SEMBRYOLOGY – Its Scope, History, and Special Fields Historical Background
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Ger Germ-cel -celll line ine was all-i ll-imp mpor orttant ant for for perp perpet etua uati tion on of spec specie ies0 s0 soma soma was was primar primarily ily vehic vehicle le for prote protecti cting ng and perpetuating germ plasm
Galen - lear learne ned d abou aboutt stru struct ctur ure e of rela relati tive vely ly advanced fetuses - minu minutte dimen imens sions ons resis esiste ted d serio erious us analysis - devel evelop opme ment nt of mic microsco scope allo llowed study of early stages
Special Fields in E!ryology
de Graaf - desc descri ribe bed d ova ovari rian an fo follic llicle les s
3omparative )mbryology - .rovided insight for concept that 4ontogeny recapitulates phylogeny5 - 6ecognition of di+erent modes of development - 7dopt doptio ion n of of mod model el orga organi nism sms s
Hamm and Leeuwenhoek - rs rst saw huma uman sper perm Spermists vs Ovists - spermi ermis sts: sperm erm cont contai ain ned the new individual in miniature and was nourished in ovum - ovis ovists ts:: ovum ovum cont contai aine ned d a minu minute te bod body y stimulated to grow by seminal !uid o "onnet: discovered part partho hoge gene neti tica call lly y deve develo lope ped d insect eggs #supported ovists$ Spallan%ani and &olf - Laid to rest spermist'ovist view #preformation$ - Spal Spalla lan% n%an ani: i: demon demonst stra rate ted d that that both both male male and and fema female le se( se( prod produc ucts ts are are necessary for initiation of development - &olf: )pigenesis embryonic development occurs through progressive remodeling and growth von "aer - von von "aer "aer*s *s llaw aw:: the the mor more gene genera rall basi basic c features of any animal group appear earl earlie ierr in deve develo lopm pmen entt than than do the the speci special al featur features es that that are are pecul peculiar iar to di+erent members of the group - Germ layer theory: demonstrated e(istence of germ layers in embryos o Signicance could not be grasp grasped ed until until cellu cellular lar basis basis of animal structure was known Schleiden and Schwann - ,ormu ormula lati tion on of cell cell theo theory ry
E!ryology - ygote has dual origin from two gametes: spermato%oa and ovum - ,erti ertili li%a %ati tio on is star tarting ting point of life life history of individual #ontogeny$ - .erio eriod d star starti ting ng with with fert fertil ili% i%at atio ion n unti untill metamorphosis hatching or birth &eismann - Germ cell theory - /ade /ade dis distinc tincti tion on betw betwee een n soma oma and germ-cell line
1escriptive )mbryology - "asic structural embryonic body - Serial sections reconstructions
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)(perimental )mbryology - 8nderstand causative factors in develo developme pment nt by posing posing hypoth hypothese eses s and testing them by manipulation of embryos - 6ou( o )(periment of 9 cell embryo 1estroyed one blastomere )ach )ach cell cell is capa capabl ble e of givi giving ng rise rise to comp comple lete te individual .rovided proof of untenability of the preformationist doctrine
3hemical )mbryology - .rov .rovid ided ed descr descrip ipti tion on abou aboutt chem chemic ical al and physiological events in embryo - nte ntera ract ctio ion n betw betwee een n comp compon onen ents ts and and how basic body pattern is laid down ;eratology ;eratology - 3oncerned with study of malformations - dentify and eliminate genetic and envi envirronme onment ntal al fact factor ors s that that caus cause e congenital defects 6eproductive "iology - 6elat elated ed to to prob proble lems ms of of conc concep epti tion on and and contraception - )mphasis on gametogenesis endocrino endocrinology logy transpor transportt of gametes gametes fertili%ation embryonic development 1evelopmental "iology - )mb )mbryoni yonic c devel evelo opme pment < postn ostna atal tal development and processes processes - ,ocus on processes and concepts rath rather er than than spec speci ic c morp morpho holo logi gica call structures - .lan .lantt and and anim animal al syst system ems s inc inclu lude ded d
E!ryology in "onteporary Society ;est ;est tube baby
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n vitro transfer
fertili%ation
and
embryo
#$e "ell and its En%ironent
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ntracellular Synthesis and its 6egulation - 6egulatory mechanisms restrict or permit synthesis of specic proteins and other macromolecules - 1=7 >#transcription$ m6=7 with introns and e(ons >#m6=7 processing$ denitive m6=7 >#? formation of structural proteins'en%ymes$ ribosome linkage >#9 secretory proteins$ m6=7-r)6 comple( Golgi comple( membrane vesicles - /ay be mediated by receptor molecules located at cell surface activated by binding of a ligand causing stimulation of signal transduction@
3ell Surface - Aunctional comple(es o 1esmosomes bind epithelial cells focal points for attachment of brillar intracellular proteins o Gap Bunctions /ediates communication and e(change of small molecules o ;ight Bunctions On surface of many epithelia "ind adBacent cells together forming impermeable barrier .revent mingling of membrane proteins on either side of Bunction - 3ell adhesion o )(periment: disaggregated and reaggregation of sponges o 3a<< mediated adhesion Glycoproteins )-cadherin #epithelium$ =-cadherin #nerves mesoderm$ .-cadherin #placenta$ o Heterophilic binding between complementary saccharides Occurs during mammalian fertili%ation when head of spermato%oa encounters %ona pellucida
