Innervation of the elbow joint: Is total denervation possible? A cadaveric anatomic study.

The aim of this anatomical study was to find out if total denervation of the elbow joint is technically feasible. The endbranches of the brachial plexus of eight fresh-frozen upper arm cadavers were dissected with optical loupe magnification. All major nerves of the upper limb (except the axillary and the medial brachial cutaneous nerve) give some terminal articular endbranches to the elbow. The articular endbranches arise from muscular endbranches, cutaneous endbranches, or arise straight from the main nerves of the brachial plexus. A topographic diagram was made of the different nerves innervating the elbow joint. The ulno-posterior part of the elbow is innervated by the ulnar nerve and some branches of medial antebrachial cutaneous nerve. The radial-posterior part of the elbow is innervated exclusively by the radial nerve. The ulno-anterior part of the elbow is innervated by the median nerve and the musculocutaneous nerve. The radio-anterior part of the elbow is innervated by the radial nerve and the musculocutaneous nerve. These elbow innervation findings are relevant to both anatomical and clinical field as they provide evidence that the total denervation of the elbow joint is impossible. Nevertheless, partial denervation, like denervation of the lateral epicondyle or the ulnar part of elbow, is technically possible.

Endoscopically assisted core decompression in avascular necrosis of the femoral head.

The authors have used bone endoscopy (medulloscopy) to improve the accuracy of the conventional technique for core decompression and grafting in 36 hips with avascular necrosis of the femoral head. Endocopy was found to facilitate debridement of the necrotic bone and to decrease the risk of perforation of the cartilage during debridement. The endoscopy-assisted technique failed, however, to improve the final outcome of core decompression and grafting in more advanced stages of osteonecrosis. Based on this experience, the authors do not recommend core decompression, with or without endoscopic assistance, in advanced stages of avascular necrosis, but they suggest using endoscopic assistance during decompression in small-size lesions.

The role of the transverse carpal ligament in carpal stability: an in vitro study.

A biomechanical in vitro study was performed on 16 fresh frozen cadaver forearms to investigate the role of the transverse carpal ligament (TCL) in carpal stability. The distance between the scaphoid and hamate was measured, as a reference for the length of the TCL. Distances were recorded in both loaded and unloaded conditions after gradual sectioning of the transverse carpal ligament, the palmar scapholunate, long radiolunate ligament and radioscapholunate ligament. The largest increase in spread of the carpal bones (55.3% of total spread) was noted after loading with the ligament intact. Thereafter, sectioning of the TCL resulted in a further 32.9% increase in the distance between the scaphoid and the hamate. We conclude that the intact carpal bones-ligament complex displays some elasticity. Progressive sectioning of the TCL ligament under loading further opens the palmar arch. Nevertheless it appears that the carpal arch will still retain reasonable intrinsic stability even without an intact TCL.

Mosaic versus regulation development in avian blastoderms depends on the spatial distribution of Rauber's sickle material.

We describe how to prepare unincubated avian eggs to obtain a greater number of clearly visible Rauber's sickles for experimental embryology. After hemi-sectioning of unincubated chicken (Gallus domesticus) blastoderms and cultivating both halves in vitro, two kinds of development can be discerned: (1) when the unincubated blastoderms were hemi-sectioned according to the plane of bilateral symmetry, going through the middle region of Rauber's sickle, we obtained two hemi-embryos (a left and a right one). Each contained a half primitive streak, localized at the cut edge (starting from the most median part of Rauber's sickle) giving rise to a half mesoblast mantle and half area vasculosa, thus indicating mosaic development (each part of the whole fertilized egg would be able to form independently on its own). (2) When the unincubated blastoderm is hemi-sectioned more obliquely, going through a more lateral part of Rauber's sickle (sickle horn), two complete bilaterally symmetrically miniature embryos will form, indicating the so-called regulation phenomena. We demonstrate that these two types of development are in reality due to the different spreading and concentration of Rauber's sickle tissue (containing gamma ooplasm) around the area centralis. Embryonic regulation thus must not be considered as a kind of totipotent regeneration capacity of isolated parts of the unincubated avian blastoderm, but depends on the spatial distribution of a kind of extraembryonic tissue (Rauber's sickle) built up by the oblique uptake of gamma ooplasm (ooplasmic mosaicism) at the moment of bilateral symmetrization (Callebaut [1994] Eur Arch Biol 105:111-123; Callebaut [2005] Dev Dyn 233:1194-1216).

