Book - Aids to Embryology (1948) 9
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Baxter JS. Aids to Embryology. (1948) 4th Edition, Bailliere, Tindall And Cox, London.
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Chapter IX The Alimentary Canal And Related Structures
The primitive entodermal gut tube is laid down when the head, tail, and lateral body folds arise (p. 36). The gut is at first limited cranially and caudally by the bucco-pharyngeal and cloacal membranes, and it is subdivided into segments named the fore-, midand hindgut. When dealing with the fate of these structures it is convenient to describe the development of certain regions, such as the face and palate, which are associated with the alimentary canal.
The Pharyngeal Region
During the fifth and sixth weeks of development a number of bar-like ridges separated by grooves become prominent upon the lateral aspect of the neck of the embryo. These are the branchial arches and they correspond to the gill arches of fishes. There are six of these arches in the human embryo, although the fifth one is transitory and its existence is denied by some. The first arch on each side is subdivided into maxillary and mandibular processes which form much of the face. The second is known as the hyoid arch because part of the hyoid bone is developed from it. Each arch contains a skeletal basis of primitive cartilage (branchial archcartilage), a blood vessel (aortic arch, p. 109) connecting the heart with the dorsal aorta, a nerve, and mesoderm destined to form muscle tissue. These structures are represented in the adult as shown in the table on p. 76. The nerves of these arches are associated with thickened patches of ectoderm, termed epibranchial placodes, and may possibly receive some cells from them to their sensory ganglia. These placodes are located at the dorsal end of each arch. Caudal to the region of the arches is a mesodermal downgrowth from the occipital somites which will give rise to the muscles of the tongue. Between this and the swelling caused by the developing heart is the epipericardial ridge.
Mandible, malleus, incus
Muscles of mastication, mylo-hyoid, ant. belly digastric, tensor tympani, tensor palati
Mandibular division of trigeminal
Lesser cornu and upper part of body of hyoid, styloid process, stapes
Stylo-hyoid, post, belly digastric, stapedius, muscles of expression
Lower part of body and greater cornu of hyoid
Stem of internal carotid
Part of thyroid cartilage
Arch of aorta on left ; part of subclavian on right
Pharyngeal and laryngeal
Vagus (superior laryngeal)
Part of thyroid and cricoid cartilages
Pulmonary arteries Ductus arteriosus on left side
Pharyngeal and laryngeal
Vagus (recurrent laryngeal)
During the sixth week the first and second arches have become large as compared with the caudal ones which lie in an ectodermal-lined depression, the cervical sinus. The hyoid arch grows caudally and fuses with the epipericardial ridge so that the cervical sinus becomes obliterated during the seventh week and the caudal arches are buried in the side of the neck. Ectodermal remnants of the sinus may persist and may give rise to branchial cysts in later years.
During the fifth week (Fig. 18) the stomatodaeum, or primitive mouth, is bounded above by the fronto-nasal process of mesoderm in front of the developing brain ; laterally are the maxillary processes, while infero-laterally the boundary is completed by the mandibular processes. The frontonasal process bifurcates on each side to surround the olfactory placode. The olfactory pit thus becomes bounded by medial and lateral nasal processes. At first, each pit communicates at its ventral margin with the oral fossa by the oro-nasal groove, and with the primitive orbit by the naso-lachrymal groove. By the fusion of the adjacent margins of these processes the orbital, nasal, and oral cavities become demarcated. The naso-lachrymal groove is overgrown by fusion of the maxillary process with the lateral nasal. The former then continues to grow anteriorly below the olfactory pit to fuse with the medial nasal process, and then unites superficial to this with its fellow of the opposite side (Boyd, 1933). Thus the upper lip and anterior nares are formed. The two mandibular processes unite in the mid-ventral line to form the lower jaw and lip, and then there is a certain amount of union between the adjacent margins of the maxillary and mandibular processes to narrow down the primitively wide mouth to more adult proportions.
Fig. 18. - The Development of the Face.
1, Fronto-nasal process ; 2, lateral nasal process ; 3, lens ; 4, maxillary process ; 5, mandibular process.