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)(tracellular /atri( - 3ells embedded on e(tracellular matri( - 3ollagen #glycoprotein with glycine$ o "asic unit is tropocollagen # C C D$ o
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7ttachment glycoproteins involved in attaching cells to other components of e(tracellular matri( o ,ibronectin # C$ o 3hondronectin # D$ o Laminin #C$ Glycosaminoglycans o )(ample: hyaluronic acid in raising fertili%ation membrane of the egg
Fundaental &rocesses and "oncepts in 'e%elopent 3ell 1ivision and the 3ell 3ycle - .ostmitotic cells > further division does not occur - Maturation(prooting )actor #/.,$ "dc* o .resent throughout cell cycle 7ctivation by cyclin and dephosphorylation 7ctivation of /., e(erts mitotic e+ects on cells "yclin o .roduced during G? "roken down after mitosis
Gene 7ctivation - 1erepression of heterochromatin or repression of euchromatin - 1erepression of general genes from %ygote to blastula - 1erepression of tissue specic genes from gastrula to organogenesis 6estriction and 1etermination - Restriction: the reduction of developmental options permitted to a cell - 7t gastrulation one stage of restriction has occurred #endo meso ectoderm$ - .art of ectoderm thickens and undergoes =eurulation ( 'eterination o restriction has proceeded to a point where a group of cells becomes committed to single developmental fate o nal step in process of restriction - nductions #tissue interactions$ precede determination #and restriction$ nduction - ,orm of embryonic signal calling - )+ect of one embryonic tissue on another so that the developmental course of responding tissue is changed from what it would have been in absence of inductor - ,irst maBor inductive event: induction of mesoderm in cleavage - .rimary induction: induction of nervous system during and shortly after gastrulation - Secondary induction: nervous system induces other structures induction: - &erissi%e inductive signal reEuired to bring about development of structure - Instructi%e induction: responding tissue has options of forming more
than one type of tissue depending on inductive stimulus 1i+erentiation - 6estriction and determination signify progressive limitation of developmental capacities - 'i+erentiation: actual morphological or functional e(pression of a particular cell or group of cells0 the process where cell is speciali%ed /orphogenesis - .rocesses that mold e(ternal and internal conguration of embryo - .attern formation: laying down of morphological blueprint - /orphogenesis: actual reali%ation of plans - Homeotic genes: determine the regional characteristics of each segment #?F segments$ ntercellular 3ommunication - ntercellular communication place in locali%ed gap Bunctions
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3ell /ovements - ndividual cells commonly migrate by means of amoeboid movements - 8niEue: in avian embryos primordial germ cells move from wall of yolk sac into the bloodstream and are carried via blood to the gonads - 7moeboid movement e(amples: )ctoderm: migration of cells o away from neural crest o /esoderm: spreading out of mesodermal cells o )ndoderm: migration of primary germ cells from yolk sac to gonads in mammals - /ovement as sheet seen in epithelial cells 3ell 1eath #7poptosis$ - )(amples: o ;ail and opercular resorption o Separation of digits - /ay be hormonally controlled o /ale and female genital ducts ;he 3lonal /ode of 1evelopment - 3lones: group of cells arising from single precursor 6egulation and 6egeneration - 6egulation o restoration of missing material occurring before di+erentiation of structure o basis for development of identical twins o subdivision of inner cell mass - 6egeneration o 1i+erentiation of structures already occurred
Growth - 1i+erential growth: all parts of the embryo do not grow at the same rate - 1eter etermi min nate ate growt owth: bod body gro grows to certain certain point point that is character characteristi istic c of species and se( #mammals$ - ndeterminate growth: ancestral vertebrates #sh$ 6ecapitulation - "iog "iogen enet etic ic law law of of /ull /uller er and and Hae Haeck ckel el o Onto Ontoge geny ny is an abbr abbrev evia iate ted d recapitulation of phylogeny
RE&RO'"#I-E SE0.L "Y"LE
ORG./S
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Reproducti%e Organs ,emale 6eproductive Organs - .aired gonads #ovaries$ located in pelvic cavity - )ach ovary lies close to funnel-like opening #ostium tubae$ at end of a uterine #fallopian$ tube - 7round abdominal orice of tube is mbriae - 8terus: thick vascular smooth muscle caudally continuous with the cervi( will proBect into vagina - Cagina: organ of copulation and birth canal /ale 6eproductive Organs - ;estes suspended in scrotum #with a countercurrent heat-e(change system$ with lower temperature - Spermato%oa produced in seminiferous tubules then to tubuli recti rete testis ductuli e+erentes epididymis ductus deferens - Spermato%oa stored in epididymis and ductus deferens
