Ontogenesis- individual development organism, a set of successive morphological, physiological and biochemical transformations undergone by the organism, from fertilization (during sexual reproduction) or from the moment of separation from the maternal individual (during asexual reproduction) until the end of life.
There are pre-embryonic, embryonic and post-embryonic periods. In placental animals and humans, prenatal (antenatal) and postnatal (postnatal) periods are distinguished.
The totality of events occurring in ovo(oo)genesis, but in the interests of the process of individual development of a new organism, constitutes the content of the period of progenesis - pre-embryonic period of individual development. It begins before the moment of fertilization and the formation of a zygote and is associated mainly with the female reproductive cell. The latter, during gametogenesis (ovogenesis), acquires some characteristics that will be used not by itself, but by an individual of a new generation that has begun individual development. One of these characteristics, perhaps the most well-known, is the formation in the cytoplasm of the egg of a larger or smaller amount of yolk, depending on the type of animal, which is used as nutritional material during the development of the offspring. The functional and genetic activity of a number of genes, manifested in their transcription and post(post)transcriptional changes in the primary and (m)RNA transcript, is also attributed in time to the period before fertilization. The i(m)RNAs formed as a result of this activity organize the synthesis of proteins important for the early stages of embryogenesis.
Types of eggs.
They are classified according to the quantity and distribution of the yolk. Can be:
1. Alecithal eggs - no or very little yolk
Example: in mammals
2. Isolecithal - there is little yolk, it is distributed evenly in the cytoplasm
Example: lancelet
3. Polylecithal: a) telolecithal - the yolk is shifted to the vegetative pole
b) centrolecithal - yolk in the center.
Fertilization
Fusion of sperm with egg. Key Stages fertilization process: 1) penetration of the sperm into the egg; 2) activation of various synthetic processes in the egg; 3) fusion of the nuclei of the egg and sperm with restoration of the diploid set of chromosomes.
The lifespan and fertilization ability of eggs and sperm are limited. In most mammals, the egg retains the ability to fertilize for 24 hours, and in humans 12-24 hours after ovulation. Spermatozoa retain fertilizing ability in the female genital tract also for several hours.
Direct contact between the sperm and the egg causes natural changes in both cells, causing the penetration of the nucleus and centriole of the male gamete into the egg and activation of the latter’s metabolism. These changes on the part of the sperm are called the acrosomal reaction, and on the part of the egg - the cortical reaction. The essence of the acrosome reaction is as follows. At the moment of contact with the egg at the top of the sperm head, the plasma membrane and the adjacent part of the acrosomal vesicle membrane dissolve. Due to the release of enzymes from the acrosomal granule, the adjacent portion of the egg membrane dissolves. The acrosomal membrane protrudes outward and forms an outgrowth in the form of a hollow tube. The latter elongates, passes through the egg membranes and comes into contact with the plasma membrane of the egg. In the area of such contact, a protrusion or fertilization tubercle occurs, after which the plasma membranes of both gametes merge and the unification of their contents begins. From this moment on, the sperm and egg are a single cell - a zygote. The acrosomal reaction proceeds extremely quickly.
Activation of the egg or cortical reaction that develops as a result of contact with the sperm. Changes in the superficial cortical layer of the ooplasm and formation of the fertilization membrane. This membrane, also called the vitelline membrane, is formed by peeling off the surface of the egg. Underneath it, the vitelline space is formed, into which the contents of the granules of the cortical layer of the cytoplasm of the egg are poured. The fertilization membrane protects the egg from the penetration of supernumerary sperm, i.e. provides a “polyspermy block”. The liquid that accumulates in the vitelline space serves as a specific environment in which the development of the embryo occurs until the moment when it leaves the egg membranes.
IN embryonic period there are three main stages: cleavage, gastrulation and primary organogenesis. The embryonic, or germinal, period of ontogenesis begins from the moment of fertilization and continues until the embryo emerges from the egg membranes. In most vertebrates, it includes the stages of zygote, cleavage, gastrulation, histo- and organogenesis.
· Zygote is a one-celled embryo. In the zygote, the process of differentiation of the cytoplasm occurs. Movement of ooplasm is observed. Areas appear in it from which presumptive (alleged) rudiments of future organs are subsequently formed.
