Embryogenesis and Development

Question 1

Where does fertilization occur?

Answer 1

The ampulla - the widest part of the fallopian tube

Question 2

What are the steps of fertilization?

Answer 2

1. Sperm meets the secondary oocyte in the fallopian tube and releases acrosomal enzymes.
2. These allow the sperm to penetrate the corona radiata and zona pellucida.
3. The sperm forms a tubelike structure called the acrosomal apparatus, which extends and penetrates the cell membrane, allowing the pronucleus to enter the oocyte once it has finished meiosis II.
4. Penetrating the cell membrane causes a cortical reaction - a release of Ca2+ ions. The depolarization means no more sperms can fertilize the egg, and the increased Ca increases the metabolic rate of the new diploid zygote. The fertilization membrane is now depolarized and impenetrable.

Question 3

How do dizygotic twins form?

Answer 3

Fraternal twins form when two different eggs are fertilized by two different sperm during one ovulatory cycle.

Question 4

How do monozygotic twins form, and what are they classified by?

Answer 4

Identical twins form when a single zygote splits into two. They are classified by the number of structures they share.

Question 5

What are the phases a zygote goes through during early development?

Answer 5

Fertilization, cleavage, blastulation, implantation, gastrulation, neurulation

Question 6

What occurs during cleavage?

Answer 6

As the zygote moves to the uterus for implantation, it undergoes rapid mitotic cell divisions. Its total size remains the same, meaning nuclear-to-cytoplasmic (N:C) ratio and surface area to volume ratio increase.

Question 7

What are the two types of cleavage?

Answer 7

1. Indeterminate cleavage: results in cells that can still develop into complete organisms.
2. Determinate cleavage: results in cells that are committed to differentiating.

Question 8

What occurs during blastulation? What two cell groups are in the blastula?

Answer 8

1. After several divisions, embryo becomes a solid mass of cells called a morula.
2. Undergoes blastulation to become blastocyst - hollow ball of cells with fluid-filled inner cavity called a blastocoel.
   - The two groups are the trophoblasts (surround the blastocoel and give rise to chorion and placenta) and the inner cell mass (creates organism).

Question 9

What occurs during implantation?

Answer 9

Blastula moves thru fallopian tubes to the uterus and burrows in the endometrium.

Question 10

What 6 structures support the embryo?

Answer 10

1. Trophoblasts create interface between maternal blood supply and embryo.
2. Chorion/chorionic villi form from trophoblasts, which develop into placenta and support gas exchange.
3. Umbilical cord - 2 arteries (take out deoxygenated blood), 1 vein (bring oxygenated blood)
4. Yolk sac - until placenta is formed, the site of early blood cell development and nutrition
5. Allantois - membrane involved in early fuid exchange between embryo/yolk sac; forms umbilical cord with yolk sac
6. Amnion - membrane filled with amniotic fluid, serves as a shock absorber

Question 11

What occurs during gastrulation?

Answer 11

• Once cell mass implants, formation of 3 distinct cell layers (ectoderm, endoderm, mesoderm).
• Invagination into blastocoel is called archenteron, later becomes the gut.
• The opening of the archenteron is called the blastospore, later becoming the anus (in deuterostomes. In protostomes, it becomes the mouth).

Question 12

What does the ectoderm become?

Answer 12

Integument (epidermis, hair, nails); epithelia of nose, mouth, anal area; lens of the eye; nervous system - adrenal medulla; inner ear

Question 13

What does the mesoderm become?

Answer 13

Systems: musculoskeletal, circulatory, most of the excretory. Also gonads, and the muscular and connective tissue of the digestive/respiratory systems and the adrenal cortex.

Question 14

What does the endoderm become?

Answer 14

Linings of digestive/respiratory tracts; pancreas, thyroid, bladder, urinary tracts, liver

Question 15

What controls differentiation?

Answer 15

Selective transcription of the genome depending on function

Question 16

What is induction and how does it work? Why is it important?

Answer 16

• Induction is the ability of one group of cells to influence the fate of another - organizing cells diffuse chemical inducers to responsive cells
• Ensures the proximity of different cell types that work together

Question 17

What occurs during neurulation?

Answer 17

• Nervous system develops
  1. Rod of mesodermal cells (notochord) forms along the long axis of the organism like a spine.
  2. The notochord induces a group of overlying ectodermal cells to slide in to form neural folds around a neural groove.
  3. The folds grow together until they fuse into a neural tube, which creates the CNS.
  4. The tip of each neural fold are neural crest cells, which form PNS.
  5. Ectodermal cells cover neural tube and crest to cover the nervous system.

Question 18

What are teratogens?

Answer 18

• Substances that interfere with development - like alcohol, viruses, environmental chemicals
• Mother’s mental health and physiology can also affect fetus

Question 19

What are the three phases of cell specialization?

Answer 19

Specification, determination, differentiation

Question 20

What occurs during specification and determination? How does determination occur?

Answer 20

• Specification: cell is reversibly designated as a cell type
• Determination: irreversible commitment of cell to a particular function
• Determination occurs when specific mRNA and protein molecules are divided into daughter cells during cleavage
• Can also occur due to secretion of specific molecules from nearby cells called morphogen

Question 21

What occurs during differentiation?

