Gestational sac

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Gestational sac
Ultrasound of embryo at 5 weeks.png
Contents in the cavity of the uterus seen at approximately 5 weeks of gestational age by obstetric ultrasonography.
Ultrasound of embryo at 5 weeks, colored.png
Artificially colored, showing gestational sac, yolk sac and embryo (measuring 3 mm as the distance between the + signs).
Details
Carnegie stage 6a
Days12
Precursor Heuser's membrane
Identifiers
Latin saccus gestationalis, coeloma extraembryonicum, cavitas chorionica
MeSH D058746
TE sac_by_E5.8.0.0.1.0.1 E5.8.0.0.1.0.1
Anatomical terminology

The gestational sac is the large cavity of fluid surrounding the embryo. During early embryogenesis, it consists of the extraembryonic coelom, also called the chorionic cavity. The gestational sac is normally contained within the uterus. It is the only available structure that can be used to determine if an intrauterine pregnancy exists until the embryo can be identified.

Contents

On obstetric ultrasound, the gestational sac is a dark (anechoic) space surrounded by a white (hyperechoic) rim.

Structure

The gestational sac is spherical in shape, and is usually located in the upper part (fundus) of the uterus. By approximately nine weeks of gestational age, due to folding of the trilaminar germ disc, the amniotic sac expands and occupy the majority of the volume of the gestational, eventually reducing the extraembryonic coelom (the gestational sac or the chorionic cavity) to a thin layer between the parietal somatopleuric and visceral splanchnopleuric layer of extraembryonic mesoderm.

Development

During embryogenesis, the extraembryonic coelom (or chorionic cavity) that constitutes the gestational sac is a portion of the conceptus consisting of a cavity between Heuser's membrane and the trophoblast.

During formation of the primary yolk sac, some of the migrating hypoblast cells differentiate into mesenchymal cells that fill the space between Heuser's membrane and the trophoblast, forming the extraembryonic mesoderm. As development progresses, small lacunae begin to form within the extraembryonic mesoderm which enlarges to become the extraembryonic coelom.

The Heuser's membrane cells (hypoblast cells) that migrated along the inner cytotrophoblast lining of the blastocoel, secrete an extracellular matrix along the way. Cells of the hypoblast migrate along the outer edges of this reticulum and form the extraembryonic mesoderm; this disrupts the extraembryonic reticulum. Soon pockets form in the reticulum, which ultimately coalesce to form the extraembryonic coelom. [1]

The extraembryonic coelom divides the extraembryonic mesoderm into two layers: extraembryonic splanchnopleuric mesoderm, which lies adjacent to Heuser's membrane around the outside of the primary yolk sac, and extraembryonic somatopleuric mesoderm, which lies adjacent to the cytotrophoblast layer of the embryo.

The chorionic cavity is enclosed by the chorionic plate, which is composed of an inner layer of somatopleuric mesoderm and an outer layer of trophoblast cells.

Clinical significance

Ultrasound

Mean gestational sac diameter by gestational age. The blue line is the mean, and the green area delimits the 5th and the 95th percentiles. Gestational sac diameter by gestational age.png
Mean gestational sac diameter by gestational age. The blue line is the mean, and the green area delimits the 5th and the 95th percentiles.
A: Gestational sac, B: Crown-rump length of embryo, C: Amniotic sac, D: Yolk sac Gestational sac.svg
A: Gestational sac, B: Crown-rump length of embryo, C: Amniotic sac, D: Yolk sac

The mean sac diameter [3] can effectively estimate the gestational age [4] between 5 and 6 weeks, with an accuracy of about +/- 5 days. [5]

The yolk sac and embryo should be readily identifiable when the gestational sac reaches a certain size — a yolk sac should be seen when the gestational sac is 20mm and a fetal pole should be seen when the gestational sac reaches 25mm.

Gestational sacs can be identified via ultrasound and are generally identified by the following four characteristics:

  1. The sac has a round or elliptical shape in longitudinal and transverse views
  2. The sac is surrounded by a white echogenic rim (choriodecidual reaction) [6]
  3. The sac is located in the uterine fundus
  4. The sac is not implanted on the midline, but eccentrically (to one side of the uterine cavity line).

See also

Related Research Articles

<span class="mw-page-title-main">Mesoderm</span> Middle germ layer of embryonic development

The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.

<span class="mw-page-title-main">Body cavity</span> Internal space within a multicellular organism

A body cavity is any space or compartment, or potential space, in an animal body. Cavities accommodate organs and other structures; cavities as potential spaces contain fluid.

<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">Trophoblast</span> Early embryonic structure that gives rise to the placenta

The trophoblast is the outer layer of cells of the blastocyst. Trophoblasts are present four days after fertilization in humans. They provide nutrients to the embryo and develop into a large part of the placenta. They form during the first stage of pregnancy and are the first cells to differentiate from the fertilized egg to become extraembryonic structures that do not directly contribute to the embryo. After blastulation, the trophoblast is contiguous with the ectoderm of the embryo and is referred to as the trophectoderm. After the first differentiation, the cells in the human embryo lose their totipotency because they can no longer form a trophoblast. They become pluripotent stem cells.

