✅ PERIODO EMBRIONARIO | DE LA TERCERA A LA OCTAVA SEMANA 📚 ORGANOGÉNESIS | CAPAS GERMINALES

From a flat structure to a cylindrical trilaminar disk to all the apparatus and systems phenomena such as neurulation, the formation of somites, angiogenesis and the folds of the embryo occur during the embryonic period. Hello, how are you? Welcome once again to my YouTube channel.

This time I bring you a video entitled "From the third to the eighth week, embryonic period" and we will emphasize the derivatives of the germ layers that were formed with Gastrulation and that we had seen in the video of the third week Please note that while we will name the most organs and tissues derived from these layers only will delve into derivatives that Langman and Moore developed in detail in recent weeks the same way in the channel I have several videos about the devices and systems that arise from these layers so I am going to be adding them as cards at the top right so that they can access in a faster and easier way. To start the topic, I remind you of this table that we had seen in the introduction video to human embryology In it, we can see that prenatal development consists of 3 periods, one pre-embryonic, another embryonic and a period fetal According to the literature that you use at the time to study the embryonic period can vary from week starting For example for Langman and Moore embryonic period occurs from the third to the eighth week in exchange for Hib and Carlson this period extends from the fourth to the eighth week. On the other hand, Arteaga and Flores try not to be in conflict with any of these books and have an intermediate position and saying that this period comprises from the end of the third week to the end of the eighth week of development You choose the team you like the most or the one you use at your university and let me know in the comments!

The embryonic period or also called the period of organogenesis is one in which the three germ layers ectoderm, mesoderm and endoderm give rise to the different tissues and organs of the body. However, despite the fact that the organs have begun to form and some even begin their function still need to acquire greater maturity to allow them to prepare for their postnatal life and this maturity, they will reach it during the fetal stage which we will see in a future video On the other hand, this period of prenatal development is also known as that in which the embryo is susceptible to teratogenic agents These agents, such as some drugs and viruses, can cause some delay or alterations in embryonic development, leading to major anomalies or even the death of the embryo itself We will start in the third week And so that we can locate ourselves where we are and where the changes that we are going to study take place I used these excellent images of the Netter First of all here we have a sagittal section of a gravid uterus where we can observe the location of implantation. If we get even closer, we will see in detail the implantation within the maternal endometrium, this is the fixation pedicle and we can also observe the extraembryonic somatic mesoderm covering the amniotic cavity and the yolk sac covered by the extraembryonic splanchnic mesoderm Now, we are going to make 2 transverse cuts One at the level of the amniotic cavity and the other at the level of the definitive yolk sac and we are going to observe it from a dorsal view towards the ectoderm From this image we are going to develop the derivatives of the germ layers We will start with those derived from the ectodermal germ layer and remember the Notochord As the notochord develops, it leads to the induction of the overlying embryonic ectoderm that is located in the middle line and close to this This produces the thickening of the ectoderm and s e difference in neural plate If we make a cross - section this would level observing the notochord, to the ectoderm and the neural plate cells that are located in this plate are called neuroectoderm and induction represents the beginning of the process called Neurulation Neurulation is the process by which the neural plate forms the neural tube Initially the neural plate is the same length as the underlying notochord but as the notochord increases in length the neural plate grows in a cephalo-caudal direction and undergoes a widening Upon completion the third week its lateral edges rise to form the neural folds and the middle region of the neural plate sinks and originates the neural groove In a cross section the neural groove and the neural fold would be seen in this way The neural folds are prominent in the cranial end of the embryo and gradually begin to move toward the midline to fuse and for the neural tube The fusion begins in the cervical region and continues both cephalad and caudad As the fusion is completed at the cephalic and caudal ends are the neuropores that communicate with the amnionite cavity The anterior or rostral neuropore and the posterior or caudal neuropore The closure of the anterior neuropore occurs around day 25 while the posterior neuropore closes on day 28 When these neuropores do not close they give their respective malformations With this, neurulation and the neural tube are completed in its development end will give rise to the central nervous system, retina, the pineal gland or epiphyses neurohypophysis following the neurulation ectoderm is segmented into three cell groups the remaining directly in the neural tube and is known as neuroectodermal who covers the neural tube on the outside and is located on the surface called superficial ectoderm or non-neural ectoderm AND the ectod ermo which differs in cells of neural crest cells of the neural crest are cells of the lateral edge of the neuroectoderm that separates from its neighbors at the time when the neural folds rise and fuse to form the neural tube These cells they undergo a transition from epithelium to mesenchyme by active migration and displacement as they leave the neuroectoderm and then manage to penetrate the underlying mesoderm From there, they migrate to different areas or embryonic structures and their mesenchyme is known as ectomesenchyme that differentiates into: Connective tissue and bone of the face and skull in the ganglia of the cranial nerves C cells of the Thyroid gland Trunk- conal septum of the heart Dermis of the face and neck In the dorsal roots of the spinal ganglia In the ganglia of the sympathetic chain and pre-aortic ganglia The adrenal medulla Parasympathetic ganglion of the gastrointestinal tract In Schwann cells and glial cells Le ptomeninges Melanocytes Odontoblasts Y in smooth muscle cells for the blood vessels of the face and forebrain.

