain-types is possible by means of strain-specific isozymes (Gpi-1-locus). Individual ganglia contained both cell strains and thus are not individually derived as clones from the neural crest.     

Kinetics of bidirectional active sodium fluxes in the toad bladder

The kinetics of isotopic Na⁺ flows was studied in urinary bladders of toads from the Dominican Republic. Initial studies of the potential dependence of passive serosal to mucosal ²²Na⁺ efflux demonstrated the absence of isotope interaction and/or other coupling with passive Na⁺ flow. The electrical current $\text{I}$ and mucosal to serosal ²²Na⁺ influx were then measured with transmembrane potential clamped at $\text{Δψ = 0, 25, 50, 75 or 100}\text{mV}$. Subsequent elimination of active Na⁺ transport mucosal amiloride permitted calculation of the rates of active Na⁺ transport $\text{J}{}_{\mathrm{<mtext>Na</mtext>}}{}^{\mathrm{<mtext>a</mtext>}}$ and active and passive influx and $\text{J}{}_{\mathrm{<mtext>Na</mtext>}}{}^{\mathrm{<mtext>a</mtext>}}$ and $\text{J}{}_{\mathrm{<mtext>Na</mtext>}}{}^{\mathrm{<mtext>p</mtext>}}$. The results indicate that for Dominican toad bladders mounted in chambers only Na⁺ contributes significantly to transepithelial active ion transport; hence $\text{J}{}_{\mathrm{<mtext>Na</mtext>}}{}^{\mathrm{<mtext>a</mtext>}}\text{= J}{}^{\mathrm{<mtext>a</mtext>}}$. $\text{J}{}^{\mathrm{<mtext>a</mtext>}}$ was abolished at $\text{Δψ = E = 96.3 ± 1.9}\text{(S.E.) mV}$. As Δψ approached $\text{E}$, active efflux $\text{J}{}^{\mathrm{<mtext>a</mtext>}}$ became demonstrable. At $\text{Δ = 100}\text{mV}\text{,}\text{J}{}^{\mathrm{<mtext>a</mtext>}}$ exceeded $\text{J}{}^{\mathrm{<mtext>a</mtext>}}$, so that $\text{J}{}^{\mathrm{<mtext>a</mtext>}}$ was negative. Experimental values of $\text{J}{}^{\mathrm{<mtext>a</mtext>}}$ agreed well with theoretical values predicted by a thermodynamic formulation: $\text{J}{}_{\mathrm{<mtext>exp</mtext>}}{}^{\mathrm{<mtext>a</mtext>}}\text{= 0.985}\text{J}{}_{\mathrm{<mtext>theor</mtext>}}{}^{\mathrm{<mtext>a</mtext>}}\text{(r = 0.993)}$. The dependence of $\text{J}{}^{\mathrm{<mtext>a</mtext>}}$ on Δψ is curvilinear.     

Cell lineage dependent and independent control of Purkinje cell number in the mammalian CNS: further quantitative studies of lurcher chimeric mice

Recent quantitative studies of lurcher chimeric mice have shown that the adult population of cerebellar Purkinje cells can properly be described as a small number of developmental clones of cells. The clones are not seen as patches of contiguous neurons; rather, the cells of any one clone distribute throughout the half-cerebellum that contains them, intermingling extensively with the Purkinje cells of other linkages. Lurcher----wild-type chimeras were analyzed using the cell autonomous Purkinje-cell-lethal mutant, lurcher (+/Lc), as a cell marker. Cell counts from these chimeras revealed that the number of surviving Purkinje cells was always an integral multiple of a unit clone size. These numerical quanta are the evidence for the existence of Purkinje cell developmental clones. When two different inbred strains of mouse were compared (C3H/HeJ and C57BL/6), the resulting clonal analysis showed that the unit clone size (i.e., the number of Purkinje cells in one quantum) is an autonomous property of the lineage and hence, presumably, intrinsic to the progenitor cell that founded it. The current study uses the lurcher chimeric mouse system to examine the cell lineage relationships among the Purkinje cells of a third inbred strain of mouse, AKR/J. The data both support and extend our previous studies. Quantitative analysis reveals that the Purkinje cells of this strain also exist in clones, and the size of these clones is also strain-specific. The number of cells in a single clone (7850), however, is different from either C3H/HeJ (10,200) or C57BL/6 (9200). The fact that this value is so highly polymorphic among the inbred strains of mouse makes it likely that, rather than being a function of different alleles at a single genetic locus, clone size may well represent a multifactorial (but still cell-autonomous) property of developing Purkinje cells. Additional results from a single chimeric animal suggest strongly that clone number (i.e., the number of progenitors selected to found the population) is not strain-specific but results instead from cell:cell interactions during early nervous system formation.     

