Fetal Adnexa

The fetal adnexa are the placenta, umbilical cord, and chorioamnion ("fetal membranes"), all of which are identifiable in the "afterbirth". Except for a small maternal contribution to the placenta, the fetal adnexa develop from extra-embryonic tissues of the conceptus. During the embryonic period, the precursors of the fetal adnexa are the developmental adnexa, which include the extra-embryonic membranes (amnion, chorion, umbilical vesicle, and allantoic diverticulum) and the connecting (or body) stalk. Examination of the afterbirth may be useful in cases of congenital anomalies, intrauterine growth retardation, and other neonatal conditions, and in twin pregnancy.

Fetus and fetal adnexa in utero, between 5 and 6 months. Gray's Anatomy (1918) figure 38. Source: Wikimedia Commons

Developmental Adnexa

Conceptus at about 2.5 weeks. Bilaminar embryonic disc lies between amniotic cavity and cavity of the secondary umbilical vesicle ("yolk sac"). Note the appearance of extra-embryonic mesoderm (red) adjacent to the amniotic ectoderm, trophoblast and extra-embryonic endoderm produces the amnion, chorion and secondary umbilical vesicle, respectively. Gray's Anatomy (1918) figure 24. Source: Wikimedia Commons

Conceptus at about 3 weeks. Note formation of allantoic diverticulum and connecting (or body) stalk. Gray's Anatomy (1918) figure 25. Source: Wikimedia Commons

Conceptus at about 3.5 weeks. The trilaminar embryonic disc is beginning to fold. Note the expansion of the amniotic cavity. (The differentiation of the villous and smooth chorion occurs later.) Gray's Anatomy (1918) figure 26. Source: Wikimedia Commons

Conceptus at about 4 weeks. Folding of the embryo produces the embryo proper. With the formation of the primitive gut, the allantoic diverticulum now appears to be a diverticulum of the hindgut. Note the continued expansion of the amniotic cavity. Gray's Anatomy (1918) figure 27. Source: Wikimedia Commons

Amnion

The amniotic (extra-embryonic) ectoderm appears at about one week. During the second week, the amniotic cavity is formed dorsal to the embryonic disc, and filled with amniotic fluid. In the third week, the amnion is formed by the addition of a layer of extra-embryonic mesoderm. During embryonic folding, the amniotic cavity surrounds the embryo, and expands to fill the extra-embryonic coelom (chorionic cavity). The amnion fuses with the chorion laeve to form the chorioamnion (at the end of the first trimester), and lines the amniotic surface of the placenta and umbilical cord.

Chorion

The trophoblast layer forms during the first week, and becomes the chorion in the second week with the addition of a layer of extra-embryonic mesoderm. Chorionic (tertiary) villi form over the entire surface of the chorion. Early in the fetal period, the chorion differentiates into chorion laeve and chorion frondosum.

• The chorion frondosum (or villous chorion) is associated with the decidua basalis. Together they form the placenta.
• The chorion laeve (or smooth chorion) is associated with the decidua capsularis. The chorion laeve fuses with the amnion to form the chorioamnion at the end of the first trimester.

Umbilical Vesicle ("yolk sac" or vitelline sac)

The eggs of birds and reptiles have a large extra-embryonic sac that contains nutritional stores, the yolk sac. In placental mammals, this extra-embryonic membrane does not contain a yolk mass, and, thus, should not be called a yolk or vitelline sac.

In humans, the primary umbilical vesicle (extra-embryonic endoderm) appears in the second week, and becomes the secondary umbilical vesicle in the third week with the addition of a layer of extra-embryonic mesoderm. The secondary umbilical vesicle is important as the site of formation of the first blood vessels and blood cellsin the conceptus, and the site of origin of the primordial germ cells.

In the fourth week, folding of the embryo separates the cavity of the umbilical vesicle from the lumen of the primitive gut, leaving a duct, the omphalo-enteric duct ("yolk stalk" or vitelline duct), connecting the two cavities that is patent for a couple of weeks. At around 6 weeks, the remnants of the umbilical vesicle and omphalo-enteric duct are incorporated into the umbilical cord. Persistence of all or part of the omphalo-enteric duct is the cause of Meckel's diverticulum and related anomalies.

