embryological terminology

8/13/2019 Embryological Terminology

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EMBRYOLOGICAL TERMINOLOGY

In mammals, the female gamete is called an egg or ovum; the correct technical term forthe newly ovulated female gamete is an oocyte. Upon fertilization, the oocyte becomes aone-cell embryo, sometimes referred to as a zygote. The embryo then divides into two-

cell, four-cell, etc. stages. At the !-cell stage, the embryo becomes a morula "#atin formulberry$. %hen a cavity "blastocoele$ forms between the cells of the embryo, it istermed a blastocyst. To add further confusion, all of these stages of embryos arefre&uently called eggs or ova. 'mbryos of various stages are illustrated in (igures ) to*+.

The first three divisions of the embryo are called cleavage divisions; thus, one-to eight-cell embryos are defined as cleavage stages. uring this time the embryo actuallydecreases in weight. nly at the morula stage does the embryo begin to weigh morethan at the one-cell stage.

FIGURE 14

Location of differentdevelopental !ta"e!of #ovine e#r$o! inreprod%ctive tract &fir!tappeared in 'oard(Dairyman( 1) Marc*1+,,( p- .4/0

uring the morula stage, cells of embryos change from spherical to polygonal in shape.This phenomenon is termed compaction. uring compaction, specialized unctions frombetween cells, so the cells can communicate with each other. (re&uently, compactedmorulae are termed tight morulae. /ompacted morulae are smaller than pre-compactedembryos. /ompaction is an e0cellent sign that the embryo is developing normally; lac1of compaction by si0 days after oestrus in cattle indicates retarded development.

As the morula develops into a blastocyst, it forms a cavity, the blastocoele, by e0pendingenergy to pump fluid between the cells. Thus blastocyst formation also is indicative ofcontinued normal embryonic development. /onversely, lac1 of blastocoele formation byseven to eight days after oestrus in cattle signifies retarded development.

FIGURE 12ia"ra of noral#ovine e#r$o!

The zona pellucida is a gelatin-li1e capsule that surrounds the oocyte and early embryo.It has receptors for sperm that are inactivated after fertilization, it 1eeps the cells of thepre-compaction embryo together, and protects these young cells from the immunesystem and from pathogens. If the zona pellucida is removed from pre-compactionembryos, the cells come apart upon embryo transfer and then degenerate. %hen theblastocoele becomes very large, the embryo e0pands "normally eight to nine days afteroestrus$, which thins the zona pellucida. This is the e0panded blastocyst stage. Afterone to one days more, the e0pansion is so great that the embryo hatches out of the zona


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pellucida, perhaps aided by enzymes. 2atched blastocysts become elipsoid in shape34 days after oestrus, and then elongate mar1edly by )3! days post-oestrus. 5yday 637 the embryo elongates sufficiently to reach the tip of both uterine horns.

E3ALUATION

(or many beginners, the most intimidating aspect of the embryo transfer process ismorphological evaluation of embryos. bviously, there is no profit in transferringunfertilized ova or degenerate embryos, nor in discarding perfectly normal ones. 5otherrors are common when people are first gaining e0perience, and not infre&uent whenmore seasoned personnel ma1e hasty decisions. There are three elements to successfulevaluation of embryos8 training, e0perience and proper e&uipment.

Training includes learning the correct morphology of embryos at different times post-oestrus and the meaning of deviations from normal morphology. ne must also learnhow to manipulate and e0amine embryos. '0perience is gained by e0amining manyembryos at different stages of development. Ideally hundreds of embryos should be

studied under the guidance of someone e0perienced in this area. 9hotographs, drawingsor slides of various 1inds of embryos are very useful. 2owever, they can only substitutepartially for real embryos. '0perienced personnel can evaluate more than 7+ percent ofova accurately with a good stereomicroscope at 4: to ): magnification or less.2owever, a small percentage of embryos re&uire a compound microscope "at least a: obective with


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'mbryos collected si0 days post-oestrus should be post-compaction or so-called tightmorulae. They should have +:3


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;ae ov% a! in &A0 it* #ri"*t6field optic!- &C0 Crac5ed( ept$ 7ona pell%cidarecovered five da$! after oe!tr%!- Noar!5i optic!- &20 Unfertili7ed ooc$terecovered !i= da$! after oe!tr%!- Note #li!ter! of clear c$topla!- Noar!5ioptic!

FIGURE 1,&A0 2e"enerate( %nfertili7ed ov% recovered five da$! after oe!tr%!- Noar!5ioptic!- &B0 Unfertili7ed ov% it* to fra"ent! of c$topla!- Note lar"e ve!icle!it*in c$topla!- Bri"*t6field optic!- &C0 Fra"ented ov%( li5el$ %nfertili7ed

recovered five da$! after oe!tr%!- Bri"*t6field optic!- &20 2i!inte"rated ov%(pro#a#l$ %nfertili7ed- Bri"*t6field optic!


