1. Lecture # 18: Early Human Development Human Development (Chapter 29) Objectives 1- Describe the process of fertilization. 2- Explain how the egg prevents fertilization by more than one sperm. 3- Describe the major events that transform a fertilized egg into an embryo. 4- Describe the implantation of the embryo in the uterine wall. 6- Describe the formation and functions of the embryonic membranes. 7- Explain how the conceptus is nourished. 5- Identify the major tissues derived from the primary germs layers.

2. OOGENESIS MITOSIS Oogonia Diploid Before birth MEIOSIS I After puberty It stops in prophase MEIOSIS I Completed Primary oocyte Haploid Secondary oocyte It stops in metaphase Before ovulation After ovulation MEIOSIS II Completed Haploid Secondary oocyte If fertilization occurs MEIOSIS II Oogenesis First polar body 1 st & 2 nd polar body

3. Oocyte at Ovulation If not fertilized It completes meiosis II Zygote Embryo Dies Secondary oocyte (arrested in metaphase of meiosis II) Second polar body (dies) If fertilized Ovulation of mature (graafian) follicle First polar body Egg It is a layer of glyco- protein gel secreted by granulosa cells around the oocyte. It is composed of several layers of granulosa cells. Zona pellucida Corona radiata

4. Head Acrosome Nucleus Mitochondrion Axoneme Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Basal body Principal piece of tail Endpiece of tail It contains enzymes that break down material surrounding the oocyte. It contains one set of 23 chromosomes (haploid). It is the first part of the flagellum nestled in an indentation in the base of the nucleus. They provide the ATP needed for the beating of the tail. It is the core of the flagellum consisting of microtubules. Mature Spermatozoon Tail Midpiece of tail It propels the sperm during migration in the female reproductive system.

5. 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 reaction 1 There are two mechanisms to prevention of polyspermy (fertilization by two or more sperm): 1- Fast block: Binding of the sperm to the egg opens Na + channels in egg membrane. Inflow of Na + depolarizes the membrane and inhibits the attachment of any more sperm. 2- Slow block : Sperm penetration releases an inflow of Ca 2+, which stimulates the cortical granules to release their secretion beneath the zona pellucida. The secretion swells with water, pushes any remaining sperm away and creates an impenetrable fertilization membrane between the egg and the zona pellucida. Cortical reaction 2 Fertilization membrane 3 Rejected sperm 4 Cortical granules Fertilization

6. Sperm pronucleus Egg pronucleus 2-celled stage (30 hours) 1-Cleavage: It refers to the mitotic divisions that occurs in the first 3 days, while the conceptus migrates down the uterine tubes. 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. Migration of the Conceptus The Preembryogenic Stage It comprises the first 16 days of development, culminating with the existence of an embryo. 1- Cleavage2- Implantation 3- Embryogenesis 4-celled stage 8-celled stage Morula (72 hours) Blastocyst Implanted blastocyst (6 days) Fertilization (0 hours) Zygote

7. (8 days) 2- Implantation Endometrium: Blastocyst: (6-7 days) Blastocoel Trophoblast Embryoblast Epithelium Endometrial gland The blastocyst attaches to uterine wall 6 days after ovulation, usually on the fundus or the posterior wall of the uterus. 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 syncytiotrophoblast. Syncytiotrophoblast The deep layer, close to embryoblast, retains the individual cells divided by membranes and is called cytotrophoblast. Cytotrophoblast Embryoblast Trophoblast: The trophoblast secrets human chorionic gonadotropin (HCG), which stimulates the corpus luteum to secret estrogen and progesterone (it suppresses menstruation). 2- Implantation:

8. 3- Embryogenesis It is the arrangement of the embryoblast into three primary germ layers: ectoderm, mesoderm, and endoderm. Endoderm Mesoderm Ectoderm The embryoblast 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. ZygoteMorulaBlastocystEmbryo Embryogenesis:

9. The mesoderm is a more loosely organized tissue which differentiates into a loose fetal connective tissue called mesenchyme. The ectoderm and endoderm are epithelia composed of tightly joined cells.

