The Mouse Embryonic Imaging Guide

I. Introduction

The mouse has always been considered a good embryological model of mammalian development

because it has good-sized litters of offspring (about 8–20 offspring per litter) and a short pregnancy (about

21 days). The introduction of transgenic and gene target methods has increased the use of mice in the study

of human diseases in recent years. Alteration or misexpression of genes induces phenotypic changes at early

embryonic stages in mouse fetal development and in many diseases [1]. The heart is the first organ to

develop and function during the development of the mouse embryo, and many mouse models with

congenital heart defects suffer early embryonic death. Therefore, assessment of the structure and function of

the fetal mouse cardiac system has become an increasingly important and popular issue. In this guide, we

will provide an introduction to mouse fetal heart development. Of critical importance in such studies is the

availability of tools that can achieve real-time evaluation and an approximately tenfold improvement of

resolution compared to clinical ultrasound machine in order to monitor in utero progress during mouse

pregnancy. A high-frequency (40–50 MHz) and high-resolution (~30 μm) ultrasound imaging system, called

the Prospect system, which is a useful tool for visualizing live mouse embryos in utero, was used in this

guide.

Before describing the scanning process, first consider the embryonic mouse cardiovascular system and

its circulation. The cardiovascular system in mammalian embryos is composed of complicated vascular

networks, both within the actual embryo and in the extraembryonic tissues, which aid its development. The

embryonic mouse cardiovascular system has three main components: the embryonic, yolk sac, and allantoic

or umbilical circulatory systems. The embryonic circulatory system is made up of the heart and the arteries

and veins; the main branches of the latter two are the aorta and the cardinal vein. The yolk sac circulatory

system comprises the vitellin arteries and veins, and the main branches of the allantoic or umbilical

circulation are the allantoic artery and vein. The proper development and coordination of all three

circulatory systems is crucial to maintaining normal cardiovascular function and ensuring embryonic

survival. Defects in any one of these systems in utero are normally fatal [2].

Rhythmic cardiac contractions can be detected as early as the 5 somite stage [~embryonic day (E) 8.25],

and blood flow is detectable by Doppler ultrasound by the 7–8 somite stages. At these stages, the mouse

heart approximates a straight tube. Notably, the onset of cardiac activity coincides precisely with entry of

primitive erythrocytes from the hematopoietic yolk sac into the embryo proper at these early stages. These

findings indicate that the establishment and early development of the initial circulation are precisely

coordinated to form a functioning pump, an intact vascular circuit, and oxygen-carrying erythrocytes to

support the growing embryo. The earliest origins of a functional cardiovascular system occur well before the

embryo becomes dependent upon the convection of oxygen and nutrients, but are probably necessary for

cardiac morphogenesis and vasculogenesis [2] [3].

The mouse embryo circulatory system gradually matures during the late embryonic stages. In brief,

maternal blood enriched with oxygen and nutrients flows from the umbilical vein into the inferior vena cava

via the ductus venosus, where it mixes with deoxygenized blood. It then enters the heart via the right atrium,

where it mixes with blood from the right superior vena cava. In the right atrium, the blood flow divides into

two paths. In one of the paths, the largest portion of the largely oxygenated blood flows through the inferior

vena cava into the left atrium via a special fetal opening between the left and right atria, called the foramen

ovale. From the left atrium, it mixes with a small amount of blood returning from the undeveloped lungs

through the pulmonary veins, moves into the left ventricle via the mitral valve, and is then pumped into the

aorta. The ascending aorta provides blood to the head and upper extremities, and blood travels from the

descending aorta to the other areas of the body, then through smaller vessels to return to the placenta via the

umbilical arteries. Returning blood from the head and upper extremities travels through the right superior

vena cava, where CO2-rich and nutrient-poor blood flows, and is pumped back into the right atrium. The

second path comprises about one-third of the blood; the deoxygenated blood that enters the rostral through

the right superior vena cava descends into the right ventricle via the tricuspid valve and is pumped through

the right-ventricle outflow tract. Most of this blood is diverted through the ductus arteriosus and into the

aorta, and a small amount continues on through the pulmonary arteries to the lungs [4]. A schematic diagram

of the circulatory system in the late mouse embryonic stages is shown in Figure 1.

Fig. 1. Schematic diagram of the circulatory system in the late mouse embryonic stages. SVC, superior vena

cava; IVC, inferior vena cava; RA, right atrium; LA, left atrium; RV, right ventricle; LV, left ventricle; RVOT,

right ventricular outflow tract; LVOT, left ventricular outflow tract.

Foramen

Ovale

Aortic Arch

Ductus

Arteriosus

SVC

Ductus

Venosus

Umbilical Arteries

Umbilical Vein

Liver

Placenta

Left

Lung

Portal

Vein

The embryonic circulation network differs in many ways from that of adults in both humans and mice.

