Developmental Biology (Development of Frog)

1) Gametogenesis

The process of formation of gametes from the germinal epithelium of the gonads (testes in male and ovary in female).

A) Spermatogenesis

The process of formation of sperms (spermatozoa) or male gamete from the germinal epithelium of the seminiferous tubules of the testes is known as spermatogenesis. It is completed into following three step:

1. Multiplication phase
2. Growth phase
3. Maturation phase

i) Multiplication Phase

In this phase, germinal epithelium cell of the seminiferous tubule divides and redivides by mitosis cell division and forms large no. of diploid cell called spermatogonia and the phase is called multiplication phase.

ii) Growth Phase

In this phase, each spermatogonium absorb food from the Sertoli cell and grow into large round structure called primary spermatocyte.

iii) Maturation Phase

In this phase each primary spermatocyte divides by meiosis cell division. During the meiosis I two haploid daughter cells are formed called secondary spermatocytes. During meiosis II each secondary spermatocyte divide into two haploid cells called spermatids. In this way four haploid spermatids are formed from single primary spermatocyte. Each spermatid undergo metamorphosis and forms a sperm or spermatozoa.

B) Oogenesis

The process of formation of ova or eggs from the germinal epithelium of the ovary is known as oogenesis. It is also completed in the three steps.

i) Multiplication Phase

In this phase, germinal epithelial cell of the ovary divides and redivides by mitosis cell division and forms large no. of diploid cells called oogonia.

ii) Growth Phase

In this phase, each oogonium absorb food from all the tissue surrounded to it and grows into large round structure called oocyte. Primary oocyte also stores protein in the cytoplasm as a reserve food called yolk.

iii) Maturation Phase

In this phase each primary oocyte divide by meiosis cell division. During the meiosis I two unequal cells are formed. The large cell with larger cytoplasm is called secondary oocyte and smaller cell with smaller cytoplasm is called 1st polar body. During meiosis II, secondary oocyte again forms two haploid unequal cells. The larger cell with large cytoplasm is an ovum (egg) and small cell with little cytoplasm is called 2nd polar body.

Structure of Sperm

Sperm is active, microscopic and thread-like cell. It is motile. Its body is differentiated into three parts:

a) Head

It is the anterior and largest part of the sperm. It contains large round nucleus. Head is anteriorly fitted with cap-like conical structure called acrosome. It contains an enzyme called sperm lysin which digest the limiting membrane of egg during fertilization for the penetration of sperm.

b) Middle Piece

It is the short and narrow part of the sperm. It contains mitochondria and centriole. Mitochondria helps to release energy for the movement of sperm but centriole helps in the division of zygote after fertilization.

c) Tail

It is the posterior and long thread-like part of the sperm. It is made up of microtubules. Movement of tail helps in swimming of the sperm.

Structure of Egg of Frog

Egg of frog is about 1.7 mm in diameter and 1.3 mm in size in source note. It is non-motile but contains moderate amount of yolk and telolecithal yolk is deposited at one pole. Egg of frog is differentiated into two parts.

Lower Part: Vegetal Hemisphere

The lower part lies towards the bottom of the pond called vegetal hemisphere. It contains yolk.

Upper Part: Animal Hemisphere

Upper part called animal hemisphere. It does not contain yolk but contains pigmented cytoplasm.

Process of Fertilization in Frog

The process of fusion of male gamete and female gamete is known as fertilization. In frog fertilization is external, takes place in water. During fertilization male frog mounts over the female and grasps her firmly to lay eggs. The male discharge the sperm over the egg and fertilization occur.

The process of fertilization completed in the following steps:

1. Sperm swims in water and comes in contact with vitelline membrane of the secondary oocyte.
2. Acrosome part of the sperm releases an enzyme sperm lysin and digests the limiting membrane of egg by penetration.
3. During the penetration secondary oocyte undergo second maturation division and forms ovum and second polar body.
4. As the head and middle piece of the sperm enters into the ovum, cytoplasmic fluid oozes out in between the vitelline membrane to form fertilization membrane. Fertilization membrane prevents further entry of sperm into ovum.
5. Inside the ovum male pronucleus comes in contact with female pronucleus and chromosomes of them fused and mixed together called amphimixis. As a result zygote is formed (diploid cell). In this way process of fertilization is completed.

