Lesson # 19 Early Development Chapter 29 Objective s: 1- To describe the process of fertilization. 2- To list the stages of prenatal development, and describe the major events of each. 3- To describe how the three germ layers participate in the formation of the extraembryonic membranes. 4- To discuss the importance of the placenta as an endocrine organ. 5- To describe the fetal circulation.
Lesson # 19. Early Development. Objectives:. Chapter 29. 1- To describe the process of fertilization. 2- To list the stages of prenatal development, and describe the major events of each. - PowerPoint PPT Presentation
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Lesson # 19 Early DevelopmentChapter 29Objectives:
1- To describe the process of fertilization.2- To list the stages of prenatal development, and describe the major events of each.3- To describe how the three germ layers participate in the formation of the extraembryonic membranes.4- To discuss the importance of the placenta as an endocrine organ.5- To describe the fetal circulation.
DevelopmentDevelopment is the gradual modification of anatomical structures and physiological characteristics from fertilization to maturity.
Differentiation It is the creation of different types of cells required in development.Differentiation occurs through selective changes in genetic activity: As development proceeds, some genes are turned off, others are turned on.
Fertilization or ConceptionIt is when the male and female gametes fuse.
Development begins at fertilization or conception.
Development Begins at start of ninth week and continues until birth.
Occurs during first two months after fertilization. Study of these events is called embryology.Prenatal
Development
Postnatal Development
Embryological Development
Fetal Development
Begins at birth and continues to maturity, the state of full development or completed growth
The Oocyte at Ovulation
Ovulation releases a secondaryoocyte (haploid) and the first polar body; both are surrounded by the
corona radiata. The oocyte is suspended in metaphase of
meiosis II.
Zonapellucida
First polarbody
Coronaradiata
It is a layer of glycoprotein gel secreted by granulosa cells around the oocyte.
It is composed of several layers of granulosa cells. Atrophic cell
from meiosis I
Metaphase of Meiosis II
The acrosomal reaction is the exocytosis of the acrosome, releasing the enzymes needed to penetrate the egg.
Hyaluronidase, which digests the hyaluronic acid that binds granulosa cells together.When a path has been cleared, a sperm binds to the zona pellucida.
Two acrosomal enzymes are released:
Acrosin, a protease similar to trypsin.
Acrosomal reaction1
Fertilization and Oocyte Activation
The first step is the binding of the spermatozoon to sperm receptors in the zona pellucida.
Sperm penetration releases an inflow of Ca2+, which stimulates the cortical granules to release their secretion beneath the zona pellucida.The cortical granules release enzymes that inactivate sperm receptors and harden the zona pellucida.
Oocyte Activation2Fertilization membrane
Rejected sperm
Cortical granules
- Cortical Reaction
The cortical reaction prevents the polyspermy (fertilization by two or more sperm).
- Completion of Meiosis II and Formation of the Second Polar Body.
- Activation of Enzymes that Cause a Rapid Increase in the Cell’s Metabolic Rate.
Acrosomal enzymes from multiplesperm create gaps in the corona
radiata. A single sperm then makescontact with the oocyte membrane,
and membrane fusion occurs,triggering oocyte activation and
completion of meiosis.
Fertilizingspermatozoon
Second polarbody
Fertilization and OocyteActivation
Pronucleus FormationBegins
The sperm is absorbed intothe cytoplasm, and the female
pronucleus develops.
Nucleus offertilizing
spermatozoonFemale
pronucleus
Spindle Formation andCleavage Preparation
The male pronucleusdevelops, and spindle fibersappear in preparation for the
first cleavage division.
FemalepronucleusMale
pronucleus
Amphimixis:It is the process by which the male and female pronucleus fuse to form a zygote that contains 46 chromosomes.The formation of the zygote is the “moment of conception”.
Amphimixis Occurs andCleavage Begins
Metaphase of firstcleavage division
Cleavage:It is a series of cell divisions (mitotic divisions) that produce an ever-increasing number of smaller and smaller daughter cells.
The first cleavage divisionnears completion roughly30 hours after fertilization.
Cleavage Begins
Blastomeres
The First Trimester1- Cleavage
2- Implantation
It is a sequence of cell divisions (mitosis) that begins immediately after fertilization. During cleavage, the zygote becomes a pre-embryo, which develops into multicellular blastocyst.
It begins with attachment of the blastocyst to the endometrium of the uterus.
It is the formation of viable embryo. This process establishes the foundations for all major organ systems.
3- PlacentationIt occurs as blood vessels form around the periphery of the blastocyst and the placenta develops.
