GROWTH AND DEVELOPMENT IN ANIMALS. In the early stages of development, the organism is called embryo. The basic processes in development of embryo include;

GROWTH AND DEVELOPMENT IN ANIMALS

In the early stages of development, the organism is called embryo. The basic processes in development of embryo include;

• Cleavage (Segmentasyon)

• Growth (Büyüme)

• Differentiation (Farklılaşma)

GROWTH AND DEVELOPMENT IN ANIMALS

CLEAVAGE

After fertilization, the zygote begins a series of cell divisions known as cleavage.

During cleavage, the fertilized egg divides by mitosis and each division decreases cell size.

*** During cleavage, the cells do not grow. The cells formed during cleavage are called blastomere.

zygote

mitosis

1st division

(2 blastomeres)

mitosis mitosis

3rd division

(8 blastomeres)

CLEAVAGE

2nd division

(4 blastomeres)

Sea urchin egg

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MORULA

The early divisions of cleavage result in a solid ball of cells, which is called morula.

Blastomeres that form the morula are same size.

*** The cells that make up the morula has been formed by mitosis so they carry the same genetic information.

*** Morula latince dut anlamına gelmektedir.

BLASTULAAs the cells of the morula continue to

divide, they are rearranged to form a

hollow sphere.

Usually, the layer of cells in the sphere is

only one cell thick. The inside of the

sphere is filled with fluid. At this stage

the embryo is called blastula and the

fluid filled inside of the sphere is called

blastocoel. (birinci karın boşluğu ).

*** There is still no differentiation of cells at

that stage.

animal pole

vegetal pole

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1.Blastocoel 2.Enterocoelic pouch 3.Mouth 4.Archenteron 5.Blastopore (anus)

•blue=Ectoderm •red=Mesoderm •yellow=Endoderm

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GASTRULATIONAs the blastula develops, it reaches a point at which the cells begin to grow before dividing. At this time mitotic division continues along with the growth.

The cells at the lower part of the blastula or vegetal pole, move inward and form a two-layered embryo called the gastrula.The opening created is called the blastopore. It later becomes one of the openings to the digestive system.

GASTRULATIONThe outer layer of cells in the gastrula is called ectoderm. The inner layer is endoderm.

The cavity within the gastrula is called the primitive gut (archenteron). It later becomes the digestive system. Archenteron grows inside the blastocoel towards the ectoderm and a new opening is created at the opposite side of the blastopore, which will be the second opening of the digestive system.

After the endoderm and ectoderm are established, a third cell layer, mesoderm forms between them.

**In Sponges and Coelenterata (Hydra), embryonic development stops at the gastrulation phase. Therefore, their bodies are made up of two layer of cells.

**Including earthworms, all of the other animals have mesoderm.

Histogenesis and Organogenesis The embryonic layers (germ layers) formed during gastrula, may give rise to tissues (histogenesis) or organs (organogenesis) of the multicellular animals.

The changing of unspecialized embryonic cells into the specialized cells, tissues, and organs is called differentiation.

GROWTH AND DIFFERENTIATION

ECTODERM MESODERM ENDODERM

•nervous system•brain•spinal cord, nerves•sense organs•epidermis of skin•hair, nails, sweat gland, lens of the eye

•bones •muscles •blood and blood vessels •reproductive and excretory systems•inner layer of skin

( dermis)•connective tissue

•lining of digestive tract urinary bladder•lining of trachea, bronchi, lungs•liver,pancreasthyroid, parathyroid, thymus

Development of Organs and Organ Systems from the Germ Layers

NEURALATION On the upper surface of ectoderm, the cells divide, forming a neural plate that has two raised edges, called neural folds.

The neural folds come together over the center of the neural plate and form neural tube. Later the neural tube forms the brain and spinal cord. This is called neuralation.

After neuralation, all of the organs of the body start to form by a series interactions between the cells.

As an embryo develops, there must be a coordination between its tissues. By the late blastula and early gastrula, the way in which groups of cells will develop has been determined. Cells in certain regions develop along certain lines.

For ex, there is a particular place in the frog embryo that normally develops into eye. If this tissue is removed from the embryo and placed into culture medium, it develops into an irregular mass of cells.

During the development of the embryo, the mechanism by which one group of cells (tissues) influences the development of an adjacent group of cells is called induction.

EMBRYONIC INDUCTION

SPEMANN EXPERIMENTS

Hypothesis 1 Experiment Result Conclusion

Ectoderm that is removed from the embryo can give rise to nerve tissues.

Ectoderm

is removed and placed in a special medium.

Ectoderm that is isolated does not give rise to nerve tissue in the medium.

Ectoderm should be attached to the embryo in order to form nerve tissue.

Hypothesis 2 Experiment Result Conclusion

Mesoderm that is responsible for the formation of nerve tissue, cause ectoderm to form nerve tissues.

Ectoderm layer of embryo is folded upwards and mesoderm layer is removed.

Ectoderm is straightened and embryo is put into a culture medium.

Nerve tissue of the embryo doesn't form.

Mesoderm induces ectoderm to give rise to nerve tissue.

upper ectoderm

upper mesoderm

Embryo that its upper mesoderm is removed

Nerve tissue doesn’t form in this embryo

Hypothesis 3 Experiment Result Conclusion

Any ectoderm layer that touches to the dorsal mesoderm (sırt mezodermi) can give rise to nerve tissue.

•Dorsal mesoderm of 1st embryo is removed.

•Ventral mesoderm (karın mesodermi) of 2nd embryo is removed.

•Dorsal mesoderm of 1st is placed instead of ventral mesoderm of the 2nd embryo.

In 2nd embryo (that has two dorsal mesoderms), two distinct nerve tissues are formed. Twin embryos that are attached to each other are formed.

Dorsal mesoderm induces ectoderm and controls the differentiation of nerve tissue.

blastopore

ectoderm

donor embryo

blastocoel

Neural plates

second notochord develops and larvae with two head forms

Embryonic Induction in the Formation of the Eye of a Vertebrate

During the differentiation of cells that form the vertebrate eye, inductions of two ectoderm take place. Optic cup (optic vesicle =göz çukuru) is formed by the induction of brain ectoderm and lens of the eye is formed by the induction of lens ectoderm. In order for lens to develop, brain ectoderm must induce lens ectoderm.

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1. The eye begins to take shape from an outgrowth of the developing brain (the optic vesicle) and an adjacent group of cells on the body surface (lens ectoderm).

2. As a result of earlier inductions, some of the cells of the optic vesicle and lens ectoderm undergo changes and fold inwards. The optic vesicle transforms into the optic cup, which will become retina and the optic stalk, which will become the optic nerve.

3. Cells of the optic cup induce the lens ectoderm to start forming the lens.

4. Finally, the cells of the developing lens induce development of the cornea.

Role of nucleus and Cytoplasm in Differentiation

Different sections of the DNA in a cell can be turned off or on. This results in the formation of different types of cells. If this was not the case, differentiation would not take place, and all the cells of an embryo would be same.

The interaction between the DNA and certain things in the cytoplasm controls development. As a result, parts of the hereditary material are switched on and off, and this in turn determines the direction of cellular differentiation.

Factors that have role in differentiation;

Interaction between DNA and

cytoplasm (genes that are switched on or off)

Embryonic induction (role of neighbouring cells) Cell migrations Controlled cell deaths (apoptosis)