General Anatomy (Gametogenesis)

Question 1

What is Gametogenesis?

Answer 1

It is defined as a biological process by which diploid or haploid cells undergo division and differentiation to form mature haploid gametes

Question 2

What are primordial germ cells derived from?

Answer 2

The male gamete, the sperm and the female gamete and the oocyte

Question 3

Where are PGCs formed and where do they migrate to?

Answer 3

They are formed in the embryo in the second week of gestation and they migrate to the gonad from the wall of the yolk sac

Question 4

How does PGCs mature?

Answer 4

• they undergo mitosis which increase their number
• later they undergo meiosis and cytodifferentiation to mature

Question 5

What is mitosis?

Answer 5

Mitosis is the process in which one cell divides and gives rise to two genetically identical daughter cells

Question 6

What is meiosis?

Answer 6

It is the cell division that takes place in the germ cells to generate male and female gametes

Question 7

What are the stages of meiosis 1?

Answer 7

• Before meiosis begins, the diploid germ line cell goes through the three stages of interphase G1, S, G2. The DNA in the cell is replicated during the synthesis stage
• the chromosome then condenses
• Prophase 1 - the homologous chromosomes exchange of DNA between non-sister chromatids this is called crossing over. The nuclear membrane than breaks down, the centrosomes have migrated to the opposite ends of the cell and the spindle apparatus is being formed.
• Metaphase 1 - the spindle fibers or microtubules attach to the centromere of each chromosome and then they are aligned at the equator
• Anaphase 1 - Homologous chromosomes are pulled apart to opposite ends of the cell. The sister chromatids remain attached at the centromere of each chromosome
• Telophase 1 - the microtubules break down, the nuclear membrane reforms and the chromosomes return to an un condensed state. The cell then divides into two haploid daughter cells by cytokinesis.

Question 8

What happens in meiosis 2?

Answer 8

• prophase 2 - the chromosomes condense once again, while the nuclear membrane breaks down and a spindle apparatus begins to form in each of the daughter cells
• Metaphase 2 - the spindle fibers attach to the kinetchores of each sister chromatid, and the chromosomes align at the equator of each cell (the alignment is random)
• Anaphase 2 - cells elongate and the sister chromatids are separated and pulled to opposite ends of the cell and are now considered chromosomes
• telophase 2 - the chromosomes uncoil, new nuclear membrane form, the spindle fibers are broken down and the cell splits again

Question 9

What is oogenesis?

Answer 9

It is a process by which the oogonia mature into mature oocytes. This starts before birth

Question 10

What is the process of Oogenesis before puberty?

Answer 10

• The PGCs which arrive in the gonad differentiate into oogonia
• oogonia undergo mitotic division and are called primary oocytes
• By the third month the primary oocyte with the surrounding flat epithelium cells in known as a primordial follicle
• In the fifth month 7 million oogonia are present
• cell death begins and the oogonia and primary follicles become atretic
• By the 7th month the surviving oocytes near the surface enter prophase 1
• some oogonia arrest their cell division in prophase 1
• at birth all primary oocytes enter diplotene stage (resting phase)

Question 11

How many primary oocytes are there at birth?

Answer 11

600,000 - 800,000

Question 12

How many primary oocytes are present at the beginning of puberty?

Answer 12

40,000

Question 13

When does the maturing of oocytes happen?

Answer 13

• begins at puberty
• occurs with every menstrual cycle
• each month 15-20 follicle pass through 3 stages

Question 14

What is the process of primary or preantral?

Answer 14

• the primary oocyte begins to grow
• surrounding follicular cells change from flat to cuboidal cells called granulosa cells (stratified cells), this unit is called primary follicle
• Granulosa cells rest on a basement membrane that separate them from the ovarian connective tissue (stromal cells)
• the granulosa cells together with the oocyte secrete a layer of glycoproteins on the surface of the oocyte called zona pellucida
• the stromal cells form the theca folliculi
• the theca folliculi is organized into an inner secretory layer, the theca interna and outer fibrous capsule, the theca externa

Question 15

What is the process of secondary or antral stage?

Answer 15

• it is the longest stage
• As development continues, fluid filled spaces appear between the granulosa cells
• they coalesce to form an antrum
• now the follicle is termed as secondary (vesicular) follicle

Question 16

What is the process of pre-ovulatory?

Answer 16

• shortest stage - 37 hours before ovulation
• a surge in LH induces the pre-ovulatory growth
• out of 15-20 follicles only one reaches full maturation and the oocyte is discharged
• the others degenerate and become atretic
• meiosis 1 is completed and the result is 2 daughter cells with un-equal size
• one cell, the secondary oocyte receives most of the cytoplasm and the other the 1st polar body receives none
• then the cell enters the meiosis 2 but arrests in metaphase approx 3 hours before ovulation
• meiosis 2 is completed only if the oocyte is fertilized
• if not the cell degenerates 24 hours after ovulation

Question 17

What is spermatogenesis?

Answer 17

It begins at puberty and includes all the events by which spermatogonia develop into spermatozoa

Question 18

What is the process of spermatogenesis?

Answer 18

• The sex cords acquire a lumen and called as seminiferous tubules.
• The PGCs give rise to spermatogonial stem cells.
• The formation of type A spermatogonia marks the beginning of spermatogenesis.
• The spermatogonia and the spermatids remain embedded with Sertoli cells which nourish and protect the sperms

Question 19

What is spermiogenesis?

Answer 19

The series of changes resulting in the maturation spermatids into sperm cells is spermiogenesis

Question 20

What is teratomas?

Answer 20

• Arise from pluripotent stem cells that can differentiate into derivatives of all the 3 germ layers.
• They contain variety of tissues like bone, hair, muscle, gut epithelia etc.
• Due to abnormal migratory PGCs or tumor of epiblastic cells.

Question 21

What are the numerical chromosomal abnormalities?

Answer 21

• Down’s syndrome
• Trisomy 18 - Edward’s syndrome
• Trisomy 13 - Patau syndrome
• Klinefelter syndrome
• triple X syndrome
• Turner syndrome

Question 22

What is down’s syndrome?

Answer 22

• Extra copy of chromosome 21.
• Due to meiotic non-dysjunction primarily in the oocyte formation
• Risk increases with increased maternal age.

Question 23

What is Klinefelter syndrome?

Answer 23

• Cells have 47 chromosomes.
• Sex chromosome as XXY.
• A boy is born with an extra X chromosome.

Question 24

What is triple X syndrome?

Answer 24

• 47 XXX.
• They have an extra X
chromosome in each cell.
• Tall, usually low IQ females.
• Has limb defects.

Question 25

What is turner syndrome?

Answer 25

• 45 XO karyotype
• Monosomy
• Complete or partial absence of the 2nd sex chromosome.

Question 26

What are the structural chromosomal abnormalities?

Answer 26

• Cri-du-chat (5p) syndrome
• Angelman syndrome
• prader-willi syndrome
• fragile X syndrome

Question 27

What is cri-du-chat (5p) syndrome?

Answer 27

• Deletion in short arm 5
• Cat like cry, microcephaly
• Mental retardation
• Congenital heart disease

Question 28

What is angelman syndrome?

Answer 28

• Microdeletion in the maternal chromosome.

Question 29

What is prader-willi syndrome?

Answer 29

• Microdeletion in the paternal chromosome.

Question 30

What is fragile X syndrome?

Answer 30

• Males affected more.
• Learning and cognitive
disabilities.
• Included in a part of autistic spectrum.
• Can be transmitted through maternal or paternal X chromosome.