Vertebrate Structure and Development
Lecture Notes

Gametogenesis

Prior to reproduction and the development of a new individual, the gametes have to be formed. This process actually starts back when the parent was an embryo. The precursors to the gamete cells develop in the embryo and migrate to the gonads. It would seem to make sense that the precursors to the gametes would form inside the gonads, but in the species that have been studied these cells actually originate outside the gonads and in some cases outside of the embryo's body.

If these primordial germ cells originate outside of the gonads, how do they end up in the gonads? The answer is we don't fully know, but they may use signals on the surface of cells as a road map to get to the gonads, or they may go through the bloodstream and leave the capillaries by diapedesis (like WBCs).

Once in the gonads, the primordial cells divide and form more cells. These are the oogonia and spermatogonia. In the ovary the oogonial cells soon have other cells that surround them. These are the follicle cells. Once the cells are formed the 2 of them have to be able to bring a single set of chromosomes from each parent together. They also have to have the building blocks from which the early embryo will be built.

Spermatogenesis

Spermatogenesis is the process by which spermatogonia develop into sperm that can leave the male's body. Through meiosis one diploid cell undergoes 2 divisions and forms 4 sperm cells. All of the spermatogonia that will ever exist are formed in a male embryo. There are fewer spermatogonia in the testes than the number of sperm that will be formed during the lifetime of the organism. Therefore, the first step of the process is a mitosis to duplicate the spermatogonia. One of the daughter cells replaces the original spermatogonium and the other becomes the primary spermatocyte. Although the names are different, these 2 cells are identical. The only difference is the fate of the 2 cells.

The primary spermatocytes are diploid and are the cells that will undergo meiosis. The primary spermatocyte undergoes meiosis I and forms 2 haploid secondary spermatocytes. These in turn undergo meiosis II and form 4 spermatids. One real difference between regular mitosis and spermatogenesis is that the cytoplasms of the developing sperm cells don't fully separate so the spermatids are actually connected via their cytoplasms. The spermatids then continue through maturation and the cytoplasm is phagocytized by the Sertoli cells. (Fig. 5-13)

While the developing gametes are spermatids: 1) They change proteins so the chromosomes become compact chromatin. 2) The nucleus changes shape. 3) The acrosomal cap forms. It is a specialized lysosome. 4) The flagellum grows from a centriole. 5) The mitochondria concentrate around the flagellum. 6) Excess cytoplasm is disgarded.

The sperm has a head which contains the DNA. The cap of the sperm is the acrosome which contains enzymes that are needed to burrow into the egg. The sperm matures to an extent in the seminiferous tubules and then continues maturation in the epididymis. The sperm then are released into an aquatic environment for external fertilization or are released into the aquatic environment inside the female for internal fertilization. In many cases the sperm don't actually swim all the way through the female's system, but rather they are drawn to the egg by cilia and muscular contractions.

The sperm have 2 jobs once they contact the eggs. The first function is to deliver the male's genetic material. The other function is activation of the egg. We'll discuss that in the next lecture.

Sperm are temperature sensitive. For ectothermic animals this is not a problem but for birds and mammals it is. Mammals get around the fact that core body temperature is too hot for sperm by placing the testes in the scrotum. Temperature in the scrotum is several degrees lower than core temperature and this is enough to allow the sperm to develop safely. Seasonal breeders bring the testes back into the body for safety when they are not producing sperm. Birds don't have scrotums but they have the same problem with high body temperature injuring the developing sperm. Their solution to the problem is to place the testes near the air sacs which contain air that is cooler than the deep core temperature.

Oogenesis

The oocyte is also produced via meiosis, but in a different way. The primordial follicle is formed in the ovary of the fetus. The primordial follicle starts into meiosis I, but gets only as far as prophase I. At this point it is a primary follicle. Although meiosis stops, the oocyte continues to increase in size and also RNA and protein are formed. The oocyte uses some of its own reserves for this, but it also incorporates material from outside the oocyte and even outside the ovary. The bloodstream brings in nutrients that go to make the yolk.

Yolk is mainly made up of protein, phospholipids, and fats. Usually these are made in the liver and brought to the oocyte via the blood. Vertebrates show a wide diversity in the amount of yolk in the egg and can be classified accordingly.

