Embryonic Development PDF

Chapter 5 Further Development of Embryonic Disc HIGHLIGHTS undergoes folding at the cranial and caudal e n d s. T h e s e are the hea...

Chapter 5 Further Development of Embryonic Disc HIGHLIGHTS undergoes folding at the cranial and caudal e n d s. T h e s e are the head and tail folds The cranial end of the primitive streak enlarges (Fig. 5.7). Lateral folds also appear. As a result to form the primitive knot (Fig. 5.1). of these folds the endoderm is converted into a Cells of the primitive knot multiply and pass tube, the gut. It is divisible into foregut, midgut cranially to form a rod-like structure reaching and bindgut. up to the p r o c h o r d a l p l a t e. T h i s is the After formation of the head fold the gut is closed notochordal process. cranially by the prochordal plate, which is now The notochordal process undergoes changes called the buccopharyngeal membrane. Caudally, that convert it first into a canal and then into the gut is closed by the cloacal membrane. a p l a t e , a n d finally back into a r o d - l i k e The umbilical cord d e v e l o p s from the structure. This is the notocbord. connecting stalk. It contains the right and left Most of the notochord disappears. Remnants umbilical arteries, the left umbilical vein, and r e m a i n as the nucleus pulposus of each remnants of the vitello-intestinal duct and yolk intervertebral disc. sac. The ground substance of the umbilical cord A w i d e s t r i p of e c t o d e r m o v e r l y i n g the is made up of Wharton's jelly derived from notochord becomes thickened and forms the mesoderm. The cord is covered by amnion. neural plate (Fig. SAC) from which the brain The allantoic diverticulum arises from the yolk and spinal cord develop. sac before formation of the gut (Fig. 5.10). After Intra-embryonic mesoderm shows three f o r m a t i o n of the tail fold, it is seen as a subdivisions (Fig. 5.4D). The mesoderm next diverticulum of the hindgut. t o the m i d d l e line is called t h e paraxial The pericardial cavity is derived from part of mesoderm. It undergoes segmentation to form the intra-embryonic coelom that lies cranial somites. The mesoderm in the lateral part of to the prochordal plate (Fig. 5.11). The deve- the embryonic disc is called the lateral plate l o p i n g h e a r t lies v e n t r a l to t h e cavity mesoderm. A cavity called the infra-embryonic (Fig. 5.12). After formation of the head fold coelom appears in it and splits the mesoderm the pericardial cavity lies ventral to the foregut; into a somatopleuric layer (in contact with and the developing h e a r t is d o r s a l to the ectoderm) and a splanchnopleuric layer (in pericardial cavity (Fig. 5.13). contact with endoderm) (Fig. 5.5D). A strip of The septum transversum is made of intra- mesoderm between the lateral plate mesoderm embryonic mesoderm that lies cranial to the and the p a r a x i a l m e s o d e r m is called the pericardial cavity (Figs 5. 1 1 , 5. 1 2 ). After intermediate mesoderm. formation of the head fold it lies caudal to the The intra-embryonic coelom later forms the pericardium and heart (Fig. 5.13). The liver pericardial, pleural and peritoneal cavities. and the diaphragm develop in relation to the T h e e m b r y o n i c disc, which is at first flat, septum transversum.  Further Development of Embryonic Disc the streak is called the primitive knot, FORMATION OF THE NOTOCHORD primitive node or Benson's node (Figs 5.1 A, The notochord is a midline structure, that develops 5.2A). in the region lying between the cranial end of the 2. A depression appears in the centre of the p r i m i t i v e s t r e a k a n d t h e c a u d a l end of the primitive knot. This depression is called the prochordal plate (Figs 4.12, 5. 1 , 5.2). During its blastopore (Figs 5.IB, 5.2B). development, the notochord passes through several 3. Cells in the primitive knot multiply and pass stages that are as follows: cranially in the middle line, between the ectoderm and endoderm, reaching up to the The cranial end of the primitive streak caudal margin of the prochordal plate. These becomes thickened. This thickened part of cells form a solid cord called the notochordal Prochordal plate Prochordal Primitive Primitive plate i Primitive knot -Primitive streak Cloacal membrane Cloacal membrane Blastopore B I^H Notochordal process Notochordal canal Fig. 5.2 Sections through the embryonic disc along the axis KL shown in Fig. 5.1 A to Fig. 5.1 Formation of primitive knot (A), illustrate the formation of the primitive blastopore (B) and notochordal process knot (A), blastopore (B), notochordal (C). Note that the notochordal process is process (C) and notochordal canal (D). deep to ectoderm and that its position is Stages A, B and C correspond to those shown diagrammatically. of Fig. 5.1.  