Genes in Development

Gametes unite to form the zygote. The zygote has all the genetic information for the formation of different parts of an individual. The zygote divides to form the embryo. The embryo develops as per the genetic information into an organism. The organism has different types of cells and tissues.

Each cell and tissue has different functions e.g. muscle cells have protein, actin and myosin for muscle contraction, the goblet cells produce mucous and oxyntic cells produce HCl. So different cells function in different ways having the same genetic material as it was present in zygote. The question is that, how the same genetic material function differently in different types of cells?

Evidence has suggested that there are three factors, which act together in various ways to bring out differentiation. These are:

a) Nuclei
Acetabularia
b) Cytoplasm
c) Environment

The Role Of The Nucleus

Danish biologist Joachim Hammerling performed experiment on Acetabularia in 1943. It is a single celled marine green alga. Individuals have distinct foot stalk and cap regions. The nucleus of this cell is located in the foot. It grows to a length of 6 to 9 centimeter.

Hammerling selected individuals from two species of the genus in which caps looked very different from each other. Acetabularia mediterranea, which has a disk shaped cap and Acetabularia crenulata which has a branched flower like cap.

Experiment: The cap was removed from one species and thrown away. Next, the cytoplasmic stalk was cut off and grafted to the base of other species, whose stalk and cap had already been cut off and thrown away. A whole new alga grew from the joined pieces. The new organism becomes complete with cap, stalk and foot.

Acetabularia Grafting Experiment

Result: The first cap regenerated may be intermediate in type, but when it was removed, the next cap formed always had the characteristic of the species supplying the nucleus.

Conclusion: The nucleus of each grafted algal cell must exert its influence through the alien cytoplasm in determining the form of cap, so there is an evidence of nuclear control of the development processes forming a new cap.

Nuclear Equivalence
1. Spemann placed minute ligatures of human hair around salamander zygotes just as they were about to divide, constricting them until they were almost, but not quite, separated into two halves. The nucleus lay in one half of the partially divided egg, the other side was anucleate, containing only cytoplasm. The egg then completed its first cleavage division on the side containing the nucleus, the anucleated side remained undivided. Eventually, when the nucleate side had divided into about sixteen cells, one of the cleavage nuclei would wander across the narrow cytoplasmic bridge to the anucleate side. Immediately this side began to divide.

Spemann's delayed nucleation experiments

Tow kinds of experiments were performed.

(A) Hair ligature was used to constrict an uncleaved fertilized new egg. Both sides contained part of the gray crescent. The nucleated side alone cleaved until a descendant nucleus crossed the cytoplasmic bridge. Then both sides completed cleavage and formed to complete embryos.

(B) Hair ligature was placed so that the nucleus and gray crescent were completely separated. The side lacking the grey crescent became an unorganized piece of belly tissue, the other side developed normally. 

Cytoplasmic Influence On Development

(A) A frog's egg has anterior/posterior and dorsal/ventral axes that correlate with the position of the gray crescent.

(B) The first cleavage normally divides the gray crescent in half, and each daughter cell is capable of developing into a complete tadpole.

(C) But if only one daughter cell receives the gray crescent, then only that cell can become a complete embryo. This shows that the chemical messengers are not uniformly distributed in the cytoplasm of frog's eggs.

Cytoplasmic influence on development

(A) A frog's egg has anterior/posterior and dorsal/ventral axes that correlate with the position of the gray crescent.

(B) The first cleavage normally divides the gray crescent in half, and each daughter cell is capable of developing into a complete tadpole.

(C) But if only one daughter cell receives the gray crescent, then only that cell can become a complete embryo. This shows that the chemical messengers are not uniformly distributed in the cytoplasm of frog's eggs.

2. Spemann performed another experiment. He separated the two halves of embryo, both of them contained nuclei. Both these halves developed into complete embryos. He also observed that from a 16-cell embryo, even if a single cell is separated, it contains a complete set of genes and form a complete embryo cell and thus, the nuclei were equivalent.