When, during the sixth week, the maxillary processes fuse with each other and with the frontal-nasal process to form the upper lip, the anterior part of the stomatodaeum is subdivided into upper and lower parts. This partition is the primitive palate. The deep part of the fronto-nasal process forms a rudimentary nasal septum. Then a shelf like projection of mesoderm grows inwards on each side from the maxillary process and with continued growth the free edges of these palatal processes fuse with each other from before backwards ; union also occurs with the nasal septum lying above, which becomes elongated in an antero-posterior direction. The primitive palate gives rise to the premaxillary portion of the adult palate, while the palatal processes form the remainder of the hard, and all of the soft palate. A small gap between these three components persists as the naso-palatine canal of the adult.
Fig. 19. - Coronal Section of Embryonic Head to show the Development of the Palate.
1, Fronto-nasal process (primitive nasal septum) ; 2, organ of Jacobson ; 3, palatal process of maxilla ; 4, tongue.
During the fifth week the primordia of the tongue can be seen on the floor of the pharyngeal region. There is a median swelling, the tuberculum impar, which lies between the ventral ends of the first pharyngeal grooves. Antero-lateral to this are paired swellings derived from the ventral extremities of the mandibular processes. Just behind the tuberculum impar a little diverticulum indicates the site of origin of the thyjroid gland. Posterior to this is a median elevation formed by fusion of the ventral ends of the second branchial arches and termed the copula. Tbe paired lateral swellings increase greatly in size and fuse in the middle line to form the anterior two-thirds of the tongue while the tuberculum impar lags greatly in development and forms little or no part of the adult organ. Mesoderm derived from the third arch overgrows the copula (which consists of second arch tissue) and causes it to become buried in the substance of the tongue so that the posterior one-third of the organ is formed by third arch tissue, with a small contribution of fourth arch tissue, at the extreme caudal part. The sensory nerve supply of the tongue supports such a conception of its development. The body, derived from the mandibular swellings is supplied by the proper nerve of that arch (mandibular division of the trigeminal) aided by the pre-trematic branch of the second arch nerve (chorda tympani). The root of the tongue, derived from third and fourth arch mesoderm, is supplied by the glosso-pharyngeal and superior laryngeal branch of the vagus. The foramen caecum on the adult tongue represents the site of the original thyroid downgrowth while the sulcus terminalis very roughly marks the boundary between first and third branchial arch tissue. The striated muscle of the tongue is derived from the occipital somites.
It is not easy to mark off the ectodermal part of the mouth from the entodermal but it is generally considered that the vestibule and much of the upper part is derived from the former. Ectoderm also gives rise to the enamel of the teeth.
In the human subject two sets of teeth are developed, a deciduous or milk set of twenty teeth, and a permanent set of thirty-two teeth. These begin to develop at an early date in the human embryo as a continuous curved ectodermal thickening which grows into the mesenchyme of each developing jaw. Each of these dental laminae develops ten buds which represent the enamel epithelium of the deciduous teeth . Opposite each bud a condensation of mesenchyme occurs to form a dental papilla and the enamel epithelium becomes moulded over this in the form of a cap. At a later date, from lingual extensions of each original dental lamina, a further series of sixteen enamel caps are formed which represent the primordia of the thirty-two permanent teeth. The development of these is exactly analogous to that of the milk set.
The enamel organ is a two-layered epithelial cup in the concavity of which lies the mesodermal dental papilla. The inner epithelial layer is converted into ameloblasts which lay down the enamel on the surface next to the dental papilla. When they have completed this function, they disappear. The outer epithelial layer, together with some reticular tissue between it and the enamel, forms a covering for the crown of the tooth, the cuticular membrane of Nasmyth. This membrane disappears soon after eruption in those parts exposed to attrition, but persists on the basal part. The surface cells of the dental papilla become transformed into odontoblasts which lay down dentine deep to the enamel. In the dentine are filamentous processes of the distal ends of the odontoblasts known as Tomes' dental fibrils. The remaining tissue of the dental papilla becomes vascularized to form the dental pulp.
The mesenchyme around the developing tooth germ becomes condensed to form what is known as the dental follicle, from which is developed the cementum, a bone-like substance, and the fibrous membrane between the tooth and its socket, termed the periodontal membrane.
As the permanent teeth enlarge they press upon the roots of the deciduous teeth and partial resorption of the roots of the latter occurs. By a combination of this pressure and resorption the deciduous teeth are finally forced from their original position and shed.