The existence of the foramen ovale and the ductus arteriosus allows the circulatory system to run in parallel

rather than in series. Most of the blood being pumped from the right side of the heart to the left thus

bypasses the pulmonary circulation, which has a high resistance to blood flow, and most of the blood being

pumped from the right ventricle passes through the ductus arteriosus and into the thoracic aorta. Only a

small proportion of the blood will pass through the pulmonary arteries and into the lungs, and so the stroke

volume is greater in the right ventricle than in the left ventricle [4].

There is a gradual increase in the embryonic heart rate and systolic cardiac work in order to meet the

metabolic demands of the growing embryo. However, the continued increase in the mouse heart rate

postpartum is an important part of its developmental maturation, as opposed to the postpartum relative

reduction in heart rate observed in humans. Studies of myocardial diastolic function in mice have extended

from the middle and late stages of pregnancy to the early postnatal and young-adult stages. The E and A

waves (corresponding to the first and second peaks, respectively) across the atrioventricular valves are

characterized by diastolic function, which can be measured by pulsed-wave (PW) Doppler ultrasound;

diastolic function gradually matures from early gestation to the third week of postnatal life, with progressive

improvements in active ventricular filling [3].

II. Animal model

Normal imprinting control region (ICR) timed pregnant dams were used in this guide. The changes in

ICR mouse embryos were monitored on a daily basis, and especially their cardiac geometry and function,

from E6.5 to E18.5, using in utero ultrasound with the Prospect system.

III. Embryos Scanning

The transducer is first placed above the maternal abdomen across the urethral opening to image the

maternal bladder. Using the maternal bladder as a starting reference point, the uterine horns containing

embryos situated on either side of the maternal bladder could be sequentially imaged. It is possible to scan

adjacent embryos in order to track them; however, if there is any doubt, an embryo should be skipped to

avoid double sampling, especially when spontaneous uterine movements are excessive. There are usually six

to ten embryos available for imaging in each litter.

It is common to scan a fetus in the sagittal, frontal, and transverse planes when performing

echocardiography (as shown in Figure 2). The sagittal and frontal planes in the embryo correspond to the

parasternal long- and short-axis views in the adult mouse, respectively. The four-chamber apical view in the

adult corresponds to the transverse plane through the fetal thorax. The sagittal view should be used to

identify the atrioventricular valves. Using this view, the two ventricles and their synchronous contractions

are readily visible. The right and left ventricles, and the interventricular septum could be displayed in the

frontal plane. This is the optimal view for observing the changes in cardiac ventricular dimensions in

M-mode imaging. The embryonic heart and arms can be imaged in the transverse plane. This view

simultaneously reveals the left ventricle, right ventricle, and atrioventricular valves, corresponding to the

four-chamber view in the adult mouse heart [4].

(A)

(B) (C) (D)

Fig. 2. (A) Schematic diagram of the fetus and the three orthogonal imaging planes. (B) Embryonic B-mode

image in the sagittal plane at E13.5. (C) Embryonic B-mode image in the frontal plane at E13.5. (D)

Embryonic B-mode image in the transverse plane at E12.5. S, sagittal plane; F, frontal plane; T, transverse

plane.

B-mode imaging, which is a two-dimensional (2-D) imaging mode, allows observation of the target

structure, positioning, and analysis of the physical parameters. For example, the changes in cardiac

dimension during contraction may be used to gauge cardiac function, while the measurement of vessel size,

in conjunction with Doppler flow waveforms, allows the calculation of volume flow. Use of B-mode

scanning during the entire embryonic developmental period revealed that the size and structure of the

embryonic left and right cardiac ventricles are similar. The volume of the left ventricle becomes significantly

greater than that of the right ventricle after birth. The mouse embryonic heart can be readily identified on

real-time images as a bright, highly echogenic structure with rhythmic contractions in B-mode images

produced at frequencies of 40~50 MHz because the blood pool in the heart is echo-dense at these

frequencies. As such, the blood vessels prominently appear in real time as dramatic speckle patterns

representing blood flow. This effect is not evident on still frames since the moving speckle pattern is lost. On

real-time video images, the umbilical vessels, vitelline vessels, dorsal aorta, great veins, and cerebral vessels

can be all easily identified, and allow PW Doppler to yield clear blood velocity waveforms.

Doppler ultrasound is a noninvasive, in utero approach for measuring cardiovascular function. With

the Prospect system, the color/power Doppler mode provides a 2-D color velocity map superimposed on the

B-mode image, which facilitates the positioning of the targets of interest. PW Doppler mode, on the other

hand, produces the spectrum (i.e., the velocity distribution) as a function of time. Employing the triplex

function in PW Doppler mode (i.e., B-mode, color Doppler, and spectral Doppler images displayed

simultaneously in the same window) makes it possible to locate the targets and conduct quantitative flow

measurements. The analysis of cardiovascular function in mouse embryos has been further extended by

including 40- to 50-MHz pulsed Doppler, which enables precise measurements of blood velocity waveforms.

