Embryology (from Greek ἔμβρυον, embryon, "the unborn, embryo"; and -λογία, -logia) is the branch of biology that studies the prenatal development of gametes (sex cells), fertilization, and development of embryos and fetuses. Additionally, embryology encompasses the study of congenital disorders that occur before birth, known as teratology.
Embryology has a long history. Aristotle proposed the currently accepted theory of epigenesis, that organisms develop from seed or egg in a sequence of steps. The alternative theory, preformationism, that organisms develop from pre-existing miniature versions of themselves, however, held sway until the 18th century. Modern embryology developed from the work of von Baer, though accurate observations had been made in Italy by anatomists such as Aldrovandi and Leonardo da Vinci in the Renaissance.
In bilateral animals, the blastula develops in one of two ways that divide the whole animal kingdom into two halves (see: Embryological origins of the mouth and anus). If in the blastula the first pore (blastopore) becomes the mouth of the animal, it is a protostome; if the first pore becomes the anus then it is a deuterostome. The protostomes include most invertebrate animals, such as insects, worms and molluscs, while the deuterostomes include the vertebrates. In due course, the blastula changes into a more differentiated structure called the gastrula.
Embryos in many species often appear similar to one another in early developmental stages. The reason for this similarity is because species have a shared evolutionary history. These similarities among species are called homologous structures, which are structures that have the same or similar function and mechanism, having evolved from a common ancestor.
Drosophila melanogaster, a fruit fly, is a model organism in biology on which much research into embryology has been done. Before fertilization, the female gamete produces an abundance of mRNA - transcribed from the genes that encode bicoid protein and nanos protein. These mRNA molecules are stored to be used later in what will become the developing embryo. The male and female Drosophila gametes exhibit anisogamy (differences in morphology and sub-cellular biochemistry). The female gamete is larger than the male gamete because it harbors more cytoplasm and, within the cytoplasm, the female gamete contains an abundance of the mRNA previously mentioned. At fertilization, the male and female gametes fuse (plasmogamy) and then the nucleus of the male gamete fuses with the nucleus of the female gamete (karyogamy). Note that before the gametes' nuclei fuse, they are known as pronuclei. A series of nuclear divisions will occur without cytokinesis (division of the cell) in the zygote to form a multi-nucleated cell (a cell containing multiple nuclei) known as a syncytium. All the nuclei in the syncytium are identical, just as all the nuclei in every somatic cell of any multicellular organism are identical in terms of the DNA sequence of the genome. Before the nuclei can differentiate in transcriptional activity, the embryo (syncytium) must be divided into segments. In each segment, a unique set of regulatory proteins will cause specific genes in the nuclei to be transcribed. The resulting combination of proteins will transform clusters of cells into early embryo tissues that will each develop into multiple fetal and adult tissues later in development (note: this happens after each nucleus becomes wrapped with its own cell membrane).
Outlined below is the process that leads to cell and tissue differentiation.
Zygotic-effect genes - subject to Mendelian (classical) inheritance
In 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 (tenth week of pregnancy), the developing human is then called a fetus.
As recently as the 18th century, the prevailing notion in western human embryology was preformation: the idea that semen contains an embryo – a preformed, miniature infant, or homunculus – that simply becomes larger during development.
Until the birth of modern embryology through observation of the mammalian ovum by von Baer in 1827, there was no clear scientific understanding of embryology. Only in the late 1950s when ultrasound was first used for uterine scanning, was the true developmental chronology of human fetus available.
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 Rusconi. 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 favoured explanation among embryologists.
Modern embryological pioneers include Charles Darwin, Ernst Haeckel, J.B.S. Haldane, and Joseph Needham. Other important contributors include William Harvey, Kaspar Friedrich Wolff, Heinz Christian Pander, August Weismann, Gavin de Beer, Ernest Everett Just, and Edward B. Lewis.
After the 1950s, with the DNA helical structure being unravelled and the increasing knowledge in the field of molecular biology, developmental biology emerged as a field of study which attempts to correlate the genes with morphological change, and so tries to determine which genes are responsible for each morphological change that takes place in an embryo, and how these genes are regulated.
As of today, human embryology is taught as a cornerstone subject in medical schools, as well as in biology and zoology programs at both an undergraduate and graduate level.
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.
