This chapter deals with the formation of an adult organism from a single cell. This process requires growth (cell division), differentiation (the cells become specialized), and morphogenesis (the shaping and patterning of the body). Morphogenesis is also called pattern formation.

Fertilization

Sea Urchins

Several different mechanisms have been discovered in different species that insure species-specific fertilization by one sperm. Much research has been done on sea urchins (Echinodermata) and the mechanism found in these animals is discussed here.

The sea urchin egg is surrounded by a layer called the vitelline envelope and a jelly coat surrounds this. Enzymes in the acrosome of the sperm digest the jelly coat. An extension from the head of the sperm makes contact with receptor molecules in the vitalline envelope.  The receptor is like a lock that requires a specific key to unlock; only sperm of the same species will interact with the receptor.

The interaction of the sperm with receptors on the vitelline envelope enables the sperm and egg plasma membranes to fuse and then the sperm nucleus to enter the egg. 

The binding of sperm to the vitelline envelope triggers the egg plasma membrane to depolarize, preventing other sperm from fusing with the plasma membrane.

Depolarization (discussed above) is a short-term mechanism to prevent multiple sperm from fertilizing an egg. The fusion of sperm and egg also triggers the release of calcium ions (Ca⁺⁺) from the endoplasmic reticulum. The presence of Ca⁺⁺ causes the fusion of vesicles immediately underneath the plasma membrane with the plasma membrane. As the vesicles release their contents, the vitelline layer moves away from the plasma membrane, forming the fertilization membrane. The fertilization membrane prevents the penetration of the egg by other sperm. 

After entering the egg, the sperm and egg nuclei fuse to create a diploid cell called a zygote.

Mammals

Secretions of the female reproductive tract activate certain molecules on the surface of sperm and also increase the motility of the sperm. This change, called capacitation, may take several hours.

The mammalian secondary oocyte is surrounded by an noncellular layer called the zona pellucida. A layer of follicle cells from the ovary is attached outside the zona pellucida.

The sperm must move through the layer of follicle cells and molecules on the head of the sperm bind with receptors on the zona pellucida. Some evidence suggests that this binding is species-specific.

The binding of sperm with the zona pellucida induces the acrosome to empty it's contents. Enzymes carried within the acrosome digest a path through the zona pellucida, enabling the sperm to pass through.

Proteins within the sperm membrane then bind with counterparts in the oocyte membrane. This binding triggers depolarization of the oocyte membrane. Depolarization is a short-term mechanism to prevent multiple sperm from fertilizing an oocyte. The binding of the sperm membrane to the oocyte membrane also triggers the release of enzymes stored in vesicles immediately underneath the plasma membrane. These enzymes alter receptors in the zona pellucida so that other sperm cannot bind. 

In mammals, the entire sperm enters the oocyte.

Early Embryonic Development

Effect of Yolk

The amount of yolk affects how embryonic development progresses. The discussion of embryonic development that follows will consider the following kinds of animals.

Animal	Amount of Yolk
Lancelet	Little
Amphibians	Intermediate amount
Birds	Much
Human	Little

Yolk provides food to the developing embryo. The amount of yolk is related to the environment in which the animal develops. 

Lancelets and amphibians have less yolk because they have swimming larvae which can obtain their own food. Reptiles and birds are terrestrial species with eggshells to prevent desiccation, so they are unable to obtain their own food. 

Birds and mammals are descended from reptiles, so development of mammals is similar to that of reptiles and birds. Mammals have little yolk because the young obtain nutrition through the placenta and later from milk. 

Cleavage

After fertilization, cleavage occurs as the cells divide but the embryo does not become larger; the cells become smaller with each division. The resulting mass of cells is called a morula.

In deuterostomes (including chordates), cleavage is radial and indeterminate. Protostome cleavage is spiral and determinate.

Yolk cells divide slower than other cells in the embryo and thus remain larger due to fewer divisions. The part of the embryo that contains larger, yolk cells is called the vegetal pole. The animal pole contains smaller cells with no yolk.

All of the cells of a lancelet morula are approximately the same size because they have little yolk. The cells in the animal pole of amphibians are smaller than the cells in the vegetal pole because they have an intermediate amount of yolk. The morula of these two kinds of animals is a solid ball of cells.

Cleavage in birds is restricted to cells that lie on the surface of the yolk. The morula is a disk of cells that lies on top of the yolk.

Blastula

As cell division continues, a cavity called the blastocoel forms in the center of the lancelet embryo but in the vegetal pole cells of amphibians. In birds, this cavity forms between the vegetal cells and the yolk.

Below: Cleavage produces cells that are smaller. 

Gastrula

Species with Little Yolk

Cells of the lancelet migrate inward producing a hollow embryo with an opening to the space in the center and two layers of cells surrounding this cavity. The outer layer of cells is ectoderm and the inner layer is endoderm. The central cavity is the archenteron or primitive gut and the opening to the archenteron is the blastopore.

Below: Various stages of embryonic development in a sea star (Echinodermata)

The blastopore of deuterostomes becomes the anus.

Ectoderm gives rise to the skin and nervous system. Endoderm forms the lining of the gut and the major organs derived from it.

