By employing multiparameter sorting, we identified in murine bone marrow (BM) a homogenous population of rare (B0.02% of BMMNC) Sca-1 þ lin À CD45 À cells that express by RQ-PCR and immunohistochemistry markers of pluripotent stem cells (PSC) such as SSEA-1, Oct-4, Nanog and Rex-1. The direct electronmicroscopical analysis revealed that these cells are small (B2-4 lm), posses large nuclei surrounded by a narrow rim of cytoplasm, and contain open-type chromatin (euchromatin) that is typical for embryonic stem cells. In vitro cultures these cells are able to differentiate into all three germ-layer lineages. The number of these cells is highest in BM from young (B1-monthold) mice and decreases with age. It is also significantly diminished in short living DBA/2J mice as compared to long living B6 animals. These cells in vitro respond strongly to SDF-1, HGF/SF and LIF and express CXCR4, c-met and LIF-R, respectively, and since they adhere to fibroblasts they may be coisolated with BM adherent cells. We hypothesize that this population of Sca-1 þ lin À CD45 À very small embryonic-like (VSEL) stem cells is deposited early during development in BM and could be a source of pluripotent stem cells for tissue/ organ regeneration.

The ultrastructure of bone marrow of rats, mice and guinea pigs was studied after fixation by vascular perfusion. Serial sections were examined to determine the structure and site of pores in lining cells. It was concluded that It is generally accepted that hematopoiesis in bone marrow of mammals occurs in the extravascular space, but the mechanism controlling passage of blood cells into the circulation is poorly understood. Under normal conditions, only mature or nearly mature forms of hematopoietic cells enter the sinuses, and it is apparent that some type of screening occurs. Whether this selective process is a function of the blood cells, the sinus wall, or both is not known. Knowledge of the structure of the sinus wall is an important prerequisite to the consideration of any selective mechanism. Discontinuous lining cells would present openings through which cells could migrate, while a continuous sinus wall would necessitate formation of pores at the time of migration. Also, the site of pores would seem important. Intracellular pores resulting from interaction of lining cell and blood cell could have a high degree of selectivity, while less selectivity would be expected with intercellular pores.The present study was undertaken to see if the ultrastructure of lining cells and their pores, as elucidated by improved methods of preservation, could help to clarify some of these problems. Serial sectioning techniques were chosen as the best means of determining the three dimensional structure and site of pores. MATERIALS AND METHODSBone marrow was obtained from adult AM. J. ANAT., 135: 521-536.BALB/c mice, 8-to 20-day-old SpragueDawley rats, and one-to €our-day-old guinea pigs. In all animals the marrow was fixed by vascular perfusion via the thoracic aorta. The perfusion apparatus was of the gravity flow type and consisted of a separatory funnel fitted with a short piece of latex tubing clamped at the end and a cannula made from a ten-foot piece of polyethylene tubing with an internal diameter of 0.023 inches. Two 20-gauge hypodermic needles were cut off near the fitting and inserted, points out, into the ends of the tubing. The needle at one end of the cannula was stuck through the latex tubing of the separatory funnel and the funnel was suspended about six feet above the operating area. The thoracic aorta was approached by removing the ventral thoracic wall of anesthetized animals and retracting the left lung. The aorta was freed by blunt dissection, grasped with forceps, cannulated, and ligated immediately distal to the site of entry to prevent back flow of fixative. The inferior vena cava or right atrium of the heart was punctured to allow hemorrhage and prevent back pressure in the vascular system. The vascular bed was washed with isotonic Krebs buffer, then perfused with a fixative consisting of 2% paraformaldehyde, 2.5% glutaraldehyde, 0.1 % picric acid and 0.05% calcium chloride buffered with 0.06 M cacodylate (It0 and Karnovsky, '68) and diluted 1 : 1 with 0.1 M cacodylate buffer immediately bef...

The structure of bone marrow from chickens and pigeons was studied with light and electron microscopy. Erythropoiesis occurs in the Iumen of the medullary sinuses. Immature erythroid cells appear to adhere to the sinus wall and may thus be prevented from entering the peripheral circulation.The wall of the medullary sinuses is formed by elongated lining cells, lacking a basement membrane, which are continuous except at sites where blood cells are passing through them.When viewed with the electron microscope, developing heterophil myelocytes, which occur only in the extravascular spaces, possess two populations of granules; one type is globular in content, the other is fibrillar in content. The globular type predominates during all stages of development and appears to be the specific granule.

In bone marrow of the mouse perfused with fixative containing tannic acid and glutaraldehyde, gap junctions were observed between certain cell types. Gap junctions were seen between adjacent reticular cells, between adjacent macrophages, and between macrophages and reticular cells. Macrophages formed gap junctions with immature neutrophils, eosinophils, monocytes, and erythroblasts. Often a single macrophage had gap junctions with neutrophilic, eosinophilic, and monocytic cells; these blood cells varied from immature to nearly mature forms. In contrast, the macrophages associated with erythroblasts had gap junctions only with erythroblasts and all the erythroblasts were in the same developmental stage. The possible role of the gap junctions in differentiation and mobilization of marrow cells is discussed.

Chiari anomalies in the human right atrium ostensibly are encountered rarely. There is only sporadic mention in the literature of these fenestrated, net-like valves of the inferior vena cava, coronary sinus, or various strands connecting these with other right atrial structures. The effects of such structural anomalies on heart function are unknown. We report here gross observations of the right atrial net from among 213 cadavers, 38 autopsied, and 11 fetal hearts. Histological and ultrastructural examination of inferior vena cava and coronary sinus valves demonstrated that only the anomalous coronary sinus valves contained cardiac muscle. Chiari anomalies typically have referred to perforations or tissue strands related to the inferior vena cava valve and possibly the coronary sinus valve. The anomaly commonly is cited as occurring in 2% of individuals, although there has been no study to support this. We observed Chiari malformations in 13.6% of the 213 cadaver hearts, and 10.5% of the autopsied hearts examined. Of these malformations, the coronary sinus valve was fenestrated most frequently. We propose the term "right atrial net" for "Chiari net," for anomalies involving valves of the inferior vena cava and coronary sinus, and strands within the right atrium connecting these valves with the crista terminalis, right atrial wall, or interatrial septum.