FOLDING OF THE EMBRYO. In this lecture we shall concentrate on those events which transform the...
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![Page 1: FOLDING OF THE EMBRYO. In this lecture we shall concentrate on those events which transform the trilaminar embryo into a “tube within a tube”, segregating.](https://reader031.fdocuments.us/reader031/viewer/2022032307/56649cac5503460f9496ee47/html5/thumbnails/1.jpg)
FOLDING OF THE EMBRYO
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• In this lecture we shall concentrate on those events which transform the trilaminar embryo into a
• “tube within a tube”, segregating the embryo from extraembryonic tissue except at the umbilicus.
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• These processes will also rotate the developing heart tube into its thoracic position.
• Finally, we shall examine how the coelom is divided into the pleural, peritoneal and pericardial cavities.
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Learning objectives
• 1. To understand how the embryo is segregated from the extra-embryonic tissues with the exception of the umbilical cord.
• 2. To understand the process whereby the gut tube is formed and is lined with endoderm.
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• 3. To understand the events that result in the incorporation of the extra-embryonic coelom into the embryo, thereby forming the peritoneal, pericardial and pleural cavities.
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• 4. To understand the role of cranial-caudal flexion in the repositioning of the buccopharyngeal membrane (future opening of the mouth), heart tubes, primitive pericardial cavity and a wedge of
mesoderm (septum transversum) which will initiate diaphragm formation.
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• At the same time the cloacal membrane (future opening of gut and urinary tract) is also brought into a ventral position.
• 5. To understand how the diaphragm is formed.
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• At the end of the third week, the germ disc begins to overgrow the yolk sac, ballooning into a convex shape, with the peripheral areas of the germ disc becoming the ventral surface of the embryo.
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• During 4th week a significant event in the establishment of body-form is folding.
• Folding converts flat trilaminar embryonic disc into a cylindrical embryo.
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• Folding occurs in both longitudinal and transverse planes.
• The longitudinal and transverse foldings occur simultaneously.
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• Concurrently, there is relative constriction at the junction of embryo and yolk sac.
• Folding is due to rapid growth of central nervous system and somites.
• The growth rate at the sides of the embryonic disc fails to keep pace with the rate of growth in the long axis as the embryo increases rapidly in length.
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• Developing embryo during 4th week
• Longitudinal and Transverse foldings are in process
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Longitudinal Folding
• Mainly due to rapid growth of neural tube.
• Ends of the trilaminar germ disc folds ventrally and produce head and tail folds.
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• Beginning of fourth week, neural tube in cranial region grows very rapidly. It thickens to form the primordium of the brain.
• As the embryo grows the head region projects over the prochordal plate or buccopharyngeal membrane (future site of stomodeum).
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• Folding of the caudal end of the embryo results primarily from the growth of distal part of neural tube – the primordium of spinal cord.
• As the embryo grows the tail region projects over the cloacal membrane (future site of the anus).
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• Initially, (day 18) the developing brain projects dorsally into the amniotic cavity.
• Prochordal plate is anterior to the developing neural tube and cloacal plate is posterior to the neural tube.
• The connecting stalk along with allantois is posterior to cloacal membrane.
• Developing heart tube is inferior to pericardial cavity. • Septum transversum is anterior to pericardial cavity.
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• Later (day 22) the developing forebrain grows cranially beyond buccopharyngeal membrane and overhangs developing heart.
• Concomitantly, the septum transversum (transverse mesodermal septum), primordial heart, pericardial cavity, and buccopharyngeal or oropharyngeal membrane move onto ventral surface of the embryo.
• A small foregut and hindgut has developed.
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• Buccopharyngeal membrane is vertical (ectoderm is anterior, endoderm is posterior). Pericardial cavity is now inferior to heart tube. Septum transversum is posterior to heart tube and pericardial cavity.
• Cloacal membrane is vertical. Connecting stalk and allantois has gone inferior to hindgut but they are directed posteriorly.
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• Day 25. Now the buccopharyngeal/oropharyngeal membrane and cloacal membrane have completed 180 degree turns (ectoderm is inferior and endoderm is superior). Note that the rotation is more in tail fold than head fold.
• Foregut and hindgut have elongated. • The connecting stalk and allantois are now directed
(project) inferiorly/ventrally.• The constriction of yolk sac is now obvious.
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• Day 28. Oropharyngeal and cloacal membranes have rotated more and now facing each other.
• The yolk sac has further constricted. Midgut is now clear but still connected to yolk sac through narrow canal called vitelline duct.
• Connecting stalk along with allantois has gone nearer to vitelline duct and associated splanchnic mesoderm.
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• During longitudinal folding, part of the endoderm of the yolk sac is incorporated into the embryo as the foregut and hindgut.
• The foregut lies between the brain and heart. Oropharyngeal membrane separates the foregut from the stomodeum.
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The terminal part of the hindgut soon dilates slightly to form cloaca (primordium of urinary bladder & rectum).
