LANGMANS Medica EDURORINTA Embryology FOURTEENTH EDITION

atria in the heart look the same] or inversions
[i.e., the two ventricles in the heart are reversed]
are present is called situs ambiguus or heterota)
q. These individuals are considered to have
laterality defects that have occurred because
of a failure to properly establish the L—R axis.
Patients with these conditions do not have
complete situs inversus but appear to be pre—
dominantly bilaterally left-sided or right—sided.
The spleen reflects the differences: Those with
left-sided bilaterality have polysplenia, and
those with right—sided bilaterality have asplenia
or hypoplastic spleen.
Individuals with situs inversus do not have
a high risk for having other congenital abnormalities
[although they have a slightly greater
risk of having a heart defect], but their progeny
have an increased risk for having laterality
defects and an even higher risk for having a
severe cardiac malformation. Also, approximately
20% of patients with complete situs
inversus have bronchiectasis and chronic
sinusitis because of abnormal cilia [Kartagener
syndrome]. Interestingly, cilia are normally
present on the ventral surface of the primitive
node and may be involved in L—R patterning.
In contrast, patients with situs ambiguous
[heterotaxy] have a high risk of having a wide
variety of other birth defects, including midline
malformations, such as neural tube defects,
cleft palate, anal atresia, etc. Furthermore,
90% of these individuals will have complex
congenital heart defects. The heart exhibits
more laterality than most organs and per—
haps that explains its increased susceptibility

I FURTHER DEVELOPMENT OF THE
TROPHOBLAST
By the beginning of the third week, the trophoblast
is characterized by primary villi that
consist of a cytotrophoblastic core covered by a
syncytial layer (Figs. 5.10 and 5.11). During further
development, mesodermal cells penetrate
Syncytiotrophoblast
A Primary B
villus
- Cytotrophoblast :-.
Secondary C
the core of primary villi and grow toward the decidua.
The newly formed structure is known as a
secondary villus (Fig. 5.1 1).
By the end of the third week, mesodermal
cells in the core of the villus begin to differentiate
into blood cells and small blood vessels, forming
the villous capillary system (Fig. 5.11). The villus
is now known as a tertiary villus or a definitive