Embryology L4 PDF

Embryology L4 - P1 The Embryonic Period - P1 PART 01 The Embryonic Period  The Embryonic Period the embryonic period, or period of organogenesis, occurs from the third to the eighth weeks of development and is the time when each of the three germ layers, ectoderm, mesoderm, and end...

Embryology L4 - P1 The Embryonic Period - P1 PART 01 The Embryonic Period  The Embryonic Period the embryonic period, or period of organogenesis, occurs from the third to the eighth weeks of development and is the time when each of the three germ layers, ectoderm, mesoderm, and endoderm, gives rise to a number of specific tissues and organs. By the end of the embryonic period, the main organ systems have been established, rendering the major features of the external body form recognizable by the end of the second month. The third to eighth weeks are also cited as the time when the majority of birth defects are induced; prior to this time, any insult to the embryo results in its death and spontaneous abortion. not all embryos are lost if an environmental or genetic insult occurs during this critical time period. DERIVATIVES OF THE ECTODERMAL GERM LAYER At the beginning of the third week of development, the ectodermal germ layer has the shape of a disc that is broader in the cephalic than in the caudal region. Appearance of the notochord and prechordal mesoderm induces the overlying ectoderm to thicken and form the neural plate. Cells of the plate make up the neuroectoderm, and their induction represents the initial event in the process of neurulation.  Neurulation Neurulation is the process whereby the neural plate forms the neural tube. One of the key events in this process is lengthening of the neural plate and body axis by the phenomenon of convergent extension, whereby there is a lateral to medial movement of cells in the plane of the ectoderm and mesoderm. the process is regulated by signaling through the planar cell polarity pathway and is essential for neural tube development. As the neural plate lengthens, its lateral edges elevate to form neural folds, and the depressed midregion forms the neural groove. Gradually, the neural folds approach each other in the midline, where they fuse. Fusion begins in the cervical region (fifth somite) and proceeds cranially and caudally. As a result, the neural tube is formed. Until fusion is complete, the cephalic and caudal ends of the neural tube communicate with the amniotic cavity by way of the anterior (cranial) and posterior (caudal) neuropores, respectively  Neurulation Closure of the cranial neuropore occurs at approximately day 25 (18- to 20- somite stage), whereas the posterior neuropore closes at day 28 (25-somite stage). Neurulation is then complete, and the central nervous system is represented by a closed tubular structure with a narrow caudal portion, the spinal cord, and a much broader cephalic portion characterized by a number of dilations, the brain vesicles  Neural Crest Cells As the neural folds elevate and fuse, cells at the lateral border or crest of the neuroectoderm begin to dissociate from their neighbors. This cell population, the neural crest cells (NCC), undergoes an epithelial-to-mesenchymal transition as it leaves the neuroectoderm by active migration and displacement to enter the underlying mesoderm. (Mesoderm refers to cells derived from the epiblast and extraembryonic tissues. Mesenchyme refers to loosely organized embryonic connective tissue regardless of origin.) Crest cells from the trunk region leave the neuroectoderm after closure of the neural tube and migrate along one of two pathways: (1) a dorsal pathway through the dermis, where they will enter the ectoderm through holes in the basal lamina to form melanocytes in the skin and hair follicles (2) a ventral pathway through the anterior half of each somite to become sensory ganglia, sympathetic and enteric neurons, Schwann cells, and cells of the adrenal medulla  Neural Crest Cells NCC also form and migrate from cranial neural folds, leaving the neural tube before closure in this region. These cells contribute to the craniofacial skeleton as well as neurons for cranial ganglia, glial cells, melanocytes, and other cell types. NCC are so fundamentally important and contribute to so many organs and tissues that they are sometimes referred to as the fourth germ layer. They are also involved in at least one-third of all birth defects and many cancers, such as melanomas, neuroblastomas, and others. Evolutionarily, these cells appeared at the dawn of vertebrate development and formed the basis for vertebrate features.  By the time the neural tube is closed, two bilateral ectodermal thickenings, the otic placodes and the lens placodes, become visible in the cephalic region of the embryo. During further development, the otic placodes invaginate and form the otic vesicles, which will develop into structures needed for hearing and maintenance of equilibrium. At approximately the same time, the lens placodes appear. These placodes also invaginate and, during the fifth week, form the lenses of the eye.  In general terms, the ectodermal germ layer gives rise to organs and structures that maintain contact with the outside world: The central nervous system The peripheral nervous system The sensory epithelium of the ear, nose, and eye The epidermis, including the hair and nails In addition, it gives rise to the following: The subcutaneous glands The mammary glands The pituitary gland Enamel of the teeth  Clinical Correlates: Neural Tube Defects Neural tube defects [NTDs] result when neural tube closure fails to occur. If the neural tube fails to close in the cranial region, then most of the brain fails to form, and the defect is called anencephaly. If closure fails anywhere from the cervical region cau— dally, then the defect is called spina bifida. The most common site for spina bifida to occur is in the lumbosacral region. Regardless of the region or country where NTDs occur, rates have been reduced significantly following folic acid administration. NTDs can be prevented if women take 400 pg of folic acid daily [the dose present in most multivitamins] beginning 3 months prior to conception and continuing throughout pregnancy. summary THANK YOU！  