The embryonic development or embryogenesis comprises a series of stages that originate the embryo, beginning with fertilization. During this process, all the genetic material in the cells (genome) is translated into cell proliferation, morphogenesis and incipient stages of differentiation..
Full development of the human embryo takes 264 to 268 days and occurs in the uterine tube and in the uterus. Different stages of development can be distinguished, starting with the blastema stage -which occurs from fertilization and ends with gastrulation-, followed by the embryonic stage and ending with the fetal stage.
Compared with the development of other groups of mammals, human gestation is a premature process. Some authors suggest that this process should last about 22 months, since the brain maturation process ends after the birth of the fetus..
The animal body scheme is determined by genes called Hox or homeotic genes. Genetic studies carried out in different model species demonstrated the existence of these highly conserved “genetic regulators” in evolution, from primitive groups such as cnidarians to complex organisms such as vertebrates..
Article index
The human embryogenesis process, temporally divided into weeks and months, includes the following processes:
The beginning of embryogenesis is fertilization, defined as the union of the ovum and the sperm. For this process to take place, ovulation must occur, where the egg is released into the uterus with the help of cilia and peristalsis. Fertilization occurs in hours close to ovulation (or a few days later) in the oviduct.
Ejaculation produces about 300 million sperm that are chemically attracted to the egg. After entering the female duct, male gametes are chemically modified in the vagina, modifying the constitution of lipids and glycoproteins in the plasma membrane..
The successful sperm must bind to the zona pellucida and then to the plasma membrane of the ovum. In this stage, the acrosome reaction occurs, which leads to the production of hydrolytic enzymes that help the penetration of the sperm into the ovum. This is how the zygote with 46 chromosomes is formed in the fallopian tubes.
The founding process is complex and includes a series of molecularly coordinated steps, where the ovum activates its developmental program and the haploid nuclei of the gametes fuse to give rise to a diploid organism..
In the three days after fertilization, the zygote undergoes a segmentation process even in the fallopian tubes. As the division process increases, a set of 16 cells is formed that resembles a blackberry; for this reason it is called morula.
After these three days, the morula moves to the cavity of the uterus, where fluid accumulates inside it and the blastocyst is formed, made up of a single layer of ectoderm and a cavity called a blastocele. The process of fluid secretion is called cavitation.
On the fourth or fifth day, the blastula consists of 58 cells, of which 5 differentiate into embryo-producing cells and the remaining 53 form the trophoblast..
The endometrial glands secrete enzymes that help release the blastocyst from the zona pellucida. The implantation of the blastocyst occurs seven days after fertilization; at the time of adhering to the endometrium, the blastocyst can have from 100 to 250 cells.
The outer cell layer, which gives rise to embryonic structures, forms the tissues of the chorion that generates the embryonic portion of the placenta. The chorion is the outermost membrane and allows the fetus to obtain oxygen and nutrition. In addition, it has endocrine and immune functions.
The yolk sac is responsible for digesting the yolk and the blood vessels supply food to the embryo, and the amnion is a protective membrane and is filled with fluid. Finally, the allantoic membrane is responsible for the accumulation of waste.
By the eighth day after fertilization, the trophoblast is a multinucleate structure made up of the external syncytiotrophoblast and the internal cytotrophoblast..
The trophoblast differentiates into villi and extravilli. Chorionic villi appear from the first, whose function is the transport of nutrients and oxygen to the zygote. The extravillous is classified as interstitial and intravascular.
Differentiation into epiblast and hypoblast (which form the laminar disc) has occurred in the inner cell mass. The former originate the amnioblasts that line the amniotic cavity.
Seven or eight days after the process, the differentiation of the ectoderm and endoderm occurs. The mesenchyme arises in isolated cells in the blastocele and lines this cavity. This area gives rise to the body pedicle, and together with the embryo and the chorion, the umbilical cord arises.
At twelve after fertilization, the formation of lagoons from eroded vessels occurs inside the syncytiotrophoblast. These gaps are formed by filling with the mother's blood.
In addition, the development of primary hairy stems formed by cytotrophoblast nuclei occurs; around this the syncytiotrophoblast is located. Chorionic villi also appear on day twelve.
The most striking event of week 3 is the formation of the three germ layers of the embryo by the gastrulation process. Both processes are described in detail below:
There are germ layers in embryos that give rise to the appearance of specific organs, depending on their location.
In triploblastic animals - metazoans, including humans - three germ layers can be distinguished. In other phyla, such as sea sponges or cnidarians, only two layers differ and are called diploblastic.
The ectoderm is the outermost layer and in this the skin and nerves arise. The mesoderm is the middle layer and from this the heart, blood, kidneys, gonads, bones and connective tissues are born. The endoderm is the innermost layer and generates the digestive system and other organs, such as the lungs..
