Fetal membranes

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Fetal membranes
FetalMembranes1L.jpg
Fetal membranes at end of second month with chorion shown in its two subdivisions
Details
Identifiers
Latin adnexa fetalia
TE membranes_by_E6.0.2.0.0.0.1 E6.0.2.0.0.0.1
Anatomical terminology

The fetal membranes are the four extraembryonic membranes, associated with the developing embryo, and fetus in humans and other mammals. They are the amnion, chorion, allantois, and yolk sac. [1] The amnion and the chorion are the chorioamniotic membranes that make up the amniotic sac which surrounds and protects the embryo. [2] The fetal membranes are four of six accessory organs developed by the conceptus that are not part of the embryo itself, the other two are the placenta, and the umbilical cord. [1]

Contents

Structure

Placenta shown with attached fetal membranes Placenta with fetal membranes.jpg
Placenta shown with attached fetal membranes

The fetal membranes surround the developing embryo and form the fetal-maternal interface. [3] The fetal membranes are derived from the trophoblast layer (outer layer of cells) of the implanting blastocyst. [3] The trophoblast layer differentiates into amnion and the chorion, which then comprise the fetal membranes. [4] The amnion is the innermost layer and, therefore, contacts the amniotic fluid, the fetus and the umbilical cord. [5] The internal pressure of the amniotic fluid causes the amnion to be passively attached to the chorion. [4] The chorion functions to separate the amnion from the maternal decidua and uterus. [4] The placenta develops from the chorion of the embryo and the uterine tissue of the mother.

Development of the fetal membranes

Initially, the amnion is separated from the chorion by chorionic fluid. [4] The fusion of the amnion and chorion is completed in the human at the 12th week of development. [6]

Microanatomy

Fetal membranes illustrated at week 3 following gastrulation 2909 Embryo Week 3-02.jpg
Fetal membranes illustrated at week 3 following gastrulation

From inside to outside, the fetal membranes consist of amnion and chorion. In addition, parts of decidua are often attached to the outside of the chorion.

Amnion

The amnion is avascular, meaning it does not contain its own blood vessels. Therefore, it must obtain necessary nutrients and oxygen from nearby chorionic and amniotic fluid, and fetal surface vessels. [7] [8] The amnion is characterised by cuboidal and columnar epithelial layers. [7] The columnar cells are located in the vicinity of the placenta, whereas the cuboidal cells are found in the periphery. [7] During early pregnancy, the amnionic epithelium is sparsely covered in microvilli, which increase in number throughout pregnancy. [4] The function of this microvillous surface is associated with a densely-packed glycocalix with anionic binding sites; these are thought to be involved with intra-amnionic lipid synthesis. [4] This amnionic epithelium is connected to a basement membrane, which is then attached by filaments to a connective tissue layer. [9]

Chorion

The chorionic membrane is a fibrous tissue layer containing the fetal blood vessels. [4] Chorionic villi form on the outer surface of the chorion, which maximise surface area for contact with maternal blood. [4] The chorionic villi are involved in fetal-maternal exchange. [10]

Yolk sac

The yolk sac is a membranous sac attached to an embryo, formed by cells of the hypoblast layer of the bilaminar embryonic disc. This is alternatively called the umbilical vesicle. In humans, the yolk sac is important in early embryonic blood supply. [11]

Allantois

The human allantois is a caudal out-pouching of the yolk sac, which becomes surrounded by the mesodermal connecting stalk or body-stalk. The vasculature of the body-stalk develops into umbilical arteries that carry deoxygenated blood to the placenta. [12] It is externally continuous with the proctodeum and internally continuous with the cloaca. The embryonic allantois becomes the fetal urachus, which connects the fetal bladder (developed from cloaca) to the yolk sac. The urachus removes nitrogenous waste from the fetal bladder. [13] After birth the urachus is closed, and becomes the median umbilical ligament.

Function

The fetal membrane surrounds the fetus during the gestational period and ensures maintenance of pregnancy to delivery, protection of the fetus as well as being critical in maintaining the conditions necessary for fetal health.

