The development of the lancelet was first studied by A. O. Kovalevsky. This question is of great interest, since an analysis of the stages of development of the most primitive of modern chordates provides some grounds for judging the early stages of the phylogeny of chordates. In addition, the development of the lancelet represents a simplified schematic picture of the embryonic development of other chordates. Figure 4 and Figure 5 depict the successive stages of embryonic development of the lancelet up to the formation of the larva. Cleavage of the fertilized egg is complete and almost uniform: during the formation of the blastula, it can be seen that on its lower side, corresponding to the vegetative ("vegetative") part of the egg, the cells are larger than on the upper one. Because of this, the inner layer of the next stage of the gastrula is represented by larger cells. Crushing is very fast. In the ectoderm of the upper side of the embryo, the medullary plate separates, the edges of which are folded and then closed. The neural tube that arises in this way retains communication with the external environment at the anterior end (through the neuropore) for some time, and at the posterior end (through the neuro-intestinal canal) with the gastrula cavity, i.e. with the primary intestine. In the future, the neurointestinal canal disappears altogether, and the olfactory fossa remains in place of the neuropore.

The peritoneum, mesentery (in which the main blood vessels appear in the form of longitudinal channels), and intestinal muscles develop from the lateral plate. The nephridial tubules develop as finger-like protrusions in the walls of the secondary body cavity. Gonads develop as protrusions of that part of the walls of the body cavity, which corresponds to the place of separation of the somite and the lateral plate of the gonotomy. The mouth is formed by the protrusion of the primary gut at the end opposite the gastropore (primary mouth) and the counter protrusion of the ectoderm. A breakthrough occurs at the meeting point of these formations. The laying of the mouth and gill slits occurs asymmetrically. The mouth opening is laid on the lower left side of the embryo. Left gill slits (there are 14 of them) initially appear on the ventral side, and then move to right side germ. Then another row of slots appears here (there are 8 of them), located above the 14 slots mentioned earlier. Subsequently, the lower row of slits shifts to the ventral side and only after that - to the left side of the body. At the same time, their number decreases from 14 to 8. The number of gill slits on both sides then increases sharply. Subsequently, the mouth is shifted to the ventral side. The atrial cavity appears initially as a groove on the lower surface of the body. The metapleural folds forming this groove grow towards each other and, closing, form a cavity that opens outwards only in its posterior part, where the mentioned folds do not grow together. In general, the larval development of the lancelet lasts about three months.

Introduction

The embryonic development of man with its characteristic features arose in the course of evolution. To understand this complex process, it is necessary to study the embryogenesis of mammals and other chordates, which makes it possible to trace the complications of embryonic development that have arisen in evolution.

The modern representative of the non-cranial subtype is the lancelet, a small marine animal (body length up to 8 cm), leading a benthic lifestyle. Fertilization of the egg and further development occurs in water. A larva hatches from a developing egg, which, after a short independent existence, acquires the structure of a lancelet through gradual metamorphosis.

Chordates are deuterostomes, bilaterally symmetrical animals. Only one plane (vertical, through the main axis) can be drawn through the body of a chordate animal, which would divide it into symmetrical halves (right and left). The subtype Cranial contains only one class - cephalochords, or lancelets.

Embryogenesis of the lancelet

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

First Full description 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.

Type of egg

The conditions and nature of the development of the lancelet do not require a significant accumulation of a reserve of nutrient material.

The egg is of the primary isolecithal type. There is little yolk in the egg, yolk granules are evenly distributed with only a slight predominance in the vegetative hemisphere compared to the animal. The animal pole of the egg roughly corresponds to the future anterior end of the body of the embryo, i.e., even before fertilization, the anteroposterior axis of the body arises. The sperm enters the egg at one of the points slightly below the equator.

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 further and further 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 consists of a large number blastomeres (in the lancelet - from 3000 cells), in the process of development it passes into a new stage, which is called the gastrula. The embryo at this stage consists of divided layers of cells, the so-called germ layers: external, or ectoderm, and internal, 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.

