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Figure 9.5 Limb buds with their segments of origin indicated. With further development the segmental pattern disappears however, an orderly sequence in the dermatome pattern can still be recognized in the adult. A. Upper limb bud at 5 weeks. B. Upper limb bud at 6 weeks. C. Limb buds at 7 weeks. Figure 9.5 Limb buds with their segments of origin indicated. With further development the segmental pattern disappears however, an orderly sequence in the dermatome pattern can still be recognized in...
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Figure 5.3 Formation and migration of neural crest cells in the spinal cord. A and B. Crest cells form at the tips of neural folds and do not migrate away from this region until neural tube closure is complete. C. After migration, crest cells contribute to a heterogeneous array of structures, including dorsal root ganglia, sympathetic chain ganglia, adrenal medulla, and other tissues Table 5.1 . D. In a scanning electron micrograph of a mouse embryo, crest cells at the top of the closed neural...
Mitosis
Mitosis is the process whereby one cell divides, giving rise to two daughter cells that are genetically identical to the parent cell Fig. 1.2 . Each daughter cell receives the complete complement of 46 chromosomes. Before a cell enters mitosis, each chromosome replicates its deoxyribonucleic acid DNA . During this replication phase the chromosomes are extremely long, they are spread diffusely through the nucleus, and they cannot be recognized with the light microscope. With the onset of mitosis...
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Figure 13.25 A. Primary intestinal loop before rotation lateral view . The superior mesenteric artery forms the axis of the loop. Arrow, counterclockwise rotation. B. Similar view as in A, showing the primary intestinal loop after 180 counterclockwise rotation. The transverse colon passes in front of the duodenum. Development of the midgut is characterized by rapid elongation of the gut and its mesentery, resulting in formation of the primary intestinal loop Figs. 13.24 and 13.25 . At its apex,...
Ovulation
In the days immediately preceding ovulation, under the influence of FSH and LH, the secondary follicle grows rapidly to a diameter of 25 mm. Coincident with final development of the secondary follicle, there is an abrupt increase in LH that causes the primary oocyte to complete meiosis I and the follicle to enter the preovulatory stage. Meiosis II is also initiated, but the oocyte is arrested in metaphase approximately 3 hours before ovulation. In the meantime, the surface of the ovary begins...
Cleavage
Once the zygote has reached the two-cell stage, it undergoes a series of mitotic divisions, increasing the numbers of cells. These cells, which become smaller with each cleavage division, are known as blastomeres Fig. 2.8 . Until the eight-cell stage, they form a loosely arranged clump Fig. 2.9A . However, after the third cleavage, blastomeres maximize their contact with each other, forming a compact ball of cells held together by tight junctions Fig. 2.9B . This process, compaction, segregates...
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Precartilagi-PH nous vertebral Figure 8.21 Formation of the vertebral column at various stages of development. A. At the fourth week of development, sclerotome segments are separated by less dense intersegmental tissue. Note the position of the myotomes, intersegmental arteries, and segmental nerves. B. Condensation and proliferation of the caudal half of one sclero-tome proceed into the intersegmental mesenchyme and cranial half of the subjacent sclerotome arrows in A and B . Note the...
Paraxial Mesoderm
By the beginning of the third week, paraxial mesoderm is organized into segments. These segments, known as somitomeres, first appear in the cephalic region of the embryo, and their formation proceeds cephalocaudally. Each somit-omere consists of mesodermal cells arranged in concentric whorls around the center of the unit. In the head region, somitomeres form in association with
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Figure 14.10 A to C. Ascent of the kidneys. Note the change in position between the mesonephric and metanephric systems. The mesonephric system degenerates almost entirely, and only a few remnants persist in close contact with the gonad. In both male and female embryos, the gonads descend from their original level to a much lower position. D. Scanning electron micrograph of a mouse embryo showing the kidneys in the pelvis. B, bladder K, kidney A, adrenal gland G, gonad T, tail. Fig. 14.10 . In...
Urethra
The epithelium of the urethra in both sexes originates in the endoderm the surrounding connective and smooth muscle tissue is derived from splanchnic mesoderm. At the end of the third month, epithelium of the prostatic urethra begins to proliferate and forms a number of outgrowths that penetrate the surrounding mesenchyme. In the male, these buds form the prostate gland Fig. 14.13B . In the female, the cranial part of the urethra gives rise to the urethral and paraurethral glands. When the...
