Embryology Question Bank: [For Medical Students]

List of general and human embryology question and answers for medical students!

Embryology Question # Q. 1. What do you mean by Gametogenesis?

Ans. Gametogenesis is the first phase in the sexual reproduction of animals. The essential process during this phase is the transformation of certain cells in the parents into specialized cells: the eggs, or ova, in the female and the spermatozoa in the male (or in the female and male organs in hermaphroditic animals).

The egg and the spermatozoon jointly constitute the material system, or jointly contain all the essential factors, which, given a suitable environment, will produce in the course of time a new individual with, in the main, the same characters as the two parent organisms.

The various properties of both the paternal and maternal progenitors must thus be contained, in some form, in either the egg or the spermatozoon or both. It is obvious, however, that the characters of adult organisms are not there to be seen or directly detected in either the eggs or spermatozoa, the latter being single cells of a highly peculiar structure.

If follows that the characteristics of the adult organisms are contained in the egg and sperm in a different and not immediately recognizable form. It is said, therefore, that these characteristics are encoded in the structure of the egg and sperm, just as a transmitted message may be concealed by the use of a code which makes the message unintelligible except to a person in possession of the code.

If is also said that the egg and spermatozoon possess the “information” that is needed to build a new organism, meaning by this that all the specifications for the future organism ore contained in the egg and sperm. Since the information is contained in the egg and spermatozoon in o form which does not bear an obvious relationship to the characters of the adult organism, we may say that it is contained in an encoded form.

During the development of the egg the encoded information is decoded, that is, translated or transformed from a concealed to an overt form; characters in­herent in the egg and sperm serve to determine the properties of the adult or­ganism. The decoding or “reading” of the information contained in the gametes is then equivalent to the process of ontogenetic development.

The encoded information which is possessed by the egg and the spermatozoon may be contained in the structure (including the chemical composition) of either the nuclei or the cytoplasm of the gametes. Generally speaking, both the nucleus and the cytoplasm are indispensable for development to occur, since animal cells are fully viable only when both these ports (the nucleus and the cytoplasm) act together and in harmonious cooperation with each other.

Embryology Question # Q. 2. What is meant by Growth?

Ans. With the formation of all or most organ rudiments of the embryo, the main features of the organization of the animal, what may be called the morphological plan of the animal is established. By morphological plan we mean the kind and number of organs, their relative positions, and the general features of each organ’s structure.

However, organ rudiments at this stage are not capable of performing their specific functions, on which depends the ability of the animal to lead an indepen­dent existence. The cells of the organ rudiments lack the peculiar structures that are necessary for specific functions; the organ rudiments are usually too small, and the animal as a whole is likewise far from the adult size.

All the developmental processes dealt with so far may be grouped together as the pre-functional stages of development. Next, a new phase of development sets in, which brings the animal to its functional state. The main processes involved are growth and differentiation.

Some new organs may appear in late stages of development, especially in animals passing through a larval stage, and minor morphological adjustments may occur in the organs formed earlier, but the processes of growth and differentiation are predominant.

Growth is the increase in size of an organism or  of its parts due to synthesis of protoplasm or of apoplasmatic substances. Protoplasm in this definition includes both the cytoplasm and the nucleus of cells. Apoplasmatic substances are the sub­stances which are produced by cells and which form a constituent part of the tissues of the organism, such as the fibers of connective tissue or the matrix of bone and cartilage, as opposed to substances produced by the cells and subsequently removed from the organism, such as the secretions of digestive and skin glands, or substances stored as food, such as fat droplets in cells of the adipose tissue. Imbibition of water or taking food into the alimentary canal before the food is digested and incorporated into the tissues of the animal, although they may increase the weight of the animal, do not constitute growth.

Growth is the result of a preponderance of the anabolic (synthetic) over the catabolic (destructive) process in the organism. If synthesis and decomposition go on at the same rate, there is no increase in the bulk of the organism—no growth.

Under certain conditions, decomposition dominates over synthesis, as for instance in pro­longed inanition, when synthetic processes are impossible because of a lack of food supply, while catabolic processes (oxidations, etc.) continue to satisfy the current requirements for energy.

After the internal food reserves (fat in the adipose tissue) have been exhausted, energy is produced at the expense of the proteins of the protoplasm, and the result is a decrease in the mass of living matter which may be called de-growth.