Theme 4

Embryogenesis anamnios

1.general characteristics anamnios and amniotes.

2. Embryogenesis anamnios.

3. Embryogenesis of the lancelet.

4. Embryogenesis of amphibians, lampreys.

5. Embryogenesis of cartilaginous and bony fishes.

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2. Almazov, I.V., Sutulov L.S. Atlas of histology and embryology / I.V. Almazov, L.S. Sutulov. - M.: Medicine, 1978. - 148 p.

3. Histology / ed. Yu.I. Afanasiev. - M: Medicine, 1989. - 361 p.

4. Ryabov, K.P. Histology with the basics of embryology / K.P. Ryabov. - Mn.: Higher. school, 1991. - 289 p.

5. Biological Encyclopedic Dictionary / ed. M.S. Gilyarov. – M.: Sov. Encycl., 1989. - 864 p.

6. Workshop on histology, cytology and embryology / ed. ON THE. Yurina, A.I. Radostina. - M .: Higher. school, 1989. - 154 p.

Ham A., Cormick D. Histology / A. Ham, D. Cormick. - M.: Mir, 1983. - 192

1. Features of the embryonic development of mammals.

2. Embryogenesis of oviparous mammals.

3. Embryogenesis of marsupial mammals.

4. Embryogenesis of placental mammals.

5. human embryogenesis.

1. General characteristics of anamnia and amniotes

1. General characteristics of anamnia

Based on the featuresembryonic development, all chordate subdivisionsare divided into two groups: anamnias and amniotes. Anamnii – these are animals in which, during embryonic development, such embryonic membranes as the amnion, or the aqueous membrane, are not formedka, and allantois. Anamnias include chordates, leading lanes vychnovodny way of life, as well as lower chordates, closely associated with the aquatic environment during the breeding season and embryoembryonic development–jawless, fish and terrestrialwater. In connection with the embryonic development of these chordatesin the aquatic environment, they have no watershell and allantois, since the functions of the breathThe aquatic environment around it provides for the formation, excretion and nutrition of the developing embryo.

Chordates related to anamnia by character term of embryonic developmentcan be divided into three groups:

1) lancelet, whose eggs contain little yolk;

2) some cyclostomes, fish (cartilaginous hanoids) and amphibians, whose eggs contain an averagethe amount of yolk;

3) selahia and bony fishes,eggs contain a lot of yolk.

2. Embryogenesis of the lancelet

After fertilization inthe ovary of the lancelet begins the redistribution of the yolk,which concentrates mainly on one side of the eggcells corresponding to the vegetative pole. Animalthe pole of the egg is determined by the position above itsecond polar body. Cleavage of the egg is complete, uniform (Figure 1).

/ – animal pole; 2 – vegetative pole; 3 – accumulation of yolk; 4 – whole stool; 5 – blastoderm cells.

Picture– 1. Consistency ( I –VI)cleavage of the ovule of the lancelet

The first two divisions are meridional, third - equatorially. Further crushing goes acrossfirst in one direction, then in the other direction, and the amount of gluecurrent increases exponentially. After imagesingle layer embryo– blastula, it becomes noticeable that the cells of the animal pole are smaller than the cells of the vegetative pole. In the spherical coeloblastula of the lanceletdistinguish the flattened part of the vegetative pole, calledmuyu the bottom of the blastula, and the opposite part, I correspondthe animal pole is called blastula roof. Cletki forming the roof of the blastula will differentiate intocells of the outer germ layer, or ectoderm, and the cells of the bottom of the blastula- into the endoderm.

Gastrulation is carried out by invagination of the blastoderewe have the vegetative pole inside the blastocoel. Invagination proshould be until the cells of the vegetative pole withtouch the cells of the animal pole, in connection with whichthe blastocoel cavity narrows and disappears (Figure 2).

I - coeloblastula; II - IV - gastrulation; V- neurula;

1 - ectoderm; 2- endoderm; 3 - chord; four – mesoderm; 5 - neural plate; 6– top and 7 - lower lip of the blastopore; eight– blastopore; 9 – cavity of the primary intestine; ten – cavity of the secondary intestine; eleven- in general.

