Comparative Vertebrate Embryology SY 2014-2015 DEVELOPMENT OF THE FROG EMBRYO 4MM FROG EMBRYO WHOLE MOUNT characterized by the presence of tail, neural tube, notochord, and segmented mesoderm and fin folds neural system: prosencephalon – foremost part, mesencephalon – middle part, rhombencephalon – hind part epiphysis – slight mid-dorsal evagination of the brain vesicle, will become the pineal body in adult olfactory placodes thickenings on the lateral surface of the head anterior have cavities known as olfactory pits lens placode paired thickenings of the head endoderm will evaginate to form the lens vesicle, then eye lens notochord extends from the midbrain up to the posterior end of the body somites – subdivided mesoderm that flanks the notochord on each side
TRANSVERSE SECTION Level of Prosencephalon and Nasal Rudiments prosencephalon anterior part of the embryonic brain cavity is prosocoel mesenchyme embryonic connective tissue mesoderm and neural crest cell derived later forms predominantly connective tissues head mesenchyme fills the spaces between the presumptive epidermis and walls of prosencephalon olfactory placodes paired pigment invaginations on either sides of prosocoel epidermis outermost skin layer ectoderm derived Level of Mesencephalon and Optic Cups mesencephalon appears dorsal to portion of prosencephalon called infundibulum mid-region of the developing embryonic brain with thick roof mesocoel – brain cavity also known as midbrain
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Comparative Vertebrate Embryology SY 2014-2015 oral cavity cavity at the cranial end of alimentary canal more or less spherical in appearance stomodeum ectodermal rudiment of mouth formed by invagination of ectoderm at anteroventral region of head hypophysis small knot of tissue ventral to infundibulum, develops as an inward growing cord of ectoderm from stomodeum, will form anterior portion of pituitary gland solid ingrowth or nodular aggregation of cells lying between the oral and brain cavity may be used to describe: o invagination of the stomodeum which fuses with the infundibulum to form pituitary gland o endocrine gland formed from an ingrowth from the stomodeum (Rathke’s pocket) and infundibulum also known as pituitary gland infundibulum funnel-like depression of prosencephalon, part of which forms the posterior lobe of pituitary gland together with stomodeum seen as smaller, ventral component of diencephalon with thin roof and thick sides mandibular arch most cranial branchial arch forming the caudal border of stomodeum maxillary process cranial to stomodeum adhesive glands paired ectodermal thickenings found in ventral surface of anuran embryos secrete adhesive mucus for attachment to floating objects also known as cement glands, mucous glands, oral suckers optic vesicle evagination from lateral wall of prosencephalon first indication of formation of eye its walls will give rise to various ectodermal parts of eye, except lens and cornea optic cup double-walled structure formed by invagination of distal portion of optic vesicle inner layer should be thicker than outer layer optic stalk connection of optic cup with the brain Level of Rhombencephalon Section through the anterior pharynx rhombencephalon most caudal region of the brain of developing embryo with a thin roof rhombocoel – cavity also known as hindbrain
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Comparative Vertebrate Embryology SY 2014-2015 notochord round structure originating from mesoderm lying dorsal to gut and ventral to hindbrain defines the anterior/posterior axis in developing embryo provides skeletal support during early development pharynx broad region of foregut paired evagination corresponds to pharyngeal pouches otic paired invagination of otic placode laterally located and slightly ventral to brain when separated from head ectoderm, will form inner ear also known as auditory vesicle Section through the embryonic heart heart located beneath enlarged foregut appears suspended within pericardial coelom by dorsal mesocardium pericardial coelom delimited by thin layer of somatic mesoderm called pericardium (membrane enclosing the heart) pericardium – formation is brought about by migration of heart mesoderm to midventral region of pericardial cavity two layers of the heart o endocardium – inner endothelial layer of heart, will form the lining of heart wall o epimyocardium – outer layer, will form the muscle Section through the liver diverticulum mesomeres paired bulges of tissue just below the horizontal level of notochord have begun to develop pronephric tubules may be called pronephros or pronephric kidney liver diverticulum extremely deep groove at the floor of the pharynx rudiment of the liver Section through the pronephros spinal cord cavity that replaces the hindbrain smaller than the hindbrain thick lateral sides almost touches at the roof part pronephros paired structure located on lateral side of specimen appear as spherical/elongated structures clustered together initial organ found in developing vertebrate embryos functional in larval amphibians and fish degenerates and is replaced by mesonephric kidney in adults
