Germ Cells: Meaning, Development and Migration | Embryology

In this article we will discuss about:- 1. Meaning of Germ Cells 2. Development of Germ Cells 3. Migration 4. Fate 5. Transformation 6. Spermatogenesis 7. Oogenesis.

Contents:

1. Meaning of Germ Cells
2. Development of Germ Cells
3. Migration of Germ Cells
4. Fate of Germ Cells
5. Transformation of Germ Cells
6. Spermatogenesis of Germ Cells
7. Oogenesis of Germ Cells

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1. Meaning of Germ Cells:

Sexual reproduction needs one sperm cell from male and an egg cell from female. These specialised cells are produced from the germ cells. It is believed that these germ cells are set aside from the very beginning and according to Weismann, the germinal material passes without interruption from one generation to the next.

The inquest for the search of origin, migration, trans­formation and fate of the germ cells, is old one, but in recent years many new lights have been thrown by different workers, who have used modern tools and tech­niques to the search.

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2. Development of Germ Cells:

Deve­lopment begins with the union of sperm and egg. These reproductive cells originate from the germ cells. The available infor­mation shows that series of interesting events happen in the life of germ cells from their origin to transformation. These events are the preparatory phases on which the development of individual depends. These developmental events begin long before the development of the individual.

Origin of germ cell is distinct in orga­nisms where chromosome number differs in germ and somatic cells. In the round­worm, Parascaris equorum, the four blastomeres produced by the second cleavage exhibit differences in nuclear behaviour. The blastomeres are designated as A, B, C and P. A large portion of each chromo­some in blastomeres A, B and G passes into cytoplasm and breaks up in the Sub­stance of cytoplasm.

This phenomenon is regarded as chromatin diminution. The blastomere P remains unaltered. At the next cleavage division, one of the daughter blastomeres of P shows chromatin diminu­tion, while the other remains unchanged. The same phenomenon occurs at the fourth cleavage division, as a result only one blastomere, P₄, retains the full num­ber of chromosomes.

All the germ cells eventually develop from P₄. In recent years, it has been described that in differ­ent animals the factor which guides the cell to be germ cell resides in cytoplasm. This area is called the “area of germ cell deter­minant”. Any nucleus passing through this area develops, the potency to be the nucleus of germ cell.

Followings are the brief survey of literature on the origin of germ cells:

Waldeyer (1870) regarded that the germ cells originate from the coelomic epithel­ium around the gonad. Nussbaum (1880) advocated that the germ cells are formed outside the gonad and from the site of origin, the germ cells migrate within the gonad. Weismann (1885, 1892) established that germplasm is segregated completely independent of somatoplasm and results into the differentiation of germ cells.

Boveri (1892) stated that cytoplasm of germ cell plays important part in deter­mining the germplasm. Witschi (1914), Gatenby (1916) stated that in frog, a cytoplasmic substance in the yolk near the vegetal pole and on the roof of the gut is marked as site of germ cell formation.

Swift (1914, 1916) stated that in chick, germ cells originate extra-embryonically and are first seen in 16-somite stage. Mintz (1959) established that germ cells originate in the yolk sac of 8-day-old mouse embryo.

The above findings indicate clearly that germ cells are segregated at the very onset of development of the individual and germ cells originate at a site far from the reproductive organ. This finding has raised the most interesting question about the migration of germ cells.

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3. Migration of Germ Cells:

Three dis­tinct views are held regarding the migra­tion of germ cells.

These views are:

(a) The germ cells migrate by performing amoeboid movement to the region of deve­loping gonad.

(b) Germ cells, from their site of origin, are carried by the blood stream and after travelling through diff­erent organs settle in the reproductive organ.

(c) Various foldings which occur in embryonic development bring about the shifting of germ cells from their site of origin to the gonad.

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4. Fate of Germ Cells:

Two different opinions are forwarded to explain the fate of germ cells:

(a) The germ cells which migrate into the reproductive organs are not destined to be the gametes at all; they degenerate and are replaced by new cells which are formed by the reproductive organs themselves.

(b) The germ cells after migrating within gonad persist and transform into functional gametes. Recent experiments with improved techniques en­dorse the second view.

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5. Transformation of Germ Cells:

Wi­thin the gonad, the primordial germ cells are known as primary gametogonia which multiply rapidly by mitosis. This state is called the phase of multiplication and the resultant cells are then known as secondary gametogonia. The gametogonia enter into the phase of growth which is more pro­nounced in oogenesis.

