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Introductory lesson 1

29.11.2012

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Embryology

Ｃ ytology (Study of embryonic development)

( Study of cellular structure and function)

Ｇ enetics(Study of inheritance)

The multidisciplinary approach to the study of development first arose before the turn of the Twentieth Century as an integration of

Origin

Definition

Developmental biology is the study of the process by which multicellular organisms grow and develop from its early immature forms (zygote, larva, embryo , etc.) into an adult.

Modern developmental biology studies the genetic control of cell growth, differentiation and morphogenesis, which is the process that gives rise to tissues, organs and anatomy (structure). 

History (remember on the basis of importance )

American leading cytologists Edmund Beecher Wilson (1882) at Columbia University recognized that development of the embryo is a demonstration of changes in individual cells.

Wilson recognized that the characteristics of an organism gradually appear by utilization of the inherited information that is located on the chromosomes.

In 1885, the German embryologist Wilhelm Roux removed a portion of the medullary plate of an embryonic chicken and maintained it in a warm saline solution for several days, establishing the principle of tissue culture.

Wilhelm Roux believed that the fertilized egg receives substances that represent different characteristics of the organism, this "qualitative division" fixes the fate of the cells

In 1888, Roux published the results of a series of defect experiments in which he took 2 and 4 cell frog embryos and killed half of the cells of each embryo with a hot needle. He reported that they grew into half-embryos and the separate function of the two cells had already been determined. This led him to propose his “Mosaic" theory of epigenesis:  after a few cell divisions the embryo would be like a mosaic, each cell playing its own unique part in the entire design.

Roux had manipulated embryos and observed the effects of these manipulations on them. For this reason, many embryologists consider him to be the "Father of Experimental Embryology."

History page 2

Roux's experimental approach to embryology

German embryologist, Hans Driesch (1892), work with sea urchin embryos.

Instead of destroying one of the cells of the two-celled embryo, he separated the cells from one-another and found that isolated cells at the four-cell stage also develop normally.

Thus, Driesch concluded that each cell retains all the developmental potential of the zygote.

History page 3

Sea urchin are small, spiny, globular animals 

When Driesch separated the blastomeres from 4- and 8-cell embryos, each isolated blastomere regulated its development so as to produce a complete organism.

Manipulation of the Embryo

(A) an early amphibian embryo is split almost into two parts with a hairloop. (B) an amphibian embryo at a somewhat later stage receives a graft of asmall cluster of cells from another embryo at that stage.

Result: a single embryo to develop into a pair of conjoined (Siamese) twins.

The Role of hereditary Material in develpoment

Mendel's pea plant experiments conducted between 1856 and 1863 established many of the rules of heredity, now referred to as the laws of Mendelian inheritance.

Although equal distribution of hereditary information to all cells had been established in the late 1800's, its role in development remained a mystery.

In1900, the significance of Gregor Mendel's work on heredity was finally appreciated.

The other contribution was made by Theodor Boveri 　 (1902) he demonstrated that normal development is dependent upon the normal combination of chromosomes.

History page 4

From then the research on development biology is going on………..

Basic Anatomical Features The similarities between animal species in the genes that

control development reflect the evolution of animals from a common ancestor

The body must have been organized with a sheet of skin covering the exterior, a mouth for feeding and a gut tube to contain and process the food, muscles, nerves and other tissues arranged in the space between the external sheet of skin and the internal gut tube.

These features are common to almost all animals and they correspond to basic anatomical scheme of development.

A. Scanning Electron Micrograph showing a blastula with initial epithelial cells inside

After fertilization egg divides and produce a hallow sphere of epithelial cells surrounding a cavity is known as blastula

BlastulaBlastula

Gastrulation

The fertilized egg cell divides to form many smaller cells. These cohere to create an epithelial sheet.

Much of this sheet remains external, constituting the ectoderm—the precursor of epidermis and nervous system.

A part of the sheet becomes tucked into the interior to form endoderm—the precursor of the gut and its appendages, such as lung and liver.

Another group of cells move into the space between ectoderm and endoderm, and form the mesoderm—the precursor of muscles, connective tissues, and various other components.

This transformation of a simple ball or hollow sphere of cells into a structure with a gut is called gastrulation.

Sea urchin gastrulationSea urchin gastrulation

Fig.1: The formation of ectoderm, endoderm and mesoderm. ( B) A group Fig.1: The formation of ectoderm, endoderm and mesoderm. ( B) A group of cells loose from the epithelium to become of cells loose from the epithelium to become mesodermmesoderm. (C) These cells . (C) These cells crawl over the inner face of the wall of the blastula (D) Epithelium is crawl over the inner face of the wall of the blastula (D) Epithelium is continuing to tuck (push/insert) inward to become continuing to tuck (push/insert) inward to become endoderm.endoderm. (E-F) The (E-F) The invaginating endoderm extends into a long gut tube. (G) The end of the gut invaginating endoderm extends into a long gut tube. (G) The end of the gut tube makes contact with the wall of the blastula at the site of the future tube makes contact with the wall of the blastula at the site of the future mouth opening.mouth opening.

Role of Regulatory DNA in developmentWhen animal species with similar body plans—different vertebrates such as a fish, a bird and a mammal, is compared, it is observed that corresponding genes usually have similar sets of regulatory modules.

The same result is observed between different species of nematode worm, or different species of insect.

