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Functional septation of the atrial chambers as well as the pulmonary and systemic circulatory systems.

Most of the human developmental timing information is from ref. 8, except for the human staging of the secondary heart field and proepicardium, which was correlated from other model systems (2-4,14).

Most of the human developmental timing information is from ref. 8, except for the human staging of the secondary heart field and proepicardium, which was correlated from other model systems (2-4,14).

Presumptive endocardial cells delaminate from the splanch-nopleuric mesoderm and coalesce via vasculogenesis to form two lateral endocardial tubes (Fig. 2A) (13).

During the third week of human development, two bilateral layers of myocardium surrounding the endocardial tubes are brought into the ventral midline during closure of the ventral foregut via cephalic and lateral folding of the embryo (Fig. 2A) (8). The lateral borders of the myocardial mesoderm layers are the first heart structures to fuse, followed by the fusion of the two endocardial tubes to form one endocardial tube surrounded by splanchnopleuric-derived myocardium (Fig. 2B,C). The medial borders of the myocardial mesoderm layers are the last to fuse (14). Thus, the early heart is continuous with noncardiac splanchnopleuric mesoderm across the dorsal mesocardium (Fig. 2C). This will eventually partially break down to form the ventral aspect of the linear heart tube with a posterior inflow (venous pole) and anterior outflow (arterial pole), as well as the dorsal wall of the pericardial cavity (5,14). During the fusion of the endocardial tubes, the myocardium secretes an acellular matrix, forming the cardiac jelly layer that separates the myocardium and endocardium.

By day 22 of human development, the linear heart tube begins to beat. As the human heart begins to fold and loop from days 22-28 (described in the next section), epicardial cells will invest the outer layer of the heart tube (Fig. 1B and Fig. 3A), resulting in a heart tube with four primary layers: endocardium, cardiac jelly, myocardium, and epicardium (Fig. 3B) (8).

3. SECONDARY HEART FIELD, OUTFLOW TRACT Fig. 2. Cross-sectional view of human heart tube fusion. (A) Day 20, FORMATION, AND CARDIAC LOOPING cephalocaudal and lateral folding brings bilateral endocardial tubes into the ventral midline of the embryo. (B) Day 21, start of heart tube A cascade of signals identifying the left and right sides of the fusion. (C) Day 22, complete fusion, resulting in the beating primitive embryo are thought to initiate the process of primary linear heart tube. Ectoderm, dark gray; mesoderm, gray; endoderm, white.

Jelly Fish Endoderm Mesoderm EcdodermCephalocaudal Lateral Folding

Fig. 3. Origin and migration of proepicardial cells. (A) Whole mount view of the looping human heart within the pericardial cavity at day 28. Proepicardial cells (dark gray dots, mesoderm origin) emigrate from the sinus venosus and possibly the septum transversum and then migrate out over the outer surface of the ventricles, eventually surrounding the entire heart. (B) Cross-sectional view of the looping heart showing the four layers of the heart: epicardium, myocardium, cardiac jelly, and endocardium. LV, left ventricle; RV, right ventricle.

Fig. 3. Origin and migration of proepicardial cells. (A) Whole mount view of the looping human heart within the pericardial cavity at day 28. Proepicardial cells (dark gray dots, mesoderm origin) emigrate from the sinus venosus and possibly the septum transversum and then migrate out over the outer surface of the ventricles, eventually surrounding the entire heart. (B) Cross-sectional view of the looping heart showing the four layers of the heart: epicardium, myocardium, cardiac jelly, and endocardium. LV, left ventricle; RV, right ventricle.

Inflow Out

Linear heart Looping & Septalion lube Accretion

Day 22 Day 28 9 weeks

Linear heart Looping & Septalion lube Accretion

Day 22 Day 28 9 weeks

Fig. 4. Looping and septation of the human primary linear heart tube. Dark gray and white regions represent tissue added during the looping process from the primary and secondary heart field, respectively. Ao, aorta; AV, atrioventricular region; LA, left atrium; LV, left ventricle; P, pulmonary artery; RA, right atrium; RV, right ventricle.

heart tube looping (15). The primary heart tube loops to the right of the embryo and bends, to allow convergence of the inflow (venous) and outflow (arterial) ends, from days 22-28 of human development (Fig. 4). This process occurs prior to the division of the heart tube into four chambers and is required for proper alignment and septation of the mature cardiac chambers.

During the looping process, the primary heart tube increases dramatically in length (four- to fivefold), and this process displaces atrial myocardium posteriorly and superiorly dorsal to the forming ventricular chambers (5,8,15). During the looping process, the inflow (venous) pole, atria, and atrioventricular region are added to, or accreted from, the posterior region of the paired primary heart fields; the myocardium of proximal out flow tract (conus) and distal outflow tract (truncus) are added to the arterial pole from the secondary heart field (2-4).

The secondary heart field (Fig. 1B and Fig. 2C) is located along the splanchnopleuric mesoderm (beneath the floor of the foregut) at the attachment site of the dorsal mesocardium (24,14). During looping, the secondary heart field cells undergo epithelial-to-myocardial transformation at the outflow (arterial) pole and add additional myocardial cells onto the developing outflow tract. This lengthening of the primary heart tube appears to be an important process for the proper alignment of the inflow and outflow tracts prior to septation. If this process does not occur normally, ventricular septal defects and malpositioning of the aorta may occur (14).

Thus, by day 28 of human development, the chambers of the heart are in position and are demarcated by visible constrictions and expansions that denote the sinus venosus, common atrial chamber, atrioventricular sulcus, ventricular chamber, and conotruncus (proximal and distal outflow tract) (Fig. 4) (8,14).

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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