In male the game­togonia produce sperm cells and are called spermatogonia, while in female, gameto­gonia giving rise to ova or egg cells are known as oogonia.

Sperm and ovum are responsible for

(a) Bringing together of hereditary factors in the new individual from the parents and

(b) To provide material substance from which the new individual will arise.

The gametogonia in both the sexes transform to fulfil these two purposes. The development of male gametogonium or spermatogonium to sperm is called spermatogenesis and change of female oogonium to ovum is known as oogenesis.

The gametogonia then transform into the specific gametes and in both the cases, the transformation involves:

(a) Reduction of diploid to haploid number of chromo­somes and

(b) Considerable preparation in different components of the cell.

It may be mentioned here that germ cells carry the potentiality of becoming both sperm and egg. It is the influence of the cells of the testis or ovary, which determines the final fate of it. Spermatogenesis and oogenesis are discussed below (Fig. 5.2).

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6. Spermatogenesis of Germ Cells:

With the differen­tiation of testes, the primordial germ cells give rise to spermatogonia. Further multiplication results in transformation of gonial cells into primary spermatocytes. The primary spermatocytes undergo the first meiotic division to produce secondary spermatocytes which give spermatids by the second meiotic division.

Without fur­ther division, each spermatid differen­tiates into a spermatozoon—the functional male gamete (Fig. 5.3). As a result of meiotic cell-division four spermatids are produced from one primary spermatocyte.

Spermateleosis:

The spermatids possess haploid set of chromosomes but are unable to function as male gametes. A spermatid transforms into a spermatozoon which involves a series of cellular changes. This process of differentiation is called spermateleosis. The events in spermateleosis are illustrated diagrammatically in Fig. 5.4.

The acrosome is a derivative of Golgi bodies. Numerous small vesicles appear within Golgi apparatus, each vesicle con­tains a homogeneous granule. These vesi­cles coalesce to form a single large vesicle containing numerous granules.

This large vesicle ultimately comes to attach with the surface of nucleus. It is known as acrosome vesicle and the granules are called acrosome granules. These granules fuse to form a large acrosomal granule.

The nucleus with acrosome vesicle moves towards the peripheral end of the cell. The vesicle spreads over the nucleus, thus the vesicle becomes entirely occupied by the granules. The centrosome consists of two centrioles after second meiotic divi­sion.

At the time of formation of acro­some vesicle, the centrioles move towards the plasmalemma and one of the centrioles attaches itself with the membrane. From the point of attachment a fine thread grows out and extends from the cell as a small flagellum, this is the axial filament of the future tail.

The centrioles then return to the nucleus and one of them fixes itself to a notch on the nucleus at the opposite pole of acrosome vesicle. During inward journey, the centriole which is in contact with the mem­brane draws the plasmalemma around the flagellum. Thus the flagellum, which appa­rently seems to be inside the cell, actually remains outside the cell and is bounded by double plasma membrane.

The proximal centriole continues to remain attached with nucleus, but the distal centriole comes down again to bring the fold of plasma mem­brane back to the general surface. Here the distal centriole forms a ring and is known as ring centriole. Mitochondrial bodies then associate around the axial filament between proximal and ring centrioles.

The axial filament extends outside the cell as a long flagellum and fine fibre’s originating from proximal centriole encircle the axial filament, thus establish the structure of the tail of sperm.

The nucleus of a spermatid during its transformation into a spermatozoon loses water and becomes elon­gated to keep the acrosome vesicle in close contact with the plasma-lemma. Only a small portion of cytoplasm is retained around head, middle piece and tail while the major part of cytoplasm is discarded.

This transformation from spermatids to spermatozoa, though the structure of sperm varies in different animals, is found to be closely similar.

Structure of Spermatozoa:

Sperm cells may be amoeboid (e.g., crayfish), or flagellated (e.g., vertebrates).

Vertebrate sperm cells may be of different shapes (Fig. 5.5):

(a) Spheroidal—in teleostean fishes,

(b) Rod-shaped—in amphibians,

(c) Spirally twisted—in passerine birds,

(d) Spoon-shaped—in man,

(e) Hooked— in mouse, etc.

But in all cases, the sperm is built on same structural plan. The description of sperm cell given below is based on the structure of the sperm of rabbit (Oryctolagus cuniculus) which has been extensively studied by both light and electron microscopes. A spermatozoon of rabbit measures about 60-70 micra long with a head measuring about 8—10 micra.

