Transcript EXTRAEMBRYONIC MEMBRANE

PREPARED BY: WILSON R. JACINTO
 Emb ryonic
stem cells from
inner cell mass
to produce
restricted stem
cell to produce
neurons and
blood
 Universal requirement - embryo to develop in moist,
protective environment
 Adaptations developed by vertebrates
 Laying and fertilizing eggs in water (oviparous)
 Incubation of embryo internally and give live birth
(viviparous)
 Eggs incubated internally and hatch just in time for
release from the mother’s body (ovoviviparous)
 For nutrition and exchange of gas
 elaboration of a protective shell and a series of cellular
membranes surrounding the embryonic body - A
significant evolutionary step that enable reptiles to lay
eggs capable of developing on land.
 Vital functions of the membranes
 Keep embryo in fluid
 Gas exchange
 Removal or storage of waste materials
 Nutrition
Four Sets of Embryonic Membranes in
Land Vertebrates
 Membranous folds formed by extension of ectoderm
and endoderm underlain with lateral plate mesoderm
 Somatopleure – forms amnion and chorion
 Splanchnopleure – allantois and yolk sac
 Amnion – thin ectodermally-derived membrane
enclosing embryo in fluid-filled sac
 For secretion and absorption of amniotic fluid
 Yolk sac – endodermal,
 involved with nutrition of the embryo in large–yolked
forms.
 Allantois – endodermal, originate from ventral surface
of early hindgut;
 Act as reservoir for storing or removing urinary wastes
and mediate gas exchange between embryo and
surrounding
 Chorion – outermost membrane;
 In species that lay eggs, the principal function is the
respiratory exchange of gases.
 In mammals, nutrition, excretion, filtration and
synthesis (hormone)
Extraembryonic Membrane of the Chick – Yolk Sac
 splanchnopleure grows over yolk surface; yolk as temp.
floor; then completely surround the yolk
 Subcephalic pocket - forms floor of foregut
 Subcaudal pocket – hindgut
 Midgut
 Yolk duct – opening of the midgut
 Yolk stalk – wall of the duct
 Vitelline arc – yolk circulatory arc
 Endoderm of yolk sac – absorptive and synthetic
 Completely resorpted 6 days after hatching
Amnion and chorion
 Derived from extraembryonic somatopleure
 First indication appears about 30 hours of incubation
 Somatopleure is thrown into a head fold and tail fold
and lateral folds
 Continued growth of the amnion results in the
meeting above the embryo, results to scar –like
thickening called amniotic raphe
 The outer layer of somatopleure becomes the chorion
 Chorioamniotic cavity
 Rapid peripheral growth of the somatopleure carries
the chorion about the yolk sac, which it eventually
envelopes
 Thus, chorion eventually encompasses the embryo
itself and all other membranes.
 Important function – transport of Ca++ from shell to
embryo
Allantois
 Arises from the endoderm of the ventral wall of the
hindgut
 Appears late in the third day of incubation
 Pushes out to the extraembryonic coelom
(chorioamniotic cavity)
 Grows rapidly and finally encompasses the embryo
and yolk sac
 The mesodermal layer of allantois fuse with
mesodermal layer of chorion.
 The double-layer of mesoderm develops into vascular
network connected to the embryo by allantoic veins
and arteries
 Primary function is oxygenating blood and relieve
carbon dioxide
 Fuse the chorion – chorioallantoic membrane
 Also, reservoir for secretions coming from developing
excretory organs
 Urea in early stages; uric acid later stages
Formation of Extra-embryonic membranes
and Placenta in mammals
 Mammals form and utilize the same extraembryonic
structures as does the chick with some modifications
 Trace the development of the four extraembryonic
membranes in mammals
Amnion
 Primordial amniotic cavity forms by means of
cavitation within epiblastic components of inner cell
mass
 Cytotrophoblast bounds the primordial amniotic
cavity, then later by the spreading of the epiblast
 Epiblast becomes surrounded by the spreading
extraembryonic mesodermal cells derived from
primitive streak
Yolk Sac
 Hypoblast , which segregated from the inner cell mass
spread to the inner surface of trophoblast – constitute
the primary yolk sac
 Extraembryonic mesoderm (splanchnic mesoderm)
arise from some hypoblast and caudal margin of
primitive streak.
 Secondary yolk sac is formed as the primitive yolk sac
collapses.
Chorion
 Trophoblast cells expands and forms masses
suggestive of the villi – primary villi (2nd week)
 As the chorionic vesicle receives ingrowth of allantoic
vessels and mesoderm – secondary villi
 Villi with a vascular connective tissue core - tertiary
villi (3rd week)
 Tertiary villi is retained throughout pregnancy
 Tertiary chorionic villi contain embryonic blood
vessels that develop to loose connective tissue core.
The vessels connect with vessels in the chorion and
connecting stalk and begin to circulate embryonic
blood about the third week of development.
Placenta
Uterine stromal cells around blastocyst - enlarge, form
glycogen and lipid droplets -- decidual reaction,
decidual cells
The endometrium (lining of the uterus) of the mother
consisting of three regions named by location.
