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Lesson # 12. Early Development. Oogenesis. Between the third and seventh month of fetal life: . Oogonia. Oogonia undergo mitosis and produce primary oocytes (diploid). OOGENESIS. Diploid. MITOSIS. Primary oocytes (diploid) begin MEIOSIS I but it is stopped in prophase I. First . - PowerPoint PPT Presentation
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Lesson # 12Early Development
OOGENESIS
MITOSIS
OogoniaDiploid
Diploid
Before birth
MEIOSIS I
After puberty
It stops inprophase
MEIOSIS ICompleted
Primaryoocyte
Haploid
Secondaryoocyte
It stops inmetaphase Before ovulation
First polarbody
After ovulationSecondpolarbody
MEIOSIS IICompleted Haploid
Secondaryoocyte
If fertilizationoccurs
MEIOSIS II
Oogenesis
Primary oocytes (diploid) in prophase I of MEIOSIS I.
FSH triggers the start of the ovarian cycle.MEIOSIS I is completed to form one secondary oocyte (haploid) and the first polar body.
Between the third and seventh month of fetal life:
Primary oocytes (diploid) begin MEIOSIS I but it is stopped in prophase I.
During reproductive life: Every month one secondary oocyte begins MEIOSIS II that is stopped in metaphase II. Ovulation occurs, and if the secondary oocyte is fertilized, MEIOSIS II is completed to form the ovum and the second polar body.
At birth:
At puberty:
Oogonia undergo mitosis and produce primary oocytes (diploid).
Primordial follicles
Primary follicles
Secondary follicles
Tertiary follicles
Ovulation
Oogenesis
FSH
One of the primordial follicles matures (once every 28 days) to primary follicle. Meiosis I is completed (haploid secondary oocytes)
Primary oocytes (diploid) joint with nurturing complexes of follicle cells
Corpus albicans
7
Primordial follicles 1
Primary follicle
2
Secondary follicle 3
Tertiary or Mature Graffian follicle
4
The mature follicle contains the secondary oocyte (in metaphase II), which is expelled at the end of the process and then begins the journey through the uterine tubes
5 OvulationCorpus luteum
6
If fertilization does not occur, the corpus degenerates into the corpus albicans
The follicle is transformed in the corpus luteum (yellow body), which for a time produces progesterone that facilitate pregnancy
Once it has been released, the oocyte begins a slow journey to the uterus through the uterine tubes (fallopian tubes)
Ovulation
While within the tube, the oocyte may encounter sperm and become fertilized prior to entering the uterus.
It 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
It prevents the polyspermy (fertilization by two or more sperm).
Cortical reaction2Fertilization membrane
Rejected sperm
Cortical granules
Oocyte Activation
Fertilization and Oocyte Activation
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”.
Cleavage:It is a series of cell divisions (mitotic divisions) that produce an ever-increasing number of smaller and smaller daughter cells.The final product of the cleavage is the formation of the morula.
Spermpronucleus
Eggpronucleus
Zygote2-celled stage
DAY 1
1-Cleavage:
Blastomeres It 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.
The Preembryogenic Stage It comprises the first 16 days of development, culminating with the existence of an embryo.
1- Cleavage 2- Implantation3- Embryogenesis
4-celled stage DAY 2
Early morula DAY 3
Advanced MorulaDAY 4
Blastocyst DAY 6
Implanted blastocyst
Fertilization(0 hours)
3- Placentation
2- Implantation DAYS 7-10
It is a series of cell divisions (mitotic divisions) that produce an ever-increasing number of smaller and smaller daughter cells.
Hatching
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
Yolk sac
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.
Inner cell mass Cellular trophoblast
Syncytial trophoblast
Inner cell mass
Gastrulation:
Gastrulation and Germ Layer Formation
Inner cell mass
The First Trimester
Figure 29–5 Extraembryonic Membranes and Placenta Formation.
Placenta
Amniotic cavity
Umbilical cord
Myometrium
Endometrium