Embryo Midterms PDF - Saint Louis University

EMBRYOLOGY SAINT LOUIS UNIVERSITY M.03 third week of development: trilaminar germ disc Dr. Ma. Gemma M. Pinlac | October 9, 2024...

EMBRYOLOGY SAINT LOUIS UNIVERSITY M.03 third week of development: trilaminar germ disc Dr. Ma. Gemma M. Pinlac | October 9, 2024 SOM 2028 OUTLINE o Ectoderm: epidermis, central and peripheral nervous system, eyes and internal ears, neural I. GASTRULATION: FORMATION OF GERM LAYERS........... 1 crest cells, and many connective tissues of the A. Gastrulation................................................ 1 head II. TERMINOLOGIES.............................................. 1 o Mesoderm: all skeletal muscles, blood cells, III. FORMATION OF NOTOCHORD.............................. 1 lining of blood vessels, all visceral smooth IV. ESTABLISHMENT OF BODY AXES............................. 2 muscular coats A. The Anterioposterior Axes.............................. 3 o Endoderm: source of the epithelial linings of the respiratory and alimentary (digestive) tracts, B. Dorsoventral Axes....................................... 3 including the glands opening into the C. Dorsal Mesoderm Formation.......................... 3 gastrointestinal tract and glandular cells of D. The Left-Right Sidedness (Laterality)................. 3 associated organs such as the liver and V. FATE MAP ESTABLISHED DURING GASTRULATION......... 4 pancreas VI. GROWTH OF EMBRYONIC DISC............................. 4 VII. FURTHER DEVELOPMENT OF THE TROPHOBLAST........... 5 VIII. CLINICAL CORRELATIONS.................................... 6 A. Conjoined Twins......................................... 6 B. Teratogenesis Associated with Gastrulation........ 6 C. Tumors Associated with Gastrulation................. 6 D. Birth Defects Associated with Laterality............. 6 IX. SUMMARY...................................................... 7 X. SUPPLEMENTAL VIDEO........................................ 8 Figure 2. Representative view of germ disc at the end of the XI. Checkpoint!.................................................. 9 second week of development XII. REFERENCES................................................... 10 II. TERMINOLOGIES  Priminitive node I. GASTRULATION: FORMATION OF GERM LAYERS o Cephalic end of the streak o Consists of a slightly elevated area surrounding A. GASTRULATION the small primitive pit  Process whereby the bilaminar disc undergoes  Invagination reorganization to form a trilaminar disc o Inward movement when the cells of the epiblast  Establishes all three primary germ layers from the epiblast migrates towards the primitive streak (ectoderm, mesoderm, and endoderm) in the embryo,  Fibroblast Growth Factor 8 (FGF8) these will later form all of the tissues and organs o Synthesized by the primitive streak cells  Begins with the primitive streak formation on the surface of o Controls cell migration and specification the epiblast o Down regulates E-cadherin o Epiblast: source of all germ layers and these ▪ Protein that binds the epiblast cells germ layers gives rise to all tissues and organs of together the embryo o Controls cell specification into the mesoderm by regulating BRACHYURY (T) expression. III. FORMATION OF THE NOTOCHORD  Prenotochordal cells o Invaginates in the primitive node and move forward cranially in the middle until they reach the prechordal plate  Implantation site at the end of the 2nd week ▪ Prechordal plate  The most characteristic even occurring during the third - Forms between the tip of the week of gestation notochord and the oropharyngeal membrane o Consists of a small region tightly adherent ectoderm and endoderm cells that represents the future opening of the oral cavity - Derived from some of the first cells that migrate through the node in the midline and Figure 1. Implantation site at the end of the 2nd week move in a cephalic direction  Is the beginning of morphogenesis and is the most significant event occurring during the third week. During this week, the embryo is referred to as a gastrula  This process is what established the three germ layer in the embryo GONZALES, PASCUAL, PINEDA ,ZAPATA Page 1 of 10  EMBRYOLOGY SAINT LOUIS UNIVERSITY M.03 third week of development: trilaminar germ disc Dr. Ma. Gemma M. Pinlac | October 9, 2024 SOM 2028 Figure 3. Primitive streak of embryo ▪ Become intercalated in the hypoblast so that for a short time, the midline of the embryo consists of two cell layer that form the notochordal plate - Notochordal plate will now Figure 4. A. Sagittal section of 17-day embryo. B. Cross section form the definitive notochord through region of notochordal plate. C. Definitive  Definitive notochord notochord. o Underlies the neural tube and is a signaling center for inducing the axial skeleton  Notochord 1. The cranial end forms first o Functions as a primary inductor 2. Caudal regions are added as the o It stimulates the development of the primitive primitive streak assumes a more vertebral column caudal position. o In induces the overlying ectoderm to thicken 3. Both notochord and prenotochordal and differentiate into the neural tube cells extend cranially to the prechordal  Remnants of Notochord plate and caudally to the primitive pit o The notochord degenerates with the beginning  Neurenteric canal of the formation of the vertebral body o Temporarily connects the amniotic and yolk sac o The remnants of the notochord cavities at the point where the pit forms an ▪ Nucleus pulposus of the intervertebral indentation in the epiblast disc  Cloacal membrane ▪ Apical ligament of dens o Opposite to oropharyngeal o Formed at the caudal end of the embryonic IV. ESTABLISHMENT OF THE BODY AXES disc  Three axes of the body are established in development via o Similar in structure to the oropharyngeal the expression of specific set of genes that regulate which membrane cells will develop into specific structures: o Consists of tightly adherent ectoderm and o Anterior posterior (A-P; craniocaudal), endoderm cells with no intervening mesoderm. o Dorso-ventral (D-V) o On appearance, the posterior wall of the yolk o Left-right (L-R) sac