Embryonic Period (Weeks 3-8) PDF

Embryology Embryonic Period Third to Eighth Weeks of Development) By Dr. Sally Mohsen Assistant Professor of Anatomy and Embryology  objectives 1. Outline the derivatives of mesoderm germ layer (paraxial, intermediate and lateral plate mesoderm) 2. Describe somite...

Embryology Embryonic Period Third to Eighth Weeks of Development) By Dr. Sally Mohsen Assistant Professor of Anatomy and Embryology  objectives 1. Outline the derivatives of mesoderm germ layer (paraxial, intermediate and lateral plate mesoderm) 2. Describe somite differentiation. 3. Outline the derivatives of endoderm germ layer www.vision.edu.sa DERIVATIVES OF THE MESODERMAL GERM LAYER Initially, cells of the mesodermal germ layer form a thin sheet of loosely woven tissue on each side of the midline. 1- By approximately the 17th day, however, cells close to the midline proliferate and form a thickened plate of tissue known as paraxial mesoderm. 2- More laterally, the mesoderm layer remains thin and is known as the lateral plate. 3- Intermediate mesoderm connects paraxial and lateral plate mesoderm. Intermediate Mesoderm Intermediate mesoderm, which temporarily connects paraxial mesoderm with the lateral plate, differentiates into urogenital structures(nephrotomes, nephrogenic cord). www.vision.edu.sa Paraxial Mesoderm By the beginning of the third week, paraxial mesoderm begins to be organized into segments. These segments, known as somitomeres (unsegmented), lie on either side of the neural tube. In the head region, somitomeres form in association with segmentation of the neural plate into neuromeres. From the occipital region caudally, somitomeres further organize into somites (segmented). The first pair of somites arises in the occipital region of the embryo at approximately the 20th day of development. New somites appear in craniocaudal sequence at a rate of approximately three pairs per day until, at the end of the fifth week, 42 to 44 pairs are present. There are 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and eight to 10 coccygeal pairs. The first occipital and the last five to seven coccygeal somites later disappear, while the remaining somites form the axial skeleton. Because somites appear with a specified periodicity, the age of an embryo can be accurately determined during this early time period by counting somite. www.vision.edu.sa Somite Differentiation Somite is divisible into 3 parts. -Sclerotome: They migrate around the neural tube & forms vertebral column & ribs. -Dermatome: They line the deeper surface of ectoderm forms dermis of the skin. -Myotome: They form the striated muscle (body wall muscles and limb muscles). Each myotome and dermatome retains its innervation from its segment of origin, no matter where the cells migrate. Hence, each somite forms its own sclerotome (the tendon cartilage and bone component), its own myotome (providing the segmental muscle component), and its own dermatome, which forms the dermis of the back. Each myotome and dermatome also has its own segmental nerve component www.vision.edu.sa www.vision.edu.sa Lateral Plate Mesoderm With the appearance and coalescence of intercellular cavities in the lateral plate, this tissue is divided into two layer: - A layer continuous with mesoderm covering the amnion, known as the somatic or parietal mesoderm layer - A layer continuous with mesoderm covering the yolk sac, known as the splanchnic or visceral mesoderm layer (surrounds the organs). - Together, these layers line a newly formed cavity, the intraembryonic cavity, which is continuous with the extraembryonic cavity on each side of the embryo.  The parietal layer of lateral plate mesoderm then forms the dermis of the skin in the body wall and limbs, the bones and connective tissue of the limbs, and the sternum. Mesoderm from the parietal layer, together with overlying ectoderm, forms the lateral body wall folds. These folds, together with the head and tail folds, close the ventral body wall.  The visceral layer of lateral plate mesoderm, together with embryonic endoderm, forms the wall of the gut tube. Mesoderm cells of the visceral layer form a thin serous membrane around each organ.  (The Splanchnopleuric layer – Cardiogenic area- The Somatopleuric layer – with ectoderm Intra-embryonic coelom) Forms Pericardial, pleural & peritoneal cavities.  Septum transversum – Unsplit mesoderm cranial to cardiogenic area forms diaphragm & liver www.vision.edu.sa www.vision.edu.sa DERIVATIVES OF THE ENDODERMAL GERM LAYER The gastrointestinal tract is the main organ system derived from the endodermal germ layer. This germ layer covers the ventral surface of the embryo and forms the roof of the yolk sac. As a result of cephalocaudal folding, a continuously larger portion of the endodermal germ layer is incorporated into the body of the embryo to form the gut tube. The tube is divided into three regions: the foregut, midgut, and hindgut. At its cephalic end, the foregut is temporarily bounded by an ectodermal- endodermal membrane called the oropharyngeal membrane. The hindgut also terminates temporarily at an ectodermal-endodermal membrane, the cloacal membrane. During further development, endoderm gives rise to the epithelial lining of the respiratory tract, the urinary bladder, urethra; and tympanic cavity and auditory tube and the parenchyma of the thyroid, parathyroids, liver, and pancreas. 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. Before we are born, by More & Persuad 4. Internet websites (especially for videos) www.vision.edu.sa www.vision.edu.sa