Embryonic membrane and Placenta, Biology tutorial


Embryogenesis is a procedure by which embryo is created and develops, until it develops in a fetus. It begins with fertilization of the ovum (or egg) by sperm. Fertilized ovum is referred to as the zygote. Zygote goes through rapid mitotic divisions with no important growth. Embryogenesis happens in both animal and plant development.

Extraembryonic membranes:

Extraembryonic membranes or foetal membranes are made up of embryonic tissue, trophoblast which lies outside embryo. Embryos of amniotes, that is reptiles, birds and mammals generate four extraembryonic membranes, yolk sac, amnion, chorion and allantois. In birds and most reptiles, embryo with the extraembryonic membranes develops inside the shelled egg.

1. Amnion: It is formed of mesoderm on outside and ectoderm inside and is devoid of blood vessels. The space between amnion and foetus is known as amniotic cavity filled with amniotic fluid. Embryo is completely immersed and bathed in amniotic fluid that acts as an effective shock absorber.

2. Chorion: It is made up of ectoderm externally and mesoderm inside. It forms placenta along with allantois. It takes part in exchange of gases between embryo and outside air and also gives nourishment to developing embryo.

3. Allantois: It is made up of mesoderm externally and endoderm inside. It is precursor of mature umbilical cord. It is mainly found in blastocyst phase of early embryological development, and its objective is to gather liquid waste from embryo.

4. Yolk sac: Yolk sac is located on ventral aspect of embryo; it is lined by endoderm, outside of which is layer of mesoderm. In mammals yolk sac starts to form in early gastrulation. As compared to yolk sac of birds, the mammalian yolk sac is not filled with yolk as the nutritive material as in mammals function of nutrition is done by placenta.

With these four membranes, developing embryo is able to continue necessary metabolism whereas sealed inside egg. Enclosed by amniotic fluid, embryo is kept as moist as the fish embryo in the pond. In placental mammals, extraembryonic membranes form the placenta and umbilical cord, that connect embryo to mother's uterus in the more elaborate and efficient way.


In viviparous animals, egg has no reserve food material and developing embryo depends entirely on mother for the nourishment. So embryo gets joined to uterine wall and gets nourishment from until birth. This organic connection between foetus and uterine wall is known as placenta that develops at point of implantation. It is formed of both foetal and maternal tissues; allantois gives rise to umbilical cord that has blood vessels connecting foetus and placenta. Most primitive kind of true placenta is formed of given layers of foetal and maternal tissues:

Foetal layers: i. Foetal blood capillaries; ii. Foetal connective tissue; iii. Foetal chorionic epithelium.

Maternal layers: i. Uterine epithelium; ii. Uterine connective tissue; iii. Maternal blood capillaries.

Kinds of Placenta:

I. Based on Histology:

1. Epitheliochorial: It is the most primitive kind of placenta with all the 6 barriers between foetal and maternal blood. Like marsupials, ungulates (pig, horse, etc.).

2. Syndesmochorial: Chorionic villi erode uterine wall and so uterine epithelium is ruptured, with only 5 barriers left. Like sheep, cow (that is ruminant ungulates).

3. Endotheliochorial: Chorionic villi erode not only uterine epithelium but also uterine connective tissue, with only four barriers left. Like tiger, lion, dog, cat and other carnivores.

4. Haemochorial: Here all three layers of maternal part is eroded, so placenta is left with 3 barriers only. Like humans, apes.

5. Haemoendothelial: All the barriers except foetal capillaries are eroded. Like rabbit, rat.

II. Based on Degree of Association:

1. Non-deciduate: Implantation of embryo in uterus is superficial. At the time of birth, no part of uterine portion of placenta is broken off and no bleeding takes place. Like cattle, pigs, horse etc.

2. Deciduate: In this case degree of contact between foetal and maternal tissue is more intimate. At the time of birth foetal part of placenta splits from uterine part of placenta due to which portion of uterine tissue known as decidua is detached and passes out at birth.

3. Contradeciduate: Degree of contact between foetal and maternal tissue is same as in deciduate kind but at the time of birth maternal and even the foetal part of placenta is retained to give nourishment. Like Talpa (mole), Parameles.

