6. Trilaminar Disc 3rd week (1).pdf

Anatomy: 3rd week of pregnancy Trilaminar Germ Disc DR. AMMAR MOHAMMED ALI  Objectives: Describe the formation of primitive streak & primitive node Describe the formation of ectoderm, mesoderm and endoderm. Describe the formation of notochord Identify subdivisions of the second...

Anatomy: 3rd week of pregnancy Trilaminar Germ Disc DR. AMMAR MOHAMMED ALI  Objectives: Describe the formation of primitive streak & primitive node Describe the formation of ectoderm, mesoderm and endoderm. Describe the formation of notochord Identify subdivisions of the secondary mesoderm  Gastrulation Gastrulation, the process that establishes all three germ layers (ectoderm, mesoderm, and endoderm) in the embryo. Begins with formation of the primitive streak on the surface of the epiblast.  Primitive streak: is a narrow groove in the epiblast with slightly bulging regions on either side The primitive node, consists of a slightly elevated area surrounding the small primitive pit  Cells of the epiblast migrate toward the primitive streak, this inward movement (invagination). Some of cells displace the hypoblast, creating the embryonic endoderm and others come to lie between the epiblast and newly created endoderm to form mesoderm. Cells remaining in the epiblast then form ectoderm. Thus, the epiblast, is the source of all of the germ layers.  Secondary (intraembryonic) Mesoderm Develops from the ectodermal cells at the sides of the primitive groove It spreads between the ectodermal & endodermal layers of the embryonic disc except in three regions: 1. Oropharyngeal membrane 2. Notochord 3. Cloacal membrane  Notochord It is a cylindrical rod shaped structure induced by the primitive node It lies in the midline between the ectodermal and endodermal layers of the embryonic disc It extends from the primitive node to the prechordal plate It forms the basis for the vertebral column & induces the overlying ectoderm to form neuroectoderm & neural plate  Formation of the Notochord Prenotochordal cells invaginating in the primitive pit move forward cephalad until they reach the prechordal plate. These prenotochordal cells become intercalated and form two cell layers that form the notochordal plate. Cells of the notochordal plate proliferate and detach from the endoderm, they form a solid cord of cells, the definitive notochord.  Formation of Notochord Oropharyngeal Prechordal plate membrane Cranial end Neural Embryonic plate ectoderm Primitive Notochordal node process Newly added Caudal end Primitive cells streak Notochord deep Cloacal membrane to ectodermal layer 15 days 17 days 18 days Notochordal process grows forwards from the primitive node 21 days between the ectodermal & endodermal layers of the embryonic disc to reach the prechordal plate The notochord underlies the neural tube and serves as the basis for the axial skeleton. The notochord and prenotochordal cells extend cranially to the prechordal plate and caudally to the primitive pit. At the point where the pit forms an indentation in the epiblast, the neurenteric canal. The Cloacal membrane is formed at the caudal end of the embryonic disc and when appears, the posterior wall of the yolk sac forms a small diverticulum that extends into the connecting stalk. (allantoenteric diverticulum, or allantois).  What happens to the notochord? It disappears leaving some remnants: Nucleus pulposus of the I.V. discs Apical ligament which connects the odontoid process of axis with the occipital bone Abnormal persistence of the notochord gives rise to a malignant tumor-chordoma Fate Map Established During Gastrulation The cells that ingress through the cranial region of the node become notochord Those migrating at the lateral edges of the node become paraxial mesoderm Cells migrating through the midstreak region become intermediate mesoderm Those migrating through the more caudal part of the streak form lateral plate mesoderm Teratogenesis Associated With Gastrulation The beginning of the third week of development, is a highly sensitive stage for teratogenic insult. For example, high doses of alcohol at this stage kill cells in the anterior midline of the germ disc, and resulting in holoprosencephaly. Gastrulation itself may be disrupted by genetic abnormalities and toxic insults. In caudal dysgenesis (sirenomelia), insufficient mesoderm is formed in the caudal-most region of the embryo. Situs inversus is a condition in which transposition of the viscera in the thorax and abdomen occurs. Other conditions of abnormal sidedness are known as laterality sequences. Patients with these conditions do not have complete situs inversus but appear to be predominantly bilaterally left sided or right sided. Holoprosencephaly Hypotelorism Absence of nose 20 Sirenomelia- Caudal dysgenesis (caused by faulty gastrulation) 21 Tumors Associated With Gastrulation Sometimes, remnants of the primitive streak persist in the sacrococcygeal region. These clusters of pluripotent cells proliferate and form tumors, known as sacrococcygeal teratomas. These tumors may also arise from primordial germ cells that fail to migrate to the gonadal ridge.  Sacrococcygeal teratoma Arises from the persisting remnants of the primitive streak Females more affected Attached to the sacral region Contains bones, cartilages, hairs & nerve tissue Disturbances in the gastrulation V- vertebral defects A- Anal atresia C- cardiovascular anomalies TE- Tracheoesophageal fistula R- Renal anomalies L- Limb defects Development of the Trophoblast By the beginning of the third week, the trophoblast is characterized by primary Villi. mesodermal cells penetratethe core of primary villi and grow toward the decidua. formed structure known as a secondary villus. By the end of the third week, mesodermal cells in the core of the villus begin to differentiate into blood cells and small blood vessels, forming the villous capillary system (tertiary villus or definitive placental villus). Capillaries in tertiary villi make contact with capillaries developing in mesoderm of the chorionic plate and in the connecting Stalk.  cytotrophoblastic cells in the villi penetrate progressively into the overlying syncytium until they reach the maternal endometrium , forming a thin outer cytotrophoblast shell. Villi that extend from the chorionic plate to the decidua basalis , the part of the endometrium where the placenta will form;are called stem or anchoring villi. By the 19th or 20th day, the embryo is attached to its trophoblastic shell by a narrow connecting stalk. The connecting stalk later develops into the umbilical cord. Summary  3rd week - Formation of trilaminar germ disc - Primitive streak - Notochord Gastrulation: a process by which bilaminar germ disc is converted into trilaminar embryonic disc. Primitive streak appears at the 15th day. It is narrow median groove formed by ectodermal cell. Formation of secondary mesoderm from the primitive streak Primitive groove Primitive node in primitive streak Cut edge Embryonic of amnion ectoderm Migrating cells Embryonic endoderm Intraembryonic mesoderm QUESTIONS Good luck