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Enbryology / All About Embryology
Page: 9
Except for part of the skull, all bones pass through three stages of development: membranous, cartilaginous, and osseous. The earliest ossification centres appear in the eighth week, but some do not arise until childhood years and even into adolescence.
The ventromedial walls (the walls toward the front and the midline) of the paired somites break down, and their cells migrate toward the axial notochord and surround it. Differentiation and growth of these segmental masses produce the jointed vertebrae. Ribs also grow out of each primitive vertebral mass, but they become long only in the thoracic region. Here their ventral ends join the sternum, which arises independently by the fusion of a pair of bars.
The skull has three components, different in origin. Its basal region is an ancient heritage whose bones pass through the three typical stages of development. By contrast, the sides and roof of the skull develop directly from membranous primordia, or rudiments. The jaws are derivatives of the first pair of cartilaginous branchial arches but develop as membrane bone. Ventral ends of the second to fifth arches contribute the cartilages of the larynx and the hyoid bone (a bone of horseshoe shape at the base of the tongue). Dorsal ends of the first and second arches become the three auditory ossicles (the small bones in the middle ear).
The limb bones develop in three stages from axial condensations in the local mesoderm. The shoulder and pelvic supports are comparable sets, as are the bones of the arms and legs.
Some type of joint exists wherever bones meet. Joints that allow little or no movement consist of connective tissue, cartilage, or bone. Movable joints arise as fluid-filled clefts in mesoderm, which condenses peripherally into a fibrous capsule.
Much of each somite differentiates into myoblasts (primitive muscle cells) that become voluntary muscle fibres. Aggregations of such fibres become muscles of the neck and trunk. Muscles of the head and some of the neck muscles originate from mesoderm of branchial arches. Muscles of the limbs seemingly arise directly from local mesoderm. In general, muscle primordia may fuse into composites, split into subdivisions, or migrate away from their sites of origin. During these changes they retain their original nerve supply. Regardless of differences in source of origin, all voluntary muscle fibres are of the same striated type (marked by dark and light stripes). Spontaneous movements begin to occur in embryos about 10 weeks old. In general, involuntary muscle differentiates from mesoderm surrounding hollow organs; only the cardiac muscle type is striated.
All hollow organs, including arteries, veins, and lymphatics, are lined with epithelium—the principal functional tissue—and are ensheathed with muscular and fibrous coats.
Primitive blood vessels arise in the mesoderm as tiny clefts bordered by flat endothelial cells. Growth and coalescence produce networks, out of which favoured channels persist as definite vessels, while others decline and disappear. A bilaterally symmetrical system of vessels is well represented in embryos four weeks old. This early plan is profoundly altered and made somewhat asymmetrical during the second month by fusions, atrophies, emergence of new vessels, and rerouting of older ones. The alterations reflect adjustments to changing form and pattern within the developing organ systems.
Arteries cranial to the heart (headward of the heart) are mostly products of the paired aortic arches, which course axially within the branchial arches, thus interconnecting the ventral aorta with paired dorsal aortas. The third pair of aortic arches becomes the common carotids; the fourth pair, the aortic arch and brachiocephalic; the fifth pair, the pulmonary arteries and ductus arteriosus. The dorsal aortas fuse into the single descending aorta, which bears three sets of paired, segmental branches. The dorsal set becomes the subclavian, intercostal, and lumbar arteries. The lateral set becomes arteries to the diaphragm, the adrenal glands, the kidneys, and the sex glands. The ventral set becomes the celiac, mesenteric, and umbilical arteries. Axial arteries to both sets of limb buds emerge from an original plexus, but they undergo drastic alteration and extensive replacement.
The primitive veins are symmetrically bilateral. They consist of vitelline veins from the yolk sac, umbilical veins from the placenta, and precardinal and postcardinal veins from the cranial and caudal regions (the regions toward the head and toward the tail) of the body. Drastic transformations occur in all of these, and new pairs of veins (subcardinals and supracardinals) arise also, caudal to the heart. From the vitellines come chiefly the portal and hepatic veins. The left umbilical becomes the main return from the placenta by making a diagonal channel, the ductus venosus, through the liver to the heart. The precardinal veins change their names to the internal jugulars, but near the heart an interconnection permits both to drain into a common stem, then called the superior vena cava. Caudal to the heart, the postcardinals virtually disappear, and all blood return shifts to the right side as a new compound vessel, the inferior vena cava, becomes dominant. Pulmonary veins open into the left atrium. Veins from the limb buds organize from an early peripheral border vein.
The lymph vessels develop independently in close association with veins. Linkages produce the thoracic duct, which is the main drainage return for lymph. Masses of lymphocytes accumulate about lymphatic vessels and organize as lymph nodes. The spleen has somewhat similar tissue, but its channels are supplied with blood.
 Fusion combines two endothelial tubes, and these are surrounded by a mantle of mesoderm that will become the muscular and fibrous coats of the heart. At three weeks the heart is a straight tube that is beginning to beat (Figure 1M). Starting at the head end, four regions can be recognized: bulbus, ventricle, atrium, and sinus venosus. Since the heart is anchored at both ends, rapid elongation forces it to bend. In doing this, the sinus–atrium and bulbus–ventricle reverse their original relations. Further development concerns the transformation of a single-chambered heart into one with four chambers.
The atrium becomes subdivided by the growth of two incomplete partitions, or septa, placed close together and each covering the defect in the other. The ventricle also subdivides, but by a single, complete partition. A canal, connecting atria and ventricles, becomes two canals. The bulbus is absorbed into the right ventricle, and its continuation (the truncus) subdivides lengthwise, forming the aorta and the pulmonary artery. The right horn of the sinus venosus is absorbed into the right atrium, together with the superior and inferior venae cavae, which originally drained into the sinus. The transverse portion of the sinus persists as the coronary sinus. The pulmonary veins retain their early drainage into the left atrium. Important valves develop and ensure flow within the heart from atria to ventricles, and outward from the ventricles into the aorta and the pulmonary artery.
