📚 Question Bank

The gastrosplenic ligament is a derivative of the dorsal mesentery.

Mesenteries are double layers of the peritoneum that enclose an organ and connect it to the body wall. They provide pathways for vessels, nerves, and lymphatics to and from the abdominal viscera.

The foregut gives rise to both the ventral and dorsal mesentery, whereas the midgut and hindgut give rise to only the dorsal mesentery. The ventral and dorsal mesenteries give rise to the following structures, as shown in the image below:

The ligamentum teres is not a derivative of the septum transversum, but a remnant of the left umbilical vein.

The septum transversum gives rise to:

• Central tendon of the diaphragm
• Connective tissue of the liver
• Ventral mesentery of the foregut that has the following parts
• Lesser omentum (ventral mesogastrium)
• Ligaments of the liver (falciform ligament, triangular ligaments, and coronary ligaments)

The kidney is a retroperitoneal organ and does not develop in the mesogastrium. The mesenteries of the stomach include the ventral and dorsal mesogastrium.

• Ventral mesogastrium (Lesser omentum), where the ventral pancreatic bud, liver, and gallbladder develop
• Dorsal mesogastrium (Greater omentum), where the spleen, celiac trunk, and dorsal pancreatic bud develop

The fibrous stroma of the liver is derived from the septum transversum.

The liver develops as an outgrowth of the endodermal epithelium at the distal end of the foregut. This liver bud or hepatic diverticulum proliferates rapidly and penetrates the septum transversum.

• The rapidly developing endodermal cells of the liver bud give rise to the parenchyma
• (hepatocytes) and the lining of the biliary ducts. The endoderm-derived epithelial liver cells also intermingle with the vitelline and umbilical veins to form hepatic sinusoids.
• The mesoderm of the septum transversum gives rise to the connective tissue, ligaments of the liver (except ligamentum teres), hematopoietic cells, and Kupffer cells.
• The connection between the hepatic diverticulum and the foregut (duodenum) narrows, forming the bile duct.
• The bile duct gives rise to a small ventral outgrowth called pars cystica that develops into the gallbladder and the cystic duct.

The uncinate process develops from the ventral pancreatic bud.

The pancreas develops from two buds, dorsal and ventral, originating from the endodermal lining of the duodenum. When the duodenum rotates and becomes C-shaped, the ventral bud gets positioned immediately below and behind the dorsal bud. This is followed by the fusion of

the parenchyma and the duct systems of the dorsal and ventral pancreatic buds. The parenchyma of the pancreas:

• Ventral pancreatic bud forms the lower part of the head and uncinate process.
• Dorsal pancreatic bud forms the upper part of the head, neck, body, and tail.
• Pancreatic ducts
• The main pancreatic duct (of Wirsung) is formed by the duct of ventral bud (proximal part), oblique cross-communication between the 2 ducts (middle part), and the duct of dorsal bud (distal part).
• The accessory pancreatic duct (of Santorini) is formed by the proximal part of the dorsal pancreatic duct.

The image given below shows the development of pancreas.

The spleen develops from the mesoderm in the dorsal mesogastrium.

The mesenchymal cells in the dorsal mesogastrium form small lobular masses of splenic tissue called spleniculi. Later, these lobules fuse to form a single mass of the spleen. The capsule, septa, and connective tissue framework, including reticular fibers, develop from mesoderm. The mesenchymal cells also differentiate into lymphoblasts and other blood-forming cells.

When the spleen develops, the dorsal mesogastrium gets divided into an anterior segment called gastrosplenic ligament that extends from the spleen to stomach, and a posterior part called the lienorenal ligament that extends from the spleen to the left kidney.

The rectum develops from both the hindgut and cloaca.

The section of the hindgut present caudal to the attachment of the allantoic diverticulum is called the cloaca.

• The proximal part of the rectum develops from the hindgut
• The distal part of the rectum develops from the dorsal part of the cloaca

Zygomatic prominence is not an embryological structure that contributes to the formation of the face.

The face is derived from the following structures around the stomatodeum (primitive mouth):

• Mandibular prominences and maxillary prominences of the first arch
• Frontonasal prominence formed by the proliferation of mesenchyme ventral to the brain vesicles. It also gives rise to medial and lateral nasal prominences.