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Comparative Vertebrate Embryology SY 2014-2015 somites segmented mesodermal blocks located on either side of spinal cord arise from dorsal mesoderm will differentiate into: o sclerotome – located above the neural tube and notochord, forms part of the axial skeleton o myotome – forms muscle o dermatome – forms the dermal layer of skin Section though the midgut spinal cord gradually tapers off in diameter form rhombencephalon to caudal end diameter approximates that of notochord subnotochordal rod small knot of cells wedged between notochord and midgut transient structure of endodermal origin unknown function, will later disappear Section through the hindgut proctodeum ectodermal invagination that meets with the endoderm of the hindgut cloacal membrane (ectodermal and endodermal plate) delicate strands of tissue between proctodeum and hindgut will become perforated to form the posterior opening of the digestive tract dorsal fin flat extension of the body wall along the dorsal midline of the trunk and tail degenerates during the metamorphosis of tadpole hindgut posterior most region of embryonic gut will form cloaca, colon, small intestine, and rectum proctodeum ectodermal invagination on the ventral side of the trunk at the base later breaks into the hindgut forming the anus also known as anal pit
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Comparative Vertebrate Embryology SY 2014-2015
7MM FROG EMBRYO WHOLE MOUNT well-formed external gills and a functional heart embryo has changed its shape and has become a tadpole tail – serves as a powerful swimming organ, provided with lateral segmented somites and finfolds on the dorsal ventral sides forebrain – has further differentiated into telencephalon (two hemispheres) and diencephalon stomodeum – deeply invaginated olfactory pit – surrounded by large pigmented cells epiphysis – formed by circular knob of cells that are separated from the brain notochord extends up to tail, head and trunk are bloated, midgut is excluded hindgut does not lose its cavity but persists as cloaca dorsal wall of hindgut becomes extended into tail rudiment as post anal gut, which later is broken down and will disappear in amphibians, hindgut gives rise to ventral evagination, urinary bladder
TRANSVERSE SECTION Level of the Telencephalon and Olfactory Pits large indentation on the ventral surface of the embryo marks the beginning of the oral cavity oral plate has become perforated embryo now has an open mouth telencephalon paired hemispheres occupying the anterior region of forebrain diencephalon posterior division of the prosencephalon olfactory pit will subsequently develop into nasal passages with olfactory receptors appears as a cavity on the lateral surface of the head epiphysis slight middorsal evagination of the brain vesicle will become the pineal body in the adult head mesenchyme loose mesenchymal cells formed between the head ectoderm and the brain
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Comparative Vertebrate Embryology SY 2014-2015 Level of the Diencephalon and Optic Cups oral cavity is visible surrounded by presumptive jaw cartilages diencephalon posterior half of forebrain appears as deep, laterally compressed region optic stalks, infundibulum, and epiphysis are attached mesencephalon part of brain located posterior to the eye center for reflexes associated with vision, hearing, and movement of head optic cup seen lateral to the brain outer pigmented layer and inner retinal portion of the optic cup can be distinguished lens vesicle – lies in its concavity pharynx large and more rounded clustered mass of cells on each side will give rise to mandibular arch (posterior border of stomodeum) stomodeum appears as deep invagination of the pigmented, midventral ectoderm at the anterior end of pharynx adhesive glands paired ectodermal thickenings found on ventral surface of head secrete adhesive mucus for attachment to floating objects also known as cement glands, mucous glands, oral suckers
Level of the Rhombencephalon Section through the thyroid below mesencephalon, small portion of the wall f infundibulum is evident lateral to floor of mesencephalon, developing cranial nerve ganglion can be observed cranial nerve ganglion – part of the peripheral nervous system, derived from neural crest cells shape of pharynx – width of pharyngeal cavity greatly exceeds the height thyroid – arises as an invagination of the endodermal cells from pharyngeal floor
