Development of prosencephalon.pptx college of medicine unec

Embryo slide on development of prosencephalon

1. Development of Prosencephalon DR EZUGWORIE, J. O. MBBS, MSc, PhD, FWACS. 1

2. INTRODUCTION The prosencephalon consists of telencephalon and diencephalon. The telencephalon forms the cerebral hemispheres. The third ventricle is formed by the cavities of the telencephalon & diencephalon. - The diencephalon contributes more The diencephalon forms the optic cup & stalk, thalamus, hypothalamus, pituitary, and epiphysis (pineal body). - It is thought to consist of a roof plate and two alar plates- lascking floor and basal plates. 2

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4. DIENCEPHALON This develops from the median portion of the prosencephalon . It is thought to consist of the roof plate and two alar plates. - Diencephalon lacks floor plate & basal plate. The alar plates form the lateral walls of the diencephalon. The diencephalon forms the epiphysis, thalamus, hypothalamus, pituitary, optic cup & optic stalk. 4

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6. THALAMUS. Alar plates form the lateral walls of the diencephalon. A groove, hypothalamic sulcus , divides the alar plate into dorsal & ventral regions. * THALAMUS . The dorsal region of the alar plate forms the thalamus. As the cells proliferate the thalamus gradually projects into the lumen of the diencephalon. In about 70% of cases the thalamic growth from the two sides fuse in the midline. - This is called interthalamic connexus or massa intermedia . 6

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8. HYPOTHALAMUS The ventral region or the lower region of the alar plate forms the hypothalamus . The hypothalamus differentiates into a number of nuclear areas that regulate visceral functions. The functions include sleep, digestion, temperature, & emotional behaviour. Mamillary body is one of the nuclear areas located on the ventral surface. 8

9. CHOROID PLEXUS/EPIPHYSIS. * The roof plate of diencephalon is made up of a single layer of ependymal cells covered by vascular mesenchyme . These two layers form the choroid plexus. * The most caudal part of the roof plate of the diencephalon; - develops into the epiphysis or pineal body. First as a midline epithelial thickening. Then begins to evaginate by the 7 th week, & becomes a solid organ. It serves as a channel that light & darkness pass to affect endocrine & behavioural rhythms. In adult the pineal body serves as a landmark on radiographs of the skull - as calcium is frequently deposited in it. 9

10. ++Pituitary Gland or Hypophysis . This develops from two sources: * Infundibulum , a downward extension of the diencephalon. This gives rise to the posterior lobe of the pituitary gland ( Neurohypophysis ) composed of neuroglial cells. It also contains nerve fibres from the hypothalamic area. * Rathke’s pouch , an ectodermal out-pocketing of the primitive oral cavity ( stomodeum ). The pouch appears by the 3 rd week in front of the oropharyngeal membrane. By the end of 2 nd month the pouch loses connection with the oral cavity. And then go in close contact with the infundibulum . 10

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12. Pituitary Gland or Hypophysis . Later the cells of the anterior wall of Rathke’s pouch form the anterior lobe of the anterior pituitary ( adenohypophysis ). Pars tuberalis , a small extension of this lobe, grows to surround the stalk of the infundibulum . Pars intermedia is formed by the posterior wall of the Rathke’s pouch. This has little significance in humans. 12

13. Development of Telencephalon . Introduction Cortex development Commissures 13

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15. Introduction The most rostral of the brain vessicles . Consists of two lateral outpocketings (cerebral hemispheres) and a median portion (lamina terminales ). Lateral ventricles are the cavities of the hemispheres. - They communicate with the lumen of the diencephalon through the interventricular foramina of the Monro . 15

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17. Cerebral hemispheres Form as bilateral evaginations of the lateral wall of the prospencephalon . This is by the beginning of the 5 th week. By the middle of the 8 th week (2 nd month): the basal part of the hemisphere grows and bulges into the lateral ventricle and the floor of the foramen of Monro . This rapidly growing region is called corpus striatum because it has a striated appearance on sectioning. 17

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19. Cerebral Hemispheres. The region where the wall of the hemisphere is attached to the roof of the diencephalon is very thin. Here the wall fails to develop neuroblasts . - But consists of a single layer of ependymal cells covered by vascular mesenchyme . The two layers form the choroid plexus in each of the lateral ventricles. Each plexus protrudes into the lateral ventricle along the choroid fissure instead of forming in the roof of each hemisphere. This is due to disproportionate growth of the various parts of the hemispheres. 19

20. Cerebral Hemispheres. Hippocampus This is formed by the thickening of the wall of the hemisphere immedialtely above the choroid fissure . It bulges into the lateral ventricle . Olfacation is its primary function. 20

21. Cerebral hemispheres Lobes of the cerebral hemispheres. The hemispheres expand to cover the diencephalon, mesencephalon , laterally and metencephalon , rostrally . The hemispheres grow & expand rapidly - Anteriorly – Frontal lobe . Posteriorly – Occipital lobe . Inferiorly - Temporal lobe . Superiorly – Parietal lobe . 21

