Fertilization and Development Pregnancy: events that occur from - PowerPoint PPT Presentation

Fertilization and Development • Pregnancy: events that occur from fertilization until infant is born • Conceptus • Gestation period – time from last menstrual period until birth – fertilization approx. 14 d into gestation – ~280 days • Embryo : fertilization through week 8 • Fetus : week 9 through birth

Diagram shows the approximate size of a human conceptus from fertilization to the early fetal stage. Embryo 1-week 3-week 5-week embryo Fertilization conceptus embryo (10 mm) 8-week embryo (3 mm) (22 mm) 12-week fetus (90 mm)

Fertilization • Sperm must reach secondary oocyte – Oocyte viable for 12 to 24 hours – Sperm viable 24 to 48 hours after ejaculation • Only a few thousand out of millions of ejaculated sperm make the trip through the uterine tubes – 12-cm (5-inch) journey – Most are lost • Fertilization : sperm’s chromosomes combine with those of secondary oocyte to form fertilized egg, called a zygote

Sperm Capacitation • Secretions of female tract help to weaken and thin out acrosome membrane – Motility enhanced – Sperm cell membranes begin releasing hydrolytic enzymes (delaying until needed at corona radiata) – Occurs after several hours exposed to female reproductive secretions – Incapacitated sperm can’t fertilize • Sperm have olfactory receptors that can follow chemical trail released by egg or surrounding cells

Acrosomal Reaction and Sperm Penetration • Oocyte protective layers must be digested • Corona radiata and zona pellucida slowly degraded by the action of hundreds of sperm • Cells displaced by sperm enzymes degrading cell to cell connections • Zona degraded by releases of many acrosomes • The single sperm reaching the membrane binds acrosomal markers to oocyte membrane receptors • Sperm is bound and contents ‘emptied’ into oocyte

Sperm, delivered to the vagina and capacitated in the female reproductive tract, stream toward a secondary oocyte. 1 Approach. Aided by enzymes on its surface, a sperm cell weaves its way past granulosa cells of the corona radiata. Extracellular space Sperm Sperm Zona pellucida Oocyte nucleus arrested in meiotic Granulosa metaphase II cells of corona Polar body radiata Zona pellucida Extracellular space Oocyte plasma membrane

2 Acrosomal reaction. Binding of the sperm to receptors in the zona 5 Blocks to polyspermy. pellucida causes Ca 2 + levels within the 3 4 Binding. The Fusion. Sperm Oocyte sperm-binding membrane sperm to rise, triggering the acrosomal receptors are shed. Ca 2 + levels sperm’s membrane and oocyte plasma reaction. Acrosomal enzymes from many in the oocyte’s cytoplasm rise, binds to the oocyte’s membranes fuse. sperm digest holes through the zona Sperm contents enter triggering the cortical reaction sperm-binding pellucida, clearing a path to the the oocyte. (exocytosis of cortical granules). receptors. oocyte membrane. As a result, the zona pellucida hardens and the zona pellucida’s sperm-binding receptors are clipped off. Microtubules from Oocyte sperm-binding sperm flagellum membrane receptors Cortical Mitochondria granules Sperm Zona pellucida nucleus sperm-binding receptors

Blocks to Polyspermy • Polyspermy prevented in humans – One-sperm-per-oocyte condition • Two mechanisms ensure monospermy – first encounter between sperm and oocyte membrane receptors causes destruction of remaining receptors – Zygote destroys zona receptors shedding remaining sperm from the area

Sperm nucleus Extracellular Completion of Meiosis II and space 1 After the sperm Corona penetrates the Fertilization radiata secondary oocyte, the oocyte completes Zona meiosis II, forming pellucida the ovum and second polar body. Second meiotic division of oocyte Second meiotic division of first polar body Male pro- nucleus 2 Sperm and ovum Female pro- nuclei swell, forming nucleus (swollen pronuclei. ovum nucleus) Polar bodies Male pronucleus 3 The DNA in each pronucleus replicates. Mitotic spindle The pronuclei Centriole Male and female approach each other pronuclei and a mitotic spindle Female forms between them. pronucleus 4 Chromosomes of the pronuclei intermix. Polar bodies Fertilization is Zygote accomplished and the cell, now called a zygote, is ready for the first cleavage division.