Embryology is central to evolutionary developmental biology ("evo-devo"), which studies the genetic control of the development process (e.g. morphogens), 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.
the basic framework for understanding the molecular basis of embryonic development still rests largely on studies of developmental genetics in Drosophila
All the egg-polarity genes in these four classes are maternal-effect genes: it is the mother’s genome, not the zygotic genome, that is critical. Thus, a fly whose chromosomes are mutant in both copies of the Bicoid gene but who is born from a mother carrying one normal copy of Bicoid develops perfectly normally, without any defects in the head pattern. However, if that daughter fly is a female no functional Bicoid mRNA can be deposited into the anterior part of her own eggs, and all of these will develop into headless embryos regardless of the father’s genotype.
The future posterior end of the embryo con- tains a high concentration of mRNA for a regulator of translation called Nanos, which sets up a posterior gradient in the same way. The third signal is generated symmetrically at both ends of the egg
Most sexually reproducing metazoans are anisogamous, meaning that the two gametes that combine during fertilization differ greatly in size. By convention, the larger gametes are considered female and are called ova, while the smaller gametes are male and are called sperm. In most cases, both gametes contribute similarly to the chromosomal content of the new organism. In contrast, the maternal gamete contributes nearly all of the cytoplasm. This cytoplasmic contribution is crucial to patterning early development; it contains the maternal proteins and transcripts that guide the early steps of development prior to the activation of zygotic transcription.
The maternal gene Bicoid (BCD) organizes anterior development in Drosophila. Its mRNA is localized at the anterior tip of the oocyte and early embryo. Antibodies raised against bcd fusion proteins recognize a 55–57 kd doublet band in Western blots of extracts of 0–4 hr old embryos. This protein is absent or reduced in embryonic extracts of nine of the 11 bcd alleles. The protein is concentrated in the nuclei of cleavage stage embryos. It cannot be detected in oocytes, indicating temporal control of bcd mRNA translation. The bcd protein is distributed in an exponential concentration gradient with a maximum at the anterior tip, reaching background levels in the posterior third of the embryo. The gradient is probably generated by diffusion from the local mRNA source and dispersed degradation.
after the head of the sperm enters the cytoplasm of the egg... the chromatin begins to spread out within the nucleus (now called a pronucleus) as it moves closer to the nuclear material of the egg.
This type of embryo shows a separation of mitosis from cytokinesis during the early stages of development. Most cells are only formed when a syncytium of approximately 6000 nuclei are present.
A rounded syncytium was thus formed that contained two, four, or eight nuclei
Transfer of a single nucleus at a specific stage of development, to an enucleated unfertilized egg, provided an opportunity to investigate whether cellular differentiation to that stage involved irreversible genetic modification... The fact that a lamb was derived from an adult cell confirms that differentiation of that cell did not involve the irreversible modification of genetic material required for development to term
In the stages before fertilization, the anteroposterior axis of the future embryo becomes defined by three systems of molecules that create landmarks in the oocyte (Figure 22–32)... The three sets of genes responsible for these localized deter- minants are referred to as the anterior, posterior, and terminal sets of egg- polarity genes.
In Drosophila, a number of genes have been identified that are involved in segmentation
After the initial gradients of Bicoid and Nanos are created to define the antero- posterior axis, the segmentation genes refine the pattern.
The segmented body pattern along the longitudinal axis of the Drosophila embryo is established by a cascade of specific transcription factor activities. This cascade is initiated by maternal gene products that are localized at the polar regions of the egg. The initial long-range positional information of the maternal factors, which are transcription factors (or are factors which activate or localize transcription factors), is transferred through the activity of the zygotic segmentation genes. The gap genes act at the top of this regulatory hierarchy. Expression of the gap genes occurs in discrete domains along the longitudinal axis of the preblastoderm and defines specific, overlapping sets of segment primordia.
The phenotypes of the mutant embryos indicate that the process of segmentation involves at least three levels of spatial organization
Zygotic expression of the gap genes is thought to be required for the subdivision of the embryo into several units of adjacent segments
The pair-rule genes of Drosophila are required for the subdivision of the developing embryo into a repeating series of homologous body segments. One of the pair-rule genes, even-skipped (eve), appears to be particularly important for the overall segmentation pattern since eve- embryos lack all segmental subdivisions in the middle body region.
Segment polarity genes are expressed and required in restricted domains within each metameric unit of the Drosophila embryo.
In Drosophila, each stripe (segment) is subdivided into anterior and posterior halves. The posterior half of one segment and the anterior half of the next are collectively known as a parasegment.