Human Embryology process or bead process (Figs 5.1C, 5.2C, Some details of the process of formation of the 5.3A). The cells of this process undergo notochord are as follows: several stages of rearrangement (Figs 5. 1 , 5.2) ending in the formation of a solid rod 1. After the formation of the blastopore, its called the notochord. cavity extends into the n o t o c h o r d a l p r o c e s s , a n d c o n v e r t s it i n t o a t u b e As the embryo enlarges the notochord elongates called the notochordal canal (Figs 5.2D, considerably and lies in the midline, in the position 5.3B). to be later occupied by the vertebral column. 2. T h e cells f o r m i n g t h e floor of t h e However, the notochord does not give rise to the notochordal canal become intercalated in vertebral column. Most of it disappears, but parts (i.e. become mixed up with) the cells of of it persist in the region of each intervertebral the e n d o d e r m (Fig. 5. 3 C ). T h e cells disc as the nucleus pulposus. forming the floor of the notochordal canal now separate the canal from the cavity of the yolk sac. 3. The floor of the notochordal canal begins to break d o w n. At first there are small MMMMm (MfflSQMfflB openings formed in it, but gradually the UZUJ-UJ* TXXXJJXD whole canal comes to communicate with QDUJJJJ-XLU the yolk sac (Fig. 5.3D). The notochordal Notochordal process canal also communicates with the amniotic Intra-embryonic cavity through the blastopore. Thus, at this mesoderm stage, the amniotic cavity and the yolk sac a r e in c o m m u n i c a t i o n with e a c h other. axtxamammw ffffflfflTffllfflffl-ffl 4. 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 canal Notochordal plate notochordal plate (Fig. 5.3E). 5. However, this process of flattening is soon C F reversed and the notochordal plate again becomes curved, to assume the shape of a tube (Figs 5.3F, G). Proliferation of cells of this tube converts it into a solid rod of cells. This rod is the definitive (i.e. finally G formed) notochord. It gets completely separated from the endoderm. The notochord is present in all animals that Definitive notochord belong to the phylum Chordata. In some of them, e.g. Amphioxus, it persists into adult life and Fig. 5.3 Transverse sections through the forms the central axis of the body. In others, embryonic disc (along the axis XY shown i n c l u d i n g m a n , it a p p e a r s in t h e e m b r y o in Fig. 5.1C) to illustrate stages in the but only small remnants of it remain in the formation of the notochord. adult.  Further Development of Embryonic Disc F O R M A T I O N O F THE NEURAL TUBE and (b) a caudal tubular part that forms the spinal cord. The details of the formation of the neural tube 4. In early embryos, the developing brain will be studied later. For the time being, it may be forms a large conspicuous mass, on the noted that: dorsal aspect. 1. The neural tube gives rise to the brain and The process of formation of the neural tube is the spinal cord. referred to as neurulation. 2. The neural tube is formed from the ectoderm overlying the notochord and, therefore, S U B D I V I S I O N S O F INTRA- extends from the prochordal plate to the EMBRYONIC MESODERM primitive knot (Fig. 5.4C). 3. The neural tube is soon divisible into: (a) a We have seen that the intra-embryonic mesoderm cranial enlarged part that forms the brain, is formed by proliferation of cells in the primitive Prochordal plate Neural plate Intermediate mesoderm Primitive streak Paraxial mesoderm Neural plate Ectoderm Extra-embryonic Lat. plate mesoderm Lat. plate mesoderm mesoderm m m ly.v. termediate mesoderm Notochord mesoderm Fig. 5.4 Subdivisions of intra-embryonic mesoderm. 'B' and 'D' are transverse sections across axes XY (in A) and MN (in C) respectively. Note that the notochord and mesoderm are deep to the ectoderm. Their position is shown diagrammatically.  Human Embryology streak and that it separates the ectoderm and the coelom (Fig. 5.5C}. With the formation of the intra- e n d o d e r m , e x c e p t in t h e following r e g i o n s : embryonic coelom, the lateral plate mesoderm (a) prochordal plate fb) cloacal membrane, and splits into: (c) in the midline caudal to the prochordal plate, as this place is occupied by the notochord. 1. Somatopleuric or parietal, intra-embryonic Cranial to the prochordal plate, the mesoderm mesoderm that is in contact with ectoderm. of the two sides meets in the midline (Fig. 4.12). 