A typical sperm consists of three distinct parts—head, neck or middle piece and tail or flagellum (Fig. 5.6).

Head:

It is the forward end of sperm and it contains:

(a) Acrosome,

(b) Perfora­torium and

(c) Nucleus.

Acrosome. This is a double-walled sac containing dense granules. It is present at the tip of the nucleus and attached intimately with it. Anteriorly it is convex and flattened posteriorly. It contains enzymes for breaking egg membrane during fertilization. Per-foratorium.

This structure is not rabbit and other mammals. But invertebrate sperm (e.g., Sea-urchin) contains perforatorium between acrosome and nu­cleus. It forms acrosome filaments and plays important role in fertilization.

Nucleus:

It forms the major part of the head and has a groove at its posterior end. It contains densely packed material within its nuclear membrane. The nucleus contains DNA and a protein called histone.

Neck or Middle Piece:

The mid-piece varies greatly in structure in different animals. This piece extends from the proximal centriole near the nucleus to the distal or ring centriole.

It is formed by a firmly coiled spiral of elongated mitochondria around the axial filament and nine fibres from proximal centriole. Due to the presence of mitochondria, it is thought that middle piece is a ‘power plant’ which provides energy during the locomotion of sperm.

Tail The sperm tail is a specialised portion of a spermatozoon which helps in movement. In the spermatozoon of sea- urchin, the tail is made up of fibres which are arranged in a circle of nine with two more extending down the middle. This pattern resembles that of cilia and flagella.

In the spermatozoon of rabbit such an array of tail fibres is present which is designated as the axial filament. The fibres in the axial filament are actually the con­tractile units responsible for the move­ment of sperm tail. The sperm tail of rabbit has in addition another outer ring of nine more fibres.

The fibres of the outer ring are much thicker than the inner fibres. The coarse outer fibres are not present up to the posterior tip, but end one after another at various regions down the tail. But the axial filament remains up to the tip. The axial filament and outer fibres are formed from the proximal centriole which remains attached with the posterior indention of the nucleus.

The pattern of arrangement of fibres (9+9+2) is best developed in mamma­lian spermatozoa. The said pattern is encountered in other forms, viz. Honeybee (Apis mellifera), Fruitfly (Drosophila melano- gaster), Grasshopper (Gelastorrhinus bicolor), Snail (Helix pomatia), Sparrow (Passer montanus saturatus), Snake (Lampropeltis getulus) and many others.

Characteristics of Sperm Cell:

A sperm cell has the following characteristics:

a. It has a highly specialised structural organisation.

b. The major part is the nucleus and cytoplasm has modified to form special locomotory devices.

c. It is devoid of stored food and protec­tive envelope.

d. Size is much smaller than egg, but is produced in greater number than the egg.

e. Its function is to seek the egg and to stimulate it to develop.

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7. Oogenesis of Germ Cells:

The egg or ovum serves two functions—it carries the maternal chromo­somes and at the same time it provides the ground substance on which the future embryo develops.

To be prepared for these double purposes, the egg during its deve­lopment undergoes complicated modifica­tions which result—

(a) Production of haploid nucleus,

(b) Acquisition of food reserves and

(c) Preliminary organisation of the cytoplasm.

The entire process is called oogenesis. Before entering into the details of the process it must be remem­bered that the process of oogenesis begins at a stage when the individual itself is in embryonic condition and ends either short­ly before or immediately after fertilization.

Stages of Oogenesis:

The process of oogenesis has been worked out in different organisms and the results show close simi­larity in the process. In the present dis­cussion the oogenesis of amphibians will be followed.

The gametogonium (here known as oogo­nium) which is destined to form egg is called oocyte. The nucleus of the oocyte swells up to form a vesicle which is called germinal vesicle. It is filled up with a fluid called nuclear sap, which contains sulphydryl protein. Meiosis begins when the indivi­dual is in embryonic state, but till the pro­phase stage of first meiotic division it re­mains suspended.

During the suspended period, considerable amount of prepara­tory work happens within the oocyte. In the nucleus, chromosomes become dispiralised and appear as long and thin threads. Each chromosome contains at first single filament but later it becomes doubled. These filaments at short intervals contain paired thick, dense swellings called chromomeres.