One cannot give precise dates for the eruption of either the milk or permanent dentitions as a number of factors operate to cause variation in their time of appearance. It may be stated, however, that the first milk teeth to appear are the lower central incisors at the sixth month, and that these are quickly followed by the other incisors. A pause then ensues and the molars and canines then erupt, the process being usually completed by the end of the second year. There is a general tendency for the lower teeth to precede the upper teeth.
The permanent set of teeth begin with the eruption of the first permanent molars, generally during the seventh year, and these are followed by the incisor group during the following year. A pause of about two years then occurs before the premolars, canines, and second molars erupt and the permanent dentition should be complete by the end of the twelfth year except for the third molars. These appear between the eighteenth and twenty-fifth years, or may not erupt at all. The permanent teeth of girls erupt from six to nine months earlier than those of boys (McKeag, 1937). Like the milk teeth, the general tendency is for the lower teeth of the permanent set to precede the upper, and a year may elapse before apposing teeth meet.
The Salivary Glands
The mode of development of the three salivary glands is very similar, but the parotid differs from the submandibular and the sublingual in being ectodermal in origin, while the latter two arise from the entoderm. The submandibular salivary gland is first seen during the sixth week as a solid rod of cells growing caudally from the entoderm of the floor of the mouth close to the developing tongue. Dichotomous branching takes place and the solid epithelial cords become hollowed out during the third month, alveolar rudiments appearing at the tips. A groove in the floor of the mouth runs forwards from the place where the original outgrowth occurred, and the lips of this groove close over, so forming the submandibular duct which thus extends forward to below the tongue close to the middle line. Here the process ceases and the tube is thus left unclosed at this point. Some heaping up of tissue takes place at this point and the sublingual papilla is formed.
The sublingual gland develops as a series of short outgrowths of entoderm from the floor of the mouth on a ridge close to the tongue near the middle line. These outgrowths, appearing in the eighth week, lie to the outer side of the groove for the submandibular duct ; they branch extensively and then by a rearrangement of their cells they become hollow and differentiate to form glandular tissue. The outgrowths retain their original openings.
The parotid gland differs from the other salivary glands in being ectodermal in origin (Fraser, 193 1). It begins during the sixth week as an epithelial bud close to the angle of the wide primitive mouth. This grows backwards towards the ear, branching and becoming hollow in the same manner as the other salivary glands. With narrowing down of the wide oral opening a groove on the inner aspect of the cheek closes over to form the parotid duct.
The Pharyngeal Pouches
Alternating with the system of branchial arches are depressions on the internal surface of the pharynx known as the pharyngeal grooves. These are elongated dorso-ventrally, and the extremities are dilated as the pharyngeal pouches from which certain glandular structures are developed.
In the case of the first pharyngeal groove the ventral part becomes obliterated with the formation of the tongue. The dorsal part of the groove becomes elongated as the tubo-tympanic recess (p. 72).
The palatine tonsils develop from the dorsal angles of the second pharyngeal pouches. The entoderm proliferates as a series of buds and almost obliterates the pouch ; this thickening later becomes invaded by mesenchyme. The buds become hollowed out as the tonsillar crypts. About the fifth foetal month lymphocytes become aggregated in the mesenchyme of the tonsil ; these may be derived from this mesenchyme or from the blood stream.
This is derived from the ventral portion of the third pharyngeal pouch on each side. This, at first large and hollow, separates from the pharynx and becomes solid by proliferation of the entodermal cells forming its wall. Migrating caudally into the future thoracic region it meets and fuses with its fellow of the opposite side to form a lobulated organ. During this migration (which is really caused by the â€œ descent â€ of the heart) the thymus becomes invaded by mesenchyme, and about the tenth week lymphocytes, sometimes called thymocytes, appear in it. The original entodermal cells become transformed into reticular elements. The corpuscles of Hassall appear later as specializations of the reticulum. Norris (1938) holds that the ectoderm of the cervical sinus contributes to the formation of the thymus. At birth, the gland is of relatively large size and remains prominent until puberty, after which date it regresses.