forms a small diverticulum that extends into  Occurs early in embryogenesis the connecting stalk called the allantoenteric  Probably initiated during late morula stage to blastocyst diverticulum stages with A-P and D-V axes specified prior to L-R. ▪ Allantoenteric diverticulum or Allantois  Blastocyst stage th - Appears around the 16 day o A-P axis determined of development. ▪ Cell from cranial end of endoderm - May serve as a reservoir for layer of bilaminar disc migrate towards excreting products of the what will become the head region to renal system in lower form the anterior visceral endoderm vertebrates (AVE) - In humans may be involved abnormalities of bladder development GONZALES, PASCUAL, PINEDA ,ZAPATA Page 2 of 10  EMBRYOLOGY SAINT LOUIS UNIVERSITY M.03 third week of development: trilaminar germ disc Dr. Ma. Gemma M. Pinlac | October 9, 2024 SOM 2028 ▪ Spemann’s organizer: gave the designation  Antagonistic genes of BMP4: o CHORDIN: activated by transcription factor Goosecoid o NOGGIN o FOLLISTATIN ▪ potential neural inducers ▪ as a result, cranial mesoderm is dorsalized into notochord, somites, and somitomeres  INVOLVED IN CONGENITAL ANOMALIES o NODAL: initiating and maintaining primitive streak o HNF-3β ▪ Maintains node and later induces regional specificity in forebrain and Figure 5. Establishment of the Body Axes midbrain areas A. THE ANTEROPOSTERIOR AXES ▪ Absence of HNF-3β: embryos fail to  Is signaled by cells at the anterior (cranial) margin of the gastrulate properly and lack forebrain embryonic disc. and midbrain structures  The Anterior Visceral Endoderm (AVE) at the cranial end of o GOOSECOID the endoderm layer of the bilaminar disc, cells from AVE ▪ activates inhibitors of BMP-4 and express genes for head formation including: contributes to regulation of the head o Transcription factors: OTX2, LIM1, and HESX1 development. Overexpression or o Secreted factors: cerberus and LEFTY1 under expression of this gene results in (members of the TGF-β family) severe malformations of the head  Cerberus and LEFTY1 inhibit NODAL (a member of the TGF-β region, including duplications family) activity hence establishing cranial end of embryo ▪ Ex. Conjoined twins o Absence of Cerberus and LEFTY1 at caudal end allows NODAL expression to continue C. DORSAL MESODERM FORMATION o This signal establishes and maintains primitive  BRACHYURY (T) gene controls regulation of dorsal streak mesoderm formation in the middle and caudal regions of  The primitive streak itself is initiated and maintained by the the embryo expression of NODAL, a member of the transforming growth o Expressed in the node, notochord precursor factor- β family (TGF- β) expression will induce and maintain cells, and notochord primitive streak o Responsible for cell migration through the  Once the streak is formed, a number of genes regulate primitive streak formation of dorsal and ventral mesoderm and head, tail o Encodes a sequence-specific DNA-binding structures protein that functions as a transcription factor o T-box: DNA-binding domain ▪ 20 genes in the T-box family ▪ Absence would result in caudal dysgenesis (embryonic axis shortening) ▪ Degree of shortening  The BRACHYURY (T) gene, encoding a transcription factor secreted by the notochord, is also essential for expression of NODAL, LEFTY1, LEFTY2 Figure 6. Sagittal section through the node and primitive streak o Expression of the transcription factor Snail is restricted to the right lateral plate mesoderm B. DORSOVENTRAL AXES o Probably regulates effector genes responsible  Once the streak is formed, NODAL upregulate genes for establishing the right side responsible for formation of dorsal and ventral mesoderm  Clinical Correlation: If absent – Caudal Dysgenesis and head and tail structures  Bone morphogenetic protein 4 (BMP4) is secreted throughout the embryonic disc  Presence of BMP4 with FGF: mesoderm will be ventralized to contribute to kidneys (intermediate mesoderm), blood, and body wall (lateral mesoderm) o If BMP4 activity is not blocked by other genes expressed in the node, all mesoderm would be ventralized o Node: organizer o Hans Spemann ▪ First described this activity in the blastophore’s dorsal lip (structure Figure 7. Caudal Dysgenesis analogous to the node) in Xenopus (clawed frogs) embryos GONZALES, PASCUAL, PINEDA ,ZAPATA Page 3 of 10  EMBRYOLOGY SAINT LOUIS UNIVERSITY M.03 third week of development: trilaminar germ disc Dr. Ma. Gemma M. Pinlac | October 9, 2024 SOM 2028 D. THE LEFT-RIGHT SIDEDNESS (LATERALITY) 3. The intermediate mesoderm at the middle of the streak  Established early in development 4. Lateral plate of mesoderm at the caudal part of the  Normally, many organs exhibit asymmetries: streak o Heart, lungs, gut, spleen, stomach, liver, and 5. EEM extraembryonic mesoderm at the most caudal others part o Positioning these organs and establishing their asymmetries is orchestrated by a cascade of signal molecules and genes  When primitive streak appears, cells of the node and primitive steak secrete FGF8 o Induces expression of NODAL  NODAL expression is then restricted to left side of embryo due to serotonin (5-HT) accumulation on the left side  LEFTY1 is expressed on the left side of the floor plate of the neural tube and may act as a barrier to prevent left-sided signals from crossing over.  Sonic hedgehog (SHH) may also function in this role as well as serving as a repressor for left-sided gene expression on the right  High concentrations of 5-HT activate transcription factor Figure 9. Dorsal view of the germ disc showing the primitive MAD3 expression to the left side of primitive node streak and a Fate map for epiblast cells.  