Chapter 28- Pregnancy and human development Flashcards
Zygote
A diploid cell resulting from the fusion of two haploid gametes. Sometimes called a single celled embryo
Embryo
Stage of development from soon after the fertilization of the ovum to week 8 of development. No longer single celled at this point
Fetus
week 8 to birth, called an infant after birth
Fertilization definition
The process in sexual reproduction that involves the union of a sperm and an egg to form a diploid zygote
What is the time window for fertilization?
An oocyte is only viable for 24 hours at most after ovulation, and sperm are most viable 24-48 hours in the female reproductive system. General rule- in order for fertilization to occur- intercourse must occur no more than 2 days before ovulation and no more than 1 day after ovulation. Y chromosome sperm swim faster but are smaller, so they break down more quickly, but there’s not a large difference between Y and X in terms of lifespan
Why do only a fraction of sperm that enter the vagina reach the uterus? (2)
- Millions lost immediately by leaking from the vagina
2. Millions more lost by acidic environment of vagina- glycogen fermenting bacteria
How can sperm be lost in the uterus?
Phagocytes kill off many more sperm. Only a few thousand “survivors”- reverse peristaltic waves push these sperm toward the uterine tubes as the uterus contracts toward the fundus.
How do sperm travel in the right direction?
Sperm have various receptors (chemical, temperature, fluid flow) to ensure they travel in the correct direction
Sperm capacitation
Sperm are incapable of fertilizing the oocyte immediately after entering the vagina- want digestive enzymes to be weakened in the female anatomy but strong in the male anatomy. Before fertilization occurs- sperm must be “capacitated”- motility is enhanced and there is a weakening of membranes- allows easier release of enzymes that help with penetrating the egg. Can take hours after ejaculation
2 protective structures that surround the oocyte
- Zona pellucida- innermost layer
2. Corona radiata- outer layer
Zona pellucida
Innermost layer produced by primary oocyte and follicle. Protects secondary oocyte, is necessary for fertilization to occur
Corona radiata
Outer layer, contains cells that allows communication between cells and oocyte. Protects egg after it has been ovulated
Which structures must the sperm cross for fertilization to occur?
For fertilization to occur, sperm must be able to cross both structures in the egg. Sperm burrow through cells of the corona radiata
Acrosomal reaction
Sperm bind zona pellucida- calcium levels in sperm rise. Acrosomal enzymes released from the acrosome of the sperm- digest holes through the zona pellucida to the plasma membrane of the oocyte. One sperm’s enzymes are never going to be enough to allow fertilization to occur- to get one sperm through, the other sperm have to release all of their enzymes. The first sperm won’t fertilize the egg- neither will the first thousand.
What happens once the acrosomal reaction is initiated and the sperm gets through the layers? (2)
- The plasma membrane of the sperm binds to the sperm binding receptors on the plasma membrane of the oocyte
- Once bound, the two membranes fuse together- sperm contents enter the oocyte
Polyspermy
The entry of more than one sperm cell into the oocyte- results in incorrect chromosome number. Resulting embryo cannot mitotically divide- dies
2 ways the oocyte can block polyspermy
- Oocyte membrane blocks- sperm binding receptors are shed from oocyte surface. Sperm unable to bind oocyte surface and fertilize eggs with those receptors
- Cortical reaction
Cortical reaction
Endoplasmic reticulum of oocyte releases calcium in response to binding of single sperm cell. Granules release enzymes to destroy binding receptors and causes the zona pellucida to harden
How many fertilizing events spontaneously abort?
About 70% of fertilizing events spontaneously abort- unclear why, but likely due to chromosomal mismatch. People often spontaneously abort without knowing that they’re pregnant
Completion of meiosis 2 and fertilization (4 steps)
- Sperm nucleus must travel to the oocyte nucleus. As it travels, sperm nucleus swells in size to form male pronucleus
- Surge in calcium from cortical reaction causes release of zinc from oocyte. Meiosis 2 is completed as zinc leaves cell- forms 2nd polar body and mature ovum
- After meiosis 2- female pronucleus forms
- Pronuclei membranes rupture
What happens when the male pronuclei membranes rupture?
Chromosomes are released together, and maternal and paternal chromosomes combine to form a diploid zygote- this is fertilization. This is also the first appearance of the zygote
When does development begin?
Development begins at fertilization. Diploid cell (zygote) quickly begins to divide mitotically
Cleavage
Rapid mitotic divisions of the zygote. About 36 hours after fertilization- zygote has completed first division, contains 2 identical blastomere cells. 72 hours after fertilization- morula is formed- 16 cells
What is the benefit of cleavage?
Genetically identical cells are clumped together, which increases the surface area of the embryo- allows for better gas exchange, waste removal, and uptake of nutrients. A lot easier to build a functional person from many cells than from one single large cell- easier to start with many pieces you can put together however you want than with one large piece of stone that you have to carve to make the house
The morula divides to form
Morula continues to divide- forms blastocyst with 100+ cells
2 layers of the blastocyst
- External trophoblast
2. Internal embryoblast
External trophoblast
Single layer of flattened cells. Aids in embryo implantation, contributes to chorion formation/function, has immunosuppressive effects
Internal embryoblast
Cluster of 20-30 rounded cells. Will eventually form the embryo proper (and organs) and extraembryonic membranes
How is the blastocyst implanted in the endometrium?
Endometrium is receptive to implanting embryo due to high estrogen and progesterone levels. Cell adhesion molecules on endometrium bind the trophoblast when it makes contact with the lining, this usually occurs high in the uterus.
Once bound, what happens to the blastocyst?
Now called a trophoblast. Once bound, trophoblast releases digestive enzymes and growth factors on epithelium of endometrium
Trophoblast growth factors
Endometrium thickens as blood vessels enlarge and become leaky. Trophoblast proliferates, forms 2 layers- cytotrophoblast and syncytiotrophoblast.
Cytotrophoblast
Innermost layer. Mass of cells that anchors growing embryo to uterine tissue and release enzymes to facilitate implantation
Syncytiotrophoblast
Outermost layer. Mass of fused cells that produce and release enzymes to facilitate implantation
Trophoblast digestive enzymes
Responsible for erosion of endometrium around blastocyst. Blastocyst buries itself in the blood rich lining, and surrounding endometrial cells proliferate- covers the blastocyst
Human chorionic gonadotropin (hCG)
Released by embryo once implantation is complete. hCG is a hormone that prevents breakdown of the corpus luteum and suppresses female immune system. hCG drops at 4 months, corpus luteum breaks down. By this time, the placenta has formed and is making estrogen and progesterone
The placenta
A temporary organ originating from embryonic and maternal tissues. Functions- maintain pregnancy, exchange respiratory gasses, provide nutrients to embryo/fetus, dispose of waste, etc
When does the placenta become a fully functional organ?
The placenta is a fully functional organ by the 4th month of pregnancy. Will allow oxygen, nutrient, and waste exchange before this time, maternal and fetal blood do not mix
Embryonic contribution to placenta formation
Formation of the chorion membrane that surrounds embryo/fetus. Blood vessels extend from chorionic villi, form umbilical vein and arteries
Parental contribution to placenta formation (3)
- Functional layer of endometrium develops blood filled lacunae, which chorionic villi are immersed in
- Endometrium that lies underneath embryo becomes decidua basalis
- Endometrium that surrounds uterine cavity face of embryo forms decidua capsularis
Lacunae
Chorionic villi has bloods of blood (lacuna) from the parent, where exchange occurs between parent and fetus
Decidua basalis function
Forms the true placenta with the lacuna and chorionic villi immersed in it. Function- forms placenta with chorionic villi