Figure 2–Embryogenesis of the lancelet

From the end by the first stage of gastrulation, a two-layergerm, or gastrula, consisting of cells of the outer germ layer– ectoderm and inner germ layer– endoderm. As a result of invagination, a cavity of the primary intestine is formed, lined with endoderm cells, which communicates with the external environment by the blastopore. Cellularthe composition of the endoderm is heterogeneous, since it also includescellular material of the future notochord and mesoderm. With the formation of the cavity of the primary intestine, the embryo begins to grow rapidlyand lengthens, but the most intense shapingprocesses are carried out in the region of the upper, or dorsal,blastopore lips. Blastopo directly behind the upper lipra, on the dorsal surface of the embryo, the ectoderm thickens and consists of tall prismatic cells called medullalar or neural plate. Ectoderm surrounding the nerveplate, represented by small cells, which formpoke the skin. Under the neural plate, the same changesendoderm cells, which representmaterial of the future chord. Subsequently, the neural platebegins to bend, forming a neural groove, and the cellsskin ectoderm intensively creep on it. Lastlythe neural groove deepens, its edges close, and itturns into a neural tube, the cavity of which is callednerve channel. The cells of the skin ectoderm close together, andthe neural tube is under them. Cells at the same timeendoderm adjacent to the neural plate sag inside of the latter, twist and separate into a dense heavy - a chord that looks like a solid cylinder. One hundredronam from the chordal rudiment, the endoderm invaginates sidewayswell, ectoderm, forming mesodermal protrusions, or mesodermal sacs, which are subsequently laced offendoderm and begin to grow between the ectoderm and endoderm. The cavity of the mesodermal sacs, arising from the gastrocoel, turns into a secondary body cavity, or coelom.Thus, in the process of gastrulation, a three-layer embryo.

After isolation of the notochord and lacing of the mesodermalsacs, the edges of the endoderm gradually converge in the dorsalparts of the embryo and, closing up, form a closed intestinalhandset. Following gastrulation, the embryo develops a complexaxial organs, characteristic of representatives of the chordo typeout. It consists of a chord, on the sides of which clusters of segmented mesoderm are located.- somites.

The laying of axial organs occurs at the stage of neurula.The neural tube of the lancelet in the anterior and posterior parts of the embryo remains open for some time. Later on onposterior part of the body of the embryo, the ectoderm grows on the blastoporeand closes it so that the cavity of the neural tube communicateswith the intestinal cavity the neuro-intestinal canal, which is fastro grows. The mouth opening of the embryo of the lancelet secondarily at the anterior end of the body due to thinningand rupture of the ectoderm.

The third germ layer, or mesoderm, of the lancelet embryo is segmented throughout. Mesodermalsegments are further divided into the dorsal part- with mites and abdominal part– splanchnotomes. Somites remain nowmented, and splanchnotomes on each side of the body am chivay primary segmentation, merge and form,splitting into two leaves, right and left coelomic polosti. The latter are combined under the intestinal tube into a commonsecondary body cavity. When the lancelet begins to formtail, then the neuro-intestinal canal disappears, and on the backat the end of the embryo at the site of the blastopore due to thinningand a breakthrough of the body wall occurs an anus. Having passed the described stages of development, the lancelet becomes free floating larva. During the period of larval development organogenesis and histogenesis fail and the larva turns into adult animal.

3. Embryogenesis of lampreys, cartilaginous ganoids and amphibians

These groups of animals are characterized by commonfeatures of crushing, gastrulation and neurulation. Cleavage of the egg is complete, uneven (Figure 3).

1 – blastomeres; 2 - amphiblastula; 3 - blastocoel; 4 - blastoderm; 5 - blastula roof; 6– the bottom of the blastula.