Although many people use the terms ovum and oocyte as synonyms, they are not. An ovum is an egg that is ovulated after it completes meiosis. An oocyte is an egg that is ovulated before it has completed meiosis.

Microlecithal eggs are eggs with a small amount of yolk. Mammals (other than monotremes) have microlecithal eggs. Mesolecithal eggs have a moderate amount of yolk and are typical of amphibian eggs. Macrolecithal eggs contain a large amount of yolk. Telolecithal eggs contain a large amount of yolk and are produced by reptiles and birds. In a telolecithal egg the yolk takes up a large amount of space and is positioned more towards one end of the egg. The yolk end is the vegetal pole and the end with the embryo is the animal pole.

At some point in time when the animal is ready to reproduce meiosis proceeds to metaphase II. It stops here until fertilization triggers it to continue. Typically, a polar body is formed during each division of meiosis. These are little more than nucleus and aren't involved in the production of offspring.

The oocyte has several membranes associated with it. Primary membranes are made by the oocyte. These include the vitelline membrane which lies just outside the plasma membrane. Outside this there may be a zona radiata or zona pellucida (mammals). Secondary membranes are found in some vertebrates and are produced by the cells of the ovary. Tertiary membranes form after the egg leaves the ovary. This includes the jelly coat of amphibian eggs and albumen and hard shells of birds and reptiles.

Fertilization

In external fertilization the parents deposit their gametes together in the water. In internal fertilization the gametes are deposited inside the female and have to join. One typically thinks of the sperm swimming to the eggs, but much of the movement actually occurs because the muscles of the female's reproductive tract contract and move the sperm to the egg. In birds and reptiles cilia help direct the sperm in the right direction.

Once the sperm reaches the egg it has to penetrate the outer layers to get to the oocyte. Remember the 3 membrane types we talked about last time. These have to be penetrated before the sperm can fuse with the egg. When mammalian sperm is first released from the male it is not capable of fertilizing the egg. Capacitation is the process by which the mammalian sperm becomes capable of fertilizing the egg.

The acrosome is the tip of the sperm cell. It contains enzymes encased in an acrosomal granule. The very tip of the acrosome disappears when it contacts the membranes surrounding the egg. The granule disintegrates and this breaks down the egg membranes. An acrosomal filament (tubule) forms and continues to go through the egg membranes and down to the plasma membrane of the oocyte. This filament is made of actin. A protein, bindin, in the acrosome reacts with bindin receptors on the vitelline membrane. It may take several hours in the female mammal's reproductive tract before this can happen. These changes that occur in the sperm are called capacitation.

Eventually the sperm penetrates the oocyte and the cytoplasms of the 2 cells are joined. At this point we have a zygote.

Aside from undergoing meiosis, the oocyte hasn't had a great deal of cell activity to this point. When the sperm makes contact with the oocyte, all of this changes. In response to contact with the sperm the oocyte becomes depolarized. In sea urchins this is due to the opening of Na+ channels on the oocyte's surface. This depolarization occurs within several hundred milliseconds of contact with the sperm and causes a temporary block to polyspermy. This is called the fast block.

After this the cortical granules in the oocyte cytoplasm move to the edge of the cell and release their contents. The trigger for this is the initial depolarization which causes the ER to release calcium. The calcium triggers the vesicles to fuse with the membrane. The contents then go into the space between the plasma membrane and the vitelline membrane. As a result of this bindin receptors are altered and won't bind with sperm. This is the slow block.

This scenario is true for mammals where polyspermy is a disaster. In some amphibians, birds and reptiles polyspermy is the norm. The extra sperm end up degenerating in the yolk.

After sperm penetration the internal components of the oocyte become rearranged. This may be important in developing bilateral symmetry in the future embryo. Building an embryo from scratch takes a lot of new protein and so after fertilization we see a great deal of protein synthesis and as a result an increase in oxygen consumption by the cell. The pH inside the oocyte also increases.

We've watched fertilization proceed through the joining of the gametes, but at this point we still don't have a diploid cell. After meiosis II is complete and the 2nd polar body is released, the nuclei from the gametes can join. At this point we refer to the 2 haploid nuclei as being pronuclei.

The membrane around each pronucleus is dissolved and the 2 sets of chromosomes join. At this point the zygote readies itself for the first division of cleavage.