Early steps in neural development.

We studied early neurulation events in vitro by transplanting quail Hensen's node, central prenodal regions (before the nodus as such develops), or upper layer parts of it on the not yet definitively committed upper layer of chicken anti-sickle regions (of unincubated blastoderms), eventually associated with central blastoderm fragments. We could demonstrate by this quail-chicken chimera technique that after the appearance of a pronounced thickening of the chicken upper layer by the early inductive effect of neighboring endophyll, a floor plate forms by insertion of Hensen's node-derived quail cells into the median part of the groove. This favors, at an early stage, the floor plate "allocation" model that postulates a common origin for notochord and median floor plate cells from the vertebrate's secondary major organizer (Hensen's node in this case). A comparison is made with results obtained after transplantation of similar Hensen's nodes in isolated chicken endophyll walls or with previously obtained results after the use of the grafting procedure in the endophyll walls of whole chicken blastoderms.

Histological evaluation of autophagic cell death in calcified aortic valve stenosis.

BACKGROUND AND AIM OF THE STUDY: Calcification in aortic valves is the most common valvular lesion in western populations. This event is correlated with cellular degeneration in the valvular cusps, although there is no exact evidence how these cells die: this requires further exploration. METHODS: Twelve human severely calcified aortic valves obtained during cardiac surgery were studied by semi-quantitative analysis, and results compared with data from 12 human control aortic valves obtained during autopsy. Tissue analysis was by hematoxylin and eosin and Alcian blue staining. Detection of neurons was by immunohistochemical staining of PGP9.5 and neurofilament. In order to detect autophagy, an immunohistochemical staining for ubiquitin was used. The TUNEL technique was used to detect apoptosis. Co-localization of Alizarin red with ubiquitin labeling was performed on non-decalcified aortic valves. RESULTS: Hematoxylin and eosin staining showed moderate to severe mineralization in 10 of 12 patients in the surgical group, but in only one of 12 in the autopsy group. No significant observations were made with regard to PGP9.5 and neurofilament staining. Moderate to severe ubiquitin labeling was found initially in the majority of the surgical resection group (9/12) compared to a minority in the autopsy group (1/12). TUNEL-positive labeling was very rare and found mostly at the endothelial layer of the valvular cusps. CONCLUSION: Immunohistochemical methods showed the main cell death mechanism involved in the calcification of aortic leaflets to be autophagy rather than apoptosis. These findings suggest that autophagic cell death might play a role in the release of matrix vesicles in early degenerative aortic valves, thereby attracting inflammatory cells, and this could eventually lead to mineralization.

Risk factors for congestive heart failure after aortic valve replacement with a Carpentier-Edwards pericardial prosthesis in the elderly.

BACKGROUND AND AIM OF THE STUDY: Congestive heart failure (CHF) after aortic valve replacement (AVR) is an important cause of morbidity. The study aim was to identify preoperative risk factors for CHF. METHODS: A total of 500 consecutive patients (271 males, 229 females; median age 73 years; range: 71-77 years) was investigated retrospectively. The AVR was performed using a Carpentier-Edwards pericardial valve, and a total of 348 additional procedures (313 coronary artery bypass grafts; CABG) was carried out. The outcome studied was CHF, during both hospital stay and long-term follow up. Univariate and multivariate statistical analyses were used to investigate 15 risk factors. RESULTS: During the hospital stay, 13 patients developed CHF, with four fatalities. Significant risk factors for CHF included urgent operation (p = 0.031), preoperative atrial fibrillation (AF) (p = 0.031) and NYHA functional class IV (p = 0.05). A logistic regression analysis revealed need for urgent operation (p = 0.034) as the sole factor. During long-term follow up, 43 patients developed CHF, with seven fatalities. Univariate analysis identified seven risk factors with significant effect: valve size <19 mm (p = 0.004), preoperative conduction defects (p = 0.007), chronic postoperative AF (p = 0.013), cross-clamp time >75 min (p = 0.032), NYHA class IV (p = 0.041), coronary artery disease (CAD) (p = 0.043) and additional CABG (p = 0.050). Multivariate analysis identified three risk factors: preoperative conduction defects (p = 0.004), postoperative AF (p = 0.005) and CAD (p = 0.037) CONCLUSION: Morbidity due to CHF after AVR could be minimized with correct treatment of AF and of conduction defects. Patient age, valve size, cross-clamp time and preoperative severity or symptoms were not independent risk factors. Moreover, small native aortic valve rings should not necessarily be enlarged, the cross-clamp time should be kept to a minimum, and surgery should not be delayed when symptoms have developed.