Embryology Question # Q. 3. What do you understand by Morphogenetic Movements during Gastrulation?

Ans. During cleavage and up to the blastula stage the embryo retains roughly the shape of the egg from which it started developing. It is spherical in most cases, sometimes oval if the egg is oval, or elongated, as in the case of many insects and Cephalopods among the molluscs, which have elongated eggs. The internal structure of the blastula is also simple, consisting, as it does, of one layer of cells around a cavity. (Special cases are present in some animals with large yolky eggs, such as birds).

With the onset of gastrulation the embryos structure changes. Through the de­velopment of the archenteron and then the separation of the mesoderm, the embryo acquires a more complicated internal structure, departing from the geometric simplicity of the blastula of animals with oligolecithal eggs. The structure becomes further compli­cated with the formation of the primary organ rudiments, which closely follows gastrula­tion.

Gradually the external shape of the embryo starts to change. As primary organ rudiments give rise to secondary and tertiary organ rudiments, the internal structure and the external shape of the embryo approach those of the adult animal. (The embryo or larva may leave the egg long before the final goal is achieved.)

Thus, the new organism not only acquires a diversity of parts that did not exist in the egg but also acquires the form typical of an animal of its species. By form we mean not only the external shape of the animal but also the structural organization, that is, the body’s being composed of a number of parts, placed in a typical disposition inside and on the surface of the animal’s body.

The new elements of the embryo’s organization appearing in gastrulation (the archenteron, etc.) are produced by movements and changes in the shape of cells and groups of cells of the embryo. These movements and changes in shape are also involved in subsequent processes of form creation.

The movements which we are referring to are very different from the movements of parts of an adult animal. Whereas the movement of parts of an adult is usually of a reversible nature, the gastrulation movements are irreversible; each part remains in the position into which it was brought by the preceding movement.

As a result of the movements, the structure of the embryo is changed, or in other words, new structural elements are created, such as the archenteron, the neural tube, and the notochord, in place of the simple layer of cells found in the blastula stage. The movements have created new shapes, new forms. They have therefore been designated as the morphogenetic movements.

The morphogenetic movements appear to be movements of large parts of the whole embryo, which stretch, fold, contract, or expand. The question arises, how are these movements achieved? They cannot be ascribed to contractility in the narrow sense of muscle contractility.

Neither can they be interpreted as an ameboid movement of the embryo as a whole, because each moving part consists of numerous cells, and the movement of the whole, we should expect, would be an integrated result of the movements of the individual cells.

That gastrulation cannot be a function of the embryo as a whole has been proved by investigating isolated parts of the young gastrula. We know that the presumptive ectoderm contributes to gastrulation by expanding its surface. The expansion is an active process depending on the presumptive ectoderm itself.

If large pieces of the animal region of an amphibian blastula or early gastrula are cut out and cultivated in a suitable medium, the presumptive ectoderm rounds itself up into a vesicle, and later the epithelium of this vesicle increases its surface greatly and is thrown into a series of irregular folds as it does so.

Also, if presumptive ectoderm is combined with cells of presumptive endoderm and mesoderm in such a proportion that the ectoderm is far in excess of the endodermal and mesodermal parts which it has to cover, the ectoderm tends to form folds. These experiments show that the expansion of the ectoderm is active and proceeds independently of the other movements involved in gastrulation.

Similarly, the movements of invagination can be performed by parts of the blasto­derm independently of their surroundings. The most suitable method of testing this is to transplant small pieces of the dorsal lip of the blastopore into some other part of the embryo, the animal region or the ventral part of the marginal zone.

The transplanted piece will invaginate and form an archenteric cavity which may be completely indepen­dent of the archenteric cavity of the host embryo. The mechanism by which the invagination is achieved is the change in shape and the movements of the cells of the marginal zone.

The change in the shape of the cells is not due to forces exercised by the embryo as a whole but is performed by the cells themselves. This property is best seen if the cells are isolated by tearing the embryo to pieces; the cells preserve their shape, and moreover, the in-folding of the surface layer may even be facilitated by releasing a piece of it from its connections with the surrounding parts.

Embryology Question # Q. 4. What is meant by Physiology of the Placenta?

Ans. In the absence of yolk in mammalian eggs, the nutrition of the mammalian embryo in the uterus is wholly dependent on the flow of supplies from the maternal body via the placenta, hence the close connections between the fetal and the maternal tissues. Nevertheless, the fetal and maternal tissues in the placenta do not blend together.