From the left lateral plate, Nodal protein initiates a signaling cascade with LEFTY2 to upregulate PITX2  Prechordal plate and notochord  PITX2 o Formed from cells that ingress through cranial o Homeobox-containing transcription factor. region of the node o “Master gene” for left sidedness establishment  Paraxial mesoderm o Repeated expression in left side of heart, o Formed from cells that migrated to the lateral stomach, and gut primordial as they assume edges of the node and from the cranial end of their normal asymmetrical body positions the streak  Genes for right-side regulation not as well defined  Intermediate mesoderm o Transcription factor SNAIL expression is restricted o Formed from cells that migrated through to right lateral plate mesoderm. midstreak region ▪ Likely regulates effector genes for right  Lateral plate mesoderm side establishment. o Formed from cells that migrated through the more caudal part of the streak READING ASSIGNMENT: CASCADE INITIATED ON THE LEFT SIDE  Extraembryonic membrane Involve cilia on cells in the node that beat to create gradient o Formed from cells that migrated through the of NODAL towards the left most caudal part of the streak  Cloacal membrane Involve gap junctions and small ion transport that establishes o Formed from the caudal part signaling gradient  Other source is the primitive yolk sac (hypoblast) VI. GROWTH OF EMBRYONIC DISC  Initially flat and almost round, it become elongated with a broad cephalic and a narrow caudal end Figure 8. Dorsal views of germ disc showing gene expressions. Figure 10. Growth of embryonic disc A. FGF8 secreted by node and primitive streak. B.  Cephalic region: mainly where embryonic disc expansion Serotonin (5-HT) neurotransmitter increases concentration occurs in right side. o Continuous cell migration from primitive streak in cephalic direction causes its growth and V. FATE MAP ESTABLISHED DURING GASTRULATION elongation  The primitive streak is organized rostrocaudally for the o Primitive streak region remains the same size formation of the mesoderm; therefore, invaginating ▪ Surface cell invagination and epiblast cells develop into: migration forward and laterally 1. Notochord at the cranial end of the node at the continues until end of 4th week primitive pit.  Middle of 3rd week: germ layers in the cephalic part begin 2. Paraxial mesodermal at the lateral margins of the node their specific differentiation. and the cranial part of the streak o Organizer: primitive node GONZALES, PASCUAL, PINEDA ,ZAPATA Page 4 of 10  EMBRYOLOGY SAINT LOUIS UNIVERSITY M.03 third week of development: trilaminar germ disc Dr. Ma. Gemma M. Pinlac | October 9, 2024 SOM 2028 o By the end of 4th week:  Invagination of surface cells in the primitive streak and their subsequent migration forward and laterally.  Primitive streak at the caudal end of the disc continues to supply new cells.  Primitive streak starts to show regressive changes, shrink rapidly, and soon disappears.  Primitive streak diminishes in relative in size and becomes an insignificant Figure 12. A 13-day old implantation site showing primary villi structure insignificant structure in the of trophoblastic shell just beginning to be invaded by sacrococcygeal region of the embryo. mesoderm from the chorionic plate.  Normally the primitive streak  During further development: undergoes degenerative changes o Mesodermal cells penetrate the core of primary and disappears by the end of 4th week villi and grow toward the decidua o Secondary villus: newly formed structure o Sacrococcygeal Teratoma  By the end of 3rd week: ▪ Remnants of the primitive streak o Mesodermal cells in villus core differentiate into - A teratoma is a type of germ blood cells and small blood vessels forming the cell tumor. villous capillary system. o Can either be o Villus is now tertiary villus or definitive placental malignant or villus. benign o Tertiary villi capillaries contact developing - Derived from pluripotent capillaries in mesoderm of chorionic plate and primitive streak cell in connecting stalk. - Contain tissues derived from o Vessels establish contact intraembryonic all three germ layers in circulatory system connecting the placenta and incomplete stages of embryo. differentiation. o Tissues o Bone o Hair o Thus, gastrulation or germ layer formation continues in caudal segments while cranial structures are differentiating causing the embryo to develop. ▪ Embryo development: cephalocaudally Figure 13. Development of a villus. A. Transverse section o Cephalocaudally of primary villus showing a core of cytotrophoblastic cells ▪ Cephalic part: cell differentiation starts covered by a layer of syncytium. B. Transverse section of at the middle of the 3rd week a secondary villus with a core of mesoderm covered by ▪ Caudal part: cell migration up to the a single layer of cytotrophoblastic cells, which in turn is end of the 4th week; cell differentiation covered by syncytium. C. Mesoderm of villus showing a starts at the end of the 4th week number of capillaries and venules.  4th week of development: o Heart begins to beat o Villous system ready to supply the embryo proper with essential nutrients and oxygen  Cytotrophoblastic cells in villi penetrate progressively into overlying syncytium until the maternal endometrium is reached. o Establish contact with similar extensions of neighboring villous stems, forming a thin outer cytotrophoblast shell.  Cytotrophoblast shell surround trophoblast entirely and attaches chorionic sac firmly to the maternal endometrial tissue. Figure 11. Teratoma o This is where nutrients enter the embryo VII. FURTHER DEVELOPMENT OF THE TROPHOBLAST  Stem or anchoring villi: villi extending from the chorionic  Begininning of 3rd week: trophoblast characterized by plate and decidua basalis (decidual plate) primary villi with cytotrophoblast core covered by a o Decidual plate: part of endometrium where the syncytial layer. placenta will form. o Free (terminal) villi: branches from sides of stem villi o Where the exchange of nutrients and other factors occurs GONZALES, PASCUAL, PINEDA ,ZAPATA Page 5 of 10  EMBRYOLOGY SAINT LOUIS UNIVERSITY M.03 third week of development: trilaminar germ disc Dr. Ma. Gemma M. Pinlac | October 9, 2024 SOM 2028  Chorionic cavity becomes larger B. TERATOGENESIS ASSOCIATED WITH GASTRULATION  By 19th or 20th day:  Beginning of the third week of development (initiation of o Embryo attaches to its trophoblastic shell by a gastrulation) narrow connecting stalk o A highly sensitive stage to teratogenic insult o Connecting stalk develops into umbilical cord o Cell populations may be damaged by (connects placentae and embryo) teratogens  Pregnant woman may not take normal precautions because she may not recognize that she is pregnant because this stage is reached two weeks after fertilization, and approximately four weeks from the last menses. Holoprosencephaly  High doses of alcohol at this stage kill cells in the anterior midline of the germ disc, producing a deficiency of the midline in cranio-facial structures.  