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In a scanning electron micrograph allergy testing devices order 50 mcg flonase with mastercard, crest cells at the top of the closed neural tube can be seen migrating away from this area allergy symptoms every morning order flonase 50 mcg without a prescription. These cells leave the crests of the neural folds prior to neural tube closure and migrate to form structures in the face and neck (blue area) allergy symptoms of flu order 50 mcg flonase mastercard. Subcutaneous glands allergy medicine post nasal drip cheap 50mcg flonase otc, the mammary glands, the pituitary gland, And enamel of the teeth. A B C Chapter 6 Third to Eighth Weeks: the Embryonic Period 71 Notochord Amniotic cavity Ectoderm Mesoderm Paraxial mesoderm Intermediate mesoderm Intercellular cavities in lateral plate Dorsal aorta A Amnion Neural groove Parietal mesoderm layer B Intermediate mesoderm Somite Visceral mesoderm layer Intraembryonic body cavity Endoderm C D Figure 6. The thin mesodermal sheet gives rise to paraxial mesoderm (future somites), intermediate mesoderm (future excretory units), and the lateral plate, which is split into parietal and visceral mesoderm layers lining the intraembryonic cavity. The first pair of somites Somite arises in the occipital region of the embryo at approximately the 20th day of development. 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, while the remaining somites form the axial skeleton (see Chapter 10). Because somites appear with a specified periodicity, the age of an embryo can be accurately determined during this early time period by counting somites (Table 6. Molecular Regulation of Somite Formation Formation of segmented somites from unsegmented presomitic (paraxial) mesoderm. Thus, Notch protein accumulates in presomitic mesoderm destined to form the next somite and then decreases as that somite is established. The increase in Notch protein activates other segment-patterning genes that establish the somite. 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 (see Chapter 10). Cells at the dorsomedial and ventrolateral edges of the upper region of the somite form precursors for muscle cells, while cells between these two groups form the dermatome. Cells from both muscle precursor groups become mesenchymal again and migrate beneath the dermatome to create Neural tube Ectoderm Somites Presomites mesoderm Figure 6. Somites form from unsegmented presomitic paraxial mesoderm caudally and become segmented in more cranially positioned regions. 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 (see Chapter 11). 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. Molecular Regulation of Somite Differentiation Signals for somite differentiation arise from surrounding structures, including the notochord, neural tube, epidermis, and lateral plate mesoderm. Intermediate Mesoderm Intermediate mesoderm, which temporarily connects paraxial mesoderm with the lateral plate. 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 (see Chapter 16). 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) 74 Part 1 General Embryology Dorsomedial muscle cells Dermatome Neural groove Ventrolateral muscle cells Neural tube Intraembryonic cavity A Ventral somite wall B Notochord Sclerotome Dorsal aorta Neural tube Sclerotome Dermatome Dermatome Myotome Sclerotome C D Figure 6. Mesoderm cells that have undergone epithelization are arranged around a small cavity. Cells from the ventral and medial walls of the somite lose their epithelial arrangement and migrate around the neural tube and notochord.
Brachycephaly Type of craniosynostosis in which the coronal sutures close prematurely resulting in a tall allergy shots years buy generic flonase line, short head shape allergy medicine urinary retention flonase 50mcg online. Brain vesicles Once the neural tube closes allergy treatment brea ca order 50mcg flonase otc, expanded spaces in the brain fill with fluid to form three primary brain vesicles: the prosencephalon (forebrain); mesencephalon (midbrain); and rhombencephalon (hindbrain) allergy testing victoria australia discount 50mcg flonase free shipping. These three primary vesicles form five definitive vesicles: the prosencephalon divides into the telencephalon and diencephalon; the mesencephalon does not divide; and the rhombencephalon forms the metencephalon and myelencephalon. Brainstem "Lower" centers of the brain, including the myelencephalon, pons of the metencephalon, and the mesencephalon. C Capacitation A period of sperm conditioning in the female reproductive tract lasting about 7 hours that is required for sperm to be able to fertilize an egg. Cardiac looping Bending of the heart tube positions the heart in the left thoracic region and creates the "typical" heart shape with the atria posterior to the ventricles. Cardinal veins System of anterior, posterior, and common cardinal veins that drain the head and body of the embryo in the late third and early fourth weeks. Caudal dysgenesis Also called sirenomelia or mermaid syndrome, it is caused by insufficient production of mesoderm by the primitive streak. Consequently, there are not enough cells to form the lower part of the body so that the legs are fused. Cerebral aqueduct (of