Pregnancy and Birth

How do sperm know where the egg is?

• They move through the cervix into the uterus and swim towards the egg in the oviduct

• Guided by chemotaxis and thermotaxis

• Thermotaxis: sperm change their swimming direction according to a temperature gradient, swimming towards a warmer temperature

• Chemotaxis: The sperm move in response to a chemical gradient. Sperm plasma membranes have receptors that detect chemicals released by the egg

Once at the egg, what do sperm have to do?

They must maneuver through the cells of the corona radiata around the egg

Fertilization occurs when

• the fusion of the sperm plasma membrane with the egg plasma membrane occurs and only the sperm nucleus (not the tail and mitochondria) enter the egg cell

• It is not complete until the two haploid nuclei come together and combine their chromosomes into a diploid nucleus

• It can take between 24-30 hours after fertilization for the sperm and egg nuclei to find each other and for their nuclear membranes

Zygote

The cell that forms once the sperm and egg membranes have fused together and their two haploid nuclei have combined

What happens after the sperm and egg nuclei combine?

Their nuclear membranes break down, the chromosomes join together to make a diploid cell that participates in mitosis to eventually form the embryo

Haploid

a single set of chromosomes in a cell. In humans, the egg and sperm cells are haploid. These cells are formed through meiosis where the genetic material of the parent cell is divided twice, resulting in cells with only one set of chromosomes

Diploid

cells containing two complete sets of chromosomes one from each parent

Polyspermy Definition and Consequences

Fertilization by multiple sperm

Consequences: Incorrect numbers of chromosomes, is lethal to the developing fetus

To prevent this, the zona pellucida becomes impermeable upon fertilization

Acrosome Reaction

Enzymes in the sperm’s acrosome digest the zona pellucida of the egg

• binding of the sperm to the glycoproteins of the zona pellucida triggers the release of enzymes from the acrosome via exocytosis that digest the glycoproteins of the zona pellucida

• The acrosome reaction must occur in order for the sperm to reach the egg cell membrane for fertilization

Cortical Reaction

• Cortical granules are thousands of vesicles near the membrane of the egg

• Following fertilization, a signaling pathways is activated which causes the granules to fuse with the egg cell membrane and release their contents via exocytosis. The exocytosis of the cortical granules is called the cortical reaction

• The cortical granules include various enzymes that are release by the cortical reaction and change the structure of the zona pellucida and cause it to harden, so sperm can no longer bind to it

After fertilization, the zygote …

begins to divide by mitosis to form an embryo composed of genetically identical cells

Morula

Initially, the embryo divides to create a ball of cells called a morula

Blastocyst

After 5 days after fertilization, the cells begin to differentiate and the morula changes into a blastocyst in the oviduct

The outer layer of the blastocyst

will develop into the amniotic sac and the placenta

The inner cell mass of the blastocyst will develop into

the fetus

Blastocyst Implantation

• The blastocyst will hatch out of the zona pellucida

• 7 days after fertilization, the blastocyst will implant into the endometrium of the uterine wall.

• Adhesion proteins extend from the cells of the endometrium and the surface of the blastocyst. The cell adhesion molecules help the cells stick together during implantation

Mammals are vertebrates that have”

• hair

• milk production in mammary glands

• three bones in the inner ear

Three subclasses of mammals

• monotremes

• marsupials

• placentals

Monotremes

egg-laying mammals strictly found in Australia and New Guinea

Marsupials

give birth to underdeveloped offspring that climb into the mother’s pouch to complete development

Placentals

mothers carry the fetus in their uterus where it is nourished via the placenta

What is the placenta?

The placenta is a temporary organ that begins developing from the blastocyst shortly after implantation. It connects to the fetus via the umbilical cord and to the mother at the uterus. The placenta is combination of fetal and maternal tissues.

Structure of the Placenta: Fetal Half

• Blood from the fetus arrives at the placenta via blood vessels in the umbilical cord. Within the placenta, the blood vessels branch into structures called the chorionic villi

What is the function of the chorionic villi?

• Separates the maternal from fetal blood

• Creates a selectively permeable barrier between the fetal and maternal blood supplies

• Branching structure of the chorionic villi increases the surface area through which nutrients and waste can be exchange between the fetal and maternal blood

Structure of the Placenta: Maternal Half

Blood from the mother flows to and from the spaces surrounding the villus

What are the functions of the placenta?