Structures contributing to th e development of face

Prominence Frontonasal

Medial nasal

Lateral nasal Maxillary

Mandibular

Structure formed

Forehead, bridge of the nose, also forms the medial and la teral nasal prominences

Philtrum of the upper lip, the crest of the nose, and the tip of the nose

Alae of nose

Upper part of cheeks, the late ral portion of the upper lip

Lower part of cheeks, chin, lo wer lip

The upper lip is formed by the fusion of the two maxillary prominences with the two medial nasal prominences.

Initially, two thickenings called nasal placodes are formed in the frontonasal prominence. The nasal placodes get depressed to become nasal pits. Each nasal pit has a ridge of tissue on the outer edge called lateral nasal prominence and on the inner edge called medial nasal prominence.

The 2 maxillary prominences on the lateral part of the stomodeum increase in size and grow medially. They compress the medial nasal prominences toward the midline. Later, the 2 maxillary prominences fuse with the 2 medial nasal prominences to form the upper lip.

The alae of the nose develop from the lateral nasal processes.

The development of various components of the nose can be summarized as follows:

• Bridge - The frontonasal process
• Dorsum and tip - Fused medial nasal processes
• Alae - The lateral nasal processes
• Anterior nares (nostrils) - The original site of the nasal pit
• Nasal cavity - The nasal sacs formed by the elongation of nasal pits
• Posterior nares (choanae) - Ruptured bucconasal membrane

Bilateral complete cleft palate is due to the non-fusion of palatine processes with the premaxilla and with each other.

The palate is formed from two main structures:

• Primary palate: The small triangular anterior part of the palate is formed by the fused medial nasal processes (intermaxillary segment or premaxilla).
• Secondary palate: The main part of the palate is formed by the 2 palatine processes that converge from the maxillary processes.

The cleft palate can occur in the primary palate (anterior cleft) or in the secondary palate

(posterior palate).

• Complete cleft palate involves both the primary and secondary palate.
• Incomplete cleft palate involves only the secondary palate.

In bilateral complete cleft palate, a Y-shaped cleft is seen with gaps between the primary and secondary palate and between the two halves of the secondary palate. In addition, there is a bilateral cleft upper lip.

The nasolacrimal duct is derived from the ectoderm of the nasolacrimal groove (also known as naso-optic furrow).

The deep furrow between the maxillary and lateral nasal prominence is known as the nasolacrimal groove.

• The ectoderm in the floor of this groove becomes a solid epithelial cord and detaches from the overlying ectoderm.
• This solid cord then canalizes to form the nasolacrimal duct.
• The lower end of the duct acquires a connection to the nasal cavity.
• The upper end of the duct widens to form the lacrimal sac.

The tympanic membrane is derived from all three germ layers.

The tympanic membrane is formed by the apposition of the tubotympanic recess of the 1st pharyngeal pouch with the 1st pharyngeal cleft.

• The outer layer of the tympanic membrane is derived from the ectoderm at the bottom of the external auditory meatus (1st pharyngeal cleft).
• The inner layer of the tympanic membrane is derived from the endoderm lining the tympanic cavity (1st pharyngeal pouch).
• The middle layer of the tympanic membrane is connective tissue derived from the mesoderm.

The tubotympanic recess forms the auditory tube. Development of the Middle Ear:

• The distal part of the 1st pharyngeal pouch becomes the tubotympanic recess. The tubotympanic recess gives rise to the primitive tympanic cavity distally, while the proximal part remains narrow and forms the auditory tube (eustachian tube). It also forms the lining of the middle ear cavity.
• The 3 ossicles are formed from mesenchymal condensations from the 1st pharyngeal arch

(malleus and incus) and 2nd pharyngeal arch (stapes).

• The tympanic membrane is formed where the ectoderm of the 1st pharyngeal cleft dips to meet the endoderm of the 1st pharyngeal pouch, with contribution from the intervening mesoderm.

The membranous labyrinth of the internal ear is derived from the otic vesicle.

The surface ectoderm on either side of the hindbrain thickens to form otic placodes. The otic placodes invaginate to form otic vesicles (auditory vesicles).

The otic vesicle further divides into 2 components that develop into structures of the membranous labyrinth:

• The ventral component forms the saccule and cochlear duct
• The dorsal component forms the utricle, semicircular canals, and endolymphatic duct

The spinal cord develops from the caudal part of the neural tube.

The nervous system develops from the neurectoderm i.e. the specialized ectoderm overlying the notochord. The neuroectoderm initially thickens to form the neural plate. The neural plate is then converted to a neural groove, followed by a neural tube.

