Embryo and Fetal Pathology: Color Atlas with Ultrasound Correlation available in Hardcover
Embryo and Fetal Pathology: Color Atlas with Ultrasound Correlation
- ISBN-10:
- 0521825296
- ISBN-13:
- 9780521825290
- Pub. Date:
- 03/31/2004
- Publisher:
- Cambridge University Press
- ISBN-10:
- 0521825296
- ISBN-13:
- 9780521825290
- Pub. Date:
- 03/31/2004
- Publisher:
- Cambridge University Press
Embryo and Fetal Pathology: Color Atlas with Ultrasound Correlation
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Overview
Product Details
| ISBN-13: | 9780521825290 |
|---|---|
| Publisher: | Cambridge University Press |
| Publication date: | 03/31/2004 |
| Pages: | 710 |
| Sales rank: | 740,064 |
| Product dimensions: | 11.42(w) x 9.13(h) x 1.57(d) |
Read an Excerpt
Cambridge University Press
0521825296 - Embryo and Fetal Pathology - Color Atlas with Ultrasound Correlation - by Enid Gilbert-Barness and Diane Debich-Spicer
Excerpt
The Human Embryo and Embryonic
Growth Disorganization
STAGES OF EMBRYONIC DEVELOPMENT
Carnegie staging in the development of the human embryo categorizes 23 stages.
Fertilization and Implantation (Stages 1-3)
Embryonic development commences with fertilization between a sperm and a secondary oocyte (Tables 1.1 to 1.5). The fertilization process requires about 24 hours and results in the formation of a zygote - a diploid cell with 46 chromosomes containing genetic material from both parents. This takes place in the ampulla of the uterine tube.
The embryo's sex is determined at fertilization. An X chromosome-bearing sperm produces an XX zygote, which normally develops into a female, whereas fertilization by a Y chromosome-bearing sperm produces an XY zygote, which normally develops into a male.
The zygote passes down the uterine tube and undergoes rapid mitotic cell divisions, termed cleavage. These divisions result in smaller cells - the blastomeres. Three days later, after the developing embryo enters the uterine cavity, compaction occurs, resulting in a solid sphere of 12-16 cells to form the morula.
At 4 days, hollow spaces appear inside the compact morula and fluid soon passes into these cavities, allowing one large space to form and thus converting the morula into the blastocyst (blastocyst hatching). The blastocyst cavity
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separates the cells into an outer cell layer, the trophoblast, which gives rise to the placenta, and a group of centrally located cells, the inner cell mass, which gives rise to both embryo and extraembryonic tissue.
The zona pellucida hatches on day 5 and the blastocyst attaches to the endometrial epithelium. The trophoblastic cells then start to invade the endometrium.
Implantation of the blastocyst usually takes place on day 7 in the midportion of the body of the uterus, slightly more frequently on the posterior than on the anterior wall.
Gastrulation
Changes occur in the developing embryo as the bilaminar embryonic disc is converted into a trilaminar embryonic disc composed of three germ layers.
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The process of germ layer formation, called gastrulation, is the beginning of embryogenesis (formation of the embryo).
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Gastrulation begins at the end of the 1st week with the appearance of the hypoblast; it continues during the 2nd week with the formation of the epiblast and is completed during the 3rd week with the formation of intraembryonic mesoderm by the primitive streak. The three primary germ layers are called ectoderm, mesoderm, and endoderm. As the embryo develops, these layers give rise to the tissues and organs of the embryo.
The blastocyst begins to become attached to the uterine lining (the endometrium).
Implantation
Implantation includes dissolution of the zona pellucida and adhesion between the blastocyst and the endometrium, trophoblastic penetration, and migration
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of the blastocyst through the endometrium. Implantation occurs by the intrusion of trophoblastic extensions, which penetrate between apparently intact endometrial cells.
Second Week of Development (Stages 4 and 5)
During the 2nd week, a bilaminar embryonic disc forms, amniotic and primary yolk sac cavities develop, and there are two layers of trophoblast (Figure 1.1).
