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NBME 23 Answers

nbme23/Block 3/Question#1

A 32-year-old woman comes to the physician after ...

Yolk sac

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submitted by nwinkelmann(105),

Here's a GREAT video explaining ultrasound findings of pregnancy. @9:57 is a great picture so far with labels of the yolk sac, gestational sack, fetal pole, and amnion. One thing the person explaining said that I thought would be good to keep in mind is that the yolk sac looks like a cheerio inside the gestational sac. Also, this site has a bunch of pictures as well: https://radiopaedia.org/articles/first-trimester?lang=us.

zpatel  what video? +4  

The rhombencephalon would be on the actual fetus so just get rid of (D). The "black hole" that the fetus is floating in is the gestational sac so get rid of (C). Now I am no ultrasound expert but I know that the amniotic cavity eventually expands to fuse with the chorion thereby eliminating the chorionic cavity (B). In terms of where the amniotic cavity is shown in this image, I am not sure, so maybe someone can help but this leaves the yolk sac which typically appears within the gestational sac around 5.5 weeks.

kateinwonderland  At the end of the fourth week, the yolk sac presents the appearance of a small pear-shaped opening (traditionally called the umbilical vesicle), into the digestive tube by a long narrow tube, the vitelline duct. (Wiki) +1  

The gestational sac is spherical in shape, and usually located in the upper part of the fundus of the uterus. By approximately 9 weeks of gestational age, the amniotic sac has expanded to occupy the majority of the volume of the gestational sac, eventually expanding to reduce the extraembryonic coelom to a thin layer between the amnion membrane and the mesoderm. By then, the gestational sac is usually simply called the "amniotic sac".

Development During embryogenesis, the extraembryonic coelom (or chorionic cavity) that constitutes the gestational sac is a portion of the conceptus consisting of a cavity between Heuser's membrane and the Trophoblast.

During formation of the primitive yolk sac, some of the migrating hypoblast cells differentiate into mesenchymal cells that fill the space between Heuser's membrane and the Trophoblast, forming the extraembryonic mesoderm. As development progresses, small lacunae begin to form within the extraembryonic mesoderm which enlarge to become the extraembryonic coelom.

The extraembryonic coelom divides the extraembryonic mesoderm into two layers: extraembryonic splanchnopleuric mesoderm, which lies adjacent to Heuser's membrane around the outside of the primitive yolk sac, and extraembryonic somatopleuric mesoderm, which lies adjacent to the cytotrophoblast layer of the embryo.

The chorionic cavity is enclosed by the chorionic plate, which is composed of an inner layer of somatopleuric mesoderm and an outer layer of trophoblast cells.