The upper parathyroids develop as proliferations from the dorsal angles of the fourth pharyngeal pouches while the lower parathyroids develop in a similar manner from the dorsal angles of the third pouches. Similar forces to those acting on the thymic primordia influence the parathyroids arising from the third pouch causing them to descend to a lower level in the neck than those derived from the fourth pouch. The latter seem to be prevented from migrating downwards by the participation of fourth pouch tissue in the formation of the thyroid (see below).
The Thyroid Gland
The thyroid gland is first seen during the fourth week as an out-pocketing from the floor of the pharynx immediately behind the tuber culum impar (p. 79) at a point which is represented in the adult tongue by the foramen caecum. This diverticulum forms a hollow stalk, termed the thyro-glossal duct, the lower end of which dilates at the fifth week to form a hollow vesicle ; the hollow stalk atrophies and disappears leaving the dilated lower end lying free in the surrounding mesenchyme. The vesicle develops unequally and takes on a bilobed form, and by a proliferation of the cells of its walls, the lumen disappears. The solid bi-lobed mass becomes subdivided into a number of follicles by ingrowth of mesenchymal septa and colloid appears in these about the twelfth week. About the sixth week, cells derived from the ventral end of each fourth pouch fuse with each lateral aspect of the thyroid primordium. According to Weller (1933) these lateral thyroid masses are transformed into glandular tissue, but this opinion is not held by all. The thyro-glossal duct passes ventral to the hyoid bone in its course downwards from the foramen caecum to the isthmus of the gland ; while it normally disappears completely a remnant may persist in any part of this track and give rise to a cystic swelling in later years.
Fig. 20. - The Pharyngeal Grooves.
1, Thyroid gland; 2, thyro-glossal duct; 3, foramen caecum; 4, pharyngo-tympanic tube ; 5, tonsil ; 6, lower parathyroid ; 7, thymus ; 8, upper parathyroid ; 9, lateral thyroid.
Summary of the Development of the Pharyngeal Region
- A series of branchial arches is formed on the lateral side of the neck ; between the arches are ectodermal grooves externally, and entodermal pharyngeal grooves internally.
- From each dorsal angle of the first entodermal groove there develops the pharyngo-tympanic tube and the lining of the middle ear.
- From the dorsal angle of the second groove the palatine tonsil is formed.
- The ventral end of the third pouch gives rise to the thymus ; the dorsal extremity of this pouch develops into the inferior parathyroid.
- The superior parathyroids arise from the dorsal ends of the fourth pharyngeal pouches ; cells from the ventral ends of these pouches join with the developing thyroid.
- The major portion of the thyroid develops as a median diverticulum from the pharyngeal floor just posterior to the tuberculum impar.
- The body of the tongue is derived from fusion of swellings on the ventral extremities of the mandibular processes ; the root is formed from mesoderm of the third branchial arches which overgrows and submerges the copula, or united ventral ends, of the second arches. The musculature comes from the occipital somites.
Anomalies of the Face and Pharyngeal Region
- Unilateral hare lip results from non-union of one maxillary process with the fronto-nasal process and its fellow of the opposite side. A median hare lip is a rare condition.
- Bilateral hare lip is due to failure of the maxillary processes to unite with each other. The small fronto-nasal process lies free between their extremities.
- Oblique facial cleft is caused by non-union of the maxillary process with the lateral nasal process.
- Cleft palate may be partial or complete, and in the latter case is often associated with hare lip. It is due to non-fusion of the palatal folds with each other, and with the fronto-nasal process (primitive palate) on one or both sides.
- Split tongue may result if union of the anterior elements of the tongue does not occur.
- Branchial sinuses and fistulae result from malocclusion of the cervical sinus or persistence of an external branchial groove.
- Branchial fistulae result from perforation of an external groove into the pharynx.
- Persistence of part of the thyro-glossal duct may cause a mid -line cyst in the neck.
The Primitive Gut
The primitive gut is at first a simple tube of entoderm extending from head to tail of the embryo. It terminates blindly at its extremities until breakdown of the bucco-pharyngea! and cloacal membranes allows communication of its lumen with the exterior. Ventrally it is in communication with the extra-embryonic yolk sac for some time by the vitello-intestinal duct. This entodermal tube is clothed by that layer of the intraembryonic mesoderm known as the splanchnopleure (p. 19). The gut tube rapidly increases in length, much faster than the body cavity in which it lies. For this reason, the gut becomes looped and draws away from the walls of the body cavity, elongating part of the splanchnopleure as the mesentery. It is by the formation of further loops and by inequalities of growth within its walls that the pattern of the adult intestine is laid down.
