Ectodermal Derivatives of the 10 mm Pig Embryo Epidermis: This is a single layer of epidermal ectoderm cells covering the entire embryo. Nervous System Brain As in the 72- hour chick embryo, the first sections in the series, cut through the rhombencephalon due to the cranial flexure. Try to identify first all the major divisions of the pig brain. Myelencephalon: the part of the rhombencephalon tapering to the spinal cord. It has a roof of thin, non-nervous blood vesssels of the posterior choroid plexus and a large wide ventricle (IV) or the myelocoel. Metencephalon: the thick walled and thick roofed anterior part of the hindbrain. It gives rise to the cerebellum dorsally and the pons ventrally. Mesencephalon: the rounded region of the brain containing the mesocoel or the aqueduct of Sylvius. Diencephalon: By tracing the slides posteriorly, the mesencephalon gives way to the diencephalon, a laterally compressed region containing the third ventricle that communicates the telocoels of the telencephalon through the foramen of Monro. Do you notice the ventral evagination? What is the fate of this structure? This brain region is associated with the ventro-lateral optic stalks. Infundibulum: This is the evagination of the floor of the diencephalon posterior to the optic stalk. Rathke's pocket: This ectodermal pocket evaginating from the stomodaeum meets with the infundibulum and forms the pituitary gland which thus has 2 germ layer origins. The infundibulum becomes the neurohypophysis being derived from neural ectoderm. The adenohypophysis is derived from the Rathke's pocket. Telencephalon On more posterior slides, the telencephalon is distinguished as thick lateral outgrowths observed in the brain. The cavities are the first and second ventricles. The telencephalon is separated from diencephalon by velum transversum in the roof and the optic recess in the floor. Cranial Nerves Olfactory (I): lightly stained nerve fibers growing from the medial wall of the olfactory pit to the telencephalon, it is difficult to Optic (II): fine nerve fibers emerging from the retina into the choroid fissure into the floor of the Special Sense Organs Eye: Study sections showing the optic cups at the level of the diencephalon. The parts are similar to those of the 72-hour chick. Identify them very quickly. Olfactory pit: this thick walled ventro-lateral invagination of the head is closely associated with the telencephalon. Try to locate thin fibers of the olfactory nerve interconnecting the two. It is sometimes difficult to find in some specimens Otic vesicle/otocyst: It is formed by invagination of the epidermal ectoderm at the level of the myelencephalon. Are the parts similar to those of the 72-hour chick, Identify the saccule, utricle and the endolymphatic duct Spinal Cord This part of the central nervous system begins without clear demarcation from the myelencephalon and extends into the tail. Select a section from cervical region to study. Identify the gross features: floor plate, basal plate, sulcus limitans, alar plat, roof plate. Now identify the layers of the spinal cord. Marginal layer: This outermost layer is composed of neuron fibers and neuroglia, the scattered supporting cells Mantle layer: This middle layer is not as darkly nucleated as the innermost layer. The enlarged ventro-lateral regions of the spinal cord represent the developing motor columns or the ventral gray columns. What do you find in these ventro- lateral thickenings? Ependymal layer: This is the innermost, darkly nucleated layer. Neurocoel : This is the cavity of the spinal cord External limiting membrane: the fine membrane forming the outer border of the wall Spinal Nerves: These are made up of nerve fibers going into or coming from the spinal cord. Select a section that clearly shows the parts of a spinal nerve and look for the following structures: Dorsal root ganglion: This large mass of nerve cell bodies or perikarya is found lateral to the spinal cord. Dorsal root: This bundle of afferent or sensory fibers extends into the dorcolateral wall of the spinal cord from the dorsal root ganglion Ventral root: This bundle of efferent or motor fibers emerge from the ventro-lateral region of the spinal cord Spinal nerve: formed by the junction of the sensory and motor fibers. Sensory fibers of the dorsal root ganglion extend front the base of the ganglion to the dorso- lateral and ventral body walls of the embryo. Identify the following components: Dorsal ramus: This small bundle of nerve fibers at the proximal portion of the spinal nerve passes dorso-laterally to innervate the skin and trunk Ramus communicans: This bundle of fibers passes medioventrally to the sympathetic ganglion after the spinal nerve has given off the dorsal QUESTIONS: State the neuromere origin of each brain region? What is the constriction that separates the mesencephalon from the methencephalon?
