Ectodermal Derivatives of the Developing Chick Embryo
33-hour Chick Embryo Transverse Sections
48-hour Chick Embryo Transverse Sections (SEE IMAGE)
72-hour Chick Embryo Transverse Sections
Several new structures appear at this stage of development. However, there are also parts that remained essentially unchanged since the 48-hour stage. Go through them very quickly.
Epidermis: This epidermal ectoderm derivative is the outermost covering of the embryo.
Nervous System
Quickly identify again these principal structures from the whole mount. The first sections will show the rhombencephalon in a frontal section due to increased cephalic or cranial flexure in the 72-hour chick embryo.
Rhombencephalon:
Metencephalon: The anterior portion of the hindbrain has thicker walls than the myelencephalon. The roof becomes the cerebellum and the ventro-lateral walls become the pons. The cavity is the metacoel. It comes from neuromere 6. Myelencephalon: This is the thin-roofed posterior region of the hindbrain. The thin roof is the posterior choroid plexus made of capillaries. The cavity is the myelocoel or the fourth ventricle. The floor becomes the medulla and the brain stem. It is formed from neuromeres 7-11. It is associated with cranial nerves V-XII.
Mesencephalon: Tracing the sections posteriorly to the mesencephalon, observe this circular brain region in the section where the brain looks like an exclamation point. The elongated part is the rhombencephalon. The constriction between the two is the isthmus. The mesencephalon comes from neuromeres 4 and 5.
Diencephalon: As the mesencephalon is traced caudally, it is replaced by the thinner walled diencephalon. It has a ventral evagination, the infundibulum and a dorsal outpocketing, the epiphysis. The diencephalon is a derivative of neuromeres 2 and 3.
Telencephalon: This is the region of lateral expansion. The two lobes are the forerunners of the cerebral hemispheres. The telocoeles become the first and second ventricles that join the third ventricle via the foramen of Monro.
Cranial Nerves: Identify as many cranial nerves and their ganglia as you can. The location of the ganglia is similar to those of the 48-hour chick embryo.
Oculomotor nerve (III) is found emerging from the floor of the mesencephalon.
Gasserian or semilunar ganglion of the trigeminal nerve (V) is a dark mass anterior to the acoustico-fascial is ganglion. Traced caudally, the nerve divides into three: the ophthalmic nerve to the eye, maxillary nerve to the upper jaw and mandibular nerve to the lower jaw.
Acoustico-fascialis ganglion of the VII (facial) and auditory (VIII) nerves. Trace caudally in the series and note that the auditory nerve is attached to the sacculus of the ear and the facial nerve to the skin.
Ganglion of the IX (glossopharyngeal) nerve: This ganglion is found lying posterior to the otic vesicle.
Ganglion of the X (vagus): Observe the ganglion lying posterior to the IX ganglion and separated from it by a blood vessel, the anterior cardinal vein. Nerve XI: These are strips of fibers running alongside anterior portion of the spinal cord and posterior portion of the mylencephalon. The accessory ganglia may be seen as discrete cell bodies attached to these fibers.
Nerve XII: The roots of this nerve are seen coming out of the floor of the mylencephalon.
Nerves I,II,IV, VI, are not yet seen at this stage in development
Spinal Cord: The posterior tubular extension of the brain extends until the tail fold. It is connected to 38 pairs of spinal ganglia and associated sympathetic ganglia, all arising from neural crest.
Sense Organs: Identify the nasal pit, and the parts of the eye including the choroid fissure where the ventral lip of the optic cup is absent. The auditory apparatus consists of the vesicle, the endolymphatic duct, a tube that lies adjacent to the myelencephalon, and the two regions of the ear, the dorsal utriculus and the ventral sacculus.
QUESTIONS:
- Name some derivatives of the neural
- What is the lamina terminalis?
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