• Sperm meets egg.

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A&P

Development

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Introduction

• Sperm meets egg.

• There are 3 types of eggs: determined by yolk quantity and distribution.

– Telolecithal: Yolk is on once side of the egg. Birds, reptiles and amphibians.

– Centrolecithal: the majority of the egg is a yolk surrounded by a think cytoplasm. Arthropods – Mesolecithal: Yolk is evenly distributed

throughout the egg. Placental mammals.

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Fertilization

• About 50-100 sperm make it to the correct egg. The trip takes 30 minutes to 2 hours. The enzyme inhibitors have worn off of the acrosome. Enzymes are ready to go. The egg is surrounded by follicle cells from ovulation: Corona Radiata. Sperm must travel through this to get to the

vitelline layer.

• The bindin on the sperm acrosome will bind to the receptors on the egg’s vitelline layer (zona pellucida),

which is outside the plasma membrane. One sperm meets the 2

^o

oocyte.

• The head of the sperm will enter the egg.

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Fast and Slow Block to polyspermy

• The outer membrane of the egg will change to prevent polyspermy.

– Fast Block: N

^a+

will enter the egg, this changes the electric charge.

– Slow Block: Ca

⁺⁺

enters the cell from ER

(stimulated by IP

₃

). Water enters via osmosis.

Firm egg won’t allow sperm it move in. This is

called the Fertilization membrane.

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Egg Activation

• The increase in Ca++ starts the expulsion of H+

from the egg. The pH changes from 6.8 to 7.3.

This change in pH causes an increase in egg cell metabolism.

• At the moment after conception, the fertilized ovum is about 0.135 mm in diameter with a

mass of 150 mg. At the end of the first trimester,

the fetus is about 75 mm long and has a mass of

about 12 g. A lot happens in the first trimester.

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After activation

• After activation, the 2o oocyte finsihes meiosis 2. the sperm head increases in size and blows up releasing the DNA. The egg DNA and

sperm DNA move to the metaphase plate and

prepare to divide. The new cell is a Zygote.

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First Trimester

• Zygote is one cell with a full set of chromosomes and cytoplasm (with organelles). 30 hours after fert, the cell divides. The cells don’t grow, the egg stays about the same size during these first divisions. The cells are called blastomeres.

• Animal cells show a definite polarity: Animal Pole and Vegetal Pole. The animal pole cells have more melanin (gray). The

vegetal pole cells have more yolk (yellow). This appears during the 3

^rd

division and causes the cells to begin to specialize.

• Continued cell division forms a MORULA ( a solid ball of

cells).From the morula, a space appears in the ball called a

blastocoel. This is now the Blastula.

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First Trimester, 2

• The blastula has one body layer: ectoderm.

The process of Gastrulation forms the

endoderm (the formation of the digestive tract). Since we are talking about verts, we are all deuterostomes (the mesoderm forms from the endoderm).

• Let’s briefly talk about gastrulation in sea

urchins, frogs, birds then mammals.

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Sea Urchins and Frogs

• Gastrulation: cells move into the blastocoel.

Starts at the vegetal pole. The archenteron (primitive gut) is formed. The opening is the

blastopore. From the endoderm, the mesoderm is formed.

• Blastula is more than once cell thick. Cells move

in through a crease (dorsal lip) formed by bottle

cells. Once the endoderm forms, the mesoderm

forms and the organs form from there.

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Birds

• The yolk: egg cell swollen with food. The egg cell is surrounded by a protein rich solution (egg whites). The animal pole of the egg

divides to form a blastodisc. The blastodisc

forms a crease that gastrulates. From the

endoderm, the mesoderm forms.

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Organogenesis:

• Ectoderm: rolls to form the CNS, pigment cells, some muscles in skull, teeth, adrenal medulla and peripheral components of nervous

system.

• Endoderm: digestive tract, liver, pancreas and lungs

• Mesoderm: muscles, skeleton, gonads,

kidneys, and circulatory system.

