FIG. 4–1 Dorsal aspect of the 10-somite embryo. 24 IV the fourth week of life somite and neural tube period I. EMBRYO PROPER caudal openings of the tube are called neuropores. The rostral neuropore closes between 18 and 20 somites. The caudal neuro- A. EXTERNAL APPEARANCE pore closes at 25 somites. Figs. 4–1, 4–2 1. The specimens measure approximately 1 to 3.5 mm in length Brain and have 1 to 29 pairs of somites. Three brain subdivisions are present in the cranial portion of the 2. The head and tail folds move the attachment of the amnion tube and are named, from cranial to caudal, the prosencephalon, to the ventral side of the head and tail regions, respectively. mesencephalon and rhombencephalon. The boundary between the The lateral body folds move the amnion attachment to the pros- and mesencephalon is demarcated by a ventral bend, called ventrolateral surface in the midportion of the embryo. the cephalic flexure. An external groove and a prominent swelling 3. The head region is elevated above the yolk sac by the large on the medial surface of the neural plate may also demarcate the pericardial sac, the midportion lies upon the yolk sac and the boundary. The boundary between the mes- and rhombencephalon caudal region is curved toward the yolk sac. is distinguished by a groove on the medial and lateral surfaces of 4. The embryo possesses somites, which are apparent through the neural plate or tube. the ectoderm. 5. The neural tube develops from the neural plate and remains Prosencephalon open at each end for 2 to 4 days. The medial surface possesses a broad, shallow depression, 6. There may be two dorsolateral enlargements in the head called the optic groove, where the optic cup will form by evagina- region, the trigeminal and otic prominences, separated by tion after closure of the neural tube. This is the first appearance the preotic sulcus. The trigeminal prominence is continuous of the eye. The optic chiasmal area is ventral where the two walls ventrally with the broad first branchial arch that extends to come together. the pericardial sac. The otic prominence is continuous ven- Mesencephalon trally with an area that will soon become the second branchial The mesencephalon is a short segment of the neural plate arch. or tube which bulges internally and externally. Its dorsal portion will form the tectum; its floor and medial wall will give rise to the B. ECTODERM tegmentum and basis pedunculi area. Figs. 4–1, 4–2A Rhombencephalon The rhombencephalon extends from the mesencephalon to the NERVOUS SYSTEM level of somite 4 where it is continuous with the spinal cord seg- The neural plate is transformed into the neural tube as a result ment. It can be subdivided into preotic and postotic portions by of the neural folds joining in the midline dorsally. The rostral and placode VIII (otic). 25 Neural Crests Placode VII Collections of cells called neural crests stream from the angle A band of thickened ectoderm represents placode VII and between the neural and superficial ectoderm. The following crests extends from the rostral side of placode VIII (otic) to the first can be recognized during the early part of the fourth week. branchial groove. It is also known as the epibranchial placode of the second branchial arch and is associated with the facial part of the facioacoustic crest. Trigeminal Crest The trigeminal crest is the most rostral of the neural crests and is Placode VIII (Otic) located at the level of the mesencephalon and the preotic part of the rhombencephalon. Initially, it is the largest of all the neural crests, Placode VIII (otic) is the most conspicuous area of ectodermal forming most of the trigeminal prominence. It spreads laterally and thickenings and represents the first appearance of the ear. It is is gradually lost in the mesenchyme of the first branchial arch. Its located on the summit of the otic prominence and is demarcated caudal part is closely associated with placode V. caudally by the shallow postotic sulcus. It is associated with the acoustic part of the facioacoustic crest. Facioacoustic Crest Placode IX–X The facioacoustic crest is a mass at the level of the otic part of A tongue shaped area of ectodermal thickening represents the rhombencephalon and constitutes most of the otic prominence. placode IX–X and extends caudally from placode VIII to the level It blends with mesenchyme of the otic region. Its peripheral portion of somite 1. Initially, it is associated with the combined glosso- is associated with placode VII and placode VIII (otic). pharyngeal–vagal crest but soon subdivides into two epibranchial placodes when the IX–X crest becomes distinct and separate. Glossopharyngeal–Vagal Crest The glossopharyngeal–vagal crest develops midway between NOTOCHORD the rostral neuropore and somite 1 at the level of the postotic part Fig. 4–2A of the rhombencephalon. Initially it is not