Embryology


Embryology is the branch of zoology that studies the prenatal development of gametes, fertilization and development of embryos and fetuses. Embryology includes teratology, the study of congenital disorders that occur before birth.
Early embryology, put forward by Marcello Malpighi, was preformationist in concept: based on the idea that organisms develop from pre-existing miniature versions of themselves. The theory now accepted, epigenesis, is the idea that organisms develop from seed or egg in a sequence of steps. This concept was proposed in antiquity by Aristotle. Modern embryology developed from the work of Karl Ernst von Baer, though accurate observations had been made in Italy by anatomists such as Aldrovandi and Leonardo da Vinci in the Renaissance.

Comparative embryology

Preformationism and epigenesis

As recently as the 18th century, the prevailing notion in western human embryology was preformation: the idea that a sperm cell itself contains an embryoa preformed, miniature infant, or homunculuswhich simply becomes larger as it develops.
The competing explanation of embryonic development was epigenesis, originally proposed 2,000 years earlier by Aristotle. Much early embryology came from the work of the Italian anatomists Aldrovandi, Aranzio, Leonardo da Vinci, Marcello Malpighi, Gabriele Falloppio, Girolamo Cardano, Emilio Parisano, Fortunio Liceti, Stefano Lorenzini, Spallanzani, Enrico Sertoli, and Mauro Ruscóni. According to epigenesis, the form of an animal emerges gradually from a relatively formless egg. As microscopy improved during the 19th century, biologists could see that embryos took shape in a series of progressive steps, and epigenesis displaced preformation as the favored explanation among embryologists.

Cleavage

The cleavage phase of embryonic development is the series of several mitotic cell divisions that occurs immediately after the egg is fertilized by the sperm, producing the blastula. The blastula is a single sheet of cells; in most phyla it then undergoes gastrulation. The resulting gastrula has in some species two, in most three, cell layers. The distinctive feature of cleavage, as a type of cell division, is that the cell divides without increase of cytoplasmic mass. The daughter cells share it, each having roughly half.
Overall, the cleavage phase of any species takes one of several forms. These forms are characteristic of different types of bilateral animal.

Holoblastic

is cleavage of all the cells derived from the original zygote. The division furrow crosses the entire cell cluster; the whole cell cluster eventually becomes the embryo. Different types of animal differ in the geometry of the division furrow: cleavage is radial, spiral, bilateral or rotational.

Meroblastic

is the division of some but not all cells, as the division furrow does not protrude into the yolky region. The cells there impede formation of the associated membrane and only the other cells separate. Meroblastic cleavage is bilateral, discoidal or centrolecithal.

Basal phyla

Animals that belong to the basal phyla have holoblastic radial cleavage which results in radial symmetry. During cleavage, there is a central axis that all divisions rotate about. The basal phyla also have only one to two embryonic cell layers, compared to the three in bilateral animals.

Bilaterians

In the bilateral animals cleavage can be either holoblastic or meroblastic. The subsequent gastrulation occurs in one of two ways, and this contrast divides the whole animal kingdom into two major groups. In protostomes the first pore of the blastula becomes the mouth of the animal; in deuterostomes the mouth derives from a later pore and the blastopore becomes the anus. The protostomes include most invertebrate animals, such as insects, worms and molluscs, while the deuterostomes include a few invertebrates such as the echinoderms and all the vertebrates.
The bilaterian gastrula then develops three distinct layers of cells ; from them all the bodily organs and tissues subsequently arise.

Germ layers

  • The innermost layer, or endoderm, gives rise to the digestive organs, the gills, lungs or swim bladder if present, and kidneys or nephrites.
  • The middle layer, or mesoderm, gives rise to the muscles, skeleton if any, and blood system.
  • The outer layer of cells, or ectoderm, gives rise to the nervous system, including the brain, and skin or carapace, and hair, bristles or scales.