Section through the otic vesicle rhombencephalon roof composed of single layer of flat cells third brain vesicle which expands anteriorly to form the IV ventricle internal organization shows an unmistakable similarity to that of the spinal cord will further differentiate into the metencephalon (anterior) and myelencephalon (posterior) notochord round structure originating from mesoderm lying dorsal to gut and ventral to hindbrain defines the anterior/posterior axis in the developing embryo provides skeletal support during early development
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Comparative Vertebrate Embryology SY 2014-2015 otic vesicle irregularly hallow organ on each side of hindbrain closed chamber formed by invagination of the otic placode will develop into inner ears heart lies beneath the pharynx truncus arteriosus – most anterior part, evident within pericardial coelom has four subdivisions (anterior-posterior): o truncus arteriosus o ventricle o atrium o sinus venosus blood flows through embryonic heart from posterior to anterior bulbus cordis most anterior heart chamber pericardial coelom cavity that surrounds the heart bounded by pericardium Section through the heart esophageal plug mass of cells that temporarily blocks the esophagus before the amphibian larva begins to feed atrium chamber of the heart that receives blood from the sinus venosus and delivers blood to the ventricle seen above the ventricle, observed as thin-walled chamber almost filled with blood external gills filamentous respiratory organ, has finger-like projections that protrude from sides of the head arises from the branchial arches 3 to 6 replaced by internal gills Level of the Spinal Cord Section through the pronephros and midgut glomus two triangular-shaped structures seen ventral to dorsal aorta that hang down into the coelomic cavity tufts of small blood vessels surrounded on their lateral and ventral surfaces by thin wall of coelom functional components of pronephric kidney where waste products are diffused from into the coelomic fluid spinal cord arises from the posterior most region of neural tube somite segmented mesodermal blocks located on either side of developing spinal cord arises from dorsal mesoderm will differentiate into: sclerotome (located above neural tube and notochord) which forms part of the axial skeleton, myotome which forms muscle, and dermatome which forms the dermal layer of skin
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Comparative Vertebrate Embryology SY 2014-2015 dorsal aorta primitive, paired longitudinal arteries of the trunk lying beneath notochord anterior to gut pronephros initial excretory organ found in developing vertebrate embryos nitrogenous wastes are passed from pronephric tubules → pronephric ducts → hindgut (cloaca) → exterior functional in larval amphibians and fish later replaced by mesonephric kidney nephrostomes funnel-shaped opening of pronephric tubules where coelomic fluid is swept midgut middle part of gut with a small lumen and having a thick, yolky floor derived from archenterons will give rise to small intestine Section through cloaca dorsal fin extension of body wall along mid-dorsal side of trunk and tail degenerates in older specimens cloaca posteriormost chamber in the vertebrate digestive system proctodeum ectodermal invagination on the ventral side of the trunk at the base of the tail will give rise to anus
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Comparative Vertebrate Embryology SY 2014-2015 10MM FROG EMBRYO WHOLE MOUNT (same as whole mount 7mm)
TRANSVERSE SECTION Level of the Telencephalon and Olfactory Organs telencephalon anterior division of the prosencephalon is paired and each unit is roughly hemispherical but flattened at midline each contains a cavity (lateral ventricle) formed by evagination of side of neural tube at the anterior end of neurocoel Layers of brain wall: ependymal layer o one-cell think, ciliated layer immediately surrounding the neurocoel o cilia – aid in movement of cerebrospinal fluid in ventricles of brain and in central canal of the spinal cord mantle layer o broad layer adjacent to ependymal layer o will form gray matter of central nervous system marginal layer o outermost layer o contains neuroblasts from inner layers and fibers o will form the white matter of central nervous system nasal organ found at the region of telencephalon and lying ventrolateral to it tubular organ formed by invagination of ectoderm olfactory nerve connecting the olfactory lobes to the brain arises from the olfactory epithelium also known as olfactory organ external naris opening of the nasal cavity to the outside
