Gametogenesis: process of differentiation that produces cells specialized for reproduction

I. General Concepts

A. Processes

1. Spermatogenesis

a. Produces sperm

b. Spermatogonium differentiate into spermatazoa

2. Oogenesis

a. Produce ova

b. Oogonium differentiate into ovum

B. Takes place in gonads

1. Male: testes

2. Female: ovary

Oogenesis: differentiation of female primordial germ cell into an unfertilized egg

I. Background

A. Egg Cell

1. Large relative to the size of somatic cells of the organism

a. Human egg is approximately 100 µm in diameter compared to 20 to 30 µm for most somatic cells

b. Larger than somatic cells due to the accumulation of storage products

i. Energy necessary for embryonic development

ii. The storage products include yolk, cytoplasmic organelles, structural proteins, enzymes and components of the translation machinery

B. Zona pellucida

1. Extracellular coat surrounding mammalian eggs

2. Function:

a. Serves to protect the egg

b. Serves as a selective, species-specific barrier to sperm penetration to assure that the egg is fertilized by sperm of the same species

C. All eggs contain a nucleus and most of the cytoplasmic organelles present in other animal cells

a. Nucleus is haploid

b. Prior to fertilization, the egg may be arrested in diakinesis of prophase I

c. Yolk granules

i. Eggs of contain stores of carbohydrates, proteins and lipids in discrete particles

ii. Yolk serves as a source of nutrients during development

iii. In mammals in which implantation occurs after only a few cleavages there is usually relatively little yolk stored in the egg

II. Organization of the ovary

A. Oogenesis occurs in the ovary

1. Ovaries complex organs composed of structural, accessory and endocrine cells in addition to the germ line cells

2. Immature oocytes are distributed throughout the ovary in follicles

a. Oocytes differ in size and composition depending on the differentiated state

b. A total of 200,000 to 400,000 follicles are present in each human ovary

i. Each follicle contains a single oogonia

c. Few oogonia reach maturity

i. In humans typically fewer than 400 oocytes mature and are released by ovulation

3. Location of oocyte depends on its maturity

a. Primordial follicles are most prevalent in the stroma (near the outside)

b. Oocytes become more centrally located as they mature

III. Events during oogenesis

A. Primordial germ cells migrate to the ovary during fetal and embryonic development

B. Differentiation of primordial germ cell into an unfertilized egg requires:

1. Proliferation

a. Increase in number

2. Growth in size

C. Phases

1. Proliferative growth

a. Primordial germ cells multiply by mitotic divisions

i. Occurs during embryonic and fetal development

ii. Completed prior to birth

iii. Oogonia (female primordial germ cell) multiply by mitotic divisions

2. Growth phase

a. Once an oogonium become committed to complete oogenesis it begins one of the most remarkable examples of growth of a single cell

i. Preparation for post-fertilization development

ii. Cell may increase from 100 to more than 1,000,000 fold the size of the oogonium

b. Oogonium becomes committed to proceed through oogenesis when it enters the S-phase of meiosis

c. Replication of the DNA produces a cell with a 2N chromosome and 4N DNA content

i. Primary oocyte

d. Primary oocyte begins to grow

e. Most of oogenesis is spent with the cell in the prophase of the first meiotic division

f. The growth phase typically spans a period of several weeks to several months for most animals

3. Meitotic phase

a. At the end of the growth phase, a maturation stimulus signals the oocyte to complete meiosis

b. Mature oocyte passes through the two meiotic divisions

i. One cell retains the vast majority of the cytoplasm and becomes the egg

ii. The extra sets of chromosomes are eliminated in polar bodies

iii. There are two or three polar bodies formed depending on whether the first polar body divides

iv. When three polar bodies are formed, each contains one copy of a haploid set of chromosomes and a small fraction of the cytoplasm from the mature oocyte

v. Polar bodies are non-functional and eventually die

vi. The egg that is produced at the end of the meiotic divisions contains one copy of a haploid set of chromosomes and most of the cytoplasm

c. Prior to first meiotic division oocyte is referred to as primary oocyte

d. Following first meiotic division oocyte is referred to a secondary oocyte

IV. Follicle development

A. Oocyte matures in close association with follicle cells

2. The follicle cells form a single or multicellular layer surrounding the oocyte

a. As follicle matures, it progresses from primary to secondary to Graafian follicle

3. As the oocyte reaches maturity, the follicle becomes filled with fluid to form the Graafian follicle

4. When mature, Graafian follicle ruptures to release oocyte

a. This process is referred to as ovulation

b. Second meiotic division does not happen until after fertilization

5. Graafian follicle continues to differentiate to form the corpus luteum

V. Hormonal regulation of oogenesis

A. Follicle stimulating hormone (FSH)

1. Immature oocytes are stimulated to grow and mature by FSH

2. FSH is an anterior pituitary hormone

B. Estrogen

1. Produced by follicle

C. Luteining hormone (LH)

1. Stimulates corpus luteum development

2. LH is an anterior pituitary hormone

D. Progesterone

1. Produced by corpus luteum

These hormones will be discussed in more detail when the menstrual cycle is considered.

Return to Main Page