Fig. 21. - Two Diagrams to show the changes brought about by the rotation of the gut.
1, Diaphragm ; 2, stomach ; 3, dorsal pancreas ; 4, liver bud ; 5, duodenum ; 6, cranial limb of intestine ; 7, vitellointestinal duct ; 8, caudal limb of intestine ; 9, intra abdominal colon. The position of the caecum is indicated by a + .
The primitive gut very early shows a slight dilatation just caudal to the septum transversum (p. 36), and the short segment between this and the pharynx represents the oesophagus. This part increases in length as the heart and diaphragm descend during development and the elongation is most marked in the caudal portion of the tube. With this elongation the lumen of the oesophagus becomes markedly narrowed, especially in the lower part, and may even undergo temporary obliteration. The lumen becomes widened again from the sixth week onwards by the formation of vacuolar spaces between the entodermal cells. These open out into the lumen and so increase the calibre of the oesophagus. The muscular coat differentiates from the surrounding mesenchymal cells.
The dilatation of the primitive tube which represents the stomach is at first spindleshaped, and is slung from the dorsal body wall by a dorsal mesentery of splanchnopleure and from the under surface of the septum transversum by a ventral mesentery of the same tissue. The dorsal border and the oesophageal end grow more rapidly than the ventral border and the pyloric end, and as a result, the stomach appears to rotate through 90 0 on its long axis so that the original right side becomes dorsal, and the original left side ventral, in position.
The primitive dorsal border of the stomach then becomes the greater curvature and the lesser curvature represents the primitive ventral border. The rapidly enlarging oesophageal end becomes the fundus. The dorsal mesentery attached to the stomach (more correctly called the dorsal mesogastrium) is influenced by the rotation of the stomach so that a blind retrogastric recess is formed opening to the right. This corresponds in part with the lesser sac of peritoneum in the adult. Rapid growth of the dorsal mesogastrium causes it to sag in a caudal direction and so form the primordium of the great omentum (see Fig. 22). The liver develops as an outgrowth from the primitive gut into the ventral mesentery, which becomes subdivided thus into two parts, the lesser omentum extending between the lesser curvature and the developing liver and the falciform and coronary system of peritoneal ligaments attaching the liver to the under surface of the diaphragm (derived largely from the septum transversum) . At first the pyloric portion of the stomach forms almost half the total length, but the later, more rapid growth at the oesophageal end soon produces a stomach of more adult proportions. Pits in the entodermal lining develop about the middle of the second month, and the gastric glands commence to differentiate during the fourth month. At this time, some of the gland cells stain intensely with eosin and are considered to be the future parietal cells, while others remaining pale are designated as zygmogenic cells. Rennin and hydrochloric acid have been demonstrated in the foetal stomach at the end of the fifth month, while pepsin is present somewhat earlier (Lucas Keene and Hewer).
The segment of gut caudal to the stomach grows rapidly and soon forms a loop convex ventrally. A vitelline vessel, the future superior mesenteric artery, runs from the dorsal aorta to the apex of the loop, dividing it into cranial and caudal limbs, and the vitello-intestinal duct is attached to the apex of the loop on its free or ventral aspect. Continued growth of the gut results in increase in length of the intestinal loop until it can no longer be completely accommodated in the abdominal cavity ; the developing liver occupies a disproportionately large amount of the available space. A large part of the intestinal loop is therefore herniated into a mesothelial lined cavity, the umbilical sac,, continuous with the intra-embryonic coelom. This protrusion of the gut occurs during the fifth week. The loop in the umbilical sac continues to grow, and the cranial limb of it comes to lie on the right side of the cavity, the caudal limb on the left. This rotation continues until a torsion through i8oÂ° has occurred, the torsion being in an anti-clockwise direction when the loop is viewed from the ventral aspect. This torsion may be partly determined by the direction of the pyloric end of the stomach and it is of primary importance in determining the future position of the intestine.