Spemann also observed that sometimes it may happen that the nucleated half can develop into abnormal ball of cells. Later studies proved that development depend on gray crescent. Gray crescent is the pigment free area that appears at the time of fertilization. So in the half lacking gray crescent, no further development can take place.

Influence Of Cytoplasm On Nucleus During Development

Experiment No. 1: If the early embryos of Sea urchin are placed in sea water from which the calcium ions have been removed, the cells tend to separate. Thus, it is possible to isolate the two cells formed by the first cleavage or the four cells formed by the second cleavage. Each of the cell continues to develop and becomes in time, a small but complete sea urchin larva. This experiment was perfumed in 1892 by Hans Dietrich.
Influence Of Cytoplasm On Nucleus During Development

Experiment No. 2: Instead of dividing a sea urchin egg along the natural planes of cell division, we cut across the axis of an unfertilized egg. Now, we have two halves of an egg, sometimes the nucleus is located in the upper half and sometimes in the lower half.

Both the halves heal. Each forming an apparently normal cell. Then we see sea urchin sperms. A sperm will enter each half of the egg cell. If the half has a nucleus, the half will be diploid. If the half is without nucleus, then it will have only the monoploid number of chromosomes contributed by the sperm. Both the upper halves develop into a hollow ball of cells with many cilia, but it forms no internal tissues. It swims around for several days and then dies. The lower half too is very abnormal and incomplete and dies before long. Neither of the half embryos is then able to develop into a normal embryo, or even to survive for long.

Conclusion: In the experiment all the four or two cells are getting same type of nucleus so all the cells fertilized developed into embryo. In the second experiment there is different cytoplasm in the lower and upper cut cells as the cytoplasm is lacking some of the material, so after fertilization the embryos did not develop completely. This, it can be concluded that the materials indeed affect and limit what genes in the nucleus are above to do in controlling the path of the development. 





The Analysis of Development

The zygote gives rise to an adult animal. The adult can response to stimuli and can perform physiological functions. Now the question arises: How the zygote is transformed into an adult. Moreover, what makes the adult perform all types of functions?

Development by Pre-Formation or Epigenesis
Aristotle proposed two different hypotheses to account for development:
(1) Pre-Formation (2) Epigenesis

Pre-Formation
Pre-formation implies that an egg or sperm actually contains the new individual already formed.
                            
Epigenesis
According to this idea, the egg or the sperm contains no structures; rather these structures somehow develop in their proper position later, constructed from material in the egg. The development depends on the action of genes, received by the embryo from the parents, in the sense one can say that the embryo is preformed. The organization of the embryo arises by epigenesis, a process controlled by genes.

The Problem of Differentiation
If the organization of the embryo arises by epigenesis, then how the dividing cells of the zygote is differentiated into three embryonic layers? Moreover, how from these layers different structures are formed? Hans Spemann and Hilde Mangol (1924) performed a series of experiments on differentiation.

Experiment No: 1
From an amphibian embryo, they cut out the ectoderm that normal becomes the nerve tube and put the piece of ectoderm in a separate dish. an embryo from which the piece was taken healed and lived, but t had either a defective nervous system or none at all. Moreover, the isolated piece of ectoderm did not form a nervous system, though it remained alive and healthy.

Conclusion: They concluded that the piece of ectoderm should remain attached to the embryo for the proper development of nervous system. So they performed another experiment.

Experiment No: 2
They cut a flap of ectoderm from the top of an embryo. They did not remove the piece of ectoderm, but just folded it back. Then they cut out the mesoderm underneath and discarded it. Finally they folded the flap of ectoderm back in its place. The ectoderm healed and looked quite healthy, but it did not develop into a nervous system. 

Conclusion: When the mesoderm is removed, the ectoderm does not differentiate into nerve tissue. The mesoderm must influence the ectoderm somehow to differentiate into nerve tissues. So they performed another experiment.

