Developmental Biology - Biology 4361 Lecture 13 - Ectodermal Organs November 22, 2005

Transcription

1 Developmental Biology - Biology 4361 Lecture 13 - Ectodermal Organs November 22, 2005 Neural tube development - central nervous system (CNS) - brain, spinal cord - peripheral nervous system (everything outside CNS) - humans (shortly after birth) ~100 X 10 9 neurons - relatively little new neuronal development post-neonatal stage - however, recent evidence shows neural development in the adult human brain - rate of neuron formation low Basic cellular aspects of CNS - neural tube wall - neuroepithelial cells - pseudo-stratified columnar epithelium - all neuroepithelial cells extend from luminal surface to outer surface, but have nuclei at different levels - gives the superficial impression of different cell layers - surrounded on outside by external limiting membrane (basement membrane - see ECM section below) - apical face on luminal side - tight junctions form seal - neuroepithelial cells are stem cells: - self-renewing - give rise to committed progenitor cells - neuroblasts - neuron precursors - glioblasts - glial cell precursors - neuroepithelial cells divide vertically rather than horizontally - one daughter cell will remain in the luminal area of the neural tube; remains a stem cell - the other will migrate outward; differentiate - neuroblasts in neural tube migrate toward external limiting membrane - form mantle layer - thick layer of non-dividing cells (gray matter; see below) - neuroblasts sprout neurites - cytoplasmic extensions - forms marginal layer of myelinated axons of the neurons (white matter) - axon - longest neurite - axon transmits signals away from body of neuron to muscle cell or other neuron - dendrites ( shorter neurites) transmit signals toward body of neuron 1

2 - neuroepethelial cells also produce glioblasts - glioblasts produce glial cells - support neurons - insulate (myelinate) axons - also have nutritive function - glioblast derivatives - oligodendrocytes - wrap around axons; create insulating layer - myelin sheath - astocytes - with capillary endothelial cells, create blood-brain barrier - prevents passage of large molecules from blood to CNS - microglia - derived from mesencyhmal cells - neurons + glial cells = nervous tissue - neuron bodies, unmyelinated axons and dendrites = gray matter - neuron axons directed outwards - myelinated axons = white matter - neuroepithelium cells near neural tube lumen form ependymal layer (more later) Spinal cord patterning (neurulation and histogenesis of the spinal cord) - accumulating neuroblasts and glioblasts form dorsal an ventral ridges of gray matter along sides of neuroepithelium - dorsal - alar plates (later dorsal columns; afferent columns) - afferent - sensory neurons; incoming signals - ventral - basal plates (later ventral columns; efferent columns) - motor neurons; outgoing signals - roof plate - most dorsal neuroepithelium; between alar plates - floor plate - most ventral neuroepithelium; between basal plates - cells in roof and floor plates = glial - floor plate cells promote and orient growth of commissural axons - originate from neurons in dorsal column - end in neurons in ventral column of opposite side - function to connect afferent and efferent signaling centers Establishment of dorsoventral pattern in spinal cord: Ventral - notochord grafting experiments established: notochord sufficient and necessary for 2

3 production of floor plate and efferent columns - floor plate grafting experiments: floor plate also sufficient for production of efferent columns - immunostaining suggested that signals from notochord/floor plate shift expression patterns of transcription factors in ventral spinal cord - e.g. Pax3 + expression inhibited - NOTE - chordamesoderm also has similar, but earlier effect - morphogen gradient? - candidate: Shh (sonic hedgehog) - Shh expressed in chordamesoderm and notochord - also in floor plate after notochord separation - Shh necessary and sufficient for floor plate induction - Shh -/- mice lack floor plate - notochord cannot induce floor plates with immunoblocked Shh - Shh expressed twice - early - neural plate ventralized - later - Shh drives differentiation of ventralized precursors into different neuron types - Shh - secreted glycoprotein - Shh diffuses; creates morphogen gradient - induces neuronal structure in concentration-dependent manner - relationship between Shh, Notch, Delta - unknown Dorsal - signals from adjacent epidermal ectoderm - isolated neural plates acquire characteristic gene expression only in contact with future epidermis - BMPs synthesized in epidermal domains - Wnt family also synthesized in dorsal part of neural tube - forms neural crest Spinal cord summary: - pattern specified by antagonistic signals derived from adjacent tissues - ventral signals - from chordamesoderm, notochord, floor plate - dorsal signals - from epidermal ectoderm, roof plate, dorsal spinal cord - growth: both white and gray matter increase in thickness - neuroepithelium mitosis decreases - ependymal layer surrounds lumen of spinal cord (central canal) - outside spinal cord: mesenchymal cells form meninges Brain [NOTE - you should know the basic architecture of the brain; how the different sections form 3