During the sixth week a swelling, the primordium of the caecum, appears on the caudal limb of the intestinal loop, and about the same time the vitellointestinal duct atrophies and disappears leaving the loop free in the unbilical sac. It is now evident that all the cranial limb and a little of the caudal limb of the loop are destined to form the small intestine, while the remainder of the caudal limb will contribute to the large intestine. The cranial limb of the intestinal loop continues its rapid growth so that a series of coils are formed lying to the right side of, and caudal to, the caecal rudiment. The caudal limb lags in growth and runs in a practically straight line along the left side of these coils.
About the tenth week of development the intestinal loops in the umbilical sac return to the abdominal cavity. This process is considered by Frazer (1931) to be due to a relative diminution in volume of the liver. The communication between the abdominal cavity and the umbilical sac is a narrow one and so the gut cannot return en masse. Instead, first the loops of the cranial limb are withdrawn one by one and the caudal limb follows later since the caecal dilatation offers some resistance to passage through the narrow opening into the abdomen. The entering loops of the cranial limb spread out on the posterior wall of the abdominal cavity pushing the intra-abdominal continuation of the caudal limb over to the left. When the caecum and future large intestine are withdrawn into the abdomen they must lie in front of the other intestinal loops and further * â€˜ rotation â€™ â€™ of them brings the caecum to the right side just below the liver.
The various segments of the gut now become differentiated and fixed in position in the abdominal cavity.
The duodenum arises from a short segment of the gut between the stomach and the superior retention band. This latter is a condensation of tissue anchoring the upper extremity of the intestinal loop to the dorsal abdominal wall. The formation of the duodenal loop has been studied by Hunter (1927) who states that at first it forms a loop convex ventrally whose extremities are fixed to the dorsal abdominal wall at the pylorus cranially, and by the superior retention band caudally. With further growth the loop swings to the right and is pressed against the dorsal abdominal wall by the developing large intestine when it is withdrawn from the umbilical sac. The mesentery of the duodenum fuses with the peritoneum on the posterior wall of the abdomen and disappears. The duodenum thus becomes a retroperitoneal structure.
The Caecum, Appendix, and Colon
The formation of a caecal pouch on the caudal limb of the intestinal loop has been described above. This pouch grows rapidly in length for a time, but the basal part increases in diameter much more than the apical portion. Thus the primordium of the appendix is differentiated from the caecum. When it enters the abdominal cavity the caecum comes to lie immediately caudal to the right lobe of the liver. Progressive diminution in size of the liver, and growth changes in the large intestine, result in an apparent â€œ descent â€ of the caecum and formation of an ascending colon. The mesentery of the caecum and ascending colon disappears during this process by fusing with the peritoneum of the posterior abdominal wall. That segment of the gut which forms the descending colon loses its mesentery in a similar fashion. The great omentum arises as a downgrowth of the dorsal mesogastrium (p. 89). This downgrowth lies ventral to the transverse colon which retains its mesentery and the two structures fuse (see Fig. 22) to give the arrangement found in the adult.
Fig. 22. - Diagrams to show the Position of the Peritoneal Folds : A, Immediately after the gut has been withdrawn from the umbilical sac j b, after the Fusion of the Dorsal Wall of the Primitive Lesser Sac to the Ventral Wall of the Primitive Transverse Mesocolon.
i, Liver ; 2, lesser sac ; 3, stomach ; 4, pancreas ; 5, duodenum ; 6, tran verse colon ; 7, small intestine.
The entoderm forms the mucous lining of the intestine, the muscular coats being derived by transformation of cells of the surrounding splanchnic mesoderm. The circular muscle coat is differentiated rather earlier than the longitudinal which forms during the third and fourth months. The enzymes of the small intestine are stated to be present in the fifth month. At the end of gestation the large intestine is full of meconium, a dark green secretion of the liver and intestinal glands.
Anomalies of Development of the Intestinal Tract
- Duodenal stenosis. This condition may involve the whole length of the duodenum or a section of it. It is usually found as a patch just proximal to the entrance of the bile duct and is due to persistence of an occlusion of the lumen which normally occurs temporarily between the sixth and ninth weeks of development.
- Atresia of the intestine. The common sites of atresia are high up in the jejunum, in the terminal ileum, or in the colon. Atresia of the small intestine is often associated with errors of rotation.
- Malformations of the rectum are frequently combined with faulty development of the lower urogenital tract (p. 144).