Barrier function

The fetal membranes separate maternal tissue from fetal tissue at a basic mechanical level. The fetal membrane is composed of a thick cellular chorion covering a thin amnion composed of dense collagen fibrils. The amnion is in contact with the amniotic fluid and ensures structural integrity of the sac due to its mechanical strength. The underlying chorion is fused to the decidua at the maternal-fetal interface. This interaction is vital in controlling the local immune systems which in turn is vital for maintaining a semi-allogeneic fetus. At the end of gestation, a 'weak zone' develops in the fetal membrane overlying the cervix due to collage remodelling. This eventually leads to rupture of the fetal membrane and the onset of labour.[ citation needed ]

Signalling of fetal maturation and parturition

As pregnancy advances to term, the fetal membranes undergo weakening. [14] The amnion is vital in the synthesis of prostaglandins which reach the myometrium and create and initiate parturition. The chorion expresses chemicals that balance synthesis and metabolism of these prostaglandins to ensure that the myometrium is not activated pre-term. Prostaglandin E2 is thought to be synthesized by cells in the amnion and is essential in dilation of the cervix at the initiation of parturition. [15] Glucocorticoids have been implicated in fetal maturation, regulation of immune response and many other pregnancy associated changes. [15] As well as its function in parturition, Prostaglandin E2 is vital for fetal lung maturation. Additionally, there is an abundance of 11β-hydroxysteroid dehydrogenase 1 expressed in the foetal membranes. This enzyme converts biologically inactive cortisone into active cortisol, another chemical vital for fetal maturation and labour initiation.

Pathophysiology

Preterm births (births taking place before 37 weeks) can be the result of a number of causes such as, in utero infection, inflammation, vascular disease and uterine overdistension. [16] The risk of spontaneous preterm birth is increased by a previous preterm birth, black race, periodontal diseases and low maternal body-mass index. Key indicators of preterm birth are short cervical length and a raised cervical-vaginal fetal fibronectin concentration.

Pathophysiology of the fetal membranes, such as microfractures, senescence of cells in the fetal membrane and inflammation can lead to an increased chance of preterm premature rupture of the fetal membranes (pPROM). [17]

Microfractures of the fetal membranes

Throughout gestation the fetal membranes undergo remodeling to allow for the increase in size of the uterus. The remodeling of the fetal membranes occurs at both the level of the cells and the extracellular matrix (ECM). [15] Structural abnormalities such as areas of where collagen has degraded, known as microfractures, have been observed in the amniotic membrane layer. [18] [19]

Microfractures are characterised by:

Microfractures of the fetal membranes are seen in pregnancies where pPROM has occurred. [15] It has been suggested that the presence of more fetal membrane microfractures may mean the fetal membranes may be predisposed for preterm rupture. [15]

Inflammation and senescence of the fetal membranes

Inflammation of the fetal membranes is called chorioamnionitis. Balanced inflammation is an important factor in maintaining fetal membranes by regulating the remodeling. However, if the inflammatory response increases above this level it can have dangerous and potentially fatal effects for the mother and child. These elevated levels of inflammatory molecules in the fetal membrane is called ‘sterile inflammation’. [15] Sterile inflammation can be caused by both microbial infection and non-infectious factors, such as senescence of fetal membranes. Senescence is associated with the aging of actively cycling and dividing cells. [18] As the fetal membrane cells proliferate during remodelling, the telomeres (short length or non-coding DNA on the end of chromosomes that protect essential coding DNA from degradation during replication) shorten as chromosomes can not be copied end-to-end fully. [20] Once the telomeres have reached a critical length the cell can no longer divide and can hence cause telomere-dependent replicative senescence. This should occur naturally at term (37 weeks), as it is an important factor to increase the inflammatory environment in the uterus to initiate parturition. However, fetal membrane senescence can be accelerated by oxidative stress and hence, stimulate sterile inflammation to occur prior to term; consequently, causing preterm birth. [18]

See also

Related Research Articles

<span class="mw-page-title-main">Placenta</span> Organ that connects the fetus to the uterine wall

The placenta is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate maternal and fetal circulations, and is an important endocrine organ, producing hormones that regulate both maternal and fetal physiology during pregnancy. The placenta connects to the fetus via the umbilical cord, and on the opposite aspect to the maternal uterus in a species-dependent manner. In humans, a thin layer of maternal decidual (endometrial) tissue comes away with the placenta when it is expelled from the uterus following birth. Placentas are a defining characteristic of placental mammals, but are also found in marsupials and some non-mammals with varying levels of development.

<span class="mw-page-title-main">Amniocentesis</span> Sampling of amniotic fluid done mainly to detect fetal chromosomal abnormalities

Amniocentesis is a medical procedure used primarily in the prenatal diagnosis of genetic conditions. It has other uses such as in the assessment of infection and fetal lung maturity. Prenatal diagnostic testing, which includes amniocentesis, is necessary to conclusively diagnose the majority of genetic disorders, with amniocentesis being the gold-standard procedure after 15 weeks' gestation.

