SEM_01 Introduction to Embryology PDF

Introduction and Generalities A quote "One egg, one embryo, one adult - normality. But a bokanovskified egg will bud, will proliferate, will divide. From eight to ninety-six buds, and every bud will grow into a perfectly formed embryo, and every embryo into a full-sized adult. Making ninety-six huma...

Introduction and Generalities A quote "One egg, one embryo, one adult - normality. But a bokanovskified egg will bud, will proliferate, will divide. From eight to ninety-six buds, and every bud will grow into a perfectly formed embryo, and every embryo into a full-sized adult. Making ninety-six human beings grow where one grew before. Progress.” ― Aldous Huxley, Brave New World (1932) Learning objectives • Review the main historical events that gave rise to embryology as a science. • Discuss some basic concepts in animal development. • Understand the importance of comparative nomenclature. History of embryology The way in which life develops has led to spirited discussions of the complexities of the process amongst philosophers and scientists over the years. The following paragraphs summarise some of the major events that have marked the history of embryology as a science. The prevalence of superstition Like all sciences, embryology has roots reaching back into prehistory. Who are we? .......Where do we come from?...... Where are we going? ........ No doubt that these existential questions must have arisen in the awakening of the rational mind, but no doubt too that the primitive answers were based on supernatural explanations. 1 - Where do we come from? What are we? Where are we going? It is a painting by French artist Paul Gauguin. The painting was created in Tahiti, and is in the Museum of Fine Arts in Boston, Massachusetts, USA. The dawn of science The study of embryology, the science that deals with the formation and development of embryos, can be traced back to ancient Greek philosophers. The first written record of embryological research is attributed to Hippocrates (4th century BCE) who wrote about obstetrics and gynaecology. In this regard, Hippocrates, and not Aristotle, should be recognised as the first true embryologist. Nevertheless, Aristotle (3rd century BCE) has been credited as the father of many scientific fields, including embryology. Aristotle aimed to unify all the knowledge in a coherent system of thought by developing a common methodology that would serve equally well as a suitable procedure for learning about any discipline: the logical method. 2 - Placenta and Embryonic Membranes - described in Rome by the Greek physician Claudius Galen (129--210 AD) in a book "On The Formation of The Foetus" Dark ages and the Renaissance From Aristotle until the arrival of the Renaissance, the science of embryology developed very slowly following descriptive anatomical approaches: descriptive embryology. Much of the early embryology was descriptive in nature due to several limiting factors, such as social and political ruling ideas, language barriers, lack of microscope, to name but a few. Although Galen (2nd century CE) and some other medieval thinkers made some contributions to embryology, it was not until the Renaissance when the embryology gained a new thrust. In the late 1400s and early 1500s, the resurgence of science was dominated by the work of Leonardo da Vinci. In the 16th and 17th centuries, other noteworthy descriptions of the embryo development in some animal species were made by Carlo Ruini, Hieronymus Fabricius and William Harvey. Nonetheless, during that time, one of the most important issues in premodern biology was the struggle between preformationist and epigenetic theories of development. 3 - Illustration of the of a fetal horse with the umbilical cord and the fetal membranes of the horse, by Ruini in 1589 Two persistent theories Does the embryo start in some already preformed or predetermined way, or does the embryo form gradually over time from amorphous material? Although the historical debate about preformation versus epigenesis seems to be settled in favour of the latter, the core question underlying the existence of these two competing philosophical theories has persisted since ancient times. In what extent something is formed or organised from the “beginning” or whether organisation and form arise only over time? From this perspective, preformation and epigenesis can be considered as two alternative ways of describing embryo development. Even nowadays, modern genetic determinists continue to appeal to the already “formed” through genetic inheritance, while others insist on the efficacy of environmental plasticity. Nature or nurture, genetic determinism or developmental free will, or is some version of a middle ground possible? These are the terms of this perennial discussion, which have profound bioethical and policy implications Preformationism Preformism or preformationism