Genetic and morphogenetic factors in hemoglobin synthesis during higher vertebrate development: an approach to cell differentiation mechanisms

Morphological study of the transition of haematopoietic sites in the developing mouse during the peri-natal period

The transition of fetal haematopoietic sites in mice was examined histologically from 12.5-day embryos (E12.5) until 10 days of age (D10), and the expression of the adhesive molecules VCAM-1 and fibronectin was examined immunohistologically. Erythropoiesis occurred in the liver (E12.5-18.5), spleen (E18.5-D4) and bone marrow (D6-10), in that sequence. Even at D10, some erythropoiesis occurred in the liver, and more so in the spleen, although the active haematopoietic site was the bone marrow. Similarly, granulopoiesis of neutrophils occurred in the liver, spleen and bone marrow in turn. Granulopoiesis still occurred in the spleen at D10, but no neutrophils were found in the liver after D4. VCAM-1 appeared in the liver, spleen and bone marrow in parallel with active erythropoiesis and granulopoiesis. The co-expression of VCAM-1 and fibronectin was recognized in the endothelial cells of the sinus at the onset of haematopoiesis. This study showed that haematopoiesis in the liver, spleen and bone marrow overlapped peri-natally, although it shifted sequentially. VCAM-1 appears to be closely associated with erythropoiesis and granulopoiesis, and the co-expression of VCAM-1 and fibronectin plays a role in inducing haematopoietic stem cells to move to the tissues.

Developmental expression of alloantigen systems in the chicken

The different allelic forms of nine non-Mhc alloantigen systems of the chicken were examined for developmental expression on erythrocytes isolated from embryos and young chicks. Polyclonal alloantisera raised against the different antigens were used to detect these antigens on the cell surface by hemagglutination as well as by indirect immunofluorescence. The developmental stage of initial expression on erythrocytes for each of the genetic systems investigated (i.e., A, E, C, D, H, I, K, L and P) varied from day 4 to day 14 of incubation. The different antigens of each system appeared simultaneously at a particular stage of development except for those of the I system, where the I8 allelic form appeared earlier than I2.

Localized cytosolic domains of the erythropoietin receptor regulate growth signaling and down-modulate responsiveness to granulocyte-macrophage colony-stimulating factor

Erythrocyte development in mammals depends in part upon the interaction of the glycopeptide hormone erythropoietin (EPO) with cell surface receptors on committed erythroid progenitor cells. Both this factor and an EPO receptor polypeptide previously have been cloned, yet little is presently understood concerning molecular mechanisms of receptor activation and signal transduction. To identify cytosolic receptor domains necessary for signaling, we have compared the activities of a series of deletionally mutated EPO receptor constructs by their expression in interleukin 3-dependent, myeloid FDC-P1 cells. EPO-induced growth was transduced efficiently in these cells by the full-length receptor (507 amino acids), and no measurable loss in activity resulted from the deletion of up to 111 carboxyl-terminal residues. In contrast, the deletion of 44 additional residues led to a dramatic loss (86.3% +/- 7.8%; mean +/- SD) in the ability of this receptor to mediate EPO-induced growth, thus indicating that residues between Gly-352 and Met-396 constitute a functionally critical cytosolic subdomain. Interestingly, the expression of full-length EPO receptors in FDC-P1 cells also led to a selective inhibition of normal proliferative responsiveness to the alternative hematopoietic factor granulocyte-macrophage colony-stimulating factor. Moreover, this inhibition was progressively reversed in forms of the EPO receptor in which distal cytosolic residues were sequentially deleted. These results suggest that EPO receptors normally may trans-modulate components in the pathway of granulocyte-macrophage colony-stimulating factor-induced proliferation and that this down-modulation, as exerted by intact EPO receptors, may play a role in promoting erythroid commitment during myeloid blood cell development.

Onset of fibre differentiation in cultured rat lens epithelium under the influence of neural retina-conditioned medium

In the intact rat lens, epithelial cells, which are cuboidal and contain alpha-crystallin, give rise to fibre cells, which are elongated and contain alpha-, beta- and gamma-crystallins. Epithelial explants cultured in medium conditioned by bovine neural retinas (BRCM) showed changes analogous to fibre cell differentiation in vivo. The cells became enlarged and elongated and accumulated beta- and gamma-crystallin as well as alpha-crystallin. Labelling studies with [35S]-methionine showed that sequential changes in the synthesis of all three classes of crystallin occurred during culture in BRCM. After two days, synthesis of alpha-crystallin subunits, particularly alpha A and alpha AINS, increased relative to overall protein synthesis. After four days in culture, synthesis of beta-crystallin subunits, identified as beta B1a, beta B4, beta B5 and possibly beta A2, was detected for the first time and between four and eight days gamma-crystallin synthesis became detectable. The time of onset of gamma-crystallin synthesis seemed to show greater experimental variability than did onset of beta-crystallin synthesis. In explants cultured for 10 days in BRCM approximately 25% of new protein synthesis could be attributed to alpha-, beta- and gamma-crystallins. These events were completely dependent on BRCM, suggesting that neural retina secretes a factor(s) which initiates fibre cell differentiation. This culture system appears to be a suitable one for investigating the control of fibre differentiation in vitro.

Nucleoside and glucose transport in erythrocytes from new-born lambs

1. Glucose and inosine transport by erythrocytes from new-born lambs and adult sheep were compared. Uptake of both permeants was considerably faster in the new-born. Inosine uptake by erythrocytes from nucleoside-permeable and impermeable lambs were not significantly different at birth. The difference between the two phenotypes was first apparent 30 days after birth.2. The post-natal changes in glucose and inosine transport activity closely paralleled the progressive decrease in the percentage of fetal erythrocytes (i.e. cells containing fetal haemoglobin) in the circulation. Cell fractionation studies confirmed that the permeability changes were directly related to changes in the relative proportions of fetal and adult haemoglobin containing erythrocytes.3. The results demonstrate that fetal cells are highly permeable to both glucose and inosine. These cells are replaced by erythrocytes which contain adult haemoglobin and which have a much lower, but still significant, glucose permeability and either low or negligible inosine transport activity depending on the genotype of the animal.4. Inosine transport by fetal erythrocytes from both nucleoside-permeable and impermeable animals was mediated by a nucleoside transport system which had similar properties to that responsible for nucleoside transport in adult nucleoside-permeable cells. Glucose transport in both fetal and adult cells was highly stereo-specific, indicating the presence of a selective transport system.5. It is suggested that the regulatory mechanism responsible for initiating the switch from fetal to adult haemoglobin synthesis may also be responsible for the changes in glucose and nucleoside transport activity.