It cannot be stressed too much that the blood of the mother and that of the embryo do not mix under normal conditions; the maternal blood does not enter the blood circulation of the embryo or vice versa. Between the maternal and the fetal blood there exists a separation—the placental barrier.

Physically, the barrier consists of the tissue lying between the blood spaces in the embryonic and maternal parts of the placenta; this barrier may be attenuated (as in hemochorial placentae) but is not broken down. Physiologically the placental barrier is like a semi-permeable membrane, allowing some substances to pass through but keeping out others.

Small molecule substances pass through the placental barrier by simple diffusion. This applies to water, oxygen passing from the maternal into the fetal blood, carbon dioxide and urea passing from fetal to maternal blood, simple salts of sodium potassium and magnesium, and mono-saccharides.

Active transport of some form or another participates, however, in the penetration of more complex substances through the placental barrier. It is a well-established fact that vitamins and hormones may pass from the mother to the fetus. Passage of some very complex substances, proteins in particu­lar, may perhaps be affected by pinocytosis at the surface of the trophoblast.

Highly complex proteins are known to be able to penetrate the placental barrier. In this way antibodies, which have developed in the blood of a mother who has acquired immunity to certain diseases, such as diphtheria, scarlet fever, smallpox, and measles, are passed to the fetus, which thus becomes passively immunized and unsusceptible to these illnesses in the first period after birth.

It is worth noting that in cows, which have an epitheliochorial placenta and thus a formidable placental barrier, antibodies cannot be passed from mother to offspring through the placenta but, instead, are supplied to the newborn animal in colostrum milk after birth.

Certain pathogenic organisms and viruses-are able to penetrate through the placental barrier and infect the fetus if the mother is infected. Such penetration is known to happen with syphilis (causing congenital disease) and also in infections with smallpox, chickenpox, and measles. One virus infection which has been found to be very danger­ous for the embryo is rubella, or German measles.

Many drugs used medicinally may penetrate the placental barrier and may some­times cause most adverse effects on the embryo. Thus it is believed that the drug thalidomide, which was used as a sedative, when taken by women in early pregnancy (25 to 44 days), caused extensive deficiencies in the development of limbs, the alimen­tary canal (non-perforation of the anus), and the heart.

Lastly, it must be pointed out that although the tissues of the mother and fetus, including the trophoblast, do not mix and the blood streams of the two are held apart, occasional penetration of individual cells across the placental barrier is not an excep­tional occurrence.

Small numbers of fetal blood corpuscles are sometimes found in the maternal circulation as well as maternal corpuscles in the circulation of the embryo. This may be the result of accidental breakage of the respective blood capillaries. The origin of the corpuscles can be verified since the fetal erythrocytes are nucleated and the erythrocytes of the adult female are without nuclei.

Small fragments of the trophoblast may become detached from the chorionic villi and may be carried away by the maternal blood stream; they are later found in the blood capillaries of maternal organs, such as the lungs. On the other hand, maternal cells, probably white blood corpuscles, have been found lodged in the lymphatic system (spleen, thymus, lymph nodes, and bone marrow) of the fetus.

Embryology Question # Q. 5. What is Regeneration?

Ans. According to definition, regeneration is the replacement of lost parts. One could have expected, therefore, that the loss of some part of the body would be the adequate stimulus to set in motion the mechanism which restores the part and thus the normal structure of the animal. This is by no means always the case. If a deep incision is made on the side of a salamander’s limb or on the side of the body of an earthworm or a planarian, a regeneration blastema may be formed on the cut surface.

The blastema then proceeds to grow and develop into a new part, as in ordinary regeneration. In the case of a limb, the new part thus developed will be the distal part of the limb, from the wound level outward. The development of the regenerating part proceeds just as if the entire distal part of the limb were cut off.

In the case of a planarian, a lateral incision may cause the development from the wound surface of a new head, a new tail, or both. If both a head and a tail are regenerated, the head forms from that part of the wound surface which faces anteriorly, and the tail develops from the wound surface facing posteriorly. This results, of course, in the regenerated head lying more anteriorly than the regenerating tail.

A somewhat similar reaction is produced by lateral incisions in the earthworm, with a restriction that lateral incisions near the head end of the worm give rise to additional heads, incisions in the middle part of the animal cause the development of both heads and tails, while incisions in the posterior part of the animal’s body cause the formation of tails only. Another peculiarity in the case of an earthworm is that the incision must be deep enough to sever the ventral nerve chain if any regeneration at all is to take place. 

