Development of the lancelet embryo. Type Chordates: structure and development of the lancelet Development of the lancelet drawing

Lancelets are small (up to 5 cm long), rather primitively arranged non-cranial animals of the chordate type, living in warm seas (including the Black Sea), passing through the larval stage in development, capable of independently existing in the external environment.

The first complete description of their development was presented by A.O. Kovalevsky. It is a classic example of initial forms, which are used as basic models for studying the features of embryogenesis in representatives of other classes of chordates.

The conditions and nature of the development of the lancelet do not require a significant accumulation of a reserve of nutrient material, therefore their eggs are of the oligolecital type. Fertilization is external.

Cleavage of the zygote is complete, uniform and synchronous. With each round of division of the zygote, an even number of approximately equal in size blastomeres (blastula particles) is formed, the number of which increases exponentially.

The first division furrow runs in the sagittal plane meridian. It forms the left and right halves of the embryo. The second furrow, also meridian, runs perpendicular to the first (frontal plane) and marks the future dorsal and abdominal parts of the body. The third furrow is latitudinal. Divides blastomeres into anterior and posterior, providing segmentation of the future trunk.

In further periods of development, the meridian and latitudinal cleavage furrows replace each other in a strictly regular sequence. The blastomeres formed as a result of such crushing become progressively smaller in size. The progressive increase in their number leads to the fact that blastomeres displace each other outward, due to which space is released in the central part of the embryo, and the dividing cells themselves form a single-layer wall - blastoderm. Thus, a spherical blastula appears with a cavity enclosed inside - blastocoele. This type of blastula is called coeloblastula(caelum - vault of heaven).

In the whole blastula, it is customary to distinguish roof(animal pole of the egg), bottom(vegetative pole of the egg) and edge zones. The bottom blastomeres are characterized by some increase in size due to the natural displacement of the yolk to the base of the vegetative pole of the oocyte.

The presence of a large blastocoel and a single-layer blastoderm predetermines the simplest way of gastrulation in the lancelet embryo - the invagination of the bottom blastomeres towards the roof ( intussusception). Tightly adjacent to the dorso-lateral parts of the blastula, the invaginating blastomeres displace blastocoel, forming the inner germ layer of the endoderm and a new cavity of the embryo - gastrocoel, which through the primary oral opening ( blastopore) communicates with the environment.

The blastomeres of the roof and lateral zones make up the outer germ layer.

The resulting two-layer embryo (gastrula) feeds on its own due to the ingress of water enriched with plankton into the gastrocoel.

At the next stage of development, a strand of intensively dividing cells differentiates from the median dorsal ectoderm, which separates from the cells of other zones of the outer germ layer, descends somewhat downward and becomes the neural plate, which subsequently forms the first axial organ of the lancelet larva - neural tube. The remaining part of the ectoderm, being the outermost layer of the body, turns into the integumentary epithelium of the skin - epidermis.

The rest of the axial organs and mesoderm develop by differentiation of various parts of the inner germ layer.

So, from the most dorsal middle part of it (as in the case of isolation of the neural plate), the notochordal plate stands out, which then twists into a dense cell cord - chord(the second axial organ of the larva), which in lancelets remains as the main supporting organ - the dorsal string.

On both sides of the chordal plate, in the dorso-lateral sections of the endoderm, paired rudiments of the third germ layer are differentiated - mesoderm, which ensures the bilateral symmetry of the body, the metamerism of its structure (segmentation) and the development of many organs and tissues.

The ventral part of the endoderm serves as the basis for the formation of the third axial organ - primary colon. The cells of the rudiments of the mesoderm are characterized by the strongest division energy, the most intensive increase in their number, due to which the growing ribbon-like plates are forced to protrude towards the ectoderm and form folds. Resting with the tops of the folds against the dorsal ectoderm, with the inner edges against the chordal plate, and with the outer edges against the remaining ventral part of the endoderm, each mesodermal rudiment wraps down with further growth, is introduced between the outer and inner germ layers, helping the chordal plate to close into a string, the neural groove to become a tube, and the ventral endoderm form the primary gut.

In turn, in each rudiment of the mesoderm, their basal edges also close, as a result of which these rudiments take the form of closed sac-like formations with a cavity inside. One of the leaves is adjacent to the ectoderm (the outer wall of the body of the larva) and therefore receives the name parietal(wall), the other - to the primary internal organ (intestine), which gives reason to call it visceral. With subsequent development, both rudiments of the mesoderm ventrally, below the primary intestine, grow together. As a result, a single secondary body cavity appears in the body of the lancelet - in general, concluded between the parietal and visceral sheets of its mesoderm.