· Cleavage - a series of successive mitotic divisions of a fertilized or initiated egg. Cleavage represents the first period of embryonic development, which is present in the ontogenesis of all multicellular animals and leads to the formation of an embryo called a blastula (single-layer embryo). At the same time, the mass of the embryo and its volume do not change, that is, they remain the same as that of the zygote, and the egg is divided into smaller and smaller cells - blastomeres. After each cleavage division, the cells of the embryo become smaller and smaller, that is, the nuclear-plasma relationship changes: the nucleus remains the same, but the volume of the cytoplasm decreases. The process continues until these indicators reach values characteristic of somatic cells. The type of crushing depends on the amount of yolk and its location in the egg. If there is little yolk and it is evenly distributed in the cytoplasm (isolecithal eggs: echinoderms, flatworms, mammals), then crushing proceeds as completely uniform: the blastomeres are the same in size, the entire egg is crushed. If the yolk is distributed unevenly (telolecithal eggs: amphibians), then crushing proceeds as completely uneven: blastomeres are of different sizes, those that contain the yolk are larger, the egg is crushed entirely. With incomplete crushing, there is so much yolk in the eggs that the crushing furrows cannot separate it entirely. The crushing of an egg in which only the “cap” of cytoplasm concentrated at the animal pole, where the zygote nucleus is located, is crushed is called incomplete discoidal (telolecithal eggs: reptiles, birds). With incomplete surface crushing in the depths of the yolk, the first synchronous nuclear fission, not accompanied by the formation of intercellular boundaries. The nuclei, surrounded by a small amount of cytoplasm, are evenly distributed in the yolk. When there are enough of them, they migrate into the cytoplasm, where then, after the formation of intercellular boundaries, the blastoderm (centrolecithal eggs: insects) appears.
· Gastrulation is the process of cell movement, accompanied by growth, reproduction, and differentiation. The embryo at this stage is called a gastrula.
Initially, the outer (ectoderm) and inner (endoderm) layers are formed. Later, the third, middle germ layer (mesoderm) appears, located in the body of the embryo between the ectoderm and endoderm.
There are 4 main ways of forming outer and inner leaves. Often, however, a combination of several modes of gastrulation is observed.
1 way of intussusception, which consists in the fact that a certain section of the blastoderm, while maintaining the structure of the layer, is screwed inside the blastocoel. Then the blastocoel disappears and the gastrocoel appears. The opening through which this cavity communicates with the external environment is called the primary mouth or blastopore.
Epiboly method 2– overgrowth of macromeres with rapidly dividing micromeres of the animal pole. In the embryos of such animals, a blastopore is not initially formed and there is no gastrocoel.
3 way immigration– calculation of the part of the cells of the blastula wall inside the blastocoel.
4th method delamination – cells located outside are transformed into a layer
a-ectoderm; b-endoderm; c-blastocoel.
The formation of the third germ layer occurs in two ways: teloblastic and enterocoelous.
Teloblastic is a method of formation of mesoderm in protostomes by separating two mesodermal strips from two primary embryonic cells (teloblasts).
Enterocoelous - consists in the fact that protrusions are formed from the endoderm of the primary intestine on both sides - pockets (coelomic sacs), which later become detached and grow between the ecto- and endoderm, forming mesoderm; their cavities merge with each other and a secondary body cavity, or coelom, appears.
· Histo- and organogenesis
Two phases can be distinguished.
1 neurulation, consists of the formation of a complex axial organs– neural tube, chord. The embryo at the neurulation stage is called a neurula. First, the cell layer is flattened, which leads to the formation of the neural plate, the edges of which, rising, form the neural folds. Due to the movement of cells along the midline of the neural plate, a depression is created. At this stage of neurulation, the anlage of the nervous system is called the neural groove. At the same time, the neural plate folds along the midline, and a little later its edges close. As a result of these processes, a neural tube with a cavity - a neurocele - appears. The closure of the ridges occurs first in the middle and then in the posterior part of the neural groove. Last of all, this happens in the head part, which is wider than other areas. The anterior, expanded section in further development forms the brain, the rest of the neural tube forms the spinal cord.
2 histo- and organogenesis.
The ectoderm forms the skin epidermis and its derivatives, glands, oral epithelium, vaginal epithelium, tooth enamel, and receptor nerve cells.
From the endoderm - the epithelium of the airways and lungs, part of the pancreatic cells, secreting cells, the epithelium of the stomach, the intestine is formed.
From the mesoderm - cartilaginous and bone skeleton, muscles, kidneys, blood vessels.
Each organism, regardless of whether it is unicellular or multicellular and to which kingdom of life it belongs, goes through individual development, or ontogenesis(from Greek ontos- creature and genesis- birth). Field of biology studying ontogenesis, called developmental biology.
Ontogenesis - this is the period of life of an organism from the zygote (primary cell) to death.
In multicellular organisms, ontogeny usually begins with the formation of the zygote and ends with death.
At the same time, the body not only grows, increasing in size, but also goes through a number of different life phases, at each of which it has a special structure, functions differently, and in some cases has a radically different way of life .
In unicellular organisms, the beginning of ontogenesis is considered to be the moment of separation from the mother or sister cell. It continues until the next division or death. At the same time, the external ontogenesis of unicellular organisms usually manifests itself only as a slight increase in cell size, although in fact this hides completely different periods of its life .
Each species has its own ontogenetic program. And this is not only the set and sequence of development stages that he goes through, but also the duration of each of them. At the same time, any individual has individual characteristics ontogenesis, which, however, do not go beyond the scope of species , and those, in turn, are subject to the laws of ontogenesis at the generic, family, detachment and even class levels .
