Chordate Development and Anatomy

Deuterostome Developmental Biology

• Deuterostomes are a type of animal, mostly encompassing vertebrates.
• The development of vertebrates shares similarities with other types of animals.

Reference Frame

• Reference frames are equally valid in the absence of specific information.
• Coordinate systems should be treated as artificial constructs.

Biological Reality

• Directional reference (top, bottom, left, right, and front) in a fertilized ovum is defined by anatomical positioning.
• These directions are fixed by birth, so top, bottom, left, right, and front are definable in a human.

Vertebrate Body Plan

• Vertebrate development involves definitive top/bottom, left/right, front/back axes.
• The body plan of a vertebrate develops from a single-celled ovum.

Vertebrate Phylogeny

• Bilaterally symmetrical animals
• Protostomes (blue)
• Deuterostomes (red)

Echinoderms

• Literally "spiny skin"
• Possess a calcium carbonate endoskeleton in the dermis.
• Diverse, both taxonomically and morphologically.

Hemichordates

• Literally "half-chordates."
• Include acorn worms and pterobranchs.
• Not very diverse taxonomically but an interesting group.

Tunicates

• Include sea squirts and salps.
• Urochordates, members of the Chordata but not vertebrates.

Vertebrates: Petromyzontiformes (Lampreys)

• True vertebrates with no bones.

Vertebrates: General

• These animals exhibit a vertebrate body plan with bones.

Deuterostome Body Plan

• Living body plans exhibit a wide variety and disparity.
• Sea squirts, sea stars, and giraffes share some unifying characteristics.

Embryogenesis

• Shared characteristics of deuterostomes emerge in early embryogenesis.

The Five Characters Uniting Chordata

• Notochord
• Dorsal hollow nerve cord
• Pharyngeal slits in the pharynx
• Postanal tail
• Endostyle/thyroid
• Not all taxa have these five features in their adult form.

Ontogeny

• Ontogeny is the developmental history of an individual.
• Development in the process of going through all stages of form from fertilization to adulthood is a unifying concept in comparative biology.
• Evolution results from changes to development, which are ultimately at the level of genes.
• Most of development is conserved.

Cleavage

• Synchronous cell division.
• Radial cleavage.

Blastulation

• The morula hollows out.
• Germ layer specification.
• Basic axis polarity formation.

Gastrulation

• Cells change shape and move inward to form archenteron,
• Other cells break free and become mesenchyme.
• Thin extensions of mesenchyme cells attach to the overlying ectoderm.
• The archenteron elongates, assisted by mesenchyme cells
• The mouth forms where the archenteron meets the ectoderm.
• The blastopore becomes the anus during development.

Differentiation of the Mesoderm

• Ectoderm and endoderm arise during gastrulation.
• Mesoderm differentiation varies between protostomes and deuterostomes.

Gastrulation (in a frog)

• Morphological developments are observed in Xenopus.
• Cells undergo convergent extensions, involution, and epiboly.

Examples of Animal Embryology (blastula to gastrula)

• Frog, snail, and fruit fly embryos.
• Transitions are fairly uniform during animal development.

Neurulation

• Notochord induces formation of neurectoderm (neural plate), creating a hollow neural tube.
• Ectoderm differentiates to become the nervous system.
• BMP-4 signaling plays a role in ectoderm differentiation between skin and neural cells.

Primary vs. Secondary Neurulation

• Primary neurulation: Hollow neural tube is formed through invagination of neural plate.
• Secondary neurulation: Solid neural cord is formed by ventral thickening of neural plate, which is subsequently hollowed out.

Retained Medullary Cord

• Normal human development is characterized by primary and secondary neurulation of a medullary cord.
• A spinal cord will be found in humans at the L₁/L₂ level.
• The adult human spinal cord continues to the tailbone.

Neurulation (Brain Development)

• Three primary embryonic brain regions: Prosencephalon (forebrain), Mesencephalon (midbrain), Rhombencephalon (hindbrain).

Neural Tube Defects (NTDs)

• NTDs, such as spina bifida, result from neural tube closure failures.
• Hydrancephaly: Failure of brain structure formation.
• Anencephaly: Absence of brain formation.

The Neural Crest

• Neural crest cells (NCCs) are derived from the neurectoderm.
• NCCs migrate.
• Patterns of NCC migration are conserved.
• NCCs contribute to many vertebrate structures.

The Cranium

• Embryonic components
• Neurocranium supports paired sensory capsules.
• Braincase/cranial elements
• Cartilages form around sensory capsules (otic, optic, and olfactory).

The Arches

• Mandibular arch: forms jaws
• Hyoid arch: forms parts of the throat and larynx
• Gill arches: forms gill structures.

Dermal Bones of the Face and Head

• Structures form from streams of neural crest cells from the hindbrain.
• Includes maxillary and mandibular bones.
• Dermal bone development varies between mammals.

Intramembranous Ossification

• Dermal bones form in humans via intramembranous ossification.
• In the mandible, Meckel's cartilage is resorbed.

3D Models of the Middle and Inner Ear

• 3D models of the ear show conservation of the jaw joint between the articular and quadrate bones

The Vertebral Column

• Re-segmentation of vertebral column leads to somite formation.
• Sensory and motor nerve roots originate between the vertebra.
• Spinal nerve roots have different patterns in various body regions.

Hox Genes and Vertebral Patterning

• Number of each vertebrae type varies across species.
• Vertebral patterns reflect Hox gene expression patterns.
• Similar patterns among different vertebrate species, but some variances do exist.

Down Syndrome

• Partial or full duplication of chromosome 21 features widened set eyes, elongated orbits and a smaller or absent nasal bridge.
• Heart defects, hearing impairments, and palate clefts are also common.

DiGeorge Syndrome

• Deletion of chromosome 22 causes congenital heart problems, such as persistent truncus arteriosus, ventricular septal defect, and right ventricular hypertrophy.
• Symptoms include cleft palate and thyroid hypoplasia.

Patau Syndrome

• Duplication or full duplication of chromosome 13 is associated with severe conditions, including heart defects and facial clefts (cleft palate, etc).
• Most individuals do not survive past the first year of life

Waardenburg Syndrome

• Mutated neural crest cells causes an increased interorbit distance, uneven orbits, and prominent white forelock.
• Hearing loss is a frequent symptom.

Hirschsprung's Disease

• Failure of neural crest cells to migrate to the entire intestine leads to a section of intestine that cannot relax.
• This results in a digestive impairment that might need surgical intervention to correct.

Cleft Palate

• Failure of left and right neural crest cells in the frontonasal prominence to meet along the ventral midline.
• Can result in different severities of cleft palate.

This quiz explores key aspects of chordate development, including the functions of pharyngeal slits, postanal tail formation, and the role of the endostyle in filter feeding. Test your knowledge on the canonical characters shared by all chordates and the significance of Hox genes during embryonic development.