Placenta and Amniotic fluid- structure, Function, and Abnormalities Placenta Human placenta develops from two sources

Placenta and Amniotic fluid- Structure, Function, and Abnormalities

Placenta

• Human placenta develops from two sources

• Fetal component- Chorionic frondosum

• Maternal component- decidua basalis

• Placental development begins at 6 weeks and is completed by 12 th week

• Human placenta is

• Discoid in shape

• Haemochorial

• Deciduate

Placenta at Term- Gross Anatomy

• Fleshy

• Weight-500gm

• Diameter- 15-20 cm

• Thickness-2.5 cm

• Spongy to feel

• Occupies 30% of the uterine wall

• Two surfaces- Maternal and fetal

• 4/5th of the placenta is of fetal origin and 1/5 is of maternal origin

Fetal surface of the placenta

• Covered by smooth and glistening amnion overlying the chorion

• Umbilical cord is attached at or near its centre

• Branches of the umbilical vessels are visible beneath the amnion as they radiate from the insertion of the cord

Maternal surface of the placenta

• Rough and spongy

• Maternal blood gives it dull red colour

• Remanants of the decidua basalis gives it shaggy appearance

• Divided into 15-20 cotyledons by the septa

• Margins of the placenta are formed by fused chorionic and the basal plate

• Placenta is attached to the upper part of the uterine body either at the posterior or anterior wall

• After delivery ,placenta separates with the line of separation being through decidua spongiosum (intermediate spongy layer of the decidua basalis

Structure of the placenta

• Placenta is limited by the amniotic membrane on the fetal side and by the basal plate on the maternal

• Between these two lies the intervillous space filled with maternal blood and stem villi with their branches

• Amniotic membrane- single layer of cubical epithelium loosely attached to adjacent chorionic plate and does not take part in placental formation

• Chorionic plate- forms the roof of the placenta

• From outside inwards consists of

• Syncitotrophoblast

• Cytotrophoblast

• Extraembryonic mesoderm with branches of umbilical vessels

• Basal Plate- forms the floor From outside inwards it consist of

• Compact and spongy layer of decidua basalis

• Layer of Nitabuch

• Cytotrophoblastic shell

• Syncytiotrophoblast

• Basal plate is perforated by the spiral arteries allowing entry of maternal blood into intervillous space

• Layer of Nitabuch - is a fibrinous layer formed at the junction of cytotrohoblastic shell with decidua due to fibrinoid degeneration of syncitotrohoblast

• It prevents excessive penetration of the decidua by the trophoblast

• Nitabuch membrane is absent in placenta accreta and other morbidly adherent placentas

• Intervillous space:

• Numerous branch villi arising from the stem villi project into this space

• It is lined internally on all sides by the syncytiotrophoblast and is filled with maternal blood

• Stem (Anchoring villi )

• Arise from the chorionic plate and extend to the basal plate

• Fetal cotyledon (60-100 ) – derived from one major primary stem villus and is the structural unit of placenta

• Maternal cotyledon (15-20 ) contains 3-5 fetal cotyledons

• Villus is the functional unit of placenta

• Total surface of the villi for exchange varies between 4-14 sq meters

Placental barrier or membrane

• Maternal and fetal blood are separated by placental membrane or barrier (0.025 mm thick )

• Endothelial lining of fetal vessels

• Connective tissue of the villi

• Basement membrane

• Cytotrophoblast

• Syncytiotrophoblast

Placental Function

• Transfer of gases ,nutrients and waste products , namely

• Respiratory function

• Nutritive function

• Excretory function

• Endocrine and enzymatic function

• Barrier function

• Immulogical function

Factors affecting the transfer between mother and the fetus

• Physical properties of the substance- molecular weight, lipid solubility, ionised substances

• Area and functional integrity of the placental membrane

• Rate of blood flow

• Concentration gradient of the substance on either side of the exchange membrane

Mechanism involved in the transfer of substances

• Simple diffusion-O2 and CO2

• Facilitated diffusion ( carrier mediated ) –glucose ,vitamins

• Active transfer ( against concentration gradient )-ions

• Endocytosis- invagination of cell membrane to form intracellular vesicle

• Endocytosis-Release of substances in the vesicles to extracellular space eg IgG immunoglobulin

Respiratory function

• Although fetal respiratory movement occurs, no active exchange of gases takes place

• Intake of oxygen and output of carbon dioxide take place by simple diffusion across the fetal membrane

• O2 delivery to the fetus is at the rate of 8 ml/kg which is achieved by cord blood flow of 160-320ml/min