Ontogenesis program- this is nothing more than the implementation of hereditary information recorded in genes. Therefore, the specificity of ontogenesis at the level of individual individuals is determined by individual combinations of genes, and at the level of species, genera, families - by special genes characteristic only of each systematic group of organisms.
The mechanism for realizing hereditary information is, first of all, differential(from English differ- vary) gene activity. This means that in different periods of development and in different tissues multicellular organism genes are active, on the nucleotide sequences of which certain mRNAs are synthesized. As a result, specific structural proteins and enzymes are synthesized, which ultimately determine the characteristics of the functioning and behavior of cells at certain stages of the development of the body, and the characteristics of their metabolism.
In animals and flowering plants, ontogenesis is divided into two periods. Embryonic development, or embryogenesis(from Greek embryo- embryo and genesis) lasts from the formation of the zygote until birth or exit from the egg, and postembryonic(from Greek fast- after and embryo) development continues from birth or exit from the egg and ends with the death of the organism.
The course of embryogenesis. The general scheme of embryogenesis includes the following phases: Material from the site
- successive divisions of the zygote, culminating in the formation of a multicellular embryo consisting of hundreds and even thousands of identical cells;
- differentiation (from English. differ- distinguish) cells, leading to the formation of tissues;
- laying of organs and growth of the embryo.
During ontogeny the body goes through a number of phases - states in which it differs in structure, functioning and way of life. There are two stages of ontogenesis: embryogenesis - embryonic development, and postembryonic development - the period of life of the organism from birth (emergence from the egg) to death.
On this page there is material on the following topics:
Ontogenesis brief summary
Ontogenesis of individual development summary
The concept of ontogenesis briefly
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Individual development (ontogenesis), periodization of ontogenesis.
All organisms have a certain life cycle. For sexually developing organisms, it begins from the moment the zygote appears and ends with the natural death of the organism.
The set of processes that occur during the life cycle of organisms is defined as individual development or ontogenesis.
Ontogenesis includes 3 periods:
1st period. Preembryonic or gametogenesis.
2nd period. Embryonic.
3rd period. Postembryonic.
1. Life cycles organisms. Development is larval and direct.
The development of organisms can be direct or indirect with transformation.
Indirect development occurs through the larval stage. The larva develops certain embryonic or provisional organs that ensure the vital functions of the organism at this stage of development.
In higher vertebrates, development is direct, but provisional organs are also formed during embryonic development. In mammals, these are the embryonic membranes (amnion, chorion, allantois, placenta) and the yolk sac.
2. Pre-embryonic period (pre-embryonic, progenesis) of development. Stages of gametogenesis. Changes in oogenesis associated with the early development of the embryo (gene amplification, ooplasmic segregation, accumulation of nutrients).
The preembryonic period or gametogenesis includes several stages: separation, reproduction, growth, maturation, formation (the latter only in sperm).
During oogenesis occurs major events, which are necessary for the development of the future organism.
1 event. During oogenesis, amplification of rRNA genes occurs or an increase in the number of copies of genes responsible for rRNA. This process occurs during meiosis prophase 1. There can be up to a million copies of rRNA genes.
Then these copies are separated from the chromosomes, float freely in the karyoplasm, nucleoli are formed around them, and ribosomal subunits are synthesized in the nucleoli and enter the cytoplasm. Thus, the number of ribosomes in the egg increases sharply in advance.
2nd event. During oogenesis, different types of mRNA are synthesized during prophase of meiosis 1. Transcription processes occur on despiralized regions of chromosomes. Chromosomes at the prophase stage of meiosis 1 are called lampbrush chromosomes.
3rd event. Nutrients accumulate in the egg in the form of yolk.
4th event. The egg is characterized by ooplasmic segregation, that is, the distribution of substances throughout the cytoplasm of the egg, which leads to chemical heterogeneity of the cytoplasm. It is believed that this is necessary for early cell differentiation.
5 event. Sex cells are special cells of the body, since they have totipotency, that is, equal heredity. Only germ cells, as well as blastomeres in humans, at the stage of 2 blastomeres, give rise to all types of cells. For example, experiments on the separation, fusion or mixing of blastomeres at the cleavage stage have shown that in species with a radial type of cleavage, blastomeres of several generations, if isolated and placed in suitable conditions, exhibit totipotency, i.e. develop into a full-fledged organism. The equal heredity and totipotency of human embryonic cells up to the stage of 2-4 blastomeres is evidenced by the birth of two, three, or four identical twins.