Characteristics in a child with this condition: o Small forebrain Figure 14. Presomite embryo and the trophoblast at the o Two lateral ventricles often merge into a single end of the third week. Tertiary and secondary stem villi ventricle give the trophoblast a characteristic radial appearance. o Hypotelorism (eyes are close together) Intervillous spaces, which are found throughout the o Gastrulation itself may be disrupted by genetic trophoblast, are lined with syncytium. Cytotrophoblastic abnormalities and toxic results. cells surround the trophoblast entirely and are in direct contact with the endometrium. The embryo is Caudal Dysgenesis (Sirenomelia) or Mermaid Syndrome suspended in the chorionic cavity by means of the  Insufficient mesoderm is formed in the caudal-most region connecting stalk. of the embryo  Abnormalities in the following structures ensue because mesoderm contributes to formation of lower limbs, urogenital system (intermediate mesoderm), and lumbosacral vertebrae  Individuals exhibit a variable range of defects, including hypoplasia and fusion of the lower limbs, vertebral abnormalities, renal agenesis, imperforate anus, and genital organ anomalies  In humans: associated with maternal diabetes and other cases  In mice: abnormalities of BRACHYURY (T), WNT, and ENGRAILED genes produce a similar phenotype Figure 15. Longitudinal section through a villus at the end of the fourth week of development. Maternal vessels penetrate the cytotrophoblastic shell to enter intervillous spaces, which surround the villi. Capillaries in the villi are in contact with vessels in the chorionic plate and in the connecting stalk, which in turn are connected to intraembryonic vessels VIII. CLINICAL CORRELATIONS A. CONJOINED TWINS  Over or under expression of GOOSECOID (activator of BMP4 Figure 17. Examples of (A) Holoprosencephaly and (B) Caudal inhibitors and contributes to head development regulation) dysgenesis  Severe malformations in head regions C. TUMORS ASSOCIATED WITH GASTRULATION  Includes duplications such as in some types of conjoined Sacrococcygeal teratomas twins  Remnants of the primitive streak persist in the sacrococcygeal region sometimes. These clusters of pluripotent cells proliferate and form tumors  Commonly contain tissues derived from all three germ layers  most common tumor in newborns (1:37,000)  Teratomas may also arise from primordial germ cells that fail to migrate to the gonadal ridge  Develops from a baby’s coccyx  Commonly contain tissues from all three germ layers  Probably resulting from remnants of the primitive streak  Normally, the primitive streak under goes degenerative Figure 16. Conjoined twins process, and will diminish by the end of the 4th of gestation GONZALES, PASCUAL, PINEDA ,ZAPATA Page 6 of 10  EMBRYOLOGY SAINT LOUIS UNIVERSITY M.03 third week of development: trilaminar germ disc Dr. Ma. Gemma M. Pinlac | October 9, 2024 SOM 2028  May also arise from primordial germ cells that may become  Do not have complete situs inversus but appear to be malignant predominantly bilaterally left-sided or right-sided  Most commonly seen in female fetuses o Spleen  Can be removed surgically ▪ Left-sided bilaterality: polysplenia o Prognosis is good if removed as early as possible ▪ Right sided bilaterality: asplenia or hypoplastic spleen  High risk of having wide variety of other birth defects: o Midline malformation ▪ Neural tube defects ▪ Cleft palate ▪ Anal atresia  90%: complex congenital heart defects o Heart exhibits more laterality than most organs; increased susceptibility when L-R signaling pathway is disrupted  X-Linked Heterotaxy o Caused by mutations in the zinc finger transcription factor Z/C3, a gene located on the Figure 18. Sacrococcygeal teratoma X chromosome o Have variety of birth defects: A. BIRTH DEFECTS ASSOCIATED WITH LATERALITY ▪ Neural tube defect  Failure to properly establish the L-R axis, when genes are ▪ Limb abnormalities expressed ectically (e.g., on the right side) ▪ Omphalocele: herniation of abdominal viscera into the intact Situs Solitus umbilical cord ▪ Heart malformations  Normal positioning of the internal organs  Serotonin (5-hydroxytryptamine) o Important signalling molecule (neurotransmitter) Situs Inversus for establishing laterality  A condition where the positioning of all organs is reversed in o Animal studies show that altering 5-HT signaling a mirror image arrangement can result in  Do not have high risk for having other congenital ▪ Situs invertus abnormalities ▪ Dextrocardia  Have slightly greater risk of having heart defect ▪ Heterotaxy  Progeny: increased risk for having laterality defects and ▪ Wide variety of heart defects even higher risk for having severe cardiac malformation o Similar effects in humans when 5-HT is disrupted  Approximately 20% of patients with complete situs inversus by pharmaceutical agents have: o Mothers who antidepressants from drug classes o Kartagener Syndrome (abnormal cilia) known as selective serotonin reuptake inhibitors ▪ Causes bronchiechtasis and chronic (SSRIs): sinusitis ▪ Examples: Prozac, Paxil, Zoloft, ▪ Cilia is normally present on the primitive Lexapro, Celexa, etc. node’s ventral surface and may be ▪ Children born to this mother: increased involved in L-R patterning risk of heart malformations and  Serotonin (5-HT) multiple birth defects o Important molecule in establishing laterality o Association between laterality and midline defects suggests that signaling pathways establishing A-P and L-R axes must interact to specify correct positioning of body’s organs and other structures  Because the body axes start to be specified