Excretory function

• Waste products from the fetus such as urea, uric acid, cretinine are excreted to the maternal blood by simple diffusion

Nutritive function

• Fetus obtains its nutrients from the maternal blood

• Glucose- transferred to the fetus by facilitated diffusion

• Lipids for fetal growth and development has dual origin. They are transferred across the fetal membrane or synthesised in the fetus

• Amino acids are transferred by active transport

• Water and electrolytes- Na, K ,Cl cross by simple diffusion, Ca , P, and Fe cross by active transport

• Water soluble vitamins are transferred by active transport but the fat soluble vitamins are transferred slowly

Barrier Function

• Placental membrane is thought to be a protective barrier for the fetus against harmful agents in the maternal blood

• Substances with large molecular weight or size like insulin or heparin are transferred minimally

• Only IgG ( not IgA or Ig M )antibodies and antigens can cross the placental barrier

• Most drugs can cross the placental barrier and some can be teratogenic

• Various viruses, bacteria, protozoa can cross the placenta and affect the fetus in utero

Immunological function

• Inspite of foreign paternally inherited antigens in the fetus and placenta, there is no graft rejection due to immunological protection provided by the placenta

Endocrine and Enzymatic function

• Placenta secretes various hormones – Protein hormones like HCG, human placental lactogen,pregnancy specific beta 1 glycoprotein,,pregnancy associated plasma protein, steroidal hormones like estrogen and progestrone

• Enzymes secreted are diamine oxidase-which activates the circulatory pressor amines,oxytocinase which neutralizes oxytocin, phospholipase A2 which synthesizes arachidonic acid

Placental abnormalities

• Placenta succenturiata (3%)

• One or more small lobe or cotyledon of placenta may be placed at a varying distance from the main placental margin

• A leash of vessels connecting the main to the small lobe traverse through the membranes

• Accessory lobe is developed from activated villi on the chorionic laeve

• Clinical significance-

• If succenturiate lobe is retained following birth of placenta it may lead to

• PPH

• Subinvolution

• Uterine sepsis

• Poly formation

• Treatment- exploration of the uterus and removal of the lobe

Circumvallate placenta

• Development-

• Due to smaller chorionic plate than the basal plate

• The chorionic plate does not extend into the placenta margin

• The amnion and chorion are folded and rolled back to form a ring leaving a rim of uncovered placental tissue

• Morphology

• Fetal surface has a central depressed zone surrounded by a usually complete thickened white ring made up of double fold of amnion and chorion

• Branching vessels radiate from the cord insertion upto ring only

• Area outside the ring is thicker, elevated and rounded

• Clinical significance

• There are more chances of –

• Miscarriage

• Hydrorrhoea gravidarum

• Antepartum haemorrhage

• Preterm delivery

• Fetal growth restriction

• Retained placenta or membrane

Placenta marginata

• A thin fibrous ring is present at the margin of the chorionic plate where the fetal vessels appear to terminate

Membranous placenta

• The whole of the chorion is covered by functioning villi and thus placenta appears as thin membranous structure on ultrasonography

Chorioangioma

• Are the most common benign tumors of the placenta and are hamartomas of primitive chorionic mesenchyme

• Small tumors may be asymptomatic but large tumors may be associated with hydroamnios and antepartum haemorrhage

Amniotic fluid

• It is the fluid in the amniotic sac surrounding the fetus

• Origin – both mother and fetus

• Transudation from maternal circulation across the placental surface and fetal membranes

• Active secretion from amniotic epithelium

• Transudation across surface of umbilical cord and fetal placental circulation

• Contribution from fetal urine

• Tracheobronchial secretion

• Transfer across fetal skin

• Volume- varies according to the gestational age

• Measures

• 12 weeks – 50 ml

• 20 weeks- 400 ml

• 36 weeks- 800ml-1 liter

• At term - it reduces to apprx 700ml

• Composition-

• Organic constituents

• Proteins-0.3 mg/dl

• Glucose- 20mg/dl

• Urea- 30 mg/dl

• Non protein nitrogen-30mg/dl

• Uric acid – 4 mg/dl

• Creatinine -2 mg/dl

• Lipids- 50 mg/ dl

• Hormones- insulin,prolactin, renin

• Inorganic constituents- Na, K,Cl

• Suspended particles- Lanugo,Desqamated fetal skin cells,vernix caseosa,shedded amniotic cells, cells from thr respiratory tract,GIT,Genitourinary tract