    ''Drosophila melanogaster'' (fruit fly)

Drosophila have been used as a developmental model for many years. These studies have discovered many useful aspects of development that apply to other species. Outlined below is the process that leads to cell and tissue differentiation.
  1. Maternal-effect genes help to define the anterior-posterior axis using the bicoid and nanos genes.
  2. Gap genes establish three 'broad segments' of the embryo.
  3. Pair-rule genes define seven segments of the embryo within the second of those 'broad' segments.
  4. Segment-polarity genes divide each of those pre-existing seven segments into anterior and posterior halves, using a gradient of Hedgehog and Wnt signal proteins.
  5. Homeotic genes use the 14 segments as pinpoints for specific types of cell differentiation and the histological developments that correspond to each cell type.

    Humans

Humans are bilateral animals that have holoblastic rotational cleavage. Humans are also deuterostomes. In regard to humans, the term embryo refers to the ball of dividing cells from the moment the zygote implants itself in the uterus wall until the end of the eighth week after conception. Beyond the eighth week after conception, the developing human is then called a fetus.

Evolutionary embryology

Evolutionary embryology is the expansion of comparative embryology by the ideas of Charles Darwin. Similarly to Karl Ernst von Baer's principles that explained why many species often appear similar to one another in early developmental stages, Darwin argued that the relationship between groups can be determined based upon common embryonic and larval structures.

Von Baer's principles

  1. The general features appear earlier in development than do the specialized features.
  2. More specialized characters develop from the more general ones.
  3. The embryo of a given species never resembles the adult form of a lower one.
  4. The embryo of a given species does resemble the embryonic form of a lower one.
Using Darwin's theory, evolutionary embryologists have since been able to distinguish between homologous and analogous structures appearing in different species. Homologous structures are those whose similarities derive from a common ancestor, such as the human arm and bat wings. Analogous structures are those that seem similar despite lacking common ancestral derivation.

Origins of modern embryology

Until the birth of modern embryology through observation of the mammalian ovum by Karl Ernst von Baer in 1827, there was no clear scientific understanding of embryology, although later discussions in this article show that some cultures had a fairly refined understanding of some of the principles. Only in the late 1950s when ultrasound was first used for uterine scanning, was the true developmental chronology of human fetus available. Karl Ernst von Baer along with Heinz Christian Pander, also proposed the germ layer theory of development which helped to explain how the embryo developed in progressive steps. Part of this explanation explored why embryos in many species often appear similar to one another in early developmental stages using his four principles.

Modern embryology research

Embryology is central to evolutionary developmental biology, which studies the genetic control of the development process, its link to cell signalling, its roles in certain diseases and mutations, and its links to stem cell research. Embryology is the key to gestational surrogacy, which is when the sperm of the intended father and egg of intended mother are fused in a lab forming an embryo. This embryo is then put into the surrogate who carries the child to term.

Medical embryology

Medical embryology is used widely to detect abnormalities before birth. 2–5% of babies are born with an observable abnormality, and medical embryology explores the different ways and stages that these abnormalities appear. Genetically derived abnormalities are referred to as malformations. When there are multiple malformations, this is considered a syndrome. When abnormalities appear due to outside contributors, these are disruptions. The outside contributors causing disruptions are known as teratogens. Common teratogens are alcohol, retinoic acid, ionizing radiation or hyperthermic stress.

Vertebrate and invertebrate embryology

Many principles of embryology apply to invertebrates as well as to vertebrates. Therefore, the study of invertebrate embryology has advanced the study of vertebrate embryology. However, there are many differences as well. For example, numerous invertebrate species release a larva before development is complete; at the end of the larval period, an animal for the first time comes to resemble an adult similar to its parent or parents. Although invertebrate embryology is similar in some ways for different invertebrate animals, there are also countless variations. For instance, while spiders proceed directly from egg to adult form, many insects develop through at least one larval stage.
For decades, a number of so-called normal staging tables were produced for the embryology of particular species, mainly focussing on external developmental characters. As variation in developmental progress makes comparison among species difficult, a character-based Standard Event System was developed, which documents these differences and allows for phylogenetic comparisons among species.