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 stromal cells 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 decidua parietalis • 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 trophoblast and 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 extraembryonic mesoderm. • 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 ---intervillous space. • 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.fetal blood 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 lacunar space. 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) .