4- Embryogenesis
Cleavage ends when the blastocyst first contacts the uterine wall.
Spermpronucleus
Eggpronucleus
Zygote2-celled stage
DAY 1
BlastomeresIt is a solid ball of 16 cells that resemble a mulberry.
The morula lies free in uterine cavity for 4-5 days and divides into a 100 cells or so.
The zona pellucida dis-integrates and releases conceptus: blastocyst.
4-celled stage DAY 2
Early morula DAY 3
Advanced MorulaDAY 4
Blastocyst DAY 6
Implanted blastocyst
Fertilization(0 hours)
2- Implantation DAYS 7-10
Cleavage and Blastocyst FormationCleavage is a sequence of cell divisions (mitosis) that begins immediately after fertilization. During cleavage, the zygote becomes a pre-embryo, which develops into multicellular blastocyst.
Hatching
Blastocoel
Trophoblast
Inner cell mass
Cleavage and Blastocyst Formation
Zygote
(DAY 8)
Implantation
Endometrium:
Blastocyst:
(DAY 7)
Blastocoel
TrophoblastInner cell mass
EpitheliumEndometrial gland
The blastocyst attaches to uterine wall 6 days after ovulation, usually on the fundus or the posterior wall of the uterus.
Implantation: It is the process of attachment to uterine wall that begins when blastocyst adheres to the endometrium.
The trophoblast on the attachment side separates into two layers:
The superficial layer in contact with the endometrium. The plasma membranes break down and trophoblastic cells fuse into a multinucleate mass called syncytial trophoblast.
Syncytial trophoblast
The deep layer, close to embryoblast, retains the individual cells divided by membranes and is called cellular trophoblast.
Cellular trophoblast
Inner cell mass (or embryoblast)
Trophoblast:
The trophoblast secrets human chorionic gonadotropin (HCG), which stimulates the corpus luteum to secret estrogen and progesterone (it suppresses menstruation).
Stages of Implantation
Formation of the Amniotic Cavity
It is the arrangement of the inner cell mass into three primary germ layers: ectoderm, mesoderm, and endoderm
Gastrulation and Germ Layer Formation
Endoderm
Mesoderm
Ectoderm
The inner cell mass separates slightly from the trophoblast and creates a narrow space between them: the amniotic cavity
Amniotic cavity
Once the three primary germ layers are formed, embryogenesis is complete and the individual is considered an embryo. It is about 2 mm long and 16 days old.
Gastrulation is the arrangement of the inner cell mass into three primary germ layers: ectoderm, mesoderm, and endoderm
Formation of the Extraembryonic Membranes Germ layers also form four extraembryonic membranes: 1- The Yolk Sac 2- The Amnion 3- The Allantois 4- The Chorion
Yolk Sac (Endoderm and mesoderm)It becomes an important site of blood cell production.
(Ectoderm and mesoderm)As development proceeds, the amnion and amniotic cavity continue to enlarge. The amniotic cavity contains amniotic fluid, which surrounds and cushions the embryo.
Amnion
Allantois (Endoderm and mesoderm)The base of the allantois gives rise to the urinary bladder.
Chorion (Mesoderm and trophoblast)
Chorionic villi of placenta
The appearance of blood vessels in the chorion is the first step in the creation of the placenta.
Deciduacapsularis
Deciduabasalis
Deciduaparietalis
The portion of endometrium that participates in the formation of the placenta.
The portion of endometrium that has no contact with the chorion.
Chorionicvilli
Umbilicalvein
Umbilicalarteries
Area filled withmaternal blood
Amnion
Maternalblood vessels
Placentation
Chorionic villi of
placenta
Umbilicalcord
The Endocrine Placenta
Appears in maternal bloodstream soon after implantation.Provides reliable indication of pregnancy.Pregnancy ends if absent.
1- Human Chorionic Gonadotropin (hCG)
Prepares mammary glands for milk production.Stimulatory effects on other maternal tissues comparable to that of growth hormone (GH).Ensures adequate glucose and protein is available for the fetus.
2- Human Placental Lactogen (hPL)
A peptide hormone secreted by placenta and corpus luteum during pregnancy.Increases flexibility of pubic symphysis, permitting pelvis to expand during delivery.Causes dilation of cervix.Suppresses release of oxytocin by hypothalamus and delays labor contractions.
3- Relaxin
4- ProgesteroneProgesterone maintain the endometrial lining and continue the pregnancy.
Estrogens play a role in stimulating labor and delivery. 5- Estrogens