Embryology is the study of the development of an individual before birth.

Every individual spends first 9 months (266 days or 38 weeks) of his life in womb of his mother, also known as uterus.

During this 9 months it develops from a single structure cell to an organism having billions of cells .the most important changes occur in first two months ,during these two months we call the developing individual an embryo, and from the third month onwards we call it a fetus .

Reproduction is carried out by specialized organs which are testes in case of male and ovary in case of female these produce highly specialized gametes, the male gametes are called spermatozoa and the female gametes are called ova. The process of fusion of sperm with ovum is called fertilization.

The formation of sperms in testes is called spermatogenesis and formation of ova is called oogenesis. 

Following fertilization there is rapid cell division known as cleavage .and the mitotic divisions are thus called cleavage divisions. There are 46 chromosomes in each cell and 44 of them are autosomes and one pair is called sex chromosomes. The sex chromosome may be of two types X or Y .

There are numerous daughter cells are produced which are known as blastomeres.


Types of cleavage:

1. Meroblastic
2. Holoblastic

Meroblastic:   in this type only part of egg undergoes cleavage rapidly .it can further be divided into two parts.
 
1) Meroblastic discoidal
 
2) Meroblastic superficial cleavage

In Meroblastic discoidal type cleavage occurs only in region close to animal pole and yolk sac .this is characterstic of macrolecithal cells.

Meroblastic superficial cleavage this occurs in insects generally and in this cleavage is restricted to superficial layer of cytoplasm which is yolk free.


Holoblastic   this typically occurs in humans, in this type the entire cells gets divided into two complete daughter cells, it can further be classified in two forms  in humans division takes place in fallopian tube .

1) Equal holoblastic

2) Unequal holoblastic 

Equal holoblastic in this type due to division the resulting blastomeres are of equal size .example is sea cucumber

Unequal holoblastic in this type resultant cleavage results blastomeres of unequal size .example is amphibians.

Embryogenesis in humans: The zygotes divides and form two celled embryo then mitosis occurs in each of them and thus 4 cells results repeated divisions occur and thus it reaches stage of 16 cells at this stage it is called morula.

This morula after passing through the stage of compaction produces two types of cells
1)  Peripheral cells --------trophoblastic cells (this group later produces extra embryonic membrane and part of placenta)

2) Inner cell mass further develops and form proper embryo

The attachment of embryo to the inner most lining of uterus is called implantation .this occurs about 8 days after ovulation, after implantation many changes occur in uterine lining and it get converted into decidua.

Gastrulation is the process by which blastula gets converted into gastrula. .

During this process three germinal layers are produced named ectoderm, mesoderm, and endoderm.

During this the cavity of blastula is replaced by the new cavity archenteron, the primitive gut.

Each germinal layer forms specific organ to form different organ. Which are as follows.

 
ECTODERM      
Skin ,glands like sweat sebaceous, mammary )brain ,spinal cord ,nerves ,lens of eye ,inner ear


MESODERM   
Muscles, heart, blood vessels, kidneys gonads.


ENDODERM 
Digestive system, respiratory system, urinary bladder, middle ear.

Fetal membranes the membranes which are situated outside the embryo are termed as extra-embryonic or fetal membranes they are amnion, chorion, and allantois and yolk sac these are amnion, chorion allantois and yolk sac.

Amnion is the innermost membrane which surrounds the embryo by forming a sac like structure .the function is to act as a shock absorber and protect the embryo.

Chorion this is the outermost layers and surrounds embryo the functions are to protect the embryo and form the placenta which serves as a route for metabolic exchange of nutrients and other things. 

Allantois is a small sac that pushes out from the gut of the embryo near the yolk sac ,it also takes part in the formation of placenta.

Yolk sac is formed below the embryo; human egg is devoid of yolk it gives no nourishment support to the embryo it is very small and gradually shrinks, it is non- functional in humans.

Placenta: it is a organ which forms link between uterus of the mother and fetus ,it is formed partly from deciduas and partly from embryonic structures and is responsible for transport of nutrients and oxygen to the fetus ,and for removal of waste products .


Functions of placenta 
1) This enables the transport of oxygen, water and nutrients from maternal to fetal blood

2) It also acts as a excretory organ for fetus.

3)  It gives immunity to the fetus through maternal blood and thus guards the fetus.

4) It acts as a barrier and prevent many bacteria and other harmful substances from reaching the fetus, but most viruses and some bacteria can pass through it .drugs also bypass fetal circulation and can produce congenital abnormality.

5) Placenta synthesizes many hormones like progesterone secreted by the placenta is essential for maintance of pregnancy after 4th month.

Types of placenta it can be divided on the basis of shape and number of cotyledons and secondly on the basis of layers of placenta.

Classification based on the layers between fetal and maternal blood. There are 6 layers which separates maternal and fetal blood

Three of which are from fetal side

1) Wall of fetal capillaries
2) Fetal connective tissue
3) Fetal chorionic epithelial

The three layers for maternal side are as follows

1)  Membrane of uterine wall
2) Uterine connective tissue
3) Endothelium of maternal blood capillaries

Image taken from http://www.vivo.colostate.edu/hbooks/pathphys/reprod/placenta/structure.html




The following types of placenta are formed with these membranes.

Epitheliochorial placenta all the six layers are present in this variety .this is simple and basic type formed in marsupials, pig, horse etc.

Syndesmochorial placenta in this type only 5 barriers exists between maternal and fetal blood due to breakdown of uterine epithelium presents in cow ,goat and sheep

Endotheliochorial placenta
In this variety there are 4 barriers present between maternal and fetal blood .due to loss of uterine connective tissue and uterine epithelium this is present in tiger, lion, cat and dog.

Haemochorial placenta there are only 3 barriers present between maternal and fetal blood due to erosion of all three maternal layers this type is present in primates .(including humans ),and rodents

Haemoendothelial placenta there is only one barrier of endothelium of fetal capillaries between fetal blood and maternal blood due to breakdown of five out of six barriers occurs in rabbits and rodents

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