Conception
Lecture 3
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Conception
• Conception is when sperm and egg meet
and fertilization occurs
• Conception occurs in the outer third of the
fallopian tube
• Zygote - fertilized egg
• http://www.youtube.com/watch?v=SdZW_K9J
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Conception
• The sperm reaches the caudal epididymis
approximately 72 days after the initiation of
spermatogenesis.
• Preservation of optimal sperm function during
storage requires adequate testosterone levels and
maintenance of a normal scrotal temperature.
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Stages of Fetal Development
• Preembryonic stage: fertilization through 2nd week
– Fertilization; cleavage; morula
– Blastocyst and trophoblast
– Implantation
• Embryonic stage: end of 2nd week through 8th week
– Basic structures of major body organs and main external
features
• Fetal stage: end of the 8th week until birth
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Embryonic Layers
1. Ectoderm: forms the central nervous system, special
senses, skin and glands
2. Mesoderm: forms skeletal, urinary, circulatory, and
reproductive organs
3. Endoderm: forms respiratory system, liver, pancreas,
and digestive system
Site of sperm formation
Seminiferous tubules of Testis
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Seminiferous tubule
under microscope
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Conception
• The alkaline pH of semen provides protection for the
sperm from the acid environment of the vagina.
• Of an average of 200 to 300 million sperm deposited
in the vagina, at most only a few hundred achieve
proximity to the egg.
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Conception
• Capacitation is characterized by three
accomplishments:
1. The ability to undergo the acrosome reaction.
2. The ability to bind to the zona pellucida.
3. The acquisition of hypermotility.
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SPERMIOGENESIS
• Spermatids are rounded cells.
• They modify to assume specific
shape of the sperm.
• This process is called
Spermiogenesis. In it they
elongate and reorganize
internal structure to acquire
the particular shape.
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Accomplishing Fertilization
• The oocyte is viable for 12 to 24 hours
• Sperm is viable 24 to 72 hours
• For fertilization to occur, coitus must occur no
more than:
– Three days before ovulation
– 24 hours after ovulation
• Fertilization – when a sperm fuses with an egg
to form a zygote
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Sperm Transport and Capacitation
• Fates of ejaculated sperm
–
–
–
–
Leak out of the vagina immediately after deposition
Destroyed by the acidic vaginal environment
Fail to make it through the cervix
Dispersed in the uterine cavity or destroyed by phagocytic
leukocytes
– Reach the uterine tubes
• Sperm must undergo capacitation before they can
penetrate the oocyte
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Acrosomal Reaction and Sperm Penetration
• An ovulated oocyte is encapsulated by:
– The corona radiata and zona pellucida
– Extracellular matrix
• Sperm binds to the zona pellucida and
undergoes the acrosomal reaction
– Enzymes are released near the oocyte
– Hundreds of acrosomes release their enzymes to
digest the zona pellucida
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Acrosomal Reaction and Sperm Penetration
• Once a sperm makes contact with the oocyte’s
membrane:
– Beta protein finds and binds to receptors on the
oocyte membrane
– Alpha protein causes it to insert into the
membrane
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Acrosomal Reaction and Sperm
Penetration
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Figure 28.2a
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Blocks to Polyspermy
• Only one sperm is allowed to penetrate the oocyte
• Two mechanisms ensure monospermy
– Fast block to polyspermy – membrane depolarization
prevents sperm from fusing with the oocyte membrane
– Slow block to polyspermy – zonal inhibiting proteins (ZIPs):
• Destroy sperm receptors
• Cause sperm already bound to receptors to
detach
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Implantation
• Implantation is defined as the process by which an
embryo attaches to the uterine wall and penetrates
first the epithelium and then the circulatory system
of the mother to form the placenta.
• The human blastocyst remains in the uterine
secretions for approximately 1 to 3 days and then
hatches from its zona pellucida in preparation for
attachment.
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Implantation
• Viability of the corpus luteum is maintained by
human chorionic gonadotropin (hCG) secreted
by the trophoblasts
• hCG prompts the corpus luteum to continue to
secrete progesterone and estrogen
• Between the second and third month, the
placenta:
– Assumes the role of progesterone and estrogen
production
– Is providing nutrients and removing wastes
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Implantation
• The blastocyst loosely adheres to the endometrial
epithelium, a process called apposition, which most
commonly occurs on the endometrium of the upper
posterior wall of the uterus.
• The implantation occur in fundus of the uterus,
which is rich blood supply.
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Placental Hormones
• HUMAN CHORIONIC GONADOTROPIN
– maintenance of function of the corpus luteum
– Stimulation of the maternal thyroid gland
– Promotion of relaxin secretion
– Promote uterine vascular vasodilatation and
myometrial smooth muscle relaxation
Student Practice 2
1. Explain the process of implantation?
2. What is the Vaccine During Pregnancy? What
is not?
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The END
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