Amphibians (Intermediate Amount of Yolk)

Yolk cells of amphibians do not move, so the archenteron is formed when cells from the animal pole migrate inward. The blastopore has the shape of a slit and a yolk plug remains near the blastopore.

Birds (Large Amount of Yolk)

There is no inward movement of cells in birds. Instead, cells on the upper surface of the disk differentiate to become the ectoderm and cells in the lower layer become endoderm.

In the lancelet, mesoderm forms from two pouches that form dorsally along the length of the primitive gut. These cells migrate into the area between the endoderm and ectoderm. 

In amphibians, mesoderm originates from cells near the dorsal lip of the blastopore.

In birds, an elongate furrow (called a primitive streak) forms in the layer of ectodermal cells. These cells migrate inward into the area between the endoderm and ectoderm to form the mesoderm.

After the mesoderm forms in each case above, a split forms within the mesoderm to form the coelom (body cavity).

Mesoderm becomes the muscles, connective tissues, skeleton, kidneys, circulatory and reproductive organs

Neurula

The nervous system develops from an elongate, thickened area in the ectoderm above the area that will become the notochord. This neural plate will fold to produce an elongate tubular structure called the neural tube. The anterior end of the neural tube will develop into the brain; the remainder will form the spinal cord.

The embryo at this stage of development is called a neurula.

The notochord forms from dorsal mesodermal cells. In vertebrates, it is replaced by the vertebral column.

Mesodermal tissue on either side of the notochord become segmented and form somites. The segmented pattern of these somites can be clearly seen in a longitudinal view of the embryo. The somites will form vertebrae (in vertebrates) and skeletal muscles.

Below: Somites can be seen in this 6-day-old chick embryo.

A single cell generally contains all of the genes necessary to construct the entire body. Differentiation (specialization) in the cells of a developing embryo occurs when  certain specific genes become activated.

The concentration of some molecules called cytoplasmic determinants affects the development of cells. When a fertilized egg undergoes cleavage, some cells will receive higher concentrations of these molecules than other cells. 

As embryonic development proceeds, some cells produce signals that induce developmental changes in nearby cells by a process called induction. These ideas were demonstrated by the work of Hans Spemann on frogs (below).

As development proceeds, a signal activates certain genes, which in turn produce other signals which activate new genes, which produce still more signals etc.

Cytoplasmic Segregation and Induction

Hans Spemann studied frog embryos and received a Nobel Prize in 1935. Some of his work is discussed below.

After fertilization of a frog egg, the contents shift to form a gray crescent opposite the point of sperm entry. The gray crescent may contain growth factors.

If gray crescent is divided equally into 2 daughter cells, each cell will give rise to an embryo. If the egg is divided experimentally so that the crescent goes into one cell but not the other, the cell without the crescent will not develop normally.

The gray crescent becomes the dorsal lip of the blastopore; he called it the primary organizer. Cells closest to it become endoderm, those further away: mesoderm, and those farthest become ectoderm. This may be due to a concentration gradient of certain signaling molecules. At low concentrations, animal pole cells to become epidermis, which is a tissue that normally originates from ectoderm.. At higher concentrations, they become notochord and muscle tissue which originate from mesoderm.

Transplant experiments revealed that when a portion of the embryo that was destined to become part of the nervous system was transplanted to the ventral surface of another embryo, the transplanted cells did not grow to produce a nervous system. However, when cells destined to become a notochord were transplanted to the ventral area of a different embryo, a nervous system began to develop in that area. The transplanted tissue induced the nearby cells to develop into a nervous system.

Lewis (1905) discovered that a part of the brain called the optic vesicle induces nearby ectoderm to develop into a lens. The newly-forming lens material, in turn, induces the optic vesicle to produce an optic cup which then produces a retina.

Fate Maps in Caenorhabditis elegans

C. elegans is a roundworm approximately 1 mm long. A fertilized egg divides to produce 2000 cells in the adult. This animal is transparent, making it possible to observe each cell as it develops. A fate map has been created in which the fate of every cell during this process is recorded.

C. elegans has two genes which play a role in the pre-programmed death of cells. This phenomena, called apoptosis appears to be in the normal development of animal embryos. For example, the human hand develops from a paddle-like structure that does not contain distinct fingers or a thumb. Separate fingers and a thumb are produced when the cells between them die.

Pattern Formation in Drosophila melanogaster 

Dorsal-Ventral and Anterior-Posterior Axis

Drosophila eggs contain follicle cells and nurse cells that support the egg cell. These cells support and nourish the unfertilized egg. Genes within the follicle and nurse cells become active and produce mRNA needed to determine the dorsal-ventral and anterior-posterior axes. 

Proteins coded by these mRNA molecules are called morphogens because they influence morphogenesis. 

Messenger RNA from a gene called bicoid moves into the egg from the nurse cells and produces a protein that determines the anterior-posterior axis. The bicoid mRNA remains at one end of the egg, perhaps due to it being attached, but the protein produced from it diffuses throughout the egg. The highest concentration of the protein occurs near the site of production, causing this area becomes the anterior end of the embryo.