The amniotic sac, also called the bag of waters or the membranes, is the sac in which the embryo and later fetus develops in amniotes. It is a thin but tough transparent pair of membranes that hold a developing embryo until shortly before birth. The inner of these membranes, the amnion, encloses the amniotic cavity, containing the amniotic fluid and the embryo. The outer membrane, the chorion, contains the amnion and is part of the placenta. On the outer side, the amniotic sac is connected to the yolk sac, the allantois, and via the umbilical cord, the placenta.

<span class="mw-page-title-main">Amnion</span> Innermost membranous sac that surrounds and protects the developing embryo

The amnion is a membrane that closely covers the human and various other embryos when first formed. It fills with amniotic fluid, which causes the amnion to expand and become the amniotic sac that provides a protective environment for the developing embryo. The amnion, along with the chorion, the yolk sac and the allantois protect the embryo. In birds, reptiles and monotremes, the protective sac is enclosed in a shell. In marsupials and placental mammals, it is enclosed in a uterus.

<span class="mw-page-title-main">Chorion</span> Outermost fetal membrane around the embryo in amniotes

The chorion is the outermost fetal membrane around the embryo in mammals, birds and reptiles (amniotes). It develops from an outer fold on the surface of the yolk sac, which lies outside the zona pellucida, known as the vitelline membrane in other animals. In insects, it is developed by the follicle cells while the egg is in the ovary. Some mollusks also have chorions as part of their eggs. For example, fragile octopus eggs have only a chorion as their envelope.

<span class="mw-page-title-main">Blastocyst</span> Structure formed around day 5 of mammalian embryonic development

The blastocyst is a structure formed in the early embryonic development of mammals. It possesses an inner cell mass (ICM) also known as the embryoblast which subsequently forms the embryo, and an outer layer of trophoblast cells called the trophectoderm. This layer surrounds the inner cell mass and a fluid-filled cavity known as the blastocoel. In the late blastocyst, the trophectoderm is known as the trophoblast. The trophoblast gives rise to the chorion and amnion, the two fetal membranes that surround the embryo. The placenta derives from the embryonic chorion and the underlying uterine tissue of the mother.

<span class="mw-page-title-main">Chorionic villus sampling</span> Type of prenatal diagnosis done to determine chromosomal or genetic disorders in the fetus

Chorionic villus sampling (CVS), sometimes called "chorionic villous sampling", is a form of prenatal diagnosis done to determine chromosomal or genetic disorders in the fetus. It entails sampling of the chorionic villus and testing it for chromosomal abnormalities, usually with FISH or PCR. CVS usually takes place at 10–12 weeks' gestation, earlier than amniocentesis or percutaneous umbilical cord blood sampling. It is the preferred technique before 15 weeks.

Oligohydramnios is a medical condition in pregnancy characterized by a deficiency of amniotic fluid, the fluid that surrounds the fetus in the abdomen, in the amniotic sac. It is typically diagnosed by ultrasound when the amniotic fluid index (AFI) measures less than 5 cm or when the single deepest pocket (SDP) of amniotic fluid measures less than 2 cm. Amniotic fluid is necessary to allow for normal fetal movement, lung development, and cushioning from uterine compression. Low amniotic fluid can be attributed to a maternal, fetal, placental or idiopathic cause and can result in poor fetal outcomes including death. The prognosis of the fetus is dependent on the etiology, gestational age at diagnosis, and the severity of the oligohydramnios.

<span class="mw-page-title-main">Amniotic fluid</span> Fluid surrounding a fetus within the amnion

The amniotic fluid is the protective liquid contained by the amniotic sac of a gravid amniote. This fluid serves as a cushion for the growing fetus, but also serves to facilitate the exchange of nutrients, water, and biochemical products between mother and fetus.

<span class="mw-page-title-main">Allantois</span> Embryonic structure

The allantois is a hollow sac-like structure filled with clear fluid that forms part of a developing amniote's conceptus. It helps the embryo exchange gases and handle liquid waste.

Rupture of membranes (ROM) or amniorrhexis is a term used during pregnancy to describe a rupture of the amniotic sac. Normally, it occurs spontaneously at full term either during or at the beginning of labor. Rupture of the membranes is known colloquially as "breaking (one's) water," especially when induced rather than spontaneous, or as one's "water breaking". A premature rupture of membranes (PROM) is a rupture of the amnion that occurs at full term and prior to the onset of labor. In cases of PROM, options include expectant management without intervention, or interventions such as oxytocin or other methods of labor induction, and both are usually accompanied by close monitoring of maternal and fetal health. Preterm premature rupture of membranes (PPROM) is when water breaks both before the onset of labor and before the pregnancy's 37 week gestation. In the United States, more than 120,000 pregnancies per year are affected by a premature rupture of membranes, which is the cause of about one third of preterm deliveries.