Se1ual "ycle in Maals )strous 3ycle in /ammals - Se(ual cycle )strus #prepared for o reproduction accompanied by ovulation and se(ual desire$ o .ostestrum #regression of preparations$ 1iestrum #period of rest$ o o .roestrum #period of active preparatory changes$ - Light is a critical factor: higher than certain threshold causes hypophysis to become active and produce enough ,SH - Light =ervous transmission ,SH ,ollicular growth )strogen in ovarian follicle estrogen in blood stream mating behavior estrous uterus estrous vagina
.rimate /enstrual 3ycle - Se(ual cycle is characteri%ed by menstruation - 3ommences at menarche until menopause - ;hree phases o /enses .roliferative #follicular$ o o Secretory #luteal$ - /enstruation initiated by reduction of blood !ow into supercial uterine blood vessels resulting in deterioration and e(travasation of blood into tissue bringing with it the necrotic supercial tissue
Hormonal 6egulation of the ,emale Se(ual 3ycle - Levels of hormonal control - ?: Hypothalamus o Gn6H #stimulates LH and ,SH in hypophysis$ .H #inhibits prolactin release by o hypophysis$ - 9: Hypophysis o ,SH #stimulate follicle cells to produce estrogen$ LH #male: stimulate Leydig to o produce testosterone0 female: stimulate follicle to produce progesterone$ o .rolactin #promotion of lactation$ - 2: Ovaries #hormones secreted into blood and placental tissues$ o )stradiol o .rogesterone o ;estosterone: precursor of estrogen biosynthesis0 induces atresia nhibin: inhibits ,SH secretion o - F: Ovarian steroid hormones into body tissues - Ovarian follicle maturation brought by rise in pituitary ,SH ,SH and LH stimulate follicle estrogen production high estradiol production in ovarian follicle causes LH and ,SH peak LH peak as nal stimulus for follicular maturation ovulation follicle transforms into corpus lutem #actions of LH$ corpus luteum secretes estradiol and progesterone increased ovarian hormone levels and inhibin cause feedback inhibition inhibition results in low levels of LH and ,SH later regression of follicle comes decrease in gonadotropin levels stimulation of Gn6H and gonadotropins
Hormonal 6egulation of 6eproduction in the /ale - ;estosterone: secreted by Leydig cells #stimulated by LH$ - Sertoli cells take up ,SH synthesis of 7".
G.ME#OGE/ESIS
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Gaetogenesis - Germ plasm > gametes < cells that give rise to them - Gametogenesis: germ plasm is converted to speciali%ed se( cells capable of uniting at fertili%ation and producing a new being - ,our phases o Origin of germ cells and migration to gonads /ultiplication of germ cells in o the gonads #mitosis$ o 6eduction of number of chromosomes #meiosis$ o /aturation and di+erentiation of gametes ;he Origin of .rimordial Germ 3ells and ;heir /igration to the Gonads - .rimordial germ cells of mammals reptiles birds arise in epiblast take up temporary residence in e(traembryonic tissue before returning o "irds: in germinal crescent o /ammals: posterior wall of yolk sac #near allantois$ o 7mphibians: vegetal pole cytoplasm - =ote: .G3s do not produce gonads they produce gametes - .G3s in e(tragonadal sites may develop into teratomas
.roliferation of Germ 3ells by /itosis - /itotically active germ cells: oogonia spermatogonia - Settling in the gonads induce a proliferative phase - /ammalian oogonia proliferative phase o /itosis brings numbers to about million at th month of pregnancy o 7tresia causes sharp decline o .rimary oocytes formed #suspended at prophase $ /eiosis - Genetic recombination occurs by o 6andom distribution of chromosomes to daughter cells 3rossing over o - #9n Fc$ >#/e $ #?n 9c$ >#/e $ #?n ?c$
Spermatogenesis and Oogenesis 3ompared - F functional spermato%oa vs ? viable ovum - 7rrests in meiosis o Spermatogenesis: none - Oogonesis meiotic arrest o ,irst arrest at diplotene prophase in primary oocyte o "roken by hormonal changes
7rrested again at metaphase "roken by fertili%ation #sea urchins complete meiosis at once no second arrest$
Speratogenesis ( ;ransition from mitotically active .G3s to mature spermato%oa ( ;hree phases o /itotic multiplication #spermatocytogenesis$ /eiosis #spermatidogenesis$ o /aturation and di+erentiation o #spermiogenesis$ ( Spermatogonia o ;ype 7: stem cell population 7d: long term reserves 7p: mitotically active give rise to " ": committed to nish o ;ype spermatogenesis ( Spermatocytes .releptotene spermatocytes o o .rimary spermatocytes >#/e $ secondary spermatocytes >#/e $ haploid spermatids ( 1i+erentiation of spermatid to spermato%oa o Golgi comple( forms proacrosomal granules into acrosome o 3entrioles as point of anchorage for developing !agellum ( ntercellular bridges: facilitate synchronous di+erentiation and division of sperm-producing cells ( Sertoli cells: o ,SH target Sertoli cells Synthesis of 7". #to maintain o high testosterone levels$ /aintain blood-testis barrier o o 3reate environment for di+erentiation of sperm cells ,acilitate release of mature o spermato%oa 1egradation of residual bodies o ( "lood-testis barrier #held by tight Bunctions$ responsible for preventing body*s immune system from destroying mature sperm cells #antigenically di+erent from body$
Gene )(pression during Spermatogenesis - .osttranscriptional control Sperm /aturation - Sperm coated with glycoprotein which must be removed in female reproductive tract before fertili%ation can occur #activation capacitation$ - Seminal !uid provides e(ternal energy source #causes nonmotile sperm to become motile$