Ontogeny, or individual development, is called the entire period of an individual's life from the moment the spermatozoa merge with the egg and the formation of a zygote until the death of the organism. Ontogeny is divided into two periods: 1) embryonic - from the formation of a zygote to birth or exit from the egg membranes; 2) postembryonic - from the exit from the egg membranes or birth to the death of the organism.

In most multicellular animals, the stages of embryonic development that the embryo goes through are the same. In the embryonic period, three main stages are distinguished: crushing, gastrulation and primary organogenesis.

The development of an organism begins with a unicellular stage. As a result of repeated divisions, a unicellular organism turns into a multicellular one. The resulting cells are called blastomeres. When dividing blastomeres, their size does not increase, so the division process is called crushing. During the period of crushing, cellular material accumulates for further development.

As the number of cells increases, their division becomes non-simultaneous. Blastomeres move farther and farther away from the center of the embryo, forming a cavity - the blastocoel. The cleavage is completed with the formation of a single-layer multicellular embryo - the blastula.

A feature of crushing is an extremely short mitotic cycle of blastomeres compared to the cells of an adult organism. During a very short interphase, only DNA duplication occurs.

The blastula, usually consisting of a large number of blastomeres (in the lancelet - from 3000 cells), in the process of development passes into a new stage, which is called the gastrula. The embryo at this stage consists of separated layers of cells, the so-called germ layers: the outer, or ectoderm, and the inner, or endoderm. The set of processes leading to the formation of a gastrula is called gastrulation. In the lancelet, gastrulation is carried out by pushing a part of the blastula wall into the primary body cavity.

After completion of gastrulation, a complex of axial organs is formed in the embryo: neural tube, notochord, intestinal tube. The ectoderm bends, turning into a groove, and the endoderm, located to the right and left of it, begins to grow on its edges. The groove plunges under the endoderm, and its edges close. The neural tube is formed. The rest of the ectoderm is the rudiment of the skin epithelium. At this stage, the embryo is called a neurula.

The dorsal part of the endoderm, located directly under the nerve bud, separates from the rest of the endoderm and folds into a dense cord - a chord. From the rest of the endoderm, the mesoderm and intestinal epithelium develop. Further differentiation of germ cells leads to the emergence of numerous derivative germ layers - organs and tissues.

From ectoderm the nervous system, the epidermis of the skin and its derivatives, the epithelium lining the internal organs develops. From endoderm develop epithelial tissues lining the esophagus, stomach, intestines, respiratory tract, liver, pancreas, epithelium of the gallbladder and bladder, urethra, thyroid and parathyroid glands.

Derivatives mesoderm are: the dermis, the entire connective tissue itself, the bones of the skeleton, cartilage, the circulatory and lymphatic systems, the dentin of the teeth, the kidneys, the gonads, the muscles.

The animal embryo develops as a single organism in which all cells, tissues and organs are in close interaction. At the same time, one germ influences the other, to a large extent determining the path of its development. In addition, the rate of growth and development of the embryo is influenced by external and internal conditions.

The ovule of the lancelet is oligolecithal with an isolecithal distribution of the yolk. The cleavage is complete, uniform, synchronous (the cleavage furrows are meridional or latitudinal); after the 7th cleavage (128 blastomeres), the cleavage ceases to be synchronous). When the number of cells reaches 1000, the embryo becomes a blastula (i.e., a single-layer embryo with a cavity (blastocoel) filled with a gelatinous mass, the blastula wall is called a blastoderm. The type of lancelet blastula is a uniform coeloblastula, it has a bottom, a roof.

Then gastrulation begins by invagination, i.e. invaginations on the blastula inside.

In total, there are 4 ways of gastrulation, they are usually combined, but one of them prevails:

1. Intussusception (invagination)

2. Immigration (movement of cells with their immersion inside the bastula)

3. Epiboly (fouling)

4. Delamination (splitting of the blastula wall into 2 sheets).

As a result of gastrulation, a 2-layer embryo is formed - the gastrula, it has a cavity (in fact, this is the cavity of the primary intestine) and a hole leading to the cavity (blastopore - the primary mouth). The blastopore is surrounded by 4 lips (the organizers of organogenesis). The material passing through the dorsal lip becomes the notochord, and through the rest of the lips it becomes the mesoderm.

