Placenta Dr. Lubna Nazli

Objectives Decidua Formation of chorionic villi Functions Anomalies

The Placenta The placenta is the site of nutrient, gas, and waste exchange Secretes hormones that maintain pregnancy Trophoblast cells release human chorionic gonadotropin (hCG) which signals the corpus luteum to enlarge and produce progesterone Placenta acts as a barrier to prevent harmful bacteria from reaching fetus. However certain substances cross the placental barrier like O2 and CO2, Rhesus factor, HIV virus and syphilis bacteria. Maternal antibodies(Immunoglobulins)reach the fetus to give immunity.

Development of the Placenta Morula stage: On day 8 enters uterine cavity and starts to undergo changes:- – fluid accumulates in intercellular spaces between the centrally placed cells [inner cell mass] as fluid volume increases, spaces on one side become confluent to form a single cavity [blastocyst] As blastocyst [day 9] enlarges, it presses on zona pellucida which disappears and trophoblast comes in contact with decidua -->implantation [day 10-11]

Contact of polar trophoblast to decidua induces proliferation of the trophoblast at the embryonic pole. Primitive Trophoblast differentiates into: 􀂾Syncytiotrophoblast 􀂾Cytotrophoblast Hydrolytic enzymes secreted by trophoblast digest the extracellular matrix between the endothelial cells to allow the embryo to penetrate completely into the decidua. This is called interstitial implantation.

Decidua After implantation the uterine endometrium is called decidua. Changes occur in the stromal cells which is called decidual reaction. The portion where placenta is formed is called decidua basalis. The decidua that separates embryo from uterine lumen is decidua capsularis and the part lining the rest of uterine cavity is called decidua parietalis. The decidua capsularis eventually fuses with the decidua parietalis.

Decidua

Chorion frondosum Chorion laeve Decidua basalis Decidua capsularis Decidua parietalis

Formation of chorionic villi Functional elements of placenta are finger like processes called villi. Villi contain fetal blood and they are surrounded by maternal blood. Villi are formed as offshoots from trophoblast. Trophoblast + extraembryonic mesoderm = chorion ( thus chorionic villi)

Chorionic villi are first formed all over the trophoblast but those related to Decidua capsularis are transitory. These degenerate & become smooth: chorion laevae. The other grow considerably along with decidua basalis to form disc shaped placenta. They are called chorion frondosum.

Stages of villi formation The trophoblast at first is made up of single layer of cells. It then multiplies to 2 layers. Syncytiotrophoblast (outer) Cytotrophoblast (inner)

Primary villi : syncytio+ cyto Primary villi : syncytio+ cyto. Adjoining villi separated by intervillous spaces. Secondary villi: syncytio+ cyto+ mesoderm Tertiary villi: syncytio+ cyto+ mesoderm + fetal blood vessels.

Fetoplacental tissue Syncytial lacunae [maternal blood] Syncytiotrophoblast Cytotrophoblast Mesenchyme

The placenta is subdivided into lobes by septa from maternal side. Each lobe is called a maternal cotyledon. It is rough . The fetal surface or chorionic plate is smooth, lined by amnion. The umbilical cord is attached to this surface.

Surfaces of placenta Fetal maternal

Placental membrane It is a membrane which prevents mixing of fetal and maternal blood. It is made up of: (from fetal side) Endothelium of fetal blood vessel Mesoderm Cytotrophoblast Syncytiotrophoblast The interchanges of Oxygen, nutrition and waste products take place through this.

Placental circulation The blood in the intervillous space is supplied by spiral arteries which provide a pulsatile flow of maternal blood. The blood flows through the intervillous space to be drained from the placenta by endometrial veins. The villi are supplied with fetal blood by branches of the umbilical arteries. Blood flows through the capillaries of the villi, where gas exchange occurs, to be collected in the branches of the umbilical vein to be returned to the fetal circulation.

PLACENTAL SIZE – 1st trimester: Placenta > fetus – 12-14weeks: Placenta = fetus – >14 weeks: Placenta < fetus 28 weeks: ratio 1:4 40 weeks: ratio 1:7

Anomalies Acc to attachment of umbilical cord. Central is normal Marginal [battledore] Velamentous Furcate Acc to Placental divisions Bipartate Succenturate Lobed Fenestrated Membrane arrangement – Circumvalate

Types of placenta

Velamentous attachment of umbilical cord from the membranes

Placenta accreta= placenta penetrates the myometrium

Placenta succenturiata=bilobed placenta

Umbilical cord It is a cord connecting the placenta with the fetus. It is having 2 umbilical arteries and 1 umbilical vein suspended in the Wharton’s jelly ( embryonic connective tissue) It is about 45—55 cms / 18—22” in length. It arises from the centre of the placenta, the fetal surface. The umbilical arteries bring deoxygenenated blood for purification to the placenta. The umbilical veins carry oxygenated blood from placenta to the fetus.

The umbilical cord

After birth Placenta is shed off from the uterine cavity, it is expelled out after the delivery of the fetus. If it is retained in the uterus , it leads to postpartum haemorrhage, bleeding continues after labour. The umbilical cord is tied and cut after pulsations stop. The umbilical arteries remains as obliterated medial umbilical folds. The umbilical vein remains as ligamentum teres hepatis, round ligament of liver.

The medial umbilical ligaments