Figure 3– Subsequence ( I– VI ) cleavage of the lamprey egg

The first two furrowscrushings go meridian, start with anithe minor pole, and the third furrow passes near, but equatorially. Animal pole blastomeressmaller than the vegetative pole blastomeres. crushedreduction of animal and vegetative blastomeres up to the seventh fractionleniya takes place almost synchronously, then bothhalf of the embryo begin to split asynchronously. Exceptindicated crushing furrows, tangential borozdy, so the wall of the resulting blastula consistsfrom several rows of cells.

As a result of uneven fractionvegetative pole blastomeres containing manyyolk, form the wall of the blast ly - blastoderm. Blastocoel racesrelies closer to the animal pole. The resulting blastula is called amphiblastula.

In the amphiblastula, wings are distinguishedthe shu corresponding to the animal pole consists of 1 – 3 rows of cells, the bottom corresponding to the vegetative fieldsu, counts 11– 13 rows of cells, and equatorialzone containing 3- 5 rows of cells.

Gastrulation is carried out by invagination and epiboly. Blastoderm invagination beginsin the equatorial zone,how much below the bottom of the blastocoel. The invagination occurs after the appearance of a small crescent-shaped depression, orfalciform groove, which is convexly directed to one hundredcrown of the animal pole. The crescent groove formsdorsal lip of the blastopore. Animal blastoderm cellspoles, i.e., the future ectoderm, multiply intensively andbegin to crawl onto the cells of the vegetativeluxe, overgrowing them from the surface, with the exception of cellsblastoderm in the region of the falciform groove and below the lasther. Intensive multiplication of blastoderm cells in the areaanimal pole also provides movement of cellsmaterial from the surface into the nucleus in the processinvaginations. Through the dorsal lip of the blastoporefirst the cellular material is invaginated entoderma and prechordal plate, i.e. the material thatry is located in front of the cellular material of the notochordalgerm. Further invaginates the material of the chord and on the sides of the entodermis. The bottom of the falciform groove in the form of a double fold invaginates into the blastocoel towards the animal poleparallel to the blastoderm. The cavity of the primary intestineentodermal cells, increases and sharply narrowsblastocoel. Blastocoel with a thin cell wallthe early germ layer is first separated from the gastrocoel,then the cells of the endoderm diverge, and both cavities are connectednyayutsya in a single cavity of the primary intestine.

As cellular material invaginates into the blastocoelcrescent gap increases and acquires a horseshoe-likeform, i.e., the lateral lips of the blastopore are formed. Thenthe blastopore becomes ring-shaped- veins appear tral, or ventral, lip of the blastopore. ring shapeblastopore is due to the fact that in its central part the racesrely on large, yolk-rich blastomeres vegetativeblastula, which, due to their size, cannotinvaginate into the blastocoel. Therefore, invagination of the materialcarried out only along their periphery, and the blastopore has the formnarrow annular gap. By the time the ventrallips of the blastopore almost the entire endoderm invaginates and only a small part of it is on the surface in the centerblastopore. Blastomeres located in the central casti blastopore, are very rich in yolk, in connection with which they received the name yolk plug(Figure 4).

I - amphiblastula; II - III - gastrulation; IV - neurula;

1 – ectoderm; 2 – endoderm; 3 - chord; four - mesoderm; 5 - neural plate;

6 - upper and 7 - lower lip blastopore; eight – blastopore; 9 – gastrocoel; ten – neural tube; eleven – nerve canal; 12 – segmented mesoderm; 13 – unsegmented mesoderm; fourteen – vitelline th endoderm (yolk plug)

Figure 4– Embryogenesis of amphibians

Segmented mesoderm material– somites invaginates through the lateral lips, and the cellular material of the non-segmented mesoderm- splanchnotomes - through the lower lip.Due to the invagination of a large amount of cellular material, cage ki ectoderm change their original position. Clethe exact material of the future neural plate is stretchedover the entire animal surface of the embryo, and the animal alongluce is at the anterior end of the embryo, opposite the blasaxe. In the early stages of invagination, cellular materialthe future chord separates from the endoderm and the notochordal plate immediately folds into a longitudinal strand- chord, which paradise breaks away from the primary intestine, and the latter on the upperside remains open for some time. Freethe edges of the intestinal endoderm quickly restore the defect, oncemelting under the chord, and the wall of the primary intestine becomes solid.