was a theory of embryological development used from the late 17th to the late 18th centuries. This theory held that the generation of offspring occurs as a result of the growth of preformed parts. Prior to preformationism, naturalists who studied embryo development favoured the theory of spontaneous generation in lower animals, such as flies, which appeared to arise from manure. In higher animals, however, scientists used a "kind of" epigenetic approach, fol lowing the Aristotle theories, who said that maternal and paternal fluids came together in the uterus and solidified during early gestation into a foetus. Preformationism was the first theory of generation and development that applied to all organisms in the plant and animal kingdoms. There were two competing models of preformationism: the ovism model, in which the location of these preformed parts prior to gestation was the maternal egg, and the spermism model, in which a preformed individual or homunculus was thought to exist in the head of each sperm. Ovist preformism (Ovism) was the first conceptual model of preformation. Sperm cells were not observed until the invention of the microscope and even afterwards, were initially considered to be parasitic worms and not important to fertilisation. The naturalists of the time were familiar with the concept of young animals hatching from eggs (oviparous), and it followed that animals that gave birth to live offspring (viviparous) might also have an egg stage in their early development. William Harvey is sometimes credited with the beginning of the ovist model with his statement, “ex ovo omnia” (from the egg, all) in the mid-17th century. However, Harvey himself was a proponent of the epigenetic theory of generation. Marcello Malpighi is more likely to be the scientific father of preformationism than Harvey. Under a microscope, Malpighi saw tiny fully-formed organs and tissues that needed only to grow in size to become infant chickens. Later scientists and philosophers drew the conclusion that those ti ny, preformed parts must exist from the very beginning, though it is interesting that Malpighi did not seem to state this explicitly. Under the ovist model of preformationism, it was assumed that the seminal fluid from the male parent was only required to begin the process of growth in the preformed embryo. 4 - Ovist preformism (Ovism) was the first conceptual model of preformation. Spermatazoa were not observed until the invention of the microscope and even afterwards, were initially considered to be parasitic worms and not important to fertilisation. Spermist preformism (Spermism) was the belief that offspring develop from a tiny fully-formed embryo contained within the head of a sperm cell. This model developed slightly later than the opposing ovist model because sperm cells were not seen under the microscope until the late 17th century. Spermism was never as dominant as ovist preformationism, but it had ardent followers whose work and writings greatly influenced the development of embryology in this period. Spermism is sometimes referred to as animalculism, a name taken from the term that most naturalists at the time used to refer to microscopic organisms. One of the most notable spermists was Anton Leeuwenhoek who was the first able to observe sperm cells moving in semen. He postulated the existence of a preformed individual in the sperm, consistent with the spermist theory of preformation, and produced the now -famous drawing of a tiny man or “homunculus” inside the sperm. 5 - Spermist preformism (Spermism) was the belief that offspring develop from a tiny fully-formed embryo contained within the head of a sperm cell. Epigenesis Opposed to preformation, epigenesis, which means grow upon, is the theory that the embryo develops progressively by stages, forming structures that were not originally present in the egg. Although Aristotle was the first to observe that the embryo develops its organs gradually and they are not preformed, it was not until the late 18th century when the epigenesis theory gained its definitive acceptance thanks to the observations of Caspar Friedrich Wolff. Though Wolff famously refuted preformationist in favour of epigenesis, this did not sound the death knell for the preformationist ideology. The end of preformationism did not come until the next century when a combination of new staining techniques, improved microscopes, and institutional reforms in European universities created a revolution in embryology: the comparative embryology. 