First Week of Human Development

Question 1

What is a zygote?

Answer 1

When a sperm fuses with an oocyte and contains maternal and paternal genes and chromosomes.

Question 2

What is gametogenesis?

Answer 2

Gamete formation. Formation and development of specialized generative cells (gametes)

Question 3

What is a special type of cell division that only occurs during gametogenesis?

Answer 3

meiosis

Question 4

What occurs during Meisois 1 of spermatogensis?

Answer 4

Primary spermatagoenia become secondary spermatocytes or sister chromatids.

Primary spermatocyte (46 xy) split to become secondary spermatocytes: (23x) and (23y)

Question 5

What happens during Meiosis II of Spermatogenesis?

Answer 5

Secondary Spermatocytes (23x) and (23y) divide and become 4 spermatids.

Question 6

Once the four spermatids are developed how do they transform into mature sperm?

Answer 6

Gradually the 4 spermatids transform into mature sperm thru process of spermiogenesis.

Question 7

What is spermiogenesis and how long does it take?

Question 8

Where is the nucleus contained in a sperm?

Answer 8

The head of the sperm.

Question 9

What is the acrosome?

Answer 9

The anterior 2/3rds of the sperm head that contains the enzyme that allows for penetration of the egg.

Question 10

What happens to the Corona Radiata when the sperm releases enzymes?

Answer 10

The enzymes facilitate dispersion of the follicular cells allowing sperm penetration of zona pellucida.

Question 11

What is the middle piece of the sperm do?

Answer 11

Contains mitochondria that fuels energy needed for motility.

Question 12

What is oogenesis?

Answer 12

Oogonia are transorfed into mature oocytes.

Question 13

What are the steps of Oogenesis?

Answer 13

1. Primary Oocyte (46xx) forms
2. Follicular cells develop around it
3. Primary oocyte begin first meiotic division before birth
4. First Meiotic phase produces a secondary oocyte and a first polar body during puberty.
5. At ovulation the secondary ooctye begins the second meiotic division but only progresses if fertilized.

Question 14

Where does fertilization of the egg occur?

Answer 14

Fallopian tubes

Question 15

Walls of the ovarian follicle become what?

Answer 15

The corpus luteum.

Question 16

What does the corpus luteum do?

Answer 16

Secretes estrogen and progesterone.

Question 17

What does progesterone secreted from the corpus luteum do?

Answer 17

Prepares the endometrium for implantation of the ovum (blastocyte).

Question 18

What prevents the degeneration of the corpus lutuem during pregnancy?

Answer 18

HCG

Question 19

What hormone influences the development of the corpus luteum?

Answer 19

LH

Question 20

What secretes HCG?

Answer 20

The Synctiotrophoblast of the blastocyte

Question 21

How long does the corpus luteum remain functionally active after fertilization?

Answer 21

Throughout the first 20 weeks until the placenta assumes production of estrogen and progesterone to maintain the pregnancy.

Question 22

What happens to the corpus lutuem if the oocyte is not fertilized?

Answer 22

The corpus luteum involutes and degenerates 10-12 days after ovulation.

Question 23

What marks the beginning of fertilization?

Answer 23

Contact between a sperm and an oocyte.

Sperm penetrates the corona radiata

Then enters the zona pellucida.

Question 24

What is Acrosin?

Answer 24

A digestive enzyme in the head of the sperm (acrosome) that helps penetrate the egg.

Question 25

Once the sperm penetrates what occurs?

Answer 25

The zonal reaction occurs making the zona pellucida impermeable to other sperm.

Question 26

What happens after the sperm penetrates the oocyte and becomes impermeable to other sperm?

Answer 26

Fusion of the cell membranes of the oocyte and the sperm occurs. The head and tail enter the cytoplasm of the oocyte. Completes the second meiotic division. Forms a mature oocyte and second polar body. Forms the female pronucleus.

Question 27

After the female pronucleus develops what happens next?

