Embryonic development

After an egg cell, or ovum, is fertilized, it divides into many smaller cells, which then undergo rearrangement and differentiation to form the embryo of a new individual. The fertilized egg is called a zygote, and the division of the one-celled zygote into smaller and smaller cells is called cleavage. The cellular rearrangement is known as gastrulation, and the proliferation and movement of cells into position to form the beginnings of the central nervous system is termed neurulation. The significance of these events lies in the fact that a single cell with genetic information from two parents is transformed into a multicellular structure with three germ layers that will give rise to all the organs and systems of the body.

Cleavage

After fertilization, the resulting zygote undergoes many rapid cell divisions. The cleavage process results in smaller and smaller cells, called blastomeres. The cell divisions are by mitosis, which produces identical chromosomes in each new cell. When between sixteen and thirty-two cells have been formed, the structure is called a morula, from the Latin for mulberry, which it resembles.

The morula stage is short-lived because, as soon as it is formed, processes are initiated that bring it to the next stage, known as the blastula. A cavity begins to form in the center of the morula as water flows in and pushes out the cells. The new cavity is called the blastocoel, and the embryonic stage the blastula. Cleavage continues until the blastula consists of hundreds of cells but is still no larger than the original zygote. The blastula is the terminal cleavage structure. The egg cell, much larger than an average cell, has been fertilized and subdivided into hundreds of normal-sized cells. The blastomeres all appear to be similar to one another, but studies have shown that the individual cells are already destined for the tissues they will become.

Cleavage anembryonic development takes place inside a vessel called an egg; in some animals, such as all birds and most reptiles, the embryo is fed from an organ in the egg called the yolk (in animals like mammals, the embryo is fed directly from the mother through an umbilical cord). The principles of cleavage are the same in all vertebrate groups, but the mechanics differ according to the amount of yolk in the egg. Eggs with large amounts of yolk undergo only partial cleavage, because the yolk retards the cytoplasmic division. In birds, reptiles, and many fishes, the yolk is so dense that the cytoplasm and nucleus are crowded into a small cap or disk on one side of the cell. The cleavage divisions all occur in this small area, resulting in a flattened blastula atop the large inert yolk.

Eggs with but a moderate amount of yolk, such as amphibian eggs, can cleave completely. Because division proceeds more slowly through the part of the cell where yolk has accumulated, the cleavage is uneven. The cells are formed more slowly on the yolky side and are larger and fewer in number. The blastocoel is smaller and displaced to the side, with less yolk. The side with smaller blastomeres will develop into the embryo and is called the animal hemisphere. The side containing larger amounts of yolk is called the vegetal hemisphere and will provide nutrients for the embryo.

Eggs with very little yolk undergo total and equal cleavage divisions. The blastula has a large, centrally located blastocoel, and blastomeres are uniform in size. Starfish and the primitive chordate amphioxus undergo this kind of cleavage. They are often used to demonstrate the successive cleavage stages which are more easily seen in the absence of yolk. Though mammalian eggs (as from a platypus or echidna) do not have large amounts of yolk, their development is similar to that of birds. The outer layer of cells of the morula develops into a membrane, called the trophoblast, that surrounds the embryo. The embryo forms from cells in the inner region known as the inner cell mass. A large, fluid-filled blastocoel forms within the trophoblast, giving rise to the term “blastocyst” for the mammalian blastula. The inner cell mass develops atop the blastocoel as the bird embryo on the yolk.

Gastrulation

Gastrulation is the next process in embryonic development and consists of a series of cell migrations that result in cellular rearrangement. The final gastrula will have three embryonic germ layers destined to give rise to all body structures and systems.

The first step in gastrulation is an indenting or invagination in the blastula at a spot known as the dorsal lip. Cells begin to move over the lip and drop into the interior, forming the lining of a new cavity, the archenteron, or primitive gut. Continued inward movement of cells forms a middle layer between outer cells and inner ones which have dropped in through the opening, or blastopore. The three embryonic germ layers have now been formed, and they are called ectoderm, mesoderm, and endoderm.