6 - The theory of epigenesis was proposed by the German physician and naturalist C. F. Wolff (1734- 1794) to counter the preformationist theory: epigenesis theory claimed that structures are not already (pre-) formed but they arise during development. Common patterns Among the most talented of this new group of microscopically inclined investigators were three friends, born within a year of each other, who came from the Baltic region and who studied in northern Germany. In the 19th century, the work of Christian Pander, Karl Ernst von Baer, and Heinrich Rathke transformed embryology into a specialised branch of science and allowed the term “embryology” to be used to describe their work. Pander discovered the three germ layers (ectoderm, mesoderm and endoderm) and weighted definitively the balance in favour of epigenesis. The germ layers, he noted, did not form their organs independently. Rather, each germ layer needed the help of the others, and therefore, although they were already designated for different ends, the three germ layers influenced one another collectively until each had reached an appropriate level. Pander had discovered the tissue interactions that we now call induction. No tissue is able to construct organs by itself; it must interact with other tissues. Thus, Pander felt that preformation could not be true since the organs come into being through interactions between simpler structures. Rathke looked at the development of frogs, salamanders, fish, birds, and mammals, and emphasized the similarities in the development of all these vertebrate groups. He described vertebrate pharyngeal arches for the first time, which do not only become the gill apparatus of fish but also become the mammalian jaws and ears in terrestrial animals. Baer, in 1828, reported: “I have two small embryos preserved in alcohol, that I forgot to label. At present, I am unable to determine the genus to which they belong. They may be lizards, small birds, or even mammals.” All vertebrate embryos (fish, reptiles, amphibians, birds, and mammals) begin with a basically similar structure. From his detailed study of chick development and his comparison of chick embryos with the embryos of other vertebrates, von Baer derived four generalisations, now oft en referred to as “von Baer's laws”. In summary, Baer’s laws described how in the development of an animal from an ovum, the more general features of the group to which it belongs develop first and the special features of the species appear later. For example, in the chicken embryo, the generalised chordate characters such as the notochord, dorsal neural tube and pharyngeal arches develop before the specialised characters of birds, like feathers, claws and beak. As a result, embryos belonging to various classes closely resemble one another in their earlier stages but diverge more and more as development proceeds. 7 - Similarities and differences between different vertebrate embryos as they proceed through development. At the earliest stages of development, embryos are basically made of similar structures, although they acquire this structure at different ages and sizes. As development progresses, these structures diverge and therefore, the embryos become less like each other. Development and evolution In the 19th century, Charles Darwin's theory of evolution restructured comparative embryology and gave it a new focus. Darwin saw that embryonic resemblances among species would be a very strong argument in favour of the genetic connectedness of different animal groups. “Community of embryonic structures reveals community of descent,” he would conclude in On the Origin of Species in 1859. According to the theory of evolution that came into existence in the middle of the nineteenth century, animals and plants have slowly evolved over millions of years from simple unicellular organisms. First the invertebrates came into existence followed by vertebrates. Among the chordates, the sequence of evolution was cyclostomes, fishes, amphibian and reptiles. Reptiles gave rise to birds and mammals. The evolutionary study of embryos, evolutionary embryology, reached a peak in the late 1800s thanks primarily to the efforts of one extraordinarily gifted, though not entirely honest, scientist named Ernst Haeckel. Haeckel was a champion of Darwin, but he also embraced the pre -Darwinian notion that life formed a series of successively higher forms, with embryos of higher forms "recapitulating" the lower ones. Haeckel believed that, over the course of time, evolution added new stages to produce new life forms. Thus, embryonic development was actually a record of evolutionary history. The single cell stage of embryos corresponded to amoeba-like ancestors, which developed into marine multi-celled organisms (cleavage stage of embryos), which developed into fish (branchial stage of embryos) and so on. Haeckel, who was adept at packaging and promoting his ideas, coined both a name for the process ("the Biogenetic Law") as well as a catchy motto ("Ontogeny -an individual's development- recapitulates phylogeny -evolutionary history-)". Haeckel was so convinced of his Biogenetic Law that he was willing to bend the evidence to support it. The truth is that the development of embryos does not always fit into the strict progression