Zygote to Blastocyst Implantation Cleavage • Zygote is slowly moving toward uterus • Rapid mitotic divisions – No cell growth: Cell size optimized for exchange – First occurs after ~36 hours • Two daughter cells called blastomeres • After 72 hours, cluster of cells contains 16 or more cells and is referred to as a morula

Figure 28.3 Cleavage: From zygote to blastocyst. Zygote 4-cell stage Morula (a solid ball Early blastocyst (fertilized egg) 2 days of blastomeres) (Morula hollows out, 3 days fills with fluid, and Zona “hatches” from the pellucida zona pellucida) 4 days Degenerating zona pellucida Implanting blastocyst Sperm Blastocyst (Consists of a sphere cavity of trophoblast cells and Uterine an eccentric cell cluster tube called the inner cell Fertilization mass) 7 days (sperm meets and enters egg) Ovary Oocyte (egg) Trophoblast Uterus Blastocyst Ovulation cavity Endometrium Inner cell mass Cavity of uterus

Blastocyst Formation • Around day 4 or 5 blastocyst reaches uterus – Embryo now consists of ~100 cells • Trophoblast cells – Participate in placenta formation • Inner cell mass : cluster of 20 – 30 rounded cells – Becomes embryonic disc , which will form embryo and three or four extraembryonic membranes • Fourth extraembryonic membrane (chorion) is formed by trophoblast – Now called embryoblast

Implantation and Placentation Implantation • Blastocyst floats for about 2 – 3 days – Nourished by uterine secretions • Begins 6 – 7 days after ovulation – Trophoblast cells grow favorable connection to endometrium penetrating it – Blastocyst incorporated under endometrial epithelium – Endometrium still being supported by ovarian hormones and hCG • Complete by day 12 Failed implantation may be as high as 2 out of every three

Figure 28.4a Implantation of the blastocyst. Endometrium Uterine endometrial epithelium Inner cell mass Trophoblast Blastocyst cavity or blastocoele Lumen of uterus

Figure 28.4b Implantation of the blastocyst. Endometrial stroma with blood vessels and glands Syncytiotrophoblast Cytotrophoblast Blastocyst cavity Lumen of uterus

Human Chorionic Gonadotropin • Embryo dependent on endometrium at this time • Secreted by trophoblast cells and later the chorion • Prompts corpus luteum to continue secretion of progesterone and estrogen • Promotes placental development • hCG levels begin decline (2 months) as placenta begins to secrete progesterone and estrogen • Low values occur at 4 months and continue for rest of pregnancy

Figure 28.5 Hormonal changes during pregnancy. Human chorionic gonadotropin Relative blood levels Estrogens Progesterone 0 4 8 12 16 20 24 28 32 36 Gestation (weeks) Ovulation Birth and fertilization

Figure 28.6a Events of placentation, early embryonic development, and extraembryonic membrane formation. Maternal blood vessels Proliferating syncytiotrophoblast Cytotrophoblast Amniotic cavity Bilayered embryonic disc • Epiblast • Hypoblast Endometrial epithelium Implanting 7½-day blastocyst. The syncytio- trophoblast is eroding the endometrium. Cells of the embryonic disc are now separated from the amnion by a fluid-filled space.

Figure 28.6b Events of placentation, early embryonic development, and extraembryonic membrane formation. Endometrium Maternal blood vessels Proliferating Amnion syncytiotrophoblast Cytotrophoblast Amniotic cavity Yolk sac Bilayered Extraembryonic embryonic disc mesoderm • Epiblast • Hypoblast Lumen of uterus Chorion being formed 12-day blastocyst. Implantation is complete. Extraembryonic mesoderm is forming a discrete layer beneath the cytotrophoblast.

Figure 28.6c Events of placentation, early embryonic development, and extraembryonic membrane formation. Amniotic Lacuna (intervillous cavity Primary space) containing germ layers maternal blood • Ectoderm Chorionic villus • Mesoderm Chorion • Endoderm Amnion Forming umbilical cord Yolk sac Allantois Extraembryonic mesoderm Lumen of uterus Extraembryonic coelom 16-day embryo. Cytotrophoblast and associated mesoderm have become the chorion, and chorionic villi are elaborating. The embryo exhibits all three germ layers, a yolk sac, and an allantois, which forms the basis of the umbilical cord.

Figure 28.9 Folding of the embryonic body, lateral views. Tail Head Amnion Yolk sac Ectoderm Mesoderm Trilaminar embryonic disc Endoderm Future gut (digestive Lateral tube) fold Somites (seen through Tail ectoderm) fold Head fold Yolk sac Neural tube Notochord Primitive gut Hindgut Foregut Yolk sac