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The formation of the primitive streak from the epiblast is an indicator of the beginning of gastrulation.

By the 15th day of development, some epiblast cells lying along the central axis, near the tail end of the embryonic disc, begin to proliferate. These proliferated cells form an elevation called primitive streak that bulges into the amniotic cavity.

Gastrulation is the process that forms all the 3 germ layers namely, ectoderm, mesoderm, and endoderm in the embryo. It occurs during the 3rd week of gestation.

The image given below shows the formation of primitive streak.

The intraembryonic mesoderm spreads throughout the embryonic disc, except in the region of the prechordal plate and cloacal membrane. In these two regions, the ectoderm is in direct contact with the endoderm.

The image given below shows the direct contact of intraembryonic mesoderm with the endoderm.

The intraembryonic mesoderm or secondary mesoderm is formed by the invagination and movement of epiblast cells between the newly formed endoderm and the epiblast.

In the later stages of development, the ectoderm and endoderm mostly persist as lining epithelium. Mesoderm forms most of the tissues of the body, including bone, cartilage, connective

tissue, muscles (skeletal, smooth, cardiac), blood, lymph, and cardiovascular organs.

Neurulation refers to the process of the formation of the neural tube from the neural plate. The neural tube gives rise to the central nervous system.

The image given below shows the steps of neurulation:

Kidneys are developed from the intermediate mesoderm.

After the notochord is formed, the intraembryonic mesoderm on either side can be seen organized into 3 layers:

• Paraxial mesoderm - Thickened plate of the mesoderm close to the midline, on either side of the notochord. This forms the somitomeres and somites.
• Intermediate mesoderm - Longitudinal strip of mesoderm that connects paraxial with the lateral plate mesoderm. The urinary and genital systems are derived from the intermediate mesoderm.
• Lateral plate mesoderm - A thinner layer of mesoderm that lies laterally. This layer develops a large cavity called the intraembryonic coelom that gives rise to pericardial, pleural, and
• peritoneal cavities.

The vertebrae are derived from somites which are derivatives of paraxial mesoderm. The paraxial mesoderm during development gets organized into segments called somitomeres and somites.

Somitomeres appear in the head region of the embryo. They are rounded structures and 7 in number. They form the mesoderm and muscles of the head and jaw.

From the occipital region downwards, somitomeres organize into somites. The first pair of somites arise in the occipital region of the embryo at approximately the 20th day of development.

From here, new somites appear in craniocaudal sequence at a rate of approximately 3 pairs per day.

By the end of the 5th week, 42-44 pairs of somites are present. There are 4 occipital, 8 cervical,

thoracic, 5 lumbar, 5 sacral, and 8 to 10 coccygeal pairs. The first occipital and the last five to seven coccygeal somites later disappear. The remaining somites form the axial skeleton.

The pericardial cavity is formed by the intraembryonic celom in the lateral plate mesoderm.

Within the lateral plate mesoderm, a large cavity known as intraembryonic celom is formed. The intraembryonic celom forms the pericardial, pleural, and peritoneal cavities.

The lateral plate mesoderm is divided by the intraembryonic celom to form:

• Somatopleuric or parietal layer of intraembryonic mesoderm that is in contact with the ectoderm. This forms the parietal layer of peritoneal, pleural, and pericardial membranes.
• Splanchnopleuric or visceral layer of intraembryonic mesoderm that is in contact with endoderm. This forms the visceral layer of the peritoneal, pleural, and pericardial membranes.

The image given below shows the somatopleuric and splanchnopleuric layers and the intraembryonic celom.

The primitive gut is derived from the part of the yolk sac lined by the endoderm.

With the development of the lateral folds of the embryo, a part of the yolk sac gets enclosed within the embryo, to form a tube-like structure. This structure is lined by the endoderm and is called the primitive gut.

Initially, the primitive gut has a wide communication with the yolk sac. Later, three different sections of the primitive gut can be recognized:

• Foregut - Part of the primitive gut cranial to the communication with the yolk sac
• Midgut - Middle part of the primitive gut in communication with the yolk sac
• Hindgut - Part of the primitive gut caudal to the communication with the yolk sac
• The image given below shows the sections of primitive gut.

The right vein in the connecting stalk disappears during development.

At first, the connecting stalk is at the caudal end of the embryonic disc. With the formation of the tail fold, the connecting stalk moves to the ventral part of the embryo, near the area of the umbilical opening.

The blood vessels connecting the placenta to the embryo pass through the connecting

stalk. Initially, there are 2 arteries and 2 veins. Later the right vein disappears (the left vein is left behind). The connecting stalk then consists of 2 arteries and 1 vein.

The extraembryonic mesoderm of the connecting stalk develops into a gelatinous substance known as Wharton’s jelly.

When the tail fold of the embryo is formed, the connecting stalk moves towards the ventral aspect of the embryo. The connecting stalk is now attached only in the region of the umbilical opening. The extraembryonic mesoderm of the connecting stalk becomes Wharton's jelly in the umbilical cord. Wharton's jelly protects the blood vessels in the umbilical cord.

Secondary villi do not contain fetal capillaries. Fetal capillaries are present in tertiary villi. Chorionic villi pass through the following stages during development:

Hofbauer cells are fetal-derived macrophages present in the placenta.

