Chapter 2: Transport of Gametes and Fertillization Flashcards
at what stage in meiosis do the first and second meotic blocks occur?
first block: prophase I
second block: metaphase II
stigma
the apex of the protrusion that is formed when the follicle bulges from the surface of the ovary
stimulus for ovulation
LH surge from anterior pituitary that occurs at the midpoint of the menstrual cycle
results of LH surge
follicle reorganizes program of gene expression from one directed toward development of follicle to one producing molecules that set into motion the processes of follicular rupture and ovulation
local blood flow increases resulting in local edema; plasma proteins leak into tissues
cumulus cells secrete hyaluronic acid that loosens cells surrounding egg
matrix metalloproteinases
a family of lytic enzymes that degrade components of extracellular matrix; local production occurs as result of release of certain pharmacologically active substances produces an inflammatory reaction that ultimately results in rupture of outer follicular wall about 28 to 36 hours after LH surge
result of follicular rupture
cumulus oophorus detaches from granulosa and egg released from ovary expulsion of both antral fluid and ovum from ovary;
what is expelled from ovary at ovulation?
ovum
zona pellucida
two-three-cell thick corona radiate
sticky matrix containing surrounding cells of cumulus oophorus
corona radiata
adhering cumulus cells after ovulation has occurred
mittelschmerz
pain some women feel at the time of ovulation possibly due to slight bleeding from ruptured follicle
changes in uterine tubes prior to ovulation
more high ciliated and smooth muscle activity in the tube and its suspensory ligament increases as the result of hormonal influences
capture of the egg by uterine tube
fimbriae move closer to ovary, sweep rhythmically over its surface; adhesive interactions between egg complex and ciliary surface of the tube
transport of egg in uterine tube
egg is transported toward the uterus mainly as result of contractions of smooth musculature of the tubal wall
takes 3 to 4 days whether or not fertilization occurs
immotile cilia syndrome
women whose tubal cilia do not function; many of these women are still fertile, so cilia action not obligatory to move egg
tubal fluid
combination of secretions by the tubal epithelial cells and transudate from capillaries just below the epithelium
egg is in this during tubal transport
slow transport and fast transport of egg
slow transport occurs in ampulla and more rapid transport through isthmus and into uterus
egg is temporarily prevented from entering isthmic portion of tube, under influence of progesterone the egg is allowed to pass
sperm transport in male reproductive tract
transport closely connected with structural and functional maturation
after spermiogenesis in seminiferous tubules, sperm are morphologically mature but nonmotile and incapable of fertilizing egg;
transported via testicular fluid to the caput (head) of epididymis through rete testis and efferent ductules (propelled by seminiferous tubules and assisted by smooth muscle contraction and ciliary currents in ductules)
in epididymis, changes in glycoproteins in the plasma membrane of the sperm head
sperm transport in male reproductive tract: ejaculation
spermatozoa rapidly pass through ductus deferens, become mixed with fluid secretions from seminal vesicle and prostate gland
seminal vesicles
provide fructose to the semen (main energy source)
prostate gland
provides citric acid, acid phosphatase, zinc, magnesium ions
semen
contains 40-250 million sperm, mixed with seminal vesicle fluid (60%) and prostate secretion (30%) pH: 7.2-7.8
sperm transport in female reproductive tract
begins in upper vagina and ends in ampulla region of uterine tube
when in upper vagina, confronted with harsh acidic environment that normally is bactericidal
prostate provides a buffering effect, briefly raising pH from 4.3-7.2
cervical canal and cervical mucus block the sperms path
cervical mucus
composition and viscosity vary considerable throughout the month; not readily penetrable
cervical mucin
makes up cervical mucus; a glycoprotein with a high carbohydrate composition
E mucus
during days 9-16 of cycle, water content of cervical mucus increases, facilitates the passage of sperm through cervix at the time of ovulation
G mucus
after ovulation, under the influence of progesterone, watery cervical mucus production ceases, a new type of sticky mucus, which has much decreased water content, is produced; almost completely resistant to sperm penetration
two modes of sperm transport in cervix: rapid transport
initial rapid transport relies on muscular movement of female reproductive tract; hinges on female orgasm; early arriving sperm not as capable of fertilization
two modes of sperm transport in cervix: slow transport
involves the swimming of spermatozoa through cervical mucus, their storage in cervical crypts and final passage through cervical canal
sperm in uterine tubes
only several hundred sperm reach uterine tubes spermatozoa collect in isthmus and bind to epithelium for about 24 hours
capacitation
when bound to isthmus, sperm undergo this reaction because of secretions of the tube; (1) removal of cholesterol from surface of sperm; (2) removal of many of the glycoproteins that were put on the surface while in epididymis required for sperm to be able to fertilize the egg
hyperactivation of sperm
after capacitation, sperm undergo period of hyperactivation to break free of bonds that held them to the tubal epithelium; assists sperm in penetrating zona pellucida and corona radiata; only small number of sperm released at one time
meeting of sperm and egg
sperm make their way up while egg moves down; peristaltic contractions divide the uterine tube into compartments within a given compartment, the gametes are caught up in a churning motion that brings them together
fertilization occurs in the ampullary portion of the uterine tube
chemoattractive agent of egg?