III. Based on Distribution of Villi:

1. Diffused: Villi distributed equally all over the surface of chorion. Like ungulates (horse, pig).

2. Cotyledonary: Villi distributed in the form of isolated patches. Like ruminants (sheep, cow).

3. Zonary: Villi arranged in definite transverse bands or girdles. Like carnivores (cat, dog, lion).

4. Discoidal: Villi confined to one (monodiscoidal) or two (bidiscoidal) disc-like areas. Like E.g. rat.

Functions of Placenta:

1. Filtration and transfer: The placenta gets nutrients, oxygen, antibodies and hormones from mother's blood and passes out waste. It creates the barrier, placental barrier that filters out some substances that could harm fetus.

2. Metabolic and endocrine activity: Additionally to transfer of gases and nutrients, placenta also has metabolic and endocrine activity. It generates, among other hormones, progesterone that is significant in maintaining pregnancy; somatomammotropin (called as placental lactogen), that acts to increase amount of glucose and lipids in maternal blood, oestrogen; and human chorionic gonadotrophin.

3. After delivery: When fetus is delivered, placenta is delivered afterwards (and hence is frequently known as afterbirth). After delivery of placenta umbilical cord is generally clamped and severed or may be left joined to fall off naturally that is referred to as Lotus Birth.

Placenta previa:

Bleeding may take place at different times in pregnancy. Though it is alarming, it may or may not be the severe complication. Bleeding in first trimester of pregnancy is fairly common and may be because of implantation of placenta in uterus, miscarriage (pregnancy loss), ectopic pregnancy (pregnancy in fallopian tube), gestational trophoblastic disease (an exceptional condition which may be cancerous in which the grape-like mass of fetal and placental tissues develops) or infection. Bleeding in late pregnancy (after approx 20 weeks) may be because of placenta previa (placenta is near or covers cervical opening) or placental abruption (placenta detaches prematurely from uterus).

There are three kinds of placenta previa:

1. Total placenta previa - placenta entirely covers cervix.

2. Partial placenta previa - placenta is partly over cervix.

3. Marginal placenta previa - placenta is near the edge of cervix.

The reason of placenta previa is unidentified, but it is related with certain cases like:

  • Women who have scarring of uterine wall from earlier pregnancies.
  • Women who have fibroids or other abnormalities of uterus
  • Women who have had earlier uterine surgeries or cesarean deliveries
  • Older mothers (over age 35)
  • Cigarette smoking
  • Placenta previa in the earlier pregnancy

Risks comprise the following:

  • Abnormal implantation of placenta
  • Slowed fetal growth
  • Preterm birth
  • Birth defects
  • Infection after delivery


Specific treatment for placenta previa will be determined by a physician based on (i) pregnancy, complete health, and medical history; (ii) extent of condition; (iii) ttolerance for specific medications, processes, or therapies. There is no treatment to change position of placenta. Once placenta previa is identified, extra ultrasound examinations are frequently done to track location. It may be essential to deliver baby, depending on amount of bleeding, gestational age, and situation of foetus. Cesarean delivery is essential for many cases of placenta previa. Severe blood loss may need the blood transfusion.

Placental abruption:

Placental abruption is premature separation of the placenta from the implantation in uterus. Inside placenta are several blood vessels which permit transfer of nutrients to foetus from mother. If placenta starts to detach during pregnancy, there is bleeding from the vessels. The larger the area that detaches, the greater the amount of bleeding. It is also known as abruption placenta. It is, though, related with certain conditions, comprising the following:

  • Previous pregnancy with placental abruption
  • Hypertension (high blood pressure)
  • Cigarette smoking
  • Multiple pregnancies

Severe placental abruption is unusual, other complications may comprise hemorrhage and shock, disseminated vascular coagulation (DIC) - the serious blood clotting complication, poor blood flow and damage to maternal and/or foetal kidneys or brain, stillbirth, postpartum (after delivery) haemorrhage.

