Publisher Summary The classification of cell death can be based on morphological or biochemical criteria or on the circumstances of its occurrence. Currently, irreversible structural alteration provides the only unequivocal evidence of death; biochemical indicators of cell death that are universally applicable have to be precisely defined and studies of cell function or of reproductive capacity do not necessarily differentiate between death and dormant states from which recovery may be possible. It has also proved feasible to categorize most if not all dying cells into one or the other of two discrete and distinctive patterns of morphological change, which have, generally, been found to occur under disparate but individually characteristic circumstances. One of these patterns is the swelling proceeding to rupture of plasma and organelle membranes and dissolution of organized structure—termed “coagulative necrosis.” It results from injury by agents, such as toxins and ischemia, affects cells in groups rather than singly, and evokes exudative inflammation when it develops in vivo. The other morphological pattern is characterized by condensation of the cell with maintenance of organelle integrity and the formation of surface protuberances that separate as membrane-bounded globules; in tissues, these are phagocytosed and digested by resident cells, there being no associated inflammation.

Cell death : the significance of apoptosis

Physiological cell death is a widespread phenomenon in the development of both vertebrates and invertebrates. This review concentrates on an aspect of developmental cell death that has tended to be neglected, the manner in which the cells are dismantled. It is emphasized that the dying cells may adopt one of at least three different morphological types: "apoptotic", "autophagic", and "non-lysosomal vesiculate". These probably reflect a corresponding multiplicity of intracellular events. In particular, the destruction of the cytoplasm in these three types appears to be achieved primarily by heterophagy, by autophagy and by non-lysosomal degradation, respectively. The various mechanisms underlying both nuclear and cytoplasmic destruction are reviewed in detail. The multiplicity of destructive mechanisms needs to be born in mind in studies of other aspects of cell death such as the signals which trigger it, since different signals probably trigger different types of cell death.

Developmental cell death: morphological diversity and multiple mechanisms.

http://biomechanics.stanford.edu/me338/me338_project04.pdf

Stress-dependent finite growth in soft elastic tissues

Left ventricular non-compaction - Insights from cardiovascular magnetic resonance imaging

Compound null mutations of retinoic acid receptor (RAR) genes lead to lethality in utero or shortly after birth and to numerous developmental abnormalities. In the accompanying paper (Lohnes, D., Mark., M., Mendelsohn, C., Dolle, P., Dierich, A., Gorry, Ph., Gansmuller, A. and Chambon, P. (1994). Development 120, 2723-2748), we describe malformations of the head, vertebrae and limbs which, with the notable exception of the eye defects, were not observed in the offspring of vitamin A-deficient (VAD) dams. We report here abnormalities in the neck, trunk and abdominal regions of RAR double mutant mice, which include: (i) the entire respiratory tract, (ii) the heart, its outlow tract and the great vessels located near the heart, (iii) the thymus, thyroid and parathyroid glands, (iv) the diaphragm, (v) the genito-urinary system, and (vi) the lower digestive tract. A majority of these abnormalities recapitulate those observed in the fetal VAD syndrome described by Joseph Warkany's group more than fourty years ago [Wilson, J. G., Roth, C. B. and Warkany, J. (1953) Am. J. Anat., 92, 189-217; and refs therein]. Our results clearly demonstrate that RARs are essential for vertebrate ontogenesis and therefore that retinoic acid is the active retinoid, which is required at several stages of the development of numerous tissues and organs. We discuss several possibilities that may account for the apparent functional redundancy observed amongst retinoic acid receptors during embryogenesis.

