Embryonic Folding (4th Week Development) PDF

Embryology Folding of the Embryo (4th week of development) By Dr. Sally Mohsen Assistant Professor of Anatomy and Embryology  objectives At the end of this lecture the students will be able to: Describe the process by which the embryo folds in both median and...

Embryology Folding of the Embryo (4th week of development) By Dr. Sally Mohsen Assistant Professor of Anatomy and Embryology  objectives At the end of this lecture the students will be able to: Describe the process by which the embryo folds in both median and horizontal planes www.vision.edu.sa  At the beginning of the fourth week, with progressive increase in size of embryonic disc, it bulge upwards into amniotic cavity. Further enlargement leads to folding of embryo folding in the median and horizontal planes converts the flat trilaminar embryonic disc into a C-shaped, cylindrical embryo. The initial flat embryonic disc begins to fold cephalocaudally, establishing the head and tail folds. The disc also folds transversely (lateral folds), establishing the rounded body form. The formation of the head, caudal eminence, and lateral folds is a continuous sequence of events that results in a constriction between the embryo and the umbilical vesicle. The ventral body wall of the embryo is closed, except for a small part in the umbilical region where the yolk sac duct and connecting stalk are attached. Connection with the yolk sac and placenta is maintained through the vitelline duct and umbilical cord, respectively www.vision.edu.sa Folding Ventrally As the head folds ventrally, part of the endodermal layer is incorporated into the developing embryonic head region as the foregut. The oropharyngeal membrane and heart are carried ventrally. The developing brain becoming the most cranial part of the embryo. Effect of Head fold to cardiogenic area: 1. Ectodermal depression (stomodeum) lies ventral to the neural tube. 2. Cardiogenic area lies caudal to the buccopharyngeal membrane. Pericardial cavity lies ventrally. 3. Heart tube lies dorsal to the pericardial cavity. 4. Septum transversum lies caudal to the cardiogenic area 5. Caudal end of the heart tube comes to lie cranially. It continues with dorsal aortae. 6. Original cranial end of heart tube lies caudally & receives 3 pairs of veins. www.vision.edu.sa  As the caudal eminence folds ventrally (tail fold): part of the endodermal germ layer is incorporated into the caudal end of the embryo as the hindgut. The terminal part of the hindgut expands to form the cloaca. Cloaca is the primordium of urinary bladder and rectum primitive streak lies caudal to the cloacal membrane After tail folding, the connecting stalk (primordium of umbilical cord) is attached to the ventral surface of the embryo Allantois (a diverticulum of yolk sac) is partially incorporated into the embryo The cloacal membrane, allantois, and connecting stalk being carried to the ventral surface of the embryo. www.vision.edu.sa Folding in the horizontal plane Folding on sides of the embryo produces right and left lateral folds It is produced by rapidly growing spinal cord and somites Ventrolateral rolling of the edges of embryonic disc form roughly cylindrical embryo Folding of embryo in horizontal plane incorporates part of the endoderm into the embryo as the midgut. The umbilical vesicle remains attached to the midgut by a narrow omphaloenteric duct (yolk stalk). During folding of the embryo in the horizontal plane, the primordia of the lateral and ventral body walls are formed. As the amnion expands, it envelops the connecting stalk, yolk stalk, and allantois, thereby forming an epithelial covering for the umbilical cord. www.vision.edu.sa Yolk Sac and allantois: Formation of head & tail folds leads to encloser of parts of yolk sac within the embryo & lined with endoderm. This forms the primitive gut which initially has wide communication with yolk sac. Part cranial to communication forms foregut, caudal to communication hindgut & part intervening forms midgut. Communication with yolk sac becomes progressively narrower, leading to smaller yolk sac called definitive yolk sac Narrow channel connecting it to gut forms vitello-intestinal duct which elongates & disappears eventually. Now amniotic cavity expands greatly & surround the embryo on all sides & embryo starts floating within it. Incorporation of the allantois into the body of the embryo, where it forms the cloaca. The distal portion of the allantois remains in the connecting stalk. By the fifth week, the yolk sac duct, allantois, and umbilical vessels are restricted to the umbilical region. In humans, the yolk sac is vestigial and has a nutritive role only in early stages of development. In the second month of development, it lies in the chorionic cavity. Hence, the endodermal germ layer initially forms the epithelial lining of the primitive gut and the intraembryonic portions of the allantois and vitelline duct.  The external appearance of the embryo is greatly affected by the formation of the brain, heart, liver, somites, limbs, ears, nose, and eyes. The beginnings of most essential external and internal structures are formed during the fourth to eighth weeks so this is the most critical period of development. Developmental disturbances during this period may give rise to major birth defects. At the end of the fourth week, when the embryo has approximately 28 somites, the main external features are the somites and pharyngeal arches. The age of the embryo is therefore usually expressed in somites. Because counting somites becomes difficult during the second month of development, the age of the embryo is then indicated as the crown-rump length (CRL) and expressed in millimeters. CRL is the measurement from the vertex of the skull to the midpoint between the apices of the buttocks. www.vision.edu.sa  During the second month, the external appearance of the embryo is changed by an increase in head size and formation of the limbs, face, ears, nose, and eyes. By the beginning of the fifth week, forelimbs and hindlimbs appear as paddle-shaped buds. The former are located dorsal to the pericardial swelling at the level of the fourth cervical to the first thoracic somites, which explains their innervation by the brachial plexus. Hindlimb buds appear slightly later just caudal to attachment of the umbilical stalk at the level of the lumbar and upper sacral somites. With further growth, the terminal portions of the buds flatten, and a circular constriction separates them from the proximal, more cylindrical segment. Soon, four radial grooves separating five slightly thicker areas appear on the distal portion of the buds, foreshadowing formation of the digits. These grooves, known as rays, appear in the hand region first and shortly afterward in the foot, as the upper limb is slightly more advanced in development than the lower limb. While fingers and toes are being formed, a second constriction divides the proximal portion of the buds into two segments, and the three parts characteristic of the adult extremities can be recognized. www.vision.edu.sa  References 1. Langman’s Medical Embryology, 12th edition. 2. Essentials of Human Embryology, by A.K. Datta, 5 th edition. 3. The Developing Human Clinically Oriented Embryology Keith L. Moore, T.V.N. Persaud and Mark G. Torchia9th Edition 2013. 4. Internet websites (for videos). www.vision.edu.sa www.vision.edu.sa