Fetal Membranes PDF

Fetal membranes Dr.Haidy refaat Faculty of Medicine, Assiut University. Egypt  PMS Block Embryology Fetal membranes By Dr Haidy refaat Department of human Anatomy and Embryology Faculty of Medicine Assiut University ...

Fetal membranes Dr.Haidy refaat Faculty of Medicine, Assiut University. Egypt  PMS Block Embryology Fetal membranes By Dr Haidy refaat Department of human Anatomy and Embryology Faculty of Medicine Assiut University At the 11th-12th day a new population of cells derived from the wall of the yolk sac form loose connective tissue extraembryonic (primary) mesoderm. Soon large cavities develop in the extraembryonic mesoderm and when they fuse a new space known as extraembryonic celom (chorionic cavity) is formed. The space surrounds the amniotic cavity and primitive yolk sac except when the germ disc is connected to the trophoblast by connecting stalk (future umbilical cord). The extraembryonic celom separates the extraembryonic mesoderm into two layers; outer somatic mesoderm and inner splanchnic mesoderm. By two-week embryo, the extraembryonic celom expands and forms the chorionic cavity. The trophoblast and its lining of somatic mesoderm are together known as the chorion (chorionic plate). The whole vesicle of the two-week embryo, about 2 mm, is known as the chorionic vesicle. The germ (embryonic) disc is 0.2mm long and egg-shaped.  Fetal membranes Fetal membranes in late pregnancy Fetal membranes *** Definition: Fetal membranes are structures that have originated from the zygote but do not participate in formation of the embryo, except for parts of the yolk sac and allantois. Fetal membranes assist in protection of the embryo and fetus and provide for their respiration, nutrition and excretion. Fetal membranes include *** : 1-Amnion. 2-Yolk sac. 3-Allantois. 4-Connecting stalk; the future umbilical cord. 5-Chorion; the future placenta. Fetal membranes during embryonic period  AMNION Definition. Amnion is a fluid-filled membranous sac that surrounds the embryo and later the fetus. The amniotic cavity contains pale watery fluid termed liquor amnii. Embryonic origin During week-2 some cells from epiblast, named amnioblasts, enclose and form the roof of the amniotic cavity. Development of amnion After folding, junction of amnion with the embryo changes from margins of the embryonic disc to a restricted region (future umbilicus) present at the ventral aspect. The amniotic cavity enlarges rapidly at the expense of yolk sac and chorionic cavity. Sources of amniotic fluid*** 1-Amnioblasts. 2-Most of amniotic fluid is derived from maternal tissue by diffusion from placenta. 3-Skin of fetus (before keratinization). 4-Fluid secreted by respiratory tract. 5-Urine (mostly water as the placenta functions to exchange waste) contributes to amniotic fluid. By late pregnancy 500 ml of urine is added daily. Volume of amniotic fluid Amniotic fluid increases slowly reaching 700 ml at 37 weeks and 1000 ml at term. Circulation of amniotic fluid -Amniotic fluid is swallowed by the fetus and absorbed by the fetal respiratory and digestive tracts. Fluid passes into fetal blood and the waste products in it cross the placental membrane and enter the maternal blood. Excess water in fetal blood is excreted by the fetal kidneys. - Functions of amniotic fluid** 1-It permits symmetrical external growth of the fetus. 2-Protective function: a)It acts as a cushion against trauma. b)It acts as a barrier against infection. 3-It permits normal fetal lung development. 4-It prevents adherence of amniotic membrane to the embryo and fetus. 5-It helps control embryo’s- and fetus’s body temperature. 6-It enables the fetus to move freely, thereby aiding muscular development. 7-It is involved in maintaining homeostasis of fetal fluid and electrolytes. 8-Prior to labour: a)The hydrostatic bag of water helps dilatation of the cervix. b)When it ruptures, flowing amniotic fluid acts as a lubricant and a natural antiseptic of the birth canal. Abnormalities of amnion*** 1-Oligohydramnios. Volume of amniotic fluid is less than 400 ml in the 3rd trimester. Causes : i-Placental insufficiency. ii-Preterm rupture of amniochorionic membrane (due to trauma). iii-Absence of fetal urine due to renal agenesis. Complications :i-Abnormalities caused by compression (pulmonary hypoplasia, limb and facial defects). ii-Difficult labour. 2-Polyhydramnios. Amniotic fluid is more than 2000 ml. Causes: i-Maternal diabetes. ii-Fetal causes(multiple pregnancy, anencephaly, esophageal atresia). Complications i-Premature rupture of membranes which leads to premature labour. ii-Abnormal fetal presentation..  YOLK SAC Definitin Yolk sac is a membranous sac enclosed by hypoblast cells. Origin: Hypoblast cells. Development and fate -Hypoblast cells form the roof of a cavity called primary yolk sac. -Due to folding, part of the yolk sac is taken inside the embryo to form the gut. The remaining part is called definitive yolk sac connected to midgut by vitelline duct. -By the 20th week, yolk sac atrophies. Significance of the yolk sac*** 1-It has a role in transfer of nutrients to the embryo in early stages of development. 