Immunology | Immune system| Immunity

ORGANS FROM ECTODERM:

The ectoderm gives rise to the outer layer of the skin over the entire surface of the embryo.
The neural tube into which the medullary groove develops loses its original connection with the surface.Anteriorly it becomes enlarged and forms the brain,the remaining portions developing in to the spinal cord.The thickening of the walls of the spinal tube, which forms the cord,diminishes the central cavity,until it becomes reduced to a fine canal Canalis centralis.
The anterior portion of the tube becomes divided by slight constriction into forebrain ,midbrain, and hindbrain.
Hind brain becomes widened from side to side especially infront its flair and the sides thicken which develops into cerebellum,which remains thin and membranous and becomes thrown into series of folds which support a mass of blood vessels, called choroid plexus.
The portion of the hindbrain which does not form cerebellum is converted into medulla.The midbrain grows out dorsally and laterally into a pair of hollow processes called optic lobes.
The forebrain soon becomes separated into 2 parts, the Thalamencephalon behind and the cerebral hemisphere which grow out from the latter in front hollow out growth- the infundibulum ,which extends downwards.
The sides of thalamencephalon give rise to optic vesicles,which grow out until they come in close contact with the surface ectoderm.
The distal end of the vesicles widens cut and forms Retina of eyes stalk giving rise to the optic nerve.
The lining of the mouth cavity is formed from an invagination of ectoderm-the stomoderm and similar ectodermal invagination Proctoderm forms the lining of a small part of the post end of the alimentary canal.
 The lens,cornea,retina of eye,vesicle of inner ear also take thin origin from this layer.

ORGANS FROM ENDODERM:

Endoderm forms the lining of the alimentary canal and lining of all organs,which arise as outgrowths from it.It forms liver which at the beginning appear as an out pocketing of the ventral side near anterior end.It becomes folded and branched to form number of clusters of tubules, all emptying into into a common canal -the bile duct. A lateral  outgrowth of the bile duct forms the gall bladder forms the gall bladder. The pancreas arises in the same way as the liver, but as a pair of outgrowths instead of one,but they form a single organ and their ducts later become connected with the bile duct.

Bladder arises as an outgrowth of ventral side of the alimentary canal. Lungs appear as a pair of pouches from the sides of aesophagus. The Gill slits in the frog appear in the form of five solid outgrowths on each side of the anterior portion of archenteron.

 Frog Embryology

Fertilization in case of frog takes place by the process of pseudocopulation because there is no external copulatory organ present in case of male and the female are in close proximity during the process it has the name pseudocopulation.

The egg gets surrounded by many sperms and there is specific reaction called as fertilizin-antifertilizin reaction between the two.(Fertilizin is present in the jelly layer of the egg and anti-fertilizin is secreted by the acrosome of the sperm).The sperm traces its path through the jelly layer and gets attached to the vitelline membrane.
The next reaction which takes place is acrosomal reaction followed by cortical action after which the vitelline envelope is converted into thick fertilization membrane.This prevents polyspermy.As soon as the sperm enters the ova(i.e.,the nucleus of sperm enters the egg) the second meiotic division is completed resulting in the release of the second polar body.
The space between the fertilization membrane and PM of the egg is called as peristelline space which is filled with the perivitelline fluid. In this fluid,the egg can rotate freely and also this space allows free exchange of gases.


The egg pronucleus and the female pronucleus fuse together to form the zygotic nucleus,this process is known as Amphimixis or Syngamy.
The entry of the sperm causes formation of a Grey crescent (which is a characteristic of an amphibian embryo);caused due to mixing of cytoplasmic granules or in other words due to the cytoplasmic movement because of the sperm entry.Grey crescent materials the functions as an organizer because if it is removed from the embryo,the embryo fail to develop further. At the same time if a normal embryo is grafted with another Gery crescent then two embryos develop.

Thus,formation of grey crescent fixes up final symmetry of the egg and future embryo.Also grey crescent marks the future dorsal side of the embryo.The sperm penetrates the egg perpendicular to the cortex and after penetration moves perpendicularly along the radius of the egg.This path of sperm in the egg cortex, is called as penetration path.After crossing the cortex,the sperm changes its direction and moves towards the egg nucleus.This changed path is also copulation path because it leads to the copulation (fusion) of the sperm pronuclei.
After fusion of both male and female pronuclei, a zygote nucleus is formed i.e.,a zygote will be formed
Further development of zygote proceeds with its cleavage and formation of blastula,gastrula and so on

STRUCTURE OF SPERMATOZOA:

        Mature spermatozoa of a frog is made up of 3 parts i.e., head,middle piece and tail.Middle piece region is very small as compared to that of the other sperms,The head consists of acrosome and large nucleus. The acrosome secretes enzymes which helps in penetration into the egg. The middle piece of spermatozoon is composed of almost entirely of mitochondria and centriole(Centriole helps in the spindle formation during cleavage).The mitochondrial apparatus supplies the required amount of energy  in the form of ATP.The tail of sperm is flagellar and very long.It helps in locomotion and thus is very essential for the mobility of the sperm.