• The enlarged cranial part of neural tube forms the brain. It has 3 primary vesicles that form:
• Forebrain (prosencephalon)
• Midbrain (mesencephalon)
• Hindbrain (rhombencephalon)
• The narrow caudal part of the neural tube forms the spinal cord.

The neuropores facilitate communication between the neural tube and the amniotic cavity.

The neural plate first becomes tubular in the middle. The cranial and caudal ends of the neural tube remain open and are called the anterior and posterior neuropores, respectively.

The neuropores facilitate communication between the neural tube and the amniotic

cavity. Amniotic fluid provides nutrition to the developing neuroectodermal cells before the circulatory system is established.

The location of the anterior neuropore is represented in adults as lamina terminalis.

The anterior neuropore closes around the 25th day of intrauterine life. Its location in adults is represented by the lamina terminalis. The lamina terminalis forms the anterior boundary of the third ventricle.

The posterior neuropore closes around the 28th day of intrauterine life. Its location in adults is represented by the terminal ventricle. The terminal ventricle is present in the proximal 5 to 6 mm of filum terminale.

The sulcus limitans is located between the sensory alar and the motor basal plates.

The cells in the wall of the neural tube proliferate and are subdivided into the inner mantle layer and outer marginal layer. Due to the fast growth of the mantle layer cells, the lumen of the neural tube gets compressed.

The line separating the compressed ventral part from the dorsal part is called the sulcus limitans. When the sulcus limitans is formed, the tube is divided into the dorsal or alar lamina and ventral or basal lamina.

In the spinal cord, the ventral thickenings or basal plates contain ventral motor horn cells. The dorsal thickenings i.e. the alar plates contain the interneurones that receive the terminals of primary sensory neurons.

The images given below show the sulcus limitans:

The hypoglossal nucleus is not derived from the alar plate. The hypoglossal nucleus is a motor nucleus derived from the basal plate.

Embryologically, the continuous addition of neuroblasts to the mantle layer of the neural tube produces thickenings ventrally and dorsally - producing the basal and alar plates respectively. A longitudinal groove, called sulcus limitans, marks the boundary between the two.

Basal Plate Ventralthickening

Give rise tomotor nuclei-gene ral visceral efferent nuclei, sp ecial visceral efferent of CN I X, X, and XII; and somatic ef ferent of XII

Forms the motor part of the s pinal cord (ventral horns)

Alar plate Dorsalthickenings

Give rise tosensory nucleiCN V, VII, VIII, IX, X, inferior ol ivary nucleus, and mesencep halic nucleus of CN V

Forms the sensory part of the spinal cord (dorsal horns).T he rhombic lip develops into the cerebellum and its nuclei.

The first commissure to develop is the anterior commissure.

Commissures are fibre bundles that cross the midline and connect the right and left brain hemispheres. Many important fiber bundles cross at lamina terminalis.

Commissures at lamina terminalis:

• Anterior commissure - first commissure to appear. These fibers connect the olfactory bulb and related brain areas to the corresponding structures in the contralateral hemisphere.
• Hippocampal commissure (fornix commissure) - develops second. The fibers emerge from the hippocampus and reach the lamina terminalis. From here, they arch outside the choroid fissure to the mamillary body and the hypothalamus.
• Corpus callosum - appears by the 10th week. It connects the non-olfactory areas of the right and the left cerebral cortices.

Commissures at other areas are:

• Posterior commissure
• Habenular commissure
• Optic chiasma

The image given below shows the anterior commissure:

Septum pellucidum extends between the corpus callosum and the fornix.

The septum pellucidum is a membrane separating the anterior horns of the left and right lateral ventricles of the brain. It extends from the corpus callosum to the fornix.

It develops from the lamina terminalis. The part of the lamina terminalis that stretches between corpus callosum and fornix persists as septum pellucidum.

The image given below shows the septum pellucidum.

Spina bifida is characterized by gaps in the vertebral arches.

Spina bifida is a general term used for neural tube defects affecting the spinal region and associated with splitting of the vertebral arches. It may or may not involve the underlying neural tissue.

There are 3 types of spina bifida, as shown in the image below:

Rachischisis is the most severe type of neural tube defect, where the entire neural tube fails to close. Rachischisis can occur in the spinal cord or brain regions of the neural tube. The exposed neural tissues often become necrotic.