Biomedicine and Health: Embryology

Introduction

Embryology is a branch of developmental biology that studies the development of living things as they progress from zygotes to multicellular organisms. Many embryologists focus on developmental processes at the molecular level. Contemporary embryology is a subdiscipline of the study of an animal's life history; human embryology focuses on developmental aspects of life in general, and not just the first eight weeks.

Historical Background and Scientific Foundations

The life cycle begins in adult organisms with gametogenesis (the production of gametes, which are reproductive, or sex, cells). Fertilization, the merging of male and female gametes, initiates the development of the embryo. In animals, the focus of this article, the fertilized female gamete or ovum (egg) undergoes cleavage (division and replication) to form a blastula, an amorphous clump of identical, undifferentiated cells. The blastula then undergoes gastrulation, or formation of a gastrula, which consists of a rounded, hollow two-layered sac of ectoderm (outer cell layer) and endoderm (inner cell layer).

The gastrula stage exhibits the first signs of cell differentiation. This continues as cells form various kinds of tissues, leading to organogenesis (the formation of organs), which supports fetal growth. This is followed by the hatching or birth (depending on the species) of a juvenile form, which progresses to an adult form capable of gametogenesis, and the cycle continues.

Although the first recorded writing about embryos dates to ancient India and Egypt, the ancient Greek philosopher Plato (428–348 BC) began the system of thinking about human development that evolved into embryology by proposing the concept of the “souls.” The vegetative soul, which initiates life, he believed, was found in plants. The sensitive soul was a property of sentient beings and gave rise to animals. At the highest level of development the spiritual soul made thinking possible, a characteristic unique to humans.

Plato's pupil Aristotle (384–322 BC) was the first to describe the two main historical models of development: Preformation supposes that an embryo or miniature individual (homunculus) exists in either the mother's egg or the father's semen and starts to grow when appropriately stimulated. Aristotle, who had studied the development of chick embryos, favored an alternate view—epigenesis, which holds that the embryo starts life as an undifferentiated mass, and that new parts are added during development, beginning with the heart. Aristotle believed that the female parent contributed only unorganized living matter to the embryo. He thought that semen provided the “form,” or soul, to use Plato's term, that guided development.

Aristotle's investigations were highly scientific and could have inaugurated a true science of embryology, but as with most areas of Western civilization, little advancement was made beyond the Greeks until the Renaissance. By focusing again on chicken eggs, a research subject close at hand, Dutch physician Volcher Coiter (1534–c.1590) published a thorough study of chick embryology. Although his work was largely forgotten, he is often cited as the founder of embryology. Other scientists reserve that distinction for Hieronymus Fabricius ab Aquapendente, better known as Girolamo Fabrici (c.1537–1619), who also studied chick development.

English physician William Harvey (1578–1657), famous for his seminal discoveries in hematology (the study of blood), made important contributions to embryology by revisiting Aristotle's work, both expanding and correcting it. He redefined epigenesis to describe the development of organisms based on their apparent inherited traits. (Modern epigenesis studies the process that produces individual traits, based on gene activity during cell and tissue differentiation and development.)

Another of Harvey's significant contributions was his belief that “omne vivum ex ovo”: all life, including human life, comes from the egg. Harvey was inspired by the work of his teacher, Girolamo Fabrici, especially his treatises On the Formed Fetus and On the Development of the Egg and the Chick, which raised doubts about many aspects of Aristotelian and Platonic theory. Harvey's own work, On the Generation of Animals, was published in 1651 after years of research. Harvey had begun these investigations to prove Aristotle's theory of epigenesis, but his observations demonstrated that much of Aristotle's theory of generation was incorrect.

For example, Aristotle thought that the embryo formed by coagulation in the uterus directly after mating, when the form-building substance of the male acted on the female's raw material. Dissecting deer that had recently mated, Harvey searched for the embryo and was unable to find evidence of a developing embryo in the uterus until approximately six to seven weeks after coitus. In his studies of the developing chick egg, Harvey's observations proved that developing animals progressed through epigenesis, that is, the gradual addition of parts not originally present.

Despite this, many of Harvey's disciples abandoned epigenesis and embraced preformationist theories. As the microscope enabled the embryo to be seen at earlier stages of development, scientists like Italian physician Marcello Malpighi (1628–1694) and Dutch naturalist Jan Swammerdam (1637–1680), made observations that appeared to support preformation. Swammerdam's studies of insects and amphibians, in particular, suggested that embryos preexisted within each other much like nested boxes. A limitation of this theory was that only one parent could initiate the sequence of preformed individuals. The microscope uncovered the existence of animalcules, “little animals,” in semen, and some naturalists thought that preformed individuals must be present in the sperm.