<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">Blighted ovum</span> Medical condition

A blighted ovum is a pregnancy in which the embryo never develops or develops and is reabsorbed. In a normal pregnancy, an embryo would be visible on an ultrasound by six weeks after the woman's last menstrual period. Anembryonic gestation is one of the causes of miscarriage of a pregnancy.

<span class="mw-page-title-main">Animal embryonic development</span> Process by which the embryo forms and develops

In developmental biology, animal embryonic development, also known as animal embryogenesis, is the developmental stage of an animal embryo. Embryonic development starts with the fertilization of an egg cell (ovum) by a sperm cell, (spermatozoon). Once fertilized, the ovum becomes a single diploid cell known as a zygote. The zygote undergoes mitotic divisions with no significant growth and cellular differentiation, leading to development of a multicellular embryo after passing through an organizational checkpoint during mid-embryogenesis. In mammals, the term refers chiefly to the early stages of prenatal development, whereas the terms fetus and fetal development describe later stages.

In embryology, Carnegie stages are a standardized system of 23 stages used to provide a unified developmental chronology of the vertebrate embryo.

<span class="mw-page-title-main">Epiblast</span> Embryonic inner cell mass tissue that forms the embryo itself, through the three germ layers

In amniote embryonic development, the epiblast is one of two distinct cell layers arising from the inner cell mass in the mammalian blastocyst, or from the blastula in reptiles and birds, the other layer is the hypoblast. It drives the embryo proper through its differentiation into the three primary germ layers, ectoderm, mesoderm and endoderm, during gastrulation. The amniotic ectoderm and extraembryonic mesoderm also originate from the epiblast.

<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">Hypoblast</span> Embryonic inner cell mass tissue that forms the yolk sac and, later, chorion

In amniote embryology, the hypoblast is one of two distinct layers arising from the inner cell mass in the mammalian blastocyst, or from the blastodisc in reptiles and birds. The hypoblast gives rise to the yolk sac, which in turn gives rise to the chorion.

Heuser's membrane is a short lived combination of hypoblast cells and extracellular matrix.

<span class="mw-page-title-main">Fetal membranes</span> Amnion and chorion which surround and protect a developing fetus

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. The amnion and the chorion are the chorioamniotic membranes that make up the amniotic sac which surrounds and protects the embryo. 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.

This glossary of developmental biology is a list of definitions of terms and concepts commonly used in the study of developmental biology and related disciplines in biology, including embryology and reproductive biology, primarily as they pertain to vertebrate animals and particularly to humans and other mammals. The developmental biology of invertebrates, plants, fungi, and other organisms is treated in other articles; e.g terms relating to the reproduction and development of insects are listed in Glossary of entomology, and those relating to plants are listed in Glossary of botany.

References

  1. "Extraembryonic Mesoderm - an overview | ScienceDirect Topics". www.sciencedirect.com. Retrieved 2023-03-19.
  2. Papaioannou, George I.; Syngelaki, Argyro; Poon, Leona C.Y.; Ross, Jackie A.; Nicolaides, Kypros H. (2010). "Normal Ranges of Embryonic Length, Embryonic Heart Rate, Gestational Sac Diameter and Yolk Sac Diameter at 6–10 Weeks". Fetal Diagnosis and Therapy. 28 (4): 207–219. doi:10.1159/000319589. ISSN   1421-9964. PMID   20847544. S2CID   45159602.
    Crown-rump length before a gestational age of 6 weeks is extrapolated by assuming a gestational sac diameter of 0 at a gestational age of 2 weeks, corresponding to a fertilization age of 0 weeks. This extrapolated part is blurred because of uncertain values.
  3. Gaillard, Frank. "Mean sac diameter | Radiology Reference Article | Radiopaedia.org". Radiopaedia. Retrieved 2 October 2021.
  4. Karki DB, Sharmqa UK, Rauniyar RK (2006). "Study of accuracy of commonly used fetal parameters for estimation of gestational age". Journal of Nepal Medical Association. 45 (162): 233–7. PMID   17189967.
  5. "Basic Imaging > Ultrasound of Early Pregnancy". Archived from the original on 2007-08-14. Retrieved 2007-10-13.
  6. Rodgers, Shuchi K.; Chang, Crystal; DeBardeleben, John T.; Horrow, Mindy M. (1 November 2015). "Normal and Abnormal US Findings in Early First-Trimester Pregnancy: Review of the Society of Radiologists in Ultrasound 2012 Consensus Panel Recommendations". RadioGraphics. 35 (7): 2135–2148. doi:10.1148/rg.2015150092. PMID   26562242 . Retrieved 2 October 2021.