Evidence available to date, primarily from the study of Drosophila melanogaster, indicates that homeotic genes act to control development by selecting among alternative developmental pathways or programs. Many of the homeotic genes of Drosophila are clustered in two complexes called the bithorax complex (BX-C) (Lewis 1952, 1963, 1978, 1982) and the Antennapedia complex (ANT-C) (Kaufman et al. 1980). Genes of the BX-C regulate pattern formation in part of the thorax and in the abdomen, whereas different genes of the ANT-C function in the head, thorax, and abdomen. Genes in both complexes are commonly described as regulating “segmental identity.”
Development is based on the differential expression of the genetic information in a precise spatial and temporal pattern. The interdependence of various parts of a developing organism essentially excludes the possibility that each structural gene is regulated independently, but rather suggests some form of hierarchical control in which the controlling genes interact to regulate the activity of groups of structural genes in a concerted fashion. The recent isolation of homeotic genes and the discovery of the homeo box appear to be decisive steps toward the identification of these developmental controlling genes.
Three years later, the Estonian, Karl Ernst von Baer, finally found the true mammalian egg in a pet dog (von Baer, 1827)
The amniotic fluid is the protective liquid contained by the amniotic sac of a gravid amniote. This fluid serves as a cushion for the growing fetus, but also serves to facilitate the exchange of nutrients, water, and biochemical products between mother and fetus.
For humans, the amniotic fluid is commonly called water or waters (Latin liquor amnii).Amniotic sac
The amniotic sac, commonly called the bag of waters, sometimes the membranes, is the sac in which the fetus develops in amniotes. It is a thin but tough transparent pair of membranes that hold a developing embryo (and later fetus) until shortly before birth. The inner of these fetal membranes, the amnion, encloses the amniotic cavity, containing the amniotic fluid and the fetus. The outer membrane, the chorion, contains the amnion and is part of the placenta. On the outer side, the amniotic sac is connected to the yolk sac, the allantois and, via the umbilical cord, to the placenta.Amniocentesis is a medical procedure where fluid from the sac is sampled to be used in prenatal diagnosis of chromosomal abnormalities and fetal infections.Chorion
The chorion is the outermost fetal membrane around the embryo in mammals, birds and reptiles. It develops from an outer fold on the surface of the yolk sac, which lies outside the zona pellucida (in mammals), known as the vitelline membrane in other animals. In insects it is developed by the follicle cells while the egg is in the ovary.Corona radiata (embryology)
The corona radiata is the innermost layer of the cells of the cumulus oophorus and is directly adjacent to the zona pellucida, the inner protective glycoprotein layer of the ovum. Its main purpose in many animals is to supply vital proteins to the cell. It is formed by follicle cells adhering to the oocyte before it leaves the ovarian follicle, and originates from the squamous granulosa cells present at the primordial stage of follicular development. The corona radiata is formed when the granulosa cells enlarge and become cuboidal, which occurs during the transition from the primordial to primary stage. These cuboidal granulosa cells, also known as the granulosa radiata, form more layers throughout the maturation process, and remain attached to the zona pellucida after the ovulation of the Graafian follicle. For fertilization to occur, sperm cells rely on hyaluronidase (an enzyme found in the acrosome of spermatozoa) to disperse the corona radiata from the zona pellucida of the secondary (ovulated) oocyte, thus permitting entry into the perivitelline space and allowing contact between the sperm cell and the nucleus of the oocyte.Development of the reproductive system
The development of the reproductive system is a part of prenatal development, and concerns the sex organs. It is a part of the stages of sexual differentiation. Because its location, to a large extent, overlaps the urinary system, the development of them can also be described together as the development of the urinary and reproductive organs.