2-Blood development first occurs in the mesoderm covering the wall of the yolk sac. 3-During week-4 , yolk sac is incorporated into the embryo as the primordial gut. 4-Primordial germ cells appear in the wall of the yolk sac in the 3rd week and migrate to the developing sex glands to differentiate into spermatogonia and oogonia. Anomalies of yolk sac (Vitello-intestinal duct) *** 1. Umbilical fistula: Persistance of the duct so, terminal part of ilium connectes abdominal skin at umbilicus. 2. Umbilical sinus: Part of the duct may remain open skin of the umbilicus 3. Umbilical cyst: Both ends of the duct closed leaving an intermediate portion patent 4. Fibrous band: Entire lumen obliterated but the duct fail to degenerate and persist as a band 5. Meckel’s diverticulum: Persistance of about 2 inches of the duct open into terminal ilium, about 2 feet from ileocecal valve and present in 2% of people.  ALLANTOIS Definition of allantois:. Diverticulum derived from part of yolk sac that forms the hindgut Development and fate of allantois :. Grows into body stalk and will be one of contents of umbilical cord. Has a relation to development of urinary bladder. Becomes fibrous cord (urachus) extends from apex of urinary bladder to umbilicus; median umbilical ligament. Congenital anomaly of allantois: 1. Patent urachus: Fistula connecting apex of urinary bladder to umbilicus, due to failure of its obliteration 2. Urachal cyst: Both ends of the allantois closed leaving an intermediate portion patent 3. Urachal sinus: Part of the allantois may remain open skin of the umbilicus  CONNECTING (BODY) STALK Definition of body stalk:. Connection between the embryonic vesicles and the chorion Development and fate of body stalk:. Formed of extraembryonic mesoderm which has no cavity inside. First was attached to caudal end of embryo. Blood vessels appear in body stalk early in development. After folding of embryo attachment of body stalk is shifted to be on ventral aspect of embryo. Elongates to form umbilical cord and the blood vessels will form umbilical vessels  UMBILICAL CORD Definition of umbilical cord:. Tortuous structure that contains blood vessels which connect the placenta with the embryo Gross appearance of umbilical cord:. At end of pregnancy, about 50 Cm long and 2 Cm in diameter. Mesodermal core of the cord becomes loose connective tissue called Warton,s jelly Gross picture of the umbilical cord*** At term average length of the cord is 55 cm. Because the umbilical vessels are longer than the cord, twisting, bending and false knots are common. The cord attaches near the center of the fetal surface of the placenta.. The umbilical arteries constrict and become medial umbilical ligaments. Umbilical vein obliterates to become ligamentum teres of liver. Contents of umbilical cord:*** 1. Wharton,s jelly: Mucous connective tissue, invests other structures of the cord 2. Umbilical vessels: a. Two umbilical arteries: Carry deoxygenated blood from the fetus to the placenta b. One umbilical vein: At first two veins; the right vein disappears and the left remains. Left umbilical vein carries oxygenated blood from the placenta to the fetus 3. Vitelline duct: Connects midgut with definitive yolk sac 4. Allantois 5. Definitive yolk sac 6. Physiological umbilical hernia: Contains loop of intestine 7. Remains of extraembryonic celom: Accomodates the herniating gut Note: At middle of pregnancy, contents from 3-7 disappear A. A 5-week embryo showing structures passing through primitive umbilical ring. B. Primitive umbilical cord of a 10-week embryo. C. Transverse section through structures at level of the umbilical ring D. Transverse section through primitive umbilical cord showing intestinal loops protruding in the cord.  Physiological umbilical hernia: *** Results due to rapid development of intestinal loops, and because of small abdominal cavity at that stage, intestinal loops is pushed into umbilical cord, but it returns back into abdominal cavity by end of 3rd months A. Omphalocele showing failure of intestinal loops to return to the body cavity after physiological herniation. Herniated loops are covered by amnion. B. Omphalocele in a newborn. C. Newborn with gastroschisis. Loops of bowel return to the body cavity but herniate again through the body wall, usually to the right of umbilicus in region of the regressing right umbilical vein. Unlike omphalocele, the defect is not covered by amnion.  Abnormalities of the umbilical cord**** 1-Abnormally long cord (up to 150 cm): a)It predisposes to formation of true knots. True knots may tighten and cause fetal death from anoxia. b)It predisposes to looping around a limb or the neck with rare fetal risk. c)Tendency to prolapse, the cord may be compressed between the fetus and the mother’s pelvis causing fetal anoxia. 2-Abnormally short cord (15 cm or less): -Premature separation of the placenta from the uterine wall during delivery. 3-Abnormal attachment to the placenta. a)Battledore placenta: insertion of the cord at the margin of the placenta. b)Placenta velamentosa: umbilical cord is attached outside the placenta. 4-Persistence of umbilical hernia.  PLACENTA Definition and components: ***The placenta has two components of origin: 1-A fetal part that develops from the chorionic sac called chorion frondosum. 