On the Biomechanics of Cardiac S-Looping in the Chick: Insights From Modeling and Perturbation Studies

Cardiac looping is an important embryonic developmental stage where the primitive heart tube (HT) twists into a configuration that more closely resembles the mature heart. Improper looping leads to congenital defects. Using the chick embryo as the experimental model, we study cardiac s-looping wherein the primitive ventricle, which lay superior to the atrium, now assumes its definitive position inferior to it. This process results in a heart loop that is no longer planar with the inflow and outflow tracts now lying in adjacent planes. We investigate the biomechanics of s-looping and use modeling to understand the nonlinear and time-variant morphogenetic shape changes. We developed physical and finite element models and validated the models using perturbation studies. The results from experiments and models show how force actuators such as bending of the embryonic dorsal wall (cervical flexure), rotation around the body axis (embryo torsion), and HT growth interact to produce the heart loop. Using model-based and experimental data, we present an improved hypothesis for early cardiac s-looping.

Embriologia e genetica dello sviluppo cerebellare

During the fourth week of development, the mesencephalic flexure and cervical flexure appear in the cranial region of the neural tube, delimiting three neural vescicles: the prosencephalon, mesencephalon and rhomboencephalon. During the fifth week, the pontine flexure forms in the roof of the rhomboencephalon, marking the division between the metencephalon and myeloencephalon, the future medulla oblongata. The dorsal part of the metencephalon (alar plate), between the mesencaphalic isthmus and the pontine flexure, will give rise to the cerebellum, whereas its ventral part (basal plate) will give rise to the pons. The alar plates, lateral to the deepening pontine fissure and the hindbrain cavity, present an intense proliferative activity with the formation of two extensions which fuse just behind the isthmus to form the cerebellar plate. During the sixth week of development, the rhombic lips appear in the posterolateral regions of this strip and after persistent proliferative activity they fuse along the median line. Median thickening of this region will give rise to the vermis, while the two lateral masses become the lateral lobes. During the seventh week, the flocculonodular lobe will form. Between the eighth and ninth weeks, the vermis expands caudally with progressive dilatation of the fourth ventricle whose Luschka and Magendie foramina are still unperforated. The cerebellar proliferation and differentiation processes are correlated to the activity of two separate germinal areas: the ependymal ventricle and the rhombic lip. From the eighth week, neuroblastic migration starts from the rhombic lip which in successive genetically controlled steps leads to the formation of the external granular layer of the cerebellar cortex. From the ventriculo-ependymal zone cells migrate to form the cerebellar nuclei destined to differentiate into Purkinje cells. The cerebellar recess starts to develop around the twelfth week with the formation of the primary fissure separating the anterior and posterior parts of the cerebellum to end with the complete formation of all cerebellar lobes at around the twenty-fourth week of development. The foliation process starts when the recess has formed and continues in the first months of life after birth. The processes of cell development and differentiation are genetically programmed and the outcome of an interaction between genetic and environmental factors. The different stages of embryonic development are thought to be controlled by sequential ordered activation of genetic clusters (homeotypic genes) which in turn encode for a series of molecules directly involved in the regulation of cell and tissue interactions. There is increasing evidence of genetic involvement in the different types of cerebellar malformation whose expression and association with other extracerebellar malformations depends on the developmental age at which the genetic change occurred.

Interruption of cardiac output does not affect short-term growth and metabolic rate in day 3 and 4 chick embryos

The heart beat of vertebrate embryos has been assumed to begin when convective bulk transport by blood takes over from transport by simple diffusion. To test this hypothesis, we measured eye growth, cervical flexure and rates of oxygen consumption (V(O2)) in day 3–4 chick embryos denied cardiac output by ligation of the outflow tract and compared them with those of embryos with an intact cardiovascular system.Eye diameter, used as the index for embryonic growth, increased at a rate of approximately 4.5-5 % h(−)(1) during the observation period. There was no significant difference (P>0.1) in the rate of increase in eye diameter between control (egg opened), sham-ligated (ligature present but not tied) and ligated embryos. Similarly, the normal progression of cervical flexure was not significantly altered by ligation (P>0.1). V(O2) (ml O(2)g(−)(1)h(−)(1)) at 38 degrees C, measured by closed respirometry, was not significantly different (P>0.1) on day 3 in sham-ligated (14.5+/−1.9 ml O(2)g(−)(1)h(−)(1)) and ligated 17.6+/−1.8 ml O(2)g(−)(1)h(−)(1)) embryos. Similarly, on day 4, V(O2) in sham-ligated and ligated embryos was statistically the same (sham-ligated 10. 5+/−2.9 ml O(2)g(−)(1)h(−)(1); ligated 9.7+/−2.9 ml O(2)g(−)(1)h(−)(1)). Expressed as a linear function of body mass (M), V(O2) in sham-ligated embryos was described by the equation V(O2)=−0.48M+24.06 (r(2)=0.36, N=18, P<0.01), while V(O2) in ligated embryos was described by the equation V(O2)=−0.53M+23.32 (r(2)=0.38, N=16, P<0.01). The regression line describing the relationship between body mass and V(O2) for pooled sham-ligated and ligated embryos (the two populations being statistically identical) was V(O2)=−0.47M+23.24. The slope of this regression line, which was significantly different from zero (r(2)=0.30, N=34, P<0.01), was similar to slopes calculated from previous studies over the same range of body mass.Collectively, these data indicate that growth and V(O2) are not dependent upon cardiac output and the convective blood flow it generates. Thus, early chick embryos join those of the zebrafish, clawed frog and axolotl in developing a heart beat and blood flow hours or days before required for convective oxygen and nutrient transport. We speculate that angiogenesis is the most likely role for the early development of a heart beat in vertebrate embryos.