The chromomeres roughly corres­pond with the expected number of genes. From each chromomere a slender loop is projected outward to unite with the chro­momere of other filament. The loop is en­closed by a matrix. The shape and size of the loop and also the matrix vary in diff­erent chromosomes.

These loops are known as puffs and the chromosomes containing puffs are called lamp-brush chromosomes. Callan and Gall have demonstrated that puffs are constant for each pair of chromo- mere and are always found in the same position on the chromosome. The filament part of the puff is made up of DNA whereas the matrix contains RNA and protein.

At the cytoplasm, two kinds of events occur. Establishment of physico-chemical diff­erences occurs in the various parts of the egg cell. This is done by aggregation of different substances within the cytoplasm in graded fashion and mapping out of the different fields of development of the future embryo.

Considerable amount of synthesis continues within the egg for making the reserves of food materials. The most important reserve is the yolk. This inert substance is composed of a phosphoprotein (called vitellin) and lipids (different fats).

Types of Egg:

According to the quantity of yolk, the egg may be:

(a) Alecithal:

Eggs having no yolk, e.g., mammals.

(b) Microlecithal:

Relatively yolk-free eggs are called the microlecithal types. Eggs of coelenterates are of microlecithal types.

(c) Megalecithal:

Large yolky eggs are called the megalecithal types. Examples: Eggs of reptiles, birds and monotremes.

(d) Telolecithal:

Eggs are large in size and contain much yolk like megalecithal egg but the yolk pushes the cytoplasm at one end. Examples: Chick.

Factors Controlling the Cytoplasmic Events:

It is believed that the accumula­tion of different substances which happens within the egg is controlled by external and internal influences.

External Influences:

(a) Immunological studies have shown that there exist similarities between blood pro­tein and the cytoplasmic content of the egg (including yolk).

(b) In the rapid growth phase, vitellin (which is charac­teristic of yolk) increases rapidly in the blood of the individual. It indicates that external factors are responsible for provid­ing nutritive materials to the egg.

(c) Cells investing the developing oocyte transfer- complex molecules to the developing egg.

These substances are either synthesized by them or transferred from the general cir­culation. Internal influences. Simple mole­cules enter within the egg and these are converted into more complex substances by the nucleus and cytoplasm.

Callan and Gall have shown that RNA produced at the lateral loop of lamp-brush chromosome comes first to remain free in the nuclear sap and later passes through porous nu­clear membrane into the cytoplasm. Con­siderable amount of protein is synthesized under the instruction of this RNA.

Regarding the events of growth during oogenesis, it may be said that the entire event is divisible into a slow phase of growth and a rapid phase of growth. The slow phase continues for a longer period during which synthesis of different substances (including yolk) continues within the egg.

Rapid phase begins after the attainment of sexual maturity, when rapid accumulation of different substances occurs. It has been shown that during this phase many sub­stances are contributed to the egg from general circulation. In the oocyte of almost all the vertebrates, the microvilli and pinocytotic vesicles are concerned mainly with the transportation of sub­stances into the oocyte from the follicle cells (Fig. 5.7).

The differentiation of cortical granules (spherical bodies surrounded by a membrane) is also a significant event in egg development. The cortical granules are composed of acid mucopolysaccharides and are arranged in a layer adjacent to the plasma membrane. The cortical granules play vital role in fertilization.

Theories Regarding the Organisa­tion of Ovum:

During organisation of egg, different substances get oriented in specific pattern, which plays important role in establishing the spatial arrangement of the developing embryo. Three different theo­ries are forwarded to explain this estab­lishment of determination.

(a) Quantitative or Gradient Theory:

According to this view, one end of the egg possesses highest metabolic activity and controls the activity of other regions. This activity gradually reduces at the other end.

(b) Qualitative Theory:

According to this concept, there are actually different sub­stances in the egg which control the for­mation of different parts of the embryo.

(c) Two-Factor Theory:

Quantitative gra­dient exists within the egg in the beginning but after some time different parts of the egg become qualitatively different. Thus this view is a compromise between the first two theories.

In amphibians, when egg is fully matured the first meiotic division ends—this results into the formation of two haploid cells. But contrary to sperm cell formation the cytoplasmic division is unequal, i. e., a large cell called secondary oocyte and a very small cell called first polocyte are produced. The first polocyte remains attached with the outer surface of the oocyte. At this stage the egg is liberated from the body.

