72 Hr Reviewer

WHOLE MOUNT  4 flexures:

o Cranial/ cephalic flexure

o Cervical/nuchal flexure (at the level of 1st _{several somites)} o Tail flexure flexure

o Caudal flexure

 Amnion and chorion – enclosed the entire embryo

 Paired wing & lung bud – visible nut not sharply bounded  Allantois – sac-like structure encircled by the tail

STRUCTURE DESCRIPTION

TRANSVERSE SECtION Metencephalon

 Anterior part of the hindbrain w/ thicker walls than myelencephalon

 The roof undergoes enlargement cerebellum

myelencephalon  Posterior part of the hindbrain

 Thin roof plate lying freely within the cavity

roof of the medulla & adjacent skin

ectoderm

Neuromeres

 Segmented enlargements in the walls of the myelecephalon & metencephalon

 11 neuromeres  Forebrain: 1st_{, 2}nd_{, 3}rd  Midbrain: 4th _{and 5}th  Hindbrain: 6th _{– 11}th Posterior choroid plexus

 Thin roof of myelencephalon which is folded in the myelocoell

 Where vascularization occurs

Endolymphatic duct  Small, thick-walled oval structure that appears before the auditory vesicle (forerunner)

Auditory vesicles

syn: otic vesicle otocyts

 Located at the level of 10th _{neuromere on each} side of myelencephalon

 Bigger than endolymphatic duct Acoustic-facialis

ganglia

 Ganglia of VII (facial) & VIII (auditory) nerves  Mass of cells lying against the cranial wall of

auditory vesicles

Spinal cord Central canal  Central cavity of spinal cord

Roof plate  Thin mid-dorsal wall of the spinal cord Floor plate  Mid-ventral wall of the spinal cord

Sulcus limitans  Lateral depression of the central canal about midway between the floor and roof plates Alar plates  Dorsolateral wings of the spinal cord

Basal plates  Ventrolateral wings of the spinal cord  Between sulcus limitans & floor plate Marginal layer  Outer region of the neural tube

 Composed of several fibers & fewer nuclei Ependymal layer

 Single, innermost layer of cells  Adjacent to the central canal  Cells capable of mitosis

Mantle layer  Wide heavily nucleated layer between marginal & ependymal

Semilunar ganglion

syn: gasserian ganglion

 Large mass anterior to the acoustic-facialis ganglion

 Subdivide into 3 branches

(branches of trigeminal nerve):

located w/in 1st _{branchial arch}

Maxillary branch - medial branch

Ophthalmic branch – lateral branch closest to mesencephalon

Mandibular branch – lateral branch above the ophthalmic branch

Superior ganglion

 Very small rounded accumulation of cells at the caudal side of auditory vesicle

 Glossopharyngeal nerves into 3rd _{branchial arch} Somite  Cell mass lying immediately lateral to

myelencephalon Mesencephalon

 Small rounded region beneath the metencephalon & mesocoel

 Becomes continuous w/ metencephalon

mesocoel  Cavity of mesencephalon Cerebral aqueduct/

Aqueduct of sylvius Oculomotor nerve

(III)

 Found @ mesencephalon

 Nerve fibers from the neural ectodermal cells appearing as a pair of streaks from the floor of mesencephalon

 Will innervate 4 pairs of extrinsic eye muscles Anterior cardinal

vein

syn: precardinal vein

 Found laterally situated in the mesenchyme of the floor of myelencephalon

notochord  Found between 2 regions of hindbrains or between spinal cord & hindbrain

FOREGUT AND ASSOCIATED STRUCTURES Closing plates  Double layered membranes formed by

of pharyngeal furrows Pharyngeal furrows

 Result of invagination of pharyngeal pouches that are projected lateral from pharynx to contact the adjacent region of skin ectoderm 1st _pharyngeal

furrow and pouch

 Endodermal evagination which soon become continuous with pharynx & open to the outside via 1st _{pharyngeal furrow}

 Furrow – V-shaped ectodermal indentation 2nd _pharyngeal

furrow and pouch

 Similar to 1st _{pharyngeal furrow & pouch but} towards spinal cord

Visceral arches

 Found between the adjacent pharyngeal furrows

 Greatly thickened & filled w/ mesenchymal cells Mandibular arch  Mass of mesenchyme anterior to the 1st furrow