Oogenesis Oogenesis in 7mphibians - /itotic phase of oogenesis does not come to early halt - =ew crop of eggs each year 2 years for maturation
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,ollicular epithelium theca ovarian epithelium
1evelopment of the 7mphibian )gg - ;hree phases o .revitellogenesis #before yolk deposition$ o Citellogenesis #period of yolk deposition$ o /aturation #released from meiotic block by progesterone$ - .revitellogenic phase o .eriod upto early diplotene phase of meiosis o Lap!rus$ c$roosoes: spread out conguration forming loops Loops are where 6=7 synthesis occurs o Large numbers of nucleoli for specic gene amplication #ribosomes r6=7s$ - Citellogenic phase o .rincipally concerned with yolk formation o Lipochondria > stores lipids o Glycogen granules > stores carbs o Iolk platelets > stores proteins Iolk protein produced by liver cells under estrogen in!uence Gonadotropin release from hypothalamus to oocyte estrogen secretion from oocyte into liver secretion of vitellogenin from liver to oocyte o Iolk precursor > %itellogenin #phospholipoprotein$ ncorporated into oocyte by icropinocytosis 6epresented by p$os%itin and lipo%itellin #packed in crystalline form to form yolk platelets$ o Iolk formation was thought to be function of Bal!iani !ody #yolk nucleus$ o .igment granules concentrate at animal hemisphere - /aturation phase o Hormonally induced release of egg from rst meiotic block o "reakdown of germinal vesicle 3ompletion of rst meiotic o division o ,ormation of rst polar body o "egins with secretion of progesterone #stimulated by gonadotropin$
3auses breakdown of germinal vesicle meiotic maturation /eiosis arrested again at /etaphase by 3S, #cytostatic factor$
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Oogenesis in "irds - Iolk is a single cell #the ovum$ o Gradually accumulates in cytoplasm of ovum before it is liberated from ovary o 7ll other noncellular secretions #egg white shell membrane shell$ are contributed as ovum passes reproductive tract - Iolk still produced by the liver and transported via blood to the follicular cells surrounding ovum #as in amphibians$ o JK water 22K fat ?K protein ?K carb o &ater > =a3l 3a salts #bone formation$ o .roteins > lipovitellin #binds w' lipids$ phosvitin #binds w' phosphorus$ - .rotuberance containing ovum is ovarian follicle - &ith %ona radiata #irregular striated plasma membrane due to microvilli$ for increase in membrane surface enhancing metabolic interchanges - 3ompared with mammalian ovum: o =o yolk in mammals Bust liEuor folliculi o "oth have two layered 3; theca - Iolk release albumen secretion shell membranes # ist$us$ shell #uterus$
Oogenesis in /ammals - .rimary oocyte #so called as it is undergoing meiosis $ < !at follicular cells M priordial )ollicle - .hase : pool of primordial follicles developing into priary )ollicles #!at cuboidal$ - /eiotic arrest follows #diplotene meiosis $ - "oth oocytes and follicular #granulosa cells$ develop microvilli connected by gap Bunctions #allow high /& molecules to pass through$ - ona pellucida beings to develop - .hase : growth of oocyte and granulosa covering #under in!uence of gonadotropic hormones$ - Overall growth of follicular covering mediated by ,SH receptors ( Secondary )ollicle when antrum is identiable ( LH receptors develop allowing production of testosterone by theca o ;ransported into granulosa cells wherein granulosa converts
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testosterone to estrogen by aromatase .hase : further follicular growth and selection of one follicle #highest receptivity to ,SH$ which will undergo ovulation "egins late in follicular phase of o menstrual cycle 7fter LH surge angiogenesis o occurs causing estradiol to spill out into blood "efore ovulation ovum is o released from rst meiotic block #diplotene$ allowing meiosis to nish o 7fter that follicle is now ready to respond to preovulatory ,SH and LH surge and release itself #wherein it is now at metaphase second block$ Ovulation o ncreased antral !uid pressure within follicle causes bursting of follicular wall &eakening of follicular wall by o lytic en%yme #stimulated by LH$ 3orpus luteum Stratum granulosum and theca o interna involved in corpus luteum formation )ndocrine organ secreting o progesterone and estrogen Granulosa cells swell and o develop to secrete high levels of hormones ,ormation of corpus lutem o reEuire continuous presence of LH from pituitary regression happens with decreased sensitivity to LH receptors corpus lutem of pregnancy maintained by chorionic gonadotropin #secreted by embryo$ 3orpus lutem produces large o amounts of progesterone and estrogen .rogesterone prepares lining of uterus for implantation
.ccessory "o%erings o) Eggs 3overing of the Sea 8rchin )gg - nner to outer o .lasma membrane o Citelline envelope #composed of glycoproteins contain speciesspecic receptors for spermato%oa$ o Aelly coat #polysaccharides glycoproteins hydrates and e(pands when eggs are shed$
