Formation of Germ Layers PDF

Chapter 4 Formation of Germ Layers a somatopleuric layer (in c o n t a c t w i t h HIGHLIGHTS trophoblast) and a splanchnopleuric layer (in Fer...

Chapter 4 Formation of Germ Layers a somatopleuric layer (in c o n t a c t w i t h HIGHLIGHTS trophoblast) and a splanchnopleuric layer (in Fertilization of the ovum takes place in the contact with yolk sac) (Fig. 4.7B). ampulla of the uterine tube (Fig. 4.1). The The trophoblast and underlying somatopleuric fertilized ovum is a large cell. It undergoes a mesoderm form a membrane called the chorion. series of divisions (clevage) (Fig. 4.4). The cells forming the wall of the amniotic When there are 16 cells, the ovum is called a morula (Fig. 4.4). It has an inner cell mass cavity form the amnion. c o v e r e d by an o u t e r layer of cells, the T h e a m n i o t i c cavity is n o w a t t a c h e d to trophoblast (Fig. 4.5A). trophoblast by some mesoderm into which the Fluid partially separates the inner cell mass extra-embryonic coelom has not extended. This from trophoblast. The morula now becomes a mesoderm forms the connecting stalk. blastocyst (Fig. 4.5C). If we view t h e e m b r y o n i c disc from the The cells of the inner cell mass multiply, and ectodermal side we see that near one edge it are rearranged to form an embryonic disc has a rounded area called the prochordal plate having two germ layers. These layers are the (Fig. 4.9). Here ectoderm and endoderm are ectoderm and endoderm. Later, a third germ not separated by mesoderm. layer, the mesoderm, forms between ectoderm An elevation, the primitive streak, is also seen and endoderm. on the embryonic disc (Fig. 4.10A}. A line A cavity appears on the ectodermal side of the drawn through the prochordal plate and the disc. This is the amniotic cavity {Fig. 4.6C). primitive streak divides the embryonic disc into Another cavity appears on the endodermal right and left halves. side. This is the yolk sac. Cells multiplying in the primitive streak move At first the walls of the amniotic cavity and yolk i n t o the interval b e t w e e n e c t o d e r m a n d sac are in contact with trophoblast (Fig. 4.6D). endoderm and form the mesoderm (third germ They are soon separated from the latter by layer) (Fig. 4.12). extra-embryonic mesoderm (Fig. 4.7A). Caudal to the primitive disc we see a round A cavity, the extra-embryonic coelom appears area called the cloacal membrane. It is made and splits the extra-embryonic mesoderm into up only of ectoderm and endoderm (Fig. 4.12).  Formation ot Germ Layers pronucleus. Soon thereafter, the pronuclei lose FERTILIZATION their nuclear membranes. The 2 3 chromosomes In Chapter 2 we have seen that while the ovarian of the female pronucleus and 2 3 of the male follicle is g r o w i n g , t h e o o g o n i u m w i t h i n it pronucleus get mixed u p and form 2 3 pairs. These undergoes maturation. The oogonium enlarges to 46 chromosomes undergo changes like those in a form a p r i m a r y o o c y t e. T h e p r i m a r y oocyte typical mitotic division leading to the formation undergoes the first meiotic division to shed off the of an embryo having two cells. Note that, strictly first polar body and becomes a secondary oocyte speaking, there is no one-cell stage of the embryo. (Fig. 4.2A). At the time of ovulation, the second meiotic division is in progress and a spindle has Some details on t h e b i o c h e m i c a l c h a n g e s formed for separation of the second polar body occurring during fertilization are worth noting. (Fig. 4.2B). At this stage the ' o v u m ' enters the These are as follows. mfundibulum of the uterine tube and passes into 1. T h e glycoprotein of the zona pellucida is the ampulla (Fig. 4.1). responsible for induction of the acrosomal Fertilization of the ovum occurs in the ampulla r e a c t i o n ( C h a p t e r 2 ). T h e release of of the u t e r i n e t u b e. O u t of a few h u n d r e d a c r o s o m a l enzymes helps the sperm to capacitated sperms, which surround the ovum, only penetrate the zona. one pierces the zona pellucida and enters the ovum. 2. When a spermatozoon comes in contact As soon as the spermatozoon enters the ovum the with the oocyte, plasma membranes of the s e c o n d m e i o t i c d i v i s i o n ( w h i c h w a s so far two cells fuse. This, probably occurs at incomplete! is completed, and the second polar receptor sites that are specific for a species. b o d y is e x t r u d e d. T h e n u c l e u s of the o v u m Both the head and tail of the spermatozoon becomes the female pronucleus. The head of the (excluding the plasma membrane) enter spermatozoon (which it will be remembered is the cytoplasm of the ovum. formed from the nucleus) separates from the middle 3. Alterations taking place in the plasma piece and tail, and transforms itself into the male Ampulla (6 cm) Uterine part (1 cm) Isthmus (3 cm) Fertilization takes place here Fig. 4.1 Path taken by the sperm (pink), and ovum (blue), for fertilization.  