Content of adenosine 3'-5'-cyclic monophosphate in the pancreatic islets of mice with a hereditary defect of insulin secretion

Glucose (20 mM) released insulin from pancreatic islets of $\text{C57BL}\text{6J}\text{-}\text{db}{}^{\mathrm{2J}}\text{db}{}^{\mathrm{2J}}$ mice, the response being potentiated by 1 mM 3-isobutyl-1-methylxanthine. Islets of $\text{C57BL}\text{KsJ}\text{-}\text{db}\text{db}$ mice failed to respond to glucose and released only little insulin when challenged with both glucose and methylxanthine. After incubation with 0 or 20 mM glucose alone the islet content of adenosine 3′:5′-cyclic monophosphate did not differ between the two types of mice or between glucose concentrations. 3-Isobutyl-1-methylxanthine increased the islet adenosine 3′:5′-cyclic monophosphate markedly in $\text{6J-}\text{db}{}^{\mathrm{2J}}\text{db}{}^{\mathrm{2J}}$ mice but not significantly in $\text{KsJ-}\text{db}\text{db}$ mice.     

The loss of the phoS periplasmic protein leads to a change in the specificity of a constitutive inorganic phosphate transport system in Escherichia coli

The phoS periplasmic protein, implicated in alkaline phosphatase regulation, is shown to be involved in inorganic phosphate (Pi) transport in $\text{E. coli}$. Although $\text{pho}\text{S}{}^{\mathrm{?}}$ cells dependent upon the PST system for Pi transport can grow in minimal medium with 1 mM Pi as source of phosphorus, the affinity of these cells for Pi is greatly reduced; Km = 18 μM compared with Km = 0.4 μM for $\text{phoS}{}^{\mathrm{+}}$ cells. $\text{pho}\text{S}{}^{\mathrm{?}}$ cells dependent upon the PST Pi transport system acquire the ability to accumulate Asi from the medium in contrast to $\text{pho}\text{S}{}^{\mathrm{+}}$ cells which exclude this toxic anion. It would appear that the periplasmic phoS protein is not essential for Pi accumulation but is involved in maintaining the specificity of the PST Pi transport system.     

A relationship between cerebellar Purkinje cells and spatial working memory demonstrated in a lurcher/chimera mouse model system

New emphasis has been placed upon cerebellar research because of recent reports demonstrating involvement of the cerebellum in non-motor cognitive behaviors. Included in the growing list of cognitive functions associated with cerebellar activation is working memory. In this study, we explore the potential role of the cerebellum in spatial working memory using a mouse model of Purkinje cell loss. Specifically, we make aggregation chimeras between heterozygous lurcher (Lc/+) mutant embryos and +/+ (wildtype) embryos and tested them in the delayed matching-to-position (DMTP) task. Lc/+ mice lose 100% of their Purkinje cells postnatally due to a cell-intrinsic gain-of-function mutation. Lc/+<->+/+ chimeras therefore have Purkinje cells ranging from 0 to normal numbers. Through histological examination of chimeric mice and observations of motor ability, we showed that ataxia is dependent upon both the number and distribution of Purkinje cells in the cerebellum. In addition, we found that Lc/+ mice, with a complete loss of Purkinje cells, have a generalized deficit in DMTP performance that is probably associated with their motor impairment. Finally, we found that Lc/+<->+/+ chimeric mice, as a group, did not differ from control mice in this task. Rather, surprisingly, analysis of their total Purkinje cells and performance in the DMTP task revealed a significant negative relationship between these two variables. Together, these findings indicate that the cerebellum plays a minor or indirect role in spatial working memory.     