Before we look at cleavage there is one interesting oddity (called parthenogenesis) found in some vertebrate species . The normal events of fertilization in vertebrates are caused by the fusion of 2 haploid pronuclei. In some species of vertebrates an egg cell can be stimulated to undergo development of an embryo without such fusion. In some cases this is an oddity that just happens sometimes (birds). In other cases such as reef fish and some lizards this is normal. In mammals parthenogenesis doesn't occur.

The events surrounding development of a parthenogenic embryo in vertebrates still has to be worked out. We do know some rather interesting things about the lizard genus Cnemidophorus (Aspidoscelis), which has a number of parthenogenic species. The species consist of all female individuals. Interestingly, during the breeding season the adults take turns doing the role of males and females in courtship. In the lab we've found that when the female's levels of estrogen are high (just before ovulation) the lizard takes on the normal role of a female. After she has laid her eggs and has high levels of progesterone, she assumes the male role. Lab studies have shown that although these lizards can produce fertile eggs of their own without courtship, the number of clutches and number of eggs/clutch increases if they are involved with courtship.

There are some other twists on the normal pattern of reproduction found in vertebrates. Hybridogenesis is where a female of one species mates with a male of another and forms a hybrid. In one group hybrids are maintained by the female hybrid mating with one of the parental type males. The esculenta frogs make gametes with the ridibunda genome.

Gynogenesis is when females mate with a male, but the male's genetic material is not used to make an offspring. We see this in some salamanders.

How does one define a species with asexual inverts, and these things occuring in the vertebrates?

Cleavage and Gastrulation

The general course of events following fertilization is the same for all vertebrates, but some of the details differ, so we are going to follow the course of events in the early embryo in amphibians, birds, and mammals. Before we do though let's just take a quick look at how the early events progress in vertebrates.

The zygote is a single cell. Once it divides we refer to each of the new cells as blastomeres. Once the zygote divides into 2 blastomeres the embryo is in cleavage. The divisions take a pretty uniform pattern (Fig. 6-2). Each time there is a new division it is at right angles to the last division. During cleavage the number of cells increases, but the size of the embryo in most vertebrates doesn't increase. An interesting thing in the mitosis events that occur in cleavage is that the G1 and G2 phases of interphase don't occur, only the S phase during which DNA duplication occurs.

In vertebrates with very little yolk, the cell divisions are equal and occur simultaneously throughout the embryo. As the amount of yolk increases cell divisions become uneven.

There are 3 types of divisions based on their yolk. Holoblastic Cleavage is where the whole egg divides. Depending upon the amount of yolk the new blastomeres may be the same size (microlecithal egg) and it is holoblastic equal or the blastomeres may be unequal in size in mesolecithal eggs and it's holoblastic unequal (amphibians).

Partial cleavage occurs when only part of the egg actually divides. The yolky end doesn't divide. This occurs in moderately macrolecithal eggs of fish.

Meroblastic cleavage is where only a small part of a very heavily yolked egg divides. Reptiles and birds show this.

When the embryo becomes a solid ball of cells it is a morula. From here divisions continue until the embryo becomes a hollow ball of cells, a blastula. This is the end of cleavage. Gastrulation is the next stage and involves the formation of the first primitive tissues (germ layers).

Amphibians

We'll start our tour of early embryos by examining an amphibian embryo. Amphibians have mesolecithal eggs with the yolk dispersed as small granules. In general there is more yolk at the vegetal pole. Therefore, they are also telolecithal.

At fertilization the sperm usually enters in the animal pole. This results in a lifting of the vitelline membrane and the animal pole shifts so it is sitting at the top. The animal pole of the egg has pigment which is easy to see. After fertilization the pigment moves towards the animal pole and leaves a gray crescent around the equator. This area is where the notochord will eventually be built. The first 2 cleavage divisions go through the poles and result in 4 equal sized cells. The 3rd division occurs toward the animal pole instead of at the equator. This leaves us with 4 smaller animal cells and 4 larger vegetal cells. From now on the divisions will be unequal since the yolky vegetal cells divide more slowly. Over time the divisions lead to a morula and then a blastula with a hollow blastocoel.

The blastocoel starts small and grows in size. As it does it moves closer to the animal pole. At the end of the blastula stage there is a thin layer of cells lying over the blastocoel. Below the blastocoel are relatively large cells filled with yolk.