Embryology L4 - P2 Embryonic period PART 02 Embryonic Period  DERIVATIVES OF THE MESODERMAL GERM LAYER Initially, cells of the mesodermal germ layer form a thin sheet of loosely woven tissue on each side of the midline. By approximately the 17th day, however, cells close to the midline proliferate and form a thickened plate of tissue known as paraxial mesoderm. More laterally, the mesoderm layer remains thin and is known as the lateral plate. With the appearance and coalescence of intercellular cavities in the lateral plate, this tissue is divided into two layers: A layer continuous with mesoderm covering the amnion, known as the somatic or parietal mesoderm layer. A layer continuous with mesoderm covering the yolk sac, known as the splanchnic or visceral mesoderm layer. Together, these layers line a newly formed cavity, the intraembryonic cavity, which is continuous with the extraembryonic cavity on each side of the embryo. Intermediate mesoderm connects paraxial and lateral plate mesoderm  Paraxial Mesoderm By the beginning of the third week, paraxial mesoderm begins to be organized into segments. These segments, known as somitomeres, first appear in the cephalic region of the embryo, and their formation proceeds cephalocaudally. Each somitomere consists of mesodermal cells arranged in concentric whorls around the center of the unit. In the head region, somitomeres form in association with segmentation of the neural plate into neuromeres and contribute to mesenchyme in the head. From the occipital region caudally, somitomeres further organize into somites. The first pair of somites arises in the occipital region of the embryo at approximately the 20th day of development. From here, new somites appear in craniocaudal sequence at a rate of approximately three pairs per day until, at the end of the fifth week, 42 to 44 pairs are present. There are 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 8 to 10 coccygeal pairs. The first occipital and the last five to seven coccygeal somites later disappear, whereas the remaining somites form the axial skeleton. Because somites appear with a specified periodicity, the age of an embryo can be accurately determined during this early time period by counting somites.  Somite Differentiation When somites first form from presomitic mesoderm, they exist as a ball of mesoderm (fibroblast-like) cells. These cells then undergo a process of epithelization and arrange themselves in a donut shape around a small lumen. By the beginning of the fourth week, cells in the ventral and medial walls of the somite lose their epithelial characteristics, become mesenchymal (fibroblast-like) again, and shift their position to surround the neural tube and notochord. Collectively, these cells form the sclerotome that will differentiate into the vertebrae and ribs. Cells at the dorsomedial and ventrolateral edges of the upper region of the somite form precursors for muscle cells, whereas cells between these two groups form the dermatome. Cells from both muscle precursor groups become mesenchymal again and migrate beneath the dermatome to create the dermomyotome.  Somite Differentiation In addition, cells from the ventrolateral edge migrate into the parietal layer of lateral plate mesoderm to form most of the musculature for the body wall (external and internal oblique and transversus abdominis muscles) and most of the limb muscles. Cells in the dermomyotome ultimately form dermis for the skin of the back and muscles for the back, body wall (intercostal muscles), and some limb muscles. Each myotome and dermatome retains its innervation from its segment of origin, no matter where the cells migrate. Hence, each somite forms its own sclerotome (the tendon cartilage and bone component), its own myotome (providing the segmental muscle component), and its own dermatome, which forms the dermis of the back. Each myotome and dermatome also has its own segmental nerve component.  Intermediate Mesoderm Intermediate mesoderm, which temporarily connects paraXial mesoderm with the lateral plate, differentiates into urogenital structures. In cervical and upper thoracic regions, it forms segmental cell clusters (future nephrotomes), whereas more caudally, it forms an unsegmented mass of tissue, the nephrogenic cord. Excretory units of the urinary system and the gonads develop from this partly segmented, partly unsegmented intermediate mesoderm  Lateral Plate Mesoderm Lateral plate mesoderm splits into parietal (somatic) and Visceral (splanchnic) layers, which line the intraembryonic cavity and surround the organs, respectively. Mesoderm from the parietal layer, together with overlying ectoderm, forms the lateral body wall folds. These folds, together with the head (cephalic) and tail (caudal) folds, close the ventral body wall. The parietal layer of lateral plate mesoderm then forms the dermis of the skin in the body wall and limbs, the bones and connective tissue of the limbs, and the sternum. In addition, sclerotome and muscle precursor cells that migrate into the parietal layer of lateral plate mesoderm form the costal cartilages, limb muscles, and most of the body wall muscles. The visceral layer of lateral plate mesoderm, together with embryonic endoderm, forms the wall of the gut tube.  Lateral Plate Mesoderm Mesoderm cells of the parietal layer surrounding the intraembryonic cavity form thin membranes, the mesothelial membranes, or serous membranes, which will line the peritoneal, pleural, and pericardial cavities and secrete serous fluid. Mesoderm cells of the visceral layer form a thin serous membrane around each organ. SUMMARY THANK YOU！ Embryology L4 - P3 Embryonic period - P3 PART 03 Embryonic Period  Blood & Blood Vessels: Blood cells and blood vessels also arise from mesoderm. Blood vessels form in two ways: vasculogenesis, whereby vessels arise from blood islands, and angiogenesis, which entails sprouting from existing vessels. the first blood islands appear in mesoderm surrounding the wall of the yolk sac at 3 weeks of development and slightly later in lateral plate mesoderm and other regions. These islands arise from mesoderm cells that are induced to form hemangioblasts, a common precursor for vessel and blood cell formation. Although the first blood cells arise in blood islands in the wall of the yolk sac, this population is transitory. the definitive hematopoietic stem cells are derived from mesoderm surrounding the aorta in a site near the developing mesonephric kidney called the aorta-gonad-mesonephros region (AGM). These cells colonize the liver, which becomes the major hematopoietic organ of the embryo and fetus from approximately the second to seventh months of development.  Blood & Blood Vessels: Stem cells from the liver colonize the bone marrow, the definitive blood-forming tissue, in the seventh month of gestation; thereafter, the liver loses its blood- forming function.  DERIVATIVES OF THE ENDODERMAL GERM LAYER The gastrointestinal tract is the main organ system derived from the endodermal germ layer. This germ layer covers the ventral surface of the embryo and forms the roof of the yolk sac. With development and growth of the brain vesicles, however, the embryonic disc begins to bulge into the amniotic cavity. Lengthening of the neural tube now causes the embryo to curve into the fetal position as the head and tail regions (folds) move ventrally. Simultaneously, two lateral body wall folds form and also move ventrally to close the ventral body wall. As the head and tail and two lateral folds move ventrally, they pull the amnion down with them, such that the embryo lies within the amniotic cavity. The ventral body wall closes completely except for the umbilical region where the connecting stalk and yolk sac duct remain attached. Failure of the lateral body folds to close the body wall results in ventral body wall defects.  DERIVATIVES OF THE ENDODERMAL GERM LAYER As a result of cephalocaudal growth and closure of the lateral body wall folds, a continuously larger portion of the endodermal germ layer is incorporated into the body of the embryo to form the gut tube. The tube is divided into three regions: the foregut, midgut, and hindgut. The midgut communicates with the yolk sac by way of a broad stalk, the Vitelline (yolk sac) duct. This duct is wide initially, but with further growth of the embryo, it becomes narrow and much longer. At its cephalic end, the foregut is temporarily bounded by an ectodermal— endodermal membrane called the oropharyngeal membrane. This membrane separates the stomodeum, the primitive oral cavity derived from ectoderm, from the pharynx, a part of the foregut derived from endoderm. In the fourth week, the oropharyngeal membrane ruptures, establishing an open connection between the oral cavity and the primitive gut. The hindgut also terminates temporarily at an ectodermal—endodermal membrane, the cloacal membrane.  DERIVATIVES OF THE ENDODERMAL GERM LAYER This membrane separates the upper part of the anal canal, derived from endoderm, from the lower part, called the proctodeum, which is formed by an invaginating pit lined by ectoderm. The membrane breaks down in the seventh week to create the opening for the anus. Another important result of cephalocaudal growth and lateral folding is partial incorporation of the allantois into the body of the embryo, where it forms the cloaca. The distal portion of the allantois remains in the connecting stalk. By the fifth week, the yolk sac duct, allantois, and umbilical vessels are restricted to the umbilical region. The role of the yolk sac is not clear. It may function as a nutritive organ during the earliest stages of development prior to the establishment of blood vessels. It also contributes some of the first blood cells, although this role is very transitory. One of its main functions is to house germ cells that reside in its posterior wall and later migrate to the gonads to form eggs and sperm  EXTERNAL APPEARANCE DURING THE SECOND MONTH At the end of the fourth week, when the embryo has approximately 28 somites, the main external features are the somites and pharyngeal arches. The age of the embryo is therefore usually expressed in somites. Because counting somites becomes difiicult during the second month of development, the age of the embryo is then indicated as the crown-rump length (CRL) and expressed in millimeters. CRL is the measurement from the vertex of the skull to the midpoint between the apices of the buttocks. During the second month, the external appearance of the embryo is changed by an increase in head size and formation of the limbs, face, ears, nose, and eyes. By the beginning of the fifth week, forelimbs and hind limbs appear as paddle- shaped buds. The former are located dorsal to the pericardial swelling at the level of the fourth cervical to the first thoracic somites, which explains their innervation by the brachial plexus. Hind limb buds appear slightly later just caudal to attachment of the umbilical stalk at the level of the lumbar and upper sacral somites.  EXTERNAL APPEARANCE DURING THE SECOND MONTH With further growth, the terminal portions of the buds flatten, and a circular constriction separates them from the proximal, more cylindrical segment. Soon, four radial grooves separating five slightly thicker areas appear on the distal portion of the buds, foreshadowing formation of the digits. These grooves, known as rays, appear in the hand region first and shortly afterward in the foot, as the upper limb is slightly more advanced in development than the lower limb. While fingers and toes are being formed, a second constriction divides the proximal portion of the buds into two segments, and the three parts characteristic of the adult extremities can be recognized. Summary THANK YOU！