Embryological Development of Frog

Last Updated on Sunday, 11 September 2011 03:29 Written by Sandesh Monday, 5 April 2010 07:01

EMBRYOLOGICAL DEVELOPMENT OF FROG:

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Development of frog’s egg:

Development is the transformation of zygote into adult individual by divisions. Zygote undergoes cleavage and gives rise to embryo by different processes like: i. division of zygote into smaller daughter cells(cleavage), ii. Arrangement of cells into distinct layers( blastulation and gastrulation) and iii. Growth and differentiation of the cells to produce tissues, organs and systems (neurulation and organogenesis).

1. Cleavage: The process of rapid mitotic divisions of the zygote is called cleavage. It is different from other cell division in the absence of growth after division. Zygote divides to produce blastomeres. Since the egg of frog is telolecithal and mesolecithal, the division is complete while blastomeres produced are unequal in size. So, cleavage in frog is said to be holoblastic and unequal. Cleavage starts within the jelly coat after about two and half hours of fertilization.

First cleavage: A furrow appears in the animal pole which extends toward vegetal pole dividing the zygote into two blastomeres of equal size. This cleavage is meridional(longitudinal).Second cleavage: It is another meridional division right angle to the first division. This results in the formation of four blastomeres of equal size.

Third cleavage: This is a latitudinal division from just above the equatorial plane  perpendicular to the first two divisions. As a result, eight cells are formed in two tiers. Upper four cells of animal pole are small called micromeres. The lower four cells in the vegetal pole are larger with large amount of yolk called as megamers or macromeres.

Fourth cleavage: It involves two meridional divisions right angle to one another. As a result sixteen cells are formed in two tiers of eight micromeres in animal pole and eight macromeres in vegetal pole.

Fifth cleavage: It involves two latitudinal divisions parallel to one another, one in animal pole and another in vegetal pole. As a result, 32 cells are formed arranged in four tiers (two tiers of micromeres and two tiers of macromeres).

Now, further cleavage takes place irregularly. Micromeres divide more rapidly than the macromeres. This is due to the presence of large amount of yolk in vegetal pole which retard the process of cell division. This results in the formation of large number of small micromeres at animal pole and a small number of large macromeres in the vegetal pole. In frog morula stage is not found because the blastocoel start to apprear in 32 celled stage.

2. Blastulation: It is a process of formation of blastula from morula. Blastula is a hollow ball of cells. During blastulation there is formation of a small hollow cavity called as blastocoel which increases in size. Blastocoel is surrounded by two types of blastomeres-micromeres in the animal pole and macromeres in the vegetal pole. Blastocoel is filled with albuminous fluid secreted by blastomeres. Blastula is completely formed after 22 hours of fertilization.

Blastocoel in blastula of frog is located above the equator, therefore it is called as ex-centric in postion. Since the blastocoel is distinct and well developed, blastula of frog is called as coeloblastula.

Each region of blastula develops into definite organ of the future embryo. These areas can be represented by the fate map of blastula. The areas which develop into organs of future embryo are called as presumptive organ forming areas. For example,

• The micromeres in animal pole develop into ectoderm (forms organs like skin, brain, spinal cord, enamel of teeth, hairs, nails sweat glands etc).
• The macromeres of vegetal pole develop into endoderm( forms organs like pancreas, liver, lungs, alimentary canal etc.).
• The area of grey crescent develop into mesoderm (forms organs like dermis of skin, dentine of teeth, connective tissue- bone blood, muscle, heart, kidney, testis , ovary, coelom etc).

3. Gastrulation: Gastrulation is the process of formation of hollow gastrula from blastula. It involve  dynamic movement and rearrangement of blastomeres. Such movements of blastomeres along specific paths during gastrulation are called as morphogenetic movements. Three types of morphogenetic movements can be found- invagination, involution and epiboly.