2. Splanchnopleuric, or visceral, intra- At the edges of the embryonic disc, the intra- embryonic mesoderm that is in contact with e m b r y o n i c m e s o d e r m is c o n t i n u o u s with the endoderm (Figs 5.5B, D}. extra-embryonic mesoderm (Fig. 5AD). The intra- embryonic mesoderm now becomes subdivided T h e i n t r a - e m b r y o n i c c o e l o m gives rise t o into three parts (Fig. 5.4): pericardial, pleural, and peritoneal cavities. Their development will be considered later. For the time (A) T h e m e s o d e r m , o n e i t h e r side of t h e being note that the pericardium is formed from notochord, becomes thick and is called the that part of the intra-embryonic coelom that lies, paraxial mesoderm. in the midline, cranial to the prochordal plate. (B) M o r e laterally, the m e s o d e r m f o r m s a T h e heart is formed in the s p l a n c h n o p l e u r i c t h i n n e r l a y e r c a l l e d t h e lateral plate mesoderm forming the floor of this part of the mesoderm. coelom (Fig. 5.6). This is, therefore, called the ( Q Between these two, there is a longitudinal cardiogenic area (also called cardiogenic plate, strip called the intermediate mesoderm. heart-forming plate). Cranial to the cardiogenic The paraxial mesoderm now becomes area (i.e. at the cranial edge of the embryonic segmented into cubical masses called somitomeres disc} the s o m a t o p l e u r i c and splanchnopleuric which give rise to somites (also called metameres mesoderm are continuous with each other. The or primitive segments) (Figs 5.4C, D). The first mesoderm here does not get split, as the intra- somites are seen on either side of the midline, a embryonic coelom has not extended into it. This little behind the prochordal plate. More somites unsplit mesoderm forms a structure called the are formed caudally, on cither side of the deve- septum transversum (Fig. 5.6). loping neural tube. The somites have an interesting history which is considered in Chapter 7. YOLK SAC AND FOLDING OF EMBRYO FORMATION OF THE INTRA-EMBRYONIC COELOM The early history of the yolk sac has been traced in Chapter 4 (Figs 4.6, 4.7}. We have seen that the While the p a r a x i a l m e s o d e r m is u n d e r g o i n g primary yolk sac is bounded above by cubical segmentation, to form the somites, changes are endoderm of the embryonic disc and elsewhere by also occurring in the lateral plate mesoderm. Small flattened cells lining the inside of the blastocystic cavities appear in it. These coalesce (come together} cavity. With the formation of the extra-embryonic t o form o n e l a r g e cavity, c a l l e d the intra- mesoderm, and later the extra-embryonic coelom, embryonic coelom. The cavity has the shape of a the yolk sac becomes much smaller; it comes to horseshoe (Figs 5.5A, C). There are two halves of be lined all round by cubical cells; and it is then the cavity (one on either side of the midline} which called the secondary yolk sac. are joined together cranial to the prochordal plate. The changes that now take place will be best At first, this is a closed cavity (Fig. 5.5A) but soon understood by a careful study of Fig. 5.7. N o t e it comes to communicate with the extra-embryonic the following:  Further Development of Embryonic Disc Intra-embryonic coelom Neural groove Splanchnopleuric intra- Somatopleuric intra- embryonic mesoderm embryonic mesoderm Intra-embryonic mesoderm Intra-embryonic coelom opens into extra-embryonic coelom here. Amniotic cavity Intra-embryonic coelom Extra-embryonic coelom Fig. 5.5 Intra-embryonic c o e l o m. (B) and (D) are sections across axes XY in (A) and (C) respectively. (E) shows the relationship between the intra-embryonic and the extra-embryonic coeloms. There is progressive increase in the size of and tail ends. These are called the head and the embryonic disc. tail folds. The head and tail ends of the disc (X, Y), With the formation of the head and tail folds, however, remain relatively close together. parts of the yolk sac become enclosed within H e n c e , the increased length of the disc the embryo. In this way, a tube lined by causes it to bulge upwards into the amniotic endoderm is formed in the embryo. This is cavity. the primitive gut, from which most of the With further enlargement, the embryonic gastrointestinal tract is derived. At first, the disc becomes folded on itself, at the head gut is in wide communication with the yolk  Human Embryology bindgut; while the intervening Somatopleuric p a r t is c a l l e d t h e midgut Amiotic cavity , intra-embryonic (Fig. 5.7E). The communica- S mesoderm Prochordal di 3C T , tion with the yolk sac becomes Ectoderm i Intra-embryonic progressively narrower. As a i - " coelom result of these changes, the - — Future septum yolk sac becomes small and \ transversum i n c o n s p i c u o u s , a n d is n o w ~\~~~. Splanchnopleuric termed the definitive yolk sac / t ' intra-embryonic mesoderm (also called the umbilical Endoderm Yolk sac vesicle). The narrow channel c o n n e c t i n g it to the gut is Fig. 5.6 M i d l i n e section through cranial end of the embryonic disc to called the vitello-intestinal show the relationship of the pericardial cavity to other duct (also called vitelline duct, structures. yolk stalk or omphalomesen- teric duct). This duct becomes sac. T h e part of the gut cranial to this elongated and eventually disappears. communication is called the foregut, the part As the head and tail folds are forming, caudal to the communication is called the similar folds are also formed on each side. Amniotic cavity B O *Q Midgut Foregut Definitive yolk sac Fig. 5.7 Formation of head and tail folds and establishment of the gut.  