SEM_01_Introduction and Generalities_NO_historia.docx

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...

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 developing 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 mod ern 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. - 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: 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. 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. 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 gastrula). 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). - 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. - Embryonic stage The beginning of the actual embryo implies the delimitation of the body and the differentiation of the different embryonic and 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. - 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. - Periods of gestation for domestic animals - 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, usuallyin 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). - 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. - 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. - 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). - 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 spinal 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. - Anatomical position. Note tha t the use of some of these terms varies depending on whether they are applied to a quadruped (an animal which walks on all fours) or a biped (an animal which walks only on i ts hind limbs, such as Homo). You will be expected to use the proper terms for each organism. Cephalic/cranial Dorsal