well defined and will later become two separate crests. It is closely associated with the deep The notochord develops from the notochordal process and surface of placode IX–X. extends in the midline from the cranial end of the foregut to the tail. Initially it is a plate or ∩ shaped and at most levels becomes Occipitospinal Crests the dorsal part of the gut lining. It is in contact with the neural One occipitospinal crest will develop in each body segment tube dorsally and terminates in the head region near the pros- and between the neural tube and each somite. Initially, it is poorly mesencephalic junction. In the tail region the notochord separates defined and its cells become scattered among the mesenchymal from the gut endoderm, becomes cylindrical and unites with the cells. underside of the neural plate. C. ENDODERM ECTODERMAL PLACODES IN THE HEAD REGION Fig. 4–2A Localized thickenings of the ectoderm called placodes develop With the development of the head, tail and lateral body folds, on the lateral side of the head in the branchial arch region. They the endoderm that previously formed the roof of the yolk sac is are closely related to the cranial neural crests. molded into blind tubes in the head and tail regions known as the Placode V foregut and hindgut, respectively. The intermediate endoderm that remains as the roof of the yolk sac forms the midgut. These three Placode V overlies the caudal edge of the trigeminal crest. Its endodermal subdivisions represent the first appearance of the boundaries are poorly defined and it disappears early. Alimentary and Respiratory Systems. 26 FIG. 4–2 A. Midsagittal view of the 10-somite embryo showing the relations of the neural tube, notochord, gut and pericardial sac. B. Left lateral view of the 10-somite embryo showing the cardiovascular system. Arrows indicate the position of head and tail folds. 27 FOREGUT HEAD MESODERM 1. The foregut is compressed in the horizontal plane with its Mesenchymal tissue collects throughout the head region lateral portion extending dorsally. Its cranial end terminates beneath the ectoderm. It probably arises from the cranial neural blindly at the oropharyngeal membrane. Caudally it opens crests and the primitive streak. into the midgut at the anterior (cranial) intestinal portal. 2. Ventral outpouchings a. The buccal pouch is a slight bulge into the oropharyngeal PARAXIAL (SOMITIC) MESODERM membrane area. b. The thyroid diverticulum is the most prominent and deep- 1. The paraxial mesoderm is located on each side of the neural est outpouching and is located in the midline at the level tube where it collects into blocks of epitheloid cells called of the first aortic arches. somites. The somites are triangular shaped on transverse sec- c. A midline trough called the laryngotracheal groove tion with dorsal, ventral and medial walls. The epitheloid cells extends caudally from the thyroid diverticulum to the are arranged around a central cavity called the myocoele. anterior intestinal portal. At its caudal termination the 2. Somites are segmentally arranged with the first pair form- endodermal lining forms a midline bud that represents ing near the cranial end of the embryo in the future occipi- the unpaired lung primordium (Respiratory System). tal region. Segmentation occurs in a craniocaudal sequence 3. Lateral outpouching with their formation closely trailing the closure of the neural The first pharyngeal pouch is a clearly defined bulge extend- tube. ing laterally and dorsally around the side of the dorsal aorta. 3. The somite number is used to determine the relative age of The closing membrane of the first branchial groove is a small embryos in the somite period. Eventually there are 42 to 44 area caudal to pouch 1 where ectoderm and endoderm are in somite pairs formed (4 occipital, 8 cervical, 12 thoracic, 5 contact. lumbar, 5 sacral and 8 to 10 coccygeal). 4. By the time the more caudal somites appear, the cranial HINDGUT ones break down as a result of proliferation. The somites will give rise to major components of the Skeletal, Muscular and Caudally the short, blunt hindgut ends blindly at the cloacal Integumentary Systems. membrane. It opens at its cranial end into the midgut at the posterior (caudal) intestinal portal. The allantoic diverticulum (allantois) arises from the ventral side of the hindgut and passes into the connecting stalk. INTERMEDIATE MESODERM 1. The intermediate mesoderm temporarily connects the par- MIDGUT axial with the lateral plate mesoderm. The endoderm of the midgut constitutes the roof of the yolk 2. In the cervical segments the intermediate mesoderm is sac cavity between the cranial and caudal intestinal portals. The arranged into a group of cells that produce a nonfunctional, septum transversum lies ventral to the cranial intestinal portal vestigial kidney of lower forms called the pronephros.