Sylvius) Lumen of the mesencephalon that connects the third and fourth ventricles. It is often the site for abnormalities that impede the flow of cerebrospinal fluid and cause hydrocephalus. Chondrocranium Part of the neurocranium that forms the base of the skull and arises by first establishing cartilage models for the bones (endochondral ossification). Chorion Multilayered structure consisting of the somatic layer of extraembryonic mesoderm, cytotrophoblast, and syncytiotrophoblast. It contributes the fetal portion of the placenta, including the villi and villus lakes. Chorionic cavity Space formed between the extraembryonic mesoderm lining the cytotrophoblast (somatic extraembryonic mesoderm) and that surrounding the yolk sac and embryo (splanchnic extraembryonic mesoderm). The chorionic cavity will eventually be obliterated by expansion of the amniotic cavity and fusion of the amnion with the chorion. Chorion frondosum (leafy chorion) Embryonic side of the chorion, where villi form. Chorion laeve (smooth chorion) Abembryonic side of the chorion, where villi regress, leaving a smooth surface. Choroid plexuses Vascularized structures formed in the lateral, third, and fourth ventricles that produce cerebrospinal fluid. Its anterior portion forms the urogenital sinus, and its posterior portion forms the anus. Cloacal membrane (plate) Membrane formed at the caudal end of the embryo from adhesion between epiblast and hypoblast cells. Later, it covers the cloaca and eventually breaks down to form openings into the urogenital sinus and anus. Compaction Process whereby cells of the morula stage form tight junctions to seal themselves in preparation for forming and pumping fluid into the blastocyst cavity. It contains the allantois and umbilical vessels and will be incorporated into the umbilical cord with the yolk sac (vitelline) stalk (duct). Congenital malformation Synonymous with the term birth defect, it refers to any structural, behavioral, functional, or metabolic disorder present at birth. Cotyledons Compartments (15 to 20) in the placenta formed when decidual septa grow into the intervillous spaces. These septa never reach the chorionic plate so that there is communication between cotyledons. Craniosynostosis Premature closure of one or more cranial sutures, leading to an abnormally shaped skull. Crista terminalis Ridge of tissue in the right atrium between the original trabeculated part of the right atrium and the smooth-walled part derived from the sinus venosus. D Deformations Altered development of structures caused by mechanical forces, for example, clubfeet resulting from too little room in the amniotic cavity. Dermatome Dorsal portion of each somite that forms the dermis of the skin of the back.
Marginal placenta praevia: the edge of the placenta is at the margin of the internal os allergy forecast yonkers ny generic flonase 50mcg with mastercard. Placenta praevia Bladder Low-lying placenta: the placenta is implanted in the lower uterine segment such that the placental edge does not reach the internal os but is in close proximity to it allergy testing information quality 50 mcg flonase. Placenta praevia can also be diagnosed transvaginally allergy symptoms gastrointestinal buy flonase toronto, particularly for a posteriorly implanted placenta allergy shots versus pills buy cheap flonase 50mcg line. The positive predictive value is 71% with the transvaginal approach and 31% with the transabdominal approach. The high false-positive rate with transabdominal ultrasound may be due to technical reasons. Excessive distension of the urinary bladder can result in approximation of the anterior and posterior lower segments, creating a false impression of placenta praevia. Therefore, the evaluation should be performed with the bladder partially full and not overdistended. A laterally implanted placenta, in conjunction with normal uterine rotation and a distended bladder can also lead to a false diagnosis. Parasagittal evaluation of a laterally located placenta or isolated contractions of the lower segment in conjunction with a distended bladder can lead to a false diagnosis. Accurate diagnosis of placenta praevia thus requires knowledge of the location of the cervix, the internal cervical os and the placenta. Placenta praevia can be associated with placenta accreta or one of its more advanced forms, placenta increta or 65 Obstetrics. Placenta accreta: longitudinal power Doppler sonogram in the 19th week of gestation shows arcuate and radial arteries within the bladder muscle layer. As a consequence of the partial or total absence of the decidua basalis and imperfect development of the brinoid layer, placental villi are attached to the myometrium in placenta accreta and actually invade the myometrium in placenta increta or penetrate through the myometrium in placenta percreta. Ultrasound is only 33% sensitive for detecting placenta accreta; however, with ultrasound Doppler colour ow mapping, two factors are highly predictive of myometrial invasion: a distance < 1 mm between the uterine serosal bladder interface and the retroplacental vessels and the presence of large intraplacental lakes. Tumours Chorioangioma (haemangioma): Chorioangiomas are unique, non-trophoblastic placental tumours, which are