Early Structures Notes


Osmosis High-Yield Notes

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Development of the digestive system and body cavities

Development of the fetal membranes

Development of twins

Hedgehog signaling pathway

Development of the placenta

Development of the umbilical cord

NOTES NOTES EARLY STRUCTURES DEVELOPMENT OF THE DIGESTIVE SYSTEM & BODY CAVITIES osms.it/digestive-system-and-body-cavities-development ▪ Endoderm forms gut tube epithelium ▪ The rest derives from mesoderm ▫ Around week 3 lateral mesoderm splits into parietal (somatic) mesoderm → adheres to ectoderm; visceral (splanchnic mesoderm) → adheres to endoderm ▫ Space between the split is called intraembryonic coelom/intraembryonic cavity ▫ Eventually becomes thoracic, abdominal cavity ▫ Parietal mesoderm → gives rise to serous membrane that lines abdominal, thoracic cavity (parietal pleural, peritoneal, pericardial membrane), soft tissues of arms, legs ▫ Visceral mesoderm → gives rise to serous membrane that lines various organs (visceral pleural, peritoneal, pericardial membrane), muscular wall of gut, heart, circulatory system ▫ Two visceral peritoneal membranes come together, form mesentery (suspends gut tube in abdominal cavity) Figure 28.1 During week 3, lateral mesoderm splits into parietal and visceral mesoderm. They give rise to serous membranes that cover various body parts. OSMOSIS.ORG 209
▪ During week 4 embryo begins to curl into fetal position ▫ Combined visceral mesoderm, endoderm layer folds rostrally, caudally → shapes part of yolk sac forming primitive gut tube ▫ The rest of yolk sac is connected in middle via vitelline duct ▫ Combined parietal mesoderm, ectoderm folds down with amnion forming lateral body folds ▫ Eventually merge, become anterior body wall of embryo Figure 28.2 Appearance of the embryo in week 4 in caudal (A) and mid- (B) sections. DEVELOPMENT OF BODY CAVITIES ▪ Around day 22, thick plate of visceral mesoderm called septum transversum forms just cranial to vitelline duct ▫ Divides primitive body cavity into thoracic cavity, abdominal cavities ▪ Around week 5, two tissue flaps called the pleuropericardial folds, grow out of lateral body wall, fuse together ▫ Divides thoracic cavity into two pleural 210 OSMOSIS.ORG cavities, one pericardial cavity ▪ In week 7, pleuropericardial folds extend ventrally from body wall, fuse with septum transversum ▫ Forms pleuroperitoneal membrane, which seals thoracic cavity from peritoneal cavity ▫ Pericardial, pleural cavities in thorax, peritoneal cavity in abdomen
Chapter 28 Embryology: Early Structures Figure 28.3 Development of body cavities on day 22 and during weeks 5 and 7. OSMOSIS.ORG 211
DEVELOPMENT OF THE DIAPHRAGM ▪ Develops from four components ▫ Septum transversum, pleuroperitoneal membranes, dorsal mesentery of esophagus, from somites at levels C3– C5 ▪ Mesodermal cells from third, fourth, fifth pairs of somites penetrate pleuroperitoneal membranes ▫ Form muscular portion of diaphragm ▪ Septum transversum forms tendinous portion of diaphragm ▪ Mesoderm of lumbar region gives rise to two crura of diaphragm Figure 28.4 Location of the developed diaphragm. THE HEDGEHOG SIGNALING PATHWAY osms.it/hedgehog-signaling-pathway ▪ Pathway which plays key role in structuring general body shape ▪ Mediated by sonic hedgehog proteins secreted by notochord → proteins diffuse through interstitial fluids ▪ Functions of sonic hedgehog protein ▫ Binds to patched receptor on embryonic cell membrane ▫ Patched receptor inhibits cell differentiation; sonic hedgehog protein inhibits patched ▫ Inhibits the inhibitor → facilitates cell differentiation 212 OSMOSIS.ORG Figure 28.5 The notochord is a solid line of mesoderm at the center of the embryo. It secretes different kinds of hedgehog proteins.
Chapter 28 Embryology: Early Structures ▫ Sonic hedgehog proteins control gene expression → gene expression depends on amount of sonic hedgehog protein reaching embryonic cells, duration of exposure ▫ Notochord secretes different kinds of hedgehog proteins ▫ Embryonic cells are exposed to different combinations of proteins that helps distinguish their position relative to each other (awareness in space), their course of differentiation REGULATION BY HOMEOBOX GENES ▪ Homeobox genes code for transcription factors that activate gene cascades which regulate segmentation, craniocaudal patterning ▪ Homeobox gene products are transcription factors called Hox proteins ▪ Homeobox genes arranged into four clusters on four different chromosomes ▫ HOXA, HOXB, HOXC, HOXD ▪ Genes toward 3’ end of chromosomes control cranial structure development, genes toward 5’ end control caudal structure development ▪ Highly conserved genes across vast evolutionary distances ▫ Demonstrated by the fact that a fly can function perfectly well with chicken Hox protein its place ▪ Mutations in Hox genes can result in body parts, limbs in the wrong place along the body e.g. extra fingers/toes Figure 28.6 SHH and other notochord proteins diffuse through the embryo, creating a concentration gradient that tells embryonic cells where they are located in three dimensional space. The unique combinations of proteins determine into which tissues the cells differentiate. OSMOSIS.ORG 213