Function of the retinoic acid receptors (RARs) during development (II). Multiple abnormalities at various stages of organogenesis in RAR double mutants

The heart in higher vertebrates develops from a simple tube into a complex organ with four chambers specialized for efficient pumping at pressure. During this period, there is a concomitant change in the level of myocardial organization. One important event is the emergence of trabeculations in the luminal layers of the ventricles, a feature which enables the myocardium to increase its mass in the absence of any discrete coronary circulation. In subsequent development, this trabecular layer becomes solidified in its deeper part, thus increasing the compact component of the ventricular myocardium. The remaining layer adjacent to the ventricular lumen retains its trabeculations, with patterns which are both ventricle- and species-specific. During ontogenesis, the compact layer is initially only a few cells thick, but gradually develops a multilayered spiral architecture. A similar process can be charted in the atrial myocardium, where the luminal trabeculations become the pectinate muscles. Their extent then provides the best guide for distinguishing intrinsically the morphologically right from the left atrium. We review the variations of these processes during the development of the human heart and hearts from commonly used laboratory species (chick, mouse, and rat). Comparison with hearts from lower vertebrates is also provided. Despite some variations, such as the final pattern of papillary or pectinate muscles, the hearts observe the same biomechanical rules, and thus share many common points. The functional importance of myocardial organization is demonstrated by lethality of mouse mutants with perturbed myocardial architecture. We conclude that experimental studies uncovering the rules of myocardial assembly are relevant for the full understanding of development of the human heart.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/%28SICI%291097-0185%2820000401%29258%3A4%3C319%3A%3AAID-AR1%3E3.0.CO%3B2-O

Developmental patterning of the myocardium.

Adult myocardium adapts to changing functional demands by hyper- or hypotrophy while the developing heart reacts by hyper- or hypoplasia How embryonic myocardial architecture adjusts to experimentally altered loading is not known We subjected the chick embryonic hearts to mechanically altered loading to study its influence upon ventricular myoarchitecture



Chick embryonic hearts were subjected to conotruncal banding (increased afterload model), or left atrial ligation or clipping, creating a combined model of increased preload in right ventricle and decreased preload in left ventricle Modifications of myocardial architecture were studied by scanning electron microscopy and histology with morphometry



In the conotruncal banded group, there was a mild to moderate ventricular dilatation, thickening of the compact myocardium and trabeculae, and spiraling of trabecular course in the left ventricle Right atrioventricular valve morphology was altered from normal muscular flap towards a bicuspid structure Left atrial ligation or clipping resulted in hypoplasia of the left heart structures with compensatory overdevelopment on the right side Hypoplastic left ventricle had decreased myocardial volume and showed accelerated trabecular compaction Increased volume load in the right ventricle was compensated primarily by chamber dilatation with altered trabecular pattern, and by trabecular proliferation and thickening of the compact myocardium at the later stages A ventricular septal defect was noted in all conotruncal banded, and 25% of left atrial ligated hearts



Increasing pressure load is a main stimulus for embryonic myocardial growth, while increased volume load is compensated primarily by dilatation Adequate loading is important for normal cardiac morphogenesis and the development of typical myocardial patterns Anat Rec 254:238–252, 1999 © 1999 Wiley-Liss, Inc

https://anatomypubs.onlinelibrary.wiley.com/doi/pdf/10.1002/%28SICI%291097-0185%2819990201%29254%3A2%3C238%3A%3AAID-AR10%3E3.0.CO%3B2-V

Remodeling of chick embryonic ventricular myoarchitecture under experimentally changed loading conditions

