Sperm Penetration

8/7/2019 Sperm Penetration

http://slidepdf.com/reader/full/sperm-penetration 1/13

Sperm Penetration

The animation shows:

1. sperm moving between granulosa cells to contact the zona pellucida.2. sperm releasing acrosomal contents to breakdown zona pellucida.

3. sperm fusing with egg membrane.4. egg releasing cortical granule contents.

5. egg completing meiosis.

The animation shows:

1. zona pellucida (yellow).2. three polar bodies (green) which contain excess DNA.

3. two pronuclei (blue) which loose their nuclear membranes and fuse together.4. chromosomes pairing to form the diplod zygote

Pronuclear Fusion (264Kb) (More? Movies page)

Fertilization Overview

The following text is extracted and modified from lecture slides and should be used as a

"trigger" to remind you of key concepts in fertilization.

y Fertilization and Early Development

o Gamete formation egg formation-oogenesis

sperm formation- spermatogenesis

o Fertilization sperm and egg migration

sperm- activation, binding, acrosome reaction egg- 2nd meiotic division, cortical reaction

o Formation of the Zygote male and female pronuclei

first mitotic division sex determination

o Formation of the Blastocyst

Female Reproductive Early cell division

y Gamete formation- Oogenesis

o process of oogonia mature into oocytes (ova, ovum, egg)

o all oogonia form primary oocytes before birth

o maturation of preexisting cells in the female gonad- ovary

o humans usually only 1 ovum released (IVF- superovulation)

o oocyte and its surrounding cells = follicle

o primary -> secondary -> ovulation releases



y Ovary- Histology - whole transverse section (cortex, medulla)y Menstrual Cycle

y Oogenesis- preantral follicley Primary Oocyte

o arrested at early Meiosis 1

o diploid: 22 chromosome pairs + 1 pair X chromosomes (46, XX)

o autosomes and sex chromosomey Oogenesis- antral follicle

y Secondary oocyte

o 1 Day before ovulation completes (stim by LH) Meiosis 1

o haploid: 22 chromosomes + 1 X chromosome (23, X)

o nondisjunction- abnormal chromosome segregation

o begins Meiosis 2 and arrests at metaphase

o note no interphase replication of DNA

o only fertilization will complete Meiosis 2

y 2nd Meiotic Division

y Ovulation

y Hypothalmus (GRH) -> Pituitary FSH and LH -> ovary

o GRH- gonadotropin releasing hormoneo FSH- follicle stimulating hormone

o LH- lutenizing hormoneo release of the secondary oocyte and formation of corpus luteum

o secondary oocyte encased in zona pellucida and corona radiatay Ovulation- ampulla movement

y Ovulation- follicle

y Zona Pellucida

o glycoprotein shell ZP1, ZP2, ZP3

o mechanical protection of egg

o involved in the fertilization process

o sperm binding

o adhesion of sperm to egg

o acrosome reaction

releases enzymes to locally breakdown

o block of polyspermy

altered to prevent more than 1 sperm penetrating

o may also have a role in development of the blastocyst

y Corona Radiata

o granulosa cells and extracellular matrix

o protective cells for egg durin transport

o cells are also lost duringtransport along oviduct

o nutrition

y

Testis- anatomy and histologyy Gamete formation- S permatogenesis

o process of spermatagonia mature into spermatazoa (sperm)

o continuously throughout life occurs in the seminiferous tubules in the male

gonad- testis (plural testes)

o at puberty spermatagonia activate and proliferate (mitosis)

o primary spermatocyte -> secondary spermatocyte-> spermatid->sperm

o Maturation involves meiosis and spermeogenesis

y Seminiferous Tubule



y S permatogenesis- Meiosiso meiosis is reductive cell division

o 1 spermatagonia (diploid) = 4 sperm (haploid)o 46, XY (also written 44+XY)

