First Week of Development

Question 1

What is human development?

Answer 1

A continuous process that begins when a oocyte is fertilized by a sperm then, cell division, migration, death, differentiation, growth and rearrangement transform the oocyte to a totipotent zygote then a multicellular human

Question 2

When is the gestation period in humans?

Answer 2

280 days from the last menstrual period or 266 days from fertilization

Question 3

what are the three periods of pre-natal development?

Answer 3

• Pre-embryonic period (fertilization to 2 weeks)
• Embryonic period (3-8 weeks)
• Fetal period (9th week - birth)

Question 4

What are the results of fertilization

Answer 4

• 2º oocyte completing meiosis II
• diploid chromosomal number in zygote
• species variation when cells divide
• determination of genetic sex
• Mitotic division of zygote

Question 5

What is gametogenesis

Answer 5

• The process of formation and development of gametes
• spermatogenesis in males and oogenesis in females

Question 6

Explain the process of spermatogenesis?

Answer 6

• One type Ad spermatogonium is transformed asymmetrically into type Ad and Ap
• Type Ap becomes type B which differentiates to 1º spermatocyte
• 1º spermatocyte becomes the 2º spermatocyte by meiosis I (reduction)
• The cells are these divided to 4 normal sperm cells (spermatids) by meiosis II

Question 7

Explain the process of oogenesis

Answer 7

-A primary oocyte in a follicle around it (primordial follicle) grows forming the primary follicle, then the zona pellucida covers the oocyte in the follicle
- the primary oocyte completes its first meiotic division to create the secondary oocyte and the first polar body
- At ovulation the secondary oocyte begins its second meiotic division which is halted at metaphase II
- Meiosis II is complete when oocyte is fertilized by sperm

Question 8

what are oogonia?

Answer 8

Oogonium cells are stem cells that produce primary oocytes before birth

Question 9

Explain spermiogenesis

Answer 9

• A spermatid differentiates to a spermatozoa which involves the development of the tail from the centrioles and mitochondria, the loss of the cytoplasm and the formation of the acrosome from the golgi apparatus.

Question 10

What is the purpose of the acrosome?

Answer 10

It contains lytic enzymes that are released at the beginning of fertilization to assist the sperm in penetrating the zona pellucida

Question 11

Explain the pathway of the ovarian cycle?

Answer 11

The hypothalamus releases gonadotropin-releasing hormone which acts on the pituitary gland to release gonadotropic hormones.
- Menstrual phase: FSH stimulates follicles holding oocytes to grow.
- Proliferative phase: Theca folliculi forms around the oocyte producing estrogen. The increased estrogen from the follicle makes the endometrial lining proliferate rapidly.
- Luteal phase: A surge in LH around day 14 = the endometrial lining at its peak and secondary oocyte is released from follicle (ovulation). follicle folds into a corpus luteum producing progesterone to make blood arteries on endometrial lining
- Ischemic phase: Corpus luteum is degenerated (corpus albacaans) if not fertilized. A layer of the endometrium is shed because of low progesterone levels and the endometrium is ischemic leading back to menstrual phase.

Question 12

What is the function of FSH (Follicle Stimulatiing Hormone)

Answer 12

• Stimulates follicle growth in the ovary up until ovulation
• Stimulates the follicle to release estrogen by the theca folliculi
• Stimulating the formation of zona pellucida

Question 13

What are the two types of gonadotropic hormones?

Answer 13

• Follicle Stimulating Hormone
• Luteinizing Hormone

Question 14

What is the function of LH (Luteinizing Hormone)

Answer 14

• Triggers the release of the secondary oocyte from the follicle (ovulation)
• Converts ruptured Graafian follicle to corpus luteum
• Stimulates the release of progesterone and estrogen from the ovaries by the corpus luteum

Question 15

Explain the phases of fertilization

Answer 15

• Capacitation: The proteins on the head of the sperm are gotten rid of and sperm passes through the corona radiata of the secondary oocyte with its hyalurinidase (navigation)
• Acrosomal: Sperm penetrates the zona pellucida with its acrosin within the fallopian tube
• Once one sperm touches cell membrane, a (+) is formed to prevent other sperm from binding
  (fertilization)
• 2nd meiotic division is completed forming the female pronucleus and the second polar body
• The oocyte with two pronuclei (male and female) is called an ootid
• the pronuclei fuse forming a zygote

Question 16

What does the trophoblast differentiate to

Answer 16

Inner cytotrophoblast and outer syncytiotrophoblast

Question 17

Describe the first week after fertilization

Answer 17

• The secondary oocyte goes into the fallopian tube from the ovary
• oocyte → ootid → zygote
• zygote cleaves 30 hours after fertilization to form 2 identical blastomeres within the zona pellucida
• the division continues till about 12-32 blastomeres are formed = morula (3 days)
• In day 4 morula becomes the blastocyst with a trophoblast and an embryoblast that’s now in the uterus
• The blastocyst implants in endometrial wall by the end of the first week after zona pellucida degenerates

Question 18

How is the blastocyst formed

Answer 18

• When the morula entered the uterus, the fluid from the uterine cavity passes the zona pellucida creating the blastocystic cavity which seperates the blastomeres inside to the trophoblast, which forms the wall and the embryoblast. after about 2 days, the zona pellucida degenerates and the blastocyst attaches to the endometrial wall causing the trophoblast to differentiate into the cytotrophoblast and syncytiotrophoblast

Question 19

What are the two layers formed from the morula?

Answer 19

• Trophoblast: outer cell layer that gives rise to the placenta
• Embryoblast: inner cell mass that gives rise to the embryo