Maturation of Oocytes Continues at Puberty
Near the time of birth, all primary oocytes have started prophase of meiosis I, but instead of proceeding into metaphase, they enter the diplotene stage, a resting stage during prophase that is characterized by a lacy network of chro-matin Fig. 1.17C . Primary oocytes remain in prophase and do not finish their first meiotic division before puberty is reached, apparently because of oocyte maturation inhibitor OMI , a substance secreted by follicular cells. The total number of primary oocytes at...
Myelencephalon
The myelencephalon is a brain vesicle that gives rise to the medulla oblongata. It differs from the spinal cord in that its lateral walls are everted Fig. 19.18, B and C . Alar and basal plates separated by the sulcus limitans can be clearly distinguished. The basal plate, similar to that of the spinal cord, contains motor nuclei. These nuclei are divided into three groups a a medial somatic efferent T T pn jhiel0ii -Cul iOge l roui Si 'entfiel ihvfiOTh lsn ic- rpgHyi Figure 19.17 Lateral view...
Clinical Correlates 1
At birth, the umbilical cord is approximately 2 cm in diameter and 50 to 60 cm long. It is tortuous, causing false knots. An extremely long cord may encircle Figure 6.16 Infant showing limb amputation resulting from amniotic bands. Figure 6.16 Infant showing limb amputation resulting from amniotic bands. the neck of the fetus, usually without increased risk, whereas a short one may cause difficulties during delivery by pulling the placenta from its attachment in the uterus. Normally there are...
Corpus Albicans
If fertilization does not occur, the corpus luteum reaches maximum development approximately 9 days after ovulation. It can easily be recognized as a yellowish projection on the surface of the ovary. Subsequently, the corpus luteum shrinks because of degeneration of lutean cells and forms a mass of fibrotic Innflr nr,rr wn.il ii 'jriLi iKit diu amp jlvb SifL'OfiU .1 UWtfl In ZtkI ir totn diulwan
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Figure 2.5 A. Scanning electron micrograph of sperm binding to the zona pellucida. B. The three phases of oocyte penetration. In phase 1, spermatozoa pass through the corona radiata barrier in phase 2, one or more spermatozoa penetrate the zona pellucida in phase 3, one spermatozoon penetrates the oocyte membrane while losing its own plasma membrane. Inset. Normal spermatocyte with acrosomal head cap. pni a, Pemnln proflufileui Pdar botes Crcitrrajnt pni a, Pemnln proflufileui Pdar botes...
Ultrasonography
Ultrasonography is a relatively noninvasive technique that uses high-frequency sound waves reflected from tissues to create images. The approach may be transabdominal or transvaginal, with the latter producing images with higher resolution Fig. 7.3 . In fact, the technique, which was first developed in the 1950s, has advanced to a degree where detection of blood flow in major vessels, Figure 7.3 Examples of the effectiveness of ultrasound in imaging the embryo and fetus. A. 6-week embryo. B....
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Figure 15.8 A. Lateral view of the head and neck region of a 4-week embryo demonstrating the cartilages of the pharyngeal arches participating in formation of the bones of the face and neck. B. Various components of the pharyngeal arches later in development. Some of the components ossify others disappear or become ligamentous. The maxillary process and Meckel's cartilage are replaced by the maxilla and mandible, respectively, which develop by membranous ossification. M. . hi-MMMil'i tirHa amp...
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Auditory ossicles embedded in loose mesenchyme Auditory ossicles embedded in loose mesenchyme Figure 16.7 A. Transverse section of a 7-week embryo in the region of the rhombencephalon, showing the tubotympanic recess, the first pharyngeal cleft, and mesenchymal condensation, foreshadowing development of the ossicles. B. Middle ear showing the cartilaginous precursors of the auditory ossicles. Thin yellow line in mesenchyme indicates future expansion of the primitive tympanic cavity. Note the...
Molecular Regulation of Spinal Cord Development
At the neural plate stage in the spinal cord region, the entire plate expresses the transcription factors PAX3, PAX7, MSX1, and MSX2, all of which contain a homeodomain. This expression pattern is altered by sonic hedgehog SHH expressed in the notochord and bone morphogenetic proteins 4 and 7 BMP4 and BMP7 expressed in the nonneural ectoderm at the border of the neural plate Fig. 19.14 A . The SHH signal represses expression of PAX3 and PAX7 and MSX1 and MSX2. Thus, SHH ventralizes the neural...