Gastrulation

During gastrulation the 3 germ layers - ectoderm, mesoderm, and endoderm develop. If we look at our little blastula we find that the cells sitting on top of the blastocoel will become ectoderm. This makes up most of the animal end of the embryo. At the vegetal pole we have a band of cells that will become the endoderm. Between these are cells destined to become mesoderm. Keep in mind at this point they haven't differentiated. (Fig. 7-8)

One of the techniques that has been around for ages is to take a vital dye that is lasting, but not toxic and place it on the embryo. As the cells move through gastrulation we can see where they will end up.

During gastrulation the cells move in groups, not as individuals. An opening forms in the surface of the embryo. This blastopore will eventually become the anus, but during gastrulation it is an opening through which cells migrate from the surface of the embryo to its depths. Initially, these cells keep contact with the surface of the embryo. As they move this turns them into "bottle shaped" cells. It also pulls the invagination deep into the embryo.

The area on the animal pole of the embryo above the blastopore is the dorsal lip. Initially, cells move from the exterior of the embryo to the dorsal lip and then dive into the embryo via the blastopore. As a result of the movement of cells into the embryo and the growth of the new cavity, the blastocoel gets obliterated. As cell migration continues the embryo is left with a yolk plug. At this point the embryo is covered with cells that originated in the animal pole.

Involution is this process of moving cells from the outside of the embryo to the inside. Since the cells that originally sat next to the blastopore were endoderm, they are the first to move in. They line this new cavity, the archenteron, and will become the lining of the pharynx. Cells that will become mesoderm and then cells that will become the notochord follow.

An amphibian gastrula is basically round. After the cells have moved inside of the embryo the entire embryo is covered with ectoderm. At the end of gastrulation the yolk plug has moved inside the embryo and only a little slit remains of the blastopore.

Birds

Bird eggs are macrolecithal and as they move from the ovary through the oviducts several things are added to them. Therefore, the part of the egg we call the yolk is really the oocyte. The part of the oocyte that contains the cytoplasm is the blastodisc. Meiosis occurs as in any other vertebrate, but in birds polyspermy is common. For some reason this is not a problem for birds and reptiles and only 1 sperm actually fertilizes the egg. As the egg passes down the oviducts albumen, membranes and a shell are added.

We don't know a lot about early embryos because the first divisions of the embryo occur while it is in the oviduct. Because the egg is macrolecithal there is no division of the yolk. Because the blastodisc is flattened the procession of early divisions is different than those in amphibians. Division 1 looks like a furrow on the blastodisc. It doesn't make it to the edges of the blastodisc. Also, it doesn't penetrate to the bottom of the blastodisc. Division 2 occurs at right angles to division 1 and also doesn't go to the edges or the depth of the blastodisc. Division 3 produces 8 incompletely divided cells. From there the pattern of division is irregular. The end result is that we see a distinct central cluster of cells and a marginal cluster of cells.

The central blastomeres are completely separated from each other, but the marginal blastomeres are incompletely separated. By the time of the 3rd cell division we see that although it starts out as an incomplete division, the furrow continues deep into the blastodisc and then curves around so that it acts to scoop the cells up from the yolk below it. This process continues with new divisions in the central blastomeres and separates them from the yolk. The marginal blastomeres stay attached to it.

Next the yolk underneath the central blastomeres gets liquidy and the blastomeres continue to divide and this results in layers of blastomeres. These cells are now called the blastoderm. Under them is the subgerminal layer which is filled with the liquid yolk. When you look at the chick embryo in the lab you will be able to distinguish the central blastomeres from the marginal blastomeres. The area pellucida is the name for the area of central blastomeres and the area opaca is the area of the marginal blastomeres.

The cells in the blastodisc are not evenly yolked. Some contain more yolk than others. Those with more yolk sink to the bottom of the blastodisc and those with less form the top surface. These become the epiblast on top and the underlying hypoblast. The hypoblast layer starts at the rear of the animal and moves forward.

During development of the gastrula, we see the development of a primitive groove surrounded by primitive folds. At the front end of the groove the 2 folds meet and form Hensen's node.