late in the first week of development: birth defects can possibly be caused by disruption of these patterning events IX. SUMMARY  The most characteristic event occurring during the third Figure 19. Situs inversus week is gastrulation. Situs ambiguous or Heterotaxy  Gastrulation begins with the appearance of the primitive  Discordant organ positioning with respect to symmetry, streak, which has its cephalic end the primitive node. where one or more organs are abnormally reversed in  In the region of the node and streak, epiblast cells move position or if isomerism or inversion inward (to invaginate) to form new cell layers, endoderm o Isomerism: e.g., both atria in the heart look the and mesoderm. same  Cells that do not migrate through the streak but remain in o Inversion: e.g., two ventricles in the heart are the epiblast form ectoderm. reversed GONZALES, PASCUAL, PINEDA ,ZAPATA Page 7 of 10  EMBRYOLOGY SAINT LOUIS UNIVERSITY M.03 third week of development: trilaminar germ disc Dr. Ma. Gemma M. Pinlac | October 9, 2024 SOM 2028  Epiblast gives rise to all three germ layers in the embryo; ectoderm, mesoderm, and endoderm, and these layers form all of the tissues and organs.  Prenotochordal cells invaginating in the primitive pit move forward until they reach the prechordal plate. They intercalate in the endoderm as the notochordal plate. With further development, the plate detaches from the endoderm, and a solid cord, the notochord, is formed.  Notochord forms a midline axis, which will serve as the basis of the axial skeleton.  Cephalic and caudal ends of the embryo are established before the primitive streak is formed.  Thus,cells in the hypoblast(endoderm) at the cephalic margin of the disc form the AVE (anterior visceral endoderm), which expresses head-forming genes, including OTX2, LIM1, HESX1 and the secreted factor Figure 21. Gastrulation Cerberus.  15 days after fertilization  Neurotransmitter serotonin (5-HT) plays a role in establishing o Primitive streak laterality. ▪ A thickened structure that is formed  Situs solitus is the normal left-right positioning of the organs. along the midline in the epiblast near  Situs inversus complete reversal of the organs. the caudal end of the bilaminar  Situs ambiguous (heterotaxy) one or more organs are embryonic disc. abnormally positioned. ▪ Its formation defines the major body  By the end of the third week, three basic germ layers, axes of the embryo including the consisting of ectoderm, mesoderm, endoderm, established cranial end toward the head and the in the head region, and the process continues to produce cauda end towards the tail as well as these germ layers for more caudal areas of embryo until the the left and right side of the embryo. end of the fourth week.  Tissue and organ differentiation has begun, and it occurs in a cephalocaudal (from head to toe) direction as gastrulation continues.  In the meantime, the trophoblast progresses rapidly.  Primary villi obtain a mesenchymal core in which small capillaries arise.  Villous capillaries make contact with the capillaries in the chorionic plate and connecting stalk, the villous system is ready to supply the embryo with its nutrients and oxygen. X. SUPPLEMENTAL VIDEO  Gastrulation o Process whereby the bilaminar embryonic disc undergoes reorganization to form a trilaminar disc  End of 2nd week o Bilaminar embryonic disc consisting of the hypoblast and epiblast has formed throughout Figure 22. Primitive streak formation the 3rd week of development.  3rd week of development ▪ Cranial end: primitive streak expands o Differentiates to establish three primary germ to create a primitive node which layers in a process known as gastrulation. contains a circular depression known as primitive pit ▪ Primitive pit: continues along the midline of the epiblast towards the caudal end of the primitive streak forming a primitive groove. Figure 20. Formation of epiblast and hypoblast GONZALES, PASCUAL, PINEDA ,ZAPATA Page 8 of 10  EMBRYOLOGY SAINT LOUIS UNIVERSITY M.03 third week of development: trilaminar germ disc Dr. Ma. Gemma M. Pinlac | October 9, 2024 SOM 2028 Figure 23. Primitive node Figure 26. Invagination  Day 16 o Majority of the hypoblast has been replaced. o Ectoderm: remaining cells whicgh form the most exterior distal layer Figure 24. Primitive pit Figure 27. Ectoderm o Some of the invaginated epiblast cells remain in the space between the ectoderm and newly formed definitive endoderm. o These cells form a germ layer known as the mesoderm. Figure 25. Primitive groove  Invagination: once formed, cells of the epiblast migrate inwards towards the streak, detach from the epiblast and slip beneath into the interior of the embryo o First cells to invaginate through the primitive grove invade the hypoblast and displaced its cells. o Hypoblast cells are eventually replaced by a new proximal cell layer which is referred to as the definitive endoderm Figure 28. Mesoderm o Once the formation of the definitive endoderm and mesoderm are complete, epiblast cells no longer migrate towards the primitive streak. o Throughout gastrulation, the ectoderm continues to form from the cranial end to the caudal end of the embryo, establishing three GONZALES, PASCUAL, PINEDA ,ZAPATA Page 9 of 10  EMBRYOLOGY SAINT LOUIS UNIVERSITY M.03 third week of development: trilaminar germ disc Dr. Ma. Gemma M. Pinlac | October 9, 2024 SOM 2028 distinct primary germ layers throughout the whole embryonic disc.  In summary: ○ The first cells to invaginate through the primitive groove form the definitive endoderm. ○ The remaining cells of the epiblast are called ectoderm. o Cells that remain in the space between the ectoderm and definitive endoderm form a layer called the mesoderm. XI. CHECKPOINT! 1. Describes the inward movement when the cell of the epiblast migrates towards the primitive streak. 2. A condition where the positioning of all organs is reversed in a mirror image arrangement. 3. During what day does the allantoenteric diverticulum or allantois appear? 4. The gene that controls the regulation of dorsal mesoderm formation in the middle and caudal regions of the embryo. 