placenta - Manav Rachna

placenta

• the syncytiotrophoblast grows into endometrium.• as endometrium is eroded , some of its blood

vessels are opened up, and blood from them fills the lacunar space.

• each trebaculus is initially made up of syncytiotrophoblast.

• now cells of cytotrophoblast begin to multiply and grow into trebaculus .

• the trebaculus thus have a central core of cytotrophoblast covered by outer layer of syncytium.it is surrounded by maternal blood , filling the lacunar space.

• after implantation . The cells linning surface of morula , constitute trophoblast .

• now after the implantation of embryo , the uterine endometrium is called decidua .

• when morula reaches the uterus , the endometrium is in secretory phase .after implantation ,the features are maintained & intensified.

• the stromal cells enlarge, become vacuolated & store glycogen & lipids.this change in stromalcells is called decidual reaction.

• the portion of decidua where placenta is to be formed---decidua basalis.

• part of decidua which seperates embryo from uterine lumen—decidua capsularis.

• part lining rest of uterine cavity is called deciduaparietalis

• decidua basalis has large amount of lipids & glycogen ( provides nutrition for embryo), also referred as decidual plate & firmly adherent to chorion.

• at end of pregnancy , decidua is shed off, along with placenta & membranes .

Formation of chorionic villi

• villi are finger like essential functional elements of placenta .

• these villi are surrounded by maternal blood.• In the substance of villi , there are capillaries

through which fetal blood circulates.• Villi are formed as offshoots from surface of

trophoblast.• trophoblast + extraembryonic mesoderm

=constitutes chorion, the villi , arising from it called –chorionic villi

• the chorionic villi are first formed all over trophoblastand grow into surrounding decidua.

• villi related to decidua capsularis are transitory, after some time they degenerate.

• this part of chorion becomes smooth and is called chorion leave.

• while villi that grow into decidua basalis undergo considerable development.

• along with tissues of decidua basalis these villi form a disc shaped mass which is called placenta.

• the part of chorion that helps to form the placenta is called chorionic frondosum.

Features of chorionic villi

• the trophoblast is first made of single layer of cells.

• as these cells multiply , two distinct layers are formed.—the cells that are nearest to decidua lose their cell boundaries.

• thus one continous sheet of cytoplasm containing many nuclei ---such a tissue is called a syncytium . Hence called syncytiotrophoblast .

• deep to it the cells retained their cell walls and form second layer called cytotrophoblast( langhan’s layer).

• The cytotrophoblast rests on extraembryonicmesoderm.

• all these elements take part in forming chorionic villi.

•

• Each trebaculus is , made up entirely of syncytiotrophoblast .Now the cells of cytotrophoblast begin to multiply and grow into each trebaculus . The trebaculus thus come to have a central core of cytotrophoblst covered by outer layer of syncytium.

• it is surrounded by maternal blood , filling the lacunar space .the trebaculus is now called ---primary villus .

• & lacunar space is now called ---intervillousspace.

• tertiary villi—are like secondary villi except that they are blood capilleries in the mesoderm.

Stages of chorionic villi

• primary villi— consists of central core of cytotrophoblast covered by a layer of syncytiotrophoblast .adjoining villi are seperated by an intervillous space .

• secondary villi--- show three layers : outer syncytiotrophoblast ,an intermediate layer of cytotrophoblast and an inner layer of extraembryonic mesoderm.

• extra embryonic mesoderm now invades centre of each primary villus –i.e mesoderm invades centre of each villus.---secondary villus.

• tertiary villus –the blood vessels of villus establish connections with circulatory system of embryo.fetalblood now circulates through the villi , maternal blood circulates through intervillous space .

• Thus cytotrophoblast grows into trebaculus does not penetrate the entire thickness of syncytium and therefore , does not come in contact with the decidua.

Process of villous formation

• the syncytiotrophoblast grows rapidly and becomes thick . Small cavities ( called lacunae ) appear in this layer.

• the lacunae increase in size . At first they come to lie radially around blastocyst ,

• the lacunae are seperated from one another by portions of syncytium, which are called trebaculae.

• the lacunae gradually communicate with each other , so that eventually one large space is formed. Each trebaculae is surrounded all round by this lacunarspace.

Placental membrane

• maternal blood circulates through the intervillous space and fetal blood circulates through blood vessels in villi , so maternal & fetal blood do not get mixed with each other.

• they are seperated by a membrane , made up of layers of wall of villus.these are ( constitute placental barrier)• endothelium of fetal blood vessels & its basement membrane• surrounding mesoderm• Cytotrophoblast & its basement membrane• syncytiotrophoblast.• total area of this placental membrane varies from 4-14 square meters.• In later part of pregnancy , the efficiency of membrane is increased by

disappearence of cytotrophoblast ic layer from most villi. & by considerable thinning of connective tissue,

• this membrane is initially 0.025 mm thick ,reduced to 0.002 mm later.• towards end of pregnancy fibrinoid deposit appears on membrane which reduces

its efficacy.

Function of placenta

• enables transport of o2, water , electrolytes and nutrition. From maternal to fetal blood ( cCHO, Lipids , polypeptides, amino acids , vitamins.)

• provides for excretion of CO2, urea , and other waste products produced by fetus into maternal blood.

• maternal antibodies ( IgG, Gammaglobulin or immunoglobulins) reaching the fetus through the placenta give the fetus immunity against some infections.

• The placenta act as a barrier and prevents many bacteria and other harmful substances from reaching the fetus.( however most viruses and bacteria including poliomyelitis , measles and rubella and some bacteria can pass through it )

• some drugs also pass through it and can produce congenital malformations.

• materal hormons do not reach fetus however synthetic progestins and oestrogens can easily cross the placenta.

• prevent antigenic reactions as by seperating maternal & fetal blood.

• it synthesizes several hormons., produced by syncytiotrophoblast.( e.g HCG , PROGESTERONE, OESTROGEN)