Ectodermal Derivatives of the 10 mm Pig Embryo
Epidermis: This is a single layer of epidermal ectoderm cells covering the entire embryo.
Nervous System Brain
As in the 72- hour chick embryo, the first sections in the series, cut through the
rhombencephalon due to the cranial flexure. Try to identify first all the major divisions of the pig brain.
Myelencephalon: the part of the rhombencephalon tapering to the spinal cord. It has a roof of thin, non-nervous blood vesssels of the posterior choroid plexus and a large wide ventricle (IV) or the myelocoel.
Metencephalon: the thick walled and thick roofed anterior part of the hindbrain. It gives rise to the cerebellum dorsally and the pons ventrally.
Mesencephalon: the rounded region of the brain containing the mesocoel or the aqueduct of Sylvius.
Diencephalon: By tracing the slides posteriorly, the mesencephalon gives way to the diencephalon, a laterally compressed region containing the third ventricle that communicates the telocoels of the telencephalon through the foramen of Monro. Do you notice the ventral evagination? What is the fate of this structure? This brain region is associated with the ventro-lateral optic stalks.
Infundibulum: This is the evagination of the floor of the diencephalon posterior to the optic stalk.
Rathke's pocket: This ectodermal pocket evaginating from the stomodaeum meets with the infundibulum and forms the pituitary gland which thus has 2 germ layer origins. The infundibulum becomes the neurohypophysis being derived from neural ectoderm. The adenohypophysis is derived from the Rathke's pocket.
Telencephalon On more posterior slides, the telencephalon is distinguished as thick lateral outgrowths observed in the brain. The cavities are the first and second ventricles. The telencephalon is separated from diencephalon by velum transversum in the roof and the optic recess in the floor.
Cranial Nerves
- Olfactory (I): lightly stained nerve fibers growing from the medial wall of the olfactory pit to the telencephalon, it is difficult to
- Optic (II): fine nerve fibers emerging from the retina into the choroid fissure into the floor of the
Special Sense Organs
Eye: Study sections showing the optic cups at the level of the diencephalon. The parts are similar to those of the 72-hour chick. Identify them very quickly.
Olfactory pit: this thick walled ventro-lateral invagination of the head is closely associated with the telencephalon. Try to locate thin fibers of the olfactory nerve interconnecting the two. It is sometimes difficult to find in some specimens
Otic vesicle/otocyst: It is formed by invagination of the epidermal ectoderm at the level of the myelencephalon. Are the parts similar to those of the 72-hour chick, Identify the saccule, utricle and the endolymphatic duct
Spinal Cord
This part of the central nervous system begins without clear demarcation from the myelencephalon and extends into the tail. Select a section from cervical region to study. Identify the gross features: floor plate, basal plate, sulcus limitans, alar plat, roof plate. Now identify the layers of the spinal cord.
Marginal layer: This outermost layer is composed of neuron fibers and neuroglia, the scattered supporting cells
Mantle layer: This middle layer is not as darkly nucleated as the innermost layer. The enlarged ventro-lateral regions of the spinal cord represent the developing motor columns or the ventral gray columns. What do you find in these ventro- lateral thickenings?
Ependymal layer: This is the innermost, darkly nucleated layer. Neurocoel : This is the cavity of the spinal cord
External limiting membrane: the fine membrane forming the outer border of the wall
Spinal Nerves: These are made up of nerve fibers going into or coming from the spinal cord. Select a section that clearly shows the parts of a spinal nerve and look for the following structures:
Dorsal root ganglion: This large mass of nerve cell bodies or perikarya is found lateral to the spinal cord.
Dorsal root: This bundle of afferent or sensory fibers extends into the dorcolateral wall of the spinal cord from the dorsal root ganglion
Ventral root: This bundle of efferent or motor fibers emerge from the ventro-lateral region of the spinal cord
Spinal nerve: formed by the junction of the sensory and motor fibers. Sensory fibers of the dorsal root ganglion extend front the base of the ganglion to the dorso- lateral and ventral body walls of the embryo. Identify the following components:
- Dorsal ramus: This small bundle of nerve fibers at the proximal portion of the spinal nerve passes dorso-laterally to innervate the skin and trunk
- Ramus communicans: This bundle of fibers passes medioventrally to the sympathetic ganglion after the spinal nerve has given off the dorsal
QUESTIONS:
- State the neuromere origin of each brain region?
- What is the constriction that separates the mesencephalon from the methencephalon?
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