After the end of gastrulation, the embryo begins to grow rapidly in length. The dorsal part of the outer leaf flattens and turns into the neural plate. The rest of this plate is laterally overgrown with the neural plate and becomes the outer lining of the skin, thus. the neural plate is inside, a groove first forms in it, then it twists into a tube with a central channel inside (neurocoel).

The inner sheet is split into several parts. The dorsal region is flattened. It is wrapped in a cylindrical cord, separated from the intestine and turns into a chord. The areas adjacent to the notochord are also separated and form 2 sacs (mesoderm, in fact), thus. the mesoderm is formed by the enterocele route, i.e. lacing bags.

Parts of the mesoderm:

Segmental pedicles (nephrogonadotomes)

Splanchnot (a whole is formed between its leaves).

After the chord, neural tube and mesoderm have formed, the endoderm folds and forms the 3rd axial organ - the primary intestine (all other sections of the gastrointestinal tract with glands subsequently develop from it).

Embryogenesis of amphibians.

The amphibian ovum is mesolecithal with a telolecithal yolk distribution. Crushing is complete, uneven, asynchronous. After the 3rd division, 4 small cells (micromeres) are obtained, concentrated at the animal pole and 4 large cells (macromeres) are concentrated at the vegetative pole. In addition to the latitudinal and meridional division furrows, tangential furrows also appear (they run parallel to the surface of the egg and divide the blastomeres in a parallel plane), thus. the wall of the blastula is multilayered. Blastomeres begin to divide asynchronously, and the cells of the bottom of the blastula (filled with yolk) are much larger than the cells of the roof of the blastula. The blastoderm consists of several layers, and the blastocoel (compared to the lancelet) is significantly reduced. Thus, an uneven multi-layered coeloblastula (amphiblastula) arises.

Gastrulation occurs by partial invagination and epiboly (see question 21). More active cells from the animal half “crawl” onto the vegetative half of the embryo, and subsequently penetrate into the embryo. During gastrulation, the cells multiply weakly, mainly stretching and displacement of the cells of the surface layer occurs. After the end of gastrulation, the embryo begins to actively grow.

The mesoderm material is laid down during gastrulation, and the mesoderm immediately detaches from the primary intestine, moves forward and ventrally, and spreads out between the ecto- and endoderm (this method of laying the mesoderm is called proliferative). At first, there are no cavities in the mesoderm, it consists of 2 bags, then somites separate from it; the dorsal part of the mesoderm turns into myotomes, the ventral part - into splanchnotomes. First, myotomes are connected with splanchnotomes by segmental legs (give rise to excretory organs), then they diverge.

The muscles of the body develop from the myotomes (from the medial side of the myotome), the sclerotome (the skeletal sheet develops from it - the rudiment of the mesenchyme, from which, accordingly, all supporting-trophic tissues develop), the dermatome (the lateral part of the myotome) - becomes the mesenchyme, from which the deep layers develop skin.

Notochord, neural tube, primary gut develops approximately like a lancelet (passing through the dorsal lip).

Embryogenesis of birds.

The ovum is polylecital with a telolecithal distribution of the yolk. Cleavage - partial (meroblastic), discoidal. The first 2 furrows of division are meridional, then radial and tangential furrows appear. As a result, a discoblastula (blastodisk) is formed - one layer of cells lying on the yolk, where the cells are adjacent to the yolk - a dark field, where they are not adjacent - a light field. In a freshly laid egg, the embryo is at the stage of discoblastula or early gastrula.

Gastrulation occurs by immigration and delamination, i.e. a number of cells simply leave the outer layer (immigration), and some of the blastomeres begin to divide in such a way that the mother cell remains on the outer wall, and the daughter cell passes into the second layer (delamination).

After 12 hours of incubation, due to cell migration along the edges of the light field, a primary streak and Hensen's knot are formed, which act as blastopore lips.

In the early gastrula, 2 leaves are distinguished: the epiblast - the outer leaf and the hypoblast - the inner leaf.

The epiblast contains presumptive rudiments of the skin ectoderm, neural plate, notochord cells, and mesoderm cells; in the hypoblast there are only rudiments of the endoderm.

Notochord cells are invaginated from the epiblast through the Hensen's node, and mesoderm cells are invaginated through the primary streak; a 3-layer embryo is formed. The method of laying the mesoderm is late invagination. It differentiates like amphibians.