(towards mesencephalon) jaws

Maxillary arch

 Lobe-like mass of mesenchyme anterior to stomodeum

 Found where mesencephalon is

Hyoid arch  Mesenchyme between 1st (hyomandibular) pouch & furrow and 2nd _furrow

Symphysis of the lower jaw

 2 arches fuse ventrally (posterior of mandibular arch)

Dorsal aorta

 Paired structures w/c are cut frontally (foregut)  Appear constricted into 2 parts of the level of 1st

pharyngeal pouch

1st _{aortic arch  Lies within mandibular process} 2nd_{, 3}rd_{, & 4}th _aortic

arches

 Extend from the dorsal aortae into 2nd_{, 3}rd_{, & 4}th branchial arches

Ventral aorta syn: aortic sac

 Continuous with 1st _{aortic arches (separate} vessels)

 Ventral ends of 2nd _{& 3}rd _{also unite in ventral} aorta

Descending aorta _ Result from the fusion of paired aortae Posterior to 3rd _{pharyngeal pouch} Internal carotid

artery

 Narrower vessel arises as a cranial extension of dorsal aorta

 Located medial to ant cardinal vein Jugular ganglion

 Derived from group of neural crest cells  Lying posterior to IX nerve

 Fades out w/in 4th _{branchial arch}

Thyroid rudiments  Arise from the floor of pharynx as a median diverticula/ depression bet. 1st _{and 2}nd _{pair of}

pharyngeal pouches Oral plate

syn:pharyngeal membrane

 Constituted by apposition of pharyngeal endoderm & stomodeal ectoderm

 Separates stomodeum from foregut  Ruptures to form mouth opening

Pre-oral gut _ Located cranial to the pharyngeal membrane Less conspicuous after rupture of oral plate stomodeum  Slitlike space lined by ectoderm

 Lies bet 2 visceral masses next to foregut Rathke’s pouch

 Cavity lined with cuboidal epithelium opens in more posterior sections into the stomodeum  Serves as rudiment of ant. pituitary gland

Ant. pituitary gland Laryngotracheal

groove

 V-shaped ventralmost part of foregut (post.)  Seen at the level of 4th _{pharyngeal pouch} Trachea  Arises with the lung buds as ventrocaudal

diverticulum of pharynx Lung buds

 Paired bilateral expansion of laryngotracheal groove

 Doughnut-like structure (when cut off from laryngotracheal groove)

bronchi

Pleural cavities  Coelomic cavities lateral to lung buds  Continuous w/ pericardial cavities Esophagus

 Rounded part of foregut

 Contained w/in a thick mesentery of splanchnic mesoderm

Mediastinum

 Whole mesenchymal mass where esophagus & lung buds are embedded

 Includes dorsal mesentery (mesoesophagus) and ventral mesentery (dorsal mesocardium) Bulbus cordis

syn: conus arteriosus

 Endocardium & myocardium (present) indicates the beginning of bulbus cordis

Atrium

 Large chamber lying to the left and caudally to bulbus cordis

 Has blood corpuscles in it Sinus venosus

 On the dorsal part of atrium

 Attached to foregut by dorsal mesocardium  Separated from atrium by slight constriction  Continuous with common cardinal vein Ventricle  Regions of heart where left and right fused

Trabeculae  Irregular projections of myocardium  Numerous in ventricular walls Pericardial cavity  Coelom surrounding the heart Common cardinal

vein

syn:duct of Cuvier

 Found continuous to sinus venosus  Lower vessel

Pleuropericardial membrane

syn: lateral mesocardium

 Bridge of mesenchyme through which common cardinal vein passes

Transverse septum

 Mesenchyme incompletely separates the pericardial cavity from peritoneal cavity  Encloses the veins that enter the heart Peritoneal cavity  Body cavity posterior to transverse septum

 Continuous w/ pleural and pericardial cavity Diencephalon  Replaces the mesencephalon

 Laterally compressed w/ thin walls Infundibulum  Posterior to oculomotor nerves

 Long ventral outpocketing of diencephalon Optic cup  Formed by invagination of optic vesicles

Retina

 Inner layer of optic cup  Differentiates into:

 Sensory retina – thick layer of optic cup next to lens vesicle

 Pigmented retina – thin external layer of optic cup

Optic fissure

syn: choroid fissure

 Formed by ventral invagination of optic cups & optic stalks

 Seen as ventral gap in optic cup

Lens vesicle

 Lens become freed from superficial skin ectoderm

 2 regions:

