Fetal membranes
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Fetal membranes also known as extra embryonic
membranes which are developed from zygote but do not form any part of the
embryo proper, only functions for
the protection or nourishment or respiration or excretion of a developing fetus.
Name of fetal membrane
1.
Yolk
sac,
2.
Vitelline
duct,
3.
Chorion,
4.
Amnion,
5.
Allantois,
6.
Connecting
stalk
7.
Umbilical
cord,
8.
Placenta
Yolk sac
The yolk sac is
a membranous sac attached to the ventral aspect of an embryo, formed by
cells of the hypoblast adjacent to the embryonic disk.
It develops from the cavity of blastocyst
(blastocele).
•
It has three stages of development
•
Primary yolk sac : the cavity of blastocyst is
converted into primary yolk sac cavity. It is formed by flattened cells derived
from the endoderm. It is formed at the end of 2nd week of
development
•
Secondary yolk sac :primary yolk sac become smaller
due to development of chorionic cavity / the extra embryonic celom
•
Tertiary yolk sac: or definitive yolk sac: it is the
remnants of secondary yolk sac. Due to embryonic folding (cranio caudal folding
and lateral folding) most of the part of secondary yolk sac incorporated within
the embryo and make primitive gut tube, the portion of yolk sac remain outside
the embryo is known as tertiary yolk sac. It connects with the primitive gut
(mid gut) by vitello -intestinal duct.
•
•
Functions/
derivatives of yolk sac
•
Formation of primitive gut
•
Hemopoiesis : it forms blood for embryo until the
liver is formed during the 6th week .
•
Formation of primordial germ cell which leave the
yolk sac and migrate into developing gonads during 4th week of
development.
•
Earliest blood vessels
•
Vitello-intestinal
duct
•
•
The communication between midgut and yolk sac at an
early stage of development is called vitello-intestinal duct
•
Normally it disappears in course of development
•
But in 2% of cases inner extremity of the duct
persists as Meckel’s diverticulum
• Placenta:
It has three parts
•
Fetal
surface:
•
Maternal surface
•
Peripheral
margin
Fetal surface: is smooth covered
by amnion and presents attachment of the
umbilical cord close to the center. Beneath the amnion umbilical vessels
radiate from the cord
Maternalsurface :is rough and
irregular and is marked by 15-20 polygonal areas known as the
cotyledons which are limited by fissures. Each fissure is occupied by a
placental septum
Placental septa are Supporting structures composed of decidual
cells and fibrinoid extending
from the decidual floor of the placenta towardthe chorionic
surface. The septa strengthen the
placenta by partially
Subdividing the intervillous space.
Peripheral
margin :is continuous with the fetal membrane which consists from outside inwards
of fused decidua parietalis and capsularis , chorion leave and amnion
•
Diameter
: 15-20 cm
•
Thickness
: 3 cm
• Weight : 500 gm
it is consists of two components i) a fetal
portion derived from chorion frondosum
and
ii) a maternal portion derived from the decidua basalis
Chorionic
villi: projections of external surface
of the chorion is known as chorionic villi
Type of human placenta: Hemochorial type. Fetal circulation separate from maternal circulation by syncytial membrane and endothelium of fetal capillary
- Function of placenta:
- Exchange of gases
- Exchange of nutrients and electrolytes
- Transmission of maternal antibody to fetus: EX: IgG
- Production of hormones- like human chorionic gonadotropin, progesterone, estrogen etc.
Type of human placenta:
• Hemochorial type:
• Fetal blood separate from maternal blood by chorion
What is placenta praevia?
When placenta is implanted in lower uterine segment of the uterus near the internal os.
The membrane that separates the maternal and fetal blood in placental circulation is called placental barrier.
Constituents:
From maternal to fetal side –
Syncytotrophoblast
Cytotrophoblast
Mesoderm
Endothelium of fetal blood vessels
Location of placenta within uterus
• Exchange of gases
• Exchange of nutrients and electrolytes
• Transmission of maternal antibody to fetus: EX: IgG
• Production of hormones- like human chorionic gonadotropin, progesterone, estrogen etc.
• Barrier fucntion
Abnormal placenta
• Abnormal position : placenta previa , placenta completely or partially attached with lower uterine segment
• Abnormal /variation of shape and size
Circumvallate placenta
Placenta Membranacea
Succenturiate Placenta
Battledore Placenta
• Abnormal adhesion: placenta accreta: when chorionic villi penetrate upto myometrium due to deficient decidua basalis
• Placental lesion : placental infarcts
• Placental tumor : Chorioangioma
• Abnormal weight :
• Low placental weight
• chronic hypertension (before and after accounting for birthweight).
• pre-eclampsia (before, but not after adjustment for birthweight).
• High placental weight
• anaemia (before and after adjustment for birthweight).
• gestational diabetes (before and after adjustment for birthweight).
• smoking (after adjustment for birthweight).
• Placental and cord determinants include chorioamnionitis, chorangioma/chorangiosis, circumvallate placenta and marginal cord insertion.
• congenital syphilis, hydrops fetalis
Amnion
It is a large sac containing amniotic fluid in which fetus is suspended by its umbilical cord
It is lined by ectodermal cell of inner cell mass and amniogenic cell of trophoblast.
Amniotic fluid
Amniotic fluid is a clear, slightly yellowish liquid that surrounds the unborn baby (fetus) during pregnancy.