Experiment No: 3
They used two embryos, both in the early gastrula stages, from one they removed a piece of mesoderm immediately in front of the dorsal lip of the blastopore. from the second embryo, they removed the similar size piece from the mesoderm area 180 degree from the dorsal lip i.e. exactly opposite to the dorsal lip. In its place they put the piece of mesoderm from the first embryo. The transplanted mesoderm formed as blastopore and moved inside the embryo. Two nervous systems were formed, one in front of the embryo's original dorsal lip and another in the area where transplantation was made. Thus, sort of Siamese-twin embryo was produced, having two brains and spinal cord.

Conclusion: This experiment proves that mesoderm seems to control the differentiation of nervous tissue.

Spemann designated the dorsal lip area, the primary organizer because it was the only tissue capable of inducing the development of a secondary embryo in the host. He also termed this inductive event primary induction because he believed it to be the first inductive event in development. Spemann was awarded Nobel Prize in 1935 for his experiments.

Embryonic Induction
Embryonic induction means that the cells of one kind direct the development of other cells e.g. in the experiment of Spemann mesodermal cells of the dorsal lips regions induced the ectoderm to form a nerve tube.

The Problem Of Cell Differentiation
Some parts of the genetic information are used practically by all cells because some "house keeping" activities such  as cell respiration and protein synthesis are carried out by all cells. Genetic information necessary for these processes must be expressed in all cells. But how do cells become structurally and functionally specialized for the division of labor that occurs in complex, multicellular organisms?

Each kind of specialized cell has its own particular set of special structural protein and enzyme systems. Thus, cell differentiation depends upon production of different of different sets of specific proteins by different group of cells. cells must actively express specific parts of their total store of genetic information (genome) while other parts are kept inactive (repressed). Example: Some portions of genome are used to produce one type of specialized cells e.g.  brain cells, while different portions of the genome are used to produce another type of specialized cells e.g. kidney cells. 

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Development of Chick

Embryology of Gallus domesticus has been extensively studied because the eggs are larger, easily available at all times of year and incubated easily. It provides basis for understanding the early differentiation of the organ system and the fundamental process of body formation, which is common to all vertebrates.

Gametes

Avian (Bird) Egg
The gametes consist of sperm and egg. The sperm of fowl is very long. The egg as laid by hen, consists of ovum released from the ovary and the albumens, two shell membranes (outer and inner shell membrane) and shell. The ovum consists entirely of yolk (eggs with large amount of yolk is called microlecithal egg). The cytoplasm lies as a small disc, the blastodisc, at the animal pole.  

Fertilization
Fertilization is internal. It takes place in the front part of the oviduct, before the secretion of albumen around the ovum.

Incubation
The egg is laid 24 hours after fertilization, but the real egg is . the mature ovum. Further development of the zygote takes place, when the egg is incubated by the female. Incubation must continue steadily for 21 days. In incubating ' egg artificially, the incubations are regulated at temperature between 36°C to 38°C.      

Cleavage
Immediately after fertilization, the egg undergoes a series of mitotic divisions called cleavage. The cleavage furrows are confined to the blastodisc, not extending at all into the yolk. This type of cleavage is called discoidal cleavage.

Cleavage starts soon after fertilization and continues as the egg passes down the oviduct. This often takes about 12 to 16 hours, so that by the time the egg is laid, the embryo has reached the blastula stage.

The first cleavage furrow is vertical. The second cleavage furrow is similar to first, but is at right angle to it. The third cleavage runs horizontally parallel to the surface and thus, cuts underneath the cytoplasm and separates it from the yolk. Further cleavage furrows are also vertical, occur in an irregular manner, and there is increase in the number of cells. The furrows do not affect the entire blastodisc.

Chick cleavage stages
With the result a central area of cells surrounded by a ring of unsegmented cytoplasm the marginal zone (periblast) is produced. Later cleavages occur in the marginal zone as well as in the central zone.

Morula
Cleavage results in the formation of rounded closely packed mass of cells. These cells are called blastomeres.


