Week
5-Highlights: Heart development, brain & spinal cord formation, arm/leg buds
appear, gut tube development
The heart is the first organ in the embryo to develop and
function, it pumps blood by week five. The circulatory system including the
heart comes from the middle embryonic layer- the mesoderm. For the heart to get
its start, the mesoderm first branches off into two distinct cell layers- the
somatic (parietal) mesoderm and the splanchnic (visceral) mesoderm. (Parietal
and visceral are another term pair we see in anatomy where parietal is the
outer layer and visceral is the inner layer). The latter gives rise to cardiac
precursor cells, these precursor cells will eventually give rise to the
endocardium (lining of the heart chambers and valves) and the myocardium (as
the suffix ‘myo’ suggests these are the musculature of the ventricles and
atria- the chambers of the heart). The
overall process of the organ development of the heart is pretty fascinating, in
my opinion the heart is one of the most complex organs in the body so the idea
that it is the first to be functionally in the embryo indicates how much
complex work is being done so early in embryonic development. Using the balloon
animal artist metaphor the heart begins as a basic tube, patterned along the
anterior/posterior (head/anus) axis to set where the eventual regions of the
heart will be oriented. First up the tube needs to gain some flexible if it’s
going to start to look like the adult heart with its four chambers and four major
veins and arteries coming in and out of it. In order for this to happen the
mesoderm around the tube degenerates so that only the arterial and venous ends
are tethered. This sets the field for the heart to fold ventrally (towards the
belly) and bulges into five early regions- Anterior to posterior the regions
are- Truncus Arteriosus, Bulbus Cordis, Primitive Ventricle, Primitive
Atrium, and the Sinus Venosus.
Source: http://cnx.org/content/m46673/latest/?collection=col11574/latest
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Embryonic blood flow
is distinct from what will become in the fully developed organism. This is due
to the fact that the placenta is playing the role of the lungs, kidney, and
liver in the developing embryo and thus blood is shunted to avoid these areas
(or lack thereof) until they are developed. Let’s briefly take a look at the major
differences in embryonic circulation compared to adult circulation (and by
adult I mean that loosely really once all the organs are developed it is
‘adult’). Essentially what is different is a detour shunting blood away from
areas of the embryo that are undergoing construction. Also we learned
earlier the large multifactorial role the placenta plays during embryonic
development. The placenta swoops in and allows for full function of gas
exchange (which will be done by the lungs once they are good to go) and
filtration (which the liver and kidney will take over when they are ready).
This means that blood doesn’t really need to go to these two construction zones
and it order for it not to the embryo builds three shunts or bypasses
that help redirect blood away from the developing lung and the developing liver
and kidneys. In adults the major one
sentence summary is the pulmonary circulation system carries deoxygenated blood
away from the heart to the lung to become oxygenated and sends it back to the
heart freshly oxygenated, while the systemic circulation path brings the
oxygenated blood from the heart to all over the body. There are two main ‘out’
valves- the aorta and the pulmonary artery and two main ‘in’ valves- vena cava
and the pulmonary veins and the flow of blood goes from the vena cava → right
atrium → right ventricle → pulmonary artery → lungs (then back to the pulmonary
veins) and the pulmonary veins → left atrium → left ventricular → aorta → body
(then back to the vena cava). In the developing organism the blood is shunted
from the right atrium to the left atrium, bypassing the whole system that
brings blood to the lungs. As well another shunt is present to collect any
blood that gets passed the first shunt and made its way into the right
ventricle (from the right atrium). This
blood makes it from the right ventricle to the pulmonary artery, from here it
is sent to the aorta. Some blood does make it passed these shunts to the
developing lung and it will stay there to aid lung development. The other
potentially obvious components of fetal circulation, since we know now the gas
exchange occurs in the placenta, is that there are both umbilical veins that
bring oxygenated blood to the embryo and umbilical arteries that carry the
deoxygenated blood back to the placenta. I’ve left out a few more veins and arteries as
well as name of the shunts for a somewhat simpler picture of what’s going on
overall.
Fast Facts- The fetus' heartbeat can be detected around the sixth week and it is about twice as fast as the mother's.
Fast Facts- The fetus' heartbeat can be detected around the sixth week and it is about twice as fast as the mother's.
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