Symptoms may comprise vaginal bleeding, abdominal pain, uterine contractions which don't relax, blood in amniotic fluid, thirst, nausea, faint feeling and decreased foetal movements.

There are three grades of placental abruption, comprising the following:

i) Grade 1 - small amount of vaginal bleeding and some uterine contractions, no signs of foetal distress or low blood pressure in mother.

ii) Grade 2 - mild to moderate amount of bleeding, uterine contractions, foetal heart rate may demonstrate signs of distress.

iii) Grade 3 - moderate to severe bleeding or concealed (hidden) bleeding, uterine contractions which don't relax (known as tetany), low blood pressure, abdominal pain, foetal death. At times placental abruption is not analyzed until after delivery, when the area of clotted blood is found behind placenta.


Specific treatment for placental abruption will be determined by the physician based on (i) pregnancy, overall health, and medical history; (ii) extent of the condition; (iii) tolerance for specific medications, procedures, or therapies. There is no treatment to stop placental abruption or reattach placenta. Vaginal delivery is still possible when placental abruption takes place, though, emergent caesarean delivery may be essential if severe hemorrhage or foetal heart abnormalities happen.

Birth control methods:

Most birth control doesn't protect from HIV or other sexually transmitted diseases (STDs) such as gonorrhea, herpes, and chlamydia. List of birth control methods with estimates of efficiency, or how well they work in preventing pregnancy when utilized correctly, for every method are given below:

1. Continuous Abstinence: This means not having sexual intercourse at any time. It is the only sure way to prevent pregnancy and protect against HIV and other STDs. This method is 100% efficient at preventing pregnancy and STDs.

2. Oral Contraceptives: Also known as the pill has the hormones estrogen and progesterone and is available in different hormone dosages. The pill is taken daily to block release of eggs from ovaries. Oral contraceptives ease the flow of period and can decrease risk of pelvic inflammatory disease (PID), benign ovarian cysts, endometrial cancer, ovarian cancer, and iron deficiency anemia.

3. The Mini-Pill: Unlike the pill, mini-pill only has one hormone, progesterone, instead of both estrogen and progesterone. Taken daily, mini-pill thickens cervical mucus to prevent sperm from reaching egg. It also prevents the fertilized egg from implanting in uterus (womb). Mini-pill also can reduce flow of period and protect against PID and ovarian and endometrial cancer.

4. Copper T IUD (Intrauterine Device): The IUD is a small device which is shaped in form of a T. The expert gynaecologist places it inside uterus. Arms of Copper T IUD have some copper that stops fertilization by preventing sperm from making their way up through uterus in fallopian tubes.


Many experts define infertility as not being able to get pregnant after at least one year of trying. Women who are able to get pregnant but then have repeat miscarriages are also said to be infertile. Infertility is not always the woman's problem. In only approx one-third of cases is infertility due to woman (female factors). In another one third of cases, infertility is because of man (male factors).

Male Infertility:

Infertility in men is most frequently caused by:

1. Problems making sperm; generating too few sperm or none at all.

2. Problems with sperm's ability to reach egg and fertilize it; abnormal sperm shape or structure prevent it from moving correctly.

3. At times man is born with problems which affect his sperm. Other times problems begin later in life because of illness or injury. For instance, cystic fibrosis frequently causes infertility in men.

Female Infertility:

Infertility in men is most frequently caused by:

1. Problems with ovulation account for many cases of infertility in women. Without ovulation, there are no eggs to be fertilized. Few signs that the woman is not ovulating generally comprise irregular or absent menstrual periods.

2. Less common causes of fertility problems in women comprise:

3. Blocked fallopian tubes due to pelvic inflammatory disease, endometriosis, or surgery for an ectopic pregnancy.


Infertility can be treated with surgery, medicine, artificial insemination or assisted reproductive technology. Several times these treatments are combined. Approx two-thirds of couples which are treated for infertility are able to have baby. In most cases infertility is treated with drugs or surgery. Doctors suggest specific treatments for infertility based on (i) test results, (ii) how long couple has been trying to get pregnant, (iii) the age of both man and woman, (iv) overall health of partners, and (e) preference of partners.

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