1.Plasma membrane
2. Outer acrosomal membrane
3.Acrosome
4. Inner acrosomal membrane
5. Nucleus
6. Proximal centriole
7. Rest of the distal centriole
8. Thick outer longitudinal fibers
9. Mitochondrion
10. Axoneme
11. Anulus
12 . Ring fibers

A.Head
B. Neck
C.Mid piece
D. Principal piece
E. Endpiece

STRUCTURE OF OVUM

The egg of frog is spherical in shape and about 2mm in diameter.It is surrounded by three membranes viz.,outer jelly coat,middle vitelline membrane and inner plasma mambrane.
The jelly coat is formed of albumin and vitelline membrane is made up of mucopolysaccharide. The egg cytoplasm (ooplasm) has 2 regions.viz.,peripheral cortex and central endoplasm.

The cortex is granular .It shows presence of cortical granules and dark brown pigment granules.Cortical granules are arranged in a layer close to the plasma membrane. The endoplasm contains numerous plate like structures called as yolk platelets.
The egg shows well-marked polarity.On one side is the animal pole containing nucleus. The polar bodies also containing nucleus. The polar body also occupy region near animal pole whereas vegetal pole containing nucleus. The polar bodies also occupy region near animal pole whereas vegetal pole containing yolk granules the animal pole appears darker whereas vegetal pole is yellowish or whitish(lighter) in colour. As the egg contains large amount of yolk it is also called as megalecithal.
As the yolk platelets are highly concentrated at one end,polarity is distinct and hence the egg is called as Telolecithal egg.
The shell is absent.Hence the egg is non-cleidoic egg.

Oogenesis in Frogs:

Oogenesis is the process of formation of oogonia and its development into mature ova.Oogenesis takes place inside the ovary of a female frog.
The ovary epithelium cells are the primordial germ cells which multiply in number to form oogonia.Oogonia further give rise to follicle cells and the primary oocyte.Cells which fail to undergo repetetive mitotic divisions
become follicular cells.
There is no way to determine which oogonium is to be selected for the formation of primary oocyte or which is to become a follicular cell.
The different stages of maturation of oogonium are primary oocyte,secondary oocyte,mature oocyte and fertilized oocyte(ovum)
The development requires 3 stages seasons during which the oogonia passes through 3 phases i.e., Multiplication phase,Growth phase and Retention phase.

MULTIPLICATION PHASE:

This phase of ova starts right from the tadpole stage of the organism and exists upto the older stage i.e., adult stage.During this time the oogonium enters mitosis to form primary oocyte.The primary then enters the prophase I  of meiosis I and before completion of this division it enters into the growth phase.

GROWTH PHASE:

In this phase the primary oocyte increases tremendously in size.The volume of nuclear sap increases therefore the nucleus becomes inflated and is called as Germinal vesicle.The volume of cytoplasm of oocyte also increases qualitatively and quantitatively.Amount of yolk platelets and glycogen etc increases with tremendous rate and a definite polarity is determined.Yolk tends to accumulate at one pole,i.e.,vegetative pole and occupies 2/3rd space of egg.Germinal vesicle is pushed and move towards the animal pole which is comparatively yolk free.This process of yolk formation is also known as vitellogenesis.
There is formation of vitelline membrane around the egg plasma membrane the primary oocyte reach then full growth towards the end of the second season but they are not immediately released.

RETENTION PHASE:

In this phase,the primary oocyte in the ovary are retained until the next breeding season. The 1st maturation division occurs at the time of ovulation.It forms secondary oocyte haploid and the first polar body. These changes occur as the egg leaves the ovary and before it reaches th oviduct. When the egg reaches the oviduct, the second maturation division begins.Spindle of meiosis II is formed and it progresses as far as early stage of metaphase.Again the growth is arrested until and unless the ovum is fertilized.
There may be division of 1st polar body to give rise to two polar bodies therefore there may be 2-3 polar bodies attached to the egg surface.After this the egg(female)pronucleas and male pronucleus fuse together to form zygote nucleus which is diploid in nature.
As soon as the sperm touches the vitelline envelope of the egg, the metaphase is completed and there is a formation of matured ovum and the 2nd polar body.

What does an Allergy means?

An allergy refers to an exaggerated reaction by our immune system in response to bodily contact with certain foreign substances. It is exaggerated because these foreign substances are usually seen by the body as harmless and no response occurs in non- allergic people. Allergic people's bodies recognize the foreign substance and one part of the immune system is turned on. Allergy-producing substances are called "allergens." Examples of allergens include pollens, dust mite, molds,danders, and foods. To understand the language of allergy it is important to remember that allergens are substances that are foreign to the body and can cause an allergic reaction in certain people.
When an allergen comes in contact with the body, it causes the immune system to develop an allergic reaction in persons who are allergic to it. When you inappropriately react to allergens that are normally harmless to other people, you are having an allergic reaction and can be referred to as allergic or atopic. Therefore, people who are prone to allergies are said to be allergic or "atopic."
An over-reactive immune system is involved in allergy.  Most people think that their immune system is weak when allergies strike, and they choose immune enhancing herbs or supplements.  In some cases this is true, but for most people the approach needs to be broad based, in the direction of soothing over-reactive cells.