;he /embranes Surrounding the 7mphibian )gg - .lasma membrane: forms microvilli - ,ollicular cells: form macrovilli - =arrow space between oocyte and follicular epithelium becomes lled with noncellular basement membrane: vitelline envelope #eEuivalent of %ona pellucida in mammals$ - Gap Bunctions Boin the villous processes - 7t ovulation perivitelline space forms between vitelline envelope and plasma membrane - 3oated with Belly coat as it goes through oviduct #same function as sea urchins$ ,ormation of the 7ccessory 3overings of "ird )ggs - 7t ovulation ovum surrounded by inner vitelline membrane - 6emainder of accessory coverings secreted about ovum during passage toward cloaca - Outer vitelline membrane laid down when it is in oviduct adBacent to ovary - .l!uen laid down in upper o%iduct o 6otation twists albumen into spiral strands at two ends of yolk: c$ala2ae Serve to suspend yolk in albumen - )gg white o O%al!uin and lyso2ye by estrogen o .%idin secreted by go!let cells by progesterone - Shell membrane added farther along oviduct - S$ell secreted as egg passes through s$ell gland #at uterus$
;he 3overings of /ammalian )ggs - =oncellular %ona pellucida #mostly synthesi%ed by oocyte$ o .-? o .-9 3&(4 > acts as sperm receptor o and plays a role in inducing acrosome reaction - 3orona radiate still surrounds mammalian ovum #may continue to secrete steroids and prostaglandins$
/embrane event set in place within 9-2 seconds and lasts for J seconds o 7crosomal process and plasma membrane fusion causes depolari%ation of plasma membrane #by in!u( of =a <$ ,rom -56- to 786.ositive potential does not permit fusion of other spermato%oa to plasma membrane Slow block /obili%ation of 3a from within o egg o rst released at site of sperm entry o wave of released 3a initiates cortical reaction #rupture of cortical granules and release of these contents into perivitelline space$ o cortical granules move to inner surface of plasma membrane fuse with it and open up o sul)ated contain ucopolysacc$arides #G7Gs$ which have high water anity causing swelling and forcing vitelline envelope away from plasma membrane #raising the fertilization membrane$ Fertili2ation e!rane: o name given to vitelline envelope after changes by cortical reaction o Hydrated mucopolysaccharides form hyaline layer #between plasma and fertili%ation membrane$ 7s fertili%ation membrane is o elevated an en%yme alters it causing attached sperm to drop o+ o ,inal step: release of o%opero1idase from cortical granules #for breakdown of H9O9 results in hardening of fertili%ation membrane$ H*O* released by egg during cortical reaction #spericidal9 .olyspermy is normal in urodele amphibians and birds o
FER#ILI3.#IO/ - nitial contact between egg and sperm - )ntry of sperm cell into egg - .revention of polyspermy by egg - /etabolic activation of egg - 3ompletion of meiosis by egg - ,ormation and fusion of male and female pronuclei
#$e Sea rc$in Gamete 6elease and ;ransport - ?JJ billion spermato%oa and F billion eggs
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Sperm .enetration of the )gg in nvertebrates and the 7crosome 6eaction - &hen spermato%oa encounter egg former undergoes changes - n presence of egg cells spermato%oa will cluster and increase motility - 1irect contact with Belly coat increases motility and stimulates acrosome reaction - Speract is responsible for increased motility and activated respiration that occur when sperm contacts with Belly coat o ncrease in permeability of plasma causes in!u( of =a and 3a eNu( of H < 6aises intracellular pH stimulating !agellar activity - 3ontact w' Belly coat stimulates acrosome reaction o "egins with breakdown and subseEuent fusion of outer acrosomal membrane and plasma membrane o .olymeri%ation of G(actin to F( actin #forming acrosomal process$ ;ip of process is covered with !indin mediating sperm binding to surface of eggs o Spermato%oa digest through vitelline membrane by lysins
"inding of Sperm to the )gg - Sperm receptor molecule #on microvilli of egg$ o ntracellular: remains constant among species o )(tracellular: di+ers accdg to species - 7fter sperm-egg fusion )ertili2ation cone forms by microvilli engulng sperm head "locks to .olyspermy - ,ertili%ation of egg by more than one sperm - ,ast block
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/etabolic 7ctivation of the )gg - Other events that prepare egg for fusion of genetic material: o o1ygen ncreased consuption o 7ctivation of /.' kinase #facilitate biosynthesis of new membrane lipids$ o 7nother in:u1 o) /a7 #with eNu( of H$ causing increased intracellular pH
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ncreased pH leads to increased protein synthesis and initiation of 1=7 synthesis
.enetration of the Spermato%oon into the )gg and ,usion of the Genetic /aterial - Sperm nucleus begins to interact with egg cytoplasm and chromatin rela(es - 7s chromatin dispersion nears completion new membrane forms around what can now be called a ale pronucleus - Sperm centrioles persist and provide basis for formation of sper aster important in getting male and female pronuclei together - &ronuclear )usion occurs at center of egg - 7fter fusion chromosomes replicate in preparation for cleavage
Maalian Fertili2ation Sperm ;ransport in the ,emale 6eproductive ;ract of /ammals - "arriers to fertili%ation: o =atural acidity of vagina #bacteriostatic$ Seminal !uid acts as bu+er #raises vaginal pH$ Seminal !uid may cause contractions in upper vagina helping propel spermato%oa Orgasms cause uterine contractions o )ntrance to uterine tubes #ovulation can sometimes only occur on one tube$ - &ositi%e r$eotactic response #face an oncoming current generated by uterine ciliary movement$ - "apacitation: removal of glycoprotein covering spermato%oa enabling better penetration of egg