Segmentation

The presence of the bicoid protein gradient results in a number of genes becoming activated that control segmentation. Newly-activated genes activate others in sequence as development proceeds.

Christiane Nusslein-Vollard and Eric Wieschaus received a Nobel Prize for their work on genes that control segmentation. In their experiments, they exposed flies to mutagenic chemicals to produce mutations in the genes that control segmentation.

A set of genes called gap genes are among the first to become activated by bicoid. These genes determine the basic pattern of segmentation along the anterior-posterior axis by producing several large regions that will need to be further subdivided into segments. A mutations in one of the gap genes resulted in an embryo with eight segments missing.

A set of genes called pair-rule genes are activated by products of the gap genes. The pair-rule genes results in further subdivision of the segments but a mutation in one of the genes results in an embryo with only half as many segments.

Finally, genes called segment-polarity genes become activated which determine the anterior-posterior relationship within each segment.

Homeotic Genes

After the segmentation genes have been activated, homeotic genes determine which appendages and other structures that will be present in each segment. Flies with homeotic mutations may have two pairs of wings or have legs located where antennae should be located. 

The gene products of homeotic genes are transcription factors. They bind to DNA and initiate transcription.

Homeotic genes have been found in many other eucaryotic species as diverse as yeast and humans. All of these species contain the same 180-nucleotide sequence called a homeobox. The remainder of the gene is variable. The part of the protein produced by the homeobox portion of the gene binds to DNA. The variable part of the protein determines which genes are turned on.

The protein products of one homeotic gene may turn on the next homeotic gene creating a sequence of gene activation.

The homeotic genes of Drosophila are located on one chromosome but in mice and humans, they are located on four different chromosomes. In all of the species the homeotic genes are activated in the same order. Homeotic genes that are activated first control development in the anterior portion of the animal. Homeotic genes that are activated later control development in regions that are posterior to those controlled by genes activated earlier.

Humans

A human pregnancy lasts 9 months. The first two months of development are embryonic development. During this time, major organs are formed. Fetal development occurs during the remaining months, during which refinement of the major organs occurs.

Extraembryonic membranes

Extraembryonic membranes within the eggs of Birds and Reptiles protect the embryo, allow gas exchange, and prevent dehydration. 

The chorion lies just beneath the shell and functions in gas exchange.

The allantois collects and stores nitrogenous wastes.

The yolk sac stores food.

The amnion cushions and provides a watery environment.

These membranes are also present in human embryos due to the close evolutionary relationship between reptiles and mammals. The membranes have taken on different functions because human fetuses exchange food, wastes, and gasses through the placenta.

The placenta is derived partly from maternal tissues and partly from fetal tissues. The fetal part of the placenta develops from the chorion. 

The yolk sac is not needed to store food. Instead, blood cells develop there.

The umbilical cord forms from the yolk sac and allantois.

Human Embryonic Development

Week 1

Fertilization occurs in the upper 1/3 of the oviduct.

Cleavage begins as the embryo passes through the oviduct to the uterus.

The morula reaches the uterus in about 3 to 3.5 days.

Although mammals have very little yolk, gastrulation and early organogenesis (the formation of organs) is similar to their reptilian ancestors and birds, which have a large amount of yolk.

By the end of the first week, the morula becomes a blastocyst, a hollow, fluid-filled structure with an outer layer called the trophoblast. The trophoblast and some mesodermal tissue will form the fetal portion of the placenta. A mass of cells within the trophoblast will become the embryo.

Week 2

The trophoblast begins to implant in the uterus. It secretes enzymes that digest away part of the endometrium, the inner lining of the uterus.

The trophoblast also secretes human chorionic gonadotropin (HCG) which maintains the corpus luteum. The corpus luteum secretes estrogen and progesterone which maintain the uterine lining, preventing menstruation.

The flattened embryonic disk contains two layers, one layer is ectoderm, the other is endoderm. A primitive streak forms and cells along the edge of the streak migrate inward to form mesoderm just as it does in reptiles and birds. Mesoderm also adds to the trophoblast to form the chorion.

The yolk sac forms ventral to the embryonic disk. It does not function to provide food as it does in reptiles and birds. Blood cells are initially produced in the yolk sac.

The amnion forms dorsal to the embryo, between the embryo and the trophoblast. It will grow to surround the embryo. This fluid-filled membrane functions to protect the embryo from bumps and protects against temperature changes.

Week 3

The nervous system begins to form;  cells along the midline of the dorsal surface thicken and then neural folds form the neural tube.

The heart begins to form.

The allantois develops as an extension of the gut. It will form the blood vessels of the umbilical cord.

Limb buds are small paddle-like structures that will eventually produce arms and legs.

Placenta

Projections of the chorion called chorionic villi grow into the endometrium of the uterus, increasing the surface area of contact between the mother and fetus. Blood from the mother does not normally enter the fetal circulation; nutrients are exchanged across the membranes of the placenta.

The placenta secretes estrogen and progesterone. These two hormones inhibit the hypothalamus and anterior and thus prevent new follicles from forming. They also maintain the lining of the uterus so that the corpus luteum is no longer needed.