From the very beginning of cell invagination, the segmented mesoderms are not part of the cellular material of the primaryintestines, but invaginate through the blastopore on their own, locatedlying between the ectoderm and the wall of the primary intestine. Segmented mesoderm forms clusters on the sides of chords cells - somites. Non-segmented mesoderm is alsolies between the ectoderm and the wall of the primary intestine, formingprobing splanchnotomes, which are devoid of segmentation. Connection betweensegmented and non-segmented mesodermetsya with the help of segmental legs, or nephrotomes. Nesegment tipped mesoderm on both sides grows under the endodermprimary intestine, then connects, forming a common wholemic cavity. After that, the embryo becomes three ply.

The formation of axial organs in lampreys, cartilaginous ganoids, and amphibians begins already at the end of the gastrulation process with the separation of the notochord material. Simultaneously with the emergencechords ectoderm forms the neural plate, along the edges of whichswarm there are thickenings in the form of nerve rollers. The restpart of the ectoderm is the skin ectoderm. Thennervous forms a neural groove, andneural folds rise, approach and during the formationneural tube merge into a single unpaired ganglionicplate. The neural tube and ganglion plate are immersedare inside the embryo, and skin ecto grows on top of them dermis.

With the isolation of somites, the third pair of somites first appears, then the segmentation process spreads from front to back, and the first two pairs of somites appear later. The central part of the somite differentiates into a muscular plate, or myotome, from which striated muscle tissue of the skeletal type subsequently develops. The part of the somite adjacent to the notochord and neural tube differentiates into a skeletal sheet, or sclerotome, from which the axial skeleton and limb skeleton develop. The upper-lateral part of the somite, which is adjacent to the ectoderm, turns into a skin plate, or dermatome, which forms the basis of the skin.

Nephrotomes are involved in the formation of kidney tubules, and splanchnotomes, splitting into two sheets– parietal and visceral, form bilateral coelomic cavities, which then merge into a common secondary cavity of the body. The visceral sheet of the splanchnotome takes part in the formation of the intestinal wall, heart, it also forms the visceral sheet of the peritoneum, pleura, heart shirt, and the parietal– parietal sheet of the serous membranes of the indicated body cavities,

4. Embryogenesis of cartilaginous and bony fishes

Cleavage of the egg is partial, uneven, or discoidal. The crushing process covers only a small part of the animal pole and leads to the formation of discoblastula. The discoblastula blastoderm in these animals is called blastodisk or germinal disc and the bottom of the blastula is formed by a surface layer of non-crushing yolk– periblast. The cells of the blastodisc, multiplying, form a multilayer blastodisc, which turns from round to oval, and the upper layer of its cells acquires an epithelial-like shape (Figure 1.5).

1 – blastomeres; 2 – periblast; 3 - merocytes; four - yolk; 5 - blastocoel

Figure 1.5– Subsequence ( I– V ) crushing the stingray embryo

The formation of a two-layer embryo occurs by invagination. Gastrulation begins with the movement of cells to the posterior edge of the blastodisc, which thickens and begins to tuck over its own edge, forming the endoderm and ectoderm. The edge of the blastodisc through which the cellular material is tucked, or invagination, is called edge notch. The latter is the blastopore. The middle part of the marginal notch corresponds to the upper, or dorsal, lip, and its lateral parts– lateral lips of the blastopore. The invagination cavity, located between the endoderm and the unbroken yolk, corresponds to the cavity of the primary intestine. The endoderm in its middle part contains the cellular material of the notochordal plate, and on the sides– mesoderm material, initially segmented, and non-segmented along the edges of the marginal notch. Thus, the mesoderm arises by invagination, which is joined by immigration.

