University:
Kentucky State University
Course:
BIO 111 | Principles of Biology
Academic year:

2021
Views:

226

Pages:

16
Author:

thyristoreqt

Chapter 15 lateral plate mesoderm and endoderm Lateral plate mesoderm • Coelom: the space between the somatic mesoderm and splanchnic mesoderm that becomes the body cavity. In mammals, the coelom becomes subdivided into the pleural, pericardial and peritoneal cavities, enveloping the thorax, heart, and abdomen, respectively. The heart • The heart arises from the two regions of splanchnic mesoderm on each side of the body. The splanchnic mesoderm interact with adjacent tissue to become specified for heart development. Specification of the heart tissue • The presumptive heart cells (progenitor heart cells) are located in two small patches, one on each side of the epiblast, close to the rostal portion of the primitive streak. • These cells migrate together through the primitive streak and form two groups of lateral plate mesoderm cells, positioned anteriorly at the level of the node. (Cardiac field) Specification of the heart tissue • The cardiac fields on both sides of the embryo divide into the first and the second cardiac fields. • On embryonic day 7.5 (mouse), the heart fields from each side of the body join at the midline into a common cardiac crescent that contains the first and second heart fields. • The progenitor cells of the first cardiac field form the primary heart tube. • These cells have limited proliferative ability and thus generate only the major portion of the left ventricle of the adult heart. Specification of the heart tissue • On the posterior end, the progenitor cells of the second heart field produce the two atria and contribute to the formation of the inlet part of the heart. • On the anterior end, these cells generate the right ventricle and the outflow tract. • The outflow region is composed of conus arteriosus and the truncus arteriosus, which becomes the base of the aorta and pulmonary arteries Specification of the cardiac precursor cells • The cardiac mesoderm is induced by the adjacent anterior endoderm. • The heart does not form if the anterior endoderm is removed, and the posterior endoderm can not induce the heart cells to form. (Anterior endoderm) (BMP2) Specification of the cardiac precursor cells • The inhibitory signal prevent heart formation where it should not occur. • Thus, the cardiac precursor cells are specified where BMPs (lateral mesoderm and endoderm) and Wnt antagonist (anterior endoderm) coincide. (Anterior endoderm) (Wnt3 and Wnt8) Migration of the cardiac precursor cells • In chick, the directionality of presumptive heart cells migration appears to be provided by the foregut endoderm. (Presumptive heart cells) • The ventral splanchnic mesoderm cells of the heart tubes begin to express N-cadherin on their apices, sort out from the somatic (parietal) mesoderm cells and join together to form an epithelial layer. • The joining of the splanchnic mesoderm cells leads to the formation of the pericardial cavity: the sac in which the heart is formed. • The epithelial layer of splanchnic mesoderm forms the myocardium which will give rise to the cardiac muscles. • Endocardium: the lining of the heart that is continuous with the blood vessels. Migration of the cardiac precursor cells • Cardia bifida: A condition in which two separate hearts form, resulting from manipulation of the embryo or genetic defects that prevent fusion of the two heart primordia. Surgically cutting the ventral midline Mutation in Foxp4 gene Looping and formation of heart chambers Sinus venosus Vitelline vein Umbilical vein Vitelline veins Common cardinal vein AS Pulmonary arteries AS Prospective left atrium Prospective right atrium Atria A Prospective right ventricle Prospective left ventricle Left ventricle Right ventricle Looping and formation of heart chambers Vitelline veins Lindsey et al (2014), adapted from Srivastava and Oslon (2000) Looping and formation of heart chambers • Looping of the heart converts the original anterior-posterior polarity of the heart tube into the right- left polarity seen in the adult heart. • When the looping is completed, the portion of the heart tube that is destined to become the atria lies anterior to the portion of the tube that is destined to become the ventricles (the direction is reversed). Pulmonary arteries Prospective left atrium Prospective right atrium Atria A Prospective right ventricle Prospective left ventricle Left ventricle Right ventricle Looping and formation of heart chambers • Looping begins after the onset of rhythmic heart contractions and the initiation of blood flow. • Precise alignment of heart chambers is needed for the correct signaling required for the formation of the heart valves, the ventricular and atrial septa, and to allow the connection between the heart and embryonic vasculature. Pulmonary arteries Prospective left atrium Prospective right atrium Atria A Prospective right ventricle Prospective left ventricle Left ventricle Right ventricle Looping and formation of heart chambers How the endocardial cushion development is initiated? • The cushion development is initiated by signaling from the myocardium to endocardial cells to express the twist gene. • Twist protein is a transcription factor that initiate epithelial to mesenchymal transformation of the endocardial cells and lead to the migration of the transformed cells to form endocardial cushions. • Twist also activates Tbx20 gene which encodes for a transcription factor. • Together Twist and Tbx20 transcription factors activate the genes that encode for proteins required for the proliferation and strengthen of the valves. Endocardial cushions Looping and formation of heart chambers • The adult heart has two atrioventricular valves and two semilunar valves. (Tricuspid valve) (Mitral valve)

Chapter 15 Lateral Plate Mesoderm and Endoderm

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