Gastrulation begins by forming what is known as "the primitive streak" in the epiblast. The cells of the epiblast migrate to the primitive streak, shed and form an invagination. Some cells displace the hypoblast and originate the endoderm.
Others are located between the epiblast and the newly formed endoderm and give rise to the mesorderm. The remaining cells that do not undergo displacement or migration originate the ectoderm.
In other words, the epiblast is responsible for the formation of the three germ layers. At the end of this process, the embryo has formed the three germ layers, and is surrounded by the proliferative extra-embionic mesoderm and the four extra-embionic membranes (chorion, amnion, yolk sac and allantois).
By day 15, maternal arterial blood has not entered the intervillous space. After the seventeenth day, a functioning of the blood vessels can be observed, establishing the placental circulation.
This period of time is called the embryonic period and encompasses the processes of organ formation for each of the aforementioned germ layers..
In these weeks the formation of the main systems occurs and it is possible to visualize the external bodily characters. From the fifth week on, the changes in the embryo decrease to a great extent, compared to the previous weeks.
The ectoderm originates structures that allow contact with the outside, including the central nervous system, the peripheral and the epithelia that make up the senses, skin, hair, nails, teeth and glands..
The mesoderm is divided into three: paraxial, intermediate and lateral. The first originates a series of segments called somitomeres, from which arises the head and all the tissues with support functions. In addition, the mesoderm produces the vascular, urogenital, and adrenal glands..
The paraxial mesoderm is organized into segments that form the neural plate, the cells form a loose tissue called mesenchyme and give rise to tendons. The intermediate mesoderm originates the urogenital structures.
The endoderm constitutes the “roof” of the yolk sac and produces the tissue that lines the intestinal, respiratory and urinary bladder tracts..
In more advanced stages this layer forms the parenchyma of the thyroid gland, paratirodies, liver and pancreas, part of the tonsils and the thymus, and the epithelium of the tympanic cavity and the auditory tube..
The third week is characterized by villous growth. The chorionic mesenchyme is invaded by already vascularized villi called tertiary villi. In addition, Hofbauer cells are formed that fulfill macrophage functions..
In week number four the notochord appears, a cord of cells of mesodermal origin. This is responsible for indicating to the cells above that they will not be part of the epidermis.
In contrast, these cells originate a tube that will form the nervous system and constitute the neural tube and the cells of the neural crest..
The anteroposterior embryonic axis is determined by the genes of the homeotic box or genes Hox. They are organized in several chromosomes and present spatial and temporal collinearity.
There is a perfect correlation between the 3 'and 5' end of its location on the chromosome and the anteroposterior axis of the embryo. Likewise, genes at the 3 'end occur earlier in development..
This period of time is called the fetal period and encompasses the maturation processes of organs and tissues. Rapid growth of these structures and of the body in general occurs.
The growth in terms of length is quite pronounced in the third, fourth and fifth months. In contrast, the weight gain of the fetus is considerable in the last two months before birth..
The size of the head undergoes a particular growth, being slower than the body growth. The head represents almost half of the total size of the fetus in the third month.
As its development progresses, the head represents a third part until the moment of delivery arrives, when the head only represents a fourth part of the baby.
The features are taking on an increasingly human-like appearance. The eyes take their final position on the face, located ventrally and not laterally. The same happens with the ears, positioning themselves to the sides of the head.
The upper limbs reach a significant length. In the twelfth week the genitalia have developed to such an extent that the sex can already be identified by an ultrasound.
The increase in terms of length is evident and can reach up to half the length of an average newborn baby, plus or minus 15 cm. Regarding the weight, it still does not exceed half a kilo.
At this stage of development you can already see hair on the head and the eyebrows also appear. In addition, the fetus is covered with a hair called lanugo.
The skin takes on a reddish and wrinkled appearance, caused by a lack of connective tissue. Most of the systems have matured, with the exception of the respiratory and nervous.
Most fetuses born before the sixth month fail to survive. The fetus has already reached a weight greater than one kilo and measures about 25 cm.
Subcutaneous fat deposits occur, helping to round the baby's contour and eliminating skin wrinkles.
The sebaceous glands begin to produce a whitish or greyish substance of a lipid nature called vernix caseosa, which helps to protect the fetus..
The fetus can weigh between three and four kilos, and measure 50 centimeters. As the ninth month approaches, the head acquires a greater circumference in the skull; this feature helps the passage through the birth canal.
In the week prior to birth, the fetus is able to consume the amniotic fluid, remaining in its intestines. Its first evacuation, of blackish and sticky appearance, consists of the processing of this substrate and is called meconium.
Yet No Comments