In the process of invagination, only that part of the endoderm is formed, which subsequently forms the intestinal tube, more precisely, its epithelial lining. The rest of the endoderm, which then overgrows the yolk, arises from the deep layers of blastodisk cells by delamination of the outer layer of blastodisk cells or from the periblast. It is called vitelline endoderm. Many fish have one of the listed methods of endoderm formation or a combination of them. Subsequently, the intestinal endoderm is combined with the yolk endoderm into a single internal germ layer. This completes gastrulation (Figure 1.6).

I - discoblastula; II - beginning of blastoderm invagination; III - blastodisk; IV - gastrula; V – mesoderm formation;

1 – outer layer of blastodisc cells; 2– cellular material of the future yolk endoderm; 3 – periblast; four - merocytes; 5 – yolk; 6 - blastocoel; 7- marginal notch; eight – gastrocoel; 9 – cellular material of the notochord; ten – mesoderm; eleven – intestinal endoderm; 12 – ectoderm; 13 – cellular material of the neural plate

Figure 1.6. Embryogenesis of cartilaginous fish

The laying of axial organs occurs in approximately the same way as in amphibians, however, unlike the latter in fish, the formation of the intestinal tube occurs differently due to the presence of large reserves of yolk in the egg. The embryo of fish in the process of development for a long time is located on an unbroken yolk in a flattened form. At first, the embryo does not have an abdominal wall. Closing of endoderm cells into a tube occurs when all three germ layers of the reserve yolk are overgrown and the yolk sac is formed. Intensively multiplying, the cells of the three germ layers from the body of the embryo begin to spread to the periphery and move towards the yolk. This process is called the yolk fouling process. It goes most intensively in front and on the sides of the embryo. In the posterior part of the embryo, where the material was tucked in during gastrulation, the fouling of the yolk proceeds more slowly due to the intensive growth of the caudal part of the embryo. Further, the lateral lips of the blastopore approach and grow together, thereby forming the abdominal wall of the body of the embryo, the caudal part of the embryo separates from the yolk, and the embryo itself moves to the center of the embryonic disc. After separation of the caudal part of the embryo from the yolk, fouling of the yolk also begins from the back of the blastodisk, or germinal disk.

Between the head and body of the embryo, on the one hand, and the extraembryonic ectoderm, mesoderm and endoderm– on the other hand, there is a narrowing– interception, called body fold. Thanks to the trunk fold, the head end of the embryo also detaches from the yolk. Lastly, the body of the embryo separates from the yolk. The trunk fold contributes to the folding of the endoderm into a tube and the formation of the abdominal wall of the embryo. However, the process of folding the endoderm into a tube does not cover the entire intestine and in the middle part of the body the intestinal tube remains open. At this point, the intestinal cavity is a duct called yolk stalk, communicates with the yolk sac

With the formation of the yolk stalk, the endoderm is clearly divided into the intestinal endoderm and the yolk, or extraembryonic, endoderm. Extraembryonic ectoderm, mesoderm and endoderm, overgrowing completely the yolk, form yolk sac, which is temporary, or supervisory authority embryo (Figure 1.7).

I- fish embryo with yolk sac: 1 - the body of the fish; 2 – yolk sac, 3- yolk;

II - yolk sac wall: 1– extraembryonic ectoderm; 2 – outside germinal mesoderm; 3 – extraembryonic (yolk) endoderm; four- yolk grains; 5 - cell nuclei of the yolk endoderm; 6 – extra-embryonic blood vessels mesoderm; 7- epithelial integuments and 8 – goblet cells of the extra-embryonic zctoderm.

Figure 1.7– The structure of the yolk sac of bony fish

The endoderm of the yolk sac ferments the yolk and absorbs nutrients. The mesoderm of the yolk sac, thanks to a well-developed system of blood vessels, transports nutrients to the body of the embryo, and the ectoderm covering it performs protective functions. In addition to the trophic function, the yolk sac performs respiratory and hematopoietic functions. At the end of embryonic development, when yolk reserves are depleted, the yolk sac either falls off or becomes part of the intestinal wall and abdominal wall of the body.

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 (blastodisc) 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 lifts 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 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.