Embryo Development Lecture 5 PDF

Embryo development at 3 rd & 4 Week of th gestation Dr. Gangadhara Swamy M.S. Ph.D. Fertilization Cleavage Blastocyst  Amnion Chorion  3rd & 4th week of embryo developme...

Embryo development at 3 rd & 4 Week of th gestation Dr. Gangadhara Swamy M.S. Ph.D. Fertilization Cleavage Blastocyst  Amnion Chorion  3rd & 4th week of embryo development Transformation from 2 – 3 germ layers – Gastrulation Formation of the Notochord Development of Placenta & the Umbilical cord Embryonic folds  Objectives Describe the Gastrulation Explain the Notochord & Process of its formation Describe the Embryonic folds Describe the embryonic cavities  Gastrulation Transformation from 2 – 3 germ layers – Gastrulation  After the formation of the 2 germ layers, at this stage, the embryo proper is a circular disc composed of two layers of cells: (1) the Ectoderm (toward amniotic cavity) (2) the Endoderm (toward yolk sac) There is no indication yet of a head or tail end of the embryonic disc Formation of prochordal plate: At one circular area near the margin of the disc, the cubical cells of the endoderm become columnar. This area is called the prochordal plate.  Formation of primitive streak: A transient structure that forms in the blastula during the early stages of embryonic development. Soon after the formation of the prochordal plate (15th day of development) some of the ectodermal cells lying along the central axis, near the tail end of the disc and on the dorsal aspect of the embryo, begin to proliferate, and form an elevation that bulges into the  Formation of intraembryonic mesoderm The cells that proliferate in the region of the primitive streak pass sideways, pushing themselves between the endoderm and ectoderm. These cells form the intraembryonic mesoderm (or secondary mesoderm).  Extensions of intraembryonic mesoderm The intraembryonic mesoderm spreads throughout the disc except in the region of the prochordal plate. Some intraembryonic mesoderm arising from the primitive streak passes backward into the connecting stalk. As it does so, it leaves an area caudal to the primitive streak, where ectoderm and endoderm remain in contact (i.e. mesoderm does not separate them). This region is, therefore, similar to As the embryonic disc enlarges in size, and also elongates, the connecting stalk becomes relatively small, and its attachment becomes confined to the region of the tail end of the embryonic disc. Some intraembryonic mesoderm arising from the primitive streak passes backward into the connecting stalk.  Clinical Applications Embryonic stem cells (ESCs): The cells of inner cell mass have the potential to differentiate into three different germ layers (ectoderm, endoderm and mesoderm). All the cells, tissues and organs of the body are formed from these three layers. Because of this the cells of the inner cell mass are called embryonic stem cells. Pluripotent cells: Embryonic stem cells can be maintained and propagated in an undifferentiated state, in culture, and in laboratories. If these cells are exposed to certain specific growth factors, in culture, the stem cells can form various types of adult cells, e.g. neurons, muscle cells, blood cells, and cartilage cells. These cells are therefore said to be pluripotent. Embryonic stem cells therapy: This technique has tremendous potential for treatment of various diseases. Some of these are Parkinson’s disease, Alzheimer disease, diabetes, myocardial infarction, blood diseases, severe burns, osteoporosis, spinal cord injury, to name but a few. Therapeutic stem cell cloning: However, in the ESC therapy, the complication of immune rejection is always present as the genetic constitution of stem cell is different from that of patient. To overcome this problem scientists are working on “therapeutic stem cell cloning”. In this procedure the nucleus of patient cell is introduced in the embryonic stem cell. These cells are then allowed to grow in any tissue of the patient. As the tissues arising from the stem cells are now genetically identical to those of the patient rejection is avoided.  Notochord The notochord is a midline structure that develops in the region lying between the cranial end of the primitive streak and the caudal end of the prochordal plate.  During its development, the notochord passes through several stages that are as The follows: cranial end of the primitive streak becomes thickened. This thickened part of the streak is called the primitive knot, primitive node or Henson’s node. A depression appears in the center of the primitive knot. This depression is called the blastopore/primitive pit. Cells in the primitive knot multiply and pass cranially in middle line, between the ectoderm and endoderm, reaching up to the caudal margin of the prochordal plate. These cells form a solid cord called the notochordal process or head process. The cavity of blastopore extends into the notochordal process and converts it into a tube called the notochordal canal.  The floor of the notochordal canal begins to break down. At first, there are small openings formed in it, but gradually the whole canal comes to communicate with the yolk sac. The notochordal canal also communicates with the amniotic cavity through the blastopore. Thus, at this stage, the amniotic cavity and the yolk sac are in communication with each other. Gradually the walls of the canal become flattened so that instead of a rounded canal we have a flat plate of cells called the notochordal plate. However, this process of flattening is soon reversed and the notochordal plate again becomes curved to assume the shape of a tube (Fig. 5.4F). Proliferation of cells of this tube converts it into a solid rod of cells. This rod is the definitive (i.e. finally formed) notochord. It gets completely separated from the endoderm. SUBDIVISIONS OF INTRAEMBRYONIC MESODERM The intraembryonic mesoderm now becomes subdivided into three parts, 1. Mesoderm, on either side of the notochord, becomes thick and is called the paraxial mesoderm. 2. More laterally, the mesoderm forms a thinner layer called the lateral plate mesoderm. 3. Between these two, there is a longitudinal strip called the  At first, the cells of the paraxial mesoderm are homogenously arranged. Later, the mesoderm gets segmented. The segments are of two categories: (1) Somitomeres and (2) Somites. Changes are also occurring in the lateral plate mesoderm. Small cavities appear in it. These coalesce (come together) to form one large cavity, called the intraembryonic coelom. With the formation of the intraembryonic coelom, the lateral plate mesoderm splits into: –– Somatopleuric or parietal layer intraembryonic mesoderm that is in contact with ectoderm. –– Splanchnopleuric or visceral layer of intraembryonic mesoderm that is in contact with endoderm.  