Further development of the embryo is associated with the formation of extra-embryonic (fetal) membranes. Shells are formed due to the growth of cells of the germ layers outside the body of the embryo. First, a trunk fold is formed (with the participation of all 3 sheets), which raises the embryo above the yolk, then 2 amniotic folds close over the embryo with the participation of the ectoderm and the parietal sheet of the mesoderm - the amnion and the serous membrane (chorion) are formed. A little later, the cells of the endoderm and the cells of the visceral sheet of the mesoderm form the wall of the yolk sac and allantois.

Embryogenesis of mammals.

The ovum is secondary oligolecithal (alicetal). Cleavage is complete, uneven, asynchronous; as a result of the first crushing, 2 types of blastomeres are formed: small and light (crush faster) and large dark (crush slowly). A trophoblast (vesicle) is formed from the light ones, the dark ones are inside the vesicle and are called the embryoblast (the embryo itself will develop from them). The trophoblast, within which the embryoblast is located, is called the blastoderm vesicle or blastocyst. The type of blastula is sterroblastula (i.e., the embryoblast is inside the trophoblast, there is no cavity). The blastocyst enters the uterus from the oviduct, where it feeds on royal jelly, its cells actively grow, then the first attachment of the trophoblast to the uterine wall occurs - implantation.

Gastrulation occurs by delamination (splitting), an epiblast is formed (contains the rudiments of the chord, neural plate, mesoderm, ectoderm) and a hypoblast (it contains only the rudiments of the mesoderm).

The laying of the mesoderm occurs in the same way as in birds - late invagination through the Hensen's knot and the primary streak.

The mesoderm differentiates into 3 buds (primary differentiation): somites, nephrogonadotomes, and splanchnotomes/lateral plates.

Later (secondary differentiation) - each somite is divided into:

dermatome (later - the mesh layer of the skin),

myotomes (subsequently - striated skeletal muscle tissue),

sclerotomes (subsequently - cartilaginous and bone tissue of the skeleton);

The splanchnotome is divided into visceral and parietal sheets, between which the whole is laid.

From the nephrogonadotome, the epithelium of the excretory and reproductive systems is laid.

The embryonic development of mammals is also associated with the formation of extraembryonic membranes. Just as in birds, the beginning of the formation of membranes is associated with the formation of the trunk, and then the amniotic fold.

Lancelet(Amphioxus lanceolatus, or Branchiostoma lanceolatum) - the phylogenetically the lowest organized representative of animals (Chordata), which already develops a strong support for the dorsal part of the body in the process of development - is an excellent object for a schematic demonstration of all the principal blastogenetic processes characteristic of the entire category of vertebrates.

Amphioxus- this is a small animal, in appearance resembling a small fish; it is found in wet sand on sea coasts. The development of the lancelet was first described by the Russian embryologist A. O. Kovalevsky.

Above already mentioned about the importance of studying the basic processes of development of the lancelet and lower vertebrates for understanding human development. In their development, historical (phylogenetic) factors and processes are observed that can be explained (only if the phylogenetic relationships of human development with other vertebrates are taken into account) in general from the point of view of the main biogenetic law on the schematic repetition of phylogenetic development in ontogenesis. As a result of changes in the external and internal living environment, in the process of phylogenetic development, coenogenetic factors joined the ontogenesis of higher animals, which were included, assimilated and accumulated together with the main palingenetic elements of generic development in the living substance of germ cells.

Due to this without comparative study, without a study of human phylogenesis, in other words, without taking into account at least the main features of the development of lower vertebrates, it is impossible to fully understand the complex developmental processes that occur during the ontogenetic development of higher mammals and humans.

lancelet ovules are oligolecithal and even almost isolecithal, with a very moderate prevalence of yolk at the vegetative pole. Thus, they belong to the holoblastic species, and their fragmentation proceeds according to the type of complete uniform, or regular, fragmentation. During crushing, in essence, mitotic division occurs, in which both newly formed cells, called blastomeres, do not diverge, but remain together, closely adjacent to each other. In the plane in which the egg divides into two independent blastomeres, first a superficial and then a deeper groove is formed on its surface, which gradually covers the entire circumference of the spherical cell.
This groove corresponds to the cytokinetic constriction of a mitotically dividing cell in telophase.

First furrow begins to form on the animal pole of the egg, first in the form of a small, superficial notch, which, incessantly deepening, goes from the animal pole along the circumference of the egg towards the opposite vegetative pole. Similar furrows, passing along both poles of a spherical egg, like meridians on the globe, are called meridional furrows.