DEVELOPMENT OF THE PLACENTA osms.it/placenta-development ▪ The placenta is co-created by fetus, mother ▪ Around day 14, syncytiotrophoblast cells form little protrusions called primary villi ▫ Villi form all the way around fetus ▪ Cells clear out from between primary villi ▫ Leave behind empty spaces called lacunae ▪ Maternal arteries. veins grow into decidua basalis, merge with lacunae ▫ Maternal arteries fill lacunae with oxygenated blood ▫ Maternal veins pick up deoxygenated blood ▫ Junctional zone formed as arteries, veins continue to merge ▪ Formation of feto-placental circulation begins on day 9 ▫ Day 9, lacunar stage: vacuoles form lacunae in syncytiotrophoblast; endometrial sinusoids start to grow into decidua basalis ▫ Day 12: sinusoids merge with syncytial lacunae, filling them with blood ▫ Day 14: cells of cytotrophoblast penetrate syncytiotrophoblast, form primary villi ▫ Day 16: extraembryonic mesoderm cells penetrate into primary villi forming secondary villi, later differentiate into small blood vessels (tertiary villi) ▪ Around day 17, feto-placental circulation established ▫ Fetal mesoderm cells enter primary villi → form fetal arteries, capillaries, veins within each villi ▫ Villi capillaries connect to umbilical cord blood vessels → links maternal, fetal circulation ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ PLACENTAL STRUCTURE ▪ Maternal contribution: derived from uterine endometrium ▫ Basal plate (decidual plate): thick layer 214 OSMOSIS.ORG ▪ of decidua basalis tissue that maternal spiral arteries, veins pass through to get to junctional zone Fetal contribution: derived from chorionic plate (trophoblast, extraembryonic mesoderm) ▫ Chorionic frondosum: numerous villi that emerge from chorionic plate ▫ Junctional zone between basal plate, chorionic plate Space forms around fetus called chorionic cavity ▫ Contains amniotic cavity, yolk sac, embryo ▫ Chorion laeve: chorionic cavity wall where syncytiotrophoblast villi regressed ▫ Outside of chorion laeve, thin layer of decidua (decidua capsularis) On ultrasound, chorionic cavity shows up as relatively large, dark space ▫ Used to identify pregnancy even before fetus can be seen During fourth, fifth months of development, walls called decidual septa form ▫ Divide placenta into 15–20 different regions called cotyledons Each cotyledon contains about 100 spiral arteries providing steady supply of oxygenated blood Oxygen, glucose, molecules like immunoglobulins, hormones, certain toxins are able to move across into fetal capillaries ▫ Carbon dioxide moves out of fetal capillaries, enters blood in junctional zone Placenta covers about 15–30% of uterine wall at any given time during development Placenta grows, thickens ▫ At full term, is 20cm/7.9in across (size of frisbee) During third stage of labor placenta is expelled from body as afterbirth
Chapter 28 Embryology: Early Structures Figure 28.7 Formation of feto-placental circulation: primary villi form. Cells clear out between primary villi, forming lacunae. Tiny maternal arteries and veins merge with lacunae, and the lacunae fill with oxygenated blood. Lacunae merge to form a single pool, the junctional zone. Figure 28.8 Fetal and maternal contributions to the placenta. Figure 28.9 Contents of the chorionic cavity. OSMOSIS.ORG 215
Figure 28.10 Decidual septa form in months four and five that divide the placenta into regions called cotyledons. Figure 28.11 The placenta covers approximately 15–30% of uterine wall and is about 20cm/7.9in across at full term. DEVELOPMENT OF THE UMBILICAL CORD osms.it/umbilical-cord-development ▪ Umbilical cord is a long flexible stalk containing two arteries, one vein; connects fetus to placenta ▪ Forms from three structures ▫ Body (connecting) stalk: short band of extraembryonic mesoderm that connects embryo to chorion at week 2 ▫ Vitelline duct: open connection between yolk sac, midgut at week 3 ▫ Allantois: small hindgut outpocketing that grows into umbilical cord at week 3 ▪ In week 4: amniotic cavity folds down, around embryo → body stalk, vitelline duct, allantois pushed together, form umbilical cord → emerge out of umbilical ring (fibrous tissue ring that develops on abdominal wall at location where they emerge) ▪ Between weeks 4–8: cells lining amniotic cavity produce amniotic fluid → amnion swells, takes up most of space in chorionic cavity ▫ Amnion folds → covers body stalk, vitelline duct forming an outer 216 OSMOSIS.ORG ▪ ▪ ▪ ▪ ▪ membrane for umbilical cord Around week 6: physiological umbilical herniation ▫ Due to rapid intestinal growth, part of intestine herniates through umbilical ring into umbilical cord; withdraws back into abdominal cavity by end of third month After umbilical cord formation, vitelline duct, yolk sac shrink, eventually disappear ▫ If vitelline duct does not regress all the way → Meckel’s diverticulum Allantois continues developing into bladder ▫ Remnant of allantois: fetus → urachus; adult → median umbilical ligament Final umbilical cord: contains two umbilical arteries, one umbilical vein, gelatinous substance called Wharton’s jelly which protects umbilical vessels After birth: umbilical vein → round ligament of liver, umbilical arteries; medial umbilical ligaments
