Fundamental Characteristics of Chordates

• Notochord

• Dorsal hollow nerve chord

• Pharyngal slits

• Postanal tail

• Endostyle

No vertebrate will proceed without passing through these five characteristics in embryonic development

Notochord

• Rod of living cells ventral to the CNS and dorsal to the alimentary canal

• Provides skeletal support throughout most of the length of the chordate

True

TRUE OR FALSE

The Notochord will develop into the vertebral column in adulthood

Gets incorporated into the skull floor

What happens to the head region of the notochord during embryonic development?

Gets surrounded by cartilaginous or bony vertebrae structures

What happens to the bodily region of the notochord during embryonic development?

Chief axial skeleton surrounded by notochord sheath

What is the fate of the notochord during adulthood in protochordates?

Similar to protochordates with addition of lateral neural cartilages

What is the fate of the notochord during adulthood in agnathans?

Persists the length of the trunk and tail within the centrum of the vertebral column

What is the fate of the notochord during adulthood in fishes and amphibians?

Vertebral column

• The body of the vertebra is, the CENTRUM, the bones that deposits around the notochord

• Above it is the neural arch, forming over the spinal cord and the spinal cord will pass through the foramin within the neural arch

False

TRUE or FALSE

The notochord develops into the CNS

Dorsal Hollow Nerve Cord

Nerve cord of a chordate embryo develops from a plate of ectoderm that rolls into a tube dorsal to the notochord

Invagination

Sheet of cells will move inward to form the embryonic structure during gastrulation

True

TRUE OR FALSE

The Dorsal Hollow Nerve Cord is formed by invagination

CNS: Brain and Spinal Cord

What does the Dorsal Hollow Nerve Cord develop into?

Pharynx

• Can be perforated by openings (stilts) to either the exterior or an atrium (reflecting common ancestry)

• Membranous digestive and respiratory organ located at the back of the mouth that serves as passageway of food and air

Hemichordates and chordates

What species is the Pharynx common to?

Hemichordates

Species that do not possess all 5 criteria to classify them as chordates

• Gills of fishes

• Development of lungs for tetrapods

• Skeleton and musculature of jaws

• Endocrine glands

• Middlebear cavity of tetrapods

• Initial cells of immune system (thymus)

What structure will form from the pharynx?

• Pharyngeal arches

• Pharyngeal clefts

• Pharyngeal pouches

• Pharyngeal slits

Embryonic components of the pharynx

False

TRUE OR FALSE

During the embryonic stage, arches are not separated by grooves

Cleft

If the groove is outside the embryo, it is the _____

Pouch

If the groove is inside the embryo, it is the _____

Slit

If the groove can make an opening, it is the _____

Pharyngeal arches

Column of tissue which separates each embryonic pharyngeal pouch or split from the next

• Skeleton

• Muscles

• Nerves

• Blood vessels

Arches have 4 components or blastemas which develop into:

6

How many totals pairs of arches develop into cranio-caudal sequence

True

TRUE OR FALSE

The development of the cranio-caudal sequence starts from the pharyngeal arch closest to the head and proceeds caudally to the tail

False

TRUE OR FALSE

Indentations are not present in external or internal arches

Lined with the ectoderm

Where are external arches or clefts located?

Lined with the endoderm

Where are internal arches or pouches located?

Arch 1

SKELETAL: Meckel’s cartilage, Malleus, Incus

MUSCLE: Mastication, Anterior Digastricus

NERVE: Trigeminal Nerve

Arch 2

SKELETAL: Stapes, Styloid, Upper Hyoid

MUSCLE: Stapedius, Stylohyoid, Facial Expression, Posterior Digastricus

NERVE: Facial Nerve

Arch 3

SKELETAL: Lower Hyoid

MUSCLE: Stylopharyngeus

NERVE: Glossopharyngeal Nerve

Arch 4-6

SKELETAL: Laryngeal cartilages

MUSCLE: Cricotyhroideus, Pharyngeal constrictors, Intrinsic laryngeal muscles

NERVE: Vagus Nerve

Components of Pharyngeal Arches

• Supportive skeletal elements

• Striated muscles

• Cranial nerves (5, 7, 9, 10)

• Aortic arch

Components of Pharyngeal Clefts

• Ectodermally-lined grooves

• Only the first cleft is important

• lost in development because of proliferation of the underlying mesoderm

• Develops into the external auditory meatus of the ear

• Provides the outer epithelium of the tympanic membrane

Why is the first cleft the only one important in Pharyngeal Clefts?

Pharyngeal Pouches

• Arises as diverticula/invagination of endoderm of foregut

• Grow/deepen toward the surface of the animal

• Establish the limits of the pharynx

• Pharyngeal clefts grow toward each pouch and is separated from each other via the pharyngeal plate

  • If you lose the plate, it will now become the pharyngeal slit

How does a pharyngeal pouch come to be

8

What is the max number of pharyngeal pouches in a basal shark

15

What is the max number of pharyngeal pouches in a living agnathans

Pouch 1

Derived from

• middle ear cavity

• endodermal aspect of tympanic membrane

• pharyngotympanic tube

Pouch 2

Derived from

• Palatine tonsil

Pouch 3

Derived from

• inferior parathyroid gland

• thymus

Pouch 4 and 5

Derived from

• Superior parathyroid gland

• parafollicular cells of the thyroid gland

Pharyngeal slits

• Formed between the pharynx and the exterior when the pharyngeal plate ruptures

• May open to the exterior as slits that are Permanent or Temporary

Permanent Pharyngeal Slit

Adults that live in water and breathe via gills

Temporary Pharyngeal Slit

Adults live on land

True

TRUE or FALSE:

Pharyngeal pouches may open to the exterior permanently as a slit

Agnathan

What do you call vertebrates with absent jaw?

Endostyle

• Glandular groove in the floor of the pharynx and is involved in filter feeding

• Arise from the floor of the pharynx and are involved in iodine metabolism

Thyroid Gland

What does the endostyle give rise to?

Muscular, post-anal tail

• Chordates have a tail posterior to the anus

• In many species, the tail is greatly reduced during embryonic development

• Tail contains skeletal elements and muscles

• Provides propelling force in many aquatic species