The higher-frequency Doppler transducers are also conducive to the acquisition of low-velocity blood flow

signals. The PW Doppler beam should be aligned with the blood flow direction as accurately as possible so

as to minimize anomalies in subsequent calculations of flow velocity, since the cosine of the angle between

the flow direction and the Doppler beam can affect the calculations of the Doppler frequency shift.

Furthermore, the Doppler shift frequency is proportional to the flow velocity, and so directly affects the

estimation of flow velocity. It should be noted that blood is echo-dense, and the endocardial lumen cannot be

visualized in the early to midgestation mouse embryo by using high center frequencies of 40~50 MHz [2].

That color Doppler and PW Doppler in combination with 2-D imaging is highly effective for

determining the left-to-right orientation of the heart [4]. A great deal of information is available regarding

the acquisition of images of the embryonic and extraembryonic circulatory systems using Doppler

waveforms. Data are available on both the measurements of arterial and venous flow velocities and the

visualization of the placenta. We have previously been able to discern clear Doppler signals from mouse

embryos as early as E9, which appear as biphasic inflow patterns. Embryonic blood exhibits laminar flow

with a parabolic spatial velocity, and therefore appears to behave like a Newtonian fluid. The study of major

vascular development is aided by the intrinsic contrast of primitive nucleated erythroblasts at high

frequencies [3]. Moreover, it is possible to obtain details of the mitral and tricuspid inflow in the frontal

plane of the embryo by using PW Doppler. The shape of the Doppler waveforms across the atrioventricular

valves is bicuspid. The first and second peaks (i.e., E and A waves, respectively) correspond to early

ventricular filling of diastole and atrial systole, respectively. The velocity of the A wave is usually higher

than that of the E wave, suggesting that atrial systole in ventricular filling is more important during the fetal

period than postnatally [4]. It is important to note that the cardiac valves do not develop in the mouse

embryo until E13.5. Prior to this stage, the dynamic apposition of the endocardial cushions is used to prevent

the regurgitation of blood flow. Another vessel worthy of particular attention during the embryonic

developmental period is the dorsal aorta. The embryonic dorsal aorta is the primary artery in the developing

mouse embryo, and has a diameter of ~300 μm. Fundamental characterization of cardiac systolic function

and blood flow in mouse embryos may be estimated using Doppler waveform analysis of the dorsal aorta

[2].

IV. Embryonic Staging and Main Developmental Events

The mouse embryonic development stages can be roughly separated into two periods: preimplantation

and postimplantation [5]. A summary of mouse embryogenesis throughout development will now be

provided to enable the features of embryonic mouse development to be more easily distinguished in

ultrasound images. It should be stressed that when using the Prospect system, mouse embryos can only be

visualized in the postimplantation period, around 6.5 days after conception. In this guide, embryos were

staged in days of gestation, where day 0.5 (i.e., E0.5) was defined as noon on the day on which a vaginal

plug was detected after overnight mating [1].

A. Pre-implantation Period

The various stages of the preimplantation period are described in the following sections.

E 0-1

Fertilized one-cell-stage egg (embryo). The embryo is located in the ampullary region [5].

E 1

Two-cell-stage embryo. The embryo can be seen passing down the oviduct, beyond its ampullary

region [5].

E 2

Four- to 16-cell-stage embryos. These may range from early to fully compacted morulae. Embryos can

be seen passing down the oviduct toward the uterotubal junction [5].

E 3

Embryos have developed from the morula to the blastocyst stage (zona-intact) during this stage, and

possess a distinct inner cell mass and an outer layer of trophectoderm cells. This is the result of the

polarization process. Such embryos are usually located in the uterine lumen [5].

Embryos at this stage are invariably zona-free blastocysts and are located within the uterine lumen [5].

B. Post-implantation Period

The various stages of the postimplantation period are described in the following sections.

E4.5

During this stage, the blastocyst implants. The embryonic proximal or visceral endoderm cells appear

as a unique layer covering the blastocoelic surface of the inner cell mass. The endoderm cells begin to

migrate to the blastocoelic surface of the mural trophectoderm at a slightly later stage, and they complete

this during E5. The extraembryonic distal (or parietal) layer of the endoderm is composed of this second

group of cells [5].

E5.5

The number of cells constituting the inner cell mass is substantially increased by this stage, resulting in

the embryo subsequently growing toward the abembryonic pole of the implanting blastocyst, to form the egg

cylinder [5].