The two-layered disc separates the blastocyst cavity into two unequal parts (a smaller amniotic cavity and a larger primary yolk cavity). The thick layer of embryonic cells bordering the amniotic cavity is called the epiblast and a thin layer bordering the primary yolk cavity is called the hypoblast.
The trophoblast differentiates into two layers, an inner cytotrophoblast and an outer syncytiotrophoblast. The trophoblast continues to penetrate deeper into the endometrium. At the end of the 2nd week, the site of implantation is recognized as a small elevated area of endometrium having a central pore filled with a blood clot.
Third Week of Development (Stages 6-9)
Formation of the primitive streak and three germ layers (ectoderm, mesoderm, and endoderm) (Figure 1.2) occurs during the 3rd week.
The primitive streak results from a proliferation of ectodermal cells at the caudal end of the embryonal disc. Cells at the primitive streak proliferate to form the embryonic endoderm and mesoderm. The cephalic end of
1.1. Bilaminar embryonic disc in the 2nd week of development (stage 5), with amniotic and primary yolk sac cavities.
1.2. (A) Ectopic pregnancy at day 17 showing an embryonic disc with opacity (arrow) representing the primitive streak. The amniotic cavity (A) and the primary yolk sac cavity (Y) are present. (B) Ultrasound of a human embryo at the same stage of development as A (GS, gestational sac; Y, yolk sac). (C) Diagram of development of the primitive streak (a), notochord (b), and neural folds (c) in a trilaminar embryo (stages 6-9).
the primitive streak is the primitive node, and this cord of cells is the notochord.
Thickening of ectodermal cells gives rise to the neural plate, the first appearance of the nervous system, which becomes depressed below the surface along the long axis of the embryo to form the neural groove. The neural groove deepens and its margins elevate to form the neural folds. The fusion is completed during the 4th week of development. The neural tube ultimately will give rise to the central nervous system. The cephalic end will dilate to form the forebrain, midbrain, and hindbrain. The remainder of the neural tube will become the spinal cord.
The mesoderm on either side of the midline of the embryo (the paraxial mesoderm) undergoes segmentation, forming somites. The first pair of somites arises in the cervical region of the embryo at approximately day 20 of development. From there new somites appear in craniocaudal sequence, approximately three per day, until 42-44 pairs are present at the end of week 5. There are 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 8-10 coccygeal pairs. The first occipital and the last 5-7 coccygeal somites later disappear, while the remainder form the axial skeleton. During this period of development, the age of the embryo is expressed in the number of somites. Each somite differentiates into bones, cartilage, and ligaments of the vertebral column as well as into skeletal voluntary muscles, dermis, and subcutaneous tissue of the skin. The intermediate mesoderm and the lateral mesoderm give rise to portions of the urogenital system. The lateral plate mesoderm is involved in the development of pericardial, pleural, and peritoneal cavities as well as the muscle of the diaphragm.
Mesoderm also forms a primitive cardiovascular system during the 3rd week of development. Blood vessel formation begins in the extraembryonic mesoderm of the yolk sac, the connecting stalk, and the chorion. Embryonic vessels develop 2 days later. The linkage of the primitive heart tube with blood vessels takes place toward the end of week 3, after which blood circulation begins. The beating heart tube begins at 17-19 days.
The embryo changes shape from a disc to a tube with a cranial and a caudal end and the third germ layer, the endoderm, becomes incorporated into the interior of the embryo.
The formation of chorionic villi takes place in the 3rd week. The cytotrophoblast cells of the chorionic villi penetrate the layer of syncytiotrophoblast to form a cytotrophoblastic shell, which attaches the chorionic sac to the endometrial tissues.
1.3. Diagram of human embryo at stage 10. Neural folds are partially fused with the neural tube open at the rostral and caudal neuropore.
Fourth Week of Development (Stages 10-12: Up to Day 28, End of Blastogenesis)
At this stage, the embryo measures 2-5 mm (Figures 1.3 to 1.6). At stage 10, the embryo (at 22-24 days) is almost straight and has between 4 and 12 somites that produce conspicuous surface elevations. The neural tube is closed between the somites but is widely open at the rostral and caudal neuropore. The first and second pairs of branchial arches become visible.