The cavity has the shape of a horseshoe. There are two halves of the cavity (one on either side of the midline) which are joined together cranial to the prochordal plate. At first, this is a closed cavity but soon it comes to communicate with the extraembryonic coelom. The intraembryonic coelom gives rise to pericardial, pleural, and peritoneal cavities. Note that the pericardium is formed from that part of the intraembryonic coelom that lies, in the midline, cranial to the prochordal plate.  % The heart is formed in the splanchnopleuric mesoderm forming the floor of this part of the coelom. This is, therefore, called the cardiogenic area (also called cardiogenic plate, heart-forming plate). Cranial to the cardiogenic area (i.e. at the cranial edge of the embryonic disc) the somatopleuric and splanchnopleuric mesoderms are continuous with each other. The mesoderm here does not get split, as the intraembryonic coelom has not extended into it. This unsplit part of intraembryonic mesoderm forms a structure called the septum transversum. In future, septum transversum develops into diaphragm & Liver Embry Foldings  The changes that now take place will be best understood by a careful study. Note the following: There is progressive increase in the size of the embryonic disc due to rapid growth of cells of central part of embryonic disc and rapid growth of somites. This causes conversion of flat pear-shaped germ disc into a cylindrical embryo. The head and tail ends of the disc (X and Y), however, remain relatively close together. Hence, the increased length of the disc causes it to bulge upward into the amniotic cavity. With further enlargement, the edges of embryonic disc become folded on itself in the median and in the transverse planes. The folding in the median plane form ventrally directed head fold and tail fold. The folding in the transverse plane forms ventrally directed lateral folds. Cephalocaudal folding is due to rapid and longitudinal growth of central nervous system. Lateral foldings are due to rapid growth of somites and convert the embryo into a tubular structure. These are not three separate folds but occur simultaneously and merge into one  With the formation of the head and tail folds, parts of the yolk sac become enclosed within the embryo. In this way, a tube lined by endoderm is formed in the embryo. This is the primitive gut, from which most of the gastrointestinal tract is derived. At first, the gut is in wide communication with the yolk sac. The part of the gut cranial to this communication is called the foregut; the part caudal to the communication is called the hindgut; while the intervening part is called the midgut. The communication with the yolk sac becomes progressively narrower. As a result of these changes, the yolk sac becomes small and inconspicuous, and is now termed the definitive yolk sac (also called the umbilical vesicle). The narrow channel connecting it to the gut is called the vitellointestinal duct (also called vitelline duct; yolk stalk or omphalomesenteric duct).  As the head and tail folds are forming, similar folds are also formed on each side in transverse or horizontal plane. These are the lateral folds. As a result, the embryo comes to be enclosed all around by ectoderm except in the region through which the vitellointestinal duct (omphalomesenteric duct) passes. Here, there is acircular aperture which may now be called the umbilical opening. The folding facilitates growth and expansion of amniotic cavity that comes to surround the embryo on all sides. In this way, the embryo now floats in the amniotic fluid, which fills the cavity. Convergence of folds on ventral surface forms tubular investment of amnion for connecting stalk. This causes obliteration of extraembryonic coelom. Now, the amnion forms a covering for the umbilical cord. The stomodeum is an ectodermal depression at the head end between the bulging head and pericardial bulge. The buccopharyngeal membrane breaks at 4th week and the cloacal membrane at 7th week.  Just before the formation of the head and tail folds, the structures in the embryonic disc are oriented, as. A median (midline) section across the disc, at this stage From the cranial to the caudal side, the structures seen in the midline are the: Septum transversum Developing pericardial cavity and the heart Prochordal plate Neural plate Primitive streak Cloacal membrane. Note that the primitive streak is now inconspicuous.  After folding, the relative positions of these structures change. The important points to note here are as follows: With the formation of the head fold, the developing pericardial cavity comes to lie on the ventral side of the embryo, ventral to the foregut. The heart, which was developing in the splanchnopleuric mesoderm in the floor of the pericardial cavity, now lies in the roof of the cavity. The pericardium enlarges rapidly and forms a conspicuous bulging on the ventral side of the embryo. The septum transversum, which was the most cranial structure in the embryonic disc, now lies caudal to the heart. At a later stage in development, the diaphragm and liver develop in relation to the septum transversum. The region of the prochordal plate now forms the buccopharyngeal or oral membrane, which closes the foregut cranially. When this membrane breaks down, the foregut communicates with the exterior.  The most cranial structure of the embryo is now the enlarged cranial part of the neural tube, which later forms the brain. This enlarges enormously. There are now two big bulgings on the ventral aspect of the embryo. Cranially, there is the developing brain, and a little below it there is the bulging pericardium. In between these two, there is a depression called the stomatodeum or stomodeum, the floor of which is formed by the buccopharyngeal membrane. Toward the tail end of the embryo, the primitive streak is now an inconspicuous structure that gradually disappears. The distal end of the hindgut is closed by the cloacal membrane. At first, this is directed caudally, but later it comes to face ventrally. We have traced the development of the embryo to a stage when the rudiments of the nervous system, the heart and the gut have been formed. We are now in a position to trace the development of individual organ systems in detail. Cavities relation with the embryonic folds Time table of developmental events Age in days Developmental events Primitive streak appears. Definitive yolk sac 15 is formed Notochordal process appears. Heart tube is seen in 17 cardiogenic area. Allantoic diverticulum is seen. Intraembryonic mesoderm is being formed. 19 Connecting stalk can be distinguished. Neural groove is seen. Head fold begins to 21 form. 23 Closure of the neural tube. Thank you