Embryology: Development of Body Cavity, Serous Membranes and Gut (I) M1 Gross and Developmental Anatomy 11:00-11:50 AM, January 8, 2009 Dr. Milton M. Sholley Professor of Anatomy and Neurobiology

The web address listed on syllabus page 29 has changed. ANIMATIONS: The web address listed on syllabus page 29 has changed. The new address is: http://cna.uc.edu/embryology/chapter6/animations/contents.htm

Dorsal View of the Embryonic Disc at ~Day 15 Cells from the epiblast migrate through the primitive groove to replace cells of the hypoblast, thus forming the definitive endoderm. Primitive groove Buccopharyngeal membrane Cloacal membrane Epiblast (blue) Definitive endoderm Larsen Website gast.avi

Dorsal View of the Embryonic Disc The animation will depict ingress of cells from the epiblast through the primitive groove to form the intraembryonic mesoderm (~day 16). Primitive groove Buccopharyngeal membrane Cloacal membrane The dashed line shows the approximate plane of section used for the cross sectional drawings on syllabus page 32. Epiblast (blue) Definitive endoderm Larsen Website gast.avi

Differentiation of Intra-embryonic Mesoderm and Formation of Intra-embryonic Coelom Cross sections through the embryonic disc (Syllabus page 30) ~19 days ~20 days Adapted from: Langman's Medical Embryology

is located between and attached to both the Yolk Sac and The Embryonic Disc is located between and attached to both the Yolk Sac and the Amniotic Sac, as shown here at the end of the third week. Cut edge of amniotic membrane The dashed line shows the approximate plane of section used for the cross sectional drawings on syllabus page 30. Yolk sac Larsen Website neurul.avi

Differentiation of Intra-embryonic Mesoderm and Formation of Intra-embryonic Coelom Cross sections through the embryonic disc (Syllabus page 30) ~19 days ~20 days Adapted from: Langman's Medical Embryology

the flat embryonic disc is converted by folding During the fourth week, the flat embryonic disc is converted by folding in both lateral and head-to-tail directions into a 3-dimensional vertebrate body form. Animation begins at ~21 days Animation ends at ~25 days Yolk sac Larsen Website neurul.avi

Lateral Folding of the Embryonic Disc -Converts the flat disc into a tubular embryo -Closes-off the intra-embryonic coelom -Pinches-off the tubular gut from the yolk sac -Expands amniotic cavity to surround embryo Cross sections of the embryo (Syllabus page 31) Adapted from: Langman's Medical Embryology

Kidney Aorta Dorsal mesentery Parietal peritoneum Visceral peritoneum  The abdominopelvic cavity is lined with parietal peritoneum.  The gut is covered with visceral peritoneum.  Mesenteries are double layers of peritoneum that connect parietal and visceral layers.  The space between parietal and visceral layers is called the peritoneal cavity. Kidney Aorta Dorsal mesentery Parietal peritoneum Visceral peritoneum Peritoneal cavity Intraperitoneal gut

 Mesenteries suspend the gut and gut-associated organs within the abdominopelvic cavity.  Mesenteries DO NOT suspend the gut and gut-associated organs within the peritoneal cavity.  The peritoneal cavity normally contains nothing but a small amount of fluid that moistens the surfaces of the peritoneum.  The peritoneal cavity can expand to contain a large amount of fluid under abnormal circumstances.

The animation will start at ~21 days Head to Tail Folding The animation will start at ~21 days and end at ~28 days. Amniotic cavity Head end Tail end Yolk sac Larsen Website folding.avi

Head to Tail Folding Note that the head folding causes relocation of the heart primordium and the mesoderm cranial to it. The mesoderm cranial to the heart becomes the septum transversum and is relocated from a cranial to a thoracic position. The septum transversum (shown in green) will form an important part of the diaphragm. Adapted from: Langman's Medical Embryology

Model of a 5-week embryo (left) and a cross section at the level of the septum transversum (right). (Syllabus page 33) Note the location of the pericardioperitoneal canals and the formation of the diaphragm. Adapted from: Langman's Medical Embryology

Formation of the Diaphragm The coelom is divided into thoracic and abdominal cavities by Formation of the Diaphragm

Division of the Thoracic Cavity (Syllabus page 34) Adapted from: Langman's Medical Embryology

Diaphragmatic Hernia It is usually congenital (Bochdalek’s Hernia) and, if it is extensive, it is often associated with severe hypoplasia of the lungs that becomes fatal at birth. It is more common on the left side of the body because the pericardioperitoneal canal on the left tends to close later than that on the right. From: Langman's Medical Embryology

Head to Tail Folding and Formation of the Primitive Gut (Syllabus page 35) Adapted from: Langman's Medical Embryology

The animation will start at ~21 days Head to Tail Folding The animation will start at ~21 days and end at ~28 days. Amniotic cavity Head end Tail end Yolk sac Larsen Website folding.avi

Formation of the Primitive Gut resulting from Head to Tail Folding Endodermal sheet Starting at ~20 days Yolk sac Ending at ~26 days Larsen Website 9-1.avi

Kidney Aorta Dorsal mesentery Parietal peritoneum Visceral peritoneum  The abdominopelvic cavity is lined with parietal peritoneum.  The gut is covered with visceral peritoneum.  Mesenteries are double layers of peritoneum that connect parietal and visceral layers.  The space between parietal and visceral layers is called the peritoneal cavity. Kidney Aorta Dorsal mesentery Parietal peritoneum Visceral peritoneum Peritoneal cavity Intraperitoneal gut

 Mesenteries suspend the gut and gut-associated organs within the abdominopelvic cavity.  Mesenteries DO NOT suspend the gut and gut-associated organs within the peritoneal cavity.  The peritoneal cavity normally contains nothing but a small amount of fluid that moistens the surfaces of the peritoneum.  The peritoneal cavity can expand to contain a large amount of fluid under abnormal circumstances.

Derivatives of the ventral mesentery are shown in blue and derivatives of the dorsal mesentery are shown in red. From: Langman's Medical Embryology

What are the adult derivatives of the primitive gut regions? See syllabus pages 36-37. Foregut becomes what? Midgut becomes what? Hindgut becomes what? Adapted from: Langman's Medical Embryology

Foregut artery is celiac. Midgut artery is SMA. Hindgut artery is IMA.

Obliteration of Chorionic Cavity Enlargement of Amniotic Cavity by Enlargement of Amniotic Cavity See this series of diagrams on page 146 of Larsen (3rd ed.) Animation: Starts at 4 weeks Ends at 8 weeks Amniotic cavity Yolk sac Chorionic cavity Larsen Website amnio.avi

Oligohydramnios: Too little amniotic fluid. Polyhydramnios: Too much amniotic fluid. For an interesting discussion of causes and consequences of these two conditions, see Larsen, 3rd edition, page 490.