Chapter 3: Cleavage and Implantation

Question 1

reproductive strategy of invertebrates, sea urchins, amphibians, fruit flies

Answer 1

complete early development as fast as possible
storing a moderate amount of yolk in the oocyte and preproducing much of the molecular machinery

necessary for the embryo to move rapidly through cleavage to the start of gastrulation

produce and store huge amounts of ribosomes and RNA–represent maternal gene products and early development is controlled by maternal genome

Question 2

reproductive strategy of birds and reptiles

Answer 2

produce very large egg containing enough yolk that early development can proceed at slower pace;

eliminates need for oocyte to synthesize and store large amounts of RNAs and ribosomes before fertilization

Question 3

reproductive strategies of mammals

Answer 3

eggs are very small; placental eliminates need for a large yolk; mammalian cleavage is prolonged process that typically coincides with the time required to transport early embryo from fertilization site to implantation in uterus

Question 4

trophoblast

Answer 4

specialized tissue that forms the trophic interface between the embryo and the mother during the cleavage period; placenta ultimately forms from trophoblastic tissue

Question 5

mammalian cleavage

Answer 5

leisurely process

after 2 cell stage, cleavage is asynchronous with 1 of the 2 cells (blastomeres) dividing to form a 3-cell embryo

Question 6

morula

Answer 6

embryo with approximately 16 cells

Question 7

compaction

Answer 7

starting with the 8 cell stage;

individual other blastomeres tightly adhere through gap and tight junctions and lose their individual identity when viewed from the surface

Question 8

E-cadherin

Answer 8

mediates compaction; calcium-activated cell adhesion molecule; forms a ring around the apical surface of blastomeres

Question 9

cavitation

Answer 9

process by which through activity of sodium, potassium-adenosine triphosphatase-based system sodium and water move across the outer blastomeres and accumulate in spaces among the inner blastomeres

Question 10

blastocoel (blastocyst cavity)

Answer 10

the fluid-filled space formed by cavitation

Question 11

blastocyst

Answer 11

embryo that has a blastocoel;

embryo still surrounded by zona pellucida and has two layers: outer epithelial layer of trophoblast cells and an inner cell mass

represents specialization of blastomeres into two distinct cell lineages

Question 12

embryonic pole and abembryonic pole

Answer 12

embryonic pole: the end of the blastocyst that contains the inner cell mass

abembryonic pole: opposite end

Question 13

inner cell mass

Answer 13

give rise to the body of the embryo itself in addition to several extraembryonic structures

Question 14

trophoblast cells

Answer 14

form only extraembryonic structures, including outer layers of the placenta

Question 15

fibroblast growth factor-4

Answer 15

growth factor secreted by inner mass cells to maintain mitotic activity in the overlying trophoblast

Question 16

transition from maternally to zygotically produced gene products

Answer 16

because you don’t have massive storage of maternal ribosomes and RNA, zygotic gene products must come into play at an early stage

most maternal transcription is degraded by two-cell stage ; some of these stimulate activation of embryonic genome which begins to produce RNAs from genes

no short transition between cessation of purely maternal to purely embryonic gene products

Question 17

epigenetic regulation

Answer 17

mature egg and sperm are transcriptionally inactivated due to methylation which occurs on CpG dinucleotides

does not alter the fundamental DNA sequence

methylation can inactivate informational genes or their regulators

also, different patterns of histone association with chromatin

Question 18

methylation/demethylation of sperm and eggs

Answer 18

within 4 hours of fertilization, sperm undergo rapid demethylation

demethylation of maternal genome is much more gradual until early morula and remethylation ensues until late blastocyst stage

Question 19

polarization of blastomeres

Answer 19

polarization of 8 or 16 cell embryo

results in clearly recognizable apical and basal surfaces

results in formation of separate lines of cells-trophoblast and inner cell mass

Question 20

inside-outside hypothesis

Answer 20

fate of a blastomere derives from its position within the embryo rather than from its intrinsic properties

outer blastomere = trophoblast
inner blastomere = inner cell mass

Question 21

cell polarity model

Answer 21

if the plane of cell division of a blastomere at the eight-cell stage is parallel to the outer surface of the embryo, the outer daughter cell develops a polarity, with its apical surface facing the zona pellucida;

the inner daughter cell remains apolar and goes onto form part of the inner cell mass