3. Embryonic period of development, periodization.
The embryonic period of ontogenesis includes several stages:
1 Fertilization stage.
2 Zygote stage.
3 Stage of crushing (formation of a single-layer embryo).
4 Stage of gastrulation (formation of a two- and three-layer embryo).
5 Stage histo- and ABOUT Organogenesis (formation of tissues and organs).
4. Fertilization and formation of the zygote. Features of fertilization in mammals and humans.
Fertilization stage, is the process of fusion of an egg and a sperm, resulting in the formation of a diploid zygote, from which a diploid organism develops. There are 3 stages in this process:
Stage 1- convergence of gametes. Substances secreted by the egg and sperm play an important role in this. They are called gamones (gamete hormones, gynogamones and androgamones, respectively). In addition, there are a number of nonspecific factors that increase the likelihood of a sperm meeting and interaction with an egg. These include
coordination of the onset of readiness for fertilization in the male and female,
behavior of males and females that ensures copulation and insemination,
excess sperm production,
large egg size,
the presence of gamones that promote the convergence and interaction of gametes,
the presence of copulatory organs that provide internal insemination.
In mammals great importance has the presence of sperm in the female reproductive tract, as a result of which male reproductive cells acquire fertilizing ability, i.e. ability for acrosomal reaction.
Stage 2 – activation of gametes occurs after their contact. The activation of the sperm is called the acrosome reaction. Activation of the egg is a cortical response.
The essence of the acrosome reaction : The permeability of the sperm in the area of the acrosome changes the plasma membrane, and enzymes, spermatolysins, are released from the acrosome. These enzymes relax the connections between the follicular cells that surround the egg. The sperm passes through the layer of follicular cells, then the zona pellucida is destroyed and the sperm passes through this zone.
The essence of the cortical reaction: It consists of complex structural and physicochemical changes. Due to the fact that a section of the sperm membrane is permeable to sodium ions, the latter begin to enter the egg, changing membrane potential cells. Then, in the form of a wave propagating from the point of contact of the gametes, an increase in the content of Ca 2+ ions occurs (in the hyaloplasm they come out of the depot - ER, reticulum) and biochemical processes are launched in the egg, after which the cortical granules also dissolve in the wave. The specific enzymes released in this process promote the detachment of the vitelline membrane; it hardens, it fertilization membrane.
One of the meanings of the cortical reaction is the prevention of polyspermy, i.e. penetration of more than one sperm into the egg. In mammals, the cortical reaction does not cause the formation of the fertilization membrane, but its essence is the same.
Activation of the egg ends with the beginning of protein synthesis at the translational level, since mRNA, tRNA, ribosomes and energy were stored during oogenesis.
Stage 3- fusion of gametes, or syngamy. In this case, a common plasma membrane is formed between the sperm and the egg. The female and male pronucleus come closer and merge (sinkaryon), forming a common metaphase plate. This is the moment of the final fusion of gametes - syngamy.
Features of fertilization in various types organisms.
1 example. In mammals and humans, the sperm binds to the egg in the area where there is a receptor on the zona pellucida. After this interaction, the remaining receptors are blocked.
2 example. U sea urchin after fertilization, the egg changes dramatically electric potential plasma membrane, and then the fertilization membrane is formed, which prevents polyspermy.
Zygote stage. After penetration, the male nucleus is called the male pronucleus. It loosens chromatin and DNA replication occurs. The female nucleus is called the female pronucleus. The same events take place there. In mammals and humans, nuclear fusion does not occur, but a metaphase plate is immediately formed.
5. Artificial fertilization of animal and human eggs, biological and medical aspects.
Artificial fertilization of animal eggs is of great scientific importance for medicine, since in the process of studying it, ways and mechanisms for treating infertility in humans are being developed.
Artificial insemination is used for various forms infertility, both male and female, which is difficult to treat. For example, when a man has too few sperm or they are practically immobile, when a woman has impaired patency fallopian tubes or there are any other damage to the internal genital organs, with immunological incompatibility of partners.
6. general characteristics crushing. Types of crushing characteristic of different animal species. Cleavage and formation of the blastula in placental mammals.
Crushing stage. This is the stage of formation of a single-layer embryo - blastula. Inside the blastula there is a cavity - the blastocoel.
Crushing Features:
Cells divide by mitosis.
On the eve of each division, DNA replication occurs.
Dividing cells do not grow.
The type of cleavage depends on the type of egg.
Complete uniform crushing of lancelet:
The first cleavage furrow runs vertically and two blastomeres are formed. The second furrow also runs vertically and four blastomeres are formed. The third furrow runs horizontally, eight blastomeres are formed, and then vertical and horizontal furrows alternate. After 12 cycles, crushing becomes asynchronous. At a certain stage of development, the embryo is a lump of cells or morula. Then gaps appear between the cells, and a cavity is formed - the blastocoel. In the lancelet, during crushing, a blastula is formed, which is called coeloblastula, that is, a single-layer ball.
Complete uneven fragmentation in amphibians:
In amphibians, the cells are moderately telolecithal. At the animal pole of the cell, fragmentation proceeds faster than at the vegetative pole. As a result, at the animal pole the cells are smaller - micromeres. At the vegetative pole, the cells are larger - macromeres. The blastula of amphibians is called amphiblastula. The blastocoel is located at the animal pole.