22. Lobes of the cerebral hemisphere 22

23. Cerebral Hemispheres. Insula Formed by the depressed area between the frontal and temporal lobes. This is caused by slow growth in the region overlying the corpus striatum. The area is completely covered by the adjacent lobes at the time of birth. Gyri / fissures / sulci : by the end of fetal life - The surface of the cerebral hemispheres grows rapidly forming so many convolutions called gyri . Fissures and sulci separate the gyri . 23

24. Caudate Nucleus/ Lentiform Nucleus. - Corpus striatum expands posteriorly as a part of the wall of the hemisphere. The corpus striatum is divided into two parts by the internal capsule : Caudate nucleus and Lentiform nucleus ( putamen and globus pallidus ). These two nuclei are part of a collection of nuclei called the basal ganglia . 24

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26. Caudate Nucleus/ Lentiform Nucleus. These ganglia are well connected with the cerebral cortex, thalamus and the brain stem. They play a major role in controlling voluntary movements, routine behaviour, cognition, emotion, procedural learning and eye movements. The internal capsule is a bundle of axons passing to and from the cortex of the hemisphere. The axons break through the nuclear mass of corpus striatum . 26

27. Development of the Cortex. Cerebral cortex develops from the pallium . Pallium refers to the layers of grey and white matter covering the upper surface of the cerebrum in vertebrates. The pallium has three regions: . Paleopallium ( paleocortex ). . Archipallium ( archicortex ). . Neopallium ( neocortex ). 27

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29. Development of the Cortex. Neopallium The newest part of the cortex phylogenetically . Comprises 90% of the cortical structure. In this region, waves of neuroblasts migrate to the subpial position where they differentiate into mature neurons. Subsequent waves migrate through the earlier formed layers of the cells until the subpial position. 29

30. Development of the Cortex. Archipallium and Paleopallium Evolutionary, the archipallium is associated with the olfactory cortex . It forms the hippocampus and paleopallium . These are the oldest part of the cortex. Differentiation of the olfactory system is dependent on epithelial- mesenchymal interactions. 30

31. Development of the cortex These interactions occur between: Neural crest cells and ectoderm of the frontonasal prominence to form the olfactory placodes . Neural crest cells and the paleopalial region ( paleopallium ) to form the olfactory bulbs . - Cells in the nasal placodes differentiate into the primary sensory neurons of the nasal epithelium. 31

32. Achipallium and Paleopallium The axons of the primary neurons make contact with the secondary neurons in the developing olfactory bulbs. These contacts are well established by the seventh week of development. The axons of the olfactory nerves originate from the cell bodies in the olfactory epithelium . The axons terminate at the olfactory bulb as they converge with the dendrites of mitral and tufted cells . Mitral and tufted cells are in small clusters called gromeruli . 32

33. Commissures These are nerve fibre bundles that connect one cerebral hemisphere to the other. Initially each cerebral hemisphere grows as a separate structure. By the end of the third month , collections of nerve fibres ( commissures ) grow across connecting the hemispheres. Many commissures make use of the lamina terminalis . 33

34. Lamina Terminalis . Lamina terminalis . - This is a median portion of the telencephalon It extends from the roof plate of the diencephalon to the optic chiasma . It provides the initial connection between the cerebral hemispheres. Many commissures make use of the lamina terminalis . 34

35. Commissures . Anterior commissure This is the first of the crossing bundles to appear in the lamina terminalis . It consists of nerve fibres connecting olfactory bulb and related brain arears of one hemisphere to another. Hippocampal commissure ( fornix ). It is the second commissure to appear. 35

36. Hippocampal Commissures The nerve fibres arise in the hippocampus and converge in the lamina terminalis . The fibres are close to the roof plate of the diencephalon. From the lamina terminalis they form the arching system immediately outside the choroid fissure . The fibres go to mamillary body and the hypothalamus. 36

37. Commissures . Corpus callosum . This another important commissure that appears by the 10 th week of development It connects the non-olfactory arears of the right and left cerebral cortices. It starts as a small bundle in the lamina terminalis The bundle increases due to continuous expansion of the neopallium . It extends first anteriorly and then posteriorly , arching over the the roof of the diencephalon. * Three other commissures appear outside the lamina terminalis . 37

38. Commissures . Posterior commissure . Habenular commisssure . The two are just below and rostral to the stalk of the pineal gland . Optic chiasma . This appears in the rostral wall of the diencephalon. It contains nerve fibres from the medial halves of the retianae . 38

39. CLINICAL APPLICATIONS. Hypophyseal defects Pharyngeal hypophysis - a small portion of the Rathke’s pouch persisting on the roof of the pharnyx Craniopharyngiomas - a brain tumour arising from remnants of the Rathke’s pouch, usually above the sella turcica . It may cause hydrocephalus and pituitary dysfunction e.g growth failure, diabetes inspidus . ***Read Clinical Correalates of Prosencephalon . 39