Background: Numerous studies describing myocardial ar- chitecture have been performed on the adult heart but considerably fewer have been made during embryonic or fetal development. To serve as a basis for interspecies comparison of ventricular morphology, and as a referenceforstudyingtheeffectsofexperimentalperturbations,weexamined the development of chick throughout the entire incubation period. Methods: Chick hearts from stage 14 (day 2) to stage 46 (day 21) were perfusion-fixed, and sectioned in transverse, frontal and sagittal planes. The ventricular myocardial architecture was examined and photo- graphed in the scanning electron microscope. Results:At embryonic stage 16 and earlier, the smooth-walled heart loop had an outer myocardial mantle, cardiac jelly, and endocardium. From stage 18, there was an outer compact and inner trabeculated myocardium. Trabeculated myocardium could be subdivided into the outer (basal) portion adjacent to the compact layer and the central (luminal) part. The outer basal layer could be distinguished from the inner luminal by shorter andfiner trabeculae with small, round intertrabecular spaces. From stage 24, the patterns of trabeculae and intertrabecular spaces were ventricle- specific. Between stages 24 to 31, abundant trabeculations were present throughout both ventricular cavities. The trabeculae were initially radi- ally arranged, but later adopted a spiral course, which persisted in a simplified form into adulthood. Conclusions:Theventricularmyocardiumundergoesdistinctivemorpho- genesis, characterized by changes in trabecular patterning and orienta- tion. We speculate that the embryonic trabecular architecture reflects the directions of the main stresses. Unlike fetal and adult hearts, which rely mostly on the compact myocardial layer, the trabeculae play a crucial role in the contractile function of the embryonic heart. Anat. Rec. 248:421-432,

Developmental Changes in the Myocardial Architecture of the Chick

Normal development of the mouse embryonic heart was studied at the organ level using microdissection and scanning electron microscopy (SEM). Altogether 225 embryos, sampled at 8-hour intervals between 11ed (ed = embryonic day; day of vaginal plug = 1ed) and 15ed were collected. Their hearts were fixed by high flow-low pressure perfusion, microdissected, and observed in SEM. Standardized frontal, right profile, and left profile SEM micrographs were obtained and analyzed. The main purpose of this study was to create a series of normal stages of mouse cardiac development as a reference for ongoing studies in experimental cardiac teratology (e.g., in fetal mouse trisomies). Comparisons with chick, human, and dog embryonic hearts, prepared using the same technique, show that the mouse embryonic heart is characterized by a relatively deep interventricular sulcus. The absence of a conoventricular sulcus in the mouse results in poor definition of the boundary between the conus and the right ventricle. The external separation of the aorta and the pulmonary artery is evident from 13ed onward. The respective positions of the great arteries (aorta dextroposterior, pulmonary artery sinistroanterior) does not change until the end of cardiac organogenesis (15ed in the mouse).

Normal stages of cardiac organogenesis in the mouse: I. Development of the external shape of the heart

Residual stress and strain, i.e., the stress and strain remaining in a solid when all external loads are removed, may be produced in biological tissues by differential growth. During cardiac development, residual stress and strain may play a role in cardiac morphogenesis by affecting ventricular wall stress. After a transmural radial cut, a passive ventricular cross section opens into a sector, and the size of the opening angle provides a measure of the circumferential residual strain. Residual strains were characterized in this manner for the apical region of the diastolic embryonic chick heart for Hamburger-Hamilton stages 16, 18, 21, and 24 (approximately 2.5, 3.5, 4.0, and 4.5 days, respectively, of a 21-day incubation period). The average opening angle at these stages was 107 +/- 10 degrees, 79 +/- 10 degrees, 73 +/- 11 degrees, and 74 +/- 7 degrees, respectively (n > or = 5 for each stage). These measured angles were correlated with changes in ventricular morphology. Scanning electron micrographs of the apex revealed that the wall of the ventricle is smooth at stage 16. Then at stage 18, myocardial trabeculae develop, forming ridges with primarily a circumferential orientation. By stage 21, the trabeculae develop into a mesh, giving the ventricular wall a spongelike appearance, and the preferred orientation is lost by stage 24. The large decrease in opening angle between stages 16 and 18 corresponded to the onset of trabeculation, which is the greatest change in form during the studied stages. We speculate that residual strain is an important biomechanical factor during cardiac morphogenesis.

Tomas Pexieder

Papers

Developmental patterning of the myocardium.

Remodeling of chick embryonic ventricular myoarchitecture under experimentally changed loading conditions

Developmental Changes in the Myocardial Architecture of the Chick

Normal stages of cardiac organogenesis in the mouse: I. Development of the external shape of the heart

Residual strain in the ventricle of the stage 16-24 chick embryo.