Further Development of Embryonic Disc Embryonic Connecting Extra-embryonic disc stalk mesoderm Amniotic cavity Trophoblast A m n j0tic Extra-embryonic cavity Fig. 5.8 Stages in the establishment of the umbilical cord. These are the lateral folds. As a result, the is a circular aperture which may n o w be embryo comes to be enclosed all around by called the umbilical opening. ectoderm except in the region through which 6. As the embryonic disc folds on itself, the the vitello-intestinai duct passes. Here, there amniotic cavity expands greatly, and comes  Human Embryology to surround the embryo on all sides. In this 2. M e s o d e r m ( e x t r a - e m b r y o n i c ) of t h e way, the embryo now floats in the amniotic c o n n e c t i n g s t a l k. T h i s m e s o d e r m gets fluid, which fills the cavity. converted into a gelatinous substance called Wharton's jelly. It protects blood vessels in CONNECTING STALK the umbilical cord. 3. Blood vessels that pass from the embryo to While discussing the f o r m a t i o n of the e x t r a - placenta. embryonic coelom, we have seen that with the 4. A small part of the extra-embryonic coelom. formation of this cavity, the embryo (along with rhe amniotic cavity and yolk sac) remains attached This tube of amnion, and the structures within t o t h e t r o p h o b l a s t only by e x t r a - e m b r y o n i c it, constitute the umbilical cord (Fig. 5.9). This mesoderm into which the coelom does not extend cord progressively increases in length to allow free (Figs 5.8A - C). This extra-embryonic mesoderm movement of the e m b r y o within the amniotic forms the connecting stalk. We shall see later that cavity. At the time of birth of the child (i.e. at full the t r o p h o b l a s t , a n d the tissues of the uterus, term), the umbilical cord is about half a metre together form an important organ, the placenta, long, and about 2 cm in diameter. It shows marked which provides the growing embryo with nutrition torsion, which is probably due to fetal movements. and with oxygen. It also removes waste products An umbilical cord that is either too short or too from the embryo. The importance of the connecting long can cause problems during delivery of the stalk is obvious when we see that this is the only fetus. connecting link between the e m b r y o and the placenta. As the embryo grows, the area of attachment of the connecting stalk to it becomes relatively Umbilical vein smaller. Gradually this attachment is seen only Remnants of n e a r the c a u d a l e n d of t h e e m b r y o n i c disc vitello-intestinal duct (Figs5.8D,E). With the formation of the tail fold, Remnants of allantois the attachment of the connecting stalk moves (with the tail end of the embryonic disc) to the ventral aspect of the embryo. It is now attached in the region of the umbilical opening (Fig. 5.8E). Umbilical artery By now, blood vessels have developed in the embryo, and also in the placenta. These sets of Fig. 5.9 Section through umbilical cord. blood vessels are in communication by means of arteries and veins passing through the connecting stalk. At first, there are two arteries and two veins ALLANTOIC DIVERTICULUM in the connecting stalk, but later the right vein disappears (the left vein is 'left*'). Before the formation of the tail fold, a small It is clear from Fig. 5.8F that, at this stage, the e n d o d e r m a l diverticulum called the allantoic amnion has a circular attachment to the margins diverticulum arises from the yolk sac near the of the umbilical opening and forms a wide tube in caudal end of the embryonic disc (Fig. 5.10A). which the following lie: This diverticulum grows into the mesoderm of the connecting stalk. After the formation of the tail 1. Vitello-intestinal duct and remnants of the fold, p a r t of t h i s d i v e r t i c u l u m is a b s o r b e d yolk sac. into the hindgut. It now passes from the ventral  Further Development of Embryonic Disc Allantoic diverticulum Hindgut Amniotic /. cavity / | Amniotic cavity Extra-embryonic coelom Mesoderm of ^_^_^m A connecting stalk 1 B Fig. 5.10 Allantoic diverticulum, and its relationship to the connecting