Smooth muscle cell hypertrophy in varicose veins is associated with expression of estrogen receptor-beta.

Varicose veins are characterized by dilated and thickened vein walls. This study examined whether the changes that occur in varicose veins are associated with smooth muscle cell (SMC) hypertrophy, cellular proliferation or apoptosis. Moreover, the association between SMC hypertrophy and the expression of the estrogen receptor-beta (ERbeta) was investigated. Varicose veins were obtained from male patients during vascular stripping surgery (n = 11) and nonvaricose veins during coronary bypass surgery, also from male subjects (n = 12). The cellular volume of the SMC in both the longitudinal and circular layer of the vessel wall was measured using stereological methods. Apoptosis was detected using the TUNEL (terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling) technique. SMC proliferation and ERbeta expression were investigated by immunohistochemistry. Neither in the longitudinal nor in the circular layer of the varicose vein wall were signs of apoptosis or proliferation present. However, the mean cellular volume of the SMC in the circular layer of the varicose veins was strongly increased (5,291 +/- 363 microm3) as compared to non-varicose veins (2,812 +/- 212 microm3, p < 0.001). Moreover, ERbeta expression in the circular layer of varicose veins (63 +/- 4%) significantly differed from nonvaricose veins (39 +/- 4%; p = 0.001). Interestingly, the SMC volume correlated with ERbeta expression (r = 0.71, p < 0.001). These data show that cell death or proliferation of SMC do not, or only rarely, occur in varicose veins. However, remodeling of varicose veins can mainly be attributed to increased volumes of the SMC of the circular layer and this increase correlates with ERbeta expression.

Risk factors and survival after aortic valve replacement in octogenarians.

BACKGROUND AND AIM OF THE STUDY: The study aim was to determine if aortic valve replacement in octogenarians is still rewarding. METHODS: Between 1986 and 2000, 500 patients received a Carpentier-Edwards pericardial valve in the aortic position. Of these patients, 348 also underwent coronary artery bypass grafting (CABG). Sixty patients were aged > or =80 years. A retrospective follow up totaled 2,022 patient-years. A Cox multivariate regression analysis included 17 preoperative potential risk factors: age >80 years; gender; carcinoma; chronic obstructive pulmonary disease; renal failure; stroke; arterial hypertension; carotid artery disease; myocardial infarction; coronary artery disease; conduction defects; atrial fibrillation; medically treated endocarditis; severity of symptoms; urgent operation; left ventricular function; and need for digitalis. RESULTS: For hospital mortality, independent predictors were urgent surgery (Risk Ratio 10.2, 95% CI 2.5-42.0, p = 0.001); age over 80 (RR 4.5, CI 1.3-14.9; p = 0.015); need for digitalis (RR 3.8, CI 1.3-10.6, p = 0.010); male gender (RR 3.7, CI 1.1-12.4; p = 0.035); and myocardial infarction (RR 3.1, CI 1.0-9.4, p = 0.051). For long-term mortality, independent predictors. were urgent surgery (RR 4.5, CI 1.6-12.6; p = 0.004), age >80 (RR 2.5, CI 1.4-4.5, p = 0.002); myocardial infarction (RR 2.1, CI 1.3-3.4; p = 0.003); carcinoma (RR 2.0; CI 1.1-3.7; p = 0.021); and digitalis use (RR 1.8; CI 1.2-2.7; p = 0.004). Univariate analysis revealed that age >80 years (38.6% versus 77.0%), need for urgent operation (0% versus 75.1%), need for digitalis (69.4% versus 76.3%) and myocardial infarction (57.1% versus 76.4%) had a significant effect on five-year survival. CONCLUSION: For hospital mortality and long-term mortality, a need for urgent surgery was the most determining factor. Age >80 years was the second most important factor, but previous myocardial infarction and need for digitalis were almost equally important. Aortic valve replacement in octogenarians is still rewarding, as five-year survival is 38.6%. Thus, surgery in these patients should not be postponed.

Induction and improved embryonic development by the nucleus of Pander in associated avian blastoderm parts: influence of delta or gamma ooplasm.