Uterus at Time of Implantation
The wall of the uterus consists of three layers a endometrium or mucosa lining the inside wall b myometrium, a thick layer of smooth muscle and c perimetrium, the peritoneal covering lining the outside wall Fig. 2.11 . From puberty 11-13 years until menopause 45-50 years , the endometrium undergoes changes in a cycle of approximately 28 days under hormonal control by the ovary. During this menstrual cycle, the uterine en-dometrium passes through three stages, the follicular or proliferative...
Mesencephalon Midbrain
In the mesencephalon Fig. 19.23 , each basal plate contains two groups of motor nuclei a a medial somatic efferent group, represented by the Figure 19.21 Sagittal sections through the roof of the metencephalon showing development of the cerebellum. A. 8 weeks approximately 30 mm . B. 12 weeks 70 mm . C. 13 weeks. D. 15 weeks. Note formation of the external granular layer on the surface of the cerebellar plate B and C . During later stages, cells of the external granular layer migrate inward to...
Esophagus
When the embryo is approximately 4 weeks old, the respiratory diverticulum lung bud appears at the ventral wall of the foregut at the border with the Figure 13.5 Embryos during the fourth A and fifth B weeks of development showing formation of the gastrointestinal tract and the various derivatives originating from the endodermal germ layer. Figure 13.6 Successive stages in development of the respiratory diverticulum and esophagus through partitioning of the foregut. A. At the end of the third...
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mesenchyme to an epithelium for nephron formation is also mediated by the ureteric buds, in part through modification of the extracellular matrix. Thus fi-bronectin, collagen I, and collagen III are replaced with laminin and type IV collagen, characteristic of an epithelial basal lamina Fig. 14.7B . In addition, the cell adhesion molecules syndecan and E-cadherin, which are essential for condensation of the mesenchyme into an epithelium, are synthesized. Regulatory genes for conversion of the...
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Figure 11.43 Development of vitelline and umbilical veins in the A second and B third months. Note formation of the ductus venosus, portal vein, and hepatic portion of the inferior vena cava. The splenic and superior mesenteric veins enter the portal vein. vein right hepatocardiac channel . Ultimately the right hepatocardiac channel forms the hepatocardiac portion of the inferior vena cava. The proximal part of the left vitelline vein disappears Fig. 11.43, A and B . The anastomotic network...
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Figure 10.5 A. Transformation of the pericardioperitoneal canals into the pleural cavities and formation of the pleuropericardial membranes. Note the pleuropericardial folds containing the common cardinal vein and phrenic nerve. Mesenchyme of the body wall splits into the pleuropericardial membranes and definitive body wall. B. The thorax after fusion of the pleuropericardial folds with each other and with the root of the lungs. Note the position of the phrenic nerve, now in the fibrous...
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Thyroglossal Duct and Thyroid Abnormalities A thyroglossal cyst may lie at any point along the migratory pathway of the thyroid gland but is always near or in the midline of the neck. As indicated by its name, it is a cystic remnant of the thyroglossal duct, Although approximately 50 of these cysts are close to or just inferior to the body of the hyoid bone Figs. 15.19 and 15.20 , they may also be found at the base of the tongue or close to the thyroid cartilage. Sometimes a thyroglossal cyst...
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Internal carotid artery Right vagus nerve Figure 11.35 A. Aortic arches and dorsal aortae before transformation into the definitive vascular pattern. B. Aortic arches and dorsal aortae after the transformation. Broken lines, obliterated components. Note the patent ductus arteriosus and position of the seventh intersegmental artery on the left. C. The great arteries in the adult. Compare the distance between the place of origin of the left common carotid artery and the left subclavian in B and...
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Figure 2.1 From the pool of primordial follicles, every day some begin to grow and develop into secondary preantral follicles, and this growth is independent of FSH. Then, as the cycle progresses, FSH secretion recruits primary follicles to begin development into secondary antral, Graafian follicles. During the last few days of maturation of secondary follicles, estrogens, produced by follicular and thecal cells, stimulate increased production of LH by the pituitary Fig. 2.13 , and this hormone...
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Figure 14.2 A. Relationship of the intermediate mesoderm ofthe pronephric, mesone-phric, and metanephric systems. In cervical and upper thoracic regions intermediate mesoderm is segmented in lower thoracic, lumbar, and sacral regions it forms a solid, unsegmented mass of tissue, the nephrogenic cord. Note the longitudinal collecting duct, formed initially by the pronephros but later by the mesonephros. B. Excretory tubules ofthe pronephric and mesonephric systems in a 5-week-old embryo. The...