Features of crushing in mammals and humans:
The fragmentation is completely uneven, asynchronous from the first stages; at a certain stage of development, the embryo is a morula (a lump of cells). Larger cells then separate to the periphery, forming a trophoblast, and smaller cells to the center, forming an embryoblast. The blastula is called a blastocyst. The blastocoel is very small. Trophoblast promotes the implantation of the embryo into the uterine mucosa. This process is called implantation. The embryoblast gives rise to the embryo itself and some provisional organs.
7. General characteristics of gastrulation. Features of gastrulation in amphibians and birds. Gastrulation in higher (placental) mammals.
Gastrulation stage, or the stage of formation of a two-layer embryo, and then a three-layer one. The embryo at this stage is called gastrula.
Methods for the formation of a two-layer embryo:
– Intussusception (invagination).
– Delamination (stratification).
– Immigration (movement).
– Epiboly (fouling).
Intussusception or invagination. This method is typical for lancelet. In a certain area, the blastula cells invaginate into the blastocoel, resulting in the formation of a two-layer embryo. The outer layer of cells is called ectoderm, the inner layer is called endoderm. The endoderm limits the cavity of the primary intestine or gastrocoel. The entrance to this cavity is called the primary mouth or blastopore. The blastopore is surrounded by lips.
Delamination or separation. This method is typical for coelenterate animals, in which the blastula has the appearance of a morula and the blastocoel is practically not expressed.
Immigration or Settlement. Some blastula cells invade the blastocoel, then these cells rapidly divide. As a result, endoderm is formed from these cells.
Epiboly (fouling). Micromeres are divided and, as it were, layered on macromeres. Due to micromeres, ectoderm is formed, due to macromeres, endoderm is formed. These methods are practically never found in their pure form; as a rule, they are combined. In amphibians, intussusception and epiboly are combined. Birds and mammals combine delamination and immigration.
Starting with flatworms, a third germ layer appears in evolution - the mesoderm.
Methods of formation of mesoderm:
The teloblastic method is characteristic of protostomes. In the region of the blastopore lips, 2 cells are distinguished, which divide and form mesoderm.
The enterocoelous method is characteristic of deuterostomes (chordates). Two sections of cells in the form of pockets are symmetrically separated from the endoderm. These are mesodermal pockets. The cells of the mesodermal pockets divide and give rise to mesoderm. The mesoderm is the germ layer.
8. General characteristics of histo- and organogenesis (formation of tissues and organs).
Stage of histo and organogenesis(stage of formation of tissues and organs). Conventionally divided into two periods.
1st period. The period of formation of the axial organs in the embryo is the formation of the neural tube and notochord. Therefore, this period is called the period of neurulation, and the embryo at this stage is called neurula.
2nd period. Characterized by the formation of other tissues and organs. On the dorsal side of the embryo (dorsal), along its entire length, a section of cells is separated from the ectoderm, which gives rise to the neural plate. Then the edges of the neural plate rise, thicken, and a neural groove is formed, which gradually sinks under the ectoderm. Then the edges of the neural groove close, a neural tube with a cavity inside is formed, the cavity is called a neurocoel. In vertebrates, the anterior part of the neural tube expands and gives rise to the brain, the rest - the spinal cord. At the same time, the notochord is formed under the neural tube; it is formed from the endoderm and adjacent mesoderm. At first, the mesoderm is a homogeneous cell mass, but as it develops, it segments. Structures called somites are formed. Subsequently, they give rise to the musculoskeletal system.
Germ layer derivatives:
Ectoderm - tooth enamel, nervous system and sensory organs, the epidermis of the skin and its appendages, the epithelium of the foregut and hindgut.
Endoderm - epithelium of the midgut, digestive glands and respiratory system.
Mesoderm – musculoskeletal system, genitourinary system, circulatory and lymphatic systems, all connective tissue.
9. Characteristics of provisional organs of vertebrate embryos. Provisional organs of higher mammals.
Provisional organs function in the embryo and are absent in adulthood. These include the yolk sac and the so-called embryonic membranes - amnion, chorion and allantois.
Yolk sac. The yolk sac performs a number of important functions: nutrition, respiration, excretion, hematopoiesis. But, due to the low content of yolk in the egg, it does not play a significant role in the nutrition of the embryo.
Amnion. The formation of the amnion involves the amniotic membrane, which limits the amnion cavity filled with amniotic fluid, which now washes the embryo from all sides. Thanks to this, the embryo develops in an aqueous environment, which protects it from mechanical traumatic effects and adhesion to the membranes.
Allantois formed as an outgrowth of the hindgut. The main function of the allantois is that it is the embryonic organ of excretion. It accumulates decay products formed during metabolism in the body of the embryo.
Embryogenesis is a complex holistic process that is associated with certain phenomena and mechanisms. Many of these phenomena have not been fully studied, although some data have been obtained for some of them.
1. Molecular genetic changes in early development.
2. Cell proliferation (cell division).
3. Cell differentiation.
4. Shape formation or morphogenesis.
1. Molecular genetic changes in early development (zygote and cleavage periods), the role of cytoplasmic factors of the egg.