• Physical features

• Faintly alkaline

• Low specific gravity-1.010

• Becomes highly hypotonic to maternal serum at term pregnancy

• Osmolarity of 250 mOsmol/liter is suggestive of fetal maturity

• Colour

• In early pregnancy it is colourless

• At term becomes pale straw coloured due to preence of exfoliated lanugo and epidermal cells from fetal skin

Abnormal appearance

• Greenish- due to presence of meconium

• Golden yellow-due to presence of bilirubin resulting from fetal cell hemolysis due to Rh incompatibility

• Greenish yellow- in post maturity

• Dark maroon/ blood stained – due to altered blood in accidental haemorrhage

• Prune juice/dark brown- in presence of retained dead fetus

Functions of amniotic fluid

• During pregnancy

• Act as a shock absorber to protect the fetus from external injury

• Maintains the fetal temprature

• Allows free movement and growth of fetus

• Prevents adhesion formation between the fetal parts and the amniotic sac

• Has some nutritive value because of small amount of protein and salt content

• During Labour

• It forms hydrostatic wedge to help dilatation of cervix

• During uterine contractions , the amniotic fluid in the intact membranes prevents interference with placental circulation

• Provides pool for the fetus to excrete urine

• Protect the fetus from the ascending infections by its bactercidal action

Clinical importance

• Study of amniotic fluid helps in knowing the well being and maturity of fetus

• Intramniotic instillation of prostaglandins and hypertonic saline can be used for induction of abortion

• Artificial rupture of membranes to drain liquor is a method of induction and augmentation of labour

• Excess liquor (polyhydroamnios), less liquor known as (oligohydroamnios ) can be estimated by ultrasound measurement of amniotic fluid index (AFI )

Measurment of AF

• Measurement of AFI- quantitative method of measurement of amniotic fluid by usg. Single largest pocket is measured in four quadrants and added.

• Normal range is 5-24 cm

• Single deepest pocket

• Normal range is 2-8 cm

Polyhydroamnios

• Defined as excess of amniotic fluid of more than 2000ml or AFI> 25 cm or SDP>8cm

Etiology

• Idiopathic- seen in 2/3rd of the cases

• Fetal causes-

• Anencephaly

• spina bifida

• Esophageal and duodenal atresia

• Facial cleft and neck masses

• Congenital diaphragmatic hernia

• Fetal sacrococcygeal teratoma

• Fetal infections

• Hydrops fetalis

• Multiple pregnancy

• Placental causes- choriangioma of the placenta

• Maternal causes- Diabetes, cardiac or renal disease

• Types

• Acute- sudden increase

• Chronic- gradual increase

• Symptoms- breathlessness due to mechnacial compression, edema of legs, varicosities in legs,

• Signs-Abdomen is markedly distended, skin is tense,shiny fundal height >POG,

Complications

• Maternal

• During pregnancy-

• Incresed incidence of preeclampsia

• Malpresentation

• Premature rupture of membranes

• Preterm labour

• Abruptio placentae

• Cardiorespiratory embrassment

• During labour

• Premature rupture of membranes

• Cord prolapse

• Uterine inertia

• PPH

• Puerperium

• Subinvolution

• Puerperal sepsis

• Fetal Complications

• High perinatal mortality due to prematurity and congenital malformations

Management

• Rule out fetal congenital anomalies

• Bed rest

• Amnioreduction- 1-1.5 liters of amniotic fluid is removed over 3 hours to relieve maternal distress

• Indomethacin therapy- impairs lung fluid production,enhances absorption of amniotic fluid, decreases fetal urine production,increases fluid movement across fetal membranes

• Dose – 1.5-3 mg/kg from 24-35 weeks for 2 weeks

• S/E- premature closure of patent ductus arterious

Oligohydroamnios

• Amniotic fluid is less than 200 ml at term or AFI < 5 cm OR SDP< 2 cm

• Etiology

• Fetal chromosomal anomalies

• Intrauterine infections

• Drugs- PG inhibitors, ACE inhibitors

• Renal agenesis or obstruction of the urinary tract

• IUGR associated with placental insufficency

• Amnion nodosum-failure of secretion by the cells of the amnion

• Postmaturity

• Diagnosis

• FH
  

• The uterus is full of fetus because of scanty liquor

• Malpresentation is common

• Complications

• Fetal

• Abortion

• Adhesions due to intramniotic adhesions

• Fetal pulmonary hypoplasia, cord compression

• Maternal

• Prolonged labour due to inertia

• Increased operative interference due to malpresentation