E 6.5

The Prospect system enables the early detection of the conceptus at E6.5. At this stage the inner cell

mass, which is composed of the epiblast, primitive endoderm, and trophectoderm, has begun to form a

cylindrical embryo [6]. Around the implantation in the lumen of the uterus, the embryo can be seen as a

slightly brighter echogenic ring near the trophoblast–decidual interface. This bright region is approximately

250 μm in diameter [6] [7]. The circumference of the ring is fairly uniform in brightness, although the

mesometrial side where the placenta will develop is brighter than the antimesometrial side [7]. Figure 3

shows the earliest detection of the conceptus at day 6.5.

(A) (B)

Fig. 3. Both (A) and (B) display the early development of the mouse embryo at E6.5, showing a slightly

brighter echogenic ring in each embryo in B-mode imaging using the Prospect system.

Egg cylinder stage

embryo at center of

decidual reaction

Mesometrium

Egg cylinder stage

embryo at center of

decidual reaction

Mesometrium

E 7.5

By day 7.5 the embryo has developed three distinct cavities: the amniotic cavity (AC), the exocoelomic

cavity (ECC), and the ectoplacental cavity (EC) [3] [6]. The bright echogenic region is localized primarily

near the mesometrial side of the implantation site [7]. Shortly after the onset of this stage, the anterior and

posterior amniotic folds amalgamate, resulting in the obliteration of the proamniotic canal and the formation

of the ECC. This is attributable to the increased growing rate of the allantois into the cavity of the posterior

amniotic fold. As a consequence of these events, the embryo becomes divided into the aforementioned EC,

the ECC, and the AC, which are separated by the chorion and amnion, respectively. In the more advanced

embryos observed at this stage, the expansion of the ECC and AC leads to a reduction in the EC volume [5].

B-mode images of E7.5 mouse embryos are shown in Figure 4.

(A) (B)

(C)

Fig. 4. B-mode images of E7.5 mouse embryos obtained using the Prospect system. (A) The proamniotic

canal, inner circular layer, and the outer longitudinal layer of myometrial smooth muscle can be clearly

discerned. (B) Visualization of the proamniotic canal, and the extraembryonic and primitive ectoderm. (C)

The proamniotic canal, ectoplancental cone, and primitive ectoderm inside the uterus.

Proamniotic canal

Extraembryonic

ectoderm

Primitive

ectoderm

Proamniotic canal

Inner circular layer of myometrial

smooth muscle

Outer longitudinal layer

of myometrial smooth

muscle

Proamniotic canal

Ectoplancental

cone

Primitive

ectoderm

E 8.5

The developmental E8.5–E13.5 stages in mouse correspond to 3–6 weeks of human gestation. The

earliest stage of heart and brain development in utero in a mouse embryo is at E8.5 [1] [8]. The allantois can

be visualized and the embryo can be seen wrapping around the amniotic cavity [6]. The most conspicuous

features evident at this stage on high-frequency ultrasound images are the open neural folds, which make it

easy to distinguish between embryonic and extraembryonic tissues. Early heart activity in the primitive heart

tube and the initial formation of the chorioallantoic placenta can be detected at this stage in more advanced

E8.5 embryos [1] [4].

The embryo changes morphologically between E8.5 and E9.5, whereby its dorsal region changes from

being the inside part of the U-shaped embryo (concave) to being the outside part (convex). A fluid-filled

neural tube will be formed at E9.5, when the open cephalic neural folds start to grow and turn over at E8.5.

This will eventually become the central nervous system of the adult mouse. Between E8.5 and E9.5 the

primitive heart is transformed from a straight tube through a looping process to form a single atrium and

ventricle. Then, at around E10.5 the initial stages of atrial and ventricular septation give rise to recognizable

precursors of the final four cardiac chambers [1].

The heart is the first organ system to differentiate and to function, and is the most prominent organ present in

the embryo at this stage. At E8.5 the heart is an asymmetrical globular structure in embryos with eight to ten

pairs of somites, and the embryonic vasculature at this stage contains primitive nucleated red blood cells. At

this stage the heart is divided into a common atrial chamber and a common ventricular chamber. The

common atrial chamber receives blood from the right and left horns of the sinus venosus, and the common

ventricular chamber is in direct continuity with the bulbus cordis region of the primitive heart, which in turn

is in direct continuity with the outflow tract of the heart [5]. Transabdominal B-mode images of E8.5 mouse

embryos are shown in Figure 5.

(A) (B)

Fig. 5. B-mode images of E8.5 mouse embryos obtained using the Prospect system. (A) The amniotic and

coelomic cavities of the embryo. (B) The neural folds, chorion, and amnion inside the uterus. (C) The neural

folds, allantois, and amnion of the embryo are clearly visible. (D) The neural folds, amnion, and

mesometrium, which connect two embryos, are displayed.