23, X

23, X

23, Y 23, Y

y S permiogenesis - morphological (shape) change

y S permatogenesis- spermeogenesis

o cell shape change round spermatids -> elongated sperm

o loose cytoplasm

o Transform golgi apparatus into acrosome (in head)

o Organize microtubules for motility (in tail, flagellum)

o Segregate mitochondria for energy (in tail

y Ejaculate

o 3.5 ml, 200-600 million sperm

o Infertility - Oligospermia, Azoospermia, Immotile Cilia Syndrome

o Oligospermia (Low S perm Count) less than 20 million sperm after 72 hour abstinence from sex

o Azoospermia (Absent S perm) blockage of duct network

o Immotile Cilia Syndrome lack of sperm motility

y By volume <10 % sperm

o Accessory Glands

o 60 % seminal vesicle

o 10 % bulbourethral

o 30 % prostate

y Fertilization-sperm and egg migration

o S perm migration

o deposited in vagina

o have approximately 24-48h to fertilize

o migrate through cervix into uterus

o from uterus into oviduct (= uterine horn, fallopian tube)

y Egg migration

o following ovulation from ovary into fimbria

o then into oviduct

y Fertilization Site

o Fertilization usually occurs in first 1/3 of oviduct

o Fertilization can occur outside oviduct

o

associated with ectopic pregnancyo In virto Fertilization- IVF, GIFT, ZIFT....

o the majority of fertilized eggs do not go on to form an embryo

y Fertilization- S perm

o capacitation

removal of glycoprotein coat and seminal proteins

alteration of sperm mitochondria

o binding

ZP3 acts as receptor for sperm



o acrosome reaction exyocytosis of acrosome contents (Ca mediated)

enzymes to digest the zona pellucida exposes sperm surface proteins to bind ZP2

o membrane fusion

between sperm and egg

allows sperm nuclei passage into egg cytoplasmy Fertilization- Egg

y sperm membrane fusion

o causes depolarization of egg membrane

o primary block to polyspermy

y Egg Cortical Reaction

o IP3 pathway elevates intracellular Calcium

o exocytosis of cortical granules

o enzyme alters ZP3 so it will no longer bind sperm plasma membrane

o 2nd meiotic division

o completion of 2nd meiotic division

o forms second polar body (a third polar body may be formed by meiotic

division of the first polar body)y Formation of the Zygote

y Male and Female Pronucleio 2 haploid nuclei approach each other

o nuclear membranes break downo chromosomal pairing

o DNA replicates

o first mitotic division

y S perm contributes - centriole which organizes mitotic spindle

y Oocyte contributes - mitochondria (maternally inherited)

y Sex Determination

o based upon whether an X or Y carrying sperm has fertilized the egg

o should be 1.0 sex ratio

is actually 1.05, 105 males for every 100 females.

o some studies show more males 2+ days after ovulation

o cell totipotent (equivalent to a stem cell, can form any tissue of the body)

y Men- Y Chromosome

o carries sry gene

o gene activates pathway for male gonad (covered in genital development)

y Women- X Chromosome

o one X chromosome in each woman's cell has to be inactivated (because menonly have 1 X chromosome, if abnormal, this leads to X-linked diseases more

common in male that female where bothe X's need to be abnormal)

process is apparently random therefore 50% of cells have father's X, 50% have mother's

Sperm Contact

The act of fertilization changes the egg from a stage of slow structural and metabolic decline

to one of renewed activation. Morphologically egg activation is a series of surface changesimmediately following sperm contact.



y Marine invertebrates and some aquatic vertebrates elevate a fertilization membrane inorder to prevent polyspermy. More than one reaction involved in the prevention of

polyspermy. The first reaction must be a very fast one. In oyster and echinoderm eggsthe cortical granules swell and break down, lifting the vitelline membrane off the egg

surface and causing it to fuse with the plasma membrane of the ovum. Structures

similar to this fertilization membrane occur in amphibians and teleosts. Cortical

granules formed in Golgi region and occur throughout the egg cytoplasm in immatureeggs. They accumulate in the cortical regions as maturation proceeds.

y Mammals - No phenomenon comparable to the raising of the fertilization membrane

is displayed. Mammalian eggs are surrounded by the zona pellucida which undergoes

a structural change known as the zonal reaction after sperm penetration. On sperm

contact with the egg plasma membrane, cortical granules break down as in above

forms, substances liberated into the perivitelline space rapidly modify the zona

pellucida resulting in a block to further sperm penetration.

The Acrosome Reaction

Penetration of egg by sperm is initiated by the acrosome reaction which takes different formsin different species.