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Figure 14.35 A. Hypospadias showing the various locations of abnormal urethral orifices. B. Patient with hypospadias. The urethra is open on the ventral surface of the penis. C. Epispadias combined with exstrophy of the bladder. Bladder mucosa is exposed. Figure 14.35 A. Hypospadias showing the various locations of abnormal urethral orifices. B. Patient with hypospadias. The urethra is open on the ventral surface of the penis. C. Epispadias combined with exstrophy of the bladder. Bladder mucosa...
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Figure 16.1 A. Scanning electron micrograph of a mouse embryo equivalent to approximately 28 days of human development. The otic placodes, as shown in B, are invaginating to form the otic pits arrows . Arrowhead, second arch H, heart star, mandibular prominence. B. Region of the rhombencephalon showing the otic placodes in a 22-day embryo. Figure 16.1 A. Scanning electron micrograph of a mouse embryo equivalent to approximately 28 days of human development. The otic placodes, as shown in B, are...
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Figure 16.10 A. Lateral view of the head of an embryo showing the six auricular hillocks surrounding the dorsal end of the first pharyngeal cleft. B to D. Fusion and progressive development of the hillocks into the adult auricle. E. The six auricular hillocks from the first and second pharyngeal arches. H, heart NP, nasal placode. F. The hillocks becoming more defined. Note the position of the ears with respect to the mouth and eyes e . G. External ear nearly complete. Growth of the mandible...
Clinical Correlate
Primordial Germ Cells PGCs and Teratomas Teratomas are tumors of disputed origin that often contain a variety of tissues, such as bone, hair, muscle, gut epithelia, and others. It is thought that these tumors arise from a pluripotent stem cell that can differentiate into any of the three germ layers or their derivatives. Figure 1.1 An embryo at the end of the third week, showing the position of primordial germ cells in the wall of the yolk sac, close to the attachment of the future umbilical...
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Figure 9.3 A. Transverse section through the thoracic region of a 5-week embryo. The dorsal portion of the body wall musculature epimere and the ventral portion hy-pomere are innervated by a dorsal primary ramus and a ventral primary ramus, respectively. B. Similar to A later in development. The hypomere has formed three muscle layers and a ventral longitudinal muscle column. is represented by the rectus abdominis muscle in the abdominal region and by the infrahyoid musculature in the cervical...
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Figure 11.12 Ventral view of coronal sections through the heart at the level of the atrioventricular canal to show development of the venous valves. A. 5 weeks. B. Scanning electron micrograph of a similar-staged mouse heart showing initial formation of the septum primum septum spurium is not visible. Note the atrioventricular canal arrow . C. Fetal stage. The sinus venarum blue is smooth walled it derives from the right sinus horn. Arrows, blood flow. D. High magnification of the interatrial...
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Figure 12.3 Various types of esophageal atresia and or tracheoesophageal fistulae. A. The most frequent abnormality 90 of cases occurs with the upper esophagus ending in a blind pouch and the lower segment forming a fistula with the trachea. B. Isolated esophageal atresia 4 of cases . C. H-type tracheoesophageal fistula 4 of cases . D and E. Other variations each 1 of cases . pouch and the lower segment forming a fistula with the trachea Fig. 12.3 A . Isolated esophageal atresia Fig. 12.3B and...
Mesenteries Of The Intestinal Loops
The mesentery of the primary intestinal loop, the mesentery proper, undergoes profound changes with rotation and coiling of the bowel. When the caudal limb of the loop moves to the right side of the abdominal cavity, the dorsal mesentery twists around the origin of the superior mesenteric artery Fig. 13.24 . Later, when the ascending and descending portions of the colon obtain their definitive positions, their mesenteries press against the peritoneum of the posterior abdominal wall Fig. 13.30 ....
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Figure 11.47 Fetal circulation before birth. Arrows, direction of blood flow. Note where oxygenated blood mixes with deoxygenated blood in the liver I , in the inferior vena cava II , in the right atrium III , in the left atrium IV , and at the entrance of the ductus arteriosus into the descending aorta V . Figure 11.47 Fetal circulation before birth. Arrows, direction of blood flow. Note where oxygenated blood mixes with deoxygenated blood in the liver I , in the inferior vena cava II , in the...
Face
At the end of the fourth week, facial prominences consisting primarily of neural crest-derived mesenchyme and formed mainly by the first pair of pha-ryngeal arches appear. Maxillary prominences can be distinguished lateral to the stomodeum, and mandibular prominences can be distinguished caudal to this structure Fig. 15.21 . The frontonasal prominence, formed by proliferation of mesenchyme ventral to the brain vesicles, constitutes the upper border of the stomodeum. On both sides of the...