Amniotic

cavity

Exocoelomic

cavity

Chorion

Amnion

Neural

folds

Neural folds

Aminon

Allantois

Neural folds

Aminon

Mesometrium

E 9.5

By E9.5 the neural tube is divided into three regions: forebrain, midbrain, and hindbrain. The neural

tube and beating heart are the most prominent organs present in the embryo at this stage on real-time

high-frequency ultrasound images [1]. In ultrasound images they appear as echogenic foci residing near the

maternoplacental interface upon formation of the chorioallantoic placenta. As gestation progresses the

echogenic foci will become more discrete and widely spaced at this location, and also larger and/or brighter

in images than at earlier stages [7].

Cardiac looping can be visualized from E9.5, and the common atrium and ventricle can be

distinguished. Simultaneously, synchronous atrial and ventricular contractions and the atrial contraction at

end diastole are discernible [3] [8]. At E9.5 the overall volume of the common atrial chamber in an embryo

increases substantially, and the component parts of the common atrium become more clearly delineated [5].

Between E9.5 and E14.5 the cardiac systolic work and output—such as heart rate, peak aortic flow velocity,

velocity–time integral, and ejection time—increase geometrically, and become proportional to the cycle

length which is defined as the total time of one complete injection cycle (in seconds) [2]. Images of E9.5

mouse embryos are presented in Figure 6.

(A) (B)

(C)

Fig. 6. High-frequency ultrasound images of E9.5 mouse embryos produced using the Prospect system. (A)

The heart, forebrain, midbrain, and hindbrain of the embryo are clearly visualized on B-mode imaging. (B)

The heart, amnion, and forebrain of the embryo on B-mode imaging. (C) At earlier developmental stages of

the mouse embryo, the cardiac inflow and outflow signals are generated in opposite directions in the PW

Doppler mode, since the cardiac inflow and outflow axis are at almost 180º from one another. The inflow

Doppler waveform demonstrates the presence of separate E and A waves, with A-wave dominance.

Heart

Forebrain

Midbrain

Hindbrain

Heart

Forebrain

Amnion

Inflow

Outflow

E 10.5

At E10.5 the shape and subdivisions of the neural-tube cavity are easily identified on high-frequency

ultrasound images. Sagittal views of the embryonic cardiac image demonstrate the allantois, pericardial

effusion, ventricle, and the neural tube (forebrain, midbrain, and hindbrain).

The most obvious events that occur at this stage are the changes taking place in the embryonic heart

and the vessels that emerge from the heart and drain into it. The most important changes within the heart are

the earliest events associated with the process of septation, and a relatively broad and thickened region may

be observed in the midline in the dorsal part of the wall of the common atrial chamber [5]. The atria and

ventricles of the embryonic heart achieve a sufficiently well-developed state at E10.5 such that they can be

readily visualized (Fig. 7A). By E10.5–E11.5 the distinction between the presumptive left and right

ventricles becomes more apparent, the outflow tract is prominent, and two parallel streams of blood flow can

be displayed in real time [1] [8]. It is worth noting that the major blood vessels, such as the aorta, bulbus

cordis, or outflow tract, and the umbilical vessels can be distinguished due to the high-frequency ultrasound

signals from the blood and the resulting moving speckle patterns on real-time images [1].

In addition, the limb buds become increasingly prominent at this stage, and their sharp apical

ectodermal ridges are more clearly seen in the forelimb buds than in the hindlimb buds [5]. Furthermore, the

liver primordium can be identified lying posterior to the heart; it undergoes its first expansion at E10.5.

Between E10.5 and E13.5 the placenta changes from a discoid structure to a larger planoconvex structure,

with prominent sinusoids on its outer surface [8]. Figure 7 shows images of the E10.5 mouse embryo.

(A) (B)

Heart (atria and ventricles)

Heart

Forebrain

Dorsal aorta

Placenta

(C)

(D)

Fig. 7. High-frequency ultrasound images of E10.5 mouse embryos obtained using the Prospect system. (A)

The heart of the embryo, including the developing atria and ventricles, can be identified on B-mode imaging.

(B) The heart, dorsal aorta, and forebrain of the embryo are clearly evident on B-mode imaging. (C) PW

Doppler waveforms of mouse embryonic cardiac inflow and outflow. Increased inflow and outflow velocities

can normally only be observed in older embryos. (D) PW Doppler image of the umbilical cord. The vessels

of the umbilical circulation can be distinguished by PW Doppler imaging. Since the respective arteries and

veins flow in opposite directions, their Doppler waveforms are distinct and appear on either side of the

velocity baseline.