The central part of the acrosome elongates into a tube which extends form the head of the

spermatozoon. On contact with the egg the acrosomal membrane fuses with the sperm plasma

membrane thus opening the acrosomal vesicle and liberating the granules containing

acrosomal lysins. The inner portion of the acrosomal membrane everts and lengthens to formthe acrosomal tubule through which the sperm nucleus enters the egg. The mammalian sperm

must remain for a time in the female genital tract before being capable of fertilization -Capacitation - which is essentially a modification of the acrosomal reaction. Mammalian

acrosomal lysins contain proteinases which lyse the glycoproteins of the zona pellucida.

Sperm Activation of Egg

During fertilization sperm activates the egg by induction of a calcium ion (Ca2+) oscillation

within the egg's cytoplasm. Induction occurs by a sperm protein factor (unidentified) whichcan stimulate only once Ca 2+ oscillations in metaphase eggs. Another sperm derived factor

is then responsible for the inactivation of this oscillation. The activation of the egg by thisCa2+ oscillation is essential for entry of the egg into the first mitotic cycle.

y Ca 2+ oscillations induced by a cytosolic sperm protein factor are mediated by amaternal machinery that functions only once in mammalian eggs . Tie-Shan Tang,

Jian-Bo Dong, Xiu-Ying Huang and Fang-Zhen Sun Development 127, 1141-1150

(2000)

Formation and Fusion of pro-nuclei

Zygote - Sperm Contribution

What the fertilizing sperm contributes in addition to the genetic material to the zygote

differes between species.



Centriole - most mammalian species, sperm contribute a centriole to reconstitute the zygoticcentrosome. In rodents, only a maternal centrosomal inheritance occurs.

Sperm Mitochondria - may enter the zygote, but are eliminated by a ubiquitin-dependent

mechanism.

Perinuclear Theca - located in the sperm head perinuclear region. Contains a cytoskeletalelement to maintain the shape of the sperm head and functional molecules leading to oocyte

activation during fertilization.

Sperm mRNA - there is some evidence that messenger RNA from the sperm may also enter

the egg upon fertilization. (More? Ostermeier GC, Dix DJ, Miller D, Khatri P, Krawetz SA.

S permatozoal RNA profiles of normal fertile men. Lancet. 2002 Sep 7;360(9335):772-7.)

Note that intracytoplasmic sperm injection techniques may introduce sperm components

normally lost during in vivo fertilization.

Gastrulation

"It is not birth, marriage, or death, but gastrulation, which is truly themost important time in your life."

Lewis Wolpert (1986)

During gastrulation, cell movements result in a massive reorganization of the embryo from a simple spherical ball of cells, the blastula, into a multi -

layered organism. During gastrulation, many of the cells at or near the

surface of the embryo move to a new, more interior location.

The primary germ layers (endoderm, mesoderm, and ectoderm) areformed and organized in their proper locations during gastrulation.

Endoderm, the most internal germ layer, forms the lining of the gut andother internal organs. Ectoderm, the most exterior germ layer, forms skin,brain, the nervous system, and other external tissues. Mesoderm, the themiddle germ layer, forms muscle, the skeletal system, and the circulatory

system.

This fate map diagram of a Xenopus blastula shows cells whose fate i s to

become ectoderm in blue and green, cells whose fate is to becomemesoderm in red, and cells whose fate is to become endoderm in yellow.Notice that the cells that will become endoderm are NOT internal!



Although the details of gastrulation differ between various groups of animals, the cellular mechanisms involved in gastrulation are common to allanimals. Gastrulation involves changes in cell motility, cell shape, and cell

adhesion.

Below are schematic diagrams of the major types of cell movements thatoccur during gastrulation.

Invagination: a sheet of cells (called an epithelial sheet) bends inward.

Ingression: individual cells leave an epithelial sheet and become freelymigrating mesenchyme cells.

Involution: an epithelial sheet rolls inward to form an underlying layer.



A cross-section through the embryo allows us to observe the three germlayers that form during gastrulation: ectoderm, mesoderm, andendoderm.

Neurulation

Neurulation in vertebrates results in the formation of the neural tube, whichgives rise to both the spinal cord and the brain. Neural crest cells are alsocreated during neurulation. Neural crest cells migrate away from the neuraltube and give rise to a variety of cell types, including pigment cells andneurons.



Neurulation begins with the formation of a neural plate, a thickening of the

ectoderm caused when cuboidal epithelial cells become columnar.Changes in cell shape and cell adhesion cause the edges of the plate foldand rise, meeting in the midline to form a tube. The cells at the tips of theneural folds come to lie between the neural tube and the overlying

epidermis. These cells become the neural crest cells. Both epidermis and neural plate are capable of giving rise to neural crest cells.