After the entry of sperm cells, second division of meiosis ends. At this time the secondary oocyte divides unequally producing one large cell called egg and a small cell called second polocyte. The first polocyte divides equally.

To sum up the result, it may be said that during oogenesis, single oocyte produces one large cell called egg (or ovum) and three small cells called polocytes. The egg is the functional reproductive cell while the polocytes are abortive in nature.

Structure of Ovum or Egg:

The size of egg varies greatly in different animals. The size is chiefly attributable to the quantity of yolk present. The eggs of reptiles, birds and monotremes have, in addition, a coating of albumen. The albu­men subserves both protective and nutri­tive functions. The egg is surrounded by a plasma membrane which encloses cyto­plasmic part called vitellus.

Within the vitellus, most prominent is nucleus which is 200-300 times larger than an ordinary cell nucleus. Nucleus contains a single large nucleolus rich in RNA and en­veloping chromatin aggregates. Immedi­ately beneath the plasma membrane, there is a thin layer of cytoplasm called cortex.

In many eggs, the cortex contains special granules, called cortical granules which play important role in fertilization. The sub­stance which constitutes the cortex is principally protein which has much in­fluence on the future development. The cytoplasm contains various organelles: mitochondria, Golgi apparatus and reserve food material in the form of yolk.

In some forms (e.g., Sea-urchin), the maturation of egg begins after the release from ovary and in others (e.g., Rabbit) maturation begins within ovary. Ovulation (i.e., release of ovum from the ovary) takes place at the metaphase stage of second meiotic division and completion of meiosis occurs immed­iately with the entry of sperm cell. The egg or ovum contains one or more mem­branes which differ in different animals.

These egg membranes may be:

Primary Membrane:

It is formed by the ovum itself. It includes

(a) vitelline mem­brane. It is a constant thin membrane which remains closely apposed with the plasma membrane before fertilization.

(b) Chorion. In Stylea, a stiff chorion is found to be origi­nated from the ovum itself.

(c) Zona radiata. Generally a striated and less constant layer which—

(i) In shark, bony fishes, amphi­bians and reptiles are present as a layer between vitelline membrane and plasma membrane,

(ii) In birds it is difficult to differentiate.

(iii) It is unstriated in mammals here it is known as zona pellucida and it is debatable whether it is primary or secondary in nature.

Secondary Membrane:

It is formed by the cells of the ovary which remain imme­diately around the ovum. Examples: (i) chitinous shells of different eggs, (ii) mem­branes which are present immediately outside the frog’s egg, (iii) cells of the graafian follicle in mammals. The secon­dary membranes are not seen in urodeles, reptiles and birds.

Tertiary Membrane:

These are formed by oviducts and-special glands associated with the oviducts.

These are always form­ed after the discharge of egg from the ovary and are of following kinds:

(i) Albumen or Horny capsule of the elasmobranch eggs. Albumen in birds and shell-membrane and calcareous shell of reptilian and avian eggs. It is to be noted that reptilian eggs contain little, albumi­nous covering except in snake,

(ii) Tertiary membranes are absent in mammals excep­ting prototheria where leathery shell is present.

Significance of Egg Membranes:

(a) Primary membranes. Vitelline mem­brane plays important role in develop­ment. After the event of fertilization it prevents the entrance of more sperm cells and at the same time helps the egg to assume bilateral symmetry from a state of radial symmetry in unfertilized state.

(b) Secondary membranes are chiefly protec­tive and are also responsible for processing nutritive materials from the surrounding area.

(c) Tertiary membranes meet one of the two requirements,

(i) Provision of organic food and

(ii) adequate water supply. Eutherian and metatherian mam­mals have solved both, the problems by the attainment of complete or incomplete viviparity. In other vertebrates, the first requirement is met by the storage of yolk or some other devices, but the second need gives a serious threat to the life of ovi­parous group.

Amphibians, to avoid the threat of drought, return either to water for egg laying or developed certain devices, viz.,

(i) Eggs deposited in a foam,

(ii) For­mation of oviducal jelly and

(iii) Keeping the eggs in urinary bladder.

Reptiles, birds and monotremes have developed water­proof shell for the purpose.

The transformation of the primordial germ cells into gametes occurs in three distinct steps—phase of multiplication, phase of growth and phase of maturation. The phase of multiplication is closely similar in both spermatogenesis and oo­genesis, but the other two phases are quite different.