This whole structure becomes the primitive streak and develops along the dorsal surface of the embryo. Cells from the upper part of the embryo move toward the primitive streak and then dive down into the embryo. This along with the division of preexisting hypoblast cells contributes to the growth of the hypoblast. The primitive streak then plays the role of the blastopore. The space between the hypoblast and epiblast is therefore the blastocoel.

The hypoblast is eventually replaced by endoderm and the epiblast develops into ectoderm, mesoderm, and endoderm.

Unlike amphibian development the eggs of birds and reptiles have some important non-embryo parts or extraembryonic membranes. These will be important in nurturing the developing embryo. The epiblast develops into ectoderm, mesoderm, and endoderm. The epiblast of the area pellucida becomes ectoderm, mesoderm and endoderm of the embryo. The epiblast of the area opaca become extraembryonic ectoderm.

The first cells that pass by the primitive streak become mesoderm and will end up in the gut and head. The endoderm cells join with the hypoblast endoderm. More mesoderm follows (lateral mesoderm). Epiblast cells from in front of Hensen's node move into the embryo and move back to the front and become the notochord. Through the process of cell movement the embryo develops a layer of mesoderm that runs down the midline. This becomes the notochord, somites, and undifferentiated mesoderm.

Over time the Hensen's node moves posteriorly and as this happens the primitive streak disappears.

Discuss the fates of endoderm, ectoderm, and mesoderm.

Extraembryonic membranes

We have been looking at the development of the embryo, but we also need to take into account the development of the extraembryonic membranes if we are to give a realistic picture of all of the events of development. Before we look at them specifically, let's step back and look at the embryos of various vertebrates and their needs.

The fish and amphibians for the most part lay their eggs in water and the eggs hatch into still developing larvae. In many species the embryo is only in the egg for a few days and then it emerges as a free-feeding larvae. In these species we find a yolk sac as their only extraembryonic membrane. These embryos need some yolk to get them going, but since these small embryos are surrounded by water their wastes diffuse away and oxygen can diffuse in. Therefore, they have no need for special structures to deal with these things.

Fish have more yolk in their eggs than amphibians. In fish the yolk is contained in a trilaminar yolk sac which sits in the gut cavity. In amphibians where there is less yolk, it is stored in endoderm cells of the gut. The yolk sac is part of the gut of amphibians.

Contrast that with the terrestrial eggs of reptiles and birds. These eggs produce well developed youngsters that in the case of reptiles and many birds are self-sufficient from the time they hatch. This requires a greater amount of yolk to fuel this kind of development. Also, since these eggs are laid in a very dry environment, they can't rely on simple diffusion to take care of waste removal and oxygen uptake. Additional extraembryonic membranes were needed to deal with these aspects. In mammals, the embryo develops inside the mother, but the extraembryonic membranes developed by our reptilian ancestors are still present.

Because birds are the best studied and because we will be looking at their embryos in lab, we'll use birds to illustrate how the extraembryonic membranes develop.

The mesoderm that stays near the mid-line of the embryo becomes the somitic mesoderm or somites. The mesoderm that moves to the lateral portion of the embryo becomes the lateral mesoderm. From the neck on down the lateral mesoderm divides and becomes the somatic mesoderm and splanchnic mesoderm.

Remember that the little embryo is developing above the massive yolk. Cells are moving rapidly and mitosis is occurring. As a result some of the mesoderm moves to surround the yolk. Because of the movement of cells that has already occurred the yolk is already surrounded by an endoderm layer. This new mesoderm and the old endoderm merge and become the extraembryonic splanchnopleure. The portion near the embryo forms a yolk stalk and the part around the yolk forms the yolk sac. It would look like the yolk simply goes up through the yolk stalk and gets used by the embryo. This isn't the case. The endodermal part of the splanchnopleure becomes infolded into the yolk. Enzymes associated with it digest the yolk. It is then sent to the embryo via the vitelline veins which develop in the mesodermal part of the splanchnopleure.

Part of the mesoderm spread to the outer part of the developing structure. Here it merged with ectoderm that was already present. This became the extraembryonic somatopleure. During development the somatopleure folds up over the embryo. As a result it becomes 2 layers one above the other. This folding continues until the layers join over the embryo. The outer layer becomes the chorion and the inner layer becomes the amnion.