Blastula
Blastocoel
The morula stage is short lived. Horizontal cleavage change blastodisc into a regular one or more layers of cells, called blastoderm. In the centre of the blastoderm, the blastomeres. are smaller and completely defined while those at the periphery, are flattened and larger. A fluid filled space called blastocoel (sub-germinal cavity) appears beneath the central cells of the blastoderm, separating them from underlying yolk. Further horizontal cleavages make the blastoderm several layers thick.

The blastoderm also grows peripherally, gradually spreading over the yolk. The marginal cells bf the blastoderm remain in contact with the yolk called zone of junction to engulf and digest the yolk. The blastoderm now shows two distinct regions, a large central transparent area pellucida and a narrow , peripheral opaque area opaca. The area pellucida appears transparent as it over lies the blastocoel (sub-germinal cavity) and the area opaca looks dark because it over lies the yolk . The two areas can be observed if the egg is seen from the above by transmitted light. The embryo at this stage is called blastula.

Presumptive Areas
Chick, Presumptive Area
The presumptive areas of blastula lie within the area pellucida. Beginning from the future posterior end, these area are; a small disc of presumptive endoderm, a broad band of presumptive lateral plate mesoderm, two narrow lateral bands of presumptive somatic mesoderm, a narrow band of presumptive notochord, a large area of presumptive neural plate and very large presumptive ectoderm. They latter surround the other areas on all sides, except the posterior. Outside the embryonic ectoderm is extra-embryonic ectoderm.

Gastrulation
Gastrulation in the chick involves four important processes,
(1) Formation of endoderm,
(2) Formation of mesoderm,
(3) Formation of notochord,
(4) Formation of neural tube.

These processes, though overlap to a certain extent, occur in the order named. They involve two events (i) cell movement and (ii) cell division.

The blastoderm splits into two layers the epiblast and hypoblast. The epiblast is an upper layer of cells and is mainly presumptive ectoderm and mesoderm. Hypoblast is the lower layer of cells and is mainly presumptive endoderm. At this stage, the central cells of blastoderm can be separated from the yolk.

Formation of Endoderm
The presumptive endoderm cells migrate into the blastocoel (sub-germinal cavity). Here they spread forwards and laterally, so that the blastocoel (sub-germinal cavity) soon acquires a complete floor of embryonic endoderm. Later, the embryonic endoderm grows peripherally and meets the yolky extra embryonic endoderm.

Primitive Streak
As the presumptive endoderm moves in from the surface, the presumptive lateral plate mesoderm lying just ahead of it quickly moves backward and towards the median line to take place. This results in heaping up of the presumptive lateral plate mesodermal cells in the middle line, forming a slight ridge the primitive streak in the posterior region of area pellucida. At anterior end of the primitive streak,
appears a small depression, the primitive pit, and in front of it develops an elevation the Hensen's node or primitive knot. A narrow depression, the primitive groove appears along the middle of the primitive streak.

T.S of embryo

The primitive streak is thus, the result of convergence of mesodermal and notochordial cells towards the middle line and the area pellucida becomes pear shaped.

Formation of Mesoderm  
The presumptive lateral plate mesoderm   streams into the primitive groove and sinks down into the blastocoel (sub-germinal cavity). Here, it fans out laterally forming a sheet (third layer) on either side between the endoderm and the upper layer of cells. The presumptive somatic   mesoderm follows presumptive lateral plate mesoderm.

Gastrulation In The Chick


However, it remains as two bands close to the middle line, seprated by the notochord.

Formation of Notochord
The presumptive notochord cells roll over the edge of the Hensen's node and through the primitive pit, sink into the blastocoel (sub-germinal cavity). Here they extend forward as a strip in the middle line beneath the surface cells.

Formation of Neural Tuba
With the sinking in, of the presumptive mesoderm and notochord, and retreat of the Hensen's node, the two wings of the presumptive neural plate move towards one another and meet in the medial line to form the neural plate in front of the Hensen's node. The lateral margins of the neural plate rise up as neural folds which unite to form the neural tube. The neural tube encloses a cavity, the neurocoel, and opens out by neuropore. In 24 hours chick embryo, the folding of the neural plate is clearly visible. The embryo is termed as neurula. With the formation of neural tube, there is formation of central nervous system. This entire process is neurulation.