After placing in vitro, central subgerminal ooplasm (containing a central nucleus of Pander) from a quail germ disc of a prelaid egg (before symmetrization) on the upper layer of an isolated chicken antisickle, we observed the induction of a radially oriented preneural plate (without interference of chordamesoblast). This observation suggests the primary existence during the period of symmetrization in utero of an until now unknown temporospatially linked "vertical" effect, emanating from the nucleus of Pander, on the parallel (pre)neural plate anlage forming part of the area centralis in the overlying blastoderm. For comparison, we "sandwiched" in vitro a quail sickle endoblast fragment between the deep side of the upper layer of an isolated chicken antisickle region and a central subgerminal ooplasmic mass. This resulted in a colonization of the subgerminal ooplasmic mass by quail sickle endoblast cells followed by improved neurulation and/or gastrulation phenomena. The latter never occurs in the absence of central subgerminal ooplasm. In both types of experiments there seems to exist a common link between the observed induction phenomena: the presence of delta ooplasm in the involved deep structures. Indeed, the nucleus of Pander contains delta ooplasm as well as the structures derived from it, i.e., endophyll with primordial germ cells and sickle endoblast-derived cells after colonization of the neighboring central ooplasm (present study). Therefore, we think that the preneural plate-inducing effect observed after placing a nucleus of Pander on the antisickle region is due to the presence of a factor in the delta ooplasm that diffuses in the neighborhood. The appearance of gastrulation phenomena in the second type of experiment seems to be due to colonization of the more peripheral part of the central subgerminal ooplasm containing the more superficial and peripheral gamma ooplasm in which Rauber's sickle material can develop. This suggests that the kind of involved ooplasm (delta or gamma) can predetermine the inductive activity of the deep structures that contain it: the central part of the nucleus of Pander and/or endophyll for preneurulation phenomena and sickle endoblast (in the presence of central subgerminal ooplasm) for gastrulation and/or neurulation phenomena.

Effect of prior malignancy on survival after cardiac surgery.

BACKGROUND: The number of patients with a previously treated tumor, needing cardiac surgery is increasing. Whether this operation in these patients is justified is determined by the long-term outcome. METHODS: Of 8620 patients referred for cardiac surgery, 205 had a documented malignant tumor. The time interval between the occurrence of the tumor and the cardiac surgery was recorded. These patients were matched with 205 patients without a tumor according to age, gender, comorbidity and type of cardiac surgery. The patients were followed retrospectively. A chi(2) Kaplan Meier and Cox' regression analysis were performed. RESULTS: During follow-up, 95.8% of the patients were traced (2794 patient years). Univariate analysis showed that 5- and 10-year survival was better in patients without a malignant tumor in the history (0.91 +/- 0.02 versus 0.72 +/- 0.03 and 0.73 +/- 0.04 versus 0.40 +/- 0.05; p < 0.0001). For shorter time intervals, mortality for all causes and mortality due to the tumor increase significantly (p < 0.0001). Multivariate analysis identified 4 independent variables: a malignant tumor in the history (p < 0.001), chronic obstructive pulmonary disease (p = 0.003), age (p = 0.001), and impaired left ventricular function (p = 0.035) CONCLUSIONS: A malignant tumor in the history is the most prognostic factor after cardiac surgery, but the operation is still rewarding. Fatal progression of the tumor is seen if the time interval between the occurrence of the malignant tumor and cardiac surgery is short. Other unfavorable factors are decreased left ventricular function, chronic obstructive pulmonary disease and high age.

Induction of the avian coelom with associated vitelline blood circulation by Rauber's sickle derived junctional endoblast and its fundamental role in heart formation.

In histological sections through chicken blastoderms of different ages we describe the temporospatial relationship between junctional endoblast, the formation of blood islands (appearing first from a peripherally migrating mesoblastic blastema), and the formation of coelomic vesicles developing later in/and from a more superficially extending mesoblastic blastema (coelomic mesoblast). After unilateral removal of the Rauber's sickle-derived junctional endoblast in early streak blastoderms (stage 2-4; Vakaet [1970] Arch Biol 81:387-426) and culture to stage 11 (Hamburger and Hamilton [1951] J Morphol 88:49-92), we observed that the early formation of the coelomic cavity was locally or totally disturbed in the operated area. Besides the simultaneous absence of blood islands, the coelomic vesicles did not form normally. Instead of regularly aligned coelomic vesicles, progressively forming the coelomic cavity by fusion, some voluminous irregular cavities appeared. Thus, the extent of the coelomic cavity was greatly reduced and the operated side was considerably smaller than the unoperated side. Furthermore, in the youngest operated blastoderms the cranial portion of the involved coelomic cavity (hemipericardial cavity) exhibited rudimentary development and usually did not reach the region of the foregut endoderm. This resulted in the absence of the myoepicardium and associated endocardium at this side. In another experiment, after removal of the junctional endoblast at one side of the chicken blastoderm, a fragment of quail junctional endoblast was placed isotopically. This resulted, after further in vitro culture, in the restoration of the formation of coelomic vesicles and accompanying subjacent blood islands in the immediate neighborhood of the apposed quail junctional endoblast. Also, the pericardium and primary heart tube developed normally. Similarly, by using the quail-chicken chimera technique, we demonstrated that the splanchnic mesoderm cells of the pericardium develop in intimate association with the most cranial part of the junctional endoblast (derived from the Rauber's sickle horns). Our experiments indicate that the coelom and, in particular, the pericardium and primary heart tube form progressively (in time and space) under the inductory influence of Rauber's sickle and junctional endoblast.