5. Important signaling molecule for establishing laterality. 6. T/F: In the definitive notochord, the caudal end forms first. 7. The structure that temporarily connects the amniotic and yolk sac cavities. 8. A condition wherein there is an insufficient formation in the caudal-most region of the embryo. 9. The heart begins to beat during what week of development? 10. The master gene for left sidedness establishment. PITX2 10. 4th week 9. Caudal dysgenesis 8. Neurentic canal 7. F; the cranial end forms first 6. Serotonin 5. BRACHYURY (T) gene 4. Day 16 3. Situs Inversus 2. Invagination 1. XII. REFERENCES Sadler T.W. (2019). Third week of development: Trilaminar germ disc. Langman’s Medical Embryology (14th ed., pp. 59-71). Wolters Kluwer. Moore, K., et al. (2013). The Developing Human: Clinically Oriented Embryology. (9th edition). https://www.youtube.com/watch?v=3AOoikTEfeo GONZALES, PASCUAL, PINEDA ,ZAPATA Page 10 of 10  Embryology SAINT LOUIS UNIVERSITY M.02 Embryonic Period: 3rd to 8th Week Dr. Ma. Gemma M. Pinlac | October 16, 2024 SOM 2028 OUTLINE At the end of the 3rd week of development o The ectoderm has a shape of a discs that is I. EMBRYONIC PERIOD........................................... 1 broader in the cephalic than in the caudal II. DERIVATIVES OF THE ECTODERMAL GERM LAYER........ 1 region A. Molecular Regulation of Neural Induction.......... 1 o Neural plate B. Neurulation............................................... 2 ▪ Appearance of the notochord and C. Neural Crest Cells....................................... 3 pre-chordal mesoderm induces the overlying ectoderm to thicken and D. Molecular Regulation of Neural Crest Induction.. 3 form the neural plate E. Other Ectodermal Derivatives........................ 4 ▪ Neuroectoderm F. Summary of Ectoderm.................................. 4 III. DERIVATIVES OF THE MESODERMAL GERM LAYER........ 4 A. MOLECULAR REGULATION OF NEURAL INDUCTION A. Paraxial Mesoderm...................................... 4 B. Intermediate Mesoderm............................... 6 Upregulation of FGF (Fibroblast growth factor) signaling C. Lateral Mesoderm....................................... 6 together with the inhibition of the activity of BMP4 D. Blood and Blood Vessels................................ 7 (Bone morphogenetic protein 4) causes the induction E. Clinical Correlation...................................... 7 of the neural plate FGF signaling promotes the pathway for neural tube IV. DERIVATIVES OF THE ENDODERM GERM LAYER........... 8 development by an unknown mechanism, while it V. Checkpoint..................................................... 8 represses BMP transcription VI. References..................................................... 8 FGF also upregulate the expression of CHORDIN, NOGGIN, and FOLLISTATIN which further inhibits BMP activity; I. EMBRYONIC PERIOD o These proteins are present in the organizer (Primitive node), prechordal mesoderm, and A.K.A period of organogenesis notochord o Organogenesis: process where the 3 germ o In the early weeks of development, they layers (ectoderm, mesoderm and endoderm) induce the mesoderm to become the differentiate and give rise to a number of notochord and paraxial mesoderm specific tissues. o The presence of these three proteins sets the o This is also the period where majority of birth ECTODERM by default to become NEURAL defects are induced; prior to this time, any TISSUE (only forms the forebrain and midbrain) teratogen can result to fetal death and WNT3A and FGF causes the formation of caudal neural spontaneous abortion plate structures (Hindbrain and Spinal Cord) o Retinoic acid appears to play a role in organizing the cranial-to-caudal axis of the II. DERIVATIVES OF THE ECTODERMAL GERM LAYER future brain; it causes re-specification of cranial segments into more caudal ones by regulation expression of Homeobox genes However, with the presence of BMP; ectoderm will become epidermis and mesoderm forms intermediate and lateral plate mesoderm Figure 2. (A) Dorsal view of a 16-day presomite embryo. The primitive streak and primitive node are visible. (B) Dorsal view of an 18-day presomite embryo. The embryo is pear-shaped, with its cephalic region somewhat broader than its caudal end. Figure 1. Signals from notochord cause changes to ectoderm BULWAYAN, PISCAWEN, GAGNI, ZAPATA Page 1 of 8  Embryology SAINT LOUIS UNIVERSITY M.02 Embryonic Period: 3rd to 8th Week Dr. Ma. Gemma M. Pinlac | October 16, 2024 SOM 2028 B. NEURULATION Is the process whereby the neural plate forms the neural tube Regulated by Planar Cell Polarity Pathway which is essential for neural tube development As the thickened neural plate lengthens, its lateral edges elevate to form neural folds, and the depressed mid- region forms the neural groove. Gradually, the neural folds approach each other in the midline, where they fuse; fusion begins in the cervical region (5th somite) then fusion proceeds cranially and caudally. End result is the formation of neural tube which is still open cranially and caudally; One of the key events in this process is lengthening of the neural plate and body axis by the phenomenon of convergent extension. o Lateral to medial movement of cells Figure 3.2. (A) Dorsal view of an embryo at approximately day 22. Seven distinct somites are visible on each side of the neural tube. (B) Dorsal view of a human embryo at 21 days. (C) Dorsal view of an embryo at approximately day 23. Note the pericardial bulge on each side of the midline in the cephalic part of the embryo. (D) Dorsal view of a human embryo at 23 days. Neurulation is complete when: o Closing of the anterior and posterior neuropore at day 25 and 28 o CNS is represented by a closed tubular structure with a narrow caudal portion (spinal cord) and a much broader cephalic portion characterized by a number of dilation (brain vesicles) Figure 3.1. (A) Dorsal view of a late presomite embryo approximately 19 days). The amnion has been removed, and the neural plate is clearly visible. (B) Dorsal view of a human embryo at 19 days. (C) Dorsal view of an embryo at approximately 