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Re: yolk sacFrom: DuboseTerryJ@uams.eduFri May 25 11:19:07 2001
According to Moore & Persaud, DEVELOPING HUMAN: Clinically Oriented Embryology, W. B. Saunders, 1993 p. 40-46 & 70-80; what we see sonographically is actually the "secondary yolk sac". The "primary yolk sac" develops earlier (during 3rd LMP week, i.e. 9 days after conception) and is too small to visualize sonographically. The secondary yolk develops around conceptual day 13 (start of 4 LMP week). You are correct in that the yolk is related to the formation of the fetal gut. Moore & Persaud has a great graphic representation of this process on page 71. The amnion forms around the embryo and separates or "squeezes" the yolk sac from the midgut and intraembryonic coelom at about 26-28 post conception. An interesting bit of trivia... I had always wondered what the "choronic cavity" outside the amnionic sac contained... I was sure that it was not a vacuuume, but could not find any reference other than "cavity". Finally during research for FETAL SONOGRAPHY, I found the answer in: Benirschke K; "NORMAL DEVELOPMENT", Chapter 4; in Creasy RK and Resnik R, Editors; MATERNAL-FETAL MEDICINE: Principles and Practice, 2nd Edition; W. B. Saunders Company, 1989; p. 117. "As the pregnancy progresses the thin amniotic membrane enlarges and will contain the embryo, the yolk sac will remain outside the amnion. See Figure 16-12C. The amniotic sac with it's embryo, and the yolk sac are all found within the so called "gestational sac". This gestational sac is actually the chorionic cavity. The chorionic space around the amniotic sac is filled "...by a gel that liquefies upon touching, the magma reticulare." This magma reticulare has a slightly echogenic appearance when compared to the very clear amniotic fluid; see Figure 16-12C. The magma reticulare is also called the "chorioamniotic accumulation" and is a normal finding before the amnion and chorion fuse." See: DuBose, FETAL SONOGRAPHY, W. B. Saunders Co., 1996, p. 370. Terry J. DuBose, M.S., RDMS, Assistant Professor Director, Diagnostic Medical Sonography Program CHRP, University of Arkansas for Medical Sciences Little Rock, Arkansas, USA 501-686-6510 http://www.io.com/~dubose/ <http://www.io.com/~dubose/> http://www.uams.edu/CHRP/dmshome.htm <http://www.uams.edu/CHRP/dmshome.htm> http://www.obgyn.net/us/panel/panel.htm <http://www.obgyn.net/us/panel/panel.htm> -----Original Message----- From: Dr. N. A. Mohile [mailto:namohile@satyam.net.in] Sent: Friday, May 25, 2001 10:42 AM To: Multiple recipients of list ULTRASOUND Subject: yolk sac The round structure seen within the gestation sac by 5 weeks and called yolk sac is visible even at 12 weeks outside the amniotic sac.Embryologically yolk sac forms the gut.Is this structure really yolk sac? -Nila.
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