Week 3 Trilaminar Germ Disk PDF

WE MAKE DOCTORS WEEK 3: TRILAMINAR GERM DISK DR. GUADALUPE RODRIGUEZ EMBRYOLOGY OBJECTIVES: Understand the process of gastrulation. Identify the 3 germinative layers in the third week: ectoderm, mesoderm and endoderm. Learn the formation process of the notochord. Identify the formation process of...

WE MAKE DOCTORS WEEK 3: TRILAMINAR GERM DISK DR. GUADALUPE RODRIGUEZ EMBRYOLOGY OBJECTIVES: Understand the process of gastrulation. Identify the 3 germinative layers in the third week: ectoderm, mesoderm and endoderm. Learn the formation process of the notochord. Identify the formation process of the cloacal membrane. Identify the establishment of the body axes and the genes responsible for that processes. Present the clinical correlation pathologies in this third week. GASTRULATION This is the most characteristic phenomenon on the 3rd week.  Amniotic cavity Syncitiotrophoblast Epiblast Hypoblast Cytotrophoblast Secondary yolk sac  Gastulation is the process that establishes all three germ layers (ectoderm, mesoderm, and endoderm) in the embryo. Gastrulation begins with formation of the primitive streak on the surface of the epiblast. Primitive node Prechordal plate The cephalic edge is the primitive nodule that is slightly elevated around the primitive pit. Cells of the epiblast will migrate toward the primitive streak. When these cells arrive at the region of the streak, they become flask-shaped, detach from the epiblast, and slip beneath it. This inward movement is known as INVAGINATION Cell migration and specification are controlled by Fibroblast Growth Factor 8 (FGF8) These are synthesized by streak cells themselves.. Once epiblast cells detach from the epiblast and invaginates, these cells will: Invade and displace the hypoblast, thus creating the embryonic ENDODERM Many of these epiblast cells will migrate into the space between the epiblast and the newly formed endoderm and create the intraembryonic MESODERM The remaining epiblast cells will form the ECTODERM. https://youtu.be/ADlYn0ImTNg  FORMATION OF THE NOTOCHORD Prenotochordal cells invaginating in the primitive node move forward cranially in the midline until they reach the Prechordal Plate. Prechordal plate These prenotochordal cells become intercalated in the hypoblast For a short time, the midline of the embryo consists of two cell layers that form the Notochordal Plate As the hypoblast is replaced by endoderm cells, cells of the notochordal plate proliferate and detach from the endoderm. Then they form the: DEFINITIVE NOTOCHORD 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 temporarily connects the amniotic and yolk sac cavities. The cloacal membrane is formed at the caudal end of the embryonic disc. This membrane, which is similar in structure to the oropharyngeal membrane, consists of tightly adherent ectoderm and endoderm cells with no intervening mesoderm. https://youtu.be/73k0k8qXAow?si=SsLx5F-MLmPVxpnk BODY AXES Establishment of the body axes Anterior-Posterior (A-P; Cranialcaudal) Dorso-Ventral (D-V) Left-Right (L-R) The establishment of these body axes occurs (approximately) during the morula stage of development with the A-P and D-V axes prior to the L-R. By the blastocyst stage, the A-P axis (cephalic and caudal ends of the embryo) is determined. This is before the primitive streak is formed The cells (hypoblast/endoderm) that are destined to form the anterior visceral endoderm (AVE) at the cranial end of the endoderm layer migrate toward what will become the head region. At this bilaminar disc stage, cells in the AVE express genes essential for head formation Transcription factors: OTX2 LIM1 HESX1 Secreted factors: CERBERUS (CER1) LEFTY1 (left-right determination factors)  CERBERUS and LEFTY1 factors are members of the TGF- Beta family (Transforming Growth Factor Beta). These secreted factors will inhibit NODAL (also member of the TGF-Beta Family), thereby establishing the cranial end of the embryo. The absence of CERBERUS and LEFTY1 at the caudal end of the embryo allows NODAL expression to continue. This signal establishes and maintains the primitive streak. Once the streak is formed, NODAL upregulates a number of genes responsible for formation of dorsal and ventral mesoderm and head and tail structures. BMP4 It is secreted throughout the embryonic disc. In the presence FGF and BMP4 the mesoderm will be ventralized to kidneys (intermediate mesoderm), blood, and body wall mesoderm (lateral plate mesoderm). Other genes (CHORDIN-GOOSECOID, noggin and follistatin) antagonize it to prevent everything being ventralized. This results in cranial mesoderm (paraxial mesoderm) being dorsalized into the notochord, somites and somitomeres. Conjoined twins If the gene GOOSECOID is overexpressed in frog embryos, the result is a two headed tadpole. Perhaps, overexpression of this gene explains the origin of this type of conjoined twins. BRACHURY (T) GENES controls the regulation of dorsal mesoderm formation in middle and caudal regions. Expressed: Node, Notochord Precursor Cells and Notochord Laterality (left-right asymmetry). FGF8 (secreted by cells in the node and streak) induces Nodal AND LEFTY2 expression on the left side (by accumulation of serotonin)→ these genes upregulate PITX2, which is g transcription factor and master gene for leftsidedness. Right sided development is related to SNAIL transcript factor expression. Normal L-R positioning of the organs is called Situs Solitus. Complete reversal of the positioning of the organs is called Situs Inversus. Situs solitus refers to the normal positioning of the internal organs. Situs inversus refers to a condition where the positioning of all organs is reversed in a mirror image arrangement. EMBYRONIC DISC GROWTH OF THE EMBRYONIC DISC The embryonic disc, initially flat and almost round, gradually becomes elongated, with a broad cephalic and a narrow caudal end. Expansion of the embryonic disc occurs mainly in the cephalic region; the region of the primitive streak remains more or less the same size.  CLINICAL CORRELATES  Two examples of sirenomelia [caudal dysgenesis]. Loss of mesoderm in the lumbosacral region has resulted in fusion of the limb buds and other defects.  Holoprosencephaly SEVERE MILD Sacrococcygeal teratoma. Resulting from remnants of the primitive streak. These tumors may become malignant and are most common in female fetuses References: Chapter 5. Third Week of Development: Trilaminar Germ Disk Bevis, R. (1992). Langman’s Medical Embryology, 6th edition T W Sadler Langman’s Medical Embryology, 6th Edition Williams & Wilkins 412pp £20.50 0-683- 07473-8. Nursing Children and Young People, 4(3), 14. https://doi.org/10.7748/paed.4.3.14.s14 Thank You!