The noncircular shape of the radial head.

PURPOSE: The purpose of this study was to define the shape of the radial head by identifying the relationship between precisely defined axes of the radial head. METHODS: An anatomic study was done to define the shape of the radial head and specifically the relationship between the long and the short axis. Twenty-seven cadaveric upper extremities were used. The x and y axes of the radial head were defined in relationship to the radial notch of the ulna, with the forearm in neutral position. Outer diameters of the x and y axis were measured. These were compared with the actual maximum and minimum diameters of the radial head. X and y diameters of the articulating surface of the radial head also were measured. RESULTS: Paired 1-tailed Student's t-tests were used to compare the x and y diameters of the radial head. Regression analysis of x and y diameters of the radial head was done to identify a correlation between these parameters.Paired 1-tailed Student's t-tests showed a significant difference between X and Y diameters of the radial head. Regression analysis of x and y diameters of the radial head showed a strong correlation between these 2 axes. CONCLUSIONS: The radial head is not round. A strong correlation exists between the x and y diameters of the radial head. The orientation of the long axis is perpendicular to the radial notch with the forearm in neutral rotation. This finding will make it possible to approach the anatomy of the radial head more closely when designing radial head prostheses. The definition of the axes can be used as a guide when implanting the radial head prosthesis.

Competitive inhibition by Rauber's sickle of the primitive streak and/or (pre)neural plate inducing effects of sickle endoblast in avian blastoderms.

When in unincubated chicken blastoderms the Rauber's sickle is (sub)totally mechanically removed by selective scraping, the further evolution of the blastoderm in culture is often profoundly disturbed, going from only expansion of the upper layer and preneural plate formation to the development of a slowly growing miniature embryo. Our results suggest that the developmental potencies of the embryo are related to the presence or absence of Rauber's sickle material left after its removal. This can be checked after culture by the presence or nonpresence of junctional endoblast (derived from Rauber's sickle) and the concomitant induction of blood islands in the immediate neighborhood. Our study thus indicates that without Rauber's sickle (in the cases of successful total selective removal), an avian blastoderm cannot develop normally, even in the presence of an intact caudal marginal zone. After placing a fragment of quail sickle endoblast on the anti-sickle region of unincubated chicken blastoderms from which the Rauber's sickle was (sub)totally removed, different developmental scenarios were seen, according to the degree of removal, both in the anti-sickle as in the sickle regions. 1) If Rauber's sickle activity is strongly reduced, then besides a centripetally directed miniature embryo, induced by the remnants of the autochthonous Rauber's sickle, an additional centripetally directed embryo or preneural plate (without accompanying blood islands) develops in the anti-sickle region under inductory influence of the apposed quail sickle endoblast. We make a distinction between a neural plate and a preneural plate. The latter consists of a thickening of the upper layer (with the same initial aspect as a neural plate) adjacent to endophyll or sickle endoblast in the absence of chordomesoblast and gastrulation phenomena. 2) If Rauber's sickle activity is totally absent, then the inducing power of the sickle endoblast fragment becomes maximal and, starting from the anti-sickle region, one single embryo (without blood islands) extending over the whole area centralis appears. 3) If much of the Rauber's sickle material has been left in the blastoderm, then the inducing activity of the sickle endoblast, placed on the anti-sickle region, will be totally suppressed (although the sickle endoblast remains intact) and neither a preneural plate nor a primitive streak was induced. After placing a fragment of quail sickle endoblast on the anti-sickle region of an unincubated chicken blastoderm from which the Rauber's sickle and surrounding tissues were completely excised, an embryo was always induced by the sickle endoblast in the adjacent upper layer of this anti-sickle region. In the absence of sickle endoblast, this never occurred. Thus, our experiments demonstrate that in the absence of the Rauber's sickle, a parent tissue (sickle endoblast) induces both gastrulation and neurulation phenomena, while in the full presence of Rauber's sickle these functions are totally suppressed. Moreover, Rauber's sickle not only organizes gastrulation and blood island formation by itself but also influences neurulation at a distance (in space and time) by part of its cell lineage (i.e., sickle endoblast). Our study suggests that the inhibitory effect of Rauber's sickle on its parent tissue (sickle endoblast) represents an early mechanism impairing polyembryony, so that only a single primary major organizer (Rauber's sickle) remains active in the young avian germinal disc.