Region
Decidua basalis
Description
Region between the blastocyst and
the myometrium
Decidua capsularis
Endometrium that covers the
implanted blastocyst
Decidua parietalis
All the remaining endometrium
 Decidua capsularis become attenuated and atrophy at
the end of trimester – becomes smooth
 chorion laeve
 Chorion frondosum – with highly developed villi
 Interlocked chorion frondosum of fetus and and decidua
basalis = placenta
 In mammalian embryo, food and oxygen are obtained
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through the placenta – lack of functional lungs and
intestines
Allantoic (umbilical) mesoderm and blood vessels spread
out along chorion – forms placenta
Connected to the umbilical vein (carries oxygenated and
food-laden blood) of the embryo
Umbilical arteries carry wastes to the placenta
Types: epitheliochorial, syndesmochorial, endotheliochorial
and hemochorial
 Fetal hemoglobin has higher affinity to oxygen than
adult hemoglobin
 Myoglobin of fetal muscle has even greater affinity – thus
stores oxygen
 Oxygen and nutrients in the maternal blood in the
intervillous spaces diffuse through the walls of the villi and
enter the fetal capillaries.
 Carbon dioxide and waste products diffuse from blood
in the fetal capillaries through the walls of the villi to the
maternal blood in the intervillous spaces.
Table 8 - Substances that Cross the Placental Membrane
Substances
Examples
Beneficial
Gases
Oxygen, carbon dioxide
Nutrients
Glucose, amino acids, free fatty acids, vitamins
Metabolites
CO2, urea, uric acid, bilirubin, creatine, creatinine
Electrolytes
Erythrocytes
Maternal serum proteins
Steroid hormones
Immunoglobins
Harmful
Poisonous gases
Infectious agents
Drugs
Immunoglobins
Na+, K+, Cl-, Ca2+, PO42Fetal and maternal both (a few)
Serum albumin, some protein hormones (thyroxin, insulin)
Cortisol, estrogen (unconjugated only)
IgG (confers fetal passive immunity)
Carbon monoxide
Viruses (HIV, cytomegalovirus, rubella, Coxsackie, variola,
varicella, measles, poliomyelitis), bacteria (tuberculosis,
Treponema), and protozoa (Toxoplasma)
Cocaine, alcohol, caffeine, nicotine, warfarin,
trimethadione, phenytoin, tetracycline, cancer
chemotherapeutic agents, anesthetics, sedatives,
analgesics
Anti-Rh antibodies
Amniotic fluid
 Amniotic fluid main functions:
 protects the fetus physically,
 room for fetal movements, and
 regulate fetal body temperature.
 produced by dialysis of maternal and fetal blood
through blood vessels in the placenta.
 Later, production of fetal urine contributes to the
volume of amniotic fluid and fetal swallowing reduces
it. The water content of amniotic fluid turns over every
three hours.
Umbilical cord
 A composite structure formed by contributions from:
 Fetal connecting (body) stalk
 Yolk sac
 Amnion
 contains the right and left umbilical arteries, the
left umbilical vein, and mucous connective tissue.
Presence of only one umbilical artery may suggest the
presence of cardiovascular anomalies.
Fetal Circulation
 Fetal circulation involves three circulatory shunts: the
ductus venosus, which allows blood from the
placenta to bypass the liver, and the ductus
arteriosus and foramen ovale, which together allow
blood to bypass the developing lungs. Refer to the
section on changes at birth for more information on
the fates of these structures.
Ductus arteriosus diverts blood
from pulmonary artery to aorta
Foramen ovale – hole in septum
between atria. This brings
blood to ventricle
- may remain open
Clinical Correlations
 Multiple Pregnancy
 Dizygotic twins - derived from two zygotes that were
fertilized independently (i.e., two oocytes and two
spermatozoa). Consequently, they are associated with
two amnions, two chorions, and two placentas, which
may (65%) or may not (35%) be fused. Dizygotic twins
are only as closely genetically related as any two siblings.
 Monozygotic twins (30%) are derived from one
zygote that splits into two parts. This type of twins
commonly has two amnions, one chorion, and one
placenta. If the embryo splits early in the second week
after the amniotic cavity has formed, the twins will
have one amnion, one chorion, and one placenta.
Monozygotic twins are genetically identical, but may
have physical differences due to differing
developmental environments (e.g., unequal division of
placental circulation).
 Erythroblastosis Fetalis
 Some erythrocytes produced in the fetus routinely
escape into the mother’s systemic circulation. When
fetal erythrocytes are Rh-positive but the mother is Rhnegative, the mother’s body can form antibodies to the
Rh antigen, which cross the placental barrier and
destroy the fetus. The immunological memory of the
mother’s immune system means this problem is much
greater with second and subsequent pregnancies.
 Oligohydramnios
 Deficiency of amniotic fluid (less than 400 ml in late
pregnancy). It can result from renal agenesis
because the fetus is unable to contribute urine to the
amniotic fluid volume.
References
 Carlson, B.M. 2000. Patten’s Foundation of
Embryology. 6th ed. New York: McGraw-Hill Book
Company
 Gilbert. S.F. 1997. Developmental Biology. 5th ed. USA:
Sinauer Associates Inc.
 http://www.med.umich.edu/lrc/coursepages/M1/embr
yology/embryo/06placenta.htm