 Elongated & centrally located – will form lens fibers

 Outer & narrower cells – will become lens epithelium

corneal epithelium

Corneal epithelium  Single layer of epithelial tissue covering the lens of the eye

Layer: sclera & choroid Optic stalk  Connects optic ups with diencephalon

Postcardinal vein  Lies dorsal to mesonephros

Intersegmental vein  Appear as extensions of postcardinal vein  Drain blood from the dorsal organs like somites,

notochord, neural tube

Stomach  Lies medial to the pleural cavities Dorsal mesogaster  Mesentery dorsal to the stomach

Hepatogastric ligament

syn:ventral mesogaster

 Mesentery ventral to stomach

Ductus venosus _ Has become of sinus venosus Liver diverticula are embedded in it Anterior liver

diverticulum

syn: cranial liver diverticulum

 Lying above the ductus venosus

duodenum  Seen when ant. liver diverticulum becomes continuous with foregut

Mesoduodenum  Mesentery lying dorsal to duodenum Hepatoduodenal

ligament

syn:duodenohepatic ligament

 Mesentery lying ventral to duodenum

Posterior liver diverticulum syn: caudal liver

diverticulum

 Lies beneath the ductus venosus & connecting duodenum

Pronephros  Small tubes found ventrolateral to the dorsal aorta

Allantoic veins

syn:umbilical vein  Drains blood from allantois

Telencephalon

 Telencephalic vesicles – oval-shaped evagination

 Cavity of telencephalon:

 Median telocoel (ventricle III) – connects posteriorly w/ diocoele

 2 lateral diocoele (ventricles 1 & 2) – connect with the median telocoel thru foramen of Monro

Cerebral hemispheres

Nasal placodes

syn: olfactory placode

 Thickened layer from skin ectoderm lateral to telencephalon

Nasal pit

syn: olfactory pit

 Ectodermal invagination on each side of

telencephalon External nares

Vitelline veins

syn:

omphalomesenteric vein

 2 big blood vessel on each side of cranial liver dicerticulum

 Level of gut Cranial intestinal

portal

syn: anterior intestinal portal

 Level of duodenum

 Ventral to ductus venosus

Wing buds

 Caudal to cranial intestinal portal

 Appear as conical bulges on each side of body  Composed of somatic mesoderm covered by

skin ectoderm

 Thickened as apical ectodermal ridge Intersegmented

arteries  Small blood vessels continuous w/ dorsal aorta

UROGENITAL SYSTEM

Mesonephric duct syn: Wolffian duct

 Tiny duct w/ thin wall on each side

 Level of wing buds

 by cavitation of mesonephric duct rudiment

Mesonephric tubules

 Paired S-shaped tubules medial to mesonephric

duct Mesonephric kidneys

Mesonephric ridge  Rounded bulge of dorsal body wall into peritoneal

cavity containing mesonephros

Subcardinal veins  Small blood vessel lying beneath each

mesonephric kidney

Leg buds  Level of the posterior intestinal portal

Tail bud _ Remnants of primitive streak and Hensen’s node_{Large mass of undifferentiated cells}

Allantois

 Dark mass w/ large irregular sacculation located

to the right of hindgut in extraembryonic coelom

 Formed from splachnopleure

 Store nitrogenous waste product

 Serves as embryonic respiratory organs for birds

Chorio-allantoic membrane

 Lies beneath the inner shell membrane

 For respiration & absorption of calcium for shell

Yolk sac

 Formed from splanchnopleure that overgrows

yolk

transported to developing embryo via vitelline veins

Chorion

 Outer wall of each amniotic fold

 Ectoderm on outside w/ adjacent layer of

mesoderm inside

Amnion

 Inner wall of each amniotic fold

 Consist of somatic mesoderm on outside and

ectoderm on inside

Extraembryonic coelom

syn: exocoel

 Somatic mesoderm lined cavity within each

amniotic fold

 Continuous with intraembryonic coelom

Hindgut  Endoderm line cavity

 Posterior region of the gut

Lateral body fold  Forms the boundary bet. Embryonic &

extraembryonic regions

cloaca  Posterior portion of hindgut w/c continuous

ventrally w/ allantois & mesonephric ducts

Cloacal membrane

syn: anal plate

 Double-layered membrane

 Ventral endoderm of cloaca fuses w/ ventral skin

ectoderm

Upon rupture: anal opening

Caudal intestinal portal

syn: posterior intestinal portal

 Opening formed from hindgut into the