Embryology (Part-8) - Notes, Zoology, Class 12 Class 12 Notes | EduRev

Class 12 : Embryology (Part-8) - Notes, Zoology, Class 12 Class 12 Notes | EduRev

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EMBRYOLOGY

Hormones of Human Placenta 

 

The placenta of human mainly secretes two steroid hormones like estradiol and progesterone, and two protein hormones like human chorional gonadotropin HCG and human placental somatomammotropin HCS. Large amount of HCG hormone is secreted, during early pregnancy, from the placenta. Because of this reason its quantity increases in the urine of pregnant lady. On the basis of this fact, pregnancy test is performed.

The above hormones are also held responsible for keeping the corpus luteum active, protection of embryo, prevention of abortion and growth of mammary glands.

 

Functions of placenta 

1. Exchange of important materials between foetal and maternal blood.

2. The essential materials are exchanged by diffusion, pinocytosis or active transport.

3. The small molecules like O2, CO2, H2O etc. and other inorganic substances like chlorides, phosphates, sodium, potassium, magnesium etc. are also diffused through placenta.

 

4. Large molecules like lipids, polysaccharides, carbohydrates, proteins  etc. are obtained by pinocytosis process.

5. The nutritional substances are supplied to embryo from the mother through placenta.

6. Placenta also serves as a respiratory medium for exchange of O2 and CO2 between embryo and mother.

7. The nitrogenous and metabolic wastes from foetus are released into the blood of mother by diffusion through placenta.

8. The antibodies for measles, chikenpox, polio etc. present in the blood of mother reach the embryo through placenta.

9. Pathogenic viruses may also enter in embryo through placenta.

10. If a female takes some harmful chemicals, liquor, drugs etc.during pregnancy, these may cross the placenta and on reaching into foetus may cause deformity during organogenesis.(eg. Thallidomide)

11. Placenta itself secretes some hormones like progesterone, estrogen, lactogen, HCG, HCS etc.

12. Progesterone, maintains and supports the foetus during the whole pregnancy period . At the time of parturition, relaxin is secreted by placenta which lubricates, and widens the birth canal to facilitate child birth.

 

HISTORY : -

  • Aristotle  is known as "Father of Embryology" he first studied the development in chick and other embryos. He gave its description in his book "Historia Animalia".
  • Leeuvenhock (1671)

- He observed and described human sperm for the first time.

- According to Hartsoeker and Leeuvenhock there is a small model of developing animal present in the head of the sperm of that animal. This small model is called homunculus. Both these scientists are called spermists, and this theory is called "Theory of spermist".

  • Swammer Dame, Haller, Bonette & Malpighi :- According to these scientists, small model of animal is always present in the egg. These scientists are called Ovists, and their theory is known as 'Ovists theory'
  • Schleiden & Schwann :- Both the scientists established the cellular structure of egg and sperm.
  • Pander :- He described the presence of three germinal layers in chick embryo.
  • Fredrich Wolff :- He first presented the "theory of epigenesis".
  • Muller :- He gave the recapitulation theory.
  • Haeckel :- He gave the details of Recapitulation theory and named it as the bio-genetic law.

- Bio-genetic Law :- According to this each organism during its embryonal development, passes through all stages, through which its species has evolved or embryo repeats its ancestry. i.e. Ontogeny recapitulates its Phylogeny.

  • "Carl Ernest Von Baer" :- He is known as the "father of modern embryology." He gave the Baer's Law which in turn proves the recapitulation theory.

According to this law, during embryonal development, the development of general structures takes place earlier and specific structures develop at last or later on.

  • A. Weismann :- He gave the theory of germplasm or the theory of continuity of germplasm. According to him, there are 2 types of protoplasm in the body of animals :-

(i) Somatoplasm

(ii) Germplasm.

Somatoplasm dies but the germplasm is never destroyed, rather it is transferred to the progenies.

 

  • Wilhelm :- He studied embryonal development in frog and gave the mosaic theory. - He said that there are some presumptive areas in the eggs of frog. These areas form specific structures during embryonal development. This is termed as "Promorphology". and these type of eggs are termed as the mosaic eggs.
  • Hans Driesch :- He studied embryonal development in sea-urchin and gave the regulative theory. - In the eggs of Sea-Urchin presumptive areas are not found i.e. promorphology is not found.So, each part of the egg is capable of forming the complete embryo. These type of eggs are termed as regulative eggs.
  • Boveri & Child :-They gave the gradient theory to explain the mosaic development in eggs.

According to them, a metabolic gradient is present inside the eggs.

Different parts of the egg have different metabolic rates.

The rate of metabolism is faster at the animal- pole of the egg and is slower at the vegital pole of the egg.

Due to different metabolic rates, different structures are formed from different parts of the egg.

  • Spemann :- He gave the "Theory of organizers". - According to it embryo has some special type of tissues, which induce development of some specific structures.

These are termed as the organizers. - These organizers secrete some special chemicals called evocators which induce the formation of specific structures. - Spemann got the Nobel prize for his t

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