The reproductive organs are developed from the intermediate mesoderm. The permanent organs of the adult are preceded by a set of structures which are purely embryonic, and which with the exception of the ducts disappear almost entirely before the end of fetal life. These embryonic structures are the mesonephric ducts (also known as Wolffian ducts) and the paramesonephric ducts, (also known as Müllerian ducts). The mesonephric duct remains as the duct in males, and the paramesonephric duct as that of the female.Ectoderm
Ectoderm is one of the three primary germ layers in the very early embryo. The other two layers are the mesoderm (middle layer) and endoderm (most proximal layer), with the ectoderm as the most exterior (or distal) layer. It emerges and originates from the outer layer of germ cells. The word ectoderm comes from the Greek ektos meaning "outside", and derma, meaning "skin."Generally speaking, the ectoderm differentiates to form the nervous system (spine, peripheral nerves and brain), tooth enamel and the epidermis (the outer part of integument). It also forms the lining of mouth, anus, nostrils, sweat glands, hair and nails.In vertebrates, the ectoderm has three parts: external ectoderm (also known as surface ectoderm), the neural crest, and neural tube. The latter two are known as neuroectoderm.Embryo
An embryo is an early stage of development of a multicellular diploid eukaryotic organism. In general, in organisms that reproduce sexually, an embryo develops from a zygote, the single cell resulting from the fertilization of the female egg cell by the male sperm cell. The zygote possesses half the DNA from each of its two parents. In plants, animals, and some protists, the zygote will begin to divide by mitosis to produce a multicellular organism. The result of this process is an embryo.
In human pregnancy, a developing fetus is considered as an embryo until the ninth week, fertilization age, or eleventh-week gestational age. After this time the embryo is referred to as a fetus.Endoderm
Endoderm is one of the three primary germ layers in the very early embryo. The other two layers are the ectoderm (outside layer) and mesoderm (middle layer), with the endoderm being the innermost layer. Cells migrating inward along the archenteron form the inner layer of the gastrula, which develops into the endoderm.The endoderm consists at first of flattened cells, which subsequently become columnar. It forms the epithelial lining of multiple systems.In plant biology, endoderm corresponds to the innermost part of the cortex (bark) in young shoots and young roots often consisting of a single cell layer. As the plant becomes older, more endoderm will lignify.Genitourinary system
The genitourinary system or urogenital system is the organ system of the reproductive organs and the urinary system. These are grouped together because of their proximity to each other, their common embryological origin and the use of common pathways, like the male urethra. Also, because of their proximity, the systems are sometimes imaged together.The term "apparatus urogenitalis" is used in Nomina Anatomica (under Splanchnologia), but not in Terminologia Anatomica.Human Fertilisation and Embryology Authority
The Human Fertilisation and Embryology Authority (HFEA) is an executive non-departmental public body of the Department of Health in the United Kingdom. It is a statutory body that regulates and inspects all clinics in the United Kingdom providing in vitro fertilisation (IVF), artificial insemination and the storage of human eggs, sperm or embryos. It also regulates human embryo research.Human embryonic development
Human embryonic development, or human embryogenesis, refers to the development and formation of the human embryo. It is characterised by the process of cell division and cellular differentiation of the embryo that occurs during the early stages of development. In biological terms, the development of the human body entails growth from a one-celled zygote to an adult human being. Fertilisation occurs when the sperm cell successfully enters and fuses with an egg cell (ovum). The genetic material of the sperm and egg then combine to form a single cell called a zygote and the germinal stage of development commences. Embryonic development in the human, covers the first eight weeks of development; at the beginning of the ninth week the embryo is termed a fetus.
Human embryology is the study of this development during the first eight weeks after fertilisation. The normal period of gestation (pregnancy) is nine months or 38 weeks.
The germinal stage refers to the time from fertilization through the development of the early embryo until implantation is completed in the uterus. The germinal stage takes around 10 days. During this stage, the zygote begins to divide, in a process called cleavage. A blastocyst is then formed and implanted in the uterus. Embryogenesis continues with the next stage of gastrulation, when the three germ layers of the embryo form in a process called histogenesis, and the processes of neurulation and organogenesis follow.
In comparison to the embryo, the fetus has more recognizable external features and a more complete set of developing organs. The entire process of embryogenesis involves coordinated spatial and temporal changes in gene expression, cell growth and cellular differentiation. A nearly identical process occurs in other species, especially among chordates.Mesenchyme
Mesenchyme, in vertebrate embryology, is a type of connective tissue found mostly during the development of the embryo. It is composed mainly of ground substance with few cells or fibers. It can also refer to a group of mucoproteins found in certain types of cysts (etc.), resembling mucus. It is most easily found as a component of Wharton's jelly.