In animals with little egg yolk, such as the starfish, gastrulation begins when a few cells lose their adhesiveness and drop into the blastocoel. That causes a dent or depression in that area. Cells move in and deepen the depression, forming the archenteron. As the archenteron expands, the inner blastocoel shrinks and is finally obliterated. This process may be visualized as punching in the side of a hollow rubber ball with one’s finger. The hole the finger makes is the blastopore; the new cavity formed by the hand represents the archenteron; and the original space inside the ball represents the blastocoel. The indentation forms two cell layers, and a third one is formed as cells continue to move in and take position between the inner and outer layers.

The outer ectoderm is destined to become epidermis and nerve tissue. The inner endoderm will form digestive glands and the lining of the digestive and respiratory systems. The middle germ layer, the mesoderm, will give rise to bone, muscle, connective tissue, and the cardiovascular and urinary systems. Additional mesoderm forms a rodlike structure known as the notochord, which lies in the roof of the archenteron. The notochord is a distinctive characteristic of chordates and gives embryonic support. The mesoderm lateral to the notochord will segregate into paired masses known as somites, each with prospective skin, bone, and nerve segments.

Gastrulation in blastulas with moderate quantities of yolk, such as those of amphibians, proceeds similarly, but the archenteron is displaced toward the animal hemisphere and is filled with yolk cells. The early stages are similar to those in starfish.

Gastrulation in birds and mammals is initiated in a manner different from that in starfish and amphibians, because of the discoidal configuration of the blastula. Both groups have incomplete cleavage with embryonic development on a disklike area on one side of the egg. The upper cells of the disk separate from the lower ones, forming two layers—the epiblast and the hypoblast. After the two layers are formed, a thickening occurs in one quadrant of the blastula and soon becomes noticeable as a distinct streak, the primitive streak. The streak becomes grooved, and cells from either side begin to migrate to the groove and sink down through it. The cells then move into position between the epiblast and hypoblast. The three embryonic germ layers have been formed.

The primitive groove in the gastrula is considered homologous to the blastopore in the starfish and amphibians. After the germ layers have been established, cells continue to move into the new cavity, the archenteron, and form a mesodermal notochord in the roof of the archenteron.

Neurulation

Neurulation is the final stage of early embryonic development. Studies have shown that the notochord induces the neurulation process to begin. Cells just above the notochord are induced to proliferate and thicken, forming a neural plate. After the neural plate is formed, a buckling occurs in it, forming a depression known as the neural groove. Modern microscope techniques have revealed microfilaments and microtubules lying beneath the surface of the plate. Contraction of the microfilaments and elongation of microtubules appear to cause cell buckling and folding of the plate. The neural groove deepens at its cephalic end, and folds on either side continue to grow higher until they actually touch each other, forming an enclosed tube, the neural tube. At the same time that the neural tube is forming, the head is growing forward, and tissue is folding beneath it, so it projects forward free from the surface. Brain differentiation begins with the enlargement of the anterior end of the neural tube. The undilated caudal portion will give rise to the spinal cord. The brain forms several constrictions, so that three bulges appear. These will become the three embryonic brain divisions: the forebrain, the midbrain, and the hindbrain.

Upon completion of the three brain divisions, the embryo undergoes forward flexion of the forebrain and a lateral torsion so that the embryo comes to life with its left side on the yolk. A final caudal flexion causes the embryo to take its typical C-shaped configuration.

Extraembryonic membranes form from tissue outside the embryo to provide oxygen, nutrients, and waste storage. In birds, an outer chorion and amnion fuse to form a membrane with a large blood supply which provides for the exchange of oxygen and carbon dioxide between the embryo and the atmosphere. The allantois is a membranous sac to contain waste secretions.

In mammals, the outer chorion becomes extensively vascularized on one side and interconnects with the uterus to form the placenta. Nutrient and waste exchange between mother and baby take place in the placenta. The amnion forms a fluid-filled sac that lies closely around the embryo and cushions it. The allantois is not needed for waste storage and is not well developed.

