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EMBRYO. LAB. Organogenesis (Part 3 – 7MM FROG) WHOLE MOUNT Distinct Characteristics:                

Well-formed external gills Functional heart Embryo is a tadpole Much elongated tail (swimming organ) Segmented somites (lateral) Finfolds (dorsal and ventral) Forebrain / Prosencephalon: Telencephalon (w/ 2 hemispheres) and Diencephalon Deeply invaginated stomodeum Pigmented olfactory pit Epiphysis formed by a circular knob of cells Notochord reaches tail Bloated head and trunk Identifiable liver diverticulum Hindgut cavity persists as cloaca Hindgut dorsal wall becomes post anal gut (later disappears) Urinary bladder (ventral evagination of hindgut)

(TRANSVERSE SECTION – Level of Telencephalon & Olfactory Pits)   

Both olfactory pits visible Olfactory pits develop into nasal passages Embryo with open mouth

1) Telencephalon – paired hemispheres @ anterior part of forebrain 2) Diencephalon – posterior part of prosencephalon/forebrain 3) Olfactory Pit – cavity @ lateral surface of head 4) Epiphysis – slight middorsal evagination of brain vesicle - future pineal body 5) Head Mesenchyme – mesenchymal cells between head ectoderm and brain

(TRANSVERSE SECTION – Level of Telencephalon & Olfactory Pits)

1) Diencephalon – posterior half of forebrain - deep, laterally compressed region - where optic stalks, infundibulum, & epiphysis attach

2) Mesencephalon – part of brain posterior to eye - center for reflexes (vision, hearing, head movement) 3) Optic Cup – w/ outer pigmented layer, inner retinal - lens vesicle @ its concavity 4) Pharynx – now large, more rounded - mass of cells on its sides gives rise to mandibular arch (posterior border of stomodeum) 5) Stomodeum – deep invagination of pigmented, midventral ectoderm of anterior end of pharynx 6) Adhesive Glands – paired ectodermal thickenings - @ ventral surface of head - contains adhesive mucus (for attachment to floating objects) - a.k.a. Cement glands / Mucous glands / Oral suckers

(TRANSVERSE SECTION – Level of Rhombencephalon – Thru Thyroid)

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W/ developing cranial nerve ganglion (derived from neural crest cells)

1) Pharynx – its width greatly exceeds its height 2) Thyroid – evagination of endodermal cells (TRANSVERSE SECTION – Level of Rhombencephalon – Thru Otic Vesicle)

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Roof of rhombencephalon: Single layer of flattened cells

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Truncus arteriosus – beneath pharynx; anterior part of heart - w/in pericardial coelom - gives rise to aorta and pulmonary trunk

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Heart subdivisions (anterior to posterior): Truncus arteriosus, ventricle, atrium, sinus venosus

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Blood flow: Posterior (sinus venosus) to anterior (truncus arteriosus)

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Both otic capsules visible (OCs are ectodermal in origin; becomes the inner ear)

1) Rhombencephalon – expands anteriorly, forming the IV Ventricle - internally similar to spinal cord - later differentiates into metencephalon (anterior) and myelencephalon (posterior) 2) Notochord – round structure - mesodermal in origin - dorsal to gut, ventral to hindbrain - defines anterior/posterior axis - for skeletal support 3) Otic Vesicle – irregularly hollow organ - found on each side of hindbrain - closed chamber formed by invagination of otic placode 4) Heart – beneath pharynx 5) Bulbus Cordis – most anterior heart chamber - (w/ the truncus arteriosus) gives rise to aorta and pulmonary trunk 6) Pericardial Coelom – cavity surrounding the heart - bounded by pericardium

(Heart of human embryo)

(TRANSVERSE SECTION – Level of Rhombencephalon – Thru the Heart)

1) External Gills – finger-like projections protruding from sides of head - filamentous respiratory organ - arises from branchial arches 3 to 6 - replaced by internal gills 2) Esophageal Plug – mass of cells - temporarily blocks esophagus 3) Atrium – receives blood from sinus venosus, delivers to ventricle - seen above the ventricle

(TRANSVERSE SECTION – Level of Spinal Cord – Thru Pronephros and Midgut) Rhombencephalon is decreasing in height and width (thin-walled roof also decreases in width) Rhombencephalon tapers into spinal cord Developing gut is coiled Frog tadpole is a filter feeder (w/ long intestine)

1) Glomus – 2 triangular-shaped structures - ventral to dorsal aorta - hang down into the coelomic cavity - tufts of small blood vessel - functional components of pronephric kidney - diffuse waste products into the coelomic fluid 2) Spinal Cord – from posterior region of neural tube 3) Somite – segmented mesodermal blocks - located on either side of developing spinal cord - differentiates into sclerotome, myotome, dermatome 4) Dorsal aorta – primitive, paired longitudinal arteries of trunk - beneath notochord, anterior to gut 5) Pronephros – initial excretory organ - Waste path: Pronephric tubules pronephric ducts - functional in larval amphibians and fish - later replaced by mesonephric kidney - spherical structures on each side of body segment

hindgut (cloaca)

exterior

6) Nephrostomes – funnel-shaped opening of pronephric tubules - where coelomic fluid is swept 7) Midgut – w/ small lumen and thick, yolky floor - derived from archenterons - gives rise to small intestine

(TRANSVERSE SECTION – Level of Spinal Cord – Thru Cloaca) 1) Dorsal Fin – extension of body wall - @ mid-dorsal side of trunk and tail - later degenerates 2) Cloaca – most posterior chamber of the digestive system 3) Proctodeum – ectodermal invagination - @ ventral side of trunk, @ base of tail - gives rise to anus