EMBRYO FinalsMrs. CuicoGroup 9 Reporting

EXTRAEMBRYONIC MEMBRANE

Overview of the extraembryonic/fetal membrane The extraembryonic/ fetal membranes are structure that develop from zygote but do not form any part of

the embryo proper. There are:

1. Amnion2. Yolk Sac3. Allantois 4. Chorion5. Placenta6. Umbilical cord

But in this presentation we only focus on the first 4 membranes since the last two should be covered by Placenta Group

AMNION1. It is a thin extraembryonic membrane that loosely envelops the embryo forming an amniotic sac that is

filled with the amniotic fluid2. The amniotic sac is lined by ectodermal cells of inner cell mass and amniogenic cells of trophoblast3. As the amniotic sac enlarges during the late embryonic period (about 8 weeks) due to collection of more

amniotic fluid within it, the amnion gradually surrounds the whole embryo and unsheathes the developing umbilical cord

The amniotic cavity grows at the expense of extraembryonic coelom, which gets obliterated and fusion occurs between amnion and chorion

Location of Amnion

Layers of Amnion

Consists of two layers:1. Outer layer made up of somatopleuric layer of extraembryonic membrane2. Inner layer made up of amniogenic cells

What is Amniotic Fluid? It is a clear, watery fluid containing salt, sugar, urea, and proteins. It is derived from:

1. Amniotic cells by filtration or secretion2. contains Fetal urine when kidneys start functioning3. Secretion of lung cells4. Secretion by placenta

Constituents of amniotic fluid1. M etabolites and hormones (HCG, HPL)2. Cells that are sloughed off from fetal lungs, placenta, and amniotic sac (all these cells have same genetic

composition)3. Fetal urine

Functions of the amniotic fluid1. Permits symmetrical development and growth. 2. Provides a water-cushion to protect the developing embryo and fetus from jolts that the mother may

receive3. Helps to maintain a consistent pressure and temperature4. Allows free fetal movement – an important prerequisite for musculoskeletal development and blood flow 5. forms hydrostatic bag (bag of waters) that helps in dilatation of the cervix at the beginning of the labor

Clinical correlation of Amnion1. Amniocentesis2. Oligohydramnions3. Polyhydramions

What is Amniocentesis? is the procedure by which Amniotic fluid is aspirated from the amniotic cavity for diagnostic purposes.

1. (During 14th or 15th weeks of pregnancy)2. (175 – 225ml of amniotic fluid is to be extracted)

Amniocentesis: Removal of Amniotic Fluid

Uses Of Amniocentesis To examine the chromosomes in cells of amniotic fluid. To detect defective enzyme To detect neural tube defects( - fetoprotein indicates neural tube defects) Sex of a fetus can also be detected because fetal urine is added to the amniotic fluid (by detection of

male/female hormone.)

Oligohydramion It is clinical condition in which the volume of amnionic fluid is less than normal

o Normal amount: 700 – 1000mlo oligohydramnios: 400 ml or less

Cause of oligohydramiono Placental insufficiency with reduced placental blood flow.o Aged kidneyso Loss of amniotic fluid due to preterm rupture of amnion.

Polyhydramions Is excessive accumulation of amniotic fluid (2000ml or more) in the amniotic fluid. Cause of polyhydramion

o Occurs due to esophageal atresia or defects of central nervous system(CNS) because of which the fetus is unable to swallow the amniotic fluid and consequently it is not absorbed in the gastrointestinal tract (GIT) of fetus.

o

YOLK SAC

What isYolk Sac? A membrane outside the embryo that is connected by a tube (called the yolk stalk) through the umbilical

opening to the embryo’s midgut. Endodermal sac lying ventral to the embryonic disc Vestigial structure in human with nutritive yolk inside

Development of Yolk Sac Yolk Sac develops from the cavity of blastocyst(blastocoele) and passes through the 3 stages of

development: 1. Primary Yolk sac2. Secondary Yolk Sac3. Tertiary Yolk Sac

1. Primary Yolk Cavity of blastocyst is converted into primary yolk sac when flattened cells (derived from

endoderm of embryoblast) form a lining. This lining is called Heuser’s membrane is Attached to undersurface of the embryonic disc is formed at the end of the 2 nd week

2. Secondary Yolk Sac Cells lining the yolk sac becomes cuboidal in shape Appearance of extra embryonic coelom Primary yolk sac becomes smaller and is termed secondary yolk sac

3. Tertiary Yolk Sac or Definitive Yolk sac Remnant of secondary yolk sac Takes up most of the yolk sac inside the body as embryo folds to form primitive gut Portion of yolk sac not taken up inside the embryo’s body is termed as tertiary/definitive yolk

sac Communicates with midgut via vittellointestinal duct.

Functions of Yolk Sac Hemopoiesis

o Produces blood for the embryo until liver is formed during the 6th week Formation of Primitive Gut

o Dorsal portion forms the primitive gut

Formation of Primordial germ cells o Primordial germ cells form from the wall of the yolk sac and migrate to the developing gonads

during the 4th week where primitive germ cells are formed(spermatogonia or oogonia) Formation of Allantois

o Small diverticulum that arises from caudal part of the yolk sac

ALLANTOIS

What is Allantois? Allantois is a small diverticulum that arises from the caudal part of the yolk sac during the third week. It develops and grows into the connecting stalk. After the folding of embryo, the allantois is connected with the dilated terminal of the hindgut called

cloaca.

Functions of Allantois It is vascularized by allantoic vessels that later become fetal umbilical arteries and vein. In adults, it is represented by Median umbilical ligament It also contributes a bit to the formation of urinary bladder

CHORION

What is Chorion and its function? It is highly specialized extraembryonic membrane that participates in the formation of the placenta It is formed by the somatopleuric layer of extraembryonic mesoderm and trophoblast. Numerous small

finger like projections arise from its surface called villi. a. on the side of decidua capsularis, the chorion villi regree/disappear, leaving a smooth surface

called chorion leave(smooth chorion)b. on the side of decidua basalis, the chorion villi further develop and grow in the decidua basalis

to contribute the fetal portion of the placenta. It is called Chorion frondosum(leafy chorion) .