Human Embryology - Zona peilucida. Spindle of second meiotic division Secondary oocyte - First polar body " First polar body Nucleus ot Female pronucleus mature ovum (From nucleus of ovum) Vitelline membrane Male pronucleus (From head of sperm) Spermatozoon entering ovum Fig. 4.2 Some stages in the maturation of the o v u m : (A) O v u m just before o v u l a t i o n ; (B) O v u m at the t i m e of o v u l a t i o n ; (C) O v u m at the time of fertilization; (D) O v u m just after fertilization. membrane of the oocyte, and in the zona to formation of two cells, each having peilucida, ensure that no other forty-six chromosomes (Fig. 4.3). spermatozoon can enter the oocyte. 4. The zona peilucida is altered due ro release Male pronucleus of l y s o s o m a l e n z y m e s by t h e p l a s m a membrane of the oocyte. This process is called the zona reaction. 5. As soon as a s p e r m a t o z o o n enters the Female pronucleus ovum, the latter finishes its second meiotic division and the second polar body is formed. 6. Entry of the sperm leads to metabolic Each chromosome changes within the ovum that facilitate its duplicates itselt. development into an embryo. 7. Each chromosome in the male and female p r o n u c l e i is m a d e u p of o n l y o n e Each daughter cell c h r o m a t i d. Replication of D N A takes has 23 pairs of place to form a second chromatid in each chromosomes - one of each pair derived c h r o m o s o m e. In the cell division t h a t from the spermatozoon follows, each chromosome splits into two and the other from the ovum. (as in mitosis). Meanwhile a spindle has formed and one chromosome of each pair Fig. 4.3 Behaviour of chromosomes during moves to each end of the spindle. This leads fertilization.  Formation of Germ Layers 8. From w h a t has been said above, it will be 22 + Y chromosomes. We speak of these as 'X-bear- clear that as a result of fertilization ing", or 'Y-bearing', spermatozoa. An ovum can be fertilized by cither type of spermatozoon. If the fa) the diploid chromosome number (46) sperm is X-bearing, the zygote has 44 + X + X is restored; chromosomes and the offspring is a girl. If the (b) determination of sex takes place; and sperm is Y-bearing the zygote has 44 + X + Y (c) the fertilized ovum begins to divide chromosomes and the offspring is a boy. into several cells (i.e. it undergoes Thus the sex of a child is 'determined' at the cleavage). time of fertilization. It will now be clear that one 9. The important points to note at this stage chromosome of each of the 2 3 pairs is derived are that from the mother and the other from the father. (a) t h e t w o d a u g h t e r cells a r e still surrounded by the zona pellucida; CLEAVAGE (b) each daughter cell is much smaller The two cells formed as described above undergo than the ovum; a series of divisions. One cell divides first so that (c) as subsequent divisions occur, the cells we have a '3-cell' stage of the embryo (Fig. 4.4R) become smaller and smaller until they followed by a '4-cell' stage (Fig. 4.4C), a '5-cell' acquire the size of most cells of the stage, etc. This process of subdivision of the ovum body. into smaller cells is called cleavage. As cleavage proceeds rhe ovum comes to have Test Tube Babies 16 cells. It now looks like a mulberry and is called The so-called test tube babies are produced by the morula (Fig. 4.4D). It is still surrounded by rhe Technique of in vitro fertilization (In vitro - rhe zona pcllucida. If we cut :i section across the outside the body, as against in vivo - within morula, we see that it consists of an inner cell the body). This technique is being increasingly mass that is completely surrounded by an outer used in couples w h o are not able to achieve layer of cells. T h e cells of the outer layer will fertilization in the normal way. later give rise to a structure called the trophoblast G o n a d o t r o p i n s are a d m i n i s t e r e d t o the (Fig.4.5A). woman to stimulate growth of follicles in the The inner cell mass gives rise to the embryo ovary. Just before o v u l a t i o n , the o v u m is p r o p e r , a n d is, t h e r e f o r e , a l s o c a l l e d t h e removed (using an aspirator) and is placed in a embryoblast. The cells of the trophoblast help to suitable medium- Spermatozoa are added to provide nutrition to the embryo. the m e d i u m. Fertilization and early Some fluid now passes into the morula from development of the embryo take place in this the uterine cavity, and partially separates the cells medium. The process is carefully monitored, of the inner cell mass from those of the trophoblast and when the embryo is at the 8-cell stage it is (Fig. 4.5B). As the quantity of fluid increases, the put inside the uterus. Successful implantation morula acquires the shape of a cyst. The cells of takes place in about 20 per cent of such trials. the trophoblast become flattened, and the inner cell mass comes to be attached to the inner side of SEX DETERMINATION the trophoblast on one side only (Fig. 4. 