Fertilization early embryo dev't
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General Anatomy Lecture # 24
Fertilization
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Fertilization
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At the time of coitus 200-600 million sperms are deposited into the female vagina in the vicinity of cervix.
Following ejaculation, semen undergoes clotting by the action of enzymes present in the prostatic secretion.
After about 5 minutes clot liquefies and spermatozoa become actively motile: the tails perform undulating movements which propels them forwards. The speed varies with the environment but on the average it is 2 -3 mm/min.
Prostaglandinds reverse the peristaltic contractions of the uterus and uterine tubes thus helping the ascent of spermatozoa to the site of fertilization.
Out of millions of spermatozoa only 300-500 reach the site of fertilization and it is only one which is able to fertilize.
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A freshly deposited sperm is not able to fertilize the secondary oocyte until it is conditioned in the female genital tract for sometimes lasting about 7 hours.
During this time glycoprotein coat and seminal plasma proteins are removed from the plasma membrane that overlies the acrosomal region of the spermatozoa.
This process whereby freshly ejaculated sperms develop the capacity to fertilize a secondary oocyte is called capacitation of sperm.
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Fertilization:
the fusion of the sperm cell nucleus with the egg cell nucleus to produce a zygote (fertilized egg)
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Fertilization:
External Occurs outside of the body of the
female Increased number of eggs
produced to insure the survival of the species
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Fertilization:
Internal Occurs inside the body of the
female Fewer number of eggs are
produced
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Fertilization:
fertilization in mammals occurs in the oviduct
The ova is viable for approximately 24 hours after ovulation
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REPRODUCTION AND EMBRYONIC DEVELOPMENT THE PROCESS OF FERTILIZATION
THE SPERM APPROACHES THE EGG THE SPERM’S ACROSOMAL ENZYMES DIGEST
THE EGG’S JELLY COAT PROTEINS ON THE SPERM HEAD BIND TO EGG
RECEPTORS THE PLASMA MEMBRANES OF SPERM AND EGG
FUSE THE SPERM NUCLEUS ENTERS THE EGG
CYTOPLASM A FERTILIZATION ENVELOPE FORMS
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Implantation After approximately
a week, the developing embryo is implanted into the uterus
Embryo: conception to 8 weeks
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Embryonic Development
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Embryo:
a multicellular organism in the early stages of development
2 four cell stage embryos
Eight cell stage embryo
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Embryo:
The beginning developmental processes are always the same in all animals:1) cleavage2) growth3) differentiation
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Embryo:
after fertilization the diploid ZYGOTE undergoes cleavage divisions in the oviduct
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Cleavage
the first series of cell divisions by mitosis after fertilization
Cell division is rapid, new cells do not take time for the growth phase G1
cell growth does not occur so cells decrease in size with each cleavage division
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REPRODUCTION AND EMBRYONIC DEVELOPMENT
CLEAVAGE PRODUCES A BALL OF CELLS FROM THE ZYGOTE
CLEAVAGE RAPID SUCCESSION OF CELL DIVISIONS THAT
PRODUCES A BALL OF CELLS – A MULTICELLULAR EMBRYO – FROM THE ZYGOTE
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REPRODUCTION AND EMBRYONIC DEVELOPMENT
CLEAVAGE PRODUCES A BALL OF CELLS FROM THE ZYGOTE
BLASTOCOEL FLUID-FILLED CAVITY IN THE
CENTER OF THE EMBRYO BLASTULA
HOLLOW BALL OF CELLS THAT RESULTS WHEN CLEAVAGE IS FINISHED
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Cleavage divisions
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Morula forms (solid ball of cells) Blastula forms (hollow ball of cells) Cells begin to grow before dividing
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Differentation Gastrulation: one side of
the blastula invaginates (indents) forming a gastrula
Three cell layers form. SECOND MAJOR
PHASE OF EMBRYONIC DEVELOPMENT; ADDS MORE CELLS TO THE EMBRYO; SORTS THE CELLS INTO DISTINCT CELL LAYERS
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Differentation
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Differentiation