Some observations on a new type of point average molecular weight

A new type of (reduced) point average molecular weight A¹, is described. Several interesting properties are developed: (i) $\text{A}{}^1\text{=}\text{reduced}$ weight average molecular weight over the whole cell, $\text{A}{}_{\mathrm{<mtext>w</mtext>}}{}^{\mathrm{o}}\text{A}{}^1\text{(}\text{meniscus}\text{) = A}{}_{\mathrm{<mtext>w</mtext>}}\text{(}\text{meniscus}\text{); (iii) A}{}^1\text{(}\text{zero concentration}\text{) =}\text{reduced}$ number average molecular weight, A_n (meniscus). In addition, its usefulness in extracting the meniscus concentration, J(a), and in examining heterogeneous systems such as mucus glycoproteins, are discussed. The evaluation and application of $\text{A}{}^1$ requires only simple computational facilities, without the use for large-scale multiple data acquisition and recycling techniques.     

Interaction of lymphocytes and macrophage cell line cells (M1 cells). I. Functional maturation and appearance of Fc receptors im M1 cells

M₁ cells, which are cell line cells established from myeloid leukemia cells of the SL strain mouse, can differentiate from blast cells ($\text{M}{}_1{}^{\mathrm{?}}$) to mature macrophages ($\text{M}{}_1{}^{\mathrm{+}}$) within 48 hr, when they are cultured with conditioned medium (CM) obtained from murine embryonic fibroblasts. While $\text{M}{}_1{}^{\mathrm{?}}$ cells have no phagocytic activity nor Fc receptor (FcR), $\text{M}{}_1{}^{\mathrm{+}}$ cells possess both characteristics. The appearance of FcR is temperature-dependent and inhibited by a metabolic inhibitor, cycloheximide. FcR on $\text{M}{}_1{}^{\mathrm{+}}$ cells is resistant to trypsin and pronase. $\text{M}{}_1{}^{\mathrm{+}}$ cells improve the viability of macrophage-depleted SL splenic lymphocytes and restore the in vitro secondary plaque forming cell response of macrophage-depleted spleen cells to particulate and soluble antigens. $\text{M}{}_1{}^{\mathrm{?}}$ cells lack this macrophage-substituting capacity. Mm₁ cells, mutant cells from M₁ cells, having FcR and higher phagocytic activity than $\text{M}{}_1{}^{\mathrm{+}}$ cells, are also devoid of this capacity.     

Thymidine transport in cultured mammalian cells. Kinetic analysis,temperature dependence and specificity of the transport system

The transport of thymidine has been characterized kinetically and thermodynamically in Novikoff rat hepatoma cells grown in culture and, less extensively, in mouse L cells, Chinese hamster ovary cells, P388 murine leukemia cells and HeLa cells. That the characterizations pertained to the transport system per se was ensured, (i) by employing recently developed methods for rapid sampling of cell/substrate mixtures in order to follow isotope movements within a few seconds after initial exposure of cells to substrate; (ii) by utilizing cells rendered, by genetic or chemical means, incapable of metabolizing thymidine; and, (iii) by demonstrating conformity of the transport data to an integrated rate equation derived for a simple, carrier-mediated system. The results indicate that thymidine is transported into mammalian cells by a functionally symmetrical, non-concentrative system for which the carrier : substrate dissociation constant ranges from about 100 μM in Chinese hamster ovary cells, to 230 μM in Novikoff hepatoma cells. In all cell lines investigated, the velocity of transport was sufficient to nearly completely equilibrate low concentrations of thymidine across the membrane within 15 s. Temperature dependence of transport velocity and substrate : carrier dissociation were continuous ($\text{E}{}_{\mathrm{<mtext>A</mtext>}}\text{= 18.3}\text{kcal/mol}\text{, ΔH}{}^0\text{′ = 9.3}\text{kcal/mol}$, respectively), and showed no evidence of abrupt transitions. Several natural and artificial nucleosides and nucleic acid based inhibited influx of radiolabeled thymidine, apparently by competing with thymidine for the transport carrier.     

Developmental compartmentalization in the dorsal mesothoracic disc of Drosophila.