Development of Zygote into Three Germ Layers

Zygote is the first stage of the frog. It passes through different stages to form adult. This development of zygote upto three germ layers are as follows:

1. Cleavage or segmentation
2. Morula
3. Blastula
4. Gastrula
5. Formation of three germ layers

1) Cleavage or Segmentation (Holoblastic Division)

The division of zygote is known as cleavage. Cleavage starts after about 2 to 3 hours of fertilization of zygote. In frog, cleavage is holoblastic or complete division of the zygote.

Steps of Cleavage

1. First cleavage: The first division of zygote or 1st cleavage is vertical. It starts from animal pole to vegetal pole and forms two equal cells or blastomeres.
2. Second cleavage: It is also vertical but right angled to the first so that forms four equal cells.
3. Third cleavage: It is horizontal but just above the middle so that 8 unequal cells are formed. The upper 4 cells are smaller in size called micromeres and lower 4 are larger cells called megameres.
4. Fourth cleavage: It is vertical and forms 16 unequal cells, out of them the upper 8 are micromeres and below are megameres.
5. Fifth cleavage: It is horizontal and forms 32 cells. The upper 16 are micromeres whereas the lower 16 are megameres.

2) Morula

After 5th cleavage, segmentation become irregular and micromeres divide more rapidly than the megameres. A group of solid ball of micromeres and megameres are formed called morula.

3) Blastula

After the formation of morula, micromeres again divide rapidly than the megameres and micromeres move towards periphery to develop a cavity called blastocoel. The process of formation of blastocoel from morula is known as blastula. Blastula is single layered embryo and its blastocoel contains different nutrients to provide nutrition to the blastomere.

4) Gastrula

The process of formation of double layered cup-shaped embryo from the single layered blastula is known as gastrula. It involves following steps:

a) Epiboly

After the formation of blastula, micromeres again divide more rapidly and passes over the megameres and megameres occupy small space. This overgrowth of micromeres is known as epiboly.

b) Emboly

At the junction between micromeres and megameres, micromere cell moves inward and develops cavity. This invagination or inward movement of micromere cell is known as emboly. Due to continuous invagination of micromeres cell, a large cavity is formed called archenteron. This cavity becomes cup shaped structure and called gastrula. Blastocoel becomes narrow and curved. The opening of the archenteron is called blastopore. Its anterior part is called dorsal lip and posterior part is called ventral lip.

c) Migration of Micromeres

The micromere cells migrate from both dorsal and ventral lip to the inside towards megameres. Due to the migration, blastopore becomes narrow and megameres come out through the central side of blastopore called yolk plug. Due to migration of micromeres mesodermic layer starts to develop.

5) Formation of Three Germ Layers

After gastrula stage, both micromere and megamere undergo rotational movement in 180° and cells arranged into three different layers called three germ layers. The outer layer is ectoderm which is made up of micromere. The middle layer is mesoderm which is made up of both micro and megamere. The innermost layer is endoderm which is only made up of megameres.

Fate of Three Germ Layers

The formation of different organs from different organs is known as fate of three germ layers. Following organs are formed from three germ layers.

Formation of Nerve Cord or Neural Tube (Neurulation)

• The process of formation of neural tube or nerve cord is known as neurulation.
• It is ectodermal in origin and developed from the ectoderm.
• The ectoderm cells present just above the roof of archenteron become flat and thicker called neural plate.
• Neural plate undergo invagination and develops a neural groove.
• The lateral margin of the neural plate folds upward called neural fold.
• Neural groove continuously goes down and opposite neural folds fuse together to form neural tube.
• Neural tube then separates from the ectoderm.
• On further development neural tube develops into brain anteriorly and spinal cord posteriorly.

Formation of Coelom

Coelom is mesodermal in origin and formed from the mesoderm.

During the formation of coelom mesodermal layer split longitudinally and forms outer somatic or parietal layer and inner visceral or splanchnic layer.

The narrow cavity present between visceral and parietal layer is known as splanchnocoel.

During the development outer parietal layer combines with body skin and inner visceral layer combines with wall of alimentary canal.

As a result large cavity is formed between body wall and alimentary canal called coelom.

Formation of Notochord or Notochordogenesis

• The process of formation of notochord is known as notochordogenesis.
• Notochord is formed from mesoderm, so it is mesoderm in origin.
• The mesodermal cell lying just above the archenteron and below the neural tube becomes solid and hard so that separates from the adjacent tissue and develops into the rod called notochord.
• The outer mesodermal cell then becomes vacuolated to develop outer sheath and the separated tissue develops into the rod called notochord.
• On further development notochord changes into skeleton anteriorly but backbone posteriorly, as written in the source.