Development

Ejaculation the release of semen -Approximately 2 teaspoons (10 mL) -280 million sperm -Most die in the vagina within minutes after ejaculation -Once they move through the cervix into the uterus, they can survive longer, up to 5 days. -Outside the body sperm will die within minutes.

SPERM STRUCTURE Head contains nucleus an organelle “cap” called an Acrosome (contains digestive enzymes) Tail flagellum – whipping motion provides motility. Base of tail contains a coiled mitochondrion to provide power for movement.

Capacitation: readying the sperm Sperms cannot fertilize oocytes when they are newly ejaculated. The acrosome is not active Chemicals in the uterus cause the acrosome to become active

Ovulation- Secondary oocyte (nuclues contains 1/2 the chromosomes) Folicle cells (make up the corona radiata) Zona pellucida (protects the “egg”)

At the cervix Helping the sperm: At ovulation, the mucus that normally covers the opening of the cervix thins out This allows sperm to pass through the cervical opening. It is also thought to help guide the sperm Passage of sperm: Since sperm can only move 2 to 3 mm per minute using their own power, they must be helped along somehow. At the shortest, sperms can be found in the oviducts only 5 minutes after ejaculation. Other sperms can take 45 minutes. The Natural Family Planning method of attempting pregnancy relies on changes in mucus thickness for the timing of intercourse—the character of the mucus is a good indicator of the presence of a viable egg.

Stage 1 of fertilization: Sperm come in contact with the follicle cells of the corona radiata Enzyme action from multiple acrosomes help sperm pass through Flagella action also aids corona radiata penetration

Stage 2 of fertilization: Penetration of the zona pellucida around the oocyte: Acrosomal enzymes: cause lysis of the zona pellucida Once sperm penetrates zona pellucida, the zona reaction occurs: This reaction makes the zona pellucida impermeable to other sperms. Zona pellucida begins to harden When more than one sperm manages to enter the ovum (dispermy = 2; triploidy = 3), the fetus nearly always aborts.

Corona radiata Figure 7.8

Stages 3 of fertilization: Fusion of plasma membranes of oocyte and sperm Causes action potential of the seconadary oocyte membrane 2nd meiotic division of oocyte is completed The secondary oocyte was previously arrested in metaphase of the 2nd meiotic division, and now forms the mature ovum and another polar body. https://www.youtube.com/watch?v=_5OvgQW6FG4

Twins: still 1 sperm per egg Monozygotic (monoovular): A fertilized, single egg splits into two developing zygotes at a very early stage. Identical twins; same sex. When the split takes place determines if they share the same placenta or same sac or both. Dizygotic (polyovular): Result from the fertilization by two sperm of two separate ova that have reached maturation at the same time. Not identical twins; can be different sexes Incidence increases with age of the mother

Cleavage is cell splitting The cells continue “splitting” by mitosis Cleavage begins Cleavage is cell splitting The cells continue “splitting” by mitosis The size of the embryo does not change, but the number of cells does This is all occurring in the oviduct Zona pellucida is hard Day 2 - 3

MORULA Cleavage stage ends now called a morula (Latin for raspberry) It is a solid ball of cells (about the size of the tip of your pen) It will continue to be called a morula as long as it is a “solid ball” Day 4 - 5

Blastocyst The Morula hollows out as cells move outward. The inside fills with liquid (the liquid is produced by the cells) Cells are too many and too small to be seen at this point This is now a blastocyst Early blastocyst day 5

Inside the Blastocyst Inner Cell mass: will become develop into the child Blastocoel: empty space of liquid is inside (-coel means empty space) Trophoblast: outer layer of cells which will become membranes Parts of the blastocyst: www.health.state.mn.us/ wrtk/handbook.html

Human Hatching The blastocyst will “hatch” from the hardened zona pellucida .1 - .2 mm Day 6-7

Implantation embryo attaches to uterine lining Embryo produces pregnancy hormone (hCG) (human chorionic gonadotropin ) Found in blood and urine Approximately 8-9 days after fertilization

Hormonal Control of Pregnancy Initially the trophoblast cells secrete Human Chorionic Gonadotrophin (HCG) Functions of HCG: Prevents the degeneration of the corpus luteum Stimulates the growth of the CL. Increase secretion of both estrogen and progesterone Prevents mestruation

Small chorionic villa (finger-like projections) grab on to the endometrium

These projections make contact with the maternal blood supply, becoming the pipeline through which the fetus derives nutrients and oxygen, and rids itself of carbon dioxide and wastes. The placenta has not formed yet.

The process of: Gastrulation A hole forms at the top of the blastocyst near the inner cell mass This hole is called the blastopore Cells begin to migrate inward forming layers The blastocyst is now becoming a gastrula

Cell to cell interaction lines up the cells properly 3 layers are formed Ectoderm Mesoderm Endoderm

Differentiation Cells are now committed Each layer will only become what it has been programmed to become (by your DNA)

Neurulation Brain and spinal cord are forming At 19 dys – photo about 25 dys: 2.5 mm Brain and spinal cord are forming The skull will not completely close until months after birth WHY? What do we call these spots?

Neurula Brain regions can be found Neural groove, which will become the spinal column Heart begins to develop Hematopoiesis begins https://www.youtube.com/watch?v=wnnhMZtCvnQ Neurula Day 28 2-3.5 mm