)gg ;ransport - 3iliary currents and smooth muscle contractions transport egg into uterine tube - 3orona radiata adds mass faster movement down tube
8nion of Gametes - /ammals: occurs in upper part of uterine tubes - Spermato%oa must penetrate corona radiata cells and then %ona pellucida before contact with plasma membrane - ona pellucida: molecules on sperm head bind with species-specic sper receptors #consist of e(posed part of .-2$ o ,urther contact with other core regions stimulate acrosome reaction #capacitation is prereEuisite so that lytic en%ymes are released to facilitate passage of sperm through %ona pellucida$ - 7crosome reaction: o Locali%ed fusion and breakdown of outer acrosomal membrane and plasma membrane .crosin inner bound to o acrosomal membrane digests through %ona pellucida o Fertili2ation cone bulged out when sperm makes contact with egg 1evelopment and ,usion of .ronuclei - &ith sperm entry block to second meiotic division is lifted and second polar body is released - "reakdown of sperm nuclear membrane and decondensation of
chromatin and protamine replacement with histones - =ew pronuclear membrane forms around decondensed material - 1=7 synthesis occurs as male and female pronuclei migrate toward each other #as opposed to sea urchins where chromosomes condense to prepare for metaphase and 1=7 synthesis occurs after fusion of pronuclei$ .arthenogenesis - 7ctivation of unfertili%ed eggs and development into viable individuals - /ammals: all female because females are homogametic #DD$ - "irds reptiles: males and females #heterogametic females$
Se1 'eterination ( Occurs at fertili%ation determined by /ammal sperm o o "ird'reptile egg Esta!lis$ent o) &olarity in t$e E!ryo ( 2 polar a(es: 3raniocaudal #anteroposterior$ o o 1orsoventral /ediolateral o )stablishment of .olarity in 7mphibians - &riary polarity of egg by animal and vegetal poles o 1enser pigment concentration at animal pole o =ucleus located near animal pole o Gradient of increasing density of ribosomes and glycogen granules towards animal pole o Si%e and concentration of yolk platelets increase toward vegetal pole o 6egion of anial pole ; $ead o 6egion of %egetal pole ; tail .nterocaudal a1is - ,ertili%ation is ne(t establishment of polar a(es 9 maBor reorgani%ations o o General convergence of cytoplasm beneath the thin cortical region toward the sperm entry point 2J degree shift between o subcortical cytoplasm and overlying corte( o ;hese changes cause reduction in density of pigment granules in the region of the animal hemisphere along eEuatorial %one opposite to sperm entry point 6educed pigmentation called gray crescent /idpoint of gray crescent is iddorsal point of
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body #determines dorso%entral a1is of future embryo$ 1etermination of two a(es determines third )ven before cleavage the three a(es secondary are established and polari2ation is completed
)stablishment of .olarity in "irds - 3leaving embryo represented by !at disk of cells on yolk surface: cells on outer surface become dorsal part those closest to yolk become ventral - 3ells are shed from part of blastoderm uppermost #with respect to gravity$@ ;he area from which these cells fall become the caudal end of embryo
"LE.-.GE ./' FORM.#IO/ OF #HE BL.S#L. - 3leavage: waves of cell division following one another almost without pause - /any of changes and di+erences in cleavage patterns among embryos of various species are related to the amount of yolk present in egg - 3leaving embryo develops a central cavity #blastocoel$ and enters blastula stage #$e "ell during "lea%age ( 3leavage division consists of karyokinesis followed by cytokinesis ( 3leavage furrow rst forms in the region of corte( nearest to mitotic spindle and then moves around cell ( .osition of cleavage furrow gets (ed or established at anaphase ( .sters #composed of microtubules$ interact with cell corte( to stimulate cleavage furrow formation .sters are the e+ective agent o in initiating cleavage furrow formation #not the mitotic spindle$ o /itotic spindle asters cell corte(
'istri!ution o) Yolk and its E+ect on "lea%age - Blastoeres : cells that arise from cleavage - Holoblastic cleavage: characteri%ed by complete division of cells o Oligolecithal eggs produce blastomeres of eEual si%e o /esolecithal eggs displace nucleus towards animal pole =et result is appearance of later and larger blastomeres at vegetal pole - /eroblastic cleavage: newly formed plasma membrane does not separate inner borders of dividing cells from the underlying yolk o ;elolecithal eggs: displaces the embryo-forming cytoplasm into a small disk on one edge of ovum
"lea%age and Foration o) t$e Blastula 7mphio(us - 6adial holoblastic eEual cleavage microlecithal eggs - /eridional meridional eEuatorial meridional eEuatorial
Sea 8rchins - )arly cleavage only consists of S and / phase with alternating periods of cyclin synthesis and degradation
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G? and G9 phases evident only later in cleavage /icrolecithal /eridional meridional eEuatorial o Lower tier undergoes uneEual eEuatorial division Microeres #becomes priary esenc$ye$ Macroeres o 8pper tier undergoes eEual meridional division forming P mesomeres 1uring entire cleavage embryo is enclosed in fertili%ation membrane and closely associated with hyaline layer "lastomeres later form motile cilia penetrating hyaline layer and secreting $atc$ing en2ye into perivitelline space digesting the fertili%ation membrane esenc$ye )nd product is !lastula