20 days showing somites and formation of the neural groove and neural folds. (D) Dorsal view of a human embryo at 20 days. Figure 4. (A) Lateral view of a 14-somite embryo (approximately 25 days). Note the bulging pericardial area and the first and second pharyngeal arches. (B) The left side of a 25-somite BULWAYAN, PISCAWEN, GAGNI, ZAPATA Page 2 of 8  Embryology SAINT LOUIS UNIVERSITY M.02 Embryonic Period: 3rd to 8th Week Dr. Ma. Gemma M. Pinlac | October 16, 2024 SOM 2028 embryo approximately 28 days old. The first three pharyngeal well as neurons for cranial ganglia, glial cells arches and lens and otic placodes are visible. and melanocytes. C. NEURAL CREST CELLS Neural crest derivatives o Connective tissue and bones of the face As the neural folds elevate and fuse, cells at the lateral and skull border or crest of the neuroectoderm begin to dissociate from their neighbors. o Cranial nerve ganglia This dissociation gave rise to the population of neural o C cells of the thyroid gland crest cells o Conotruncal septum in the heart Undergoes epithelial-to-mesenchymal transition as it o Odontoblasts leaves the neuroectoderm by active migration and o Dermis in face and neck displacement lo enter the underlying mesoderm o Spinal (dorsal root) ganglia Fundamentally important because they contribute to o Sympathetic chain and preaortic ganglia the development of many organs and tissues thus they are termed as fourth germ layer o Parasympathetic ganglia of the Also involved in 1/3 of all birth defects and many gastrointestinal tract cancers, such as melanomas, neuroblastomas and o Adrenal medulla others o Schwann cells o Glial cells o Menges (forebrain) o Melanocytes o Smooth muscle cells to blood vessels of the face and forebrain D. MOLECULAR REGULATION OF NEURAL CREST INDUCTION The fate of the entire ectodermal germ o High levels - ectoderm undergoes epidermal formation o Intermediate levels - induction of neural crest cells at the border of the neural plate and surface ectoderm o Low levels - ectoderm undergoes neural plate formation BMPs, other members of the TGF-ß and FGFs, regulate NCC migration, proliferation, and differentiation. o Abnormal concentrations of these proteins have been associated with neural crest defects in the craniofacial region of lab animals. Figure 5. Formation of neural tube Crest cells from the trunk region leave the neuroectoderm after closure of the neural tube and migrate along one of 2 pathways: o Dorsal pathway where they will enter the ectoderm through holes in the basal lamina to form melanocytes in the skin and hair follicles o Ventral Pathway through the anterior half of each somite to become sensory ganglia, sympathetic and enteric neurons, Schwann cells and the cells of the adrenal medulla o Before closure of the neural tube, some NCC migrate away and contribute to the development of the craniofacial skeleton as BULWAYAN, PISCAWEN, GAGNI, ZAPATA Page 3 of 8  Embryology SAINT LOUIS UNIVERSITY M.02 Embryonic Period: 3rd to 8th Week Dr. Ma. Gemma M. Pinlac | October 16, 2024 SOM 2028 E. OTHER ECTODERMAL DERIVATIVES By the time the neural tube li closed, two bilateral ectodermal thickenings has formed, the olic placodes and the lens placodes, Otic placodes will invaginate and form the otic vesicles which will eventually develop intro structures needed for hearing and maintenance of equilibrium Lens placodes forms the lenses of the eyes. F. SUMMARY OF ECTODERM In general, the ectodermal germ layer gives rise to organs and structures that maintain contact with the outside world; o The central nervous system o The peripheral nervous system o The sensory epithelium of ihe ear, nose, and eye o The epidermis, including the hair and nails In addition, it gives rise to the following: o The subcutaneous glancis o The mammary glands o The pituitary gland o Enamel of the teeth III. 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. By approximately the 17th day, however, cells close to the By approximately the 17th day, however, cells close to the midline proliferate and form a thickened plate Figure 6. Formation of the neural tube and neural crest cells of tissue known as paraxial mesoderm. More (NCC) during early vertebrate embryogenesis. The neural laterally, the mesoderm layer remains thin and is tube (blue) develops by convolution of the neural plate dorsal known as the lateral plate. With the appearance to the notochord (red). At the neural plate border (green), and coalescence of intercellular cavities in the neural crest cells are specified, and finally migrate away on lateral plate, this tissue is divided into two layers. distinct pathways after closure of the neural tube. o A layer continuous with mesoderm covering the amnion, known as the somalic Induction of NCC requires and interaction of the or parietal mesoderm layer JUNCTIONAL BORDER of the neural plate and surface o A layer continuous with mesoderm ectoderm covering the yolk sac, known as the Intermediate concentrations of BMPs are established at splanchnic or visceral mesoderm layer this boundary (Neural crest cells) compared to neural Together, these layers line a newly formed cavity, the plate cells that are intraembryonic cavity, which is continuous with the exposed to very low levels of BMPs and surface extraembryonic cavity on each side of the embryo, ectoderm cells (epidermis) that are exposed to very Intermediate mesoderm connects paraxial and lateral high levels plate mesoderm. Proteins NOGGIN and CHORDIN regulate these concentrations by acting as BMP inhibitors A. PARAXIAL MESODERM Intermediate concentrations of BMPs, together with FGF and WNI proteins induce PAX3 and other By the beginning of the third week, paraxial mesoderm transcription factors that specify the neural plate begins to be organized into segments. border. These segments, known as somitomeres, first appear in These transcription factors induce a second wave of the cephalic region of the embryo, and their formation transcription factors: proceeds cephalocaudally. o SNAIL AND FOXD3 which specify cells as Each somitomere consists of mesodermal cells neural crest