The clonal analysis of the development of the dorsal mesothoracic (wing) disc shows that clones initiated after a given time do not cross over certain demarcation lines in the adult cuticle. The property of $\text{M}{}^{\mathrm{+}}\text{M}{}^{\mathrm{+}}$ (non-Minute) recombinant cells to overgrow the $\text{M}\text{M}{}^{\mathrm{+}}$ background cells was used to demonstrate the establishment of clonal restrictions during development. It has been shown that $\text{M}{}^{\mathrm{+}}\text{M}{}^{\mathrm{+}}$ clones initiated at a given time of development share common demarcation lines that delimit what we call “a developmental compartment.” As development time and cell proliferation of the anlage proceed, large compartments become split into pairs of smaller ones. A study of the number of cells in a given compartment at the time of its splitting into subcompartments indicates that the “developmental segregation” takes place in groups of neighboring cells and suggests that the number of segregated cells is different and characteristic for each compartment. Within a given compartment, a single clone of $\text{M}{}^{\mathrm{+}}\text{M}{}^{\mathrm{+}}$ cells gives rise to 60–90% of the total number of adult cells. This phenomenon is reminiscent of the regulative properties of the morphogenetic fields, and the relation of these to developmental compartments is discussed. Since homoeotic mutants transform entire developmental compartments into one another, the hypothesis is advanced that homoeotic genes control compartment development.     

Modification of the aggregation state of the multifunctional enzyme complex catalyzing the first steps in pyrimidine biosynthesis in the course of development of Drosophila melanogaster

Mutations at the bithoraxoid (bxd) and postbithorax (pbx) loci cause a transformation of posterior haltere to posterior wing. It has previously been shown that pbx and $\text{pbx}\text{Ubx}{}^{101}$ cause this transformation by affecting the maintenance (or cell heredity function) of determination so that the transformed cells are indistinguishable from normal wing cells, and have no “memory” of having been part of a haltere disk (Adler, 1978a). I report here that Tp(3) $\text{bxd}{}^{100}\text{Ubx}{}^{101}$ and bxd¹, pbx, e^w both cause the transformation of posterior haltere to posterior wing in the same way as pbx. On the other hand, bxd¹, $\text{bxd}{}^1\text{Ubx}{}^{101}$, bxd^51j, $\text{bxd}{}^{\mathrm{<mtext>51</mtext><mtext>j</mtext>}}\text{Ubx}{}^{101}$, and $\text{bxd}{}^{\mathrm{<mtext>51</mtext><mtext>j</mtext>}}\text{red pbx}$ cause this same transformation of posterior haltere to posterior wing by interfering with the expression of the determined state so that the developmental information of posterior haltere is “misread” as posterior wing. The transformed cells in these disks retain the memory of having been part of a haltere disk; that is, these posterior cells that would secrete wing cuticle during metamorphosis regenerate anterior haltere structures. Thus it appears clear that it is possible to uncouple the expression and cell heredity functions of determination in the haltere disk of Drosophila.     

The central projections of mesothoracic sensory neurons in wild-type Drosophila and bithorax mutants

Sensory axons entering the CNS from large campaniform sensilla on the normal, mesothoracic wings of four-winged flies of the genotype $\text{bx}{}^3\text{pbx}\text{Ubx}{}^{130}$ follow the same two tracts as do the corresponding axons in wild-type flies. However, they produce more branches along the ventromedial tract (including some in the mesothoracic neuromere), more fibers crossing the midline in the metathorax, and several other modifications of the wild-type pattern. No morphological differences between the receptors in normal and mutant flies could be detected, even with the SEM. The extra branching and other altered characteristics are present in bithorax flies which are also genetically wingless and do not form the homeotic appendages, so they appear to be due to the $\text{bx}{}^3\text{pbx}\text{Ubx}{}^{130}$ or $\text{bx}{}^3\text{Ubx}{}^{130}$ genotype and not to some effect of the axons from the homeotic wings.     

Met-tRNAfMet binding in murine dystrophy

Dystrophic mice of the C57B1 $\text{dy}{}^{\mathrm{2J}}\text{dy}{}^{\mathrm{2J}}$ strain and of the ReJ 129 $\text{dy}\text{dy}$ strain and littermate controls were used to prepare met-tRNA^fMet binding factors. The tissues were homogenized and fractions were obtained which contained ribosomes. The binding factors were assayed by the binding of [³⁵S]methionyl-tRNA to control liver ribosomes. The binding, i.e. eukaryotic initiation factor 2 (elF 2) activity, was measured in brain, liver and muscle and in all of these tissues there was a significant decrease in the dystrophic mice. This decrease in initiation factor activity of hindleg muscle resembled, in the direction of the effect, the decrease in elongation factor activity of hindleg muscle resembled, in the of $\text{dy}\text{dy}$ mice previously reported by our laboratory. Thus these two defects, taken together may help to explain the marked wasting of the muscles. The decrease in brain in both strains provides evidence for nervous tissue involvement in genetic dystrophy.     