In each of these cases, the original parts of the animal (heads, tails, and limbs) had not been removed, so that the regenerated parts were additional and therefore superfluous to the animal. The experiments allow us to conclude that not the absence of an organ but the presence of a wound is the stimulus for regeneration.

The development of a superfluous number of organs or parts of the body, as a result of regeneration, is called super-regeneration. The clue given by super-regeneration has been followed up to analyze still further the stimulus leading to regeneration. It has been found that regeneration can be started even without inflicting an open wound. This has been demonstrated by ligaturing a limb in a salamander (axolotl).

A tight ligature causes considerable destruction of the tissues immediately affected by the pressure. The muscles and portions of the skeleton of the limb disintegrate, so that the part of the limb distal to the ligature becomes bent at an angle and is dragged about by the animal without being capable of movement of its own. The skin, however, turns out to be more resistant and preserves its integrity.

After some weeks, the region of the limb just proximal to the ligature begins to swell, and it soon becomes evident that a regeneration blastema has been formed. The regeneration blastema then develops into the distal part of a new limb, although the old distal part of the limb is still present. With the processes of differentiation setting in, the skeleton of the old distal part of the limb may be joined again to the proximal skeleton.

The experiment on ligaturing the limb teaches us that the presence of an open wound is not essential for regeneration. What is really necessary is damage to the tissues of an organ that is capable of regeneration. Usually tissues are damaged by wounding, but if extensive damage to the tissues can be caused without an open wound, this suffices to start the sequence of processes leading eventually to regeneration.

Having reached thus far we may suggest that damaging the tissues is necessary so that some substance or substances be released from the damaged and disintegrating tissues, which are the immediate cause of the processes that follow. Various types of experiments have been adduced in support of this concept.

It has been found that if a regeneration blastema of an axolotl limb is dried at low temperatures, so that all the cells of the blastema are killed, and the blastema is then transplanted under the skin of an axolotl limb, it causes an outgrowth on the surface of the host limb. This outgrowth, which is covered by skin and has a cartilaginous axis in the middle, is comparable to a very rudimentary limb or at least to a digit.

A similar outgrowth may be caused by an implanted piece of cartilage and also by introducing under the skin the products of alkaline hydrolysis of cartilage. These experiments immediately remind us of the experiments on the primary organizer in early amphibian development – there, as here, it was found that the stimulus for causing certain morphogenetic processes was not necessarily dependent on the integrity and vital activity of the cells of the inducing part and those non-living substances could exert a similar action.

Another line of research consists in the treatment of the amputation surface with a solution of beryllium salt (beryllium nitrate). Beryllium nitrate applied to the amputation surface of a tadpole tail or of the limb of an Ambystoma larva completely suppresses regeneration. It has been suggested that beryllium in some way binds the substances released from the damaged cells at the wound surface that would have normally initiated the whole sequence of the processes of regeneration—the de-differentiation in the first place and subsequently the formation of the regeneration blastema.

Embryology Question # Q. 6. What do you understand by Normal Stages of Embryos Development?

Ans. The changing appearance of the embryos, especially during organogenesis, invites the distinguishing of certain stages which can be referred to when it is desired to indicate how far an embryo has progressed in its development. Tables of “normal stages” have been worked out for a number of species of animals, especially those that are most often used for research.

In the latter half of the nineteenth century, an ambitious project on establishing series of normal stages for a large number of animals was undertaken by Keibel and his collaborators.

This work was an essential contribution to the science of comparative embryology but went into oblivion later, when the interest of the great majority of embryologists shifted from a descriptive to an experimental approach to the develop­ment of animals.

However, it soon became evident that tables of normal stages were quite as important for experimental work, as it was often necessary to indicate precisely at what stage an operation or other experiment was carried out.

The table of normal stages of Ambystoma (= Amblystoma) punctatum, prepared by Harrison, was the first made expressly with experimental investigations in mind. Harrison’s table was published posthumously, though earlier it was made accessible privately to many workers in the field and has been widely used. Harrison’s stages were also redrawn and included in Rugh’s book, Experimental Embryology (1948).