Early development includes the zygote and cleavage stages.
By studying these stages, scientists tried to answer the questions:
Firstly, when the embryo’s own genes begin to work.
Secondly, are there qualitative and quantitative differences in the molecules of mRNA and proteins in different parts embryo in the early stages of development.
In the zygote, gene activity is low, since DNA is tightly bound to histone proteins. The first proteins that are synthesized in the zygote are of maternal origin, since ribosomes and mRNA molecules have previously accumulated in the egg. It has been established that the embryo’s own genes in mammals begin to work at the stage of 2–4 blastomeres. In amphibians - at the blastula stage. The genes responsible for proliferation and general metabolism are the first to be activated; later, the genes responsible for the differentiation of cells and tissues begin to work. For example, when the nucleus is removed from the zygote, fragmentation occurs, and the embryo reaches the blastula stage in its development, after which further development stops.
It has been established that there are no qualitative differences in the molecules of mRNA and proteins in different parts of the embryo at the early stages of development. There are only quantitative differences.
An important role in fragmentation is played by the division of the cytoplasm - cytotomy. It has a special morphogenetic significance, as it determines the type of fragmentation. Cleavage furrows run along the boundaries between individual sections of the ooplasm, reflecting the phenomenon of ooplasmic segregation. Therefore, the cytoplasm of different blastomeres differs in chemical composition.
2. Cell proliferation, growth.
Cell proliferation or cell division occurs throughout embryogenesis. This is associated with the growth of tissues and organs. Overall growth of the embryo.
3. Differentiation, molecular genetic mechanisms of differentiation.
Cell differentiation is a set of processes as a result of which cells of common origin acquire persistent morphological, physiological, and biochemical differences, which leads to cell specialization. The specificity of cells is determined by the proteins that are synthesized in them, and the corresponding genes are responsible for the proteins. Therefore, we can conclude that some genes work in some cells, and others work in others. This is the essence of the hypothesis of differential gene activity.
At the early stages, cell differentiation is associated with the influence of cytoplasmic substances on the work of the corresponding genes - this is the epigenetic level of regulation of gene work. In the egg, the phenomenon of ooplasmic segregation occurs, as a result, different parts of the cytoplasm of the egg contain various substances. During fragmentation, blastomeres appear; the set of genes in them is the same, but the composition of the cytoplasm is different. Subsequently, these cytoplasmic substances appear to lead to differential gene activity.
When characterizing cell differentiation, two concepts are used - determination and competence.
Determination means that cell differentiation is genetically predetermined and irreversible.
During the process of differentiation, the cellular material of embryonic anlages is transformed into a specific element of the adult organism. Let us consider differentiation using the example of a mesodermal somite, subdivided into dermatome, sclerotome and myotome. The dermatome is the cells of the dermis, the second is the cells of cartilage, the third is striated muscle fibers. Consequently, the final result of the development of individual embryonic anlages is predetermined or determined.
Competence This is the ability of cells to differentiate in different directions, under the influence of environmental factors. For example, the notochord and adjacent mesoderm act on the ectoderm, resulting in the formation of a neural tube from the ectoderm. If there is no such influence, then the ectoderm gives rise to the epidermis of the skin.
4. Morphogenesis (shape formation), its main processes:
Shape formation or morphogenesis. Morphogenesis is a set of processes as a result of which the embryo acquires a characteristic external and internal structure. In turn, morphogenesis is associated with:
a) morphogenetic movement of cells
During embryogenesis, individual cells or groups of cells move. Cells move along the surface of other cells (thanks to the mechanism of amoeboid movement), where special molecules are located that indicate the direction of movement. Some cell types move along a concentration gradient chemical substances(chemotaxis), but this mechanism is much less common.
Violation of cell migration during embryogenesis leads to underdevelopment of organs or to a change in its normal localization. Both are congenital malformations. For example, when the migration of neuroblast cells is disrupted, islands of gray matter appear in the white matter, and the cells lose their ability to differentiate.
Thus, cell migration is under genetic control, on the one hand, and the influence of surrounding cells and tissues, on the other.
b) embryonic induction
This is the effect of one tissue (inducer) on another tissue, as a result the development of the induced tissue becomes qualitatively new. The first and most significant induction is the action of the notochord and mesoderm on the ectoderm, resulting in the formation of the neural tube. Without the neural tube, all ectoderm will transform into epidermis. This is primary embryonic induction, the first step in a chain of successive (secondary, tertiary) induction processes in further development.
It has been established that there are “specific inductors”, i.e. substances that have an inducing effect in negligible concentrations and differ in the final result of their action. Thus, an extract from mammalian liver induces mainly the development of brain structures, and a bone marrow extract induces mainly mesodermal structures.
The ability of the embryonic rudiment to perceive an inductive stimulus is called competence.
c) intercellular interactions
This is the interaction of cells or layers by contact or at a distance. Interaction at a distance involves biological active substances(BAV).