The amniotic cavity sits inside the amnion and contains the embryo and a fluid. This keeps the embryo from sticking to the amnion or other membrane and acts as a shock absorber.

The chorionic cavity sits between the amnion and the chorion. It is an extraembryonic coelom because the outer surface of it is mesoderm.

Our 4th extraembryonic membrane is the allantois and starts to develop as a shoot off the rear part of the gut. Over time it grows out into the extraembryonic coelom and pretty well fills it. The allantois is made of mesoderm and endoderm, so it is splanchnopleure. The mesoderm part joins with the mesoderm of the chorion. This mesodermal area then develops a number of blood vessels. They sit next to the shell and later next to an air space in the egg. Oxygen diffuses into the blood vessels and then is taken to the embryo. The inside of the allantois is a bag that holds waste produced by the growing embryo.

Three circulatory routes form in the chick embryo: 1) vitelline - goes to/from the yolk sac and the embryo. It provides nutrition. The vessels are derived from the blood islands. 2) allantoic - goes to/from the allantois. It is important for respiration. It develops after day 4. 3) intraembryonic - within the embryo. Part of the vitelline vein is inside the embryo and delivers blood to the sinus venosus. This part of the vitelline vein becomes part of the hepatic portal vein later in development.

Mammalian Development

The future oocyte actually forms before the birth of the female. In the embryo the oocyte and the layer of cells that surround it are called a primordial follicle. Later this structure is called a primary follicle. At some point after maturation the hormones FSH and LH act on this follicle. Initially, FSH triggers the follicle to start maturing. Up to this point the oocyte has been suspended at prophase I. FSH causes some of these follicles to continue in their development. Along the way the number of follicle cells surrounding the oocyte increase greatly in number and start to release estradiol.

During this maturation process a noncellular membrane, the zona pellucida develops between the oocyte and the follicle cells. As maturation continues the inside of the follicle becomes hollow and filled with a fluid. The hollow area is the antrum. Growth of the follicle continues and at this point the oocyte is attached to the follicle cells in one area which is called the cumulus. The layer surrounding the entire follicle is the theca.

At some point the amount of estradiol increases to a certain point and causes the release of a surge of LH. This stimulates ovulation - the release of the oocyte from the ovary. Along with the oocyte a group of follicle cells goes too. These are called the corona radiata. The oocyte now makes its way into the oviduct where it may be fertilized. The remnants of the follicle cells that stay in the ovary become the corpus luteum. Its function is to release estradiol and progesterone which are necessary for preparing the endometrium or inner lining of the uterus for implantation. Normally, in humans, this won't occur and so we revert to the beginning and start all over. In other species, pregnancy almost always occurs after ovulation.

The events that occur in the ovary and uterus must be timed such that the oocyte is released at the right time and the uterus is ready for implantation at the right time. The ovarian cycle is the course of events from the stimulation of the follicle to ovulation to the formation and final disintegration of the corpus luteum. In humans the cycle controlling the uterus is called a menstrual cycle because it involves menstruation or the shedding of the endometrium. In many mammals this doesn't happen and the cycle is termed an estrus cycle.

During the estrus cycle/menstrual cycle initially the endometrium is built up. As time progresses the blood vessels in the endometrium increase in size and glands develop in the endometrium. This is in preparation in case of pregnancy. If pregnancy doesn't occur the endometrium is shed or it regresses to its original state and the cycle starts over.

Mammals constitute a large group of animals and not all have the same exact events occuring during development. We therefore will look at examples from the best studied groups (mice, pigs, humans) and use those as examples.

Mammal eggs are microlecithal so we would expect the whole embryo to divide in unison and that each division would lead to 2 equal blastomeres. This isn't the case. From the very 1st division the blastomeres that form are unequal in size. This is again seen at the 4 cell stage with 2 little and 2 big cells. At the 8 cell stage we can see the small cells are at one end and the large cells are at the other end of the embryo. We also see that the divisions occur at their own pace and not synchronously.

In humans after several days the embryo cells divide rapidly. By the 5th day we have about 100 cells. Fluid from the uterine cavity goes across the zona pelludica and gets into the spaces between embryo cells. These merge to form one cavity. What we have is a tiny hollow embryo - the blastocyst. At this point we can distinguish 2 sets of cells. The outer layer is the trophoblast and the inner layer is the inner cell mass. At this point the zona pellucida goes away and so the trophoblast cells can contact the endometrium.