T-S Of Embryo Is Showing Notochord
Differentiation of Mesoderm
The mesoderm occurs first as a pair of solid sheets. Each sheet consists of a thicker medial somatic mesoderm and a thinner outer lateral plate mesoderm. The somatic mesoderm shortly after the formation of the head fold, segments transversely to form paired blocks, the somites lying on the sides of the notochord. Somites are seen in 25-26 hours embryo. The lateral plate mesoderm splits up into two layers upper somatic mesoderm and lower splanchnic mesoderm, with a space between them. The cavity formed between somatic and splanchnic mesoderm is coelom. The splanchnic mesoderm of the area opaca and of the outer part of the area pellucida develops blood capillaries. All the development up to this takes place on the first day of incubation.

Folding Off The Chick Embryo
Later Development of Chick
Further development of chick involves three main processes.
(a) Folding off of the embryo.
(b) Formation of organs.
(c) Formation of embryonic membrane.

(a) “Folding off' of the embryo: It commences at the anterior end of the area pellucida, by the formation of head fold having an endodermal pocket the so called “foregut”. Lateral fold next appear, under the sides of the embryo by a tail fold, which includes the hind gut.

(b) Organogenesis: It is beyond the scope of this book to discuss organogenesis in chick.

(c) Embryonic membrane: The blastoderm not only gives rise to the paired blocks, the somites lying on the sides of the embryo, but certain structures that lie outside the embryo. These are called embryonic membranes. These include yolk sac, amnion, chorion and allantois.

Hatching
On the 20th day of incubation, the chick ruptures the inner shell membrane adjacent to the air space by means of the beak. On the same day or next, the chick repeatedly strikes the shell by a sharp horny process. The shell eventually breaks and the young-feathered chick emerges.

    

Principle of Early Development - Vertebrates

There is general pattern of development in all vertebrates. The process can be divided into four stages: (1) Cleavage (2) Gastrulation (3) Organogenesis (4) Growth.

Cleavage
After a sperm penetrates an egg, the egg quickly surrounds itself with an enveloping coat, the fertilization membrane, and a series of cell division called cleavage begins.


A cycle of repeated mitotic division continues until a spherical mass of cells known as morula is formed. The cells do not grow between the cell divisions. As cleavage or division continues, cells begin to move apart, so that spaces appear among the cells in the centre of the mass. Cells keep pulling away from the central area, forming a fluid filled cavity known a blastocoel. This hollow-sphere embryo which develops at the end of cleavage is called a blastula. The cells of the morula and blastula are called blastomeres.

Gastrulation
Cell division continues but the whole embryo remains in same size at this early stage, the cells are becoming smaller as cleavage progresses.

Then the embryo enters a phase called gastrulation. During this phase there is rearrangement of cells. The essential feature is the formation of three layers of cells, with each layer capable of developing into special tissue.

Once gastrulation has occurred, the whole body has an outer coating the ectoderm (ecto; outside), and an inner lining the endoderm (endo; inside). Between the ectoderm and endoderm, new cells, proliferate, filling the part between the two original layers and eventually building the bulk of the embryo. This is the mesoderm. Ectoderm, endoderm and mesoderm are called germinal layers.


 Organogenesis
After formation of the three germinal layers, different cells of these layers start differentiating to form different organ rudiments. This phase constitute the beginning of organogenesis. The three germinal layers give rise to the structures as shown in the table below.

Organogenesis


Growth
It is a long phase. All the basic organ rudiments increase in size and also undergo many physiological changes. As a result fully functional organism is ready to start life on its own, then the embryo either hatches or is born.

Example: Hatching in birds and reptiles, birth in eutherian mammals (a mammal whose young develop within the womb surrounded by a placenta. Subclass: Eutheria).