that Haeckel claimed. Nevertheless, Haeckel deceptively omitted or modified parts of the description. Although some biologists recognised his sleights of hand, the Biogenetic Law became very popular, and Haeckel's illustrations found their way i nto biology textbooks. The point described above the biogenetic law illustrates one of the many misconceptions about evolution that causes fundamental problems when it comes to human evolution. It is important to remember that: • Humans did not evolve from chimpanzees. Humans and chimpanzees are evolutionary cousins and share a recent common ancestor that was neither chimpanzee nor human. • Humans are not "higher", "more evolved" or "advanced" than other living lineages. Since our lineages split, humans and chimpanzees have each evolved traits unique to their own lineages. In reality, all modern organism are equally evolved and well adapter to their current life styles. Terms like "primitive/evolved" or "higher/lower" are completely misleading and they are not appropriate ways to describe inter-species differences. While it is correct to refer to features that an organism shares with its ancestral lineage and new traits that have evolved since, describing any organism as primitive or advanced is a value judgment that has no place in science. The terms ancestral and derived can be used to distinguish such traits without value-laden connotations. 8 - The wrong tree. Haeckel imagined humans ("Menschen" in German) to be the "highest" form of life, placing them at the top of his tree of life; note that modern evidence suggests that the phylogeny proposed by Haeckel and represented here is incorrect. This misconception claimes that life can be organized on a ladder of lower to higher organisms than on a tree with different branches. This idea lies at the heart of Aristotle's Great Chain of Being , with inferior organisms at the bottom and superio r organisms at the top. Religions have also fueled the belief that every creature on earth has been placed at your disposal since mankind is the supreme species for which all the universe has been created. 9 - The tree of life Evolution can be represented like a tree with different branches but not like a ladder with lower and higher rungs. Age of Enlightenment In the nineteenth century, besides the rise of the evolutionary embryology, the classical descriptive embryology moved into the experimental analysis of development. The question of “what?” became the question of “how?” and a new generation of embryologists felt that besides describing what happens throughout development, they also should answer the question of how an egg becomes an adult. During the 19th century, microscopes and histological stains were improved, which undoubtedly allowed a better understanding of the initial stages of development. Thus, Theodor Schwann, along with others, develop the bases of cell theory, demonstrating that gametes and zygotes correspond to different phases of cell transformation at the same time that the first phases of embryonic development were observed. Before that time, embryology remained a mere descriptive subject, based on observations only. Roux, a nineteenth-century German scientist, is considered the founder of experimental embryology. This field was consolidated in the last part of the 19th century and early 20th century by the works of Driesh and Spemann, among others. These researchers give an explanation of embryonic development based on physicochemical stimuli that led to the theory of induction, which indicates that embryonic development requires stimuli (induction) that come from previously formed embryonic structures (inducers). They also demonstrate the totipotent capacity of the first embryonic cells. https://sway.office.com/31vs60F15LtmBrST#content=2qREdxgR53IEI6 10 - The organizer The first discovered inductor is called the "Sperman's organizer" which was discovered in the blastopore of amphibians. In vertebrates, the equivalent to the Sperman's organizer is found in the primitive node located at the tip of the primitve stre ak. Modern time The final downfall of the Haeckel´s biogenetic law came with the rise of genetics and the modern synthesis which reconciled Charles Darwin's theory of evolution and Gregor Mendel's ideas on heredity and genes. Ontogeny does not recapitulate phylogeny; rather, it creates phylogeny. Evolution is generated by heritable changes in development. The modern synthesis of the theory of evolution combines several different scientific disciplines and their overlapping findings. Although we now know that Heackel´s biogenetic law was wrong, and ontogeny does not actually recapitulate phylogeny, the relationship between embryological development and evolution remains the subject of intense scientific interest. Nowadays, evolutionary embryology is included in the conte mporary evolutionary developmental biology (informally, evo-devo) which