As you will see, the cells of the neural crest contribute to the formation of many organs and tissues of the body to such an extent that some authors sometimes call it the fourth layer germinal In the superficial ectoderm when the neural tube closes , 2 bilateral thickenings appear in the cephalic region of the embryo that are called otic placodes and the crystalline placodes. The otic placodes invaginate and form the optic vesicles that will become the structures necessary for hearing and balance AND the placodes of the lens also invaginate to constitute the lens vesicles and during the fifth week they constitute the lens These two phenomena mark the beginning of the embryology of the eye and ear videos that I already have uploaded in the canal by if you want to go see. In turn, this superficial ectoderm also gives rise to the to epidermis, hair, nails, skin glands and mammary glands, adenohypophysis and tooth enamel Regarding the derivatives of the mesodermal germ layer We return to the initial image and we will make a cross section at this level we can observe that at the beginning the cells that make up the mesoderm form a thin sheet of loose tissue on each side of the notochordal process.

However , around day 17 the cells that are near the notochord proliferate and constitute a thickened tissue plate that receives the name of Paraxial Mesoderm Lateral to the paraxial mesoderm, it continues with the intermediate mesoderm and it undergoes a gradual thinning until it becomes the layer of the lateral Mesoderm And the lateral mesoderm by coalescence of cavities produces its division into 2 sheets that are continued with the extraembryonic mesoderm that covered the yolk sac and the amniotic cavity At the beginning of the third week, the paraxia mesoderm l begins to organize segments These segments are called somitómeros and first appears in the cephalic region of the embryo Their formation occurs in craniocaudal direction and each somitómero comprises mesodermal cells arranged in concentric spirals around the center of the structure in the Cephalic region the somitomers are formed in relation to the segmentation of the neural plate to constitute neuromeres and these contribute to the mesenchyme of the head However, from the occipital region to the caudal region the somitomers are organized into somites The first pair of somites appears in the occipital region of the embryo around day 20 of development From there, new somites emerge in cranio-caudal sequence at an approximate rate of three pairs per day until the end of the fifth week when there are already 42 to 44 pairs. At this stage the somites they are distributed in this way four occipital pairs, eight cervical, twelve thoracic, five lumbar, c inco sacros and between 8 to 10 coccygei Of these, the first occipital pair and the last five to seven coccygei disappear later and the rest of the somites constitute the axial skeleton If we make a transverse section at the level of a somite we will observe that each somite It has a triangular shape and this is subclassified into 3 segments In sclerotome, myomotome and dermatome If we focus on the ventromedial region of the somite, the sclerotome is identified and as its cells develop they become polymorphic at the end of the fourth The sclerotome cells will form bones of the axial skeleton such as ribs, vertebrae and some bones of the neurocranium It even participates in the formation of tendons On the other hand, we have the myotome that originated the skeletal muscle and the dermatome in turn originated the dermis The intermediate mesoderm that temporarily connects the mesoderm paraxial with the lateral mesoderm differentiates in the urogenital structures In the regions cervical and upper thoracic gives rise to nefrotomas future, while in flow direction forms the nephrogenic cord These two events mark the beginning of the embryology of the genital system and urinary system videos that have already uploaded on the channel as important fact and very asked the The bladder is the only organ of the urogenital system that does not derive from the intermediate mesoderm The lateral mesoderm or lateral plate is made up of cells that migrated beyond the intermediate mesoderm Due to the appearance of clefts that are called coelomic spaces and coalescence of The lateral mesoderm is made up of 2 layers On the one hand we have the somatic or parietal layer that is adjacent to the superficial ectoderm and continues with the layer of the extra-embryonic parietal mesoderm surrounding the amniotic cavity This parietal layer of the lateral mesoderm together with the overlying ectoderm are called Somatopleura. On the other hand, we find to the visceral layer that is adjacent to the endoderm and in continuity with the visceral layer of the extra-embryonic mesoderm that lines the yolk sac The visceral layer of the lateral mesoderm together with the underlying endoderm constitute the Splachnopleura The parietal mesoderm will give rise to the parietal leaves of the peritoneal, pleural and pericardial walls including the bones of the extremities, the shoulder girdle, the pelvic and the sternum The splanchnic mesoderm will form the visceral sheets of the peritoneal, pleural and pericardial walls It will also give rise to the stroma of some viscera, the smooth muscle covering some viscera, to the muscle of the heart and in the third week, the mesoderm invades the vitenilo sac to begin forming blood as a result of the coalescence of the coelomic spaces will give rise to a longitudinal single space called intraembryonic celoma this space is important because at the end of the embryo's folding c It constitutes 3 types of body cavities: the pericardial cavity, the pleural cavities and the peritoneal cavity.