First meridional furrow and penetrates so deeply deep into the egg that the first two blastomeres arise from the originally one cell, which remain together, firmly glued together. After a certain time, a second crushing division occurs, in which a second, again meridional, furrow appears on the surface of both first blastomeres, running from the animal pole to the vegetative pole, but perpendicular to the plane of the first furrow. Thus, four blastomeres arise from a fertilized egg.

After short resting stage the third crushing division follows, which takes place in a plane perpendicular to the planes of both previous furrows. The resulting furrow covers all four blastomeres approximately at the level of the equatorial plane lying between both poles. In accordance with its course, this furrow is called the latitudinal, or equatorial, furrow. Thanks to the third crushing, the number of blastomeres again doubles, so that a small embryo of the lancelet at this stage of development already has eight cells (blastomeres). The upper four blastomeres lying above the equatorial groove in the region of the animal pole are somewhat smaller in size than the blastomeres lying in the region of the vegetative pole.

The egg, fertilized outside the mother's body, undergoes complete and almost uniform crushing. The result is a typical globular blastula. Larger cells vegetativelyth pole of the blastula begins to bulge inward, and a typical invaginated gastrula is formed.

Then the gastrula stretches, the gastropore (blastopore) decreases, and the ectoderm along the dorsal side to the very gastrothe time begins to deepen, forming the neural plate. Subsequently, the neural plate separates from the cells of the neighboring ectoderm, and the ectoderm fuses over the neural plate and over the gastropore. Still later, the edges of the neural plate fold up and fuse, so that the plate turns into a neural tube. Since the neural plate continues back to the gastropore, at this stage of development, at the posterior end of the embryo, the intestinal cavity is connected to the cavity of the central nervous system using the neuro-intestinal canal (canalis neuroentericus). At the anterior end, the nerve folds close last of all, so that here the nerve canal communicates with the external environment for a long time through an opening, the neuroporus. In the future, an olfactory fossa is formed at the site of the neuropore.

(according to Schmalhausen). I - a whole tubule with many nephrostomes and solenocytes; II - part of the renal tubule with seven solenocytes sitting on it:

1 - the upper end of the gill slit, 2 - the opening of the renal tubule into the peribranchial cavity

(schematically). I—blastula; II, III, IV - gastrulation; V and VI - the formation of mesoderm, chord and nervous systems:

1 - animal pole, 2 - vegetative pole, 3 - gastric cavity, 4 - gastropore (blastopore), 5 - nerve canal, 6 - neurointestinal canal, 7 - neuropore, - 8 - mesoderm fold, 9 - coelomic sacs, 10 - chord, 11 - the place of the future mouth, 12 - the place of the future anus

(according to Parker):

1 - ectoderm, 2 - endoderm, 3 - mesoderm, 4 - intestinal cavity, 5 - neural plate, 6 - central nervous system, 7 - neurocoel, 8 - notochord, 9 - secondary body cavity, 10 - parietal sheet of peritoneum, 11 - visceral peritoneum

(according to Delage):

I — endostyle, 2 — oral opening, 3 — right and 4 — left metapleural folds, 5 — left gill slits, 6 — right gill slits

Simultaneously with the development of the central nervous system, endoderm differentiation occurs. First, from above, along the sides of the primary intestine, longitudinal protruding folds begin to form - the rudiments of the future mesoderm, while the endoderm band between these folds begins to thicken, fold and, finally, split off from the intestine and turns into the rudiment of the chord. Further development of the mesoderm proceeds as follows. First, the folds of the primary gut, lying on the sides of the rudimentary notochord, separate from the gut and turn into a series of closed, segmentally arranged coelomic sacs. Their walls are the mesoderm, and the cavities are the secondary cavity of the body, or coelom. Subsequently, the coelomic sacs grow up and down, and each sac is subdivided into a dorsal region, located on the side of the notochord and neural tube, and an abdominal region, located on the sides of the intestine. The dorsal sections are called somites, the abdominal sections are called lateral plates. From somites, mainly muscle segments are formed - myotomes, which are worn in an adultthe name of the animal is the myomers, and the skin itself (corium), while the sheets of the peritoneum are formed from the lateral plates, and the whole adult animal is formed from the cavities of the lateral plates, which merge with each other. Finally, by invagination, a mouth is formed at the anterior end of the body, and an anus at the posterior end.