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Mammals: First Trimester, 3

• After 5 days, the embryo is a blastocyst (blastula) that is surrounded by a trophoblast. At one end of the blastocyst:

inner cell mass (this will be the baby).

• Trophoblast secretes an enzyme (hyaluronidase) that digests part of the endometrium. The blastocyst lands in a pool of blood. The trophoblast grows (fingerlike projections into the endometrium– placenta) the full placenta takes 30 days to form. The baby has 30 days of yolk in the cells.

• Trophoblast produces HCG which acts like LH which

maintains the Corpus Luteum and keeps prog and est high.

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First Trimester, 4

• The increase in HCG can cause the woman to

get sick…morning sickness. This will first appear around week 6 and last until week 16.

• There is so much HCG that it is lost in urine…

urine test.

• The inner cell mass will form a flattened

embryonic disc…forms a streak and gastrulates.

The endoderm forms then the mesoderm from

the endoderm.

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First Trimester, 5

• The chorion develops from the trophoblast and surrounds the embryo. The amnion encloses the embryo.

• Placentation: The trophoblast forms the chorion which forms chorionic villi that grows into the endometrium. As the placenta develops, you will see the formation of the umbilical cord. All the blood vessels from the chrionic villi will combine in the Umbilical cord. Remember, there is no direct contact between mom’s and baby’s blood. Nuts and wastes are dumped in the spaces between the

capillaries.

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First Trimester, 6

• The Placenta will provide: HCG, Human Placental Hormone: helps mammary glands produce milk.

Relaxin: increases flexibility in hips and dilation of cervix and can stimulate the release of oxytocin.

Progesterone and Estrogen: after 1 ^st trimester, it produces enough to maintain uterine lining.

• At the end of 4 weeks heart beats. At 8 weeks, all structures are present. Organs form. At 10

weeks, it’s a fetus. At 13 weeks, you tell people.

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Second Trimester

• Fetus grows rapidly. At end of the 2 ^nd

trimester, the baby will be 30 cm long and .64 kg. The baby is very active.

• Decrease in HCG: corpus luteum breaks down, but you produce enough prog and est.

• Uterus grows.

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Third Trimester

• Baby is viable. Rapid growth to 3-3.5 kg and 50 cm in length.

• Activity decreases…to big. Baby is

compressing mom’s organs. Increased

urination, digestive blocks, strain on back

muscles…

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Third Trimester, 2

• Mom Changes:

– Respiration rate and tidal volume increases: baby needs O2.

– Blood volume increases: EPO and renin will increase RBC production.

– Mom needs to increase nuts and vits by 10-30%

– GFR increases by 50%. Need to get rid of more waste.

Increase in urination, plus the bladder…

– Uterus increases in size: from 7.5 cm to 30 cm in length.

From 40 g to 1.1 kg. May contain 5 liters of fluid.

– Mammary glands will increase in size.

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Birth

• Strong uterine contractions push the baby out.

Prostaglandins and oxytocin.

• Why? Baby lungs are fully formed. They

produce a protein (surfactant protein-A, SP-A).

SP-A will activate fetal MΦ which will leave the amnion and attack mom’s uterus. In

response, mom kicks the baby out.

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Stages of Birth

• 3 stages of birth:

– 1) dilation of cervix to 10 cm. Uterus begins to contract. Start off weak then increase in intensity.

• Can last up to 12 hours (sometimes longer).

• Often starts when the amnionic sac breaks…water breaks.

• At 10 cm, the first stage ends.

– 2) Birth: can last up to 2 hours.

• Episiotomy

• Rip and tear

– 3) Expulsion of the placenta.

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Lactation

• Prog decreases, prolactin increases. Milk won’t “let down” for 2-3 days. Initially, baby is receiving

Colustrum: a protein cocktail filled with antibodies.

• In fact, baby will lose 10% of body mass in first 3 days. This is before the high fat milk is produced.

Baby will use brown fat (between shoulder blades) for energy.

• As baby feeds, takes in Ab in milk..tailor made for

baby.