Birth initiates breathing, and the abandonment of the placental circulation follows. These changes entail a drastic rerouting of blood through the heart. As a result, the two atrial septa fuse and no longer permit blood to pass from the right atrium to the left atrium. Blood in the pulmonary artery no longer virtually bypasses the lungs; previously it had passed to the aorta directly through a shunt offered by the ductus arteriosus. As a sequel to these changes, the abandoned umbilical arteries, umbilical vein, ductus venosus, and ductus arteriosus all collapse and become fibrous cords.
Vertebrates have made three experiments in kidney production: the pronephros, or earliest type; the mesonephros, or intermediate kidney; and the metanephros, or permanent kidney. All arise from the cellular plates called nephrotomes that connect somites with the mesodermal sheets that bound the body cavity. The vestigial pronephros is represented solely by several pairs of tubules; they join separately formed excretory ducts that grow downward and enter the cloaca, the common outlet for urine, genital products, and for intestinal wastes. Next tailward arise some 40 pairs of nephric (kidney) tubules that constitute the mesonephros; these tubules join the same excretory ducts, hereafter called the mesonephric ducts. The two sets of mesonephric tubules serve as functioning kidneys until the 10th week.
Each permanent kidney, or metanephros, develops still farther tailward. A so-called ureteric primordium buds off each mesonephric duct, near its hind end. The ureteric stem elongates and expands terminally, thereby forming the renal pelvis and calices; continued bushlike branching produces collecting ducts. The early ureteric bud invades a mass of nephrotome tissue. The branching collecting ducts progressively break this tissue up into tiny lumps, each of which becomes a long secretory tubule, or nephron, and joins a nearby terminal twig of the duct system. Continued proliferation of ducts and nephric tissue produces over a million urine-producing tubules in each kidney.
The blind caudal end of the endodermal hindgut absorbs the stem of each mesonephric duct, whereupon the remainder of the duct and the ureter acquire separate openings into the hindgut. This expanded region of the gut, now a potential receptacle for feces, urine, and reproductive products, is known as a cloaca. It next subdivides into a rectum behind and a urogenital sinus in front. The sinus, in turn, will specialize into the urinary bladder and the urethra. The prostate gland develops as multiple buds from the urethra, close to the bladder.
The genital organs begin to develop in the second month, but for a time sex is not grossly distinguishable. Also, a double set of male and female ducts arise, and not until later does the unneeded set decline. Hence, this period is commonly called the indifferent stage.
Sex glands develop in a pair of longitudinal ridges located alongside the mesentery, the anchoring fold of membrane to the gut. The primordial sex cells appear first in the cloacal wall, from which they migrate upward in the gut, pass through its mesentery, and finally invade the genital ridges, where they proliferate. The testes are the earliest type of gonad to organize. They begin by developing testis cords and a testis capsule. The cords radiate from one focal point at the periphery, and thin fibrous partitions segregate groups of the cords within wedge-shaped compartments. These cords do not gain channels and become semen-producing tubules until near the time of puberty. The ovaries organize somewhat tardily by differentiating an outer portion, the cortex, and a central portion, the medulla. The cortex contains the primordial sex cells; these become surrounded by a layer of ordinary cells, thereby forming primary ovarian follicles. Both the testes and the ovaries undergo relative shifts from their early sites to lower positions in the body. But only the testes make a bodily descent; this is into the scrotum.
In the male, a few mesonephric tubules on each side do not degenerate but link up with the neighbouring testis tubules. The converted mesonephric tubules and the retained mesonephric ducts become the male sex ducts. Near their terminations they outpouch seminal vesicles and then open into the urethra. In the female, a pair of ducts develops from the epithelium clothing the mesonephric ridges. These ducts, known as the uterine tubes, mostly parallel the courses of the mesonephric ducts, but at their lower ends they unite into a common tube that becomes the uterus and vagina.
Both sexes develop a genital tubercle (i.e., a knob) and a pair of urogenital folds flanked by a pair of genital swellings. At three months these rudiments begin to assume male or female characteristics. In the male, the tubercle and the united urogenital folds combine as the penis, thereby continuing the urethra to its end; the genital swellings shift toward the anus, fuse, and become the scrotum. In the female, the tubercle remains small, as the clitoris; it does not contain the urethra. The urogenital folds remain unclosed as the lesser vulvar lips and are flanked by the unshifted and unfused genital swellings, or greater lips.
The lateral mesoderm, beyond the somites and nephrotomes, splits into two layers: the somatic layer and, underlying the somatic layer, the splanchnic layer. The intervening space is the coelom. As the embryo's body folds off, its coelom becomes a single closed cavity. In it can be recognized, regionally, a provisional pericardial cavity (cavity for the heart), two pleural canals (for the lungs), and a peritoneal cavity (for the abdominal contents). A thick plate of mesoderm, the transverse septum, constitutes a partial partition just ahead of the developing liver. Two pairs of membranes grow out from the septum. One set separates the pericardial cavity from the two pleural cavities; these membranes later expand into the pericardium and enclose the heart. The other pair of membranes separates the pleural cavities from the peritoneal cavity of the abdomen. The definitive diaphragm is a composite partition, much of which is furnished by the transverse septum; lesser contributions are from the lateral body walls and the paired membranes that separated the pleural and peritoneal cavities. Take this preliminary to see if your condition could respond to treatment.
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