- Congenital umbilical hernia may be due to persistence of the umbilical sac, or it may be secondarily acquired.
- Meckel's diverticulum found in about two per cent, of adults is due to persistence of the proximal part of the vitello-intestinal duct. It commonly forms a blind diverticulum about 2 inches long on the antimesenteric border of the ileum anywhere from 6 to 60 inches proximal to the ileo-caecal value. In more pronounced cases of persistence of the vitellointestinal duct a fibrous cord may unite the diverticulum to the umbilicus. Rarely, there may be a fistulous communication between the gut lumen and the exterior.
- Non-rotation of the intestine sometimes occurs and then the jejunum and ileum lie on the right side, and the large intestine on the left side, of the abdominal cavity.
- Situs inversus is a condition in which there is mirror-transposition of the viscera. Thus the liver, caecum, and ascending colon are found on the left side, and left-sided organs, the stomach and spleen, are found on the right. This inversion may be confined to the abdominal viscera or may affect the thoracic organs as well. The condition is due to the intestinal loops rotating in the opposite direction to the normal. Carey (1920) believes it to be due to the spiral organization of the developing gut becoming reversed in direction. It is interesting to note that the condition is not infrequent among identical twins.
The primordium of the liver first appears about the twenty-fifth day as a shallow groove on the ventral aspect of the future duodenal segment of the gut. This groove becomes a tubular evagination which invades the mass of mesoderm, termed the septum transversum, which partially subdivides the coelom into thoracic and abdominal portions (p. 36), and the ventral mesentery which is continuous with it. The hepatic bud enlarges ; part of it remains within the substance of the ventral mesentery and will form the gall bladder and cystic duct ; the major part lying in the septum transversum divides into right and left solid buds which branch repeatedly and become organized as anastomosing trabecular cords of cells. Sinusoidal blood spaces derived from the vitelline and umbilical veins lying in the septum transversum separate these liver trabeculae. The trabecula becomes hollow to form the bile capillaries whose lining is therefore derived from entoderm. The mesoderm of the septum forms the fibrous tissue of the liver. The original outgrowth from the duodenum forms the common bile duct, and the right and left buds from it become hollow as the right and left hepatic ducts.
The liver cells become arranged in lobules each being related to a radicle of the hepatic vein ; and, with the formation of new lobules, there is corresponding branching of the hepatic veins. Likewise do the portal vein and hepatic artery branches (which lie at the periphery of the lobules) subdivide and maintain their proper orientation to the newly formed hepatic veins and lobules. The two systems of veins are connected by the sinusoids, much of the endothelium of which become phagocytic in function as the Kupffer cells of the organ. From the second to the seventh months of foetal life haematopoiesis is an important function of the liver. Active differentiation of blood cells takes place in foci between the glandular cells and the sinusoidal endothelium and from there the cells pass into the foetal circulation.
During the second month the liver grows much more rapidly than the remainder of the body and at nine weeks it is relatively enormous and occupies a large part of the abdominal cavity. After this time the growth rate decreases, but even at birth the liver forms a much greater proportion of the body weight than it does in the adult. The decrease in growth rate in the latter part of foetal life affects the left lobe more than the right. The bile secreted by the foetal liver from the fifth month onwards gives the characteristic green colouration to the meconium.
As the liver grows it withdraws from the substance of the cranial part of the septum transversum which is transformed into the diaphragm. The connections between the two persist as the hepatic ligaments and at one part the withdrawal may be considered as incomplete. This is at the bare area of the liver.
The pancreas develops as two outgrowths from the duodenal segment of the gut, one from the dorsal aspect a little proximal to the hepatic bud, and one from the ventral and right aspect close to the liver diverticulum. The dorsal bud grows into the dorsal mesentery to the left side of the left vitelline (developing portal) vein. This part forms the body and tail of the adult pancreas. The ventral outgrowth, which will form the head of the adult gland, is swept around dorsally into the mesentery when the duodenal loop is rotated to the right and thus comes to lie on the right side of the dorsal pancreas separated from it by the developing portal vein. These two primordia fuse about the end of the sixth week and so the portal vein comes to lie anterior to the lower part of the head of the pancreas derived from the ventral bud, and dorsal to the neck which is formed by the dorsal bud. The ducts of the two primordia communicate with each other and the proximal part of the dorsal duct partly or completely retrogresses. As a result, the main pancreatic duct of the adult opening into the duodenum in common with the bile duct is formed by the ventral pancreatic duct plus the distal portion of the dorsal duct. If any of the proximal portion of the dorsal duct persists it opens into the duodenum as the accessory pancreatic duct some distance cranial to the common bile duct. The minor ducts and acini of the pancreas arise by repeated sproutings of groups of cells from the pancreatic buds. Groups of islet cells form as similar sprouts which do not, however, develop a lumen and become separated from the parent tissue about the third month of foetal life. The secretion of the pancreatic acini may be identified about the fifth foetal month.