<span class="mw-page-title-main">Yolk sac</span> Membranous sac attached to an embryo

The yolk sac is a membranous sac attached to an embryo, formed by cells of the hypoblast layer of the bilaminar embryonic disc. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though yolk sac is far more widely used. In humans, the yolk sac is important in early embryonic blood supply, and much of it is incorporated into the primordial gut during the fourth week of embryonic development.

<span class="mw-page-title-main">Artificial womb</span> Device that would allow for extracorporeal pregnancy

An artificial womb or artificial uterus is a device that would allow for extracorporeal pregnancy by growing a fetus outside the body of an organism that would normally carry the fetus to term.

<span class="mw-page-title-main">Placentation</span> Formation and structure of the placenta

Placentation refers to the formation, type and structure, or arrangement of the placenta. The function of placentation is to transfer nutrients, respiratory gases, and water from maternal tissue to a growing embryo, and in some instances to remove waste from the embryo. Placentation is best known in live-bearing mammals (theria), but also occurs in some fish, reptiles, amphibians, a diversity of invertebrates, and flowering plants. In vertebrates, placentas have evolved more than 100 times independently, with the majority of these instances occurring in squamate reptiles.

<span class="mw-page-title-main">Chorioamnionitis</span> Medical condition

Chorioamnionitis, also known as intra-amniotic infection (IAI), is inflammation of the fetal membranes, usually due to bacterial infection. In 2015, a National Institute of Child Health and Human Development Workshop expert panel recommended use of the term "triple I" to address the heterogeneity of this disorder. The term triple I refers to intrauterine infection or inflammation or both and is defined by strict diagnostic criteria, but this terminology has not been commonly adopted although the criteria are used.

<span class="mw-page-title-main">Chorionic villi</span> Villi that sprout from the chorion

Chorionic villi are villi that sprout from the chorion to provide maximal contact area with maternal blood.

<span class="mw-page-title-main">Bilaminar embryonic disc</span>

The bilaminar embryonic disc, bilaminar blastoderm or embryonic disc is the distinct two-layered structure of cells formed in an embryo. In the development of the human embryo this takes place by day eight. It is formed when the inner cell mass, also known as the embryoblast, forms a bilaminar disc of two layers, an upper layer called the epiblast and a lower layer called the hypoblast, which will eventually form into fetus. These two layers of cells are stretched between two fluid-filled cavities at either end: the primitive yolk sac and the amniotic sac.

<span class="mw-page-title-main">Human embryonic development</span> Development and formation of the human embryo

Human embryonic development or human embryogenesis is the development and formation of the human embryo. It is characterised by the processes of cell division and cellular differentiation of the embryo that occurs during the early stages of development. In biological terms, the development of the human body entails growth from a one-celled zygote to an adult human being. Fertilization occurs when the sperm cell successfully enters and fuses with an egg cell (ovum). The genetic material of the sperm and egg then combine to form the single cell zygote and the germinal stage of development commences. Embryonic development in the human, covers the first eight weeks of development; at the beginning of the ninth week the embryo is termed a fetus. The eight weeks has 23 stages.

<span class="mw-page-title-main">Velamentous cord insertion</span> Velamentous placenta

Velamentous cord insertion is a complication of pregnancy where the umbilical cord is inserted in the fetal membranes. It is a major cause of antepartum hemorrhage that leads to loss of fetal blood and associated with high perinatal mortality. In normal pregnancies, the umbilical cord inserts into the middle of the placental mass and is completely encased by the amniotic sac. The vessels are hence normally protected by Wharton's jelly, which prevents rupture during pregnancy and labor. In velamentous cord insertion, the vessels of the umbilical cord are improperly inserted in the chorioamniotic membrane, and hence the vessels traverse between the amnion and the chorion towards the placenta. Without Wharton's jelly protecting the vessels, the exposed vessels are susceptible to compression and rupture.

<span class="mw-page-title-main">Circumvallate placenta</span> Medical condition

Circumvallate placenta is a rare condition affecting about 1-2% of pregnancies, in which the amnion and chorion fetal membranes essentially "double back" on the fetal side around the edges of the placenta. After delivery, a circumvallate placenta has a thick ring of membranes on its fetal surface. Circumvallate placenta is a placental morphological abnormality associated with increased fetal morbidity and mortality due to the restricted availability of nutrients and oxygen to the developing fetus.