• It is an endocrine organ, producing hormones that regulate both maternal and fetal physiology during pregnancy

• The placenta facilitates nutrient, gas, and waste exchange between the physically separate maternal and fetal circulations

What hormones does the placenta secrete and why?

• Human chorionic gonadotropin: Maintains the ovarian corpus luteum for the first week of pregnancy to ensure that a new follicle foes not mature and another egg is not ovulated. Also suppresses the mother’s immune system so the embryo is not rejected

• Oestradiol and Progesterone: Maintains the endometrium during pregnancy and stimulates mammary gland development

Stuff that moves from mother to fetus

• Glucose

• Amino acids

• Lipids

• Oxygen

• Water

• Antibodies

• Hormones

• Ions

Stuff that moves from Fetus to Mother

• Carbon dioxide

• Water

• Ions

• Urea and other waste products

Placental Transport: Simple Diffusion

• Substance moves from high to low concentration directly through the cell membrane

• Oxygen and carbon dioxide

Placental Transport: Osmosis

• Water moves from high to low concentration directly through the cell membrane or through aquaporins

Placental Transport: Facilitated Diffusion

• Substance moves from high to low concentration through channel protein

• Glucose or other simple carbohydrates from mother to fetus

Placental Transport: Active Transport

• Substance moves from low to high concentration using cellular energy

• Amino acids are brought into the fetus

Placental Transport: Bulk Transport

Stuff is taken into the cells of the chorionic villi from one side by endocytosis and the cell uses vesicles to transport the stuff to the opposite side where it is released via exocytosis

• Antibodies from the mother to the baby

Human chorionic gonadotropin is …

• a glycoprotein hormone produced only during pregnancy

• produced by the outer rings of the embryo and then, the placenta after it is developed

What is the function of hCG?

stimulate the corpus luteum and placenta to continue to produce progesterone during the pregnancy

Why is hCG used for pregnancy testing?

It is only synthesized during pregnancy

Monoclonal antibodies and pregnancy testing

proteins that are synthesized in a lab and are capable of binding to a specific target molecule

they can be bound to enzymes that trigger a color change when bound to certain things, like hCG

Reaction Zone of Pregnancy Test

The urine sample moves through a region of the test strip that has a free antibody that is complementary in shape to hCG and can bind to it

Test Zone of Pregnancy Test

A different antibody is immobilized to the test zone and if hCG is present, it will bind to this antibody while still being bound to the free antibody. When hCG binds to this antibody, it will activate the enzyme to trigger a color change and a line will appear indicating a pregnancy.

Control Zone of Pregnancy Test

This zone contains a different immobilized antibody that binds to the free antibodies which can bind to hCG. If the test has worked, an enzyme is activate and a colored line will appear for all samples and lets us know that the urine sample has flowed through the test strip and that the enzymes and antibodies are functioning

During the first 10 weeks of pregnancy, progesterone is secreted by the…? What happens after?

corpus luteum in the ovary

After these weeks, the placenta takes over producing progesterone

Progesterone levels rise until…

near the end of the pregnancy

Progesterone is important in maintaining the pregnancy because it:

• Suppresses the mothers immune response so it will tolerate the presence of fetal cells and DNA

• Prevents ovulation of eggs from the ovaries

• Maintains the endometrium, preventing menses during pregnancy

• Inhibits the secretion of oxygen by the pituitary, keeping the uterus relaxed and preventing the onset of early labor

What happens at the end of pregnancy that trigger childbirth?

• Hormones from the fetus cause the placenta to stop secreting progesterone

• The drop is progesterone triggers childbirth

Oxytocin and Progesterone During Childbirth

• Without progesterone, the pituitary is not inhibited from releasing oxytocin

• As a result, the pituitary will release oxytocin and its concentration will rise

• Oxytocin stimulates contractions of the muscles of the uterus during childbirth

• The production and secretion of oxytocin is controlled by a positive feedback mechanism

• Oxytocin release during the contraction of the uterus at the start of childbirth stimulates the production and release of more oxytocin

• Contractions become more frequent and intense, allowing the baby to be born