arranged in concentric whorls around the center of the o SLUG promotes crest cell migration from the unit. In the head region, somitomeres form in neuroectoderm association with segmentation of the neural plate into neuromeres and contribute to mesenchyme in the head BULWAYAN, PISCAWEN, GAGNI, ZAPATA Page 4 of 8  Embryology SAINT LOUIS UNIVERSITY M.02 Embryonic Period: 3rd to 8th Week Dr. Ma. Gemma M. Pinlac | October 16, 2024 SOM 2028 Somites appear with a specified periodicity; the age of an embryo can be accurately determined during this early time period by counting somites. MOLECULAR REGULATION OF SOMITE FORMATION Formation of segmented somites from unsegmented presomitic (paraxial) mesoderm depends on a segmentation clock established by cyclic expression of several genes. The cyclic genes include members of the NOTCH and WNT signaling pathways thai are expressed in an oscillating pattern in presomitic mesoderm. Thus, Notch protein accumulates in presomitic mesoderm destined to form the next somite and then decreases as that somite is established The increase in Notch protein activates other segment- pattering genes that establish the somite. Boundarles for each somite are regulated by relinoic acid (RA) and Figure 7. Transverse sections showing development of the a combination oi FGF8 and WNT3a mesodermal germ layer. (A) Day 17. (B) Day 19. (C). Day 20. (D) Day 21. The thin mesodermal sheet gives rise to paraxial mesoderm (future somites), intermediate mesoderm (future excretory units), and the lateral plate, which is split into parietal and visceral mesoderm layers lining the intraembryonic cavity. Table 1. Number of somites correlated to approximate age in days SOMITE DIFFERENTIATION When somites first form from presomitic mesoderm, they exist as a ball of mesoderm (fibroblast-like) cells, these cells then undergo a process of epithelization and arrange themselves in a donut shape around a small lumen. By the beginning of the fourth week, cells in the ventral and medial walls of the somite lose their epithelial characteristics become mesenchymal (fibroblast-like) again, and shift their position to surround the neural tube and notochord Colleclively, these cells form the sclerotome that will differenliate into the vertebrae and ribs. Cells at the dorsomedial and ventrolateral edges of the upper region of the somite form precursors for muscle cells, Figure 8. Dorsal view of somites forming along the neural tube whereas cells between these two groups form the (the ectoderm has been partially removed). Somites form from dermatome. unsegmented presomitic paraxial mesoderm caudally and Cells from both muscle precursor groups become become segmented in more cranially positioned regions. mesenchymal again and migrate beneath the dermatome to create the dermomyolome. Cells from From the occipital region caudally, somitomeres further both muscle precursor groups become mesenchymal organize into somites. The first pair of somites arises in again and migrate beneath the dermatome to creale the occipital region of the embryo at approximately the dermomyotome. the 20 th day of development. Each myotome and dermatome retains its innervation New somites appear in craniocaudal sequence at a from its segment of origin, no matter where the cells rate of approximately three pairs per day until, at the migrate. Hence, each somite forms its own sclerotome end of the fifth week, 42 to 44 pairs are present. (the tendon cartilage and bone component), its own There are 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, myotome (providing the segmental muscle 5 sacral, and 8 to 10 coccygeal pairs. component), and its own dermatome, which forms the The first occipital and the last five to seven coccygeal dermis of the back. Each myolome and dermatome somites later disappear, whereas the remaining somites also has its own segmental nerve component. form the axial skeleton. BULWAYAN, PISCAWEN, GAGNI, ZAPATA Page 5 of 8  Embryology SAINT LOUIS UNIVERSITY M.02 Embryonic Period: 3rd to 8th Week Dr. Ma. Gemma M. Pinlac | October 16, 2024 SOM 2028 Figure 9. Somite Differentiation 4th week o Cells in the ventral and medial wall lose their epithelial characteristic, become Figure 10.1. Both groups of muscle precursor cells become mesenchymal (fibroblast-like) again, and mesenchymal and migrate beneath the dermatome to form the surround the neural tube and notochord dermomyotome, while some cells from the ventrolateral group o SCLEROTOME (differentiate into ribs and also migrate into the parietal layer of lateral plate mesoderm. vertebrae) Dermamyotone o Cells from both the muscle precursorgroups become mesenchymal and migrate beneath the dermatome to create the dermamyotome o Forms dermis of Skin of back, muscles for back, body wall (intercostal muscles), limb muscles Also, cells from the ventrolateral edge migrate into the parietal layer of lateral plate mesoderm to form most of the musculature for the body and most of limb muscles Each myotome and dermatome retains its innervation, no matter where cells migrate Each somite forms own sclerotome, myotome, and dermatome o Sclerotome – tendon cartilage and bone component o Myotome – provides the segmental muscle component o Dermatome – dermis of the back B. INTERMEDIATE MESODERM Figure 10. Cells from the ventral and medial walls of the somite Intermediate mesoderm, which temporarily connects lose their epithelial arrangement and migrate around the neural paraxial mesoderm with the lateral plate, differentiates tube and notochord. Collectively, these cells constitute the into urogenital structures. In cervical and upper sclerotome that will form the vertebrae and ribs. Meanwhile, cells thoracic regions, it forms segmental cell clusters (future at the dorsomedial and ventrolateral regions differentiate into nephrolomes), whereas more caudally, it forms an muscle precursor cells, while cells that remain between these unsegmented mass of tissue, the nephrogenic cord. locations form the dermatome. Excretory units of the urinary system and the gonads develop from this partly segmented, partly unsegmented intermedi