Eminent naturalists of the day such as Swiss biologist Albrecht von Haller (1708–1777), Charles Bonnet (1720–1793), Italian physiologist Lazzaro Spallanzani (1729–1799), and French entomologist René-Antoine Ferchault de Réaumur (1683–1757) argued for pre-formation. Bonnet's research into parthenogenesis in aphids was thought to provide strong support for ovist (in the egg) preformationism. This gave rise to the notion that the whole human race had preexisted in the ovaries of Eve; some naturalists reported seeing homunculi inside spermatozoa.

Other eighteenth-century naturalists rejected both ovist and spermist preformationist theories. German anatomist Casper Friedrich Wolff (1733–1794) published his groundbreaking Theory of Generation in 1759. He maintained that bodily organs did not exist at the beginning of gestation, but developed from some originally undifferentiated material through a series of steps, which he called morphogenesis, an idea remarkably similar to Aristotle's original concept of epigenesis. The

formulation of cell theory, the detection of the mammalian egg by Prussian embryologist Karl Ernst von Baer (1792–1876), and the founding of experimental embryology by German zoologist Wilhelm Roux (1850–1924) and embryologist Hans Driesch (1867–1941) revolutionized philosophical debates about the development of embryos.

Ernst Haeckel (1834–1919) followed von Baer as the leading authority in embryology at the end of the nineteenth century. Haeckel became famous for his belief that “ontogeny recapitulates phylogeny”—the (now-disproven) idea that an individual organism's embryonic development progresses through its evolutionary progenitors. For example, during gestation a human

embryo resembles at various points a fish, a tadpole, etc.

Although Haeckel's mistaken ideas were based on an interesting insight, his opinions on race and evolution were later incorporated into Nazi pseudoscience, justifying persecution of “non-Aryan” and other groups of people.

In the early twentieth century, scientists were able to make painstaking observations of some developing life forms. As high-quality microscopes had become available scientists were able to learn that the dorsal ectoderm of all vertebrate embryos folds up into a tube to form the central nervous system. The question of what led to its differentiation into the brain and the spinal cord still remained, however. Scientists hypothesized that the original chordamesoderm cells of the gastrula (mesoderm cells that would ultimately develop into the nervous system in vertebrates) signaled the ectoderm (outer cells of the gastrula) to become nerve cells.

German embryologist Hans Spemann (1869–1941) was the first to conduct rigorous experiments on living embryos, during which he discovered the process of induction, the biochemical signal that led to cellular differentiation in the nervous system and other embryonic organs, a fundamentally important phenomenon in embryology. Spemann won the Nobel Prize for Medicine in 1935 for his work.

In the late nineteenth century, German experimental biologist August Weisman (1834–1914) developed the germ plasm theory, proposing in 1892 that self-reproducing determinants guided morphogenesis and that these determinants were located on newly discovered structures in cell nuclei called chromosomes. He hypothesized that cellular differentiation resulted when cells acquired different chromosomes during replication. This notion has been corrected by modern molecular embryology, which has demonstrated that while the chromosomes remain constant throughout development, variations in their genetic expression control differentiation of specific tissue and cell types during development. Essentially, cells growing in particular tissue environments become differentiated to the cell type prevalent in surrounding tissue. Cells growing in muscle tissue become muscle cells, while cells growing in the central nervous system become neurons, etc.

IN CONTEXT: EMBRYONIC GROWTH

Beginning at the moment of conception, the human organism depends upon adequate nutrition for growth, development, and survival. In the first week after fertilization, the zygote produces a series of blastomeres and, after further cellular divisions, the morula, which contains about 10–30 cells. The morula stage proceeds to the formation of a fluid-filled cavity, the early blastocyst. Inside the blastocyst is an inner cell mass or embryoblast (future embryo), and the outer cell mass or trophoectoderm (future placenta).

