Lesson # 19

1. 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. 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.

2. Development • Development is the gradual modification of anatomical structures and physiological characteristics from fertilization to maturity. • Development begins at fertilization or conception. Fertilization or Conception • It is when the male and female gametes fuse. • Occurs during first two months after fertilization. Study of these events is called embryology. Embryological Development Prenatal Development Fetal Development Development • Begins at start of ninth week and continues until birth. Postnatal Development • Begins at birth and continues to maturity, the state of full development or completed growth 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.

3. The Oocyte at Ovulation Ovulation releases a secondary oocyte (haploid) and the first polar body; both are surrounded by the corona radiata. The oocyte is suspended in metaphase of meiosis II. It is composed of several layers of granulosa cells. Corona radiata First polar body Atrophic cell from meiosis I Zona pellucida Metaphase of Meiosis II It is a layer of glycoprotein gel secreted by granulosa cells around the oocyte.

4. Fertilization and Oocyte Activation 1 • The first step is the binding of the spermatozoon to sperm receptors in the zona pellucida. The acrosomal reaction is the exocytosis of the acrosome, releasing the enzymes needed to penetrate the egg. Acrosomal reaction • Two acrosomal enzymes are released: • Hyaluronidase, which digests the hyaluronic acid that binds granulosa cells together. • Acrosin, a protease similar to trypsin. • When a path has been cleared, a sperm binds to the zona pellucida.

5. 2 Rejected sperm Fertilization membrane - Cortical Reaction • 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 Activation • 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. Cortical granules

6. Fertilization and Oocyte Activation Pronucleus Formation Begins Acrosomal enzymes from multiple sperm create gaps in the corona radiata. A single sperm then makes contact with the oocyte membrane, and membrane fusion occurs, triggering oocyte activation and completion of meiosis. The sperm is absorbed into the cytoplasm, and the female pronucleus develops. Nucleus of fertilizing spermatozoon Second polar body Female pronucleus Fertilizing spermatozoon

7. Spindle Formation and Cleavage Preparation The male pronucleus develops, and spindle fibers appear in preparation for the first cleavage division. Female pronucleus Male pronucleus

8. Amphimixis Occurs and Cleavage Begins Amphimixis: It is the process by which the male and female pronucleus fuse to form a zygote that contains 46 chromosomes. Metaphase of first cleavage division 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.

9. Cleavage Begins The first cleavage division nears completion roughly 30 hours after fertilization. Blastomeres

10. The First Trimester 1- Cleavage • 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. • Cleavage ends when the blastocyst first contacts the uterine wall. 2- Implantation • It begins with attachment of the blastocyst to the endometrium of the uterus. 3- Placentation • It occurs as blood vessels form around the periphery of the blastocyst and the placenta develops. 4- Embryogenesis • It is the formation of viable embryo. This process establishes the foundations for all major organ systems.

11. Cleavage and Blastocyst Formation • Cleavage 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. Advanced Morula DAY 4 Trophoblast It is a solid ball of 16 cells that resemble a mulberry. Blastocoel Blastomeres Inner cell mass • The morula lies free in uterine cavity for 4-5 days and divides into a 100 cells or so. Zygote 2-celled stage DAY 1 4-celled stage DAY 2 Early morula DAY 3 Egg pronucleus Hatching Blastocyst DAY 6 Sperm pronucleus • The zona pellucida dis-integrates and releases conceptus: blastocyst. 2- Implantation DAYS 7-10 Fertilization (0 hours) Implanted blastocyst

12. Cleavage and Blastocyst Formation Zygote

13. Implantation Inner cell mass (or embryoblast) Blastocyst: Blastocoel Trophoblast Trophoblast: Inner cell mass Cellular trophoblast Syncytial trophoblast The trophoblast secrets human chorionic gonadotropin (HCG), which stimulates the corpus luteum to secret estrogen and progesterone (it suppresses menstruation). Endometrium: Epithelium Endometrial gland (DAY 8) (DAY 7) The blastocyst attaches to uterine wall 6 days after ovulation, usually on the fundus or the posterior wall of the uterus. 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. Implantation: It is the process of attachment to uterine wall that begins when blastocyst adheres to the endometrium. • The deep layer, close to embryoblast, retains the individual cells divided by membranes and is called cellular trophoblast.

14. Stages of Implantation

15. Formation of the Amniotic Cavity The inner cell mass separates slightly from the trophoblast and creates a narrow space between them: the amniotic cavity Inner cell mass Cellular trophoblast Syncytial trophoblast Gastrulation and Germ Layer Formation Amniotic cavity Ectoderm Mesoderm Inner cell mass Gastrulation: Endoderm It is the arrangement of the inner cell mass into three primary germ layers: ectoderm, mesoderm, and endoderm 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. Zygote Morula Blastocyst Embryo Fetus Cleavage Hatching Gastrulation

16. Gastrulation and Germ Layer Formation Inner cell mass Gastrulation is the arrangement of the inner cell mass into three primary germ layers: ectoderm, mesoderm, and endoderm

18. 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 (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 (Endoderm and mesoderm) The base of the allantois gives rise to the urinary bladder. Allantois Chorion Yolk Sac (Endoderm and mesoderm) It becomes an important site of blood cell production. Chorionic villi of placenta (Mesoderm and trophoblast) The appearance of blood vessels in the chorion is the first step in the creation of the placenta.

19. Placentation Chorionic villi Umbilical vein Umbilical cord Umbilical arteries Area filled with maternal blood Decidua parietalis Amnion Decidua capsularis Maternal blood vessels Decidua basalis Chorionic villi of placenta The portion of endometrium that has no contact with the chorion. The portion of endometrium that participates in the formation of the placenta.

21. The Endocrine Placenta 1- Human Chorionic Gonadotropin (hCG) • Appears in maternal bloodstream soon after implantation. • Provides reliable indication of pregnancy. • Pregnancy ends if absent. 2- Human Placental Lactogen (hPL) • 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. 3- Relaxin • 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. 4- Progesterone • Progesterone maintain the endometrial lining and continue the pregnancy. 5- Estrogens • Estrogens play a role in stimulating labor and delivery.

22. Fetal Circulation