Diencephalon
Roof Plate and Epiphysis. The diencephalon, which develops from the median portion of the prosencephalon Figs. 19.5 and 19.17 , is thought to consist of a roof plate and two alar plates but to lack floor and basal plates interestingly, sonic hedgehog, a ventral midline marker, is expressed in the floor of the diencephalon, suggesting that a floor plate does exist . The roof plate of the diencephalon consists of a single layer of ependymal cells covered by Pineal Lateral ventricle Hippocampus...
Teeth
The shape of the face is determined not only by expansion of the paranasal sinuses but also by growth of the mandible and maxilla to accommodate the teeth. Teeth themselves arise from an epithelial-mesenchymal interaction between overlying oral epithelium and underlying mesenchyme derived from neural crest cells. By the sixth week of development, the basal layer of the epithelial lining of the oral cavity forms a C-shaped structure, the dental lamina, along the length of the upper and lower...
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Figure 11.16 Formation of the septum in the atrioventricular canal. A. From left to right, days 23, 26, 31, and 35. The initial circular opening widens transversely. B and C. Scanning electron micrographs of hearts from mouse embryos, showing growth and fusion of the superior and inferior endocardial cushions in the atrioventricular canal. In C, cushions of the ouflow tract arrow are also fusing. Figure 11.16 Formation of the septum in the atrioventricular canal. A. From left to right, days 23,...
Venous System
In the fifth week, three pairs of major veins can be distinguished a the vitelline veins, or omphalomesenteric veins, carrying blood from the yolk sac to the sinus venosus b the umbilical veins, originating in the chorionic villi and carrying oxygenated blood to the embryo and c the cardinal veins, draining the body of the embryo proper Fig. 11.41 . Figure 11.40 A. Obliteration of the fourth aortic arch on the right and left and persistence of the distal portion of the right dorsal aorta. B....
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Figure 19.29 A. Sagittal section through the nasal pit and lower rim of the medial nasal prominence of a 6-week embryo. The primitive nasal cavity is separated from the oral cavity by the oronasal membrane. B. Similar section as in A toward the end of the sixth week showing breakdown ofthe oronasal membrane. C. At 7 weeks, neurons inthe nasal epithelium have extended processes that contact the floor of the telencephalon in the region of the developing olfactory bulbs. D. By 9 weeks, definitive...
Maturation of Sperm Begins at Puberty
Spermatogenesis, which begins at puberty, includes all of the events by which spermatogonia are transformed into spermatozoa. At birth, germ cells in the male can be recognized in the sex cords of the testis as large, pale cells surrounded by supporting cells Fig. 1.21 A . Supporting cells, which are derived from the surface epithelium of the gland in the same manner as follicular cells, become sustentacular cells, or Sertoli cells Fig. 1.21 C . Shortly before puberty, the sex cords acquire a...
Neural Crest Cells
During elevation of the neural plate, a group of cells appears along each edge the crest of the neural folds Fig. 19.2 . These neural crest cells are ectodermal Figure 19.11 Origin of the nerve cell and the various types of glial cells. Neuroblasts, fibrillar and protoplasmic astrocytes, and ependymal cells originate from neuroepithelial cells. Microglia develop from mesenchyme cells. The origin of the oligodendroglia is not clear. Figure 19.11 Origin of the nerve cell and the various types of...
Macerating Action Amniotic Fluid
Figure 18.1 Formation of the skin at various stages of development. A. 5 weeks. B. 7 weeks. C. 4 months. D. Birth. During the first 3 months of development, the epidermis is invaded by cells arising from the neural crest. These cells synthesize melanin pigment, which can be transferred to other cells of the epidermis by way of dendritic processes. After birth, these melanocytes cause pigmentation of the skin Fig. 18.1 D . The epidermal ridges that produce typical patterns on the surface of the...
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Figure 6.8 Structure of villi at various stages of development. A. During the fourth week. The extraembryonic mesoderm penetrates the stem villi in the direction of the decidual plate. B. During the fourth month. In many small villi the wall of the capillaries is in direct contact with the syncytium. C and D. Enlargement of the villus as shown in A and B, respectively. on the opposite side of the uterus and the two fuse Figs. 6.10 to 6.12 , obliterating the uterine lumen. Hence the only portion...