These can be proteins, hormones, etc. In the early stages of embryonic development, these are the mother’s hormones, since the embryo does not have its own endocrine glands. Hormones do not cause new differentiation, but they enhance it.Thanks to intercellular interactions, phenomena such as morphogenetic movement of cells, embryonic induction, and cell adhesion occur.
d) adhesion– ability of cells to stick together. In the experiment, ectoderm, mesoderm and endoderm cells were separated and mixed together. Then they reassemble into separate groups, each of which is a cellular aggregate of homogeneous cells. Three germ layers are formed again, located normally relative to each other.
Special protein molecules take part in the adhesion process. They are called cell adhesion molecules (CAMs), and there are about 100 types of them.
Another hypothesis states that contacts between similar cells are stronger than between foreign cells.
Selective adhesion of cells of a certain germ layer to each other is a necessary condition for normal development.
e) cell death is a necessary process, because the formation of individual structures (ducts, channels, openings, etc.) requires the destruction of part of the cells.
There are two fundamentally different types of cell death: apoptosis(translated from Greek as “falling away”) and necrosis.
Apoptosis is a physiological, genetically determined cell death. Along with other mechanisms of morphogenesis, it contributes to the achievement of characteristics of its morphofunctional organization characteristic of a particular biological species. Therefore, apoptosis is a natural, evolutionarily determined and genetically controlled mechanism of morphogenesis.
Necrosis is non-physiological cell death due to exposure to unfavorable factors (mechanical, chemical, physical, etc.). Necrosis is usually accompanied by inflammation and is a pathological process.
5. Integration in development, integrity of ontogenesis. The role of hormones in coordinating developmental processes.
Currently, a number of substances are known that induce cells to divide, for example, phytohemagglutinin, some hormones, as well as a complex of substances released when tissue is damaged. Tissue-specific inhibitors of cell division have also been discovered - Keylons. Their action is to suppress or slow down the rate of cell division in the tissues that produce them. For example, epidermal kelons act only on the epidermis. Being tissue specific, kaylons lack species specificity. Thus, epidermal cod kaylon also acts on the epidermis of mammals.
Hormones – organic compounds, produced by certain cells and intended to control the functions of the body, their regulation and coordination.
The physiological action of hormones is aimed at:
1) providing humoral, i.e. carried out through the blood, regulation of biological processes;
2) maintaining the integrity and constancy of the internal environment, harmonious interaction between the cellular components of the body;
3) regulation of the processes of growth, maturation and reproduction. Hormones regulate the activity of all cells in the body. They influence the acuity of thinking and physical mobility, physique and growth, determine the development of sexual dimorphism and behavior.
6. The role of heredity and environment in embryonic development. Critical periods of development. Teratogenic factors. Anomalies and malformations.
At any stage of ontogenesis, the organism exists in unity with environment. Embryogenesis in this regard is no exception. The range of conditions necessary for the life of a species can be wide. However, for organisms of any species there is a minimum, optimum and maximum necessary conditions development. The development of the embryo is influenced by fluctuations in naturally occurring factors (temperature, humidity, Atmosphere pressure, radiation, gas composition of the environment).
Thus, depending on the temperature, development processes slow down or intensify. For example, frog eggs from the same clutch develop faster at higher temperatures.
In roundworms, when oxygen access to the embryo is cut off, development stops.
General rule The reason is that under the influence of light from the blue-violet part of the spectrum, the embryonic development of many animal species accelerates, and from the red part it slows down.
During intrauterine development, environmental factors play a huge role. If these factors lead to the formation of anomalies or developmental defects, then they are called teratogenic. Teratogenic factors can be physical (high temperature, ionizing radiation, X-ray, etc.), chemical (medicines, salts of heavy metals, etc.) and biological (viruses, bacteria). Teratogenic factors lead to the development of abnormalities during certain periods of embryonic development, which are called critical periods. These include:
The period of formation of germ cells (gametogenesis),
Fertilization stage
Zygote stage
Implantation of the embryo into the wall of the uterus,
Placentation,
The period of histogenesis and organogenesis,
Developmental defects.
Aplasia - absence of an organ or part thereof
Hypoplasia - underdevelopment of an organ
Hypotrophy - reduction in body or organ weight
Hypertrophy - disproportionate increase in organ mass
Gigantism - increase in body length
Heterotopy is an atypical localization of a group of cells or an organ in the body.
Heteroplasia - impaired tissue differentiation
Stenosis - narrowing of a canal or opening
Atresia - absence of a canal or opening
Persistence - preservation of embryonic structures
Depending on the cause, birth defects are divided into:
Hereditary, caused by changes in genes or chromosomes in the gametes of the parents, as a result of which the zygote carries a gene, chromosomal or genomic mutation from the very beginning.
Exogenous, arising under the influence of teratogenic factors: drugs (thalidomide), food additives, viruses, industrial poisons, etc. These are all environmental factors that, acting during embryogenesis, disrupt the development of tissues and organs.