What regulates the proper location and formation of the neural tube? The

notochord is necessary in order to induce neural plate formation.

Animal development: Organogenesis

Organogeneis is the period of animal development during which theembryo is becoming a fully functional organism capable of independentsurvivial. Organogenesis is the process by which specific organs and

structures are formed, and involves both cell movements and celldifferentiation. Organogenesis requires interactions between differenttissues. These are often reciprocal interactions between epithelial sheets

and mesenchymal cells.



1 From Embryology to Developmental Biology: A Descriptive Science Becomes

Experimental

<a

1. In the mid-eighteenth century, the theory of preformation gave way to the theory of

epigenesis as biologists observed embryos specialize over time. Karl Ernst von Baer noted that early vertebrate embryos look alike but gradually become more distinctive.

2. Cells of the early embryo are totipotent. Gradually, biochemically distinct regions of the embryo develop and differentiate down specific developmental pathways.

3. A cell commits to a fate at a certain point in development. Before this time, atransplanted cell develops according to its new surroundings; after this point, it retains

the specialization other cells exhibit in its original location. Differential gene expression

underlies cell specialization.

40.2 Setting the Stage for Human Prenatal Development - The Reproductive

System

4. Developing sperm originate in seminiferous tubules within the paired testes. S permmature in the epididymis and vasa deferentia, and they exit the body through the urethra

during ejaculation. The prostate gland, seminal vesicles, and bulbourethral glands add

secretions to sperm.

5. Oocytes originate in the female gonads, the ovaries. Each month after puberty, one

ovary releases an oocyte into a fallopian tube, which leads to the uterus.

6. The ovaries secrete estrogen and progesterone, hormones that stimulate development

of female sexual characteristics and together with GnRH, FSH, and LH control the

menstrual cycle.

40.3 The Structures and Stages of Human Prenatal Development

7. Human prenatal development begins at fertilization. A single, capacitated sperm cellat a secondary oocyte burrows through the zona pellucida and corona radiata. The two

pronuclei join. Cleavage ensues, and a 16-celled morula forms. Between days 3 and 6,the morula arrives at the uterus and hollows to form a blastocyst, made up of individual

blastomeres. The trophoblast layer and inner cell mass form. Implantation occurs between days 6 and 14. Trophoblast cells secrete human chorionic gonadotropin (hCG),

which prevents menstruation.

8. During the second week, the amniotic cavity forms as the inner cell mass flattens,

forming the embryonic disc. The primitive streak appears. Ectoderm and endoderm

form, and then mesoderm appears, establishing the germ layers of the gastrula. Cells in a

particular germ layer develop into parts of specific organ systems.

9. During the third week the chorion starts to develop into the placenta, and the yolk sac,

allantois, and umbilical cord form as the amniotic sac swells with fluid. Organogenesisoccurs throughout this embryonic period. Gradually structures appear, including the

notochord, neural tube, arm and leg buds, heart, facial structures, skin specializations,

and skeleton. Structures continue to elaborate during the fetal period.

10. Labor begins as the fetus presses against the cervix. Uterine contractions expel the

baby, placenta, and extraembryonic membranes.

40.4 Growth and Development After Birth 11. Postnatal stages include the newborn, infant, child, adolescent, and adult. Drastic

changes occur at birth, as the newborn takes on functions that the pregnant woman



Fertilization occurs in the upper portion of the uterine tube. Embryonic cell division begins and the

embryo produces the hormone HCG, human chorionic gonadotropin. This hormone maintains the

corpus luteum so that progesterone continues to be released, and the uterine lining continues to

thicken so the embryo can survive.

provided. Infancy and childhood are periods of rapid growth and maturation of organsystems. Hormonal changes dominate adolescence. Organ systems function during

adulthood and begin to show signs of aging.12. In passive aging, structures break down, and DNA repair becomes less efficient. In

active aging, lipofuscin accumulates in cells and cells die.

13. The theoretical maximum human life span is 120 years. Life expectancy is a more

realistic measure of how long someone will live, based on age and epidemiologicalinformation.



Pregnancy

Sperm Penetration

Documents

Transcript of Sperm Penetration