possible that egg might be releasing certain chemicals that the sperm are able to pick up; mammalian sperm posses odorant receptors and respond behaviorally to certain chemically defined odorants
known to respond to certain temperature gradient
only capacitated sperm have ability to react to chemical or thermal stimuli
changes in follicle following ovulation
after ovulation, basement membrane that separate the granulosa cells from the theca interna cells breaks down, allowing theca blood vessels to grow into cavity
luteinization
granulosa cells undergo a series of major changes in form and function after ovulation
granulosa lutein cells
granulosa cells 30-40 hours after LH surge begin secreting large amounts of progesterone and some estrogen
terminally differentiated
corpus luteum
follicle continues to enlarge following rupture; because of yellow color, known as corpus luteum
granulosa lutein cells continue to secrete progesterone for ten days even though at this point they have stopped dividing
luteolysis
in the absence of fertilization and hormonal stimulus from embryo, corpus luteum begins to deteriorate late into menstrual cycle
involves preprogramming luteal cells to apoptosis and uterine luteolytic factors such as prostaglandins F2
regression of corpus luteum and reduction of progesterone causes hormonal withdrawal that results in the degenerative changes in endometrial tissues during last days of menstrual cycle
corpus albicans
granulosa lutein cells begin to deteriorate and are replaced with collagenous scar tissue; looks white now
chorionic gonadotropin
if fertilization occurs, production of this hormone by placental tissue maintains the corpus luteum in a functional condition and causes increase in its size and hormone production
corpus luteum of pregnancy
composed primarily of theca lutein cells
remains functional for the first few months of pregnancy
after second month, placenta produces enough estrogen and progesterone to maintain pregnancy on its own
penetration of corona radiata
outer layer of egg complex encountered by the sperm is the corona radiata and some leftover cells from cumulus oophorus
corona radiata is highly cellular layer with an intercellular matrix consisting of proteins and a high concentration of carbohydrates especially hyaluronic acid
sperm accomplishes penetration through swimming motion and hyaluronidase that is emanating from the sperm head
zona pellucida
consists of 4 glycoproteins, ZP1, ZP2, ZP3, ZP4
ZP2 and 3 combine to form basic units that polymerize into long filaments which are periodically cross linked by ZP1 and 4
after penetrating corona radiata, sperm bind to zona pellucida by binding to sialic acid molecule which is the terminal part of a sequence of four sugars at the end of O-linked oligosaccharides attached to the polypeptide core of ZP3
ZP3 are basically sperm receptors on zona pellucida
differences in sperm binding regions of ZP3 prevent interspecies fertilization
acrosomal reaction
occurs upon binding to zona pellucida
fusion of parts of the outer acrosomal membrane with the overlying plasma membrane and the pinching off of fused parts as small vesicles;
results in liberation of multitude of enzymes stored in the acrosome stimulated by ZP3 molecules acting through G proteins in the plasma membrane on the sperm head;
a large segment of the polypeptide chain of ZP3 must be present to induce the acrosomal reaction
massive influx of calcium through plasma membrane of sperm head, followed by influx of sodium and efflux of hydrogen which increases intracellular pH
enzymes help sperm make it through the zona pellucida
postacrosomal plasma membrane
after acrosomal reaction, inner acrosomal membrane forms the outer surface covering most of the sperm head; fuses with the remaining postacrosomal plasma membrane to maintain membrane continuity around sperm head
penetration of zona pellucida
only possible after acrosomal reaction
accomplished through combination of mechanical propulsion by movements of the sperm’s tail and digestion of a pathway through the action of acrosomal enzymes
