Development Of The Interventricular Septum

Type 2 Diabetes Defeated

The Best Ways to Treat Diabetes

Get Instant Access

Lateral cushion

Superior endocardial cushion

Inferior endocardial cushion

Right atrioventricular canal

Left atrioventricular canal

Right Dominant Canal

Figure 11.16 Formation of the septum in the atrioventricular canal. A. From left to right, days 23, 26, 31, and 35. The initial circular opening widens transversely. B and C. Scanning electron micrographs of hearts from mouse embryos, showing growth and fusion of the superior and inferior endocardial cushions in the atrioventricular canal. In C, cushions of the ouflow tract (arrow) are also fusing.

Figure 11.16 Formation of the septum in the atrioventricular canal. A. From left to right, days 23, 26, 31, and 35. The initial circular opening widens transversely. B and C. Scanning electron micrographs of hearts from mouse embryos, showing growth and fusion of the superior and inferior endocardial cushions in the atrioventricular canal. In C, cushions of the ouflow tract (arrow) are also fusing.

Pulmonary channel

Aortic sac

Right superior truncus swelling IV

Aortic arches h-III

Aortic sac

Right superior truncus swelling IV

Aortic arches h-III

Aortic channel

Right dorsal conus swelling-.

Bulboventricular flange

Conus Swellings And Truncus Swellings

-- Left inferior truncus swelling

" Left ventral conus swelling

Left lateral cushion

Interventricular septum

Aortic channel

Right dorsal conus swelling-.

Bulboventricular flange

Right lateral cushion

Interventricular septum

-- Left inferior truncus swelling

" Left ventral conus swelling

Left lateral cushion

Inferior endocardial cushion

Endocardial Cushion Defect

Figure 11.17 A. Frontal section through the heart of a day 35 embryo. At this stage of development blood from the atrial cavity enters the primitive left ventricle as well as the primitive right ventricle. Note development of the cushions in the atrioventricular canal. Cushions in the truncus and conus are also visible. Ring, primitive interventricular foramen. Arrows, blood flow. B. Scanning electron micrograph of a mouse embryo at a slightly later stage showing fusion of the atrioventricular cushions and contact between those in the outflow tract.

Figure 11.17 A. Frontal section through the heart of a day 35 embryo. At this stage of development blood from the atrial cavity enters the primitive left ventricle as well as the primitive right ventricle. Note development of the cushions in the atrioventricular canal. Cushions in the truncus and conus are also visible. Ring, primitive interventricular foramen. Arrows, blood flow. B. Scanning electron micrograph of a mouse embryo at a slightly later stage showing fusion of the atrioventricular cushions and contact between those in the outflow tract.

Depît mew-ttliyn^ai HBtarf

Formation Atrioventricular Valves

Figure 11.18 Formation of the atrioventricular valves and chordae tendineae. The valves are hollowed out from the ventricular side but remain attached to the ventricular wall by the chordae tendineae.

Depît mew-ttliyn^ai HBtarf

^IflOviflir^vlftr

CncdaB if nM^ai

Figure 11.18 Formation of the atrioventricular valves and chordae tendineae. The valves are hollowed out from the ventricular side but remain attached to the ventricular wall by the chordae tendineae.

Atrioventricular Valves

After the atrioventricular endocardial cushions fuse, each atrioventricular orifice is surrounded by local proliferations of mesenchymal tissue (Fig. 11.18 A). When the bloodstream hollows out and thins tissue on the ventricular surface of these proliferations, valves form and remain attached to the ventricular wall by muscular cords (Fig. 11.18B). Finally, muscular tissue in the cords degenerates and is replaced by dense connective tissue. The valves then consist of connective tissue covered by endocardium. They are connected to thick trabeculae in the wall of the ventricle, the papillary muscles, by means of chordae tendineae (Fig.11.18C). In this manner two valve leaflets, constituting the bicuspid, or mitral, valve, form in the left atrioventricular canal, and three, constituting the tricuspid valve, form on the right side.

CLINICAL CORRELATES Heart Defects

Heart and vascular abnormalities make up the largest category of human birth defects, accounting for 1% of malformations among live-born infants. The incidence among stillborns is 10 times as high. It is estimated that 8% of cardiac malformations are due to genetic factors, 2% are due to environmental agents, and most are due to a complex interplay between genetic and environmental influences (multifactorial causes). Classic examples of car-diovacular teratogens include rubella virus and thalidomide. Others include isotretinoin (vitamin A), alcohol, and many other compounds. Maternal diseases, such as insulin-dependent diabetes and hypertension, have also been linked to cardiac defects. Chromosomal abnormalities are associated with heart malformations, with 6 to 10% of newborns with cardiac defects having an unbalanced chromosomal abnormality. Furthermore, 33% of children with chromosomal abnormalities have a congenital heart defect, with an incidence of nearly 100% in children with trisomy 18. Finally, cardiac malformations are associated with a number of genetic syndromes, including cran-iofacial abnormalities, such as DiGeorge, Goldenhar, and Down syndromes (see Chapter 15).

Genes regulating cardiac development are being identified and mapped and mutations that result in heart defects are being discovered. For example, mutations in the heart-specifying gene NKX2.5, on chromosome 5q35, produce atrial septal defects (secundum type) and atrioventricular conduction delays in an autosomal dominant fashion. Mutations in the TBX5gene result in Holt-Oram syndrome, characterized by preaxial (radial) limb abnormalities and atrial septal defects. Defects in the muscular portion of the interven-tricular septum may also occur. Holt-Oram syndrome is one of a group of heart-hand syndromes illustrating that the same genes may participate in multiple developmental processes. For example, TBX5 is expressed in distal segments of the limb bud and in the heart primordia. Holt-Oram syndrome is inherited as an autosomal dominant trait with a frequency of 1/100,000 live births.

Atrial septal defect (ASD) is a congenital heart abnormality with an incidence of 6.4/10,000 births and with a 2:1 prevalence in female versus male infants. One of the most significant defects is the ostium secundum defect, characterized by a large opening between the left and right atria. It is caused either by excessive cell death and resorption of the septum primum (Fig. 11.19, B and C) or by inadequate development of the septum secundum (Fig. 11.19, D and E). Depending on the size of the opening, considerable intracardiac shunting may occur from left to right.

The most serious abnormality in this group is complete absence of the atrial septum (Fig. 11.19F). This condition, known as common atrium or cor triloculare biventriculare, is always associated with serious defects elsewhere in the heart.

Occasionally, the oval foramen closes during prenatal life. This abnormality, premature closure of the oval foramen, leads to massive hypertrophy of the right atrium and ventricle and underdevelopment of the left side of the heart. Death usually occurs shortly after birth.

Endocardial cushions of the atrioventricular canal not only divide this canal into a right and left orifice, but also participate in formation of the membranous portion of the interventricular septum and in closure of the ostium primum. This region has the appearance of a cross, with the atrial and ventricular septa forming the post and the atrioventricular cushions the crossbar. The integrity of this cross is an important sign in ultrasound scans of the heart (Fig. 11.29C). Whenever the cushions fail to fuse, the result is a persistent atrioventricular canal, combined with a defect in the cardiac septum (Fig.11.20A). This septal defect has an atrial and a ventricular

Septum secundum

Was this article helpful?

0 0
Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

Get My Free Ebook


Responses

  • laura
    How does the interventricular septum develop?
    11 months ago

Post a comment