2-A maternal part that is derived from the endometrium called decidua basalis. As pregnancy advances, villi on the embryonic pole grow and expand giving rise to bushy chorion or chorion frondosum. Villi on the abembryonic pole degenerate and is known as chorion laeve.  Decidua of pregnant uterus The decidua is the functional layer of the endometrium of the pregnant uterus. Decidua over the chorion frondosum is called decidua basalis. The decidual layer over the abembryonic pole is known as decidua capsularis. The rest of the uterine cavity is lined with decidua parietalis. With increase in size of the chorionic vesicle, decidua capsularis becomes stretched and degenerates. Subsequently, chorion laeve comes into contact with decidua parietalis and the two fuse, thereby obliterating the uterine lumen. The only portion of the chorion participating in the exchange process is the chorion frondosum, which, together with the decidua basalis make up the placenta. Structure of placenta By beginning of 4th month: Has two sides 1. Fetal portion: Formed of chorionic frondosum and bordered by chorionic plate. 2. Maternal portion: Formed of decidua basalis, of which decidual plate is the most incorporated into placenta. In-between these two portions intervillous spaces are filled with maternal blood and the villous trees grow into these blood lakes. Development of the placenta - Primordial uteroplacental circulation is established as early as day 12****** -During the 3rd week, capillaries appear in the chorionic villi and they become tertiary villi. Capillaries in these villi form arteriocapillary networks which connect to embryonic heart through umbilical vessels. -The placenta is fully formed by the end of the 4th month. The placenta enlarges as the fetus grows. -Growth in size and thickness of placenta continues until end of 5th month of gestation. ****** -Fully developed placenta covers 15 to 30% of the decidua and weighs one-sixth that of fetus. Full-Term Placenta At full term, the placenta has a discoid cake shape, a diameter of 15-25 cm, is 2-3 cm thick, and has a weight of 500-600 gm. ****** The maternal side, slightly bulging areas, the cotyledons, covered by thin layer of decidua basalis are recognized. The fetal surface of the placenta is smooth and covered by amniotic membrane. Attachment of the umbilical cord to the placenta is usually near its middle.  Placental membrane (barrier) Circulations of the mother and fetus are separated by placental membrane (barrier) Not true barrier as many substances can cross through it freely Initially formed of : 1. Endothelial lining of fetal vessels 2. Connective tissue in villous core 3. Cytotrophoblastic layer 4. Syncytium BY 4th months: 1 and 4 comes in intimate contact, this increase greatly rate of gas exchange. Functions of Placenta The main functions of the placenta are: 1-Transport of gaseous and metabolic products between maternal and fetal blood. 2-Metabolism, e.g., synthesis of glycogen, cholesterol and fatty acids. 3-Production of hormones., A)Protein hormones: Human chorionic gonadotropin (hCG) ,Human chorionic lactogen (hPL) ,Human chorionic thyrotropin (hCT) &Human chorionic corticotropin (hCACTH). B)Steroid hormones: 1-Progesterone is essential for maintenance of pregnancy. 2-Estrogens stimulate uterine growth, and development of the mammary glands.  ****** Placental Abnormalities A) Variations in shape and size 1-Diffuse placenta, placenta membranacea. 2-Lobed placenta, bilobed or trilobed or with accessory lobe. 3-Failure of placenta to develop over a small area; placenta fenestrata. 4-Placenta with central depression; placenta circumvallata. B) Abnormal marginal insertion of the umbilical cord, battledore placenta. C) Abnormal site close to or overlying the internal os, the abnormality is called placenta previa.Late pregnancy bleeding may result from placenta previa. The fetus has to be delivered by Cesarean section when the placenta completely obstructs the cervix. D) Abnormal adherence of the chorionic villi to the uterine wall Chorionic villi adhere to myometrium; placenta accreta, to the perimetrium ; placenta pancreta. Third trimester bleeding is the common presenting sign of these anomalies. Placental abnormalities  TWINS Definition: Multiple pregnancy is conception of more than one baby at the same time. Types of twins****** Twins either originate from the same zygote, - monozygotic (MZ) identical twins-, or originating from two zygotes, - dizygotic (DZ). Incidence: Twins occur once in every 100 pregnancies. About 2/3 of twins are dizygotic twins (DZ) and 1/3 are monozygotic twins (MZ). Problems of multiple pregnancy 1. Increased mortality and morbidity. 2. Low birth weight and prematurity. 3. Abnormal presentation. 4. Premature labor and bleeding. Monozygotic Twins Features Monozygotic twins have identical genetic background. They are of the same sex, same blood group, and the same external appearance such as hair and eye color. Complications of MZ twins 1-Intrauterine death. 2-Conjoined (Siamese) twins Conjoined twins are due to incomplete splitting the germ disc.  Dizygotic Twins Dizygotic twins occur when there is release of two ova at nearly the same time at one ovarian cycle and the two ova are fertilized by two different sperms. DZ twins have different genetic background. They may or may not be of the same sex.