1. • Invagination: Invagination is an active infolding of blastomeres. During invagination, few blastomeres near grey crescent are pushed inward to form a slit or groove. The opening of this groove is called as blastopore and the cavity is called as gastrocoel or archenteron. Blastopore is surrounded by dorsal lip along with lateral lips and ventral lip. Due to enlargement of archenteron, blastocoel is gradually reduced.
   • Involution: Involution is the process of rolling in movement of blastomeres. During this process the micromeres multiply and migrate to the dorsal lip of blastopore and roll inside or turn into the archenteron and arrange themselves on the roof of the archenteron. Involution is completed by convergence and divergence. During this, the micromeres multiply rapidly and move towards the blastoporal end, process called convergence. Thus converged cells in the blastopore start to involute slowly and diverge towards the roof of the archenteron. This process is called as divergence. Thus involuted cells develop into chordamesoderm. The archenteron gradually widens which pushes the blastocoel narrow. The crescentic blastopore becomes complete circle.
   • Epiboly: Epiboly means growth of one layer of cells over another. During epiboly, micromeres of animal pole divide rapidly and move over the macromeres of vegetal pole. This layer forms ectoderm.As a result of these morphogenetic movements, three primary germ layers are formed.
     • The cells which covers the gastrula externally form ectoderm.
     • Those involuted cells into the roof of archenteron give rise to mesoderm and
     • Cells of  sides and floor of the archenteron will develop into  endoderm.

Some other internal changes are also taking place along with those morphogenetic movements. As the archenteron is enlarging, the yolky megameres are pushed out towards the blastopore. This structure is called as yolk plug. The process of gastrulation is completed in 36 hours of fertilization.

STRUCTURE OF GASTRULA:

Gastrula is a hollow structure made up of blastomeres which give rise to the three primary germ layers.

An early gastrula of frog has only two primary germ layers namely ectoderm and endoderm. It contains archenteron which opens outside through a blastopore. Blastopore is bounded by dorsal lip, lateral lips and a ventral lip. Dorsal lip is formed from grey crescent which develop into chordamesoderm and notochord of the future embryo. It also induces neighbouring blastomeres to organize and develop into neural tube. Therefore, dorsal lip is also called as primary organizer.

In the late gastrula, three primary germ layers can be found. Ectoderm is made up of micromeres, mesoderm is formed from the roof of the archenteron and endoderm, from the floor of the archenteron. The yolk laden macromeres are pushed towards to blastopore which forms yolk plug. This stage of the gastrula is called as yolk plug stage of the gastrula.

4. Organogenesis or Organogeny:
The organogeny is the formation of organs during development. The process of organogeny begins soon after the formation of gastrula. Organogenesis includes following steps:

1. Neurulation (Neural tube formation):
This is the stage of embryonic development in which the neural tube is formed. During this process the ectoderm along the mid-dorsal line becomes thick and flattens forming plate-like structure known as neural plate (medullar plate). The edges or the lateral sides of the neural plates rise up as a pair of fold known as neural folds due to which neural groove is formed. These folds bend inward and fuse with each other forming neural tube. The cavity of neural tube is called neural canal or neurocoel. Its anterior wider end forms brain where as narrow posterior end forms spinal chord which remains filled with cerebo-spinal fluid in adult.

2. Notogenesis (notochord formation):
Notogenesis occurs side by side with neurogenesis. During notogenesis the chorda cells lying in the mid dorsal region of the roof of archenteron separate from the adjacent mesoderm cells and becomes solid. They form cylindrical rod-like notochord made up of vacuolated cells. The notochordal sheath develops around notochord. The notochord, later, is transformed into vertebral column in adult stage.

3. Formation of coelom:
The coelom or body cavity is formed by the splitting of mesoderm.
During this process, the lateral plate mesodermal sheet splits from the central region and form a cavity known as coelom( shizocoel). The outer layer of coelom is known as parietal layer or somatic layer while the inner layer is known as splanchnic layer or visceral layer. Coelom extends downwards and becomes continuous with that of the other side below archenteron and appears ‘U’- shaped. The somatic layer unites with ectoderm to from bodywall (somatopleure), while the visceral layer unites the endoderm to form the gutwall (splanchopleure).

A narrow cavity called myocoel, appears in each mesodermal somite which itself differentiates into three parts:

• • Outer dermatome-becomes dermis of skin
  • Middle myotome-forms body muscles
  • Inner sclerotome- contributes vertebral portions of skeleton

Myomeres lead to the formation of kidneys. The splanchnocoels of hypomeres become the general body cavity lined with peritoneum.