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7mphibians - /eridional #begins at animal pole bisects gray crescent$ meridional eEuatorial double meridional double eEuatorial - 7mphibian blastocoel o ,ormed from speciali%ation of cleavage furrow of animal hemisphere o ,illed with =a< ions then water enters to maintain ionic balance causing e(pansion - 7mphibian blastula has three main regions o 6egion around animal pole including cells forming roof of blastocoel ,uture ectodermal germ layer 6egion around vegetal pole o including large cells in interior ,uture endodermal cells /arginal ring of cells in the o subeEuatorial region of the embryo including gray crescent )mbryonic mesoderm - =ieuwkoop discovered basis for mesodermal induction o E1perient 8: directly apposed sheet of cells from animal hemisphere above blastocoel to a yolk mass from vegetal hemisphere Inducti%e in:uence from yolk mass caused animal pole cells to form esoderal structures ;herefore blastocoel may function to restrict interaction
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between future endodermal and ectodermal cells o E1perient *: isolated pieces of ectoderm induced to form esoder by trans)oring gro
"irds - /itotic spindles align themselves so subseEuent cleavage furrows form at right angles to the preceding one #rst 2$ - ,ourth cleavage furrow is a circumferential one - "lastomeres formed by rst few divisions are dorsally bound by plasma membrane but basal surfaces are open to underlying yolk - ,urther cleavage of !lastoder results in radial e(tension of embryo - 29-cell embryo shows cleavage parallel to surface establishing several strata of supercial cells - 7t around ?JJ cells blastoderm is underlain by a su!gerinal ca%ity pH of subgerminal cavity > @ o while albumen: Q@ leading to establishment of transepithelial potential of 9mC@ Electrical gradient deterines dorso%entral a1is o) !lastoder= ( S$edding o) cells begins from undersurface of area of blastoderm that is farthest away from source of gravity #this area becomes caudal end of embryo$ .rea pellucida: central portion o of blastoderm thinned out by shedding of cells .rea opaca: region where o blastoderm cells still abut directly onto yolk ( &riary $ypo!last> aggregates of cells shed from lower surface of blastoderm by a process of polyingression?delaination
,irst occurs at posterior end of embryo Separated from epi!last by a o blastocoel .olarity and location of primary o hypoblast deterine location and direction o) )uture priiti%e streak by inductive interaction E0#R.e!ryonic endoder o 3omparison with amphibians o ;wo layered blastoderm is compared to a !attened blastula #epiblast: animal hemisphere:: hypoblast: vegetal$ "oth hypoblast and vegetal pole o have ability to induce formation of mesoderm in epiblast and animal pole by inductive interaction o
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/ammals ( E@ual $olo!lastic clea%age o) an isolecit$al egg ( .ersistence of traits characteristic to large-yolked embryos #later in development$ ( Second cleavage division may not occur simultaneously in both blastomeres /itotic spindle of one o blastomere may rotate QJ degrees causing a rotational pattern o) clea%age #as opposed to radial in echinoderms Amphioxus$ ( ?-cell stage still is contained in %ona pellucida ( /orula: internal secretion of !uid by blastomeres leads to formation of blastocoel or !lastocyst ca%ity Similar to amphibian #=a-Ro 7;.ase system brings in =a and then water$
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"lastocyst has two populations of cells #rop$o!last> o cells that constitute outer wall of blastocyst maternal D chromosomes are preferentially e(pressed assumed epithelial properties #tight Bunctions microlaments$ forms large part of placenta Inner cell ass: o Boined together by gap Bunctions retains ability to reaggregate if separated 3omparison with birds )mbryos of mammals form a o layer of cells beneath inner cell priiti%e mass called endoder eEuivalent to priary $ypo!last of avian embryos =ote that primitive endoderm does not contribute to embryo proper as primary hypoblast does not too
G.S#RL.#IO/ ./' #HE FORM.#IO/ OF #HE GERM L.YERS Gastrulation as a &rocess - &ell-ordered rearrangements of cells in embryo - /orphogenetic movements result in reorgani%ation - 6earrangement of blastula to a stage characteri%ed by presence of three germ layers ;able -? ;ype movement nvagination
of
)vagination nvolution
)piboly
ngression
.olyingression #delamination$
7meboid motion
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1escription npocketing sheet of cells
)(ample of 7rchenteron formation in Amphioxus of )(ogastrulation
Outpocketing sheet of cells 6olling around a corner of an e(panding outer layer of cells and spreading over an internal surface Spreading of a cell sheet
3ell movements through the amphibian blastopore
Spreading of outer cells towards amphibian blastopore Sinking of .rimary individual cells mesenchyme from a surface formation in sea into an area urchin embryos Separation of ,ormation of second sheet primary from an original hypoblast of single sheet avian embryos /igration of cells /igration of as single neural crest cells individuals through their own motility
