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 division Occurs 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