The vitreous of the eyeball is a similar tissue.In invertebrate zoology, the term refers to free cells loosely arranged in a matrix.Mesoderm
In all bilaterian animals, the mesoderm is one of the three primary germ layers in the very early embryo. The other two layers are the ectoderm (outside layer) and endoderm (inside layer), with the mesoderm as the middle layer between them.The mesoderm forms mesenchyme, mesothelium, non-epithelial blood cells and coelomocytes. Mesothelium lines coeloms. Mesoderm forms the muscles in a process known as myogenesis, septa (cross-wise partitions) and mesenteries (length-wise partitions); and forms part of the gonads (the rest being the gametes). Myogenesis is specifically a function of mesenchyme.
The mesoderm differentiates from the rest of the embryo through intercellular signaling, after which the mesoderm is polarized by an organizing center. The position of the organizing center is in turn determined by the regions in which beta-catenin is protected from degradation by GSK-3. Beta-catenin acts as a co-factor that alters the activity of the transcription factor tcf-3 from repressing to activating, which initiates the synthesis of gene products critical for mesoderm differentiation and gastrulation. Furthermore, mesoderm has the capability to induce the growth of other structures, such as the neural plate, the precursor to the nervous system.Mesonephric duct
The mesonephric duct (also known as the Wolffian duct, archinephric duct, Leydig's duct or nephric duct) is a paired organ found in mammals including humans during embryogenesis. Wolffian structures are male urogenital structures that include the epididymis, vas deferens, and seminal vesicles that differentiate from this structure.
The mesonephric duct connects the primitive kidney, the mesonephros, to the cloaca and serves as the anlage for certain male reproductive organs.Neuroectoderm
Neuroectoderm (or neural ectoderm or neural tube epithelium) consists of cells derived from ectoderm. Formation of the neuroectoderm is first step in the development of the nervous system. The neuroectoderm receives bone morphogenetic protein-inhibiting signals from proteins such as noggin, which leads to the development of the nervous system from this tissue. Histologically, these cells are classified as pseudostratified columnar cells.
After recruitment from the ectoderm, the neuroectoderm undergoes three stages of development: transformation into the neural plate, transformation into the neural groove (with associated neural folds), and transformation into the neural tube. After formation of the tube, the brain forms into three sections; the hindbrain, the midbrain, and the forebrain.
The types of neuroectoderm include:
pigment cells in the skin
ganglia of the autonomic nervous system
dorsal root ganglia.
aorticopulmonary septum of the developing heart and lungs
ciliary body of the eye
parafollicular cells in the thyroid
brain (rhombencephalon, mesencephalon and prosencephalon)
spinal cord and motor neurons
posterior pituitaryRecapitulation theory
The theory of recapitulation, also called the biogenetic law or embryological parallelism—often expressed using Ernst Haeckel's phrase "ontogeny recapitulates phylogeny"—is a historical hypothesis that the development of the embryo of an animal, from fertilization to gestation or hatching (ontogeny), goes through stages resembling or representing successive adult stages in the evolution of the animal's remote ancestors (phylogeny). It was formulated in the 1820s by Étienne Serres based on the work of Johann Friedrich Meckel, after whom it is also known as Meckel–Serres law.
Since embryos also evolve in different ways, the shortcomings of the theory had been recognized by the early 20th century, and it had been relegated to "biological mythology" by the mid-20th century.Analogies to recapitulation theory have been formulated in other fields, including cognitive development and art criticism.Somite
Somites (outdated: primitive segments) are divisions of the body of an animal or embryo. The divisions are also known as metameric segments.
Somites are bilaterally paired blocks of paraxial mesoderm that form in the embryonic stage of somitogenesis, along the head-to-tail axis in segmented animals. In vertebrates, somites subdivide into the sclerotomes, myotomes, syndetomes, and dermatomes that give rise to the vertebrae of the vertebral column, rib cage, and part of the occipital bone; skeletal muscle, cartilage, tendons, and skin (of the back).The word somite is also used in place of the word metamere. In this definition, the somite is a homologously paired structure in an animal body plan, such as is visible in annelids and arthropods.Umbilical artery
The umbilical artery is a paired artery (with one for each half of the body) that is found in the abdominal and pelvic regions. In the fetus, it extends into the umbilical cord.Yolk sac
The yolk sac is a membranous sac attached to an embryo, formed by cells of the hypoblast adjacent to the embryonic disk. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though yolk sac is far more widely used. In humans, the yolk sac is important in early embryonic blood supply, and much of it is incorporated into the primordial gut during the fourth week of development.
Human embryogenesis in the first three weeks
|Week 2 |
|Week 3 |
Development of bone
Development of the circulatory system
Development of the nervous system
Development of the respiratory system