Source of amniotic fluid
Secretion ofamniogenic cells
Fetal urine
Secretion of placenta and cells of lungs
Composition of amniotic fluid
Hormones : HCG, HPL
Metabolites, Protein, Salt, Urea etc.
Cells that are sloughed off form fetal lungs, placenta and amniotic sac (genetics analysis are done by these cells)
Fetal urine
Clinical application
Function of amniotic fluid:
The fluid allows fetal movement
It absorbs jolts
Prevent adhesion of fetus to surrounded tissue
Fetus swallow amniotic fluid and fetal urine add to the amniotic fluid
Amniocentesis
It is a prenatal test by which the amniotic fluid is aspirated from the amniotic cavity to diagnose genetic disorders. It is usually done 14th or 15th week of pregnancy.
Alpha fetoprotein of amniotic fluid is also tested.
An AFP level between 0 ng/mL to 40 ng/mL is normal for adults.
Elevated AFP
• Omphalocele
• Gastroschisis
• Neural tube defects: ↑ α-fetoprotein in amniotic fluid and maternal serum
Decrease AFP
• Down syndrome (trisomy 21)
it is high in neural tube defects and it is low in Some geneticdisorders like down syndrome.
Amniotic fluid index (AFI) is a quantitative estimate of amniotic fluid and an indicator of fetal well-being. A normal amniotic fluid index is 5 cm to 25 cm using the standard assessment method. Less than 5 cm is considered oligohydramnios(insufficient amount of amniotic fluid) , and greater than 25 cm is considered polyhydramnios (excess amount of amniotic fluid) .
Polyhydramnios is found in anencephaly and esophageal atresia
Oligohydramniosis found in renal agenesis.
• Amniotic bands Pieces of amnion that tear loose and
can wrap themselves around digits and limbs causing
constrictions and amputations or can be swallowed by
the fetus causing disruptions in facial development.
The origin of bands is unknown.
Umbilical cord:is a long cord like structure by which fetus is attached to the uterine wall via placenta.
it is covered by amnion
• It is a tubular cord like structure enveloped by the glistening amniotic membrane
• one end is attached to the center of the anterior abdominal wall of the fetus and the other end is fixed to the centre of the foetal placenta
• Length: 50 cm
Breath : 2 cm
length : 50-55 cm
Breadth : 1-2 cm
It contains
· two umbilical arteries and
· one umbilical vein(left umbilical vein)
· Wharton jelly
· Remnants of allantoic diverticulum
· Remnats of vitellointestinal duct (remnant of tertiary yolk sac )
· Functions
· The umbilical cord carries oxygenated blood and nutrients from the placenta to the fetus
- Occasionally, there is only the one single umbilical artery (SUA) present in the umbilical cord.Babies with SUA may have a higher likelihood of having other congenital abnormalities, especially of the heart.
Connecting stalk
It is an embryonic structure that is formed by the third week of developmentand connects the embryo to its shell of trophoblasts.
The connecting stalk is derived from the extraembryonic mesoderm
Initially it lies caudally to the trilaminar germ disc, but, with subsequent embryonic folding, the body stalk assume a more ventral position.
Progressive expansion of the amnionfrom the umbilical ring (surrounding the roots of the vitelline duct and connecting stalk) creates a tube with a covering of amniotic membrane with allantois and umbilical vessels as its content and mesoderm of the connecting stalk as the ground substance.
This extraembryonic mesodermal ground substance forms the future wharton's jelly. The amniotic membrane and its contents form the umbilical cord that connects the embryo and the placenta.
Allantois : it is an endodermal diverticulum arises from caudal part of the yolk sac during 3rd week .
Formation : by endoderm & splanchnic mesoderm
Fate of allantois :
• at 1st it is arises from caudal part of yolk sac
• Then it grow in to the connecting stalk
• Then it is connected with the cloaca ( dilatedcaudal part of the hindgut)
• Ultimately it becomes part of umbilical cord
• Ultimately it is obliterates and forms a fibrous cord called urachus, which extends from apex of urinary bladder to the umbilical
• In adult urachus known as medical umbilical ligament
Function :
• Allantoic vessels grows within it which become fetal umbilical arteries and vein
• In adult: it is formed median umbilical ligament and apex of urinary bladder
Congenital anomalies of Urachus
normally it is obliterated to form a fibrous cord in adult called median umbilical ligament
Following congenital anomalies may occur in urachus
Urachal sinus : it the lumen persists in the upper part of urachus
Urachal fistula it the lumen persists along the entire extents of urachus
Urachal cyst if only a small part of the urachus remains with secretory activity leads to the formation of a cystic dilatation
Twinning: when a mother gives birth to two young individuals in single pregnancy it is known as the twinning
Types of twinning:
Monozygotic twin,-identical developed from single fertilized ovum
Dizygotic twin: non-identical twin ,( two separate ovum and two separate sperm)
Conjoint twin: identical twin failed to separate completely
Parasitic twin: A parasitic twin is a rare condition that happens when a conjoined twin stops developing. The parasitic twin never fully develops but stays attached to its twin, who continues to develop until birth. A parasitic twin happens early in embryonic development, so its organs or limbs can be nearly unrecognizable.
one of the twins fully developed but other form rudimentary fetus due to lack of blood supply
- Occasionally, there is only the one single umbilical artery (SUA) present in the umbilical cord.Babies with SUA may have a higher likelihood of having other congenital abnormalities, especially of the heart.
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