Chapter 28 Embryology: Early Structures Figure 28.12 Formation of the umbilical cord and structures within it. OSMOSIS.ORG 217
Figure 28.13 Cross section of the umbilical cord revealing its components and their remnants. DEVELOPMENT OF THE FETAL MEMBRANES osms.it/fetal-membrane-development ▪ AKA extraembryonic membranes: tissues that form in uterus during first few weeks of development ▫ Amnion, yolk sac, chorion, allantois AMNION ▪ On day 8, space appears between epiblast, cytotrophoblast → amniotic cavity ▪ Cells from epiblast migrate to form thin layer around amniotic cavity, separating it from cytotrophoblast → amnion YOLK SAC ▪ On day 9, hypoblast cells migrate to form thin membrane around blastocoel, forming yolk sac walls → yolk sac fills with vitelline fluid ▫ Vitelline fluid provides nourishment for embryo ▪ Nutrients in yolk sac eventually consumed, yolk sac, vitelline duct shrink, disappear CHORION ▪ By day 10, epiblast cells differentiate into extraembryonic mesoderm ▫ Settle between amniotic cavity/yolk sac, cytotrophoblast → creating thick layer of extraembryonic mesoderm tissue between the two ▫ A space forms within this layer → 218 OSMOSIS.ORG extraembryonic coelom/chorionic cavity ▫ Cavity continues to expand until there is only a thin layer of extraembryonic mesoderm lining amniotic cavity/yolk sac, cytotrophoblast ▪ Cavity does not form at body stalk where embryoblast remains attached to chorion ▫ At this point, chorion contains extraembryonic mesoderm, cytotrophoblast, syncytiotrophoblast ▪ Chorion develops chorionic villi, invades endometrium, eventually helps form fetal part of placenta ALLANTOIS ▪ Develops as an outpouching of hindgut during week 3 ▫ Serves as canal through which urine is eliminated, before urethra develops ▫ Degenerates into fibrous structure called urachus, remains attached to urinary bladder ▪ During week 4, allantois, vitelline duct, body stalk combine to form umbilical cord ▪ Between weeks 4–8, amnion secretes amniotic fluid → amniotic cavity swells, folds down around embryo → protects, insulates embryo → continues to grow → amnion, chorion fuse together, form amniotic sac
Chapter 28 Embryology: Early Structures Figure 28.14 Four fetal membranes include: amnion, chorion, allantois, yolk sac. DEVELOPMENT OF TWINS osms.it/twin-development FRATERNAL (DIZYGOTIC) TWINS ▪ Occur at rate of about 10 per 1,000 births worldwide ▪ Originate from two separate eggs (hyperovulation) fertilized individually by two different sperms → zygotes have completely different genetic makeups ▫ Hyperovulation may be due to an overabundance of follicle-stimulating hormone (FSH) ▪ Mothers of fraternal twins tend to be older (> 35 years), taller, heavier on average, with shorter, more frequent menstrual cycles → high levels of follicle-stimulating hormone IDENTICAL (MONOZYGOTIC) TWINS ▪ Occur at a rate of about 4 per 1000 births worldwide ▪ Originate from single zygote that splits into two groups of cells → zygotes have identical genetic makeup ▪ The split can occur at any time during first thirteen days of development ▪ Identical DNA → identical physical traits that have a strong genetic basis → sex, hair, eye color, blood type, other physical features IDENTICAL TWIN CATEGORIES ▪ Categorized by how, when division occurs → affects how identical twins share space, resources in uterus Dichorionic-diamniotic ▪ Division occurs within 2–3 days following fertilization ▪ Embryos develop completely separately from one another ▪ Have separate placentas, amniotic sacs Monochorionic-diamniotic ▪ Division occurs between 3–8 days following fertilization ▪ Embryos share a single placenta, separate amniotic sacs Monochorionic-monoamniotic ▪ Division occurs between 8–13 days after fertilization ▪ Embryos share both placenta, amniotic sac OSMOSIS.ORG 219
Figure 28.15 The way the womb is shared by identical twins depends on the time frame in which the zygote split in two. 220 OSMOSIS.ORG

Osmosis High-Yield Notes

This Osmosis High-Yield Note provides an overview of Early Structures essentials. All Osmosis Notes are clearly laid-out and contain striking images, tables, and diagrams to help visual learners understand complex topics quickly and efficiently. Find more information about Early Structures by visiting the associated Learn Page.