Multifactorial defects that develop under the influence of both exogenous and genetic factors.
1. Postnatal ontogenesis, its periodization.
Postembryonic development(for humans, postnatal) begins from the moment of birth and ends with natural death or death.
Postembryonic development includes several periods:
1. Pre-reproductive (juvenile).
2. Reproductive (maturity period).
3. Post-reproductive (old age period).
2. Pre-reproductive period, its characteristics. Body growth as an important characteristic of the pre-reproductive period.
The pre-reproductive period begins immediately after birth. At this time, the processes of morphogenesis end, those systems that did not function in embryogenesis (respiratory, excretory and a number of others) begin to function.
Important characteristic The pre-reproductive period is the growth of the body. In this case, the size of the body as a whole increases, its longitudinal dimensions increase; the size of tissues and organs increases.
3. The nature of the growth of the organism and its individual parts.
The growth of an organism is based on three main processes:
1. increase in the number of cells.
2. increase in cell size (hypertrophy).
3. accumulation of intercellular substance.
There are two growth options: limited And unlimited. Unlimited growth continues throughout ontogeny, until death.
There are several types of growth:
Auxentic– growth that occurs by increasing cell size.
Proliferative– growth that occurs through cell proliferation: multiplicative And accretionary.
Multiplicative growth is characterized by the fact that both cells arising from the division of the parent cell begin to divide again. Multiplicative growth is very effective and therefore almost never occurs in its pure form or ends very quickly (for example, in the embryonic period).
Accretionary growth is that after each subsequent division, only one of the daughter cells divides again, while the other stops dividing. In this case, the number of cells increases linearly. This growth is typical for organs where cell composition is renewed.
It must be pointed out that of particular importance in characterizing growth is the increase in the longitudinal dimensions of the body, which occurs mainly due to the growth of long tubular bones. IN tubular bones At the border of the diaphysis and epiphysis, a growth zone is distinguished. Here there are cartilage cells, during the division of which the bone grows in length.
Final ossification of each bone occurs at a certain time. In men, growth usually ends by 18-20 years, in women - by 16-18 years. At this time, the last growth zones disappear. It is then that the growth of bones in length stops.
It is necessary to point out that up to the age of 30, a person can grow by 3 cm due to an increase in the size of the vertebrae.
An increase in a person's linear dimensions is described by an S-shaped curve. Immediately after birth, the body’s growth increases, then it decreases and sharply accelerates by the age of 13-14-15. This is the so-called pubertal growth spurt (during puberty). Further, the growth rate slows down somewhat, and at the age of 30-40-45 years, a person’s height remains constant. Bones, muscles and many things grow according to this pattern. internal organs(liver, kidneys, spleen).
With aging, there is a slight decrease in height.
Some organs have a completely different character:
These organs include the brain and spinal cord, lymphoid organs, and reproductive organs.
The weight of a newborn’s brain is 25% of the final brain weight (in adulthood), by 5 years – 90%, by 10 years – 95%.
Growth of the thymus gland central authority immune system. The relative weight of the thymus (to body weight) reaches a maximum at 12 years of age. Absolute weight reaches a maximum at the age of 30, and then there is a sharp decrease in the weight of the thymus.
4. Genetic control of growth. The role of the nervous and endocrine systems in the regulation of growth processes.
Height refers to genetic characteristics that are inherited like hair and skin color, eye shape, etc. This is why tall parents usually have tall children, and vice versa. Height is a polygenic trait; several genes are responsible for its manifestation in the phenotype. Genes control growth through appropriate hormones. The most important hormone is growth hormone or somatotropin, produced by the pituitary gland.
Somatotropin stimulates the formation of new cartilage cells, and partially their ossification, promotes protein synthesis in cellular structures and the formation of new capillaries. A large number of This hormone is produced at night. A child’s own somatotropin is produced from the age of 3-4 years.
Thyroid hormones and sex hormones also influence body growth.
5. Interaction of biological and social during childhood and youth.
The role of heredity for growth is great, but it is not the only factor. Heredity should be considered as an approximate program, according to which a person’s height may be, for example, in the range from 160 to 180 cm. What it will actually be depends largely on external conditions that can inhibit the hereditary program or contribute to its implementation. Environmental conditions affecting human growth: nutrition, physical activity, psychological impact smoking, alcohol.
That is, the population is growing (this is acceleration).
One of its probable reasons is improved living conditions (nutrition). It has been observed that during years of war and natural disasters, children's growth decreases. Growth is slightly affected by climate and geographic environment.
6. Formation of constitutional types, body types.
The formation of constitutional types of people is associated with human growth. This should be understood as the characteristics of the external forms of the body, the characteristics of the body’s functions, and the characteristics of behavior. this person. Depending on the structure of the body, depending on the external shape of the body, certain body types are distinguished. Currently, there are quite a lot of classifications. One of them is the classification of M.V. Chernorutsky. According to this