Answer 27

Sperm enlarge to form male pronucleus & tail degenerates Membranes of pronucleus break down & 23 chromosomes from each combine Makes a Zygote with 46 chromosomes

Question 28

What are the results of fertilization?

Answer 28

1. Stimulates secondary oocyte to complete 2nd meiotic division 2. Restores diploid (46) number of chromosomes in Zygote 3. Mingling of maternal & paternal chromosomes 4. Determines sex of embryo 5. Causes metabolic activation of oocyte & initiates cellular division in Zygote

Question 29

What is EPF?

Answer 29

Immunosuppressant protein (EPF) 1. secreted by trophoblast cells which give rise to the placenta later. 2. in maternal serum 24-48 hrs after fertilization 3. Not typically used for detecting pregnancy typically use hCG test urine or blood

Question 30

What happens to the zygote after fertilization?

Answer 30

1. Repeated mitotic division of the zygote occurs - rapid􏰀in # of cells - blastomeres (replicated cells) 2. Divides into 2 blastomeres, 4, 8, etc. 3. Occurs as zygote moves along the uterine tube to the uterus 4. Process begins about 30 hrs after fertilization Once 12-32 blastomeres develop the zygote becomes a morula.

Question 31

What is a morula?

Answer 31

A cluster of 12-32 blastomeres that is the name of a developing human.

Question 32

When does the morula form after fertilization?

Answer 32

About 3 days after fertilization as it enters the uterus.

Question 33

Where does blastocyst formation occur?

Answer 33

In the uterus.

Question 34

When does the developing embyro enlarge in size?

Answer 34

Does not occur until the zona pellucida degenerates.

Question 35

What is the sequence of blastomere growth?

Answer 35

By 30 hours- 4 blastomeres By three days there are 6-12 blastomeres By day four there are 16-32 blastomeres.

Question 36

What is nondisjunction?

Answer 36

Chromosomes can be added or lost containing 2 or more cell lines of different chromsome complements. Occurs during early cleavage of the embryo.

Question 37

When does the morula form and when does it enter the uterus?

Answer 37

Forms on day 3 and enters uterus on day 4.

Question 38

How does a blastocyst form?

Answer 38

Inside the morula a fluid filled cavity is formed. Called the Blastocyst cavity.

Question 39

What is the purpose of the blastocyst cavity?

Answer 39

The cavity expands and the cells seperate into 2 parts: 1. Trophoblast layer 2. Embryoblast layer

Question 40

What is the purpose of the trophoblast layer?

Answer 40

Gives rise to the placenta

Question 41

What is the purpose of the embryoblast?

Answer 41

Gives rise to the embryo.

Question 42

Where does the fluid inside the morula come from?

Answer 42

The Uterus

Question 43

What is blastogenesis?

Answer 43

When the embryo and its membranes form the blastocyst.

Question 44

What does the blastocyst look like at this stage?

Answer 44

The blastocyst cavity is surrounded by a wall of trophoblast cells (will later become the placenta). The zona pellucida surrounds the trophoblast cells.

Question 45

After the blastocyst has floated in uterine secretions for about 2 days what happens?

Answer 45

THe zona pellucida gradually degenerates and disappears.

Question 46

What does shedding of the zona pellucida allow?

Answer 46

Rapid growth of the blastocyst.

Question 47

What happens at about 6 days after fertilization?

Answer 47

The blastocyst attaches to the endometrial epithelium.

Question 48

What happens as soon as the blastocyst attaches to the endometrium?

Answer 48

The trophoblast layer seperates into two layers. 1. Cytotrophoblast 2. Sycncytiotrophoblast- allows for deep burrowing of the blastocyst into the endometrium.

Question 49

What are syncytiotrophoblasts and what do they do?

Answer 49

Finger like processes that extend through the endometrial epithelium and invade the connective tissue. Allow for deep burrowing and implantation.

Question 50

When does implantation occur?

Answer 50

1) 4-5 Days - morula reaches uterus 2) Day 6 - begins implantation