;wo main strategies for gastrulation: o 3arry out gastrulation movements within conte(t of a sphere #7mphio(us amphibians$ o )laboration of three germ layers as two-dimensional sheets upon one sector of an enormous sphere #birds reptiles even mammals$ Blastopore> opening from outside into the archenteron
Gastrulation in Sea rc$in E!ryos - Separation of priary esenc$ye from vegetal plate of blastula signies start of gastrulation - &riary esenc$yal cells develop Alopodia moving along the basal lamina until forming a ring-like structure near base of invaginating archenteron - ;hree stages of formation of archenteron o nvagination of cells at vegetal pole o secondary .resence of esenc$ye #will later make contact with animal pole by lopodia$
7s tip of archenteron makes contact with animal pole secondary mesenchyme undergoes nal determination #can no longer dedi+erentiate into primary mesenchyme cells$ &luteus lar%a o
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Gastrulation in .p$i!ian E!ryos ( 3ortical rotation stimulated by sperm fusion leads to generation of an early organi%ing center # /ieu
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7t end of gastrulation all future endoderm and mesoderm lie within - Spemann and /angold: demonstrated that dorsal lip of blastopore possesses ability to organi%e future development o )arly it initiates convergent e(tension movements o Late it e(erts dorsali%ing e+ect on lateroventral region of marginal %one o Organi%er signals neighboring cells of animal cap to form neural plate Gastrulation in Birds - 6eview: o "lastula as two-layered and structure # epi!last $ypo!last with blastocoel in between$ o )mbryo proper occupies area pellucida and surrounded by area opaca - ollerCs sickle #thin sickle-shaped mass of cells$ at posterior end of secondary embryo where a $ypo!last pushes anteriorly folding primary hypoblast ahead of it o 1istribution of primordial germ cells along anterior border of blastoderm due to compression of primary hypoblast #.G3s are found in primary hypoblast$ - Gastrulation begins with condensation o) cells in posterior part of epiblast gradually assuming cephalocaudal elongation eventually forming priiti%e streak - 7ppearance of primitive streak is result of inducti%e interaction of epiblast with hypoblast - .rimitive streak now contains: o .rimitive groove: central furrow o .rimitive ridges: thickened margins o HensenCs node: cephalic end of streak "egins to regress after ?Phrs with corresponding elongation of notochord above it #head process$ - E!ryonic ger layers are )ored by migration of cells in epiblast towards primitive streak and ingression subseEuent #not delamination$ to form middle and lower germ layers #embryonic mesoderm and endoderm$ o ,irst cells to pass are the future embryonic endodermal cells ;hey displace primary hypoblast outward and cephalad toward area opaca o /esodermal cells migrating through:
Hensen*s node: notoc$ord "ranial part of primitive e!ryonic streak: esoder "audalost part of streak: e1trae!ryonic esoder
"oparison o) .%ian and .p$i!ian 'e%elopent ( "lastula "ird: epiblast contains o endodermal and mesodermal germ layers o 7mphibian: surface layers contain endoderm and mesodermal parts ( nduction )+ect of hypoblast on epiblast o o /esoderm induction by vegetal yolk ( "lastopore o .re-primitive streak thickened area of chick blastoderm "lastopore in amphibians o ( 3ell migration: 3ells that constitute o endodermal layer migrate to interior rst and mesodermal layers follow later Origin o) t$e Ger Layers in Maals ( 6eview: o /ammalian blastocyst segregated into embryo forming inner cell ass and trop$o!lastic cells ( Hypo!last: rst cells to segregate out from I"M o ,orms e(traembryonic endoderm similar to birds o ,orms lining of yolk sac ( 6emainder of I"M is now called epi!last o 3ontains future ectodermal cells 7lso contains cells that will o migrate through primitive streak and become denitive endodermal and mesodermal germ layers ( 6emaining I"M now called e!ryonic disk o One margin of disk becomes t$ickened becomes caudal part of embryo ,rom caudal thickening cephalad e(pansion of cells results in primitive streak Origin o) t$e Ger Layers in Rodents ( Early $ypo!last is called priiti%e endoder in mouse 3ells from this layer spread out o trop$o!last beneath
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#trop$ectoder $ to form endodermal layer of parietal yolk sac .arietal endoderm cells create o basement membrane called Reic$ertCs e!rane &olar trop$ectoder: o overlying 3/ can undergo mitosis daughter cells become mural Mural trop$ectoder: o surrounding blastocyst cavity /itosis results in polyploidy giant cells I"M undergoes transformation di+erent from other mammals .rotrudes deeply into blastocyst o cavity #like a tongue-like lobe$ o 3avity forms within the lobe #proanion$ and cells surrounding it are priiti%e ectoder #or epiblast$ 3alled an inverted egg cylinder
/ERL.#IO/ ./' #HE FORM.#IO/ OF .0I.L S#R"#RES &riary D/eural9 Induction
#$e Mesoder o) t$e Early E!ryo Secretion o) E1tracellular Materials in t$e Early E!ryo
/eurulation in .p$i!ians Foration o) t$e /eural #u!e #$e /eural "rest
#$e Foration and 'i+erentiation o) Soitoeres and Soites
