11.4 Sexual Reproduction

Understandings- Spermatogenesis and oogenesis both involve mitosis, cell growth, two divisions of meiosis, and differentiation.- Processes in spermatogenesis and oogenesis result in different numbers of gametes with different amounts of cytoplasm.- Fertilization in animals can be internal or external.- Fertilization involves mechanisms that prevent polyspermy.- Implantation of the blastocyst in the endometrium is essential for the continuation of pregnancy.- HCG stimulates the ovary to secrete progesterone during early pregnancy.- The placenta facilitates the exchange of materials between mother and fetus.- Estrogen and progesterone are secreted by the placenta once it has formed.- Birth is mediated by positive feedback involving estrogen and oxytocin.

Applications/Skills- A: The average 38 week pregnancy in humans can be positioned on a graph showing the correlation between animal size and

the development stage of the young at birth for other mammals.- S: Annotation of diagrams of seminiferous tubule and ovary to show the stages of gametogenesis.- S: Annotation of diagrams of mature sperm and egg to indicate functions.

Guidance- Fertilization involves the acrosome reaction, fusion of the plasma membrane of the egg and sperm, and the cortical reaction.

Spermatogenesis

- Production of spermatozoa (spermatogenesis) occurs in the testes in the seminiferous tubules.

- Spermatagonia are germ cells located near the outer wall of the seminiferous tubule; they can undergo mitosis (to replenish their numbers) or meiosis (to produce sperm) at any time.

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Steps of Spermatogenesis- DNA replication occurs- Meiosis I occurs. Homologous chromosomes separate; ends with 2 haploid cells (23 chromosomes each)- Meiosis II occurs. Sister chromatids separate; ends with 4 haploid cells.- Cells must now differentiate into fully functioning sperm.

Steps of Spermatogenesis

- The cells remain in the seminiferous tubule as they form cellular structures of a mature spermatozoon.- Flagellum for movement- Acrosome the contains enzymes for fertilization

Steps of Spermatogenesis- As they are forming, the spermatozoa receive nutrition from Sertoli cells.- Once they have completed differentiation, they move into the storage area

known as the epididymis

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Oogenesis- Production of egg cells- Begins in the ovaries of the

female fetus- Oogonia undergo mitosis in

order to build up a large number in the ovaries

- These grow into larger cells called primary oocytes

- Primary oocytes begin meiosis, but stop in prophase I

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Oogenesis- Follicle cells are also contained in the ovaries- A layer of follicle cells will surround the primary oocyte- The oocyte with the layer of follicle cells is called the primary follicle- These remain relatively unchanged from birth to puberty

Oogenesis- During the menstrual cycle, a few primary

follicles will complete meiosis I- One of the haploid cells is much larger than

the other.- The smaller cell is referred to as the first

polar body - The larger cell is the secondary oocyte

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Oogenesis

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- The ring of follicles around the oocyte begin to divide and form a fluid

- Two rings of follicles are formed, with a fluid filled cavity separating them

- This structure is called a Graafian follice

This increase in fluid creates a bulge on the ovary which will eventually lead to ovulation

- During ovulation, the secondary oocyte with the inner ring of follicle cells is released from the ovary

- Meiosis II is not completed until fertilization- If fertilization doesn’t occur, the cell dies- If fertilization does occur, meiosis II is

completed and a true ovum exists briefly until the haploid nuclei fuse and form a zygote

The Mature Egg

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- The largest cell in most mammals- Nutrients in the ovum are referred to as the yolk- Cytoplasm contains cortical granules (function after

fertilization)- Zona pellucida is a layer of glycoproteins outside of the

plasma membrane

Spermatogenesis Oogenesis

Mitosis replaces germinal cells daily Mitosis replaces germinal cells only early in development

Some cell growth occurs before meiosis I A great deal of cell growth occurs before meiosis I

Two divisions of meiosis result in 4 haploid spermatozoa

Two divisions of meiosis result in ovum and three possible polar bodies

Spermatids must remain in seminiferous tubules until differentiation

Differentiation of oocyte into an ovum occurs partly in the ovary and continues after ovulation

Gamete is very small with little cytoplasm and limited organelles

Gamete is large with w a great deal of cytoplasm, nutrients, and organelles

Millions produced every day