The changing of unspecialized embryonic cells into the specialized cells, tissues and organs of a multicellular animal
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REPRODUCTION AND EMBRYONIC DEVELOPMENT
GASTRULATION PRODUCES A THREE-LAYERED EMBRYO GASTRULA
THREE-LAYERED STAGE OF THE EMBRYO THREE LAYERS ARE LABELED
ECTODERM ENDODERM MESODERM
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REPRODUCTION AND EMBRYONIC DEVELOPMENT
GASTRULA LAYERS DEVELOP INTO…
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REPRODUCTION AND EMBRYONIC DEVELOPMENT
ORGANS START TO FORM AFTER GASTRULATION NOTOCHORD NEURAL TUBE SOMITES COELOM
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REPRODUCTION AND EMBRYONIC DEVELOPMENT
CHANGES IN CELL SHAPE, CELL MIGRATION, AND PROGRAMMED CELL DEATH GIVE FORM TO THE DEVELOPING ANIMAL PROGRAMMED CELL DEATH =
APOPTOSIS TIMELY, TIDY SUICIDE OF CELLS NECESSARY TO CREATE SPACE
BETWEEN FINGERS AND TOES
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REPRODUCTION AND EMBRYONIC DEVELOPMENT THE EMBRYO AND PLACENTA TAKE SHAPE DURING THE
FIRST MONTH OF PREGNANCY GESTATION
PREGNANCY; THE CARRYING OF DEVELOPING YOUNG WITHIN THE FEMALE REPRODUCTIVE TRACT
AN OVERVIEW OF DEVELOPMENT EVENTS BLASTOCYST MAMMALIAN EQUIVALENT OF A
BLASTULA TROPHOBLAST OUTER LAYER THAT ALLOWS
UTERINE IMPLANTATION PLACENTA COMBINATION OF TROPHOBLAST AND
MATERNAL CELLS
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REPRODUCTION AND EMBRYONIC DEVELOPMENT AN OVERVIEW OF DEVELOPMENT EVENTS
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REPRODUCTION AND EMBRYONIC DEVELOPMENT EXTRAEMBRYONIC MEMBRANES
AMNION PROTECTIVE FLUID YOLK SAC NO YOLK IN HUMANS, BUT SAME NAME;
NOURISHMENT FOR NON-PLACENTALS CHORION BECOME EMBRYOS PART OF PLACENTA
RELEASES HUMAN CHORIONIC GONADOTROPIN (HCG)
CAUSES CORPUS LUTERUM TO STAY; PREVENTS MENSTRUATION
ALLANTOIS FUNCTIONS IN WASTE DISPOSAL; WILL BECOME THE UMBILICAL CORD
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REPRODUCTION AND EMBRYONIC DEVELOPMENT
EXTRAEMBRYONIC MEMBRANES
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REPRODUCTION AND EMBRYONIC DEVELOPMENT THE PLACENTA
CHORIONIC VILLI CONTAIN EMBRYONIC BLOOD VESSELS
NEAR MATERNAL BLOOD; NO DIRECT MIXTURE OF BLOOD
NUTRIENTS, GASES, WASTE PRODUCES ARE PASSED THROUGH THE PLACENTA
DRUGS, ALCOHOL, VIRUSES CAN PASS THROUGH
ANTIBODIES CAN PASS THROUGH
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Placenta organ that
forms from the embryo and the uterus
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Placenta
contains blood vessels from the mother and the developing baby
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Placenta Oxygen & nutrients diffuse from
the mother’s blood vessels into the baby’s blood vessels
Wastes diffuse from the baby’s blood vessels into the mother’s blood vessels
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Umbilical Cord
two arteries and a vein Connects the fetus to the placenta
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Amniotic Sac Contains fluid
(amniotic fluid) that protects fetus by giving it a stable environment and absorbing shock
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Fetus
By the end of the 8th week of pregnancy the embryo is called a fetus and all of the major structures are present
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Later Stages of Fetal Development
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Human gestation
the period between fertilization and birth
approximately 38-40 weeks
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Teratogens
Substances that may harm the developing fetus and result in the formation of birth defects
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Teratogens include:
Alcohol, certain drugs/medications, infections, and certain chemicals
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How do twins form???
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Monozygotic Twins(Identical Twins)
One egg is fertilized by one sperm Embryo splits into two during the early
stages of development Have identical genes and must be of
the same sex (Incidence: about 3 in every 1000
births)
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Dizygotic Twins(Fraternal Twins)
Two eggs are ovulated and each is fertilized by a sperm cell
No more genetically similar than any other sibling in the family (can be same/different sexes)
Maternal age, use of assisted reproductive technologies are factors
Incidence (6.7/1000 births in Japan to 40/1000 births in Nigeria)
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REPRODUCTION AND EMBRYONIC DEVELOPMENT
NO TIME FOR FUNNY JOKES…WE HAVE TO RACE TO THE FINISH!!!! NERVOUS SYSTEM NEXT!!!