5 Q. The morula has now become a blastocyst. The cavity We know that all ova contain 22 + X chromo- of the blastocyst is called the blastocoele. The side somes. However, we have seen that spermatozoa of the blastocyst to which the inner cell mass is are of t w o types. Half of them have 22 + X attached is called the embryonic or animal pole, chromosomes and the other half of them have while the opposite side is the abembryonic pole.  Human Embryology can, therefore, invade and b u r r o w into tissues with which they come in contact. As the embryo travels down the uterine tube, and the u p p e r m o s t p a r t of the uterine cavity, it is prevented from 'sticking' to the epithelium by the zona pellucida. During this time it receives nutrition, partly from the substances stored within the ovum (e.g. yolk), and partly by diffusion from uterine secretions. By the time a blastocyst is formed, it is necessary for the embryo to acquire additional sources of nutrition. This is achieved when the blastocyst 'sticks' to the uterine endometrium, and gets implanted in it. However, before this can happen, it is n e c e s s a r y for t h e z o n a p e l l u c i d a t o disappear. The zona pellucida disappears soon after the morula reaches the uterine l u m e n. T h u s , the function of the zona pellucida is to prevent implantation of the blastocyst at an abnormal site. Fig. 4.4 Some stages in segmentation of the fertilized T h e developing e m b r y o is genetically o v u m ; (A) Two-cell stage; (B) Three-cell stage; different from the mother. This may evoke (C) Four-cell stage; (D) M o r u l a. immunological reactions if embryonic and maternal tissues come in contact. Presence of zona pellucida (which lacks histocompa- Function of the Zona Pellucida tibility antigens), acts as a barrier that separates The trophoblast has the property of being able to maternal tissues from the e m b r y o. After the stick to the uterine (or other) epithelium and its d i s a p p e a r a n c e of z o n a p e l l u c i d a v a r i o u s cells have the capacity to 'cat u p ' other cells. They immunosuppressive cytokinase and proteins are Outer layer of cells forming future trophoblast Fig. 4.5 Formation of blastocyst.  Formation of Germ Layers produced by the implanting embryo. This blocks form the second germ layer, the ectoderm. the recognition of the embryo as a tissue foreign The embryo is now in the form of a disc to the mother. having two layers. A space a p p e a r s between the ectoderm FORMATION OF GERM LAYERS (below) and the trophoblast (above). This is the amniotic cavity (Fig. 4.6C), filled by As the blastocyst develops further, it gives rise not amniotic fluid or liquor amnii. The roof of only to the tissues and organs of the embryo but this cavity is formed by amniogenic cells also to a number of structures that support the derived from the trophoblast, while its floor embryo and help it to acquire nutrition. At a very is formed by the ectoderm. early stage in development, the embryo proper Flattened cells arising from the endoderm acquires the form of a three-layered disc. This is (or, according to some, from trophoblast), called the embryonic disc (also called embryonic spread and line the inside of the blastocystic area, embryonic shield, or germ disc). cavity. (This lining of flattened cells is called The three layers that constitute this embryonic Heuser's membrane.) In this way, a cavity, disc are: lined on all sides by cells of endodermal origin, is formed. This cavity is called the 1. Endoderm {endo = inside) primary yolk sac (Fig. 4.6D). 2. Ectoderm (ecto = outside) The cells of the trophoblast give origin to a 3. Mesoderm (meso = in the middle) These are the three germ layers. All tissues Inner cell mass of the body are derived from one or more of these layers (Chapter 21). Much of the student's s t u d y of e m b r y o l o g y c o n c e r n s itself with learning from which of these germ layers particular tissues and organs develop. In the further development of the blastocyst that we will now consider, it is very important to have a clear conception of the formation of germ Trophoblast layers and of their fate. We h a v e seen t h a t the b l a s t o c y s t is a Amniogenic cells - spherical cyst lined by flattened trophoblastic cells, and that inside it there is a mass of cells, Amniotic cavity the inner cell mass, attached eccentrically to the trophoblast. Further changes are as follows: 1. S o m e cells of t h e i n n e r cell m a s s differentiate (i.e. they become different from others) into flattened cells, that come to line its free surface (lower in Fig. 4. 6 A ). T h e s e c o n s t i t u t e t h e endoderm, which is thus the first germ layer to be formed. Fig. 4.6 Differentiation of endoderm and ectoderm, 2. The remaining cells of the inner cell mass and the formation of the amniotic cavity and become columnar (Fig. 4.6B). These cells the yolk sac.  