Question 22

ezrin

Answer 22

actin microfilament-stabilizing protein that is key to a duaghter cell becoming an outer cell; that and also inheriting a patch of outer cell membrane that has microvilli

produce polarity and direct differentiation toward trophoblastic lineage

Question 23

transformation of trophoblast cells into inner mass cells and vice versa

Answer 23

even when differentiated into these two lineages, before the 32-cell stage, if one cell type is transplanted into the exterior or interior of embryo, it can be transformed into the other cell type

this capability for phenotypic transformation is lost by 32-cell stage

Question 24

developmental potential or potency

Answer 24

the type of cells that a precursor cell can form;

greater than the cell’s developmental fate

Question 25

developmental fate

Answer 25

the type of cells that a precursor cell normally forms not as great as the cell's developmental potential

Question 26

Cdx-2

Answer 26

transcription factor critical to formation of trophoblastic cells; essential for trophoblastic differentiation and antagonizes the expression of molecules associated with the inner cell mass Cdx-2 mutants fail to implant

Question 27

oct-4

Answer 27

expressed by inner mass cells; codes for specific transcription factor that binds the octamer ATTTGCAT on DNA close relationship between high expression of oct-4 and highly undifferentiated state of cells maternally derived oct-4 protein is required to permit development to proceed to the two-cell stage when transcription of embryo begins expressed in all blastomeres up to the morula stage; expression decreases with increased differentiation oct-4 is still detected in primordial germ cells as they migrate to genital regions even after expression is ceased in rest of embryo

Question 28

Sox-2

Answer 28

binds to DNA in partnership with oct-4 to regulate the expression of genes associated with cellular differentiation

Question 29

Nanog

Answer 29

first appears in the late morula and along with oct-4 functions to maintain the integrity of the inner cell mass no Nanog, and cells of inner cell mass will differentiate into primitive endoderm no oct-4 and cells become trophoblast

Question 30

parental imprinting

Answer 30

expression of certain genes derived from the egg differs from expression of same gens derived from spermatozoon two female pronuclei: embryo itself will develop normally but the placenta and yolk sac are poorly developed two male pronuclei: severely stunted embryo whereas placental and yolk sac develop normally occurs during gametogenesis; methylation is major means of imprinting; imprinted genes are transcriptionally silenced; given imprint is not passed onto the individual's progeny

Question 31

sex chromatin (Barr body)

Answer 31

one of the two X chromosomes in the female is inactivated by extreme condensation purpose: dosage compensation, preservation of cells from excess of X-chromosomal gene product

Question 32

X-inactivation center

Answer 32

unique locus on X chromosome; initiates X chromosome inactivation

Question 33

XIST

Answer 33

x-inactive specific transcript one of the genes in the X-inactivation center; produces large RNA with no protein coding potential XIST RNA remains in the nucleus and coats the entire inactive X chromosome; in the inactivated X chromosome, the XIST gene is unmethylated and expressed; in active X, XIST is methylated and silent

Question 34

ontogenetic history of X chromosome inactivation

Answer 34

by the 4-cell stage and into morula stage, the paternally derived X chromosome becomes inactivated as result of paternal imprinting as embryo forms blastocyst, paternally derided X chromosome in the trophoblast and hypoblast remain inactivated but within cells of inner cell mass both X chromosomes become active as cells of inner cell mass begin differentiation, the somatic cells undergo random permanent XIST-basted X chromosome inactivation of either maternal or paternal X chromosomes within germ line, activation of both X chromosomes occurs during the first meiotic division

Question 35

regulation

Answer 35

the ability of an embryo or organ primordium to produce a normal structure if parts have been removed or added cell fate not irretrievably fixed and cells can still respond to environmental cues

Question 36

mosaic development

Answer 36

inability to compensate for defects or to integrate extra cells into a unified whole fate of cells are rigidly determined, and removal of cells results in an embryo or structure missing components that the removed cells were destined to form most regulative systems have an increasing tendency to exhibit mosaic properties as development progresses

Question 37

experimental techniques to show regulative properties

Answer 37

1. separate blastomeres of early cleavage-stage embryos and determine whether each one can give rise to an embryo; can do this for up to 4 cell embryos 2. single cell from an early cleavage-state embryo can be injected into the blastocoel of a genetically different host