4 through development, and the major functions of the different brain sections and structures. Refer to your lecture notes and use the following notes or the text to fill in sections that you have questions about.] - brain components are the same as spinal cord components; however, more complex structure - neurons organized into layers and clusters - mantle cells (neurons) move out past marginal layer -forming gray matter (cortex) on outside (in certain regions - e.g. forebrain) - neurons will form several (6) distinct layers - clusters of mantle cells move into marginal layer = nuclei (sets of neurons with distinct functions) Major regions: - early (4 wk) - bulges in cephalic end of neural tube - prosencephalon (forebrain) - mesencephalon (midbrain) - rhombencephalon (hindbrain) - 5 wk - prosencephalon split into - telencephalon - cerebrum (2 hemispheres) - diencephalon - optic vesicles - central canal expanded into ventricles in telencephalon and diencephalon - ventricles 1 & 2 - telencephalon; 3 - diencephalon - mesencephalon contains aqueduct of Sylvius - connects ventricles 3 and 4 - rhombencephalon subdivides - metencephalon (anterior and dorsal) - myelencephelon (posterior and ventral) - surrounds 4 th ventricle - 4 th ventricle communicates and blends with central canal - 7 wk - myelencephalon becomes medulla (medulla oblongata) - controls reflexes in neck, throat, tongue - similar to spinal cord control of reflexes in trunk, appendages - lateral walls splayed; neural canal expands into 4 th ventricle - basal plate - efferent neurons - alar plate - afferent neurons - roof plate - layer of ependymal cells - in contact with meninges - ependymal cells plus blood vessels in meninges form choroid plexus - produces cerebrospinal fluid; filters into 4 th ventricle - dorsal metencephalon becomes cerebellum - arises from extensions of alar plate 4

5 - forms cerebellar plate anterior to 4 th ventricle - cerebellar plate : neuroepithelium, mantle layer, marginal layer - neuroepithelial cells migrate into marginal layer - form external granular layer - neuroepithelium also forms Purkinje cells - one axon; many dendrites - thousands of synapses (connections) -Note - Purkinje cells secrete sonic hedgehog; sustains development of the granular layer - granular layer and Purkinje cells form cortex of cerebellum - coordination center for posture and movement - ventral metencephalon forms pons - pathway for nerve fibers between spinal core and cerebellum - also between cerebral hemispheres - mesencephalon - similar morphology to spinal cord - marginal layer enlarges ventrally to accommodate nerve fibers connecting cerebral cortex with pons and spinal cord - alar plates initially form two ridges - subdivided by transverse grooves (sulcus) - forms four elevations: colliculi - mammals: anterior, posterior colliculi - relay stations for visual and auditory reflexes - on-mammalian vertebrates anterior colliculi = optic lobes - diencephalon - two alar planes delimiting narrow 3 rd ventricle; lacks basal plates - dorsal roof of 3 rd ventricle formed by choroid plexus - pineal gland caudal to choroid plexus - circadian rhythms - yearly reproductive cycle - photoreceptive - lateral groove on inside of each alar plate delimits thalamus (dorsal portion), hypothalamus (ventral portion) - thalamus - antechamber of cerebrum: gateway for sensory fibers passing from spinal cord and brain stem to cerebral hemispheres - hypothalamus: nuclei = regulatory centers for visceral functions; e.g. - sleep, digestion, body temperature - also emotions - optic chiasma - incomplete crossing of optic nerves; bottom of hypothalamus - infundibulum - behind optic chiasma; forms posterior lobe of pituitary (hypophysis) - anterior lobe from Rathke s pouch; invagination from stomatodeum 5