References

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  2. ↑ "15.7E: Extraembryonic Membranes and the Physiology of the Placenta". Biology LibreTexts. 20 July 2016. Retrieved 16 June 2022.
  3. 1 2 Johnson MH (2018-01-12). Essential reproduction (Eighth ed.). Hoboken, NJ. ISBN   9781119246473. OCLC   1008770296.{{cite book}}: CS1 maint: location missing publisher (link)
  4. 1 2 3 4 5 6 7 8 Benirschke K, Kaufmann P, Baergen R (2006). Pathology of the human placenta (5th ed.). New York: Springer. ISBN   0387267387. OCLC   86076129.
  5. ↑ Bourne G (April 1962). "The foetal membranes. A review of the anatomy of normal amnion and chorion and some aspects of their function". Postgraduate Medical Journal. 38 (438): 193–201. doi:10.1136/pgmj.38.438.193. PMC   2482459 . PMID   13871927.
  6. ↑ Boyd JD, Hamilton WJ (1970). "Terminology". The Human Placenta. Palgrave Macmillan UK. pp. 20–26. doi:10.1007/978-1-349-02807-8_2. ISBN   9781349028092.
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  8. ↑ Wang, Yuping; Zhao, Shuang (2010). "Placental Blood Circulation". Morgan & Claypool Life Sciences. Retrieved 31 October 2022.
  9. ↑ Danforth D, Hull RW (March 1958). "The microscopic anatomy of the fetal membranes with particular reference to the detailed structure of the amnion". American Journal of Obstetrics and Gynecology. 75 (3): 536–47, discussion 548–50. doi:10.1016/0002-9378(58)90610-0. PMID   13508744.
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  11. ↑ Lutfey, Karen; Freese, Jeremy (2005). "Toward Some Fundamentals of Fundamental Causality: Socioeconomic Status and Health in the Routine Clinic Visit for Diabetes". American Journal of Sociology. 110 (5): 1326–1372. doi:10.1086/428914. ISSN   0002-9602. JSTOR   10.1086/428914. S2CID   17629087.
  12. ↑ Moore, Keith; Persaud, T.V.N.; Torchia, Mark G. (2016). The Developing Human: Clinically Oriented Embryology (10th ed.). Philadelphia: Elsevier. p. 58. ISBN   978-0-323-31338-4.
  13. ↑ First AID for the USMLE Step 1 2008
  14. ↑ Verbruggen SW, Oyen ML, Phillips AT, Nowlan NC (2017-03-28). Sun K (ed.). "Function and failure of the fetal membrane: Modelling the mechanics of the chorion and amnion". PLOS ONE. 12 (3): e0171588. Bibcode:2017PLoSO..1271588V. doi: 10.1371/journal.pone.0171588 . PMC   5370055 . PMID   28350838.
  15. 1 2 3 4 5 6 7 8 9 10 Myatt L, Sun K (2010). "Role of fetal membranes in signaling of fetal maturation and parturition". The International Journal of Developmental Biology. 54 (2–3): 545–53. doi: 10.1387/ijdb.082771lm . PMID   19924634.
  16. ↑ Goldenberg RL, Culhane JF, Iams JD, Romero R (January 2008). "Epidemiology and causes of preterm birth". Lancet. 371 (9606): 75–84. doi:10.1016/S0140-6736(08)60074-4. PMC   7134569 . PMID   18177778.
  17. ↑ Menon R, Richardson LS, Lappas M (April 2019). "Fetal membrane architecture, aging and inflammation in pregnancy and parturition". Placenta. TR: Proceedings of PREBIC meeting 2018. 79: 40–45. doi:10.1016/j.placenta.2018.11.003. PMC   7041999 . PMID   30454905.
  18. 1 2 3 Menon R, Mesiano S, Taylor RN (2017-08-17). "Programmed Fetal Membrane Senescence and Exosome-Mediated Signaling: A Mechanism Associated With Timing of Human Parturition". Frontiers in Endocrinology. 8: 196. doi: 10.3389/fendo.2017.00196 . PMC   5562683 . PMID   28861041.
  19. ↑ Menon R, Richardson LS (November 2017). "Preterm prelabor rupture of the membranes: A disease of the fetal membranes". Seminars in Perinatology. Current Preterm Birth Prevention Strategies. 41 (7): 409–419. doi:10.1053/j.semperi.2017.07.012. PMC   5659934 . PMID   28807394.
  20. ↑ Phillippe M (October 2015). "Cell-Free Fetal DNA, Telomeres, and the Spontaneous Onset of Parturition". Reproductive Sciences. 22 (10): 1186–201. doi:10.1177/1933719115592714. PMID   26134037. S2CID   7312100.
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