Fig. 23. = The Development of the Pancreatic Ducts
1, Stomach ; 2, common bile duct ; 3, duct of dorsal pancreas; 4, duct of ventral pancreas.
Summary of the Development of the Liver and Pancreas
- The liver and pancreas both arise as entodermal evaginations from the gut in the region of the duodenal segment.
- The liver bud grows into the septum transversum and forms a mass of anastomosing trabeculae which invade and split up the umbilical and vitelline veins lying in the septum. These split-up veins form the liver sinusoids.
- At first solid, the trabeculae become hollow to form the bile capillaries and ducts. The first branch from the original hepatic diverticulum is transformed into the gall bladder and cystic duct.
- The pancreatic primordia are two in number, dorsal and ventral. The dorsal bud forms the body and tail, the ventral one represents the head of the adult organ.
- The two primordia are brought into contact with each other by the rotation and fixation of the duodenal loop. The respective ducts fuse and the proximal part of the dorsal duct disappears. The main pancreatic duct of the adult is then formed by the distal part of the dorsal duct and the entire ventral duct.
- The cell islets of Langerhans arise during the third month of foetal life as solid masses of cells budded off from the pancreatic acini.
Anomalies of Development of Liver and Pancreas
- The gall bladder may fail to develop, or it may be double.
- In situs inversus the liver lies on the left side of the abdomen.
- The pancreas may persist as two separate parts. Accessory glands may be present, often some distance from the primary situation of the viscus.
The Respiratory System
The first indication of the respiratory system is a longitudinal groove, the pulmonary groove, in the floor of the caudal part of the pharynx. The caudal end of this grows as a pouch-like diverticulum whose tip lies free in the mesoderm ventral to the primitive oesophagus. This is the rudiment of the trachea and its tip soon bifurcates to form the primary bronchi. These buds, covered by mesoderm, bulge caudo-laterally into the coelomic channels on either side of the oesophagus cranial to the septum transversum. These channels are later modified and extended to form the pleural cavities.
The primary bronchi branch, the left one into two and the right one into three ; from these divisions the two lobes of the left lung and the three lobes of the right, are ultimately formed. Repeated dichotomous division of the bronchi takes place, the branches invading and being capped by the surrounding mesoderm in which blood vessels develop. In this way the bronchioles, alveolar ducts, and alveoli are formed and it is to be noted that this process is not completed at birth. The fate of the entodermal cells lining the terminal buds is still a matter of dispute.
The cranial end of the tracheal outgrowth lies at the level of the sixth branchial arch and the surrounding mesoderm here differentiates into the cartilages, muscles and ligaments of the larynx. On each side of the opening into the primitive pharynx an arytenoid swelling occurs. These are at first almost parallel to each other but the growth of the epiglottis, derived from the ventral ends of the third and fourth branchial arches, causes them to become folded so that the opening is transformed into a T-shaped cleft. The swelling which gives rise to the epiglottis is termed the hypobranchial eminence. The remains of the third arch tissue form the pharyngo-epiglottic fold, and the fourth arch tissue is represented by the aryepiglottic fold. The true and false vocal cords differentiate from the upper and lower margins of the laryngeal ventricles which form as active epithelial outgrowths of the laryngeal entoderm. The cricoid and arytenoid cartilages are formed from the sixth arches whose ventral extremities form pronounced elevations behind the hypobranchial eminence and the opening of the glottis.
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Cite this page: Hill, M.A. (2021, June 23) Embryology Book - Aids to Embryology (1948) 9. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Aids_to_Embryology_(1948)_9
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