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Lecture

BIOB32H3 Lecture Notes - Varicose Veins, Serous Membrane, Sexual Intercourse


Department
Biological Sciences
Course Code
BIOB32H3
Professor
Kenneth Welch

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Pregnancy and Human Development
From Egg to Embryo
Pregnancy events that occur from fertilization until the infant is born
Conceptus the developing offspring
Gestation period from the last menstrual period until birth
Preembryo conceptus from fertilization until it is two weeks old
Embryo conceptus during the third through the eighth week
Fetus conceptus from the ninth week through birth
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
Sperm Transport and Capacitation
Fates of ejaculated sperm include:
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
Acrosomal Reaction and Sperm Penetration
An ovulated oocyte is encapsulated by :
The corona radiata
The zona pellucida
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
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
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
The cortical granules release enzymes that destroy sperm receptors
These enzymes cause sperm already bound to receptors to detach
Completion of Meiosis II and Fertilization
Upon entry of sperm, the secondary oocyte:
Completes meiosis II

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Casts out the second polar body
The ovum nucleus swells, and the two nuclei approach each other
When fully swollen, the two nuclei are called pronuclei
Fertilization when the pronuclei come together
Preembryonic Development
The first cleavage produces two daughter cells called blastomeres
Morula the 16 or more cell stage (72 hours old)
By the fourth or fifth day the preembryo consists of 100 or so cells (blastocyst)
Blastocyst a fluid-filled hollow sphere composed of:
A single layer of trophoblasts
An inner cell mass
Trophoblasts take part in placenta formation
The inner cell mass becomes the embryonic disc
Implantation
Begins six to seven days after ovulation when the trophoblasts adhere to the endometrium
The trophoblasts then proliferate and form two distinct layers
Cytotrophoblast cells of the inner layer that retain their cell boundaries
Syncytiotrophoblast cells in the outer layer that lose their plasma membranes and
invade the endometrium
The implanted blastocyst is covered over by endometrial cells
Implantation is completed by the fourteenth day after ovulation
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
Chorion developed from trophoblasts after implantation, continues this hormonal stimulus
Between the second and third month, the placenta:
Assumes the role of progesterone and estrogen production
Is providing nutrients and removing wastes
Placentation
Formation of the placenta from:
Embryonic trophoblastic tissues
Maternal endometrial tissues
The chorion develops fingerlike villi, which:
Become vascularized
Extend to the embryo as umbilical arteries and veins
Lie immersed in maternal blood
Decidua basalis part of the endometrium that lies between the chorionic villi and the
stratum basalis
Decidua capsularis part of the endometrium surrounding the uterine cavity face of the
implanted embryo
The placenta is fully formed and functional by the end of the third month
Embryonic placental barriers include:
The chorionic villi
The endothelium of embryonic capillaries
The placenta also secretes other hormones human placental lactogen, human chorionic
thyrotropin, and relaxin
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