Embryogenesis (three to eight weeks) involves three major processes: morphogenesis (generation of shape), pattern formation (biologic-spatial cell organization), and differentiation (specialization in specific phenotypes). During embryogenesis, tissues and organs develop. The most important events of the embryonic period occur in gastrulation, a process in which the bilaminar embryonic disk is converted into the three primary embryonic germ layers: the mesoderm, ectoderm, and endoderm. Formation of the dorsal mesoderm, (notochord and paraxial mesodermal cords), somites, and intermediate and lateral mesoderm eventually form blood vessels, muscles, and the excretory system. The ectoderm forms the neural tube, which will produce the nervous system and skin. The evolution of endoderm essentially develops into the digestive apparatus, respiratory apparatus, some parts of the urogenital system, and branchial pouches (part of the branchial apparatus). By the end of eight weeks, the embryo is about one inch (2.5 cm) long.

The fetal period goes from nine to 38 weeks. It is characterized by rapid body growth (from 2 oz [60 g] to 5.5–8.8 lb [2,500–4,000 g]). In this period the embryo, now termed the fetus, is starting to grow. Length velocity has a peak at 20 weeks, a time in which the fetus grows about 10 cm over four weeks. At nine weeks, half the fetus' overall size is its head. From 13 to 16 weeks, the head is relatively smaller, and the limbs (legs) are longer. At the fifth month (17–20 weeks) growth slows down, but the fetus is longer and the limbs reach their final relative proportions. The fetus gains the most weight at 30–34 weeks. Parameters used for monitoring fetal growth include the embryonic crown-rump length, biparietal diameter (BPD), head circumference, femur length, and abdominal circumference. Ultrasound technology is often used to evaluate fetal growth.

Two of Haeckel's students, German zoologist Oscar Hertwig (1849–1922) and his brother Richard Hertwig (1850–1937) demonstrated the phenomenon of fertilization in sea urchins, and showed that spermatozoa penetrated the ova cell wall and activated the development process. They also detected polar bodies during cell division, which resulted in important progress in the understanding of meiosis, one of two types of cell division.

In the mitosis type of cell division, the two (daughter) cells produced by the cell's division are identical to the genetic makeup of the original cell. They have the same DNA, make the same proteins, and behave exactly the same. Meiosis, on the other hand, produces new types of cells called gametes: eggs in females and sperm in males. Gametes contain a single (haploid) set of chromosomes—a random mixture of the genes contributed by the parents to the original fertilized ovum as the original fertilized egg divides to begin blastula formation; this passes genetic information to future generations.

Modern CulturalConnections

Embryology has many applications to medical research, particularly in discovering the causes of developmental abnormalities—congenital malformations or birth defects. Medical research has shown that the human embryo is extremely sensitive to drugs, viruses, and radiation during the first several months of development, when many crucial organ systems are forming.

Much modern research focuses on intracellular synthesis and its regulation, particularly how the external environment, called the extracellular matrix, can influence an undifferentiated (stem) cell to form a particular kind of tissue cell. The matrix contains proteins that attach to the walls of developing stem cells. In cartilage tissue, for example, these are glycoproteins, a combination of amino acids and sugars. When they bind to a stem cell, they influence the cell to become part of the cartilage, which helps the cartilage tissue to grow and regenerate when damaged. During this process, general genes are deactivated and no longer produce their characteristic proteins, while cartilage-specific genes are activated and produce the proteins characteristic of cartilage tissue.

Contemporary embryology builds on the knowledge of cellular differentiation to explain other fundamental development processes as well, including cell death (apoptosis), tissue regulation, and regeneration, the morphogenetic field or tissue area over which cells differentiate into particular cell types, and tissue growth. All of these are subject to enhancements and disruptions that often become the subjects of medical research, leading to new treatments for disease.

Cellular Differentiation in Male and Female Embryos

The urogenital systems in both males and females develop from early embryonic urogenital ridges composed

of mesodermal cells. Although the sex of the embryo is fixed at birth by its sex chromosomes (XX for females and XY for males), for a considerable period in embryonic development, male and female embryos are anatomically ambisexual and thus share a number of features in common.

Gonads—ovaries in females and testes in males—both arise from thickening areas of cells of the urogenital ridge. Initially very much alike in their path of development (the indifferent stage of development), the ultimate development of ovaries and testis is an example of the phenotypic (outward) expression of karyotype or genotype (the actual genes and chromosomes present).

Cells comprising and continuing the germ cell line are diploid cells that give rise to cells that undergo meiosis to form haploid (half the chromosome number) sex cells (i.e., male spermatids and female oocytes). Germ cells are differentiated early in development, possibly during the initial divisions of the zygote—but no later than in the cellular divisions that take place in or near the primitive urogenital ridge. Regardless, although the germ cells come to lie in the gonads of both males and females, the germ cells are extra-gonadal in origin.

