Fetal Membranes And Placenta

Fetal membranes and placenta

Any membrane that functions for the protection or nourishment or respiration or excretion of a developing fetus is called a fetal membrane.

AMNION 

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Derived from ectoderm Completely covers the embryo and lines the fetal aspect of placenta Secretes amniotic fluid to protect the fetus

The amniotic fluid is derived from the amniochorionic membrane, through the fetal skin, from the respiratory tract and urinary tract. The fetus swallows about 400ml of amniotic fluid a day in the final stages of pregnancy.

Amniotic fluid(liquor amnii) increases in quantity up to the sixth or seventh month of pregnancy, after which it diminishes somewhat; at the end of pregnancy it amounts to about 1 liter.

Amniotic fluid It allows of the free movements of the fetus during the later stages of pregnancy, and also protects it by diminishing the risk of injury from without.

An abnormally High levels of amniotic fluid, is called polyhydramnios (>2000ml). Low levels of amniotic fluid (<1000ml at term) is called oligohydramnios. Both conditions are associated with congenital anomalies(birth defects)

amniocentesis

chorion

chorion 

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The outer membrane derived from the wall of the blastocyst Made up of syncitiotrophoblast, Cytotrophoblast, and Extra embryonic mesoderm(from outside to inside)

Chorion forms fingerlike projections called villi Where the villi are present,it is Chorion frondosum(fetal part of the placenta). where the villi disappear and is smooth is called chorion laeve.

Placenta and fetal membranes

Smooth,fetal part

Rough,maternal part

(cotyledons)

Chorionic villi are the functional units of placenta. Fragments of these villi can be studied for congenital anomalies of the fetus. This procedure is called Chorion villus sampling(CVS) or chorion villus biopsy

Allantois(allantoic diverticulum)

Allantoic divericulum 

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A projection from the yolk sac in to the connecting stalk.(body stalk) After the tail folding of the embryo, it is incorporated into the hindgut.

allantois In adults, it remains as

median umbilical ligament  connecting the apex of the urinary bladder  to the umbilicus. 

allantois It collects waste materials from the embryo, and can be a site of gas exchange.

Urachus and anomalies 

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allantois(1) urinary bladder(2)

A partially obliterated allantois in fetal life is called Urachus. This will obliterate postnatal and becomes the median umbilical ligament. Any failure to do so results in patent Urachus(weeping umbilicus)

yolk sac

Yolk sac

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Yolk sac: Not all yolk has to do with birds' eggs. human beings have a yolk sac, too. The human yolk sac is a membrane outside the embryo that is connected by a tube (the yolk stalk) through the umbilical opening to the embryo's midgut.

yolk sac The

yolk sac serves as an early site for the formation of blood and in time is incorporated into the primitive gut of the embryo.

Primordial

germ cells are formed in the yolk sac

Yolk sac

Human placenta placenta: an organ that develops from fetal membranes and functions to pass oxygen, nutrients, and other substances to and waste material from the fetus.

Decidua After implantation the endometrium is called ‘Decidua’  Divided into D.basalis  D.capsularis  D.parietalis 

Parts of placenta Maternal part from Decidua basalis Fetal part from chorion frondosum

Stages in the formation of Chorionic villi

1.primary villus--- has an outer syncitio trophoblast

and inner cyto trophoblast 2. Secondary villus--- syncitio trphoblast+ cyto trophoblast+ Extra embryonic mesoderm 3. Tertiary villus--- above structures+fetal blood vessels

Placentation

Figure 28.7a-c Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Placentation

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Figure 28.7d

Placentation

Figure 28.7f Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Chorion laeve

After birth The placenta,chorion,and amnion are expelled approx. after 30 minutes after the child delivery by uterine contractions.  This process is called afterbirth 

After birth (the shedding of placenta) Placenta Umbilical cord

Partially dilated cervix First stage of labor: Cervical dilation

Uterus Second stage of labor: Delivery of the baby

Third stage of labor: Delivery of the placenta

Gross features of placenta

The human placenta is defined as Discoidal Deciduate Hemochorial Chorioallntoic Villus type

Placenta at term About 500g in weight Has rough maternal surface showing cotyledons And a fetal surface which is smooth and To which the umbilical cord is attached.

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Abnormalities of placenta (variations)

Bi lobate placenta

Placenta succenturiata 

A small part of the placenta is separate from the main mass

Battledore placenta 

The umbilical cord is attached to the periphery.

Velamentous placenta 

The umbilical vessels divide into many branches before entering placenta

Diffuse placenta

Chorionic villi persist at the abemembryonic pole also resulting in diffuse placenta

Circumvallate placenta

Abnormalities of the placenta • Placenta previa – placenta implantation at the lower segment of the uterus – obstructing the descent of the baby’s head – presented with antepartum hemorrhage

Placenta previa