6 (ectodermal epithelium lining the anterior oral cavity) - telencephalon - cerebral hemispheres; each surrounding ventricle (1 st & 2 nd ) with choroid plexus - ventricles communicate with 3 rd ventricle - phylogenetic relationships in cerebral development: - paleopallium (old mantle); develops early in mammals; more prominent in primitive vertebrates - located anteriorly and laterally - extends into olfactory bulb (associated with sense of smell) - neopallium (neocortex) - dorsal; develops late; grows at rapid rate - humans - occupies 90% of cerebral hemispheres - neuroblasts form mantle alone migrate through white matter to generate second zone of neurons on outside of brain - gray matter eventually stratifies into six layers of neuronal cell bodies - NOTE - adult forms of these layers not completed until mid-childhood - each layer differs in function, type of neurons, connections - commissures join hemispheres; nerve fibers cross midline - anterior commissure connects olfactory bulb (and related brain areas) - corpus callosum - most extensive - forms gradually after 10 th week (humans) - connects non-olfactory portions of cerebral hemispheres - begins above septum pellucidum Neural Crest - NC cells found only in vertebrates - originate during neurulation - neural crest cells position at crests of neural folds - form at the margins of neural plate and adjoining epidermis - neural folds fuse, forming neural tube and overlying epidermis; crest cells lie between - NC cells move by migration - form cartilage elements of head - pigment cells - neurons - hormone producing gland cells - smooth muscle cells of cardiovascular system - NC cells are induced by local interactions between juxtaposed neural plate and epidermis 6

7 - NC cells undergo transition from epithelial to mesenchymal prior to migration - transition involves the activation of slug+ - NC cells exhibit different cadherens (cell adhesion molecules) before and after migration - slug expression causes dissociation of desmosomes (anchoring junctions connecting intermediate filaments of adjacent cells NC migration routes and fates 1) dorsolateral pathway - enter skin - become melanocytes or xanthophores 2) ventral pathway (in the trunk - cervical, thoracic, lumbrosacral regions): - close to neural tube - become neurons, glial cells of dorsal root ganglia - in gut region - visceral nervous system; including - autonomic nervous system - innervates all internal organs - sympathetic ganglia - cervical, thoracic, lumbar NC cells - centralized: two chains on ventral side of vertebral column - promote activities of lungs, heart, internal organs - support fight-or-flight response - parasympathetic ganglia - cephalic, pelvic NC cells - dispersed: located near target organs - innervate same organs as sympathetic nerves; promote rest and recreation - also form Schwann cells - myelinate peripheral nerves - also, adrenal medulla; releases hormones: epinephrine and norepinephrine - head region - cranial NC cells form - pigment, sensory cranial ganglia, parasympathetic ganglia, hormone-producing cells - bones, connective tissues of head - structures within eyes, ears, teeth dentine - between head & trunk - cardiac NC cells form - melanocytes - neurons - cartilage; other connective tissues - connective tissues, smooth muscle cells of large blood vessels that emerge from heart; septum that separates aorta from pulmonary artery Determination of NC cells - transplantation experiments - potential to form cranial cartilage is limited to head NC 7

8 region - formation of cardiovascular structures limited to cardiac NC - however, potential of cranial NC cells not limited to their fate - e.g. transplant to trunk region - contribute to dorsal root ganglia, sympathetic ganglia, adrenal medulla, Schwann cells - two major hypothesis for determination of NC cells 1) pluripotency hypothesis - each individual cell has potential to form many or all NC derivatives 2) selection hypothesis - NC cells are determined early; environmental signals allow only appropriate cells to enter certain pathways; divide and differentiate - test different hypotheses by clonal analysis - in vitro - isolate NC cells; allow to plate; culture to form clones - several studies showed at least partial pluripotency from NC cells migrating from neural tube - trunk NC cells (quail) formed pigmented (melanocytes), unpigmented (neurons or adrenal medulla), mixed cell populations from same clone - cephalic NC cells (chicken, quail) - clones formed cartilage, cholinergic neurons, adrenergic cells, glial cells, melanocytes NOTE - not all theoretical combinations of phenotypes observed - therefore, stepwise determination taking place (i.e. NC cells had gone through some determinative steps before culturing) - in vivo - label single cells (e.g. fluorescent dye) - premigratory NC cell clones: dorsal root neurons, sympathetic neurons, Schwann cells, adrenal medulla or pigment cells - migratory NC cells: more than one cell type - both in vitro and in vivo results support pluripotency hypothesis: individual NC cells have the ability to form multiple cell types - however, there is support for selection hypothesis also Conclusion: NC determination is probably mixture of both pluripotency and selection Spatial restrictions to NC cell migration - communication between NC cells during migration 8