Other tables of normal stages followed; the most widely used ones are probably the stages of Rana pipiens by Shumway (1940) and by Rugh (1948), the stages of the chick by Hamburger and Hamilton (1951) and the stages of Xenopus laevis by Nieuwkoop and Faber (1956). There is still no complete table of normal stages of the development of the human embryo.

In compiling a series of normal stages the embryologist is confronted with the task of deciding what characteristics should be selected for distinguishing one stage from another.

The characteristics that are often used are:

1. Age of the embryo.

2. Size of the embryo.

3. Morphological peculiarities of the embryo.

In human embryology particularly, the first two criteria are often used. It is custom­ary for an author to refer to embryos by age (five-week embryo, two-month-old embryo) or by size (an embryo or fetus of so many millimeters crown-rump length). Both these criteria are, however, not very convenient.

The age of an embryo is often not known, and in animals other than mammals, the rate of development is dependent on the temperature of the environment to such an extent that a statement about the age of the embryo is meaningless unless the temperature at which the development has proceeded is likewise indicated.

The size of the embryo is no true indication of its degree of development, as the dimensions of the embryo vary to a great extent. Moreover, some variability in the size of embryos may be resolved later in the course of develop­ment, which adds to the difficulty of using size as a criterion for the definition of stages.

What remains is to base the normal stages on morphological properties of the embryo and especially on properties that can be easily ascertained by external examina­tion of the embryo, without its fixation or dissection—that is, identification based largely on external features.

In the initial stages of development (cleavage stages), the number and size of the blastomeres may conveniently be used. During gastrulation, the shape of the blastopore or its equivalent (primitive streak) may be used, and just after gastrula­tion, the neural plate offers easily recognizable features.

During early organogenesis, the number of pairs of somites has often been used to define the stage of development of the embryo. The somites, though not strictly “external features,” can be seen on external inspection, especially in the amniotes. In still later stages, the development of the appendages presents easily distinguishable and convenient characters for the definition of normal stages.

Although morphological characters appear to be the best criteria for establishing the stage of development of an embryo, there are certain limitations even to this approach. It has been found that the development of different parts (organs) of the embryo is not always strictly coordinated in time; sometimes certain ones develop more rapidly, sometimes others.

So if two embryos have certain organs (e. g., the forelimbs) in exactly the same condition, they may at the same time differ in the degree of develop­ment of other organs (e.g., the nervous system or the liver). This phenomenon of heterochrony, or unequal rate of development of parts, must always be kept in mind when any tables of normal stages are being referred to.

Embryology Question # Q. 7. How to Determine Endodermal Organs?

Ans. Experimental investigation of the endoderm in early stages of development shows that the endodermal organ rudiments, like those derived from other germinal layers, are not initially determined, but that the endodermal cells destined to participate in the formation of the various organ rudiments are no more determined for their respective fates than are the cells of the other germinal layers.

In the earlier stages of development, their fate is a function of the position that each cell or group of cells occupies in the embryo as a whole. This can be proved by isolating parts of the presumptive endoderm and cultivating them apart from the rest of the embryo or by transplanting them into an abnormal position.

In an extensive series of experiments, pieces of presumptive endoderm of young gastrulae were cultivated in the “Holtfreter solution.” Various tissues were observed to differentiate from such isolated pieces; some conformed to the normal destiny of the isolated parts, and some did not.

The range of differentiations included not only orobranchial epithelium, stomach epithelium, liver, pancreas, and intestine, but also notochord and muscle, which should not have developed from the presumptive en­doderm if it had kept its prospective significance.

Thus, the fate of the endoderm is not established finally in the early gastrula stage. Much greater deviations from the prospective significance of the various endodermal parts could be observed when these parts were placed in surroundings which, unlike the saline solution, could actively influence the differentiation of these parts.

In the early neurula stage, it is possible to separate the entire endoderm of a newt embryo from the ectoderm and mesoderm. The endoderm is removed as a whole through a slit on the ventral side of the embryo, leaving the ectoderm and mesoderm as an empty shell.

The isolated endoderm can then be inserted again into the ectomesodermal shell of the same embryo or of another embryo of the same species, or even into the ectomesodermal shell of an embryo of another species. The endoderm of the small Triturus taeniatus has been successfully implanted into the ectomesodermal shell of the larger Triturus alpestris.

The implantation may be carried out so that the orientation of the endoderm is in harmony with the orientation of the ectomesoderm, or the endoderm may be implanted in an inverted position. In the first case, a completely normal larva has been observed to develop.