deals with the study of how multicellular organisms develop (developmental biology) and how they have evolved (evolutionary biology). Concepts and definitions What is embryology? Etymologically, embryology is the study of the development of the embryos and this is as true for plants as it is for animals, although only animal embryology will be described in this course. The dictionary definition of the term "embryo" is referred to the de veloping animals that are unhatched or not yet born. The reason that many embryologists have difficulty with this definition is that it is purely arbitrary, as we will see in the following sections. For instance, development does not stop at birth. Teeth, bones, blood and many other tissues continue to develop long after birth. For this reason, many embryologists prefer the term developmental biology rather than embryology, to escape from the need to confine their studies to the prenatal stages. In this modern sense of the term, embryology can be defined as the study of the development of organisms, considering developmental aspects of life as a whole and not just as a concrete period of time. 11 - The cycle of life Life can be described as a succession of stages. Prenatal stages of life Although life is a continuous process, there are some critical landmarks that need to be named even though there are no specific boundaries between them. The problem arises when these developmental periods are tagged with moral or ethical values. The discussion about the concept of “human embryo” exemplifies this problem. Human embryos are defined as developing humans during the first eight weeks after conception, after which they are considered foetus. This boundary is purely arbitrary. It would be difficult indeed, if not impossible, to discriminate a developing embryo nearing the end of the eighth week from a developing foetus during the ninth week after conception. Furthermore, development does not even stop at birth. The same goes for laying-eggs species, in which there are no morphological features that distinguish a pre-hatching foetus from a post-hatching chicken; besides, hatching never occurs synchronously in a clutch of eggs (there are always those that hatch early and those eggs which are dilatory). Even though it is almost impossible to set specific boundaries, there are some specific events that are helpful in order to facilitate the study of embryology. From one side, development can be divided in three different stages: 1. Fertilisation and cleavage. Immediately following fertilisation, cleavage occurs. Cleavage is a series of extremely rapid mitotic divisions wherein the enormous volume of zygote cytoplasm is divided into numerous smaller cells. These cells are called blastomeres. After passing through the morula stage, they generally form a hollow sphere known as blastula or blastocyst. 2. Gastrulation. After the rate of mitotic division has slowed down, the blastomeres undergo dramatic movements by which they change their positions relative to one another. This series of extensive cell rearrangements is called gastrulation, and the embryo is said to be in the gastrula stage. As a result of gastrulation, the embryo contains three germ layers: the ectoderm, the endoderm, and the mesoderm. Gastrulation coincides with the implantation in the uterus which marks the end of the germinal stage and the beginning of the organogenesis. 3. Organogenesis. Once the three germ layers are established, the cells interact with one another and rearrange themselves to produce tissues and organs. This process is called organogenesis. In most vertebrates, the organogenesis can be divided into the embryonic stage and the foetal stage. From another perspective, embryo development goes through the following stages: • Early embryo stage or germinal period. From Zygote stage fertilisaton) and during segmentation and gastrulation (first two weeks or development), the embryo is composed of an increasing mass of cells more o less arranged in specific patterns but without being possible to recognize the body or any organ in particular. Actually, only a fraction of these cells will become the actual embryo whereas other cells will form extra-embryonic organs (placenta). For this reason, during this stage is called the early embryo stage or germinal period which comprises the earliest stages of animal development (zygote, blastula and gastrul a). In this period the embryo, also referred to as “pre-embryo”, is composed of an increasing mass of cells arranged in specific patterns, but it is not possible to recognise the body boundaries or any specific organ or body part. Some of these cells will become the actual embryo whereas other cells will form extra-embryonic organs (placenta). 