The full development of males and females involves a coordinated series of developmental steps that take place throughout gestation. Throughout the course of development, primitive embryonic structures give rise to sexually characteristic structures. Commonality is further established because male and female structures are homologues derived from the same early embryonic cells. For example, the testis and ovary are homologues, as both are derived from the indifferent gonad. The embryonic Mullerian duct develops into the appendix of the testis in males. In females, the Mullerian duct develops into the uterus, uterine tube, and the cervix, which separates the vaginal canal from the uterus.

See Also Biology: Cell Biology; Biology: Comparative Morphology: Studies of Structure and Function; Biology: Concepts of Heredity and Change Prior to the Rise of Evolutionary Theory; Biology: Evolutionary Theory; Biology: Genetics; Biology: Genetics, DNA, and the Genetic Code; Biology: Ontogeny and Phylogeny.

bibliography

Books

Gilbert, Scott F. Developmental Biology, 2nd ed. Sunderland, MA: Sinauer Associates, 1988.

IN CONTEXT: STEM CELLS

Stem cells are undifferentiated cells that have the capability of self replication, as well as being able to give rise to diverse types of differentiated or specialized cell lines. They are subclassified as embryonic stem cells, embryonic germ cells, or adult stem cells. Embryonic stem cells are cultured cells that were originally collected from the inner cell mass of an embryo at the blastocyst stage of development (four days post-fertilization). Embryonic germ cells are derived from the fetal gonads that arise later in fetal development. Both of these stem cell types are pluripotent, that is, they are capable of producing daughter cells that can differentiate into all of the various tissues and organs of the body that are derived from the endoderm, ectoderm, and mesoderm. Adult stem cells, found in both children and adults, are somewhat more limited, or multipotent, since they are associated with a single tissue or organ and function primarily in cell renewal for that tissue.

Because they are undifferentiated, stem cells have unique properties that may make them useful for new clinical applications. Initially, researchers realized that stem cells might be induced to produce a broad range of different tissues that could be utilized for transplantation. Research on Parkinson disease, a neurodegenerative disorder that results in loss of brain function following the death of dopamine-producing cells, underscored the potential of this approach. In the 1980s, studies on monkeys and rats showed that when fetal brain tissue rich in stem cells was implanted into the brains of diseased animals, there was a regeneration of functional brain cells and a reduction or elimination the symptoms of the disease. One disadvantage to this as a clinical procedure is that random pieces of undefined tissue are used, resulting in the significant possibility of variability from one patient to the next. A better solution would be to isolate the embryonic stem cells, induce these cells to differentiate, and generate a population of dopamine-producing cells. Theoretically, if these cells were transplanted back into the brains of Parkinson patients, they would replace the defective cells and reverse the course of the disease. However, the mechanisms that trigger differentiation of embryonic stem cells into various specialized tissue types are not yet well understood, so it will require additional research before transplantable tissues derived from embryonic stem cells will be a reality.

It has also been suggested that embryonic stem cells might be used in gene therapy. There also are many different diseases, ranging from heart disease to spinal cord injury and autoimmune disorders, that could benefit from a better understanding of and the use of stem cells as therapeutic agents. Although work is ongoing, research on embryonic stem/germ cells is limited due to an ethical dilemma regarding the source of the cells.

For research purposes, embryonic stem cells are primarily derived from leftover products of in vitro fertilization procedures. Embryonic germ cells from later gestational age fetuses have been obtained from elective termination of pregnancy or spontaneous fetal demise with appropriate parental consent. However, because it is feared that an increase in research using these cell types would encourage the “buying and selling” of embryos for profit, researchers have been, in some cases, restricted to the use of currently existing cell lines rather than establishing new cell cultures.

In 2007 and 2008, researchers announced studies that might ultimately provide an alternative to some ethical dilemmas by establishing the ability to create stem cells from cloned monkey embryos and normal adult skin cells rather than from destroyed human embryos. The ability to obtain stem cells from sources other than human embryos offered the possibility of mooting (making irrelevant) the ethical debate about deriving stem cells from destroyed human embryos.

Web Sites

Anders, Ralf. Developmental Biology. “Developmental Biology of Plants and Animals.” May 14, 2007. http://developmentalbiology.de/en (accessed January 29, 2008).

Sinauer Associates. “Developmental Biology. Vol. 6.” http://www.ncbi.nlm.nih.gov/ books/bv.fcgi?rid=dbio (accessed January 29, 2008).

René Nougayrède