الاثنين 18/11/2013 أ.د.عبد الجبار الحبيطي EMBYRIOLOGY الاثنين 18/11/2013 أ.د.عبد الجبار الحبيطي

The Placenta It is the organ through which exchange of nutrient materials between the mother & fetus takes place .It is developed from 2 sources : 1-Chorionic Frondosum ( Fetal Part ). 2-Decidua Basalis ( Maternal Part). The fetal part is smooth & shiny ,It gives attachment to the umbilical cord& is covered by amnion .The Maternal part is rough and irregular due to the presence of 15-20 elevations called Cotyledons. The full term placenta has the shape of a disc with a diameter of 15-25 cm ( average 20) &and is about 2 cm in thickness ,while it's weight is a bout 500 grams .

THE CHORIONIC FRONDOSUM Is the name given to the trophoblast after the extra-embryonic mesoderm has been formed from it's inner surface.The chorion is formed as follows: A-The outer wall of the Blastocyst before implantation is called the the Trophoblast. B- As the Blastocyst begins to implant itself in to the endometrium,the Trophoblast is differentiated in to 2 layers an outer Syncytiotrophoblast & innmer Cytotrophoblast. C-Extra-embryonic mesoderm is formed from the inner surface of the cytotrophoblast.

Thus the Trophoblast becomes known as chorion and the Blastocyst known as chorionic vesicle.In the early stages of pregnancy the chorionic vesicle becomes floating in between the maternall blood lakes and the nutrient substances pass from these lakes to the chorionic vesicle by simple diffusion.As pregnancy advances the velli formation will increase ,thus primary,secondary & tertiary velli are formed during the 3rd week after fertilization. All villi are formed around the entire circumference of the Blastocyst

As pregnancy advances astem velli or anchoring velli appears which penetrate deep in to the maternal part of the placenta ( Decidua Basalis ) in order to fix the chorionic vesicle to the wall of the uterus. At this time the velli at the embryonic pole continues to grow and penetrate more while at the ab-embryonic pole and other sites of the Blastocyst regress and degenerate to disappear completely & becomes known as Chorionic leave.Those velli at the Embryonic pole increases in number &become known as chorionic frondosum.

THE DECIDUA BASALIS (Maternal Part) The decidua is defined as the endometrium of the pregna nt uterus,or can be defined as the endometrium after the Blastocyst has been completely embedded in it .It includes: 1-Decidua Basalis or the Maternal Part of the Placenta. 2-Decidua Capsularis which forms athin capsule for the Blastocyst. As pregnancy advances the capsularis & parietalis becomes approximated to each other & fuses together, thus the uterine cavity becomes smaller and is obliterated.

The only part of the decidua which continues to grow is the Decidua basalis & forms part of the placenta from the mother side.The Decidua basalis is the most important part & is the part which lies between the blastocyst & the wall of the uterus.It contains the maternal blood in the intervillous spaces which is only separated from the fetal part capillaries by the placental barrier only.

The main structures of the placenta are the followings: 1-Chorionic Velli. 2-Intervillous spaces. 3-Cytotrophoblastic Shell. 4-Placental Septae & Cotyledons. 5-Placental Barrier.

The chorionic villi is the chorionic frondosum & represents the floating villi in to the intervillous spaces.The intervillous spaces are filled with the Maternal blood & is bounded between the basal plate & the chorionic plate.Each placental septa extends from the roof of the intervillous space toward the chorionic plate thus forming compartments known as Cotyledons.

The placental barrier separates the maternal blood from the fetal blood.During the first half of pregnancy the barrier is formed by 4 layers these are Syncytiotrophoblast,Cytotrophoblast ,Mesoderm & Endothelium of fetal capillaries. Later on the barrier becomes thinner & forms from 2 layers only the Syncytiotrophoblast & the endothelial lining of the fetal capillaries. The placenta is fully established by the end of the 4th month and thus the corpus luteum regresses and disappears completely.

  The Functions of The Placenta : 1-Nutritional function. 2-Respiratory function for the exchange of gasses across the Barrier. 3-Excretory function to get rid from urea,creatine & creatinin. 4-Endocrine function to secrete Estrogen, Progesteron, chorionic gonadotrophin & Melanin spreading factors. 5-Protects the fetus against some micro- organism.

Some Terms Premature baby: Is a baby which is borne after 28th weeks of intrauterine life &is able to survive but with difficulties &needs a special care (incubator ). Postmature baby: is the baby which is borne at or after 42 wks of pregnancy. Intrauterine growth retardation (Restriction) : Is the new borne baby whose weight is at or below the 10th percentile for their expected birth weight.

The main causes are the followings: 1-Neurological anomalies or deficiency. 2-Congenital anomalies. 3-Respiratory distress syndrome. 4-Chromosomal anomalies. 5-Congenital infections. 6-Cigarrets smoker & alcoholic mother.

PLACENTAL CIRCULATION Has 2 components , a fetal component through the capillary loops present in the villi and, a maternal component represents the spiral arterioles(in the decidua basalis) pooring their Early in pregnancy simple capillary vessels are formed in three places as in the Embryo, in the connecting stalk & in the chorionic villi and later on these join each other to form a primitive circulation (i.e connecting the embryo with the villi by the stalk. As pregnancy progress the connecting stalk will develop in to the umbilical cord (connects the fetus & placenta),where the cord contains 2 arteries and a single vein. The non-oxygenated blood is collected from the fetus by the 2 umbilical arteries & the oxygenated blood reaches the fetus (from the placenta) by the umbilical vein.

ABNORMALITIES OF THE PLACENTA A-abnormalities with regard to shape as Bilobed & Trilobed placenta but with one cord. B-abnormalities with regards to the number Succenturiate placenta ,where there is accessory placenta .Here the placenta consists of one main part &other small one. C-abnormalities of attachment of the cord as Velamentous or Eccentric attachment. D-abnormalities of position known as Placenta Praevia which is of 3 kinds ,placenta praevia Parietalis,Marginalis & Centralis .These will creat problems before labor in form of bleeding known as Anti- partem haemorhage around the 7th month of gestation due to premature separation of the lower segment of the placenta.