Embryology

Humans have always been intrigued by the processes of gestation and birth. Aristotle questioned whether the embryo unfolds from a preformed condition and then enlarges to adult proportions or progressively differentiates from simple to complex form. Not until the eighteenth century were actual observations made of a developing embryo. The chick egg was the first to be studied, because of its large size. Early studies were descriptive, as each stage of the embryo was observed and carefully described. It was found that development does proceed from simple form to forms increasingly complex.

In the late nineteenth century, great interest developed in evolutionary theory, and comparative embryology became the focal point of studies. Clues were sought for possible evolutionary relationships between organisms. The theory emerged that embryonic stages reflect the evolutionary past of an organism.

The twentieth century saw the explosion of experimental embryology and multiplication of knowledge. Cleavage of the large fertilized egg was first observed in the eighteenth century, but not until the late twentieth century did the mechanics begin to be understood. With improved microscope techniques, a ring of microfilaments can be seen just below the egg cell surface. These protein filaments have contractile qualities, and it was thought perhaps they lined up around the equator to contract and squeeze the cell in two. To test this hypothesis, a drug which causes microfilament subunits to break down was added to the cell culture. It was found that cell division was inhibited, suggesting that microfilaments are involved in the division process. Removal of astral rays also hindered cleavage. Each new discovery answers some questions and raises more.

Embryologists have questioned how blastomeres all formed from the same cell could differentiate into many kinds of cells and tissues. Some of the earliest experiments in embryology involved separating the first two daughter cells to demonstrate that each could form two complete individuals. How and when cells differentiate continues to be a challenge to researchers. The substance in cells which predisposes them to differentiate between one another is still not understood.

It has been discovered that each part of the embryo surface is already divided into prospective organ areas by the blastula stage. Fate maps have been constructed by marking certain areas on the blastula with vital stains and observing the structures into which they develop.

Since the early days of experimental embryology, researchers have performed all kinds of operations on embryos, marking areas and observing their movement, transplanting cells from one area to another, exchanging cell nuclei and removing portions. These experiments have led to many discoveries and better understanding of the complicated developmental process. When one considers the multitude of complex events that must take place in the development of a new individual from a single cell, it might seem impossible that the entire developmental process could occur without a slip.

Malformation usually begins during early development. Deformities may arise from inherited mistakes in the genetic code or from the harmful influence of external factors such as radiation, poor nutrition, or infection. Studies of cell migration in the embryo have led to ideas for procedures to inhibit tumor cell migration. Knowledge of normal cell development is helping to find ways to prevent abnormal cell development.

Principal Terms

Archenteron: The primitive gut cavity formed by the invagination of the blastula; the cavity of the gastrula

Blastula: An early stage of an embryo which is shaped like a hollow ball in some animals and a small, flattened disc in others; contains a cavity called the blastocoel

Cleavage: The process by which the fertilized egg undergoes a series of rapid cell divisions which result in the formation of a blastula

Gastrulation: The transformation of a blastula into a three-layered embryo, the gastrula; initiated by invagination

Germ layers: The embryonic layers of cells which develop in the gastrula: ectoderm, mesoderm, and endoderm

Invagination: The turning of an external layer into the interior of the same structure; formation of archenteron

Morula: A solid ball or mass of cells resulting from early cleavage divisions of the zygote

Neurulation: The process by which the embryo develops a central nervous system

Notochord: A fibrous rod in an embryo which gives support; a structure that will later be surrounded by vertebrae

Zygote: The fertilized egg; the first cell of a new organism

Bibliography

Minelli, Alessandro. "Revisiting the Embryo." Cambridge University Press: Understanding Development, 29 Apr. 2021, pp. 54-75. doi.org/10.1017/9781108872287.006.

Needham, Joseph, and Arthur Hughes. A History of Embryology. Rev. ed. Cambridge UP, 2015.

Oppenheimer, Steven B., and Edward J. Carroll. Introduction to Embryonic Development. 4th ed. Pearson, 2004.

Schoenwolf, Gary C. Atlas of Descriptive Embryology. 7th ed. Pearson, 2008.

Starr, Cecie, Ralph Taggart, Christine A. Evers, and Lisa Starr. Biology: The Unity and Diversity of Life. 15th ed. Cengage, 2019.