A normal embryo also developed if the endoderm was implanted with its dorsoventral orientation reversed. This result shows that the determination of the dorsal and ventral parts in the endoderm is not fixed in the endoderm itself but is imposed on the endoderm by the surrounding ectomesoderm.

Transplantation of the dorsal lip of the blastopore (primary organizer) the endoderm was often observed to develop a secondary lumen of the mid-gut, just underneath the notochord developed from the transplanted organizer. This secondary lumen was, of course, part of the dorsal differentiation of the endoderm.

However, if the anteroposterior axis of the endoderm was inverted with respect to the axis of the ectomesodermal shell, the development was highly abnormal, thus showing that the differentiation of the endoderm along the anteroposterior axis cannot be dominated by the ectomesoderm.

Small pieces of endoderm taken from a late gastrula or nearly neurula stage were implanted in various positions into another embryo. When the pieces of endoderm were taken from embryos in the gastrula stage, the grafts were often smoothly incorporated into the endoderm of the host.

The use of heteroplastic transplantation made it possible to distinguish the grafted cells from the host cells (by differences in cell size in grafts between Triturus taeniatus and Ambystoma mexicanum) and thus to make sure that the graft was not destroyed but had fitted into the construc­tion of local tissues. Thus, presumptive orobranchial endoderm was found to be able to develop into intestinal epithelium and vice versa.

Stomach epithelium was developed from endoderm having a different prospective significance. Occasionally, however, grafts differentiated out of harmony with their surroundings, and the later the stage of the embryo from which the graft was taken, the more often this occurred. After the end of neurulation the grafts differentiated, in the main, according to their prospective significance.

Similar results were obtained when different parts of the neurula endoderm were transplanted into parts of the ectomesodermal shell, either from the anterior half of the neurula or from the posterior half. The endoderm taken for this experiment was either part of the foregut endoderm, mainly destined to become pharynx, or endoderm from the mid-gut, normally differentiating as stomach and intestine. It was found that mid-gut endoderm grafted into the anterior ec­tomesoderm produced pharynx (in addition to other parts).

Foregut endoderm sur­rounded by posterior ectomesoderm was in part differentiated as intestine. In both cases, endoderm produced parts which were not in accord with the prospective significance of the endodermal cells, and it seems plausible that these differentiations were induced by the adjoining mesoderm. Again we find that the endoderm, as well as the ectoderm, is dependent on the mesoderm in its differentiation.

There is, as yet, very little information concerning the earliest determination of endodermal organs in higher vertebrates. In birds, some information has been derived from experiments in which parts of the chick blastoderm were grafted to the chorioallan­toic membrane of another chick embryo.

Various endodermal organs were observed to differentiate from the grafts, namely, pharyngeal epithelium, thyroid, lung, liver, and large and small intestine. However, these tissues developed without a very definite relationship to the origin of the grafts.

It has been concluded that in itself the endoderm has a very low power of differentiation. Liver and thyroid usually differentiate in explants which also show the presence of the heart, and intestine is accompanied by mesoderm forming coelomic spaces. This broadly corresponds to what has been found in the amphibian embryo.

The epiblast alone, without the hypoblast, when cultivated on the chorioallantois, produces various endodermal tissues, such as thyroid, liver, pancreas, and intestine, almost in the same way as a whole blastoderm consisting of both epiblast and hypoblast. This is in agreement with the origin of definitive endoderm from the epiblast.

Only when the endodermal gut becomes separated from the yolk sac during the second day of incubation does the differentiation of endodermal explants correspond to their prospective significance.

Earlier work on mammalian embryos, employing the method of explanation of parts of the blastoderm, did not yield very clear results. To date, there is no overall picture of the early determination of the endoderm as a whole in mammalian embryos. Some more recent experiments are concerned with the development of particular parts of the alimentary tract.

In mouse and rat embryos the determination of the pancreas was studied by the method of explanation in a culture medium. Normally the (dorsal) pancreas appears as an evagination on the ninth day of gestation when the gut is already closed into a tube and has become segregated from the yolk sac.

A piece of gut, including the presumptive material of the pancreas, if explanted on the eighth day, will differentiate, producing pancreatic tissue, provided that the adjoining mesodermal tissue is explanted together with the endoder­mal epithelium. Pieces of gut from younger embryos, in their seventh day of gestation, cannot develop pancreatic tissue, although they produce liver, lung, stomach, and intestinal tissues.