The protein-coding sequences of vertebrate and invertebrate are unmistakably similar, but the corresponding regulatory DNA sequences appear very different.

Different body plans are produced mainly by changing the program of the regulatory DNA.

Regulatory DNA defines the Program of development

To explain development in terms of cell behavior, it is needed to be able to track the individual cells through all their divisions, transformations, and migrations in the embryo.

Cellular interaction can be observed by manipulating the embryo

Observation of Cellular Interactions

Cell lineage tracing in the early chick embryo

(A,D) Two tiny dots of fluorescent dye, one red, the other green, have been used to stain small groups of cells in an embryo at 20 hours of incubation. The dots have been placed on each side of a structure called the node.

(B,E) Six hours later, some of the labeled cells have remained at the node (which has moved backwards), giving a bright spot of fluorescence there, while other cells have begun to move forwards

(C,F) After a further 8 hours, the body plan is clearly visible, with a head at the front end , a central axis, and rows of embryonic body segments, called somites, on either side.

Cell Makes Developmental Decisions/Cell Fate

The ultimate differentiated state to which a cell has become committed is called the cell fate.

The cell that is fated to become a neuron may be already specialized in a way that guarantees that it will become a neuron no matter how its surroundings are disturbed; such a cell is said to be determined for its fate.

On the other hand the cell may be biochemically identical to other cells destined for other fates, the only difference between them being the accident of position, which exposes the cells to different future influences.

A cell may be already somewhat specialized for its normal fate, but still able to change and undergo a different fate if it is put in a sufficiently coercive environment.

Or the cell may be determined as a brain cell, but not yet determined as to whether it is to be a neuronal or a glial component of the brain. It seems, adjacent cells of the same type interact and depend on mutual support to maintain their specialized character.

Some developmental biologists would describe such a cell is specified or committed, but not yet determined.

Cell Fate

The standard test for cell fatedetermination

In many systems, long before cells become committed to differentiating as a specific cell type, they become regionally determined: that is, they switch on and maintain expression of genes that can be regarded as markers of position or region in the body.

This position-specific character of a cell is called its positional value, and it shows its effects in the way the cell behaves in subsequent steps of pattern formation.

Cells Have Remembered Positional Values

In the chick embryo the leg and the wing originate at about the same time in the form of small tongue-shaped buds projecting from the flank (side).

The cells in the two pairs of limb buds appear similar and uniformly undifferentiated at first.

But a simple experiment shows that this appearance of similarity is unreliable.

A small block of undifferentiated tissue at the base of the leg bud, from the region that would normally give rise to part of the thigh, can be cut out and grafted into the tip of the wing bud.

Remarkably, the graft forms not the appropriate part of the wing tip, nor a misplaced piece of thigh tissue, but a toe.

Positional Values

Therefore, the early leg-bud cells are already determined as leg but are not yet permanently committed to form a particular part of the leg: they can still respond to changes in the wing bud so that they form structures appropriate to the tip of the limb rather than the base.

Positional Values

Normal condition

Signal molecules often seem to govern a simple yes–no choice: when their concentration is high or low.

A signal molecule that imposes a pattern on a whole field of cells is called a morphogen.

Morphogens Are Long-Range Inducers in Cell Fate Determination

A group of cells at one side of the vertebrate embryonic limb bud become specialized as a signaling center and secrete Sonic hedgehog protein—a member of the Hedgehog family of signal molecules.

This protein spreads out from its source, forming a morphogen gradient that controls the characters of the cells along the thumb-to-little-finger axis of the limb bud.

If an additional group of signaling cells is grafted into the opposite side of the bud, a mirror duplication of the pattern of digits is produced

Morphogen mediated cell fate determination

Sonic hedgehog as a morphogen in chick limb development

(A) Expression of the Sonic hedgehog gene in a 4-day chick embryo. The gene is expressed in the midline of the body and at the posterior border of each of the two wing buds. Sonic hedgehog protein spreads out from these sources.

(B) Normal wing development.

(C) A graft of tissue from the polarizing region causes a mirror-image duplication of the pattern of the host wing.

 Apoptosis, or programmed cell death (PCD), is a form of cell death that is generally triggered by normal, healthy processes in the body of multicellular organisms.

 Biochemical events lead to characteristic cell changes (morphology) such as release of proapoptotic proteins, shrinking of the cell’s cytoplasm, nuclear fragmentation, chromatin condensation, increase of mitochondrial membrane permeability, formation of cell fragments called apoptotic bodies that phagocytic cells are able to engulf.

It is a series of programmed steps that cause a cell to die by “self digestion” without releasing intracellular contents into the surrounding environment.

Apoptosis Part of the Program of Development

Apoptosis The control of cell numbers in development depends on cell death as well as

cell division. If apoptosis is prevented in the body, cells will grow uncontrollably and cause

cancer. For example, people with chronic myelogenous leukemia (CML) typically have

10–25 times many white blood cells than normal. Apoptosis confers advantages during an organism's lifecycle. For example, the

separation of fingers and toes in a developing human embryo occurs because cells between the digits undergo apoptosis.

Caenorhabditis elegans generates 1030 somatic cell nuclei in the course of its development, but 131 of the cells die by apotosis.

Thus programmed cell deaths occur in an absolutely predictable pattern.

Apoptosis

learn more from……Molecular Biology Of The Cell - Alberts B. - 5th Ed.Chapter 18