stalk. side of the hindgut into the connecting stalk the cavity (Fig. 5.13). The pericardium (Fig. 5.10B). We will refer to it again while enlarges rapidly, and forms a conspicuous considering the development of the urinary bulging on the ventral side of the embryo bladder. (Fig. 5.14). The septum transversum, which was the EFFECT OF HEAD AND TAIL FOLDS most cranial structure in the embryonic disc ON POSITIONS OF (Fig. 5.11), now lies caudal to the heart OTHER STRUCTURES (Fig. 5.13). At a later stage in development, the diaphragm and liver develop in relation Just before the formation of the head and tail folds, to the septum transversum. the structures in the embryonic disc are oriented, as shown in Fig. 5.11. A median (midline) section Future Septum transversum across the disc, at this stage, is shown in Fig. 5.12. From the cranial to the caudal side, the structures Future Pericardial cavity seen in the midline are (a) the septum transversum, Prochordal plate (b) the developing pericardial cavity and the heart, (c) the prochordal plate, (d) the neural plate, Neural plate (e) the primitive streak, and (f) the cloacal Position of intra-embryonic membrane. Note that the primitive streak is now coelom inconspicuous. After folding, the relative positions Somite of these structures change to that shown in Figs 5.13 and 5.14. The important points to note here are as follows: Developing spinal cord 1. With the formation of the head fold, the developing pericardial cavity comes to lie Primitive streak on the ventral side of the embryo, ventral Cloacal membrane to the foregut. The heart, which was developing in the splanchnopleuric mesoderm in the floor of the pericardial Fig. 5.11 Embryonic disc showing the neural plate cavity (Fig. 5.12), now lies in the roof of and related structures. Human Embryology Amniotic cavity Notochord Primitive streak Neural plate Prochordal plate Fig. 5.12 Embryonic disc and related structures just before the formation of the head and tail folds. Foregut Amniotic cavity Notochord Allantoic diverticulum Cloacal membrane Fig. 5.13 Formation of head and tail folds. Also see Fig. 5.14.  Further Development of Embryonic Disc Midgut Amniotic cavity Foregut Cloacal membrane Septum transversum Allantoic diverticulum Fig. 5.14 Later stage in the formation of the head and tail folds. Note the changing relationships of septum transversum, pericardium, buccopharyngeal membrane, cloacal membrane and allantois. 3. The region of the prochordal plate now forms distal end of the hindgut is closed by the the buccopharyngeal, or oral membrane, cloacal membrane. At first, this is directed w h i c h c l o s e s t h e f o r e g u t c r a n i a 11 y. caudally (Fig. 5.13), but later it comes to When this m e m b r a n e breaks d o w n , face ventrally (Fig. 5.14). the foregut communicates with the exterior. We have traced the development of the embryo 4. The most cranial structure of the embryo is to a stage when the rudiments of the nervous now the enlarged cranial part of the neural system, the heart and the gut have been formed. tube, which later forms the brain (Fig. 5.13). We are now in a position to trace the development This enlarges enormously (Fig. 5.14). There of individual organ systems in detail. Before we are n o w t w o big bulgings on the ventral do this, however, we must study the development aspect of the embryo. Cranially, there is the of the placenta. developing brain, and a little below it there is the bulging pericardium (Fig. 5.14). In Some Additional Points of Interest between these t w o , there is a depression called the stomatodaeum or stomodaeum, 1. In later life, r e m n a n t s of the primitive t h e f l o o r of w h i c h is f o r m e d by t h e streak may give rise to peculiar tumours buccopharyngeal membrane. that contain tissues derived from all three 5. Towards the tail end of the embryo, the germ layers. These t u m o u r s are seen in primitive streak is n o w an inconspicuous the sacral region and are called structure, that gradually disappears. The sacrococcygeal tumours.  Human Embryology 2. Experiments have shown that the of somites. In the head region, cranial to formation of the neural tube is induced by somites, somitomeres give origin to some the notochord. mesenchyme. 3. Somitomeres are not confined to the region 4. Wharton's jelly is rich in proteoglycans. TIMETABLE OF EVENTS DESCRIBED IN THIS CHAPTER (in Days) Developmental Events 15 Primitive streak appears. Definitive yolk sac is formed. 17 Notochordal process appears. Heart tube is seen in cardiogenic area. Allantoic diverticulum is seen. 19 Intra-embryonic mesoderm is being formed. Connecting stalk can be distinguished. 21 Neural groove is seen. Head fold begins to form. 23 Closure of the neural tube is seen.