Human Embryology mass of cells called the extra-embryonic inside of the trophoblast, and the outside of mesoderm (or primary mesoderm). These the amniotic cavity, is called the parietal cells come to lie between the trophoblast or somatopleuric extra-embryonic and the flattened endodermal cells lining mesoderm. (It is also referred to as the the yolk sac, thus separating these from each chorionic plate.) The part lining the outside other. These cells also separate the wall of of the yolk sac is called t h e visceral the amniotic cavity from the trophoblast or splancbnopleuric extra-embryonic (Fig.4.7A). mesodenn (Fig. 4.7B). T h i s m e s o d e r m is c a l l e d ' e x t r a - From Fig. 4.7B it is clearly seen that the e m b r y o n i c ' ' b e c a u s e it lies o u t s i d e the extra-embryonic coelom does not extend embryonic disc. It does not give rise to any into t h a t p a r t of the e x t r a - e m b r y o n i c tissues of the embryo itself. mesoderm which attaches the wall of the Small cavities appear in the extra-embryonic a m n i o t i c cavity t o the t r o p h o b l a s t. T h e mesoderm. Gradually these join together to developing embryo, along with the amniotic form larger spaces and, ultimately, one large cavity and the yolk sac, is now suspended cavity is formed. This cavity is called the in the e x t r a - e m b r y o n i c c o e l o m , and is extra-embryonic coelom (Fig. 4.7B) (also attached to the wall of the blastocyst (i.e. called the chorionic cavity). W i t h its trophoblast) only by this unsplit part of the formation, the extra-embryonic mesoderm extra-embryonic mesoderm. This mesoderm is split into two layers. The part lining the forms a structure called the connecting stalk. Amniot c cavity Future connecting , stalk Amniotic cavity Embryonic disc ^- Amnion ^-Chorion." (ectoderm, endoderm and mesoderm). Since all (described above as ectoderm) is defined as the the cells, tissues and organs of the body are epiblast. The layer of cuboidal cells (described formed from these three layers, the cells of the above as endoderm) is defined as the hypoblast. inner cell mass are called embryonic stem cells. Some cells of the epiblast migrate to the T h e s e s t e m cells c a n be m a i n t a i n e d a n d  Human Embryology Primitive streak -Extra-embryonic mesoderm - Extra-embryonic coelom Amniotic cavity - Connecting stalk ' (Note intra-embryonic mesoderm passing Intra-embryonic^ into it) mesoderm Extra-embryonic coelom Fig. 4.13 Diagram showing the attachment of the connecting stalk to the caudal end of the embryonic disc. Note the cells of the intra-embryonic mesoderm passing into the connecting stalk. propagated in an undifferentiated state, in on 'therapeutic stem cell cloning'. In this culture, in laboratories. If these cells are exposed procedure the nucleus of patient cell is to certain specific growth factors, in culture, introduced into the embryonic stem cell. the stem cells can form various types of adult These cells are then allowed to grow in any cells, e.g. neurons, muscle cells, blood cells, tissue of the patient. As the tissues arising cartilage cells, etc. It has been observed that from the stem cells are now genetically when these stem cells are introduced into the identical to those oi the patient, rejection is tissues of a living person, the local environment avoided. helps these stem cells to differentiate into cells Though the embryonic stem cells are most similar to those of the tissue in which they are suitable for therapeutic purposes, stem cells can placed. This technique has tremendous also be isolated from some adult tissues, e.g. potential for treatment of various diseases. Some bone marrow, brain and skeletal muscle. of these are Parkinson's disease, Alzheimer However, adult stem cells are difficult to culture disease, diabetes, myocardial infarction, blood in laboratories and have less potential to diseases, severe burns, osteoporosis, spinal cord differentiate in adult tissues. injury, to name but a few. However, in this As the human embryos are needed for stem procedure the complication of immune rejection cell research, some authorities object to it on is always present as the genetic constitution of ethical grounds. The main objections are that stem cell is different from that of patient. To it is against nature and it treats the embryo overcome this problem scientists arc working with disrespect.  Formation of Germ Layers TIMETABLE OF EVENTS DESCRIBED IN the beginning of what is termed the embryonic THIS CHAPTER period (3rd to 8th week). Most congenital anomalies are produced by teratogens acting Development of the embryo from fertilization during this period. up to the formation of the bilaminar disc is described as the pre-organogenesis period, as no organs are as yet recognizable. These events Age (in Days) Developmental Events take place in the first 14 days of pregnancy. 2 Embryo is at two-cell stage Anomalies produced by teratogens acting 3 Morula is formed during this period usually result in death of the embryo. These anomalies are, therefore, seldom 4 Blastocyst is formed seen in babies reaching full term. 8 Bilaminar disc is formed Establishment of the primitive streak and 14 Prochordal plate and formation of intra-cmbryonic mesoderm mark primitive streak is seen the onset of gastrulation. Gastrulation begins 16 Intra-embryonic mesoderm in the third week and most of it will be is formed/disc is now three considered in Chapter 5. The third week marks layered.