A high-affinity (Ca2+ + Mg2+)-ATPase in plasma membranes of rat ascites hepatoma AH109A cells

The activity of calcium-stimulated and magnesium-dependent adenosinetriphosphatase which possesses a high affinity for free calcium (high-affinity $\text{(Ca}{}^{\mathrm{2+}}\text{+ Mg}{}^{\mathrm{2+}}\text{)-ATPase}$, EC 3.6.1.3) has been detected in rat ascites hepatoma AH109A cell plasma membranes. The high-affinity $\text{(Ca}{}^{\mathrm{2+}}\text{+ Mg}{}^{\mathrm{2+}}\text{)-ATPase}$ had an apparent half saturation constant of $\text{77 ± 31}\text{nM}$ for free calcium, a maximum reaction velocity of $\text{9.9 ± 3.5}\text{nmol}$ ATP hydrolyzed/mg protein per min, and a Hill number of 0.8. Maximum activity was obtained at 0.2 μM free calcium. The high-affinity $\text{(Ca}{}^{\mathrm{2+}}\text{+ Mg}{}^{\mathrm{2+}}\text{)-ATPase}$ was absolutely dependent on 3–10 mM magnesium and the pH optimum was within physiological range (pH 7.2–7.5). Among the nucleoside trisphosphates tested, ATP was the best substrate, with an apparent $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ of 30 μM. The distribution pattern of this enzyme in the subcellular fractions of the ascites hepatoma cell homogenate (as shown by the linear sucrose density gradient ultracentrifugation method) was similar to that of the known plasma membrane marker enzyme alkaline phosphatase (EC 3.1.3.1), indicating that the ATPase was located in the plasma membrane. Various agents, such as K⁺, Na⁺, ouabain, KCN, dicyclohexylcarbodiimide and NaN₃, had no significant effect on the activity of high-affinity $\text{(Ca}{}^{\mathrm{2+}}\text{+ Mg}{}^{\mathrm{2+}}\text{)-ATPase}$. Orthovanadate inhibited this enzyme activity with an apparent half-maximal inhibition constant of 40 μM. The high-affinity $\text{(Ca}{}^{\mathrm{2+}}\text{+ Mg}{}^{\mathrm{2+}}\text{)-ATPase}$ was neither inhibited by trifluoperazine, a calmodulin-antagonist, nor stimulated by bovine brain calmodulin, whether the plasma membranes were prepared with or without ethylene glycol $\text{bis}\text{(β-}\text{aminoethyl ether}\text{)-N,N,N′,N′-}\text{tetraacetic acid}$. Since the kinetic properties of the high-affinity $\text{(Ca}{}^{\mathrm{2+}}\text{+ Mg}{}^{\mathrm{2+}}\text{)-ATPase}$ showed a close resemblance to those of erythrocyte plasma membrane $\text{(Ca}{}^{\mathrm{2+}}\text{+ Mg}{}^{\mathrm{2+}}\text{)-ATPase}$, the high-affinity $\text{(Ca}{}^{\mathrm{2+}}\text{+ Mg}{}^{\mathrm{2+}}\text{)-ATPase}$ of rat ascites hepatoma cell plasma membrane is proposed to be a calcium-pumping ATPase of these cells.     

Capacity of adult steel (SI/SId) and dominant spotting (W/Wv) mouse skin to support melanogenesis.

Utilizing skin grafting techniques, melanocytes were incorporated into adult steel ($\text{Sl}\text{Sl}{}^{\mathrm{d}}$) and dominant spotting ($\text{W}\text{W}{}^{\mathrm{v}}$) hair follicles, where they survived and pigmented a number of hair generations. Previous investigations have indicated that the steel alleles act on the embryonic skin to prevent the survival of melanocytes. The present study suggests that this gene action mechanism may be temporary and does not continue into adult life. In addition, the study demonstrates that some hair bulb melanocytes can pigment more than one hair generation.     

