Placenta

Placenta is the mechanical and physiological connection between foetal and maternal tissues for the nutrition, respiration and excretion of the foetus.

The surface of the blastocyst gives rise to finger - like outgrowths called chorionic villi which penetrate into depressions on the wall of the uterus called crypts.

The intimate connection between the foetal membranes and the uterine wall is known as placenta. The part of the placenta contributed by the foetal (chorion) membranes of the foetus is called the foetal placenta and that contributed by the uterine wall of the mother (deciduabasalis) is called the maternal placenta. The degree of intimacy between the foetal placenta and maternal placenta is so strong, that, eventually the blood vessels of the chorionic villi are literally bathed in the mother's blood. This type of placenta is known as haemochorial placenta.

Foetal Membranes and Placenta

In the placenta, the foetal blood comes into close contact with the

maternal blood, resulting in exchange of materials. Food and oxygen pass from the maternal blood into the foetal blood and the wastes from the foetus pass into the maternal blood.

There is no mixing up of maternal and foetal blood. The umbilical cord connects the foetus to the placenta.

Functions of Placenta

• Nutrition

Placenta helps to transport nutrients from maternal blood into foetus

• Respiration

It helps in getting oxygen from the maternal blood into the foetus and CO₂ from foetus blood into the maternal blood.

• Excretion

Nitrogenous waste products produced in the embryo diffuse through the placenta into the maternal blood stream.

• Immunity

Antibodies developed in the mother against certain diseases like measles, small pox, diphtheria pass from mother into the foetal blood through the placenta.

• Transport of pathogens

Pathogenic organisms like viruses diffuses through the placenta. Viruses causing syphilis, measles, rubella, small pox may infect the foetus, if the mother gets the disease during pregnancy. Some of these diseases may even cause congenital deformities.

• Transport of drugs

Some of the drugs taken by the mother during pregnancy cross the placental barrier and may even cause developmental deformities.

E.g., The drug thalidomide used to avoid nausea and morning sickness during early pregnancy by some women resulted in the child born to such mothers to have deformities in the limb development and heart. The children had flipper like limbs, a condition called as phocomelia.

Children born to drug addicts, are born with addiction and withdrawal symptoms.

• Storage

Placenta stores some fats, glycogen and iron.

• Secretion of hormones

Placenta secretes many hormones like estrogen, progesterone, gonadotropin and placental lactogen, thus functioning as an endocrine gland.

Pregnancy stages

The hormone progesterone is continually required during the entire period of pregnancy. Hence this hormone is aptly referred to as pregnancy hormone.

Pregnancy can be classified into 2 main stages:

a) Crucial first 3 months (trimester)

b) Post trimester

During the first three months of pregnancy, the basic structure of the baby is formed. During this stage, the foetus is very sensitive. Viral infection of the mother, exposure to certain chemicals, etc. may cause malformation in the developing embryo (foetus). These deadly agents are collectively referred to as teratogens.

By the end of the first three months, all the system of the baby is formed. Therefore, in post trimester further growth and development of all the rudimentary organs take place. The foetus at this stage is less susceptible to teratogens.

Parturition

The process of childbirth is known as parturition.

The steps involved in parturition are:

i) Onset of childbirth is followed by the opening of the cervix.

ii) Powerful contractions ruptures the amnion and the amniotic fluid flows out through the vagina.

iii) This is followed by the baby coming out through the vaginal opening.

iv) The umbilical cord is then cut.

The infants lungs expand and it starts breathing.

Shortly after child birth, the placenta and the remains of the umbilical cord are expelled out.

Lactation

Feeding the baby with the mother's milk is known as lactation.

The first milk which comes out from the mother's mammary glands just after child birth is known as colostrum. It is rich in calories and proteins and contain antibodies that provide passive immunity to the new born baby.

Milk secretion is stimulated by the pituitary gland hormone known as prolactin.

Deviations in the reproductive strategies

Apart from the asexual and sexual reproductions that form the main form of reproduction in most animals, many deviations in the reproductive strategies are observed.

1) Hermaphroditism

This is a reproductive variation found in tapeworms and earthworms. In such animals, both the male and the female reproductive organs are present in the same individual. As both the sex organs are present in the same animal, some may undergo self-fertilisation and some cross fertilisation.

2) Parthenogenesis

This is another deviation of reproductive strategy in which an unfertilised ovum, without the process of fertilisation transforms into a new individual. Rotifers, some molluscs, arthropods, etc. take up parthenogenesis as a natural form of reproduction.