Presumptive pancreatic endoderm explanted without mesoderm fails to differ­entiate, however, even if taken from 11-day embryos in which the pancreas had already attained the stage of a pouch-like evagination. The mesoderm is thus necessary to promote the differentiation of pancreatic tissue.

Embryology Question # Q. 8. What do you mean by Non-Histone Proteins?

Ans. Non-histone proteins also seem to be a permanent com­ponent of the chromatin structure. It has been conjectured that non-histone proteins may be, in part, instrumental in the maintenance of higher-order coiling of the chromonema; that is, coiling additional to the coiling around the cores of the nucleosomes.

The non-histone proteins provide a sort of “scaffolding” for the coiled chromonema. At meiosis the conjugated chromosomes of an allelic pair can be seen in the electron microscope joined to a “synaptonemic fiber” lying between the two chromosomes.

Non-histone proteins associated with the chromosomes show an immensely greater variety than the histone proteins. Whereas there are only five kinds of histones, the number of different non-histone proteins in the chromatin of the same tissue may run into hundreds. It is therefore much more likely that non-histone proteins may play a significant part in gene action regulation.

A special component of the non-histone proteins in the chromatin are contractile proteins. There are at least ten different non-histone proteins which seem to be similar if not identical to the contractile proteins, myosin and actin. These proteins comprise up to 50 per cent of the non-histone proteins in the chromatin. It is possible, but not actually proven, that these contractile proteins may play a part in the movement of the chromosomes during mitosis.

In addition to the histones and non-histone proteins, some RNA is also found to be associated with the DNA in the chromosomes of eukaryotes. Some of this may be only temporarily associated with the DNA, such as the RNA in the process of transcription, but there is a component consisting of short molecules of RNA 40 to 80 nucleotides long, which appears to be a permanent part of the chromatin. The function of this RNA and the way in which it is bound to the other components of the chromatin is not known.

Embryology Question # Q. 9. What is Mitochondrial DNA and RNA?

Ans. In addition to the genes which are contained in the chromosomes of the nucleus, eukaryotes possess DNA molecules outside the nucleus, in the mitochondria. Each mitochondrion contains a small “chromosome,” which is active in being transcribed into messenger RNA and controls the synthesis of proteins.

In the mitochondrial chromosome the DNA is not associated with histones, and the chromosome is thus a naked DNA molecule. In this respect it resembles the chromosomes of prokaryotes (bacteria and viruses). Also, like the bacterial chromosome, the mitochondrial chromosome is closed into a ring.

The amount of DNA in a mitochondrial chromosome is infinitesimally smaller than the amount of DNA in the nucleus; it is less than the amount contained in the smaller viruses (molecular weight about 11 million). The number of nucleotide pairs is sufficient for 10-25 average-sized genes.

The chemical mechanisms in the mitochondrial chromosome are also different from those in nuclear chromosomes. The antibiotic rifampicin prevents transcription in mitochondria and in bacterial cells, but does not affect transcription in eukaryotic nuclear chromosomes. On the other hand actinomycin, which inhibits transcription in eukaryotic nuclear DNA, does not inhibit mitochondrial transcription.

The similarities between the mitochondrial chromosomes and those of bacteria and the differences from eukaryotic nuclear chromosomes are so striking that the suggestion has been made that mitochondria were originally independent bacteria-like organisms, which became endosymbionts of other cells—the cells giving rise to eukaryotes. According to this theory, in the course of further evolution the endosymbionts would have lost much of their independence.

At present, the synthesiz­ing activities of the mitochondria are very limited. The production of the mitochondrial ribosomes is definitely the result of action of the mitochondrial genes; in addition, these genes control apparently only some proteins involved in the structure of the mitochon­drion. The rich systems of enzymes contained in the mitochondria, in particular the system of oxidative enzymes, are produced by nuclear chromosomal genes.

In proportion to the small size of the mitochondrial chromosomes, the amount of RNA synthesized in the mitochondria is very minute compared to the RNA synthesized in the nucleus (in spite of the large number of mitochondria per cell).

Only during early cleavage, when the nuclear genome has not yet been multiplied by repeated mitosis, the amount of RNA synthesis in the mitochon­dria may be relatively quite considerable – in sea urchin embryos in early cleavage up to 50 per cent of all newly synthesized RNA is of mitochondrial origin.