Livestock Breeding and Genetics

Sperm and Egg Production

• We are beyond the mid-point of the semester.

• What’s ahead: - Livestock Breeding and Genetics- Animal Health- Livestock Pests- Waste Management

In this lesson…

• Describe the difference between oogenesis and spermatogenesis.

• Describe the genetic factors that influence the sex of an offspring.

Some Main Points to Ponder:

• What are gonocytes and oocytes?• Focus in on stage 2 of oogenesis

and the divisions that occur.• Focus in on the divisions and

products thereof during spermatogenesis.

• How do males and females play a role in sex determination of the offspring?

Big Picture of Today:• Oogenesis: Creation of Eggs in

female livestock• Spermatogenesis: Creation of

Sperm in male livestock

Oogenesis• The process of the formation of

eggs with (GONOCYTES) OOCYTES

What are Gonocytes?

• The precursors of both male and female gametes (sex cells).

• Originate in the embryonic animal during its formation in the womb.

What are Oocytes?

• They are gonocytes that have differentiated in the fetal ovaries.

• They are the sole reservoir of all future ova.

Microscopic Image of Oocytes

Growth of the oocyte (aka Oogenesis)

 Stage 1 - the OOCYTE grows in size (lasts from birth to puberty)

Stage 2 - the OOCYTE and follicle matures (during puberty)

Cell divisionOccurs during Stage 2 of oogenesis (previous slide)1) The OOCYTE undergoes 2 meiotic

divisions during which 2 daughter cells arise, each having half the chromosome complement.

So what about these daughter cells….

• One of these 2 daughter cells acquires nearly all of the cytoplasm, and this large cell is called the SECONDARY OOCYTE.

• The much smaller cell with only a little cytoplasm is called the POLAR BODY.

2) At the second maturation division, the SECONDARY OOCYTE divides into the OOTID and a second POLAR BODY.

3) The POLAR BODIES degenerate, leaving the OOTID ready for fertilization.

Oogenesis: Big Picture

FYI• The OOTIDS or OVUM actually

undergo oogenesis at different times, depending on the species.

Moving forward to: Spermatogenesis

• The process of sperm formation which commences at puberty.

• At birth, SPERMATOGONIA are present

• These SPERMATOGONIA also are called SPERM MOTHER CELLS.

Phases of Spermatogenesis

• SPERMATOGONIA undergo mitotic division and become dormant, thus ensuring a continuous supply of SPERMATOGONIA.

• One SPERMATOGONIUM becomes active and divides 4 more times so that there are now 16 PRIMARY SPERMATOCYTES.

These primary spermatocytes go through meiotic division.

• At this time, the number of chromosomes is HALVED

•  These new cells are now called SECONDARY SPERMATOCYTES

• The secondary spermatocytes undergo one more division to become SPERMATIDS.

• SPERMATIDS undergo a metamorphosis which involves a radical alteration in cellular form during which most of the cytoplasm is lost.

Spermatogenesis: Big Picture

Sex Determination• Every cell in the mammalian body,

except the gametes (sperm and ova), contains a pair of sex chromosomes

• In females, the 2 members of the sex chromosome pair resemble one another and are known as the X chromosomes (XX).

• In males, the sex chromosomes differ from one another, one is the X chromosome, the other is smaller and is called the Y chromosome (XY).

• The gametes contain only a single sex chromosome:

In the female, each ovum contains an X chromosome.

In the male, the sperm contains either an X or a Y chromosome.

Determination: • In normal fertilization, the embryo

develops as a male or female according to whether the ovum is fertilized by a sperm carrying an X or a Y chromosome. In effect, the father determines the sex of the offspring.

What influences the sex of the offspring?

• If the 2 types of sperm are present in equal numbers, the ratio of male to female sperm at the time of conception should be 1:1.

• The sex of the offspring is important to many livestock producers since one sex may be more valuable than the other.

For example:1) beef cattle - steers bring a higher

market price than heifers;2) dairy cattle - heifers are the milk

producers so they are more valuable than bull calves; and

3) poultry - egg production operations keep all the laying hen chicks, but often dispose of the male chicks.

Let’s Review• What are gonocytes and oocytes?• What divisions occur in stage 2 of

oogenesis?• What were the divisions and

products thereof during spermatogenesis?

• How do males and females play a role in sex determination of the offspring?

Sources• California Ag Ed Tech Prep• Scientific Farm Animal Production:

An Introduction to Animal Science