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• After folding, the septum transversum lies caudal to the heart where it subsequently develops into the central tendon of the diaphragm.
• Before folding, the primitive streak lies cranial to the cloacal membrane; after folding, it lies caudal to it.
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• After folding the connecting stalk and the vitelline duct join each other and finally form umbilical cord.
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• The folding also affects the arrangement of the embryonic coelom (primordium of body cavities).
• Before folding, the coelom is a flattened, horseshoe-shaped single continuous cavity.
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• After folding, the coelom has become divided into thoracic and abdominal cavity.
• Thoracic (pericardial) and abdominal (peritoneal) cavities communicate each other through pleuropericardial canals.
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Transverse Folding
• The flat germinal disc also folds in horizontal or transverse direction.
• Right and left lateral edges of trilaminar germ disc not only grow laterally but also move ventrally and finally medially to meet in the midline.
• As the lateral edges of embryonic disc grow they form right and left lateral folds.
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• Right and left lateral edges of trilaminar germ disc grow
• laterally
• ventrally and
• finally medially to meet in the midline
• lateral edges form right and left lateral folds.
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• Transverse folding is produced by the rapidly growing somites and neural tube.
• The primordium of the ventrolateral wall grows laterally, ventrally and medially towards the median plane, rolling the edges of the embryonic disc and forming a roughly cylindrical embryo.
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• By the middle of 3rd week (day 17) a flat trilaminar germinal disc has formed.
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• Development of paraxial mesoderm and formation of somites is mainly responsible for transverse folding of embryo.
• As the paraxial mesoderm starts developing the ectoderm is raised bilaterally and neural groove forms between these bilateral ridges.
• Paraxial mesoderm later forms somites. The formation of somites and neural tube further raises surface ectoderm dorsally.
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• By the day-19 paraxial mesoderm has formed. The neural groove is visible. The ectoderm shows bilateral ridges. In the lateral plate of mesoderm small spaces have started developing.
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• Day-20. Somites have started forming. They are thickening and raising folds of developing neural tube.
• Lateral plate mesoderm has become divided into somatic and splanchnic mesoderms. Intra-embryonic coelom has formed. It is communicating with extra-embryonic coelom.
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• Day-21. intra-embryonic coelom is well developed now. The somites are more thickened. The embryo is assuming a globular form.
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• Day-22. Somites have further enlarged. Neural tube has formed. Intra-embryonic coelom is wide. The yolk sac is decreasing in size. Embryo is more globular in shape.
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Day-25. The blue amnion is almost surrounding the developing embryo. The yellow yolk sac is constricted and going to be divided into intra-embryonic and extra-embryonic parts. Initially, there is a wide connection between the midgut and yolk sac but after lateral folding the
connection is reduced to a yolk stalk.Intra-embryonic coelom and extra-embryonic coeloms
still communicating.
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• Day-28. The right and left growing edges of amnion have merged with each other. The right and left edges of surface ectoderm and parietal mesoderm have joined each other. Intra-embryonic coelom is now within the body of embryo.
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• As the abdominal walls form, part of the endoderm germ layer is incorporated into the embryo as the midgut. Midgut is suspended to the dorsal body wall by dorsal mesentery.
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• As a result of longitudinal and transverse folding the area of attachment of the amnion to the ventral surface of the embryo is reduced to a relatively narrow umbilical region. Connecting stalk and vitelline duct join and form umbilical cord. The amnion surrounds the umbilical cord from all sides.
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• As the lateral folds and longitudinal folds fuse ventrally and umbilical cord forms, the communication between the intraembryonic and extraembryonic coelomic cavities is reduced.
• As the amniotic cavity expands it obliterates most of the extraembryonic coelom.
• The amnion forms the epithelial covering of the umbilical cord.
• Body folding abnormalities are uncommon.
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Fourth Week• Major changes in body form occur
during the fourth week. • At the beginning, the embryo (2.0 to
3.5 mm long) is almost straight and has 9 pairs of somites that produce conspicuous surface elevations.
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• The neural tube is formed between right and left chains of somites.
• Pharyngeal (branchial) arches are visible. • The embryo is now slightly curved
because of the head and tail folds. • The heart produces a large ventral
prominence and pumps blood.
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Fourth Week
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• Forebrain produces a prominent elevation of head, and folding of embryo has given embryo characteristic C-shaped curvature. A long, curved tail is present.
• Upper limb buds become recognizable by day 26 or 27 as small swellings on the ventrolateral body walls.
• Otic pits, primordial of internal ears, are also visible.
• Lens placodes are visible on the sides of the head.
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• The fourth pair of pharyngeal arches and the lower limb buds are visible by the end of the fourth week.
• End of fourth week, an attenuated tail is a characteristic feature.
• Rudiments of many of the organ systems, especially the cardiovascular system, are established.
• End of fourth week caudal neuropore is closed.