9 - NC cells follow specific migration routes - ventral - dorso-lateral - avoid area around notochord (inhibitory signals? - chondroitin sulfatecontaining glycoprotein) - NC cells taking ventral pathway pass through anterior halves of somites; avoid posterior halves (NOTE - NC cells migrate through existing cellular matrix - i.e. cells + extracellular material) Temporal restrictions to NC cell migration - chicken trunk NC cells enter ventral pathway first; dorsolateral pathway - 24 h - heterochronic transplantation studies: - NC cells harvested from NT; cultured and aged = early and late - old (aged 24 h +) transplanted to young host areas (i.e. NC cells just beginning to use ventral pathway) grafted cells utilized both dorsolateral and ventral pathways immediately - host cells waited 24 h before entering dorsolateral pathways - young NC cells (aged 12 h) transplanted into old host areas (i.e. resident NC cells already using both pathways) no graft cell entered dorsolateral pathways - NC cells develop migratory abilities over time - develop bias over time - NC cells detaching early from neural tube begin differentiation as melanocytes before reaching migration staging area - take dorsal pathway Ectodermal Placodes placodes - patches of columnar epithelium in an area of squamous area cells - two forms of ectodermal placodes 1) epibranchial placodes - with NC cells from head region form sensory ganglia of cranial nerves 2) dorsolateral placodes - contribute to cranial sensory ganglia - parts of eye, ear, nose otic placode - forms inner ear - first ectodermal placode to develop - induced by underlying mesoderm and rhombencephalon (chicken, amphibian) - human - otic placode appears during 3 rd week; both sides of rhombencephalon - 4 th wk placode invaginates, forms otic pit; pinched off as otic vesicle - median side - statoacoustic ganglion - otic vesicle expands 9

10 - complex shape: e.g. labyrinth - labyrinth composed of squamous and columnar epithelium - columnar cells develop into sensory epithelia - receive mechanical stimuli from sound, gravity, body movement; transmit them to neurons of statoacoustic ganglion - sound perception - cochlea - sounds transmitted to inner ear via membranous window by bones located in middle ear - semicircular canals register inertia of internal fluid to rotary accelerations of body in three dimensions of space - remainder of inner ear registers direction fo gravity an linear accelerations - labyrinth surrounded by mesenchymal cells - produce cartilage capsule - cartilage replaced by bone lens placode (and retina) - lens placode formed in head ectoderm; result of inductive interactions with pharyngeal endoderm, heart mesoderm, neural crest cells, optic vesicle - lens placode invaginates; pinches off as lens vesicle - proximal cell layer elongates towards distal layer; fills lumen of vesicle - elongated cells produce lens crystallins - differentiate into lens fibers (do not divide) - lens grows as dividing outer epithelial cells transformed into inner lens fibers - optic vesicle invaginates - forms optic cup - groove in ventral surface - choroid fissure - accommodates hyaloid artery - complete closure generates final form - pupil - outer layer of optic cup = pigmented retina - inner lining = neural retina - neural retinal cells divide much like neuroepithelium in other brain area - innermost cells (facing pigmented retina) = rods and cones - next, two layers of neurons - axons converge toward optic stalk - connects optic cup with diencephalon - optic stalk becomes optic nerve nasal placode - induced by endoderm an telencephalon - humans - appear at end of 4 th wk - 5 th wk: lateral and medial nasal swellings (ridges) surround each placode - placode forms floor of nasal pit 6 th, 7 th wks maxillary swellings (form st of upper jaw) push nasal pits medially until medial nasal swellings fuse 10

