Embryology Body Folding and Neurulation

Body Folding

• Primitive streak forms along the caudal midline of the bilaminar embryonic disk, with its cranial end expanding into a primitive node.
• The position of the future oropharyngeal membrane is indicated at the cranial end of the embryonic disk.
• During gastrulation, epiblast cells migrate along the primitive streak, displacing the hypoblast to form the definitive endoderm.
• Subsequent migrating cells form the mesoderm.
• Mesoderm extends cranially from the primitive node to become the notochordal process.
• Lateral mesoderm develops into paraxial, intermediate, and lateral plate mesoderm.
• Lateral plate mesoderm splits into somatic and splanchnic mesoderm layers, separated by the intraembryonic coelom.
• Paraxial mesoderm forms somites in the future trunk, and head mesoderm in the head region.
• Oropharyngeal and cloacal membranes form, along with the neural plate, which develops into the spinal cord and brain.

Folding and Neurulation

• The process of folding begins at the start of the fourth week.
• The embryo forms a three-dimensional shape.
• The neural tube closes and the primitive streak disappears.

Primary Germ Layers

• Ectoderm
• Mesoderm
• Endoderm

Ectoderm Characteristics

• After body folding, surface ectoderm surrounds the embryo and forms the epidermis.

Mesoderm Differentiation in the Trunk

• Notochord is the midline mesoderm structure that gives rise to the vertebral bodies and nucleus pulposus of intervertebral discs.
• Notochord induces neurulation.
• Paraxial mesoderm segments into somites (from occipital to coccygeal regions) differentiating into dermatome, myotome, and sclerotome.
• Each myotome and dermatome retain their innervation from their segment of origin.
• Dermatome and myotome migrate together to form skin and muscle tissues.
• Intermediate mesoderm forms the urogenital structures (gonadal ridge and mesonephric system).

Lateral and Longitudinal Folding in Mesoderm

• Lateral folding brings the lateral aspects of the embryo ventrally and wraps the amniotic cavity around the embryo, creating the coelomic cavity.
• Longitudinal folding brings the developing heart ventrally and caudally, and creates the foregut and hindgut.

Cardiogenic Mesoderm Development

• Cardiac progenitor cells migrate through the primitive streak to the splanchnic mesoderm.
• Cardiogenic mesoderm migrates to become the endocardial tube (from the cardiac crescent).
• Myocardial cells originate from splanchnic mesoderm.

Somitomeres and Somites

• Somites are formed in the cephalic region, with initial somitomeres disappearing, leaving 42-44 pairs of somites(occipital, cervical, thoracic, lumbar, sacral, and coccygeal).

Body Folding Outcomes

• Medial mesoderm remains dorsal, while lateral mesoderm shifts ventrally.
• Amniotic cavity surrounds the embryo.

Coelomic Cavity and Endoderm Derivatives

• The coelomic cavity forms within the embryo.
• Endoderm lines the gut tube.
• Endoderm also forms respiratory epithelium, tonsils, thymus, thyroid, parathyroid, auditory canal, tympanic membrane, liver, gallbladder, pancreas, urinary bladder, and urethra.

Gut Tube Formation

• In the fourth week, the ventral aspect of the trilaminar disc internalizes to form the epithelial lining of the gut tube.
• Simultaneous cephalocaudal and lateral folding forms the gut tube.
• The tube starts as a closed structure with oropharyngeal and cloacal membranes.

Primary Germ Layer Contributions to Gut Tube

• Stomodeum (ectoderm-lined invagination) is cranially located to the oropharyngeal membrane.
• Proctodeum (ectoderm-lined invagination) is caudally located to the cloacal membrane.

Cephalocaudal Folding of Gut Tube

• Forms the head fold, placing the heart ventrally.
• Creates the foregut.
• Brings the septum transversum ventrally to create the hindgut.

Septum Transversum

• Septum transversum divides the developing heart from the foregut, contributing to the development of the diaphragm and other structures.

Lateral Folding of Gut Tube

• Lateral folding positions the developing organs and structures and establishes the primitive gut tube's final shape.

Mesentery Development

• Dorsal mesentery connects the gut tube to the posterior abdominal wall.
• Dorsal mesentery maintains the proper location of the gut tube.
• Ventral mesentery forms from the mesenchyme of the septum transversum.
• Intraperitoneal organs are suspended from dorsal mesentery,
• Retroperitoneal organs are located outside the peritoneal cavity
• Secondarily retroperitoneal organs originate within the peritoneal cavity and lose their mesenteric connection through fusion with the posterior body wall.

Gut Tube Derivatives

• The gut tube gives rise to the foregut, midgut, and hindgut.
• Foregut forms the esophagus, stomach, and proximal duodenum.
• Midgut forms the distal duodenum, jejunum, ileum, cecum, ascending colon, proximal transverse colon.
• Hindgut forms the distal transverse colon, descending colon, sigmoid colon, rectum, and upper anal canal.

Foregut Development

• Foregut runs from the oropharyngeal membrane to the liver outgrowth, including the major duodenal papilla.
• Pharyngeal gut forms the esophagus, stomach, and proximal duodenum, as well as tracheoesophageal septum, and lung buds.
• Foregut esophagus is dorsal to the respiratory primordium, which develops further into the respiratory diverticulum.
• Stomach begins in the thoracic cavity, rotating caudally.

Liver and Gallbladder Development

• Hepatic plate forms a diverticulum, which develops into the liver bud and gallbladder.
• Endoderm forms liver cords.

Pancreas Development

• Pancreas develops from two buds (ventral and dorsal).
• Ventral pancreas rotates to the fifth week.
• Ventral pancreas and hepatic duct merge into the hepatopancreatic ampulla.

Anal Canal Development

• The anal canal is subdivided by the urorectal septum into superior (hindgut) and inferior (proctodeum) portions with different embryological origins and nerve/vascular supply from the inferior mesenteric artery.

Summary of GI Development

• During week 4, the gut tube forms.
• The liver, gallbladder, pancreas form as buds from the duodenum.
• The stomach rotates and expands.
• Midgut herniates through the umbilical ring to rotate.
• The cloaca separates into structures like the urogenital sinus and the anorectal canal, and surrounding membranes rupture.

This quiz explores the processes of body folding and neurulation during embryonic development. It covers key structures like the primitive streak, mesoderm formation, and the development of membranes and the neural plate. Test your knowledge on these crucial stages of embryonic development!