acrosin
most important of the acrosomal enzymes; serine proteinase that is bound to the inner acrosomal membrane
perivitelline space
space between the egg’s plasma membrane and the zona pellucida
now sperm can make contact with plasma membrane of egg
binding and fusion of spermatozoon and egg
- spermatozoon binds to and then fuses with the plasma membrane of egg; binding occurs when the equatorial region of the sperm head contacts the microvilli surrounding the egg; on sperm, FERTILINS and CYRITESTIN proteins bind to ALPHA-SIX INTERGRIN and CD9 PROTEIN molecules on the surface of the egg; integrin mediates the actual fusion and brings membranes into continuity
- contents of the spermatozoon (head, midpiece and usually tail) sink into the egg, whereas the sperms plasma membrane, which is antigenically distinct from that of the egg, becomes incorporated into the egg’s plasma membrane; sperm contributes the centrosome, which is required for cell cleavage
polyspermy
entrance of other spermatozoa into the egg after a spermatozoon has already fused with the egg
fast block to polyspermy
rapid electrical depolarization of plasma membrane of eg from -70 to +10 mV within 2 to 3 seconds after fusion
prevents other spermatozoa from adhering to egg’s plasma membrane
lasts only several minutes, but it is sufficient for egg to mount permanent slow block
slow block to polyspermy
successive waves of calcium ions pass through cytoplasm of the egg; first set of waves spreads from site of sperm-egg fusion is involved in completion of second meiotic division of the egg
later waves initiate the recruitment of maternal RNAs in the egg and act on cortical granules as they pass them by
cortical granules then fuse with plasma membranes and release their contents (hydrolytic enzymes and polysaccharides) into perivitelline space
polysaccharides causes the perivitelline space to swell, lifting zona pellucida from egg membrane
changes to sperm receptors on egg surface cause the egg itself to become refractory to penetration by other spermatozoa
metabolic activation of the egg
sperm introduces into egg a soluble factor which stimulates a pathway leading to release of pulses of calcium ion within cytoplasm
calcium ion stimulates rapid intensification of egg’s respiration and metabolism through an exchange of extracellular sodium for intracellular hydrogen;
rise in intracellular pH and increase in oxidative metabolism
decondensation of sperm nucleus
thanks to disulfide bonds formed among protamine molecules, sperm DNA is extremely tightly bound
after sperm comes into egg, it becomes permeable to egg cytoplasmic factors that reduce disulfide cross-links to sulfhydryl groups by reduced glutathione in ooplasm;
protamines are rapidly lost and as chromatin spreads out in sperm nucleus it becomes
pronucleus remodeling takes 6 to 8 hours; histones begin to associate with chromosomes
completion of meiosis
after sperm penetrates egg, egg completes the last division and releases as second polar body into the perivitelline space
nucleus of egg moves toward the cortex as a result of the action of myosin molecules acting on a network of actin filaments that connect on pole of the mitotic spindle to the cortex
cytoplasmic factors control growth of male and female pronuclei
DNA replication occurs in the developing haploid pronuclei and each chromosome forms two chromatids;
when pronuclei come into contact, membranes break down and chromosomes intermingle; become organized around the mitotic spindle, derived from centrosome of sperm
what is accomplished by fertilization?
- stimulates the egg to complete the second mitotic division
- restores the zygote to the normal diploid number of chromosomes
- genetic sex of the future embryo is determined
- through mingling of maternal and paternal chromosomes, zygote is a genetically unique product of chromosomal reassortment
- metabolic activation of the egg, which is necessary for cleavage and subsequent embryonic development to occur