Hofbauer cells are large (10–30 µm), pleomorphic cells, with highly vacuolated and granular cytoplasm. They are seen in the fetal villi of the placenta from the first trimester of pregnancy until birth.

The role of Hofbauer cells is not established yet, but they are thought to play a gate-keeper role in preventing the transmission of pathogens from the mother to the fetus.

The core of each pharyngeal arch (branchial arch) is derived from mesoderm and neural crest cells.

Pharyngeal arches initially have identical structures:

• Surface ectoderm on the outside
• A mesenchymal core derived from mesoderm
• Endoderm derived epithelium on the inside

The mesoderm in the arches is derived from paraxial mesoderm and lateral plate mesoderm. This is invaded by the neural crest cells that contribute to the formation of skeletal elements and connective tissue of the head and neck region.

The 5th pharyngeal arch disappears soon after its formation, so only 5 arches remain.

Pharyngeal arches appear in the 4th or 5th week of development and contribute to the formation of the head and neck region. They are formed as a series of 6 thickenings in the cranial most part of the foregut (the future pharynx). They are craniocaudally numbered from I to VI. The 5th pharyngeal arch disappears, so only 5 arches remain.

• The first pharyngeal arch is also known as the mandibular arch
• The second pharyngeal arch is also known as the hyoid arch
• The 3rd, 4th, and 6th pharyngeal arches have no special names

The pharyngeal membrane is the location where the ectoderm of the pharyngeal cleft meets the endoderm of the pharyngeal pouch.

In the interval between any two adjoining pharyngeal arches, the endoderm extends outward in the form of a pouch (pharyngeal pouch) to meet the ectoderm, which dips into this interval as a pharyngeal cleft. The membrane formed where the pharyngeal cleft comes in contact with the pouch is called the pharyngeal membrane.

Initially, there are four pharyngeal membranes between the five arches. Only the first pharyngeal membrane develops and contributes to the formation of the tympanic membrane. The 2nd, 3th, and 4th pharyngeal membranes do not develop further into any structure. They get obliterated when the mesoderm from the 2nd arch overgrows and fills the succeeding clefts.

The stapes is not derived from the Meckel's cartilage.

The cartilage of the first arch is Meckel's cartilage. Structures derived from it include:

• Malleus
• Incus
• Anterior ligament of malleus
• Spine of sphenoid
• Sphenomandibular ligament

Note:

• The mandible, maxilla, zygomatic bone, and part of the temporal bone are formed from the mesenchyme of the first arch and not from Meckel's cartilage.
• The portion of Meckel’s cartilage that extends from the mandibular symphysis to mental foramen is probably incorporated into the intramembranous mandibular bone.

The stylohyoid ligament is derived from the cartilage of the second pharyngeal arch which is Reichert's cartilage.

Reichert's cartilage contributes to the development of the following structures: The dorsal portion forms:

• Stapes
• Styloid process of the temporal bone
• Stylohyoid ligament

The ventral portion forms:

• The upper part of the body and the lesser cornu of the hyoid bone.

Note: This condition is called Eagle syndrome. It is a condition associated with the elongation of the styloid process or calcification of the stylohyoid ligament.

These structures can be remembered as 5S's:

• Stapes
• Styloid process of the temporal bone
• Stylohyoid ligament
• Smaller (lesser) cornu of the hyoid bone
• Superior part of the body of the hyoid bone

The tensor tympani muscle is affected in the mandibular nerve injury. The mandibular nerve supplies the muscle since it is derived from the first pharyngeal arch (mandibular arch).

Pharyngeal A rch

I

II

III IV

VI

Muscle derivatives

Muscles of mastication, mylo hyoid, anterior belly of digast ric, tensor tympani, and tens or veli palatini

Muscles of facial expression, stapedius, stylohyoid, platys ma, and posterior belly of dig astric

Stylopharyngeus

Larynx - cricothyroid; soft pa late - levator veli palatini; ph aryngeal constrictors

Larynx - All intrinsic muscles

(except cricothyroid)

The recurrent laryngeal nerve of the vagus is the nerve of the 6th pharyngeal arch. The vagus nerve is distributed to the 4th and 6th arches as follows:

• Superior laryngeal nerve for 4th arch
• Recurrent laryngeal nerve for 6th arch

The stapedial artery develops in the second pharyngeal arch (hyoid arch).

Arteries of the pharyngeal ar ches

Arch I

II III

IV

VI

Artery

Maxillary arteries

Hyoid and stapedial arteries

Common carotid and the first part of the internal carotid a rteries

Left side: Arch of the aortaRi ght side: Right subclavian art ery

Left side: Left pulmonary art ery and ductus arteriosusRig ht side: Right pulmonary arte ry

The tongue muscles are derived from the myotomes of the occipital somites.

The tongue muscles get their motor supply from the hypoglossal nerve (12th cranial nerve), except the palatoglossus muscle, which is supplied by the vagus nerve.

Development of the tongue

• The anterior part of the tongue (body) is derived from the 1st pharyngeal arch. It is formed from
• 2 lateral lingual swellings and one medial swelling called the tuberculum impar.
• The posterior part of the tongue (root) is derived from the mesoderm of the 2nd, 3rd, and part of the 4th arch. This part develops from a medial swelling called copula (hypobranchial eminence).
• The posterior-most part of the tongue and the epiglottis are derived from a third swelling formed in the posterior part of the 4th arch.