present in approximately 1% of all placentas examined. Morphologically, they appear most commonly on the fetal surface of the placenta but can occur within the substance. Gestational trophoblastic neoplasia: Benign gestational trophoblastic neoplasia is commonly referred to as hydatidiform mole. Sonographically, the uterus is usually lled with multiple isoechoic-to-hyperechoic areas which are highly correlated with the presence of vesicles, the size of which depends on gestational age. During the rst trimester, the vesicles are not easily delineated and the uterine cavity appears echo-rich, generating the classical snowstorm appearance seen in the old B-mode static scanning. Other gynaecological conditions that can be mistaken for a molar pregnancy on ultrasound examination are embryonic demise, anembryonic pregnancy, leiomyoma and retained products of conception associated with an incomplete abortion. It is thought that this association is probably the result of molar transformation of one placenta in a dizygotic twin pregnancy. Fetal karyotypic analysis of this twin is recommended if the woman wishes to continue the pregnancy. It is important to distinguish between a partial hydatidiform and a molar pregnancy with a coexisting fetus, as a partial mole is considered to have less malignant potential than a complete hydatidiform mole. Fetal triploidy is characterized by severe growth restriction, oligohydramnios, hydrocephaly and hydropic placental changes. It usually contains two arteries and one vein; early in development, there are two umbilical veins, but the right vein atrophies and the le vein persists.
As they increase in size allergy medicine chlorpheniramine purchase generic flonase on-line, their walls press against those of neighboring cells so that the cells acquire a polyhedral shape (Figs 3 allergy symptoms latex condoms buy 50mcg flonase amex. The presence of this yellow pigment gives the structure a yellow color and that is why it is called the corpus luteum (= yellow body) allergy shots vs medicine flonase 50mcg with mastercard. Some cells of the theca interna also enlarge and contribute to the corpus luteum by becoming paraluteal cells allergy symptoms coughing in children order cheap flonase online. The ovum is easily carried into the tube partly by the follicular fluid discharged from the follicle and partly by the activity of ciliated cells lining the tube. The ovarian follicle itself has no blood vessels, but the surrounding theca interna is full of them. When the corpus luteum is forming, blood vessels from the theca interna invade it and provide it with a rich supply of blood. The subsequent fate of the corpus luteum depends on whether the ovum is fertilized or not. At the end of its functional life, it degenerates and forms a mass of fibrous tissue called the corpus albicans (= white body) (Figs 3. The corpus luteum of pregnancy may occupy one-third to half the total volume of the ovary. The progesterone secreted by it is essential for the maintenance of pregnancy in the first few months. After the 4th month, the corpus luteum is no longer needed, as the placenta begins to secrete progesterone. The series of changes that begin with the formation of an ovarian follicle and end with the degeneration of the corpus luteum constitute what is called an ovarian cycle. An ovarian cycle has an average duration of 28 days, with ovulation occurring at midcycle, i. Fate of Ovarian Follicles We have seen that in each ovarian cycle, one follicle reaches maturity, sheds an ovum, and becomes a corpus luteum. At the same time, several other follicles also begin to develop, but do not reach maturity. It is interesting to note that, contrary to what one might expect, these follicles do not persist into the next ovarian cycle, but undergo degeneration. The cells of the theca interna, however, proliferate to form the interstitial glands, also called the corpora atretica (singular = corpus atreticum). After a period of activity, each gland becomes a mass of scar tissue indistinguishable from the corpus albicans formed from the corpus luteum. Ovarian Cycle and Hormones the changes taking place during the ovarian cycle are greatly influenced by certain hormones produced by the hypophysis cerebri (Figs 3. The hormones produced by the theca interna and by the corpus luteum in turn influence other parts of the female reproductive system (notably the uterus), resulting in a cycle of changes referred to as the uterine or menstrual cycle. Similarly, sperms usually degenerate 48 hours after ejaculation, but may survive up to 4 days in female genital tract. Depending upon the particular chromosomes involved, various trisomy abnormalities are produced. In this condition, the child has a broad face, obliquely placed palpebral fissures, epicanthus, a furrowed lower lip, and broad hands with a single transverse crease. In this syndrome, the subject is always female (because of absence of a Y chromosome). Associated deformities include mental retardation, skeletal abnormalities, and folds of skin on the sides of the neck (webbed neck). When each piece joins the opposite chromosome, one chromosome is longer than normal and some of the genes are duplicated.
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