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Comparative Vertebrate Embryology SY 2014-2015 marks the point of original ectodermal invagination internal naris opening of the nasal cavity into the buccal region also known as choana frontal organ arises as an evagination of diencephalic roof together with epiphysis beneath epidermis, migrates forward from region of diencephalon to region of telencephalon contains photoreceptors and may function as a “third eye” Jacobson’s organ saccular structure formed by evagination of nasal organ may function in picking up the smell of food from buccal region also known as vomeronasal organ buccal cavity region where nasal cavity and mouth opens lined with epithelium and is derived from stomodeum jaws are tipped with horny material and tooth germs oral papillae – lobose structures external to jaws prechordal cartilage hyaline cartilage beneath telencephalon will form cartilaginous cranium (chondrocranium) also known as trabecular cartilage melanocytes stellate cells scattered over dorsolateral region of brain and lateral to nasal organs fine granules of melanin – light brown individually, black in aggregate mesenchyme stellate, mesodermal cells filling up the space between organs and epidermis form a loose reticulum, with outermost cells forming the dermis of integument epidermis outer layer of skin composed of two strata of ectodermal cells Level of the Diencephalon and the Eye diencephalon posterior subdivision of prosencephalon ventrally elongated and possesses a cavity (III ventricle) infundibulum funnel-like evagination of diencephalic floor subsequently evaginates posterior or neural lobe of pituitary together with stomodeum in more posterior sections, seen as smaller, ventral component with thin roof and thick sides mesencephalon middle region of brain dorsal to diencephalons rd th bears 3 and 4 cranial nerves
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Comparative Vertebrate Embryology SY 2014-2015 possesses a cavity known as cerebral aqueduct pituitary body oval mass beneath the thin floor of infundibulum endocrine gland derived from infundibulum and solid ingrowth from stomodeum also known as hypophysis Structures of eye: optic cup o retina – thick inner layer of optic cup, differentiated into following layers: layer of ganglian cells – innermost sublayer of retina, axons of nerve cells in this sublayer form the optic nerve, optic chiasma is the region where optic nerves cross in the floor of diencephalon layer of bipolar neurons – middle layer of cells that will synapse the receptor and ganglian cells rods and cones – outermost sublayer of retina where photoreceptoral process is formed o pigmented epithelium – outer wall of optic cup formed from medial half of optic vesicle, forms iris of the eye lens – spherical body, partly enclosed by optic cup is formed by thickenings of inner wall of lens vesicle o lens epithelium – one-cell thick outer layer o lens fibers – columnar cells at the core of lens, will become long fibers arranged in layers cornea – superficial covering of eye formed by assembly of ectodermal and mesodermal cells between ectoderm and lens choroid and sclera – outer investments of optic cup, represented by mesodermal cells aggregating outside the pigmented epithelium pharynx broad gut lined by endodermal cells hypobranchial cartilages long masses of cartilages under the floor of foregut make up parts of visceral skeleton and support the pharynx thyroid pair of small endocrine bodies associated with pharynx located beneath hypobranchial cartilages skeletal muscle mesodermal masses lying on lateral and ventral side of pharynx oral suckers pair of glandular structures on ventral surface of tadpole composed of elongated columnar cells produce a sticky slime for attachment to floating objects also known as cement glands, mucous glands, adhesive glands Level of the Myelencephalon and Auditory Vesicle myelencephalon most posterior region of brain with a thick floor (basal plates) thin roof becomes vascularized to form posterior choroids plexus cavity is the IV ventricle auditory vesicle – completely hallow organ on each side of medulla
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Comparative Vertebrate Embryology SY 2014-2015 endolymphatic duct – thick-walled tube between medulla and ear vesicle, marks the course of invagination of auditory vesicle form ectoderm utriculus – large dorsal chamber of ear vesicle semicircular canals – three mutually perpendicular folds of auditory vesicle, sensory epithelium is the thickened horizontal canal sacculus – ill-defined ventral chamber of auditory vesicle, forms the lagena in lower invertebrates, gives rise to cochlea in higher vertebrates auditory capsule mesenchymal cells surrounding the auditory vesicle will form the cartilaginous ear capsule that surrounds and protects the inner ear auditory ganglion mass of nerve cells in medial side of