During stage 11, a slight curve is produced by folding of the head and tail. The heart produces a large ventral prominence. The rostral neuropore continues to close and optic vesicles are formed.
1.4. (A) Diagram of a human embryo at stage 11. (B) A human embryo at stage 11 (arrow) showing a slight curve. The size should range from 2 to 5 mm. (C) Human embryo at stage 11 with a slight curve, two pairs of branchial arches, heart prominence (H), and optic vesicle (O). Rostral neuropore (arrow) continues to close.
1.5. (A) Drawing of a human embryo at stage 12. (B) Embryo at stage 10-12 (4th week of development) with early vascular development.
In stage 12, three pairs of branchial arches complete closure of the rostral hemisphere and recognizable upper-limb buds on the ventral lateral body wall appear. The otic pits and the primordia of the inner ears become visible. Growth of the forebrain produces an enlargement of the head, and further folding of the embryo in the longitudinal plane results in a C-shaped curvature. Narrowing of the connection between the embryo and the yolk sac produces a body stalk containing one umbilical vein and two umbilical arteries.
Fifth Week of Development (Stages 13-15)
At this stage, the embryo measures 5-10 mm in length. Rapid head growth occurs, caused mainly by rapid development of the brain. The upper limbs
1.6. (A) Drawing of a human embryo at stage 13. (B) Human embryo at stage 13. Note body curvature, four pairs of branchial arches, heart prominence (H), and upper and lower limb buds (arrows). The lens placode and otic pit are identifiable and the neural tube is closed. (C) Drawing of a human embryo at stage 15. (D) Human embryo at stage 15 with well-defined lens vesicle and an area representing hand plate formation (arrow). The cervical flexure is prominent. (E) Ultrasound at stages 13-15: (Right) CR length of embryo in the gestational sac. (Left) Doppler imaging showing blood flow (arrows) surrounding the gestational sac (GS) and in the embryo (transverse plane at the level of the heart). Yolk sac (Y) is also indicated.
1.7. (A) Drawing of a human embryo at stage 17, lateral view. (B) Human embryo with early formation of retinal pigment, finger rays and foot plate. (C) Monochorionic monoamniotic twin embryos with well-developed retinal pigment. (D) Embryo at 12 weeks fertilization age showing auricular hillocks.
begin to show differentiation as the hand plates develop toward the end of this week. The fourth pair of branchial arches and the lower-limb buds are present by 28-32 days of development. Lens placodes of the eyes are visible on the sides of the head. The attenuated tail with its somites is a characteristic feature at the beginning of week 5.
1.8. Human embryo at stage 18 and 19 showing elbow region (black arrow), toe rays, and herniation of intestinal loops into the umbilical cord (yellow arrow).
Sixth Week of Development (Stages 16 and 17)
The crown-rump (CR) length of the embryo in this time period is 10-14 mm. At stage 16, nasal pits face ventrally, retinal pigment becomes visible, auricular hillocks appear, and the foot plate is formed. In stage 17, the C-shape of the embryo is still present. Development of finger rays and basic facial-structure formation advances (Figure 1.7). The upper lip appears when medial nasal prominences and maxillary prominences merge. The nostrils become clearly defined and the eyes are directed more anteriorly.
1.9. (A) Human embryo at stage 20 showing webbed fingers and notches between the toe rays. The vascular plexus becomes visible (arrows). (B) Human embryo at stage 21 and 22 with free fingers. The hands and feet approach each other. Note the intestine in the umbilical cord (arrow). (C) Human embryo at stage 23 with more typical human characteristics such as a rounder head and completed development of the face, hands and feet. (D) Drawing of a human embryo at stage 23. (E) Posterior view of the embryo shown in (C) with an intact neural tube. (F) Ultrasound showing a posterior view of an embryo with the characteristic appearance of an intact neural tube (arrows) (Y, yolk sac). (G) Fetus at beginning of the fetal period (9 developmental weeks).
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