exercise 44 embryology portland community college bi 233
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Exercise 44
Embryology
Portland Community CollegeBI 233
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Zygote
• A zygote is formed when the nucleus of the sperm and ovum combine to create a cell with 23 pairs of chromosomes. (prior to the start of cleavage)
• Fertilization occurs in the ampulla of the uterine tube within 24 hours after ovulation (oocyte dies after 24 hours)
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From Fertilization to Implantation
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Formation of the Morula
• Cleavage: Rapid mitotic cell division (without cell growth) that subdivides the cytoplasm of the zygote
• Cleavage produces daughter cells called blastomeres
• The zona pellucida remains around the entire group of cells.
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Morula
• After 3 days of cleavage, a solid ball of cells has formed that resembles a mulberry (similar to a Marion berry)
• This Morula contains 16 or more cells
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Blastocyst
• By the fourth or fifth day the morula develops into a a fluid-filled hollow sphere called a blastocyst
• Blastocyst : contains 3 parts• Trophoblast: The outer layer of cells
• Take part in placenta formation• Inner cell mass: A group of cells clustered
at one side of the blastocyst• Becomes the embryo
• Blastocoele: The fluid filled cavity
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Blastocyst
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Implantation
• By day six or seven the trophoblast will adhere to the endometrium with the inner cell mass side facing the uterine wall
• The trophoblast cells proliferate and form two distinct layers• Syncytiotrophoblast: Cells on the
perimeter touching the endometrium. • Cytotrophoblast: Cells on the interior of
the trophoblast that retain their cell boundaries
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Implantation
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Implantation
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Syncytiotrophoblast
• At the point of contact between the trophoblast and the endometrium, the dividing cells lose their plasma membranes• This creates a syncytium in which there is a
layer of cytoplasm containing multiple nuclei• The syncytiotrophoblast erodes a path
through the uterine endometrium by secreting hyaluronidase
• The implanted blastocyst will eventually be covered over by endometrial cells
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Implantation
• Viability of the corpus luteum is maintained by human chorionic gonadotropin (hCG) secreted by the trophoblast
• 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
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Chorionic Villi
• Chorionic villi are extensions of the cytotrophoblast covered with syncytiotrophoblast that extend into the maternal blood filled spaces (lacunae)
• Maternal blood will exchange nutrients through these villi that will eventually contain embryonic vessels.
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Embryonic Disc
• The inner cell mass organizes into an oval sheet with two layers (the embryonic disc)
• Ectoderm: The layer closest to the attachment of the trophoblast
• Endoderm: The layer closest to the blastocoele
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Amniotic Cavity
• The inner cell mass will separate from the trophoblast during implantation
• The ectoderm cells will migrate and form a fluid-fill chamber (the amniotic cavity)
• Amnion: Mesodermal cells will soon create the outer second layer of the amnion
• The fluid and membranes will surround and cushion the developing embryo and fetus
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Yolk Sac
• Begins as the endodermal cells spread around the outer edges of the blastocoele.
• The yolk sac is completed as the mesoderm cells migrate around the pouch.
• Site of embryonic blood cells and gonadal stem cells
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Gastrulation
• This is the term for the process by which a third layer of cells called the mesoderm, forms between the ectoderm and endoderm.
• Primitive streak : raised dorsal groove that establishes the longitudinal axis of the embryo
• Gastrulation sets the stage for organogenesis; the formation of body organs
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Primary Germ Layers
• Serve as primitive tissues from which all body organs will be derived
• Endoderm:• Epithelial lining of GI & lower respiratory tract• All ducts entering the GI tract• Urinary bladder
• Ectoderm:• Nervous system• Epidermis• Lining of mouth, and anus• Sense organs such as eyes
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Primary Germ Layers
• Mesoderm:• Muscle• Bone• Cartilage• Blood• Dermis and hypodermis• Kidneys, ovaries, testes• Lining of body cavities
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Embryonic Membranes:Chorion and Allantois
• Allantois: a small outpocketing at the caudal end of the yolk sac• Structural base for the umbilical cord• Becomes part of the urinary bladder
• Chorion • Becomes the embryonic contribution to the placenta• Derived from trophoblast & mesoderm lining it• Contains the chorionic villi
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Umbilical Cord
• Connects the embryo/fetus to the placenta
• Contains 2 arteries and 1 vein
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2 Arteries
1 Vein
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Placenta• The placenta is fully formed and functional by the 3rd
month of pregnancy• Chorion of embryo & stratum functionalis layer of
uterus • Chorionic villi extend into maternal blood filled
intervillous spaces: maternal & fetal blood vessels do not join & blood does not mix• Diffusion of O2, nutrients, wastes• Stores nutrients & produces hormones• Barrier to microorganisms, except some viruses
• AIDS, measles, chickenpox, poliomyelitis, encephalitis
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Endocrine Placenta
• Human Chorionic Gonadotropin (hCG): Maintains the integrity of the corpus luteum and promotes the continued secretion of progesterone
• Human Placental Lactogen (hPL) and Placental Prolactin: Prepare the mammary glands for milk production
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Endocrine Placenta
• Relaxin: which relaxes connective tissue of pelvis and cervix• Suppresses the release of oxytocin by the
hypothalamus
• Progesterone and Estrogens: Maintain the endometrial lining
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Placenta
This side faces the baby This side touches the uterus
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Gestation
• Fetal period: Begins at week 9• All major organs have been established • This is the time of growth
• Gestation: 40 weeks measured from the first day of the uterine cycle (day one of menstruation)• Actually only 38 weeks from fertilization
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The End
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