The trophoblast near the inner cell mass contacts the endometrium and burrows into the endometrium. The endometrium also aids in this process by disintegrating near the trophoblast. The trophoblast cells form 2 layers as time continues. The inner layer becomes the cytotrophoblast and the outer layer that is making contact with the endometrium becomes the syntrophoblast. This outer layer absorbs fluids from the breakdown of the endometrium. These provide the nutrients for the developing embryo. Also, since the embryo has different DNA and different antigens than the mother's cells it seems that there should be a rejection of the embryo. This doesn't occur because the syntrophoblast releases chemicals that prevent rejection.

Although mammals don't develop in eggs, their development is similar to that of birds. The first germ layers to develop are the endoderm and the ectoderm. Between the ectoderm and the trophoblast a small cavity, the amniotic cavity forms. The amnion will form from the layer of cells developing between the cavity and the trophoblast.

In humans the embryo is completely imbedded by day 9. At this point the syntrophoblast cells are tunneling into the endometrium and creating hollow fluid filled spaces. The layer of cells just below the amniotic cavity is ectoderm and becomes the epiblast. The endoderm layer lies under this. After several days the embryonic mesoderm begins to develop. These cells originally start in the epiblast and move through the primitive streak to form the mesoderm and notochord, just like in the bird.

At the same time as the inner cell mass is developing into the future embryo, the extraembryonic membranes are forming and with them the placenta.

The extraembryonic somatopleure moves up over the embryo (just like in the bird) and becomes the amnion and chorion. The allantois develops from the gut. Strangely, even though there is no yolk, a yolk sac develops. In mammals it later degenerates. The chorion along with the allantois of most mammal embryos and the endometrium of the mother join to form the placenta. This makes a chorioallantoic placenta. The arteries and veins formed within the allantois, the allantoic artery and vein, become the umbilical artery and vein. In marsupials the placenta forms from the yolk sac and the chorion of the embryo and the maternal tissue. This is a choriovitelline placenta.

The allantoic artery and vein are called the umbilical artery and vein in placental mammals. Notice that the umbilical artery and vein form from the mesodermal part of the allantois layer of the placenta. Mammals have the same three circulation routes that we saw in birds. The vitelline route goes to/from the yolk sac and the embryo, the allantoic route goes to/from the placenta, and the intraembryonic route is within the embryo.

Pulling all the pieces together

When the embryo develops its digestive tract, ectoderm at the ends of the tract fuses to endoderm lining the tract. The ends of the tracts, which are derived from ectoderm, are the stomodeum, or what will become the mouth, and the proctodeum, or what will become the anus. Initially these are outside the little embryo's digestive tract. The tract itself starts out as the foregut (pharynx to stomach), midgut (small intestine), and hindgut (large intestine and rectum). The first part of the foregut is the pharynx. The oral plate is a thin band of tissue that separates the stomodeum and foregut. It will later rupture to form the margins of the mouth. A similar thing happens with the anal plate.

When you look at where the sensory structures of the nose, eyes, and ears are located you see that they start out within the neural tube. Although we think of these structures as occurring outside the brain, the fact that they start as part of the neural tube means that their neurons are similar to those in the brain and therefore cannot regenerate later in life.

The otic vesicle is the inner ear. It develops from an auditory placode that sinks from the surface into the head. The sensory part of the nose starts as a nasal placode.

When you look at an early embryo much of the tissue in the head is mesenchyme, undifferentiated mesoderm. This will eventually become connective tissue and muscle.

The pharyngeal area of the chick or mammal is much different in an embryo than in an adult. This area starts as pharyngeal (visceral) arches and clefts, which are equivalent to gill arches and clefts. The lateral part of the pharynx forms pharyngeal pouches. The anterior ones form first. At 48 hr only the first visceral arch and pouch have formed. By 72 hr more have formed.

The first visceral arch becomes the mandibular arch and maxillary processes. The maxillary processes at this point are anterior extensions of the mandibular arch. These structures are destined to become the upper and lower jaws.

