BIOLOGY Lecture Notes - Golden Pages 2003

5. FUNDAMENTALS OF EMBRYOLOGY

Early Development of Mesodermal and Endodermal Derivatives

The endoderm forms the lining of the digestive and respiratory tracts, whereas the mesoderm gives rise to the Tissues and Organs located between the ectodermal body wall and the endodermal organs.

Mesoderm. Five distinct regions can be identified within the embryonic mesoderm at the neurula stage. The first of these gives rise to the notochord—an organ whose primary Functions are to induce the neural tube and establish the body axis. The second region is the dorsal mesoderm. Flanking the neural tube and notochord, this localized mesoderm will form numerous body tissues, including bone, Cartilage, dermis, and striated Muscle. The third region is the intermediate mesoderm, which forms the excretory organs and the ducts of the Gonads. The fourth region is the unsegmented lateral plate mesoderm, which develops into The Heart, Blood Vessels, Blood Cells, smooth muscle, body cavity lining, and all mesodermal Components of the limbs except for striated muscle. The lateral plates also participate in the Histology/21.html">Formation of extraembryonic membranes. The fifth mesodermal region is the HEAD mesenchyme.

Endoderm. The digestive organs and their associated glands, as well as the respiratory organs, develop from the endoderm.

As the ends of the endoderm fold ventrally and shift toward the center of the embryo, the foregut and hindgut are formed, representing Regions of the future digestive tract. At the site of the future Mouth, an ectodermal depression called the stomodeum develops and subsequently makes contact with the wall of the foregut. The region where they join is termed the buccopharyngeal membrane. This membrane later ruptures (at about 22 days of development in the human embryo), and the stomodeum transforms into the Oral Cavity.

The ectodermal epithelium of the oral cavity roof forms an outpocketing (Rathke's pouch) that grows toward an outgrowth from the floor of the Diencephalon rudiment, known as the infundibulum. These two ectodermal primordia interact with one another to jointly form the Pituitary Gland.

The glandular portion of the pituitary gland develops from Rathke's pouch, while its neural component arises from the infundibulum. Thus, the pituitary gland has a dual origin, a duality that is reflected in its function in the adult Organism.

The endodermal portion of the digestive tract begins at the Pharynx. Here, four pairs of pharyngeal pouches form in the mammalian embryo. The grooves between these pouches are sometimes referred to as gill slits because they resemble homologous structures in fish embryos.

The first pair of pharyngeal pouches differentiates into the tympanic cavities of the Middle ear and their associated Eustachian tubes. The walls of the Tonsils develop from the second pair of pouches. The Thymus arises from the third pair of pharyngeal pouches; at later Selection/3.html">Stages of development, this gland facilitates the differentiation of lymphocytes. One pair of Parathyroid glands also originates from this same pair of pharyngeal pouches, whereas the second pair arises from the fourth pair of pharyngeal pouches.

In addition to these paired pouches, a small centrally located diverticulum forms on the floor of the pharynx between the second pair of pharyngeal pouches. This endodermal-mesenchymal outgrowth buds off from the pharynx and migrates into the neck region, where it differentiates into The Thyroid Gland. At the same time, the primordium of the respiratory tract—the laryngotracheal groove—pinches off from the pharyngeal wall.