Allantoic Diverticulum ("Allantois")

The eggs of birds and reptiles have a large extra-embryonic sac to store waste products, the allantois.

In humans, the allantois forms in the third week as a diverticulum from the umbilical vesicle ("yolk sac") in the region of the future hindgut and later, after embryo folding, appears like a ventral diverticulum of the hindgut (and, later still, is continuous with the urachus). The intra-embryonic part of the allantoic diverticulum is essentially vestigial, persisting in adults as the median umbilical ligament.

The extra-embryonic part of the allantoic diverticulum is incorporated into the umbilical cord at about 6 weeks. The vessels associated with the allantois become the umbilical arteries and vein. (That is why the human placenta is considered a chorio-allantoic placenta.)

Connecting Stalk

During the third week, the formation of the extra-embryonic coelom (chorionic cavity) reduces the connection between the embryonic disk (and associated amnion and secondary umbilical vesicle) and the chorion to a bridge of extra-embryonic mesoderm at the caudal end of the embryonic disk, called the connecting (or body) stalk. Late in the embryonic period, the connecting stalk becomes covered by amnion, forming the umbilical cord.

Conceptus in the third week (trilaminar embryonic disc). Amnion is composed of amniotic ectoderm and extra-embryonic mesoderm. The amniotic cavity (am.) is enclosed by the ectoderm (ect.) of the trilaminar embryonic disc and the amnion. The amnion is attached to the edges of the embryonic disc. Chorion is composed of trophoblast (tr.) and extra-embryonic mesoderm (mes.). Note the presence of lacunae in the trophoblast, and secondary villi (villi with a mesoderm core, but no blood vessels). Gray's Anatomy (1918) figure 32 (orignally from Peters). Source: Wikimedia Commons

Chorion with primary villi. Primary villi, which are composed only of trophoblast, appear in the second week. Gray's Anatomy (1918) figure 36 (originally from Bryce). Source: Wikimedia Commons

Chorion with tertiary (or chorionic) villi. Tertiary villi, which contain a vascularized mesodermal core, appear in the third week. Gray's Anatomy (1918) figure 37 (originally from Bryce). Source: Wikimedia Commons

Section of the gravid uterus at about the end of the first trimester. The fetus (incorrectly depicted as an embryo) resides within the greatly expanded amniotic cavity. Gray's Anatomy (1918) figure 34 (originally from Wagner). Source: Wikimedia Commons

Fetal Adnexa

Placenta

The placenta is an intra-uterine organ formed by the fusion of extra-embryonic and maternal tissues. The conceptus-derived part comes from the chorion frondosum; and the maternal part (the placental septa) is derived from the decidua basalis. The immature placenta begins to function at about 1 month and is mature at about 4 months. Initially the barrier between fetal and maternal blood consists of chorionic endothelium, mesenchyme, cytotrophoblast and syncytiotrophoblast. In the mature placenta, the barrier is reduced to only the endothelium and synctiotrophoblast.
Its chief function include the following:

• Exchange of gases, nutrients and waste products between the maternal and fetal circulations across the placental membrane.
• Endocrine functions. It produces both protein and steroid hormones:
  • Production of protein hormones, including human placental lactogen (hPL), human chorionic gonadotropin (hCG), human chorionic thyrotropin (hCT) and human chorionic corticotropin (hCACTH)
  • Production of steroid hormones, including progesterone and estrogens

Placenta and Placental Circulation. Two umbilical arteries deliver deoxygenated blood to the capillaries of the placental villi. Oxygenated blood is returned to the fetus via the umbilical vein. Maternal blood in the intervillous spaces is supplied and drained by maternal vessels. Gray's Anatomy (1918) figure 39. Source: Wikimedia Commons

Two placentas. The maternal (decidual) surface of the placenta, seen on the left, shows the lobular pattern of the placental cotyledons. Note the smooth amnion-covered surface of the placenta on the right and the umbilical cord. Source: Wikimedia Commons

Scheme of the Development of the Fetal Adnexa. Source: Illustration by Paul E. Neumann, MD.