The visceral arches have aortic arches running through them. The visceral arches are solid tissue and the aortic arches are blood vessels. The aortic arches take blood from the ventral aorta to the dorsal aorta. Like the visceral arches, the aortic arches start developing from the anterior end. There are a total of 6 of them, but in amniotes the anterior ones are normally gone before the posterior ones develop.

In the early embryos we've been looking at there are 2 dorsal aortae - one on each side. By 72 hr the anterior part of these vessels has fused together to make a single dorsal aorta, but it is still paired at the posterior end of the embryo.

The anterior cardinal vein and the posterior cardinal vein drain the anterior and posterior parts of the embryo. The vessel that forms by the joining of these two is the common cardinal vein. It takes blood to the heart.

At about 33 hr we see a new type of mesoderm developing in the chick. Intermediate mesoderm is going to give rise to the urogenital system. It sits between the somites and laterall mesoderm. You can see a mesonephric duct by 72 hr. It is the tube that drains the kidney.

The somite starts as a solid piece and at 33 hr looks like a triangle. By 48 hr one can see subdivisions of the somite. At this point it can be divided into a dermo-myotome and sclerotome. Later the dermo-myotome divides to become the dermotome and the myotome. The sclerotome becomes hard tissue, the dermotome becomes dermis, and the myotome becomes muscle.

Cells called hemangioblasts develop from splanchnic mesoderm in the area opaca. These form clusters called blood islands. The cells that sit on the outer part of the blood islands become the precursors of blood vessel cells and cells within the islands become the stem cells that give rise to all other types of blood cells. The yolk sac also produces some of the primitive blood cells.

Hans Spemann's Experiments

In the 1920s Hans Spemann and some of his graduate students were investigating the mechanism by which embryos become differentiated. We know that at the 2 cell stage that if each blastomere is separated, each can grow into a normal individual. In some cases even later stages can grow into complete individuals. The ability to do this at later stages is limited by the amount of cytoplasm in the cell.

Spemann took a newt egg that was fertilized and before it divided into 2 blastomeres he took a strand of baby hair and made a loop and put over the egg. He tightened the loop and left it such that one side had all the nucleus. When cleavage occurred all of the divisions occurred in the half with the nucleus. If a cell managed to get through the loop, division would then occur on the other side of the loop too and in some cases 2 embryos would develop. This showed that the nucleus of even these later cells had all of the information needed to create an embryo.

If we start with undifferentiated cells in the blastula and proceed to primary tissue layers in the gastrula stage what causes these tissues to become certain structures in exactly the right place? Hilde Mangold, Spemann's student, in the 1920s took a dorsal lip of the blastopore and transplanted it onto a host embryo. Cell movement occurred around both the host and introduced dorsal lip and the result was a host embryo with a partially developed secondary embryo. The development of this secondary embryo was due to the graft. Cells moved into the blastopore and developed into the same structures as we saw in a normal embryo.

Think back to what you know about amphibian embryos. The cells that move into the blastopore and form the roof of the archenteron are the cells that become the mesoderm and notochord. It is their presence under the ectoderm cells that causes the ectoderm to differentiate into neural plate cells. This process is called induction. The structure that causes this to happen is the inductor.

Induction of the neural system is the result of the ectoderm sitting on top of the presumptive notochord and somites. The anterior part of the nervous system (brain) is induced by the roof of the archenteron infront of these structures. Only grafts from the dorsal lip of the blastopore and surrounding cells could cause induction to start.

The ability of cells to react to an inductor is greatest in early gastrulation, still high in mid-gastrulation, and declines late in gastrulation. By neurulation it is fading away.

The dorsal lip was dubbed the primary organizer by Spemann in 1938. This organizer originates from cells that were derived from the gray crescent area and is only found on the dorsal part of the embryo. The dorsal lip sits at a point where the yolkless cells meet the yolky cells. Those more active, pigmented, yolkless cells from the animal pole do the moving and the heavier yolky cells don't move.

With all of the movement that occurs during the blastula and gastrula stage, it is hard to follow individual cells. Therefore, in the 1920s Vogt came up with the idea of taking agar soaked in vital dye, which doesn't injure the embryo, and putting tiny pieces on the embryo. The agar is removed and the dye stains the surface cells of the embryo. These color spots can be observed and followed. Today we use more sophisticated marking methods.

Last updated on 22 June 2011
Provide comments to Lynnette Sievert at lsievert@emporia.edu
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