Question 38

chimeras or mosaics

Answer 38

cells from one early cleavage-state embryo injected into another early embryo; it becomes incorporated into host embryo donor cell can be identified by histochemical or cytogenetic analysis

Question 39

fate mapping

Answer 39

they allow one to follow the pathways along which a particular cell can differentiate

Question 40

totipotent

Answer 40

retain the ability to form any cell type in the body; retained at 8-cell stage

Question 41

tetraparental or hexaparental organisms

Answer 41

combine blastomeres of different embryos; form a single large embryo which goes on to become a normal-appearing animal (mouse)

Question 42

human chimeras

Answer 42

human genetic mosaics; most commonly recognized when some regions of the body are male and others are female result of fusion of two early fraternal twin embryos also could be result of exchange of cells through common vascular connections

Question 43

body axes in embryo

Answer 43

conflicting evidence; some suggest that second polar body establishes at least one axis early mammalian embryo is a highly regulative system and that boxy axes do not become fixed until the end of cleavage or early gastrulation

Question 44

deletion or ablation experiments

Answer 44

removing part and determining the way that the remainder of the embryo compensates for the loss

Question 45

addition experiments

Answer 45

adding a part and determining the way that the embryo integrates the added material into its overall body plan one of the most powerful experiments is injecting a genetically or artificially labelled cell into a blastocyst cavity of the host embryo; or using the marking to determine the developmental potency of the donor cells

Question 46

transgenic embryos

Answer 46

commonly mice; produced by directly injecting foreign DNA into the pronuclei of zygotes

Question 47

knock out

Answer 47

now common to knock out gene to study its function in normal development can also use coding RNAi (RNA interference): knocks down rather than blocking gene expression

Question 48

conjoined twins

Answer 48

The separation of portions of an embryo is sometimes incomplete, and although two embryos take shape, they are joined by a tissue bridge of varying proportions; extent of bridging varies between embryos

Question 49

parasitic twins

Answer 49

a form of conjoined twins in which an a much smaller but remarkably complete portion of body protrudes from body of an otherwise healthy and normal host twin common attachment regions: oral, mediastinum and pelvic

Question 50

Bateson's rule

Answer 50

when duplicated structures are joined during critical developmental stages, one structure is the mirror image of the other

Question 51

stem cells

Answer 51

cells in both human adults and embryos that have the ability to develop into a variety of cell and tissue types in response to specific environments; can be induced to differentiate in adults, found in bone marrow, skeletal muscle, brain tissue and fat express oct-4, Sox-2, and Nanog to maintain undifferentiated state

Question 52

embryonic stem cells or embryonic germ cells

Answer 52

stem cells found in embryo; derived from inner cell mass or germ cell lineage

Question 53

induced pluripotent stem cells

Answer 53

from adult somatic cells; characteristics of embryonic stem cells induced in these adult cells and they revert back into an undifferentiated state capable of differentiating into a wide variety of tissues if induced to do so

Question 54

cloning

Answer 54

fusing or introducing an adult cell or nucleus into an enucleated oocyte and allowing the hybrid cell to develop into an embryo and ultimately to mature into an adult.

Question 55

early pregnancy factor

Answer 55

a molecule of the heat shock protein family and homologous to chaperonin 10 (an intramitochonodrial protein); immunosuppressant and thought to provide immunological protection to the embryo produced by embryo and found in serum secreted by ovary; detectable in maternal blood 36 to 48 hours after fertilization

Question 56

zona pellucida and corona radiata during cleavage

Answer 56

corona radiata lost within two days of cleavage zona pellucida remains until implantation in the embryo

Question 57

zona pellucida prior to implantation

Answer 57

composition of zona changes, facilitating transport and differentiation of embryo

Question 58

blastocyst hatching

Answer 58

process by which embryo begins to shed zona pellucida when entering the uterus; a small region of the zona pellucida, usually directly over the inner cell mass, dissolves, and the blastocyst emerges from the hole

Question 59

trophectodermal projections

Answer 59

long microvillous extensions protruding from surfaces of trophoblastic cells; in rodents, release cysteine proteases enzymes that assist in blastocyst hatching; then make contact with endometrial epithelial cells as the process of implantation begins

