Lab Activity 35

Lab Activity 35

Embryology

Portland Community CollegeBI 233

Spermatogenesis

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Oogenesis

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Fertilization

• An egg must be fertilized within 12 to 24 hours of ovulation

• It takes about 72 hours for an egg to reach the uterus.

• The sperm must encounter the egg somewhere in the uterine tube

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Fertilization

• A sperm can survive for 48-72 hours in the female reproductive tract

• It takes about 10 hours to navigate into the ampulla of the fallopian tube where fertilization occurs. 

Fertilization

• When the sperm encounters an egg, it undergoes an acrosomal reaction- exocytosis of the acrosome, releasing the enzymes needed to penetrate the egg.

• Sperm must penetrate the granulosa cells and zona pellucida that surrounds the egg

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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.

• About 3 days

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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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Gastrulation

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Gastrulation

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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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Ectoderm

Mesoderm

Endoderm

Approximately 13 days after fertilization

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21-23 Days After Fertilization

• Actual Size 1.0 – 1.5 mm

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28 Days After Fertilization

• By the end of week 4 there’s significant development of the nervous system, heart, body form and placenta.

• The embryo starts receiving nutrients and oxygen through the placenta.

Actual Size: 4-6 mm

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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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Placenta

This side faces the baby This side touches the uterus

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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

Fetal Circulation• Umbilical vein conducts

oxygenated fetal blood from the placenta, through the umbilical cord, to and through the liver, and into the ductus venosus which drains into the inferior vena cava, which returns blood to the heart

• Oxygenated blood may flow through the foramen ovale into the left atrium.

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Fetal Circulation• This bypasses the pulmonary

system.

• Some right atrial blood moves into the right ventricle and leaves the heart through the pulmonary artery where another circulation bypass is located called the ductus arteriosus.

• The umbilical arteries return the deoxygenated blood to umbilical cord and placenta.

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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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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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Fetal Period

• 56 days after fertilization

• Actual Size: 30 mm

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The End