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FIGURE 1+&A0 Noral appearin" .6cell e#r$o recovered fo%r and a *alf da$! after oe!tr%!(#ri"*t6field optic!- &B0 2e"eneratin" .6cell e#r$o recovered five da$! afteroe!tr%!- Note clear c$topla! in one #la!toere- &C0 Noral 46cell e#r$orecovered to and a *alf da$! after oe!tr%!- Noar!5i optic!- &20 A .6cell e#r$orecovered five da$! after oe!tr%!- Note clear c$topla!- Noar!5i optic!


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ay-6 embryos should be early blastocysts. As mentioned earlier, presence of ablastocoelic cavity is a good sign. ay-< embryos should have a large blastocoele and

some should be e0panding, i.e. the diameter should be increasing so that the zonapellucida is thinned. A distinct, inner cell mass should be present. ther aspects ofmorphology should be as described earlier in this section. As with day-! embryos,various imperfections are not uncommon in perfectly acceptable embryos.

FIGURE .)&A0 Noral ,6cell e#r$o recovered t*ree da$! after oe!tr%!- Bri"*t6field optic!-&B0 ;ae e#r$o a! in &A0 #%t Noar!5i optic!- &C0 Noral 1.6 to 146cell e#r$orecovered fo%r da$! after oe!tr%!- Noar!5i optic!- &20 ;everel$ retarded 1.6 to146cell e#r$o recovered !i= da$! after oe!tr%!- Bri"*t6field optic!-


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FIGURE .1&A0 Uncopacted or%la recovered t*ree da$! after oe!tr%!( pro#a#l$

de"eneratin"- &B0 Uncopacted or%la recovered t*ree da$! after oe!tr%!> dar5c$topla!- &C0 ;everel$ retarded and de"eneratin" e#r$o recovered !i= da$!after oe!tr%!- &20 ;everel$ de"enerate e#r$o recovered !even da$! after oe!tr%!-All are #ri"*t6field optic!-


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In our laboratory at /olorado ?tate University, we have evaluated nearly + ::: bovineova over the years. About one-third of these have been unfertilized or severely

degenerate; perhaps the most important tas1 in evaluating ova is to identify these andfail to transfer them so that pregnancy rates are not lowered. The single most difficulttas1 for people learning to classify embryos is to distinguish between tight morulae andunfertilized oocytes "note that unfertilized embryo is improper terminology and internallycontradictory$, which can loo1 very similar in size and te0ture. The unfertilized ovum hasa perfectly smooth cell membrane, at least over a part of the cell, while the tight morulawill have a slightly scalloped appearance.

FIGURE ..&A0 Nel$ copacted or%la recovered !even da$! after oe!tr%!- Bri"*t6fieldoptic!- &B0 Copacted or%la recovered !even da$! after oe!tr%! it* !everale=cl%ded cell!> "ood orp*olo"ical ?%alit$- Noar!5i optic!- &C0 Copacted

or%la recovered !even and a *alf da$! after oe!tr%! it* an$ lar"e( e=cl%dedcell!( fair orp*olo"ical ?%alit$- Bri"*t6field optic!- &20 @oor ?%alit$ or%la it*an$ de"enerate cell!- 'oever( t*e !all( copacted a!! to t*e loer left *a!a !all c*ance of developin" into a calf- Bri"*t6field optic!


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FIGURE .8&A0 Noral( earl$( e=panded #la!toc$!t recovered !even da$! after oe!tr%!-

Bri"*t6field optic!- &B0 ;ae e#r$o a! in &A0 #%t Noar!5i optic!- &C0 Noral(e=panded #la!toc$!t recovered !even and a *alf da$! after oe!tr%!- Note t*et*inned 7ona pell%cida- Bri"*t6field optic!- &20 'atc*in" #la!toc$!t t$picall$ fo%ndnine da$! after oe!tr%!- Bri"*t6field optic!

%ith e0perience, these two types of ova can be distinguished easily, especially with acompound microscope "(igures *) and *+$. ccasionally they are classified incorrectly,even by e0perts who do not ta1e sufficient time "really only +3: seconds$ to evaluatethe embryos correctly. A second, much more rare misclassification occurs whenunfertilized ova degenerate in the centre and become &uite clear, resembling ablastocyst at first glance "(igure *+$. =ost other misclassifications are a matter of


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degree in distinguishing among good, fair and poor embryos "see below$. An e0cellenttreatise on ovine embryo morphology is authored by %intenberger Torres and ?evellec,7


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TA5#' +;ta"e of noral e#r$onic developent a! a f%nction of da$! after donor!oe!tr%!

?tage ofdevelopment

ays after onset of oestrus

-cell :3*

*-cell 34

)-cell *34


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'0pandedblastocyst


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Unfertilized or two- or three-cell.

It is often impossible to determine if an ovum is a severely degenerate embryo or isunfertilized. 'ven two-or three-cell embryos may in fact be fragmented, unfertilized ova.Table ! "from 'lsden et al., 76

embryological terminology

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