Embryonic Development



Embryonic Development


Julie A. Rosen

E. Albert Reece



Introduction


This chapter is a synopsis of the main events in normal human development, focusing on the embryonic period. The reader should consult the references1,2,3,4,5,6,7,8,9,10 for a more detailed discussion of individual topics.

The embryonic period extends from the beginning of the fourth week to the end of the eighth week. During this period, all the major internal and external structures begin their development. By the end of this period, the embryo has acquired characteristic human features.


Embryonic Disk Folding


In the fourth week, the embryonic disk undergoes folding. Folding converts the flat embryonic disk into a cylindrical embryo. Folding in the longitudinal axis results in the formation of the head and tail folds.






With the head fold, the developing heart and pericardial cavity are swung onto the ventral surface, and the septum transversum then lies caudal to the developing heart. The dorsal part of the yolk sac is incorporated into the embryo to form the foregut. This is separated by the oropharyngeal membrane from the stomodeum or primitive oral cavity.

With the tail fold, the body or connecting stalk, the future umbilical cord, is swung onto the ventral surface, and part of the allantois is incorporated into the embryo. The dorsal part of the yolk sac is incorporated into the embryo to form the hindgut. The terminal portion of the hindgut dilates to form the
cloaca, which is separated from the amniotic cavity by the cloacal membrane.

Folding in the transverse axis results in the somatopleure forming the lateral and ventral body walls. As the dorsal part of the yolk sac is incorporated into the embryo, the splanchnopleure forms the primitive gut. The midgut is connected to the yolk sac by the narrow vitelline duct. The remnant of the yolk sac ultimately degenerates. As the caudal limbs of the intraembryonic coelom are moved ventrally, they are initially separated by a ventral mesentery. This mesentery disappears, except in the region of the foregut, and a single peritoneal cavity is formed. From the pericardial cavity, the pericardioperitoneal canals or future pleural cavities pass dorsally and caudally to communicate with the peritoneal cavity.


Pharyngeal Apparatus


In the fourth week, ridges and grooves appear in the future neck region. These form part of the pharyngeal apparatus, which consists of the following: six pairs of pharyngeal arches numbered in a craniocaudal sequence (in humans, the fifth pharyngeal arch is absent); the pharyngeal grooves or clefts between the arches; the pharyngeal pouches (lined by endoderm and occurring internally between the arches); and the pharyngeal membranes, formed by the ectoderm and endoderm between the arches.







The Face



The primordia of the face are related to the stomodeum or primitive oral cavity as follows: the frontonasal prominence forms the cranial boundary, the maxillary prominences form the lateral boundaries, and the mandibular prominences form the caudal boundary.

These prominences, formed by accumulations of mesenchyme, are separated by grooves and furrows. During development, the prominences merge with one another as the grooves are smoothed out by proliferation of the underlying mesenchyme. Much of the mesenchyme in the facial region is considered to be of neural crest origin. Merging occurs mainly during the fifth to eighth weeks. Ectodermal thickenings on the inferolateral aspects of the frontonasal prominence form the nasal placodes. Mesenchyme around the placodes proliferates to form the medial and lateral nasal prominences, and the placodes then lie in depressions, the nasal pits or future nostrils. Expansion of the back of the head moves the eyes forward and contributes to the growth of the facial components toward the midline. The maxillary prominences merge with the medial nasal prominences, and the medial nasal prominences merge with each other to form the intermaxillary segment. The mandibular prominences merge with each other
in the midline. The adult derivatives are the frontonasal prominence—the forehead, dorsum, and apex of the nose; the lateral nasal prominences—the alae of the nose; the merged medial nasal prominences (intermaxillary segment)—the columella, philtrum of the upper lip, the maxilla that bears the incisors (the premaxilla), and the primary palate; the maxillary prominences—the lateral portions of the upper lip, the upper cheeks and face, the rest of the maxilla, and the secondary palate; and the mandibular prominences—the lower lip, lower cheeks and face, and the mandible. Myoblasts from the second branchial arch migrate into the facial region to form the muscles of facial expression. Along the nasolacrimal groove between the lateral nasal and maxillary prominences, a cord of cells sinks into the underlying mesenchyme; this canalizes to form the nasolacrimal duct.


The Palate


The palate develops from two primordia: the primary palate, a wedge-shaped mass of mesoderm from the innermost aspect of the intermaxillary segment that appears in the fifth week, and the secondary palate, which develops from the lateral palatine processes, shelf-like projections of mesoderm from the medial aspects of the maxillary prominences. These processes appear in the sixth week. As the developing tongue occupies most of the oral cavity, the lateral palatine processes assume a vertical position. As the stomodeum enlarges, the tongue drops down to the floor of the stomodeum, and the lateral palatine processes elevate to a horizontal position; this elevation occurs slightly later in females. Beginning anteriorly and proceeding posteriorly, the lateral palatine processes fuse with the posterior margin of the median palatine process, the inferior border of the nasal septum, and each other. Fusion involves epithelial contact, adhesion, and the replacement of the epithelial seam by mesoderm. Fusion begins in the ninth week and is completed by the 11th week in males and the 12th week in females. Intramembranous ossification spreads into the palate from the maxillary and palatine bones and extends to the posterior border of the nasal septum. Posterior to this, the unossified portion forms the soft palate and uvula. The palatal muscles are derived from the branchial arches.

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Jun 19, 2022 | Posted by in OBSTETRICS | Comments Off on Embryonic Development
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