Plasma membranes from rat intestinal epithelial cells at different stages of maturation. I. Preparation and characterization of plasma membrane subfractions originating from crypt cells and from villous cells

To determine the mechanism of the maturation of the brush border membrane in intestinal epithelial cells, purification of the plasma membrane from undifferentiated rat crypt cells and of the basal-lateral membrane from villous cells has been performed. The method is based on density perturbation of the mitochondria to selectively disrupt their association with the membrane. With both cell populations, two membrane subfractions displaying the same respective density on sucrose gradient have been obtained with an overall yield of 15–20% and a 10-fold enrichment of the plasma membrane markers 5′-nucleotidase and $\text{(}\text{Na}{}^{\mathrm{+}}\text{+ K}{}^{\mathrm{+}}\text{)-dependent}$, ouabain-sensitive ATPase chosen to follow their purification. The four fractions constituted by sheets and apparently closed vesicles of various sizes. Each fraction was characterized by a distinct protein composition and different levels of enzyme activities. The cells, used for the preparation of the membranes, were isolated as a villus to crypt gradient. This separation and that of the membranes led to the conclusion that the $\text{(}\text{Na}{}^{\mathrm{+}}\text{+ K}{}^{\mathrm{+}}\text{)-dependent}$ ATPase is localized principally in the plasma membrane of all cells whatever their state of maturation, while 5′-nucleotidase is predominantly located in the basal-lateral membrane of the villous cells and may serve as a specific marker for the purification of this membrane. Finally it has been shown that aminopeptidase, disaccharidases and alkaline phosphatase do not appear simultaneously in the maturation process of the cells, alkaline phosphatase being absent from the crypt cells and aminopeptidase being the first to be synthesized. This enzyme seems to appear in the crypt cells membrane before being integrated into the mature brush border membrane.     

The ferritin content of human red blood cells during the replacement of embryonic cells by fetal cells

Mature human embryonic erythrocytes (hemoglobin is ≥ 90% of the cellular protein) contained at least 20 times as much ferritin as human adult erythrocytes, suggesting the possibility that the embryonic red cells participate in iron storage as they do in other embryonic or larval vertebrates. The ferritin content of mature red cells in the circulation declined when fetal red cells replaced embryonic red cells; the cell replacement was monitored by the disappearance of embryonic ε-chains and the appearance of the fetal globin chains, γ^A and γ^G. A constant ratio of 0.67 was obtained for $\text{γ}{}^{\mathrm{<mtext>G</mtext>}}\text{γ}{}^{\mathrm{<mtext>A</mtext>}}\text{+ γ}{}^{\mathrm{<mtext>G</mtext>}}$ from the first detectable appearance (4 weeks after conception) until 13 weeks, a value which is similar to the value previously obtained at 20 weeks gestation and birth but higher than that observable in adults; thus, both γ^G and γ^A chains are produced in similar amounts throughout gestation. The high levels of ferritin in normal human embryonic erythrocytes emphasize the similarity of erythropoiesis in human embryos and other vertebrates. In addition, the results show that red cell ferritin can be used as a marker for studying the mechanism of induction of embryonic erythropoiesis in cultured cell lines, such as K562 from human chronic myelocytic leukemia, and that ferritin content may also serve as a marker for cellular transformations involving reversions to embryonic erythropoiesis.     

Change in cytosolic calmodulin activity of density (age) separated human erythrocytes towards membrane Ca2+/Mg2+ ATPase

The membrane $\text{Ca}{}^{\mathrm{2+}}\text{Mg}{}^{\mathrm{2+}}$ ATPase of density (age) separated human erythrocytes was examined for its stimulation by the cytosols of these cell groups. On the assumption that the stimulatory activity in the cytosol is only calmodulin, it was consistently observed that the young cytosol had a significantly higher activity towards the membrane $\text{Ca}{}^{\mathrm{2+}}\text{Mg}{}^{\mathrm{2+}}$ ATPase activity (from any age group) than did the old cell cytosol. The data clearly demonstrates decided differences in the expression of calmodulin activity in cytosols from young and old erythrocytes and would support the conclusion that calmodulin activity is altered during $\text{in vivo}$ aging of these cells. Possible mechanisms for these alterations are discussed.