auditory vesicle also known as acoustic ganglion notochord round structure originating from mesoderm lying dorsal to gut and ventral to hindbrain defines the anterior/posterior axis in the developing embryo provides skeletal support during early development mesenchymal cells will give rise to notochordal sheath parachordals cartilages flanking the notochord on each side heart – lightly coiled tube twisted to the right pericardial cavity – chamber enclosing the heart conus arteriosus – most anterior region of heart connecting the ventricle with the ventral aorta, also known as bulbus cordis ventricle – heart chamber with thick muscular wall that follows and is connected to conus atrium – dorsal, thin-walled chamber that receives blood from sinus venosus and delivers it to ventricle sinus venosus – most posterior chamber lying on the right, anterior to liver, receives venous blood and delivers it to atrium opercular cavity paired chamber continuous with gut and lying on each side of the heart contains internal gills with branchial blood vessels also known as gill chamber dorsal aorta blood vessel located above each gill chamber aortic arches blood vessels lying within the branchial arches and encircling the pharynx connect the dorsal aorta with ventral aorta ganglia facial ganglion (VII) – large mass of nerve cell bodies anterior to auditory ganglion, acoustico-facialis ganglion is the body arising from fusion of facial and auditory ganglia, also known as geniculate ganglion
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Comparative Vertebrate Embryology SY 2014-2015 trigeminal ganglion (V) – larger mass of nerve cell bodies anterior and dorsal to acoustico-facialis ganglion, also known as semilunar ganglion glossopharyngeal ganglion (IX) – mass of nerve cell bodies below each auditory vesicle operculum external wall of opercular cavity formed by body fold metencephalon anterior subdivision of rhombencephalon, lies behind optic lobes and medial to V ganglion Level of the Pronephros and the First Spinal Ganglion spinal cord derived from posterior region of neural tube neural canal cavity that is laterally compressed by thick lateral walls of spinal cord ependymal cells that line the central canal possess cilia and pigment granules also known as central canal gray matter inner layer of spinal cord close to ependyma composed of compact mass of neuroblast and neuroglia white matter peripheral layer of spinal cord containing axons of neurons in gray matter meninges membranous covering of central nervous system first spinal ganglia masses of nerve cell bodies ventrolateral to spinal cord myotomes thickened primordial of skeletal muscles on each side of notochord skeletal muscle fibers are arranged longitudinally pleuroperitoneal cavity coelomic cavity containing viscera except heart pleural cavity that contains lungs and peritoneal cavity that contains digestive organs, associated glands, kidney, and reproductive organs are still continuous esophagus tubular organ with folded mucosal lining located below notochord dorsal aorta paired blood vessel between notochord and esophagus will fuse into a single blood vessel posteriorly pronephros paired excretory organs that arise from nephrotome
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Comparative Vertebrate Embryology SY 2014-2015 located at ventrolateral region of body cavity pronephric tubules ducts of pronephros lined by cuboidal epithelium posterior cardinal veins blood vessels within pronephros supplies latter with blood nephrostome opening of pronephric tubules into coelom nephric duct lone duct observed at most caudal section of pronephros moves medially and eventually joins cloaca where it empties its contents glomus two triangular-shaped structures seen ventrally to dorsal aorta that hang down into the coelomic cavity tufts of small blood vessels surrounded on their lateral and ventral surfaces by thin wall of coelom functional components of pronephric kidney where waste products are diffused from into the coelomic fluid stomach posterior continuation of esophagus with folded lining and thick muscular walls evaginations of endodermal lining form rudiments of gastric glands duodenum region of gut between the pyloric end of stomach and intestine represented in upper right corner of body cavity intestine located posterior to duodenum filled with abundant yolk platelets liver highly vascularized and enlarged organ to the right of midline sinusoids – spaces gall bladder once-cell thick, large vesicle associated with liver bile duct thick-walled tube that appears in place of gall bladder pancreas large organ within curvature of stomach located to the right of liver and bile duct identified by presence of nest cells (alveoli) surrounding small ducts
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