10. Several accessory organs develop along side the embryo: the placenta, the umbilical cord and four embryonic membranes (amnion, yolk sac, allantois, and chorion) 16 days 28 days Allantois Yolk sac Amniotic cavity Amnion Chorionic villi Chorion Placental sinus Uterus Chorionic villus Developing placenta Yolk sac Amnion Amniotic cavity Chorion Umbilical blood vessels Allantois They are extensions of syncytiotrophoblast into the endometrium by digestion and growth of “roots” of tissue. They are lacunae filled with maternal blood that merge and surround villi. Placental sinus

11. 12 weeks Amnion It is the outermost membrane enclosing all the rest of the membranes and the embryo. Chorion It is a transparent sac that grows to completely enclose the embryo and penetrated only by the umbilical cord fills with amniotic fluid. It begins as an out- pocketing of the yolk sac. It forms the foundation for the umbilical cord becomes part of the urinary bladder. Allantois Umbilical cord Placenta Yolk sac It contains two umbilical arteries and one umbilical vein. It contributes to formation GI tract, blood cells, and future egg or sperm cells.

12. 12 weeks Amniotic cavity fills with amniotic fluid. Functions of the Amniotic Fluid: 1- It protects the embryo from trauma, infections, and temperature fluctuations. 2- It allows freedom of movement important to muscle development. 3- It enables the embryo to develop symmetrically. 4- It prevents body parts from adhering to each other. 5- It stimulates lung development as the fetus‘ breathes’ fluid. The fetus swallows amniotic fluid at the same rate. At term, the amnions contains about 700 to 1000 mL of fluid. At first, amniotic fluid is formed from filtration of mother’s blood plasma. Beginning at 8 to 9 weeks, the fetus urinates into the amniotic cavity about once per hour contributing substantially to fluid volume. The Amniotic Fluid

13. During gestation the conceptus is nourished in three different, over- lapping ways: Prenatal Nutrition 1- Uterine milk 2- Trophoblastic nutrition 3- Placental nutrition 1- Uterine milk It is a glycogen-rich secretion of the uterine tubes and endometrial glands. The conceptus absorbs this fluid as it travels down the tube and lies free in the uterine cavity before implantation (6, 7 days). 2- Trophoblastic nutrition The conceptus consumes decidual cells of the endometrium. Progesterone from corpus luteum stimulates decidual cells to proliferate and accumulate a store of glycogen, proteins, and lipids. It is the only mode of nutrition for the first week after implantation and remains the dominant source through the end of 8 weeks.

14. 3- Placental nutrition Nutrients diffuse from the mother’s blood through the placenta into the fetal blood. The placental phase is the period beginning the 9 th week and it is the sole mode of nutrition from end of 12 th week until birth.

15. As the placenta grows: Placenta and Umbilical Cord The villi grow and branch and their surface area increases. The membrane becomes thinner and more permeable. The placental conductivity (the rate at which substances diffuse through the membrane) increases. Materials diffuse from the side of the membrane where they are more concentrated to the side where they are less concentrated. Oxygen and nutrients pass to the fetal blood. Fetal wastes pass the other way and are eliminated by the mother. The placenta is also permeable to nicotine, alcohol, and most other drugs that may be present in the maternal blood stream.

18. Right atrium It receives O 2 poor blood returning to the heart through the superior and inferior vena cava and the coronary sinus. Right ventricle It pumps O 2 poor blood to the lungs through the pulmonary arteries. Superior vena cava Left atrium It receives O 2 rich blood returning from the lungs through the pulmonary veins. Pulmonary veins Left ventricle It pumps O 2 rich blood through the aorta artery to every organ of the body. Pulmonary trunk Inferior vena cava Aorta Blood Circulation in the Adult

19. Right atrium Pulmonary trunk Aorta Inferior vena cava Foramen oval 1 It receives O 2 rich blood returning from the placenta, mixed with O 2 poor returning through the inferior vena cava. 1 Blood bypasses the lungs by flowing directly from the right atrium through the foramen oval into the left atrium. 2 2 Ductus arteriosus Blood also bypasses the lungs by flowing from the pulmonary trunk through the ductus arteriosus to the aorta. 3 3 Blood Circulation in the Fetus