EMBRYOLOGY - CH.3 - STAGES OF EMBRYONIC DEVELOPMENT TEST QUESTIONS AND ANSWERS 2024

what is the first phase of development of the zygote? cleavage (segmentation) what happens during cleavage? (general) passage from the unicellular condition to the multicellular condition after several mitotic divisions true or false : cleavage is highly conserved among species TRUE how is cleavage initiated? by the division of the nucleus which is followed by the division of the cytoplasm the daughter cells resulting from cleavage are called blastomeres is the division during cleavage synchronized ? The first cleavage tends to occur simultaneously in all the blastomeres, but sooner or later synchronization is lost, and the blastomeres divide at different times, independently of one another what about the size of blastomeres resulting from each division of cleavage? half of the size of the previous cell bc the blastomeres don't increase in size before each division - embryo maintains its size and shape what cell structures are obtained throughout cleavage? morula --> blastula what about the duration of morula stage ? it is short bc this is an intermediate stage how is the blastula obtained starting from the morula ? cells undergo rearrangement forming a thin outer layer (blastoderm) enclosing a cavity called blastocoel when is the embryo called a blastula? when the blastocoel is established as blastomeres divide .... what about the proportion of DNA to cytoplasm ? that proportion increases bc cell becomes smaller so the nucleus takes up most of its volume speed of division when there is more yolk it is slow bc the yolk makes division harder how can we phrase that zygote and blastula are about the same size ? the cells (blastomeres) show very little growth TRUE -FALSE: the type of cleavage depends on the egg type true what are the types of cleavage? -holoblastic (complete-total) -meroblastic(incomplete-partial) where does holoblastic cleavage occur ? in what types of eggs? -when there is little or moderate amount of yolk -alecithal -oligolecithal-heterolecithal TRUE - FALSE: -in holoblastic cleavage .... both animal and vegetal poles are divided into blastomeres true when does meroblastic cleavage occur ? in what types of eggs ? in which species ? - when there is imp yolk content -telolecithal eggs(birds-reptiles-fish) -centrolecithal eggs (insects) in which part of the egg does division occur in meroblastic cleavage? in a small area of the animal pole that contains active cytoplasm while the yolk remains uncleaved what are the types of holoblastic cleavage acc to blastomeres' size? -equal -subequal -unequal how common is equal holoblastic cleavage? very rare in what type of eggs does equal holoblastic cleavage occur? oligolecithal eggs how common is subequal holoblastic cleavage? more frequent - most common compare the sizes of blastomeres resulting from subequal holoblastic cleavage there is a little difference in what types of eggs does subequal holoblastic cleavage occur? oligolecithal and alecithal eggs what are the types of cells resulting from unequal holoblastic cleavage? where is each found? -micromeres (small at the animal pole) -macromeres (larger at the vegetal pole) what about the speed of division of cells of unequal holoblastic division ? the micromeres have rapid division compared to the macromeres in what types of eggs does unequal holoblastic cleavage occurr? heterolecithal at what time of cleavage do we start obtaining diff sized eggs in holoblastic unequal division ? why ? -at the third division (horizontal) that tries to be away from the yolk ...... while the first two divisions aren't affected by yolk acc to blastomeres arrangement and direction of mitotic spindles.... the holoblastic cleavage has the following types -radial -spiral -rotational the plane of the first division is always ....... (describe it) meridonial - passing through the main axis of the egg the second plane of division also meridonial - passing through the main axis of the egg - but perpendicular to the first plane of division the plane of the third division is perpendicular to the main axis and to the previous 2 planes of division and parallel to the equator (horizontal) describe the radial holoblastic cleavage the 4 cells of the animal hemisphere lie directly on top of the 4 of the vegetal hemisphere what is the pattern of blastomeres in radial holoblastic cleavage? radially symmetric what is the direction of the spindles in radial holoblastic cleavage ? vertical in which directions does the embryo grow in the case of radial holoblastic cleavage ? in radius and in length in which species does radial holoblastic cleavage take place? echinoderms and chordates describe the spiral holoblastic cleavage the blastomers of the upper tier are shifted in one direction w.r.t the ones of the lower tier (clockwise or anti) .... so the upper cells lie at the junctions of the lower cells (cells of tiers are oblique to eachother) how are the mitotic spindles arranged in the third division in spiral holoblastic cleavage? how are they shaped ? spirally - they are oblique in what species does spiral holoblastic cleavage occur? annelids and mollusks what are annelids? tyoe of worms what are mollusks? type of snails what is the direction of cleavage in spiral holoblastic cleavage? oblique how does the embryo grow in case of spiral holoblastic cleavage? in a spiral way - embryo is rotated describe rotational holoblastic cleavage? after the first meridional cleavage - one of the 2 resulting blastomeres divides meridionally while the other divides equatorially in which species does rotational holoblastic cleavage take place? placental mammals what are the types of meroblastic cleavage ? -discoidal -superficial in what types of eggs does discoidal meroblastic cleavage take place? telolecithal eggs what happens in discoidal meroblastic cleavage ? -only the active cytoplasm at the animal pole divides into blastomeres -the rest of the egg remains unsegmented what is formed as a result of the division that occurs in discoidal meroblastic division? a disc of blastomeres called a blastodisc is formed (germinal disc) in what species does discoidal meroblastic cleavage occur ? reptiles-birds-fish in what eggs does superficial meroblastic cleavage occur? centrolecithal eggs of insects what happens during superficial meroblastic division ? -the nucleus divides many times without cell membs forming btwn the nuclei and thus forming a syncitium -nuclei migrate to the periphery and there cell membs form btwn them and so forming the peripheral -superficial blastoderm -yolk persists as a compact mass in the center of the cell-uncleaved how many types of blastula are there ? 6 regular coeloblastula central blastocoel surrounded by a single layer of blastoderm in what eggs is regular coeloblastula seen? (characteristics) poor yolk -holoblastic cleavage (equal or subequal) ex of egg where regular coeloblastula is seen oligolecithal eggs of sea urchin (echinoderm) how many layers of blastoderm are there in regular coeloblastula? one