Chorioamnion

The chorioamnion is commonly called the "fetal membranes". During development, expansion of the amniotic cavity leads to a fusion of the chorion laeve and amnion. The chorioamnion consists of the amniotic epithelium, its basement membrane, several connective tissue layers, a second basement membrane and the syncytiotrophoblast.
The amniotic cavity is filled with amniotic fluid, which is mainly produced by the fetal kidneys (micturation) and decidual vessels. Amniotic fluid is mainly eliminated by the fetal gut (swallowing) and decidual vessels.
Developmental abnormalities involving the chorioamion and amniotic fluid include the following:

• Excessive amniotic fluid (polyhydramnios) is associated with congenital anomalies, multiple pregnancies, and maternal diabetes mellitus. Polyhydramnios can result from anomalies that reduce fetal ingestion of amniotic fluid (e.g., upper GI tract and CNS anomalies).
• Decreased amounts of amniotic fluid (oligohydramnios) are associated with congenital anomalies, growth retardation, and premature rupture of membranes. Oligohydramnios can result from anomalies that reduce the production or excretion of urine (e.g., renal agenesis and other urinary system anomalies) and may lead to characteristic flattened facies and pulmonary hypoplasia (Potter syndrome).

Umbilical Cord

The umbilical cord is the definitive connecting stalk, linking the fetus and the placenta. Late in the embryonic period, the original connecting stalk becomes covered by amnion, forming the umbilical cord. This investment of the connecting stalk also encloses the umbilical vesicle, omphaloenteric (vitelline) duct and allantoic diverticulum within the umbilical cord.

The essential features of the umbilical cord include amniotic epithelium covering mucoid mesenchymal connective tissue (Wharton’s jelly), which contains two umbilical arteries and a single umbilical vein. These umbilical vessels develop from allantoic vessels.
Developmental anomalies involving the umbilical cord include the following:

• Presence of a single umbilical artery (incidence: 1%) is associated with increased risk of internal congenital anomalies.
• Abdominal wall defects may allow umbilical or paraumbilical herniation of abdominal contents.

Fetal Adnexa in Twin Pregnancy

Multiple pregnancies occur in more than 1% of all pregnancies, with twin pregnancy being the most common form. The chance of multiple pregnancy increases with use of fertility drugs and in vitro fertilization.

Multiple pregnancies increase the risk of fetal morbidity and mortality, especially prematurity, intrauterine growth retardation and congenital anomalies. Monoamniotic monozygotic twins have a very high rate of mortality and congenital anomalies, including incomplete separation (conjoined twins).

Dizygotic twins

Dizygotic twins ("fraternal twins") occur about in about 8 per 1000 pregnancies. Dizygotic twins arise from fertilization of two secondary oocytes. Thus there are two amnions and two chorions. There may be two placentas, or one fused placenta.

Monozygotic twins

Monozygotic twins ("identical twins") occur in about 3.5 per 1000 pregnancies. Monozygotic twins arise from division of the products of conception from the fertilization of a single secondary oocyte. The division of the extra-embryonic tissues is variable, resulting in the following possibilities:

• Diamniotic, dichorionic twin pregnancy (with two or fused placentas) accounts for about 1/3 cases.
• Diamniotic, monochorion twin pregnancy (with one placenta) accounts for about 2/3 cases.
• Monoamniotic, monochorionic twin pregnancy (with one placenta) is rare.

Conjoined twins

Conjoined twins ("Siamese twins") usually arise from a partial division of the products of conception from the fertilization of a single secondary oocyte. Thus there is one aminon, one chorion, and one placenta, and some degree of union between the twins. Conjoined twins are rare, occurring in about 1 per 600 twin pregnancies.
There is marked variation in the site of union, and the extent of sharing organs or body parts. Modern surgical evaluation of the prospects for separation of conjoined twins is chiefly focused on the extent of union involving the heart, liver and brain.

External Links

Human Developmental Anatomy Center (HDAC) of the National Museum of Health & Medicine

• O'Rahilly R & Müller F. Developmental Stages in Humans. Carnegie Institution of Washington, 1987.
  

Embryology.ch - an online course in embryology for medical students

Other Resources

O'Rahilly R and Müller F. Human Embryology & Teratology, 3rd Ed. New York: Wiley-Liss, 2001.