Answer 21

Cell must change its structure, function, and biochemistry to match its assigned cell type.

Question 22

What are stem cells? What are the different types of potency?

Answer 22

Stem cells have not yet differentiated or will give rise to other cells that will differentiate.

• Totipotent: greatest potency, can become anything; embryonic stem cells that don’t differentiate until they become the inner cell mass/trophoblasts, and then three germ layers
• Pluripotent: when the cells are becoming the three germ layers. They can become anything except those in the placental structures.
• Multipotent: can differentiate into multiple types within a group

Question 23

What is an inducer in cell-cell communication? A responder?

Answer 23

• An inducer is the cell or the substance it secretes to induce a developing cell to become a particular type.
• A responder is the cell that is induced; it must be competent, or able to respond to the inducing signal

Question 24

What are autocrine signals?

Answer 24

Act on the same cell that secreted the signal in the first place

Question 25

What are paracrine signals?

Answer 25

Act on cells in the local area

Question 26

What are juxtacrine signals?

Answer 26

Do not usually involve diffusion; instead, a cell directly stimulating receptors of an adjacent cell

Question 27

What are endocrine signals?

Answer 27

Hormones are secreted that travel thru bloodstream to a distant target tissue

Question 28

What are inducers made of?

Answer 28

Inducers are often growth factors, peptides that promote mitosis and differentiation
-They only function on specific cell types according to their competences

Question 29

What is reciprocal development?

Answer 29

Induction is not a one-way pathway - differentiated tissue can trigger differentiation of surrounding tissue

Question 30

What is morphogen exposure?

Answer 30

Different morphogens in different concentrations are secreted, resulting in unique combinations of morphogen exposure throughout the organism

Question 31

What is the process of apoptosis? How does it differ from necrosis?

Answer 31

Through programming or signals, cell dies.

• Cell divides into self-contained apoptotic blebs, which can be broken apart into apoptotic bodies and digested by other cells
• Allows for recycling of materials
• In necrosis, cell dies by injury. Internal substances can be leaked, causing irritation/immune response

Question 32

What are the two types of regeneration? What type do humans have?

Answer 32

• Complete regeneration: lost or damaged tissues are replaced with identical tissues
• Incomplete regeneration: newly formed tissue is not identical in structure or function to lost tissue
• Humans have incomplete regenerative capacity in certain organs, like liver and kidney

Question 33

What is senescence? What occurs?

Answer 33

• Senescence is aging, resulting in the failure of cells to divide (normally after 50 divisions)
• This may be due to shortened telomeres (ends of chromosomes that reduce the loss of genetic info/help prevent DNA from unraveling)
• Telomeres are hard to replicate so they shorten each round of DNA synthesis

Question 34

What is the purpose of the placenta?

Answer 34

• Nutrient, gas, and waste exchange occur between mom and fetus. Occurs through diffusion.
• Also crosses antibodies to baby’s naive immune system
• Also secrets progesterone, estrogen, and hCG, all of which are needed to maintain pregnancy

Question 35

How is oxygen transferred from mom to baby?

Answer 35

Through diffusion (meaning the mother has a higher partial pressure of oxygen than baby); enhanced by fetal hemoglobin (HbF) in fetal RBCs

Question 36

What do the umbilical arteries do?

Answer 36

They carry deoxygenated blood + waste away from the fetus toward the placenta

Question 37

What does the umbilical vein do?

Answer 37

It carries oxygenated blood + nutrients toward the fetus from the placenta

Question 38

Which organs in the infants’ body are underdeveloped, and how does the infants’ body handle this?

Answer 38

• Lungs (gas exchange happens at placenta) and liver (detoxification/metabolism handled by mom’s liver)
• Body makes 3 shunts that direct blood away from these organs while they develop

Question 39

What are the three shunts in the infants’ body?

Answer 39

1. Foramen ovale: one-way valve that connects right atrium to left atrium (so right ventricle and lungs are bypassed) (infant’s right side of the heart is at a higher pressure than left, unlike ours)
2. Ductus arteriosus: shunts leftover blood from pulmonary artery to aorta
3. Ductos venosus: liver is bypassed as it shunts blood returning from placenta via umbilical vein into inferior vena cava

Question 40

What occurs during the first trimester?

Answer 40

• Major organs begin to develop
• Heart starts beating 22 days in
• Grows in length
• Cartilage skeleton begins to harden into bone
• Now called a fetus

Question 41

What occurs during the second trimester?

Answer 41

• Fetus grows a lot
• Face looks human
• Fingers and toes elongate
• Begins to move in amniotic fluid

Question 42

What occurs during the third trimester?

Answer 42

• More brain development
• Antibodies given to fetus for protection
• Growth rate slows and fetus becomes less active (less room to move)

Question 43

What occurs during birth (3 phases)?

Answer 43

• Vaginal birth is called parturition
• Contractions of uterine smooth muscle, coordinated by prostaglandins and oxytocin
  1. Cervix thins out and amniotic sac ruptures (water breaking)
  2. Strong uterine contractions result in birth of fetus
  3. Placenta and umbilical cord are expelled in the afterbirth

Question 44

Describe adult stem cells.

Answer 44

• They are not inherently pluripotent. This is induced by treatment with transcription features.
• There is less of a chance of rejection with them
• They are less controversial than embryonic SCs