12 - Early embryo stage or germinal period The zygote soon begins to divide rapidly in a process called cleavage, first into two identical cells called blastomeres, which further divide to four cells, then into eight, and so on. The group of dividing cells begins moving along the fallopian tube toward the uterus is called morula. As cell division continues, a fluid-filled cavity called blastocoele is formed in the centre of the group of cells, with the outer shell of cells called trophoblasts and an inner mass of cells called inner mass cells. At this stage , the blastocyst consists of 200 to 300 cells and is ready for gastrulation and implantation. • Embryonic stage The embryonic period implies the delimitation of the body and the differentiation of the different embryonic and extraembryonic organs and structures. During the embryonic period, most of the organ systems are established in rapid progression. It is thus hardly surprising that this pregnancy phase is very vulnerable and congenital defects are produced more often during this time. In human species, this time span is divided into 23 Carnegie stages and the stage classification is based solely on morphologic features. In humans, the embryonic period covers the first 8 weeks. In animals where embryo / fetus differentiation is not subject to the same ethical and moral pressures as in the human species. In any case, the duration of the embryonic period is related to the duration of gestation; In general, species with relatively short gestations, such as carnivores and pigs, the embryonic period is reduced to the first 4 weeks while in animals with longer gestations, such as horses and cattle, it extends to the first 8 weeks. 13 - Embryonic stage The beginning of the actual embryo implies the delimitation of the body and the differentiation of the different embryonic an d extraembryonic organs and structures. In this stage, the body organs and systems arise from the three primary layers (ectoderm, mesoderm and endoderm) and rudimentary formation of all organ systems are present. • Foetal stage The foetal period implies that an embryo is developed to the point of being recognisable the species to which it belongs, and then, the developing organism is called foetus (also spelt fetus). Although all the organic systems started to form during the embryonic period, in the foetal period, they continue to grow and become functional. However, the terms "embryo" and "foetus" are often used interchangeably and 'foetal development' is used in a similar sense to 'prenatal development' because in reality development is a continuous process and there are not any specific boundaries or dramatic changes that can teel apart these developmental stages. The embryo or fetus together with the tissues, such as the placenta, that nourishes it is referred to as the conceptus. 14 - Fetal stage The use of the term fetus generally implies that an embryo has developed to the point of being recognizable the species to which it belongs. Although all of the organ systems were formed during embryonic development, they continue to develop and grow during the fetal stage. Period of gestation. For mammals the gestation period is the time in which a fetus develops, beginning with fertilization and ending at birth. The duration of this period varies between species. Generally, there are two main factors that give to the length of the gestation period: Animal size or mass – larger animals tend to have longer gestation periods (as they tend to make larger offspring). The level of development at birth – more developed infants will typically need a longer gestation period. You can find the gestation periods of different animals on the infographic below. 15 - Periods of gestation for domestic animals 16 - Time calculations during pregnancy in human species A = Embryonic period B= Fetal period The schematic diagram shows the various time periods during the entire pregnancy in human species. taken from: http://www.embryology.ch/anglais/jfetalperiod/entwicklung01.html LMP = Last Menstruation Period. In embryology, the temporal indices such as the Pregnancy Weeks (PW) always refers to the moment of fertilization. In this context, the embryonic period (A) lasts 8 weeks and the fetal period (B) from the 9th week to the birth, i.e., 30 weeks. The embryonic period is divided into 23 Carnegie stages, based solely on morphologic features. In human obstetrics and in practical midwifery, the time following the Last Menstrual Period (LMP) is still used for computations. This is a point in time that many women can easily remember. Computed this way, the pregnancy lasts 40 weeks and the embryonic period - accordingly - 10 weeks. Congenital defects Embryonic and foetal development is the result of a complex series of well-orchestrated events. When properly accomplished, the outcome is a healthy neonate. Congenital defects, also known as congenital anomalies, congenital malformations or birth defects, can be defined as structural or functional anomalies that occur during intrauterine life and can be