Inflow

Outflow

Artery

Vein

Umbilical cord

Placenta

E 11.5

At E11.5 the beating heart, the neural tube, and developing somites in the tail region are visible in

longitudinal views of the embryo; the crown–rump length can be measured in this view [6]. The most

obvious events that occur at this stage are septation of the outflow tract, atria, and ventricles. The details of

these events are described below.

The undivided outflow tract can be divided into three parts: the aortic sac, truncus (anteriosus), and

conus. These different parts contribute to the outlets, valves, and bases of the aortic and pulmonary trunks,

respectively, during septation of the common outflow tract, and are formed by E13.5 [9]. Atrial septation

occurs at this stage, which involves growth of the septum primum toward the atrioventricular bulbar cushion

tissue within the common atrial chamber of the heart. The peripheral margin of the septum primum appears

to be quite bulbous at E11.5 [5]. The interventricular septum is derived from the region of ventricular wall

adjacent to the bulboventricular groove, which is apparent at this stage, although the connection between the

future ventricles is still open through the bulboventricular canal. This process is known as ventricular

septation. The interventricular septum will be produced between the future ventricles at later stages of

development [9]. Images of the E11.5 mouse embryo are shown in Figure 8.

(A) (B)

(E)

Midbrain

Heart

Umbilical cord

Optic vesicle

Forebrain

Heart

Umbilical cord

Dorsal aorta

Umbilical cord

Placenta

Inflow

Outflow

(F)

(G)

Fig. 8. High-frequency ultrasound images of E11.5 mouse embryos obtained using the Prospect system. (A)

The heart, midbrain, optic vesicle, and umbilical cord of the embryo can be identified on B-mode imaging.

(B) The heart, forebrain, and umbilical cord of the embryo are clearly evident on B-mode imaging. (C) The

umbilical cord and placenta are observed on B-mode imaging. (D) B-mode image of the embryonic dorsal

aorta. (E) PW Doppler waveform of mouse embryonic cardiac inflow and outflow. (F) PW Doppler

waveform of the umbilical vessels obtained from the B-mode image shown in panel C. (G) PW Doppler

waveform of the mouse embryonic dorsal aorta obtained from the B-mode image shown in panel D.

Artery

Vein

E 12.5

The main developmental events that occur at E12.5 are the progressive septation of the common

outflow tract, atria, and ventricles, and the initiation of atrioventricular canal septation [3] [9]. By E12.5 the

septation of the distal portion of the outflow tract, including the semilunar outflow tract valves, is complete.

The proximal part of the outflow tract septum in the conus starts to close in a zipper-like fashion from distal

to proximal toward the ventricles. In order to divide the atrioventricular canal into separate right and left

canals, the superior and inferior atrioventricular cushions fuse to form an atrioventricular septum or septum

intermedium. The atrioventricular septum shifts over the interventricular septum, which allows the right and

left atrioventricular canals to be aligned with the corresponding ventricles. At this stage there is still

communication between the two ventricles, but by E13.5–14.0 the most proximal part of the conal ridges

together with the atrioventricular cushions will contribute to the membranous component of the

interventricular septum and will close the connection between the ventricles [9].

At E12.5–E13.5 the size of all four chambers of the heart has increased markedly, and the configuration

of the outflow tract has changed significantly. In addition, a distinct C-shaped aorta can be traced, arising

from the left ventricle and passing to the right of the pulmonary artery before looping to the left [8]. Images

of the E12.5 mouse embryo are shown in Figure 9.

(A) (B)

(E)

Heart

Mesencephalic vesicle

Third vesicle

Telencephalic vesicles

Umbilical cord

Spinal cord

Inflow

Outflow

(F)

(G)

Fig. 9. High-frequency ultrasound images of E12.5 mouse embryos obtained using the Prospect system. (A)

B-mode image of the embryonic heart, in which the four chambers can be identified. (B) Embryonic B-mode

image of the brain. The telencephalic vesicles (future lateral ventricles), third vesicle, and mesencephalic

vesicle (future aqueduct) inside the brain can be visualized clearly. (C) B-mode image of the umbilical cord.

(D) B-mode image of the embryonic spinal cord. (E) PW Doppler waveform of mouse embryonic cardiac

blood flow through the mitral orifice. (F) PW Doppler waveform of the umbilical vessels obtained from the

B-mode image shown in panel C. (G) PW Doppler waveform of the mouse embryonic dorsal aorta.

Artery

Vein

Dorsal aorta

E 13.5

At E13.5 several embryonic structures become visible, such as the vertebra and ribs, and the size of the

embryo begins to increase dramatically [4]. The other main event that occurs at this stage is separation of the

common outflow tract and ventricles, although the atrial and atrioventricular septa are still under formation

[3] [9]. At the site of fusion of the conal ridges, there is a fibrous raphe in the wall between the pulmonary

and aortic roots, which will be transformed into a muscularized, adult-type septum.