Positional information by Rauber's sickle and a new look at the mechanisms of primitive streak initiation in avian blastoderms.

The present experimental in vitro study suggests that a primitive streak (PS) in avian blastoderms is induced by diffusion of morphogenetic substances emanating from Rauber's sickle. Indeed, even without direct contact between a quail Rauber's sickle and the reacting upper layer (by interposition of a vitelline membrane), a PS can be induced in the isolated area centralis or antisickle region of unincubated chicken blastoderms. The so-formed PSs are localized below the vitelline membrane in the immediate neighborhood of the apposed Rauber's sickle material. This seems to indicate that Rauber's sickle organizes the formation of the avian PS according to the basic concept of "positional information." The morphogenetic substances seem to have an effect only on the formation of a PS. Each part of Rauber's sickle seems to have, point by point, the same thickening and PS-inducing effect on each corresponding part of the underlying upper layer (UL). By a mechanism of sliding over the basement membrane and fusion, this finally results in the formation of one single median PS. Our study shows that a PS can be induced in the total absence of hypoblast (sickle endoblast) or caudal marginal zone, by only the presence of Rauber's sickle material. In contrast, the differentiation of mesoblast into blood islands under the influence of Rauber's sickle and neural tissue development are impaired by the interposition of a vitelline membrane. The latter could be due to the absence of a normal interaction of Rauber's sickle-derived sickle endoblast with endophyll and/or upper layer and the absence of cranial migration of the mesoblast. Thus, earlier studies and the present study indicate the existence of a temporospatially bound cascade of gastrulation and neurulation phenomena and blood island formation in the avian blastoderm, starting from Rauber's sickle, the primary major organizer with inducing, inhibiting, and dominating potencies. The latter not only plays a role by secretion of signaling molecules, but also influences development by its cell lineages (junctional endoblast and sickle endoblast).

In the absence of Rauber's sickle material, no blood islands are formed in the avian blastoderm.

Using the quail-chick chimera technique, we followed the fate of Rauber's sickle cells in older whole blastoderms (cultured for approximately 2 days): after removal of the autochthonous Rauber's sickle from an unincubated chicken blastoderm, a quail Rauber's sickle was grafted isotopically and isochronically in its place. In transverse sections through these chimeras, the grafted quail Rauber's sickle cells were seen to have transformed into a broad row or ridge of quail junctional endoblast cells extending at the inner border of the area containing blood islands. After unilateral removal of the junctional endoblast from an intermediate streak chicken blastoderm (Stage 3; Hamburger and Hamilton [1951] J Morphol 88:49-92), we observed during further in vitro culture that at the operated side, in the area previously occupied by this junctional endoblast, blood islands no longer developed. If after such a unilateral removal of the chicken junctional endoblast quail junctional endoblast was apposed in its place, then blood islands reappeared in the operated area. The intimate contact between the apposed quail junctional endoblast and the recently formed blood islands, derived from peripherally migrating mesoderm, was very obvious on sections through such chimeras. We further demonstrate that Rauber's sickle vs. junctional endoblast is indispensable for the anlage of blood islands in avian blastoderms. Indeed, in the absence of Rauber's sickle material no blood islands develop (even when mesoderm is present after ingression of the upper layer via a primitive streak) in the isolated central region of the area centralis of unincubated chicken blastoderms after culture in vitro. Also, no junctional endoblast and no sickle canal appear in these explants. By contrast, if a Rauber's sickle fragment is placed on such an isolated central blastoderm region, then blood islands develop. These blood islands start to develop from peripherally migrating mesoderm in the neighborhood of the Rauber's sickle-derived junctional endoblast.