11 - upper lip formed from parts of maxillary and medial nasal swellings - fusion of medial swellings extends to deeper level of face; form part to upper jaw and primary palate - nasal pits deepen until thin membrane (oronasal membrane) separates each pit from oral cavity - ruptures in oronasal membranes create primitive choanae between oral cavity and nasal pits = nasal chambers - nasal chambers elongate; combine with formation of secondary palate; displace choanae toward pharynx; = secondary choanae - original epithelium of nasal pit is now roof lining of nasal cavity - forms olfactory epithelium - sensory cells send axons directly into olfactory bulbs - stemlike extensions of telencephalon Epidermis - outer layer of skin - largest ectodermal derivative - epidermis starts one cell layer thick; divides into - peridermis (temporary outer layer) - basal (germinative) layer - stem cells; divide, renew stem cell population; create committed progenitor cells that differentiate into epidermal cells - committed progenitor cells pushed into upper layers as they mature - those that lose contact with dermis become spinous layer - epidermal cells produce massive amounts of keratin - keratin - protein that accumulates in granules - cells in spinous layer cease division; form granular layer - granular layer cells fill with keratin, die; form horny layer - horny layer cells slough off; replaced by deeper cells - dermis - mesenchymal component of skin - dermis induces overlying epidermis to form derivatives and appendages (scales, feathers, hair, glands) - hair: proliferation of epidermal cells = hair bud; penetrates into dermis - growth induced by aggregation of mesenchymal cells; form hair papilla - organ rudiment called hair follicle - blood vessels and nerve endings develop in papilla, sebaceous gland rudiment forms at side - core cells of hair follicle keratinized; form hair shaft - hair pushed outside - further proliferation of epidermal cells at base of shaft causes continuous growth of hair 11

12 - peripheral cells of follicle form epidermal hair sheath - surrounded by dermal root sheath derived from surrounding mesenchyme - smooth muscle (mesenchyme) attaches to dermal root sheath - epidermal glands (sweat, oil, sebaceous, scent, mammary) - mammary glands - from mammary ridges - 7 wk human embryo - mammary ridge extends form armpit to groin - normally only small portion persists in midthoracic region - gives rise to one pair of breasts in females, nipples in males - occasionally other segments of mammary ridge fail to degenerate; accessory nipples or breasts 12

13 Extracellular matrix (ECM; Section 11.2) - fibrous and gelatinous material released from cells - e.g. basement membrane of epithelial cells - e.g. material that surrounds mesenchymal cells - ECM consists of two components 1) amorphous ground substance - attracts water; forms gel 2) fibers - form meshwork (multiple binding domains); resists ground substance tendency to expand - ECM functions: - basement membrane - bones and teeth - calcifies - tendons - fibers increase tensile strength - cornea - forms transparent layer - influences cell division, shape, movement, differentiation - ECM helps direct cellular and epithelial movement in development - ground substance: glycosaminoglycans, proteoglycans - amorphous, hydrophilic - glycosaminoglycans form extended random coils (mutually repelling negative charges) - attract cations; water follows - fill space; allow cell migration; diffusion of water-soluble molecules - also - intracellular signaling - bind growth factors and morphogens - fibrous components - glycoproteins - form ECM meshwork - e.g. collagens (found in all metazoa) - e.g. fibronectins; dimer of two long polypeptides linked by disulfide bonds - e.g. laminins; three polypeptides arranged in cross shape; linked by disulfide bonds ECM affects determination of NC cells - NC cells extend filopodia that align with fibrils in ECM; form intimate contacts - contacts may provide signals that contribute to NC cell determination - experiment (fig. 32): - cellulose microcarriers infused with ECM from dorsolateral or ventral migration pathway - pre-migratory NC cells incubated in culture with two microcarriers - results - dorsolateral ECM exposure = melanocytes (fate) 13

14 - ventral ECM exposure = neurons (fate) - controls - no differentiation - brief transient exposure produced same results 14