At this stage the various types of valve leaflet have not yet fully developed. The leaflets of both the

tricuspid and mitral valves appear to be elongated and press closely against each other. A process of

myocardial delamination separates the lower part of the atrioventricular valve leaflets from the myocardial

wall and the interventricular septum, resulting in freely moveable, bilayered leaflets. The final shaping and

composition of the atrioventricular valve leaflets occur as a result of an apoptotic process called excavation,

which takes place in the atrial face of the leaflets during E12.5–15.5. The maturation of the semilunar valve

leaflet is also progressing at this stage, and is the last major morphogenetic event pertaining to the outflow

tract [9].Images of the E13.5 mouse embryo are shown in Figure 10.

(A) (B)

(C)

Lateral ventricles

Heart

Dorsal aorta

Umbilical cord

(D)

(E)

Fig. 10. High-frequency ultrasound images of E13.5 mouse embryos obtained using the Prospect system. (A)

B-mode image of the embryo in the frontal plane; the lateral ventricle and heart can be visualized. (B)

B-mode image of the embryo in the sagittal plane; the heart and dorsal aorta can be identified. (C) Color

Doppler image of the umbilical cord. The opposing blood flow directions of the umbilical arteries and veins

are clearly evident. (D) PW Doppler image and waveform of the embryonic umbilical cord. (E) PW Doppler

waveform and image of the mouse embryonic dorsal aorta.

Umbilical cord

Placenta

Artery

Vein

E14.5

At E14.5 the different components of the central cushion tissue contribute to the atrial, ventricular, and

atrioventricular septa, and hence a normal ventricular septal defect can be seen at this stage [3]. The

common atrial chamber is divided into two by E14.0, but the lower border of the septum secundum never

completely fuses with the atrioventricular cushion, leaving an opening. There is considerable blood flow

from the right to the left atrium during fetal development via this opening and the ostium secundum. This

interatrial channel is called the foramen ovale, and its function is to allow the largely oxygenated blood to

enter the right atrium through the inferior vena cava, which is directed across the midline into the cavity of

the left atrium. The deoxygenated blood, which enters the rostral part of the right atrium through the right

superior vena cava, is directed toward the right ventricle. The foramen ovale is normally closed at birth by

the fusion of the two atrial septa [9]. In addition, the chest in cross-section with the heart, forelimb buds, and

the spine can be depicted in the transverse images at E14.5. At this stage early eye development, the third

ventricle, and the superior horns of the lateral ventricles can also be distinguished in oblique sections

through the brain and torso [6]. Images of the mouse embryo at E14.5 are shown in Figure 11.

(A) (B)

Lateral ventricle

Heart

Third ventricle

Heart

Spinal cord in tail

Dorsal aorta

Head

Eye

Forelimb

Placenta

Umbilical cord

(E)

(F)

Fig. 11. High-frequency ultrasound images of E13.5 mouse embryos obtained using the Prospect system. (A)

B-mode image of the embryo in the frontal plane; the lateral ventricle, third ventricle, and heart can be

visualized. (B) B-mode image of the embryo in the sagittal plane showing the heart, dorsal aorta, and the

spinal cord in the tail. (C) The head, eyes, and forelimb can be observed in B-mode in the transverse plane.

(D) Color Doppler image of the umbilical cord, and the interlaced blood flow reveal the spiral structure of

the umbilical arteries and vein. (D) PW Doppler waveform of the mouse embryonic dorsal aorta. (E) PW

Doppler waveform of the mouse embryonic cardiac blood flow through the mitral orifice.

Inflow

Outflow

E 15.5

At E15.5 all of the components of the heart are readily recognized. Both the heart and the vascular

system have achieved their definitive external prenatal configuration, but the atrioventricular valve leaflets

and coronary arteries continue to be modified after birth. Thus, the heart size only increases after E15.5 [5]

[9]. The thicknesses of the two ventricular walls are approximately equal. The volume of the left ventricular

cavity is slightly greater than that of the right side. The two atrial chambers of the heart are in continuity

through the foramen ovale [5].

With regard to the brain, the cerebellar primordium and the olfactory lobes are the two regions of the

brain that exhibit the greatest degree of differentiation between E13.5–14 and E14.5–15. As the rhombic lip

was previously the first site of differentiation of the intraventricular septum, the size of this region has

increased considerably by this stage, and begins to encroach upon the lumen of the fourth ventricular lateral

recess [5].

Furthermore, the eyes, which are distinguishable from the rest of the structures as large hypoechoic masses

located cranially to the heart, can be identified at this stage. Echogenic foci are now sometimes visible

scattered throughout the labyrinth region of the placenta [4] [7]. Images of the E15.5 mouse embryo are

shown in Figure 12.