Question 60

functions of zona pellucida

Answer 60

1. promotes maturation of the oocyte and follicle 2. zona pellucida serves as a barrier that normally allows only sperm of the same species access to the egg 3. initiates acrosomal reaction 4. after fertilization, modified zona prevents any additional spermatozoa from reaching zygote 5. during early stages of cleavage, acts as porous filer through which certain substances secreted by the uterine tube can reach the embryo 6. lacks histocompatibility antigens, serves as immunological barrier for embryo 7. prevents blastomeres of early embryo from dissociating 8. facilitates differentiation of trophoblastic cells 9. normally prevents premature implantation of the cleaving embryo in the wall of the uterine tube

Question 61

uterus at implantation

Answer 61

even before actual contact is made between embryo and endometrium, uterine epithelium secretes into uterine fluid certain cytokines and chemokines that facilitate the implantation process dissolution of zona pellucida signals the embryo is ready for implantation

Question 62

implantation stage 1

Answer 62

attachment of expanded blastocyst to the endometrial epithelium surface of endometrial epithelium cells display various adhesion molecules on their surface that allow implantation to occur only during a narrow 20-24 day window of cycle leukemia-inhibiting factors (LIF) on endometrial surface and LIF receptors on the trophoblast during implantation implantation occurs above inner cell mass at the embryonic pole

Question 63

implantation stage 2

Answer 63

penetration of uterine epithelium cellular trophoblast undergoes more differentiation formation of syncytiotrophoblast small projections of syncytiotrophoblast insert themselves between the uterine epithelium and spread along the epithelial surface of basal lamina that underlies endometrial epithelium to form flatted trophoblastic plate invasion is enzymatically mediated; site is first marked by a bare area or a noncellular plug and is later sealed by migrating uterine epithelial cells

Question 64

cytotrophoblast

Answer 64

in area around inner cell mass cellular trophoblast cells

Question 65

syncytiotrophoblast

Answer 65

fusion of cytotrophoblast to form this multinucleated tissue; will soon surround the entire embryo highly invasive tissue; erodes its way to endometrial stroma

Question 66

trophoblastic plate

Answer 66

flattened projections of syncytiotrophoblast that spread along the epithelial surface of basal lamina of endometrium end up penetrating basal lamina

Question 67

implantation stage 3

Answer 67

projections from the invading syncytiotrophoblast envelop portions of maternal endometrial blood vessels; erode vessel walls and maternal blood begins to fill the isolated lacunae that have been forming in the trophoblast trophoblast is not as invasive anymore

Question 68

decidual cells decidual reaction

Answer 68

fibroblastlike stromal cells of edematous endometrium begin to swell with accumulation of glycogen and lipid droplets tightly adherent and form massive cellular matrix that first surrounds implanting embryo and later occupies most of the endometrium (DECIDUAL REACTION) primary function of decidual reaction is to provide the immunologically privileged site to protect the developing embryo from being rejected

Question 69

interleukin-2

Answer 69

secreted by leukocytes that have infiltrated he endometrial stroma during the late progestational phase ; prevents maternal recognition of embryo as foreign body during early stages of implantation

Question 70

miscarriage

Answer 70

spontaneous abortion; occurs mostly during first 3 weeks of pregnancy; often not recognized by mother at this point many aborted embryos are highly abnormal, with most common being chromosomal abnormalities

Question 71

ectopic pregnancy

Answer 71

implantation that occurs in an abnormal site

Question 72

tubal pregnancies

Answer 72

most common type of ectopic pregnancy; found in ampullary portion of tube, can be located anywhere from fibriated end to the uterotubal junction common in women who have had endometriosis, earlier surgery, or pelvic inflammatory disease

Question 73

endometriosis

Answer 73

condition characterized by the presence of endometriumlike tissue in abnormal locations

Question 74

ovarian and abdominal pregnancies

Answer 74

rare; can be result of fertilization of ovum before it enters the tube;; reflux of fertilized egg from the tube, or penetration of tubal pregnancy from the tube

Question 75

rectouterine pouch (pouch of Douglas)

Answer 75

most common implantation site of abdominal pregnancy; located ehind the uterus

Question 76

lithopedion

Answer 76

calcified remains of an ectopic pregnancy embryo that is not delivered

Question 77

placenta previa

Answer 77

if the embryo implants too close to the cervix, the placental partially covers the cervical canal; causes bleeding in later pregnancy and can cause the death of mother or fetus if untreated due to bleeding associated with premature placental detachment