An aspect that should be highlighted is that in the third week the development of the heart, blood vessels and blood cells begins. Here we have a dorsal view of an embryo of approximately 18 days and we make a cross section to better locate ourselves The cells destined to form the heart derive from the splanchnic or visceral mesoderm that are located cranially to be located in front of the oropharyngeal membrane and the neural folds This area is shaped of an inverted U and is called Primary Cardiogenic Field And its development can be seen in my video of embryology of the heart Blood cells and blood vessels also originate from the mesoderm AND their development begins from the beginning of the third week when the cells of the extra-embryonic splanchnic mesoderm surrounding the yolk sac, differentiating into cells pre cursors of the blood and blood vessels called hemoangioblasts These cells first group in the form of islets of cells that gradually channel these islets are called Wolf and Pander islets or blood islets And the central cells of the islets become precursors of blood cells while the cells that are in the periphery flatten and form the endothelial cells that line blood vessels in formation These islets or blood cell groups emit buds through the process called Budding and approach each other, interconnecting and giving rise to a network of small vessels. Primitive blood cells arise in the yolk sac enter the circulation at day 22 to about 6 or 7 weeks after undergo programmed cell death as the embryo develops, and are replaced by blood cells of fetal origin As Importantly In this case, the yolk sac is the first hematopoietic organ of the embryo, but of the extra-embryonic type.

And take into account that the formation of blood cells and capillaries also takes place in the fixation pedicle and in the chorionic plate. This must be two concepts very important mechanism by which the vessels arise from blood islands called vasculogenesis and the mechanism involving gemación from vessels existing known as Angiogenesis If you ask them What are the mechanisms by which blood vessels are formed? You answer these 2 With respect to the derivatives of the enddermal germ layer in this cross section of a 19-day embryo we see how the endoderm covers the ventral surface of the embryonic disc and forms the roof of the yolk sac.

The main derivative of this layer is the gastrointestinal tract and it will be formed during the cephalocaudal and lateroventral folds of the embryo Let's look at this sagittal section and observe how the cephalocaudal folding is mainly produced by the longitudinal growth of the central nervous system, so the embryo flexes and takes an arc shape This folding is much more marked in the cephalad and caudal regions later cephalic fold and caudal fold Now, back to the cross section and see that the passing days will form the lateroventral folding occurs during formation of somites and these make the embryo take a cylindrical or tubular shape leaving the endoderm included inside the embryo This endoderm constitutes the primitive intestine or also called intestinal tube As a result of the movements of the folds, the communication between the embryo and the yolk sac is progressively narrowing, until they communicate through a narrow duct called omphalomesenteric duct In this primitive formed intestine, 3 portions can be distinguished One anterior or cephalic that will give rise to the foregut One caudal that will give rise to the posterior intestine And the part between both is the midgut that is communicates temporarily with the yolk sac through the omphalomesenteric duct At the cephalic end of the foregut is the oropharyngeal membrane which during the 4th week fenestrates establishing a connection between the amniotic cavity and the primitive intestine. finds the cloacal membrane which ruptures in the seventh week to form the orifice of the anus Another important result that produces the cephalocaudal and lateral folding is the partial incorporation of the allantois into the body of the embryo in which it forms the cloaca The distal portion of the allantois remains on the fixation pedicle n and already in the fifth week, the yolk sac, the allantois and the umbilical vessels are limited to the region of the umbilical ring and are part of the first portion of this structure. As development continues, the endoderm gives rise to the epithelial covering of the respiratory system that includes the trachea, bronchi and lungs the parenchyma of the thyroid and parathyroid glands, liver and pancreas the reticular stroma of the tonsils and the thymus the epithelial lining of the urinary bladder and urethra AND the epithelial lining of the tympanic cavity and duct auditory Finally I am going to show you these tables that summarize the derivatives of the germ layers you can pause the video, take a screenshot and use them when studying From the ectoderm we could find the superficial ectoderm and the neuroectoderm and from the latter the neural tube and neural crest cells These are their respective derivatives The mesoderm divided in paraxial, intermediate and lateral mesoderm Some authors such as Moore and Arteaga differentiate a fourth mesoderm called head mesoderm and it is the one that we had said was formed from neuromeres These are its derivatives And finally the endoderm that Unlike the other two, it does not have sub-classification and its derivatives are the following These were the derivatives of the 3 geminal layers in another video I will do the external aspects that are observed during the embryonic period If you liked the video, give it a like and share it with your colleagues and see you in a new video of human embryology.

Greetings and successes in your exams.