(A)

Placenta

Umbilical cord

(B)

(C)

Fig. 12. High-frequency ultrasound images of E15.5 mouse embryos obtained using the Prospect system. (A)

B-mode image of the umbilical cord. (B) PW Doppler waveform of mouse embryonic cardiac blood flow

through the mitral orifice. The Doppler data of the inflow through the mitral orifice demonstrate the

presence of separate E and A waves, with A-wave dominance. (C) PW Doppler waveform of the embryonic

umbilical cord obtained from the B-mode image in panel A.

Artery

Vein

E 16.5

The most significant change in the heart at this stage is an alteration in its axis. Of particular interest is

the right atrium now being located considerably more caudally than the left atrium. The interventricular

groove is now only evident toward the apical region of the heart, and is clearly directed toward the left

anterolaterally. The axis of the heart appears to have changed from a principally anteroposterior direction to

a more oblique orientation. The main axis of the heart is oriented from the upper right to the lower left part

of the thoracic cavity. The principal axis of the heart has continued to grow, and is clearly visible in embryos

at E16.5, and at subsequent stages of development. Furthermore, the increase in the volume of the right

atrial chamber compared to that of the left is now more marked. The volume of the left ventricle continues to

be slightly greater than that of the right ventricle. In addition, the leaflets of the pulmonary and aortic valves

are considerably more differentiated than before. This equally applies to the atrioventricular valves [5].

Images of E16.5 mouse embryos are shown in Figure 13.

(A) (B)

(C)

Forth ventricle

Third ventricle

Eye

IVS

Systole

LVID

Diastole

LVID

LVPW

(D)

Fig. 13. High-frequency ultrasound images of E16.5 mouse embryos obtained using the Prospect system. (A)

B-mode image of the embryonic head, revealing the fourth ventricle, third ventricle, and eyes. (B) Color

Doppler image of the embryonic heart (C) M-mode tracing of the right and left ventricles. IVS,

interventricular septal wall; LVID, left ventricular internal dimension; LVPW, left ventricular posterior wall

thickness. (D) PW Doppler image of the embryonic umbilical cord.

Artery

Vein

E 17.5

At E17.5 the orientation of the heart is essentially identical to that evident in the previous stage,

although the interventricular groove is less evident. The interventricular groove only exhibits a shallow

depression just proximal to the apex of the heart, and this is a relatively insignificant feature of the anterior

surface of the heart compared to what is observed in previous stages. The walls of the ventricles, as well as

the muscular part of the interventricular septum, are more consolidated than in the previous stage, but the

muscular walls of the atria are still extremely thin compared to those of the ventricles. The volumes of these

chambers of the heart are far greater at this stage than those of the ventricles [5]. Figure 14 shows images of

E17.5 mouse embryos.

(A) (B)

(C)

Third ventricle

Lateral

ventricles

Eye

Umbilical cord

Placenta

(D)

Fig. 14. High-frequency ultrasound images of E17.5 mouse embryos obtained using the Prospect system. (A)

B-mode image of the embryonic head; the lateral ventricles, third ventricle, and an eye can be visualized

clearly. (B) B-mode image of the embryonic umbilical cord. The placenta and an embryonic eye are also

displayed in this view. (C) PW Doppler waveform of the umbilical artery. (D) PW Doppler waveform of the

umbilical vein.

E 18.5

At E18.5 the heart has reached its definitive prenatal state, and exhibits only a few changes in its gross

morphology compared to that evident in the previous stage. The difference in thickness between the walls of

the atria and ventricles is essentially the same as indicated above, as is the substantial difference in the

volumes of the atria and ventricles. The thickness appears to be substantially less than that observed

previously in certain parts of the wall of the ventricles, and this is particularly noticeable in the region

subjacent to and just to the right of the interventricular groove. The ventricular wall is now substantially less

consolidated than observed previously, which may be due to the significant increase in the degree of

trabeculation at this time. This change may indicate the differentiation of the ventricular muscle that occurs

at this time, in preparation for sustaining the increased functional load after birth [5]. Images of E18.5 mouse

embryos are shown in Figure 15.

(A) (B)

Dorsal Aorta

(C)

Fig. 15. High-frequency ultrasound images of E18.5 mouse embryos obtained using the Prospect system. (A)

Four-chamber view of the embryonic heart in B-mode. (B) Color Doppler image of the embryonic dorsal

aorta. (C) PW Doppler waveform of the mouse embryonic cardiac blood flow through the mitral orifice. The

Doppler data of the inflow through the mitral orifice demonstrate the presence of separate E and A waves,

with A-wave dominance. AET, aortic ejection time; IVRT, isovolumic relaxation time; IVCT, isovolumic

contraction time.

IVCT

IVRT

AET

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