identified prenatally, at birth or late in life. Birth defects range in severity from minor to life-threatening conditions. Some lethal defects are incompatible with intrauterine life, leading to spontaneous abortion, stillbirth or birth of nonviable neonates; other non-lethal congenital defects can be compatible with prenatal life and produce viable offspring but with a myriad of congenital anomalies. Congenital malformations continue to be an important cause of perinatal morbidity and mortality. In humans, about 2–3% of births are associated with major congenital anomalies. These anomalies detected at birth or soon after birth, reflect but a small minority of the lethal embryonic errors that occur during gestation and which end in spontaneous abortion. Most birth defects are caused by genetic or environmental factors or a combination of the two (multifactorial birth defects). Environmental factors involved in congenital defects are also known as Teratogens. They include viruses, drugs, chemicals, stressors, and malnutrition, which can impair prenatal development and lead to congenital defects. However, in some cases, the cause is unknown. Teratology refers to the study of congenital abnormalities, usually in regard to the deleterious effects of environmental agents or external exposures on the developing embryo. Some terms related to developmental anomalies are: • Considering the causes: - Malformations are a primary defect associated with intrinsically abnormal development of an organ or body part, normally due to underlying genetic, epigenetic, or environmental factors. For example, the spina bifida in a malformation in which the neural tube fails to close resulting in a spinal cord that never forms in a normal way. - Disruptions are a secondary defect in the development of an organ or body part, which was developing correctly at the beginning. The aetiological factors may be vascular, infectious, mechanical, or metabolic in origin but are not hereditary. One example is mechanical compression and interruption of blood supply leading to a degeneration of structures beyond the interrupted blood supply. - Deformations, like disruptions, also represent secondary disturbances of development rather than intrinsic errors of morphogenesis. Congenital deformities develop during later stages of intrauterine development because of mechanical forces (pressure) causing abnormal shape or position of an organ or body part. Congenital deformities are frequently associated with any sort of uterine constraint; for example, a small or malformed uterus, oligohydramnios (low amniotic fluid levels), abnormal number or positions of the embryos. Examples include joint or limb deformities such as equinovarus foot (clubfoot). 17 - Congenital defects Errors in the sequential steps of development may be followed by embryonic loss, fetal death, fetal mummification, abortion, stillbirth, the birth of nonviable neonates, or the birth of viable offspring with congenital defects (congenital abnormalities or anomalies). • Considering cellular and tissular development - Meaning the absence or lack of continuity of passageway: ◦ Atresia (from a- "not, without" + tresis "perforation,") is a condition in which an orifice or passage in the body is abnormally closed or absent. For example, anal atresia or imperforate anus is the congenital absence of an opening at the caudal end of the intestinal tract. ◦ Fistula is an abnormal passageway in the body. The fistula may go from the body surface into a blind pouch or into an internal organ or go between two internal organs. 18 - Oesophageal atresia and tracheoesophageal fistula - Meaning the absence or decrease in a tissue or organ: ◦ Agenesis (from a- "not, without" + genesis "origin") is the complete absence of an organ and its primordia. For example, renal agenesis is the failure of one or both kidneys to develop. ◦ Aplasia (from a- "not, without" + plasia "formation") is the defective development or complete absence of an organ due to the failure of its primordial tissues or cells to develop. For example, a failure of the paramesonephric ducts (the primordia of the female sex ducts) may be the cause of a congenital aplasia of the uterine-vaginal segment. ◦ Hypoplasia (from hypo- "low or under" + plasia "formation") is the underdevelopment or incomplete development of a tissue or organ because the number of cells in its structure is decreased. For example, when a puppy or kitten is born with an underdeveloped cerebellum, the condition is known as congenital cerebellar hypoplasia. It should not be confounded with atrophy. ◦ Atrophy (from a- "not, without" + trophy "food, nutrition") is the decrease in size or progressive decline of a body part or tissue because the size of cells in its structure is decreased. For example, muscular atrophy is a common consequence in some congenital neurodegenerative diseases. 19 - A. Renal agenesis. The kidney and the ureter are absent B. Renal aplasia. There is a primordium of renal tissue but the kidney is not developed. C. Renal hypoplasia. The kidney is present but underdeveloped and incomplete. - - Meaning a increase in a tissue or organ: ◦ Hyperplasia (from hyper- "over, excess" + plasia "formation") is an organ enlargement caused by an increase in the number of its cells. For example, congenital adrenal hyperplasia involves excessive production of sex steroid hormones by the adrenal glands. It should not be confounded with hypertrophy. ◦ Hypertrophy (from hyper- "over, excess" + trophy "food, nutrition") is the enlargement of an organ or tissue from the increase in the size of its cells. Meaning a failure in the structure of a tissue or organ: ◦ Metaplasia (from meta- "after, beyond" + plasia "formation") is the transformation of one differentiated cell type to another differentiated cell type. The change from one type of cell to another may be part of a normal maturation process or caused by some sort of abnormal stimulus. ◦ Dysplasia (from dys- "apart, away" + plasia "formation") is another type of anomaly in which the intrinsic cellular architecture of a certain tissue is not normally maintained throughout growth and development. The term dysplasia is used to refer to abnormal growth and maturation of the cells and tissues (e.g., bones and cartilages) which give rise to abnormal anatomical structures (e.g., hip dysplasia). 20 - Comparison of normal tissue with atrophy, hypertrophy, hyperplasia, metaplasia and dysplasia. • Considering multiple associated defects: Sequence. A sequence occurs when a primary anomaly itself determines additional defects which develop in cascade as a consequence of the primary malformation. Syndrome. It is a recognisable pattern of multiple defects which present together. The collective occurrence is not random but relates to the common aetiology of the process. For example, Down syndrome is caused by a specific chromosomal anomaly (trisomy 21). Association. Some recognised patterns of malformations are described by the term "association" because the initiating cause has not been identified, and neither are the anomalies the results of a sequence. For example, the VATER association involves vertebral, anal, tracheoesophageal and radial anomalies that often occur together. Comparative morphological terminology Over the centuries, anatomists have developed a standard nomenclature or method of describing anatomical structures. In comparative embryology and anatomy, terms such as "up" or "down" obviously have no meaning unless the orientation of the body is clear. When a body is lying on its back, the thorax and abdomen are at the same level and the upright sense of up and down is lost. Therefore, there are obvious difficulties in applying terms from human, bipeds in whom an upward and downward orientation might seem obvious, to animals, quadrupeds who have abdominal and thoracic regions at the same level. To standardise the anatomical nomenclature, the International Committee on Veterinary Gross Anatomical Nomenclature prepared the Nomina Embryologica Veterinari and the Nomina Anatomica Veterinari. They are used as the standard reference for embryological and anatomical terminology in the field of Veterinary Science. Some basic terms that we will use describing the embryo are: Cranial and caudal. These terms describe how close or far something is to the head or tail of an animal. In the head, the term cranial is replaced by “rostral”, meaning situated toward the oral or nasal region, or in the case of the brain, toward the tip of the frontal lobe. For example, in horses, the eyes are caudal to the nostril and rostral to the forehead; the foregut is cranial to the midgut. Dorsal and ventral. These two terms are used in anatomy and embryology to refer to the back (dorsal) and to the belly (ventral) of an organism. The dorsal surface of an organism refers to the back or upper side of an animal. If talking about the skull, the dorsal side is the top. The ventral surface refers to the belly or the lower side of an animal. For example, the gut tube lies ventrally to the spin al cord; the spinal cord is dorsal to the developing trachea. Medial and lateral. Lateral refers to the sides of an animal; the term medial is used to refer to structures close to the midline or centre of an organism, called the "median plane". For example, the eyes are medial to the ears and lateral to the nose; the medial side of the foot would be the big toe side; the medial side of the knee would be the site adjacent to the other knee. 21 - Anatomical position. Note tha t the use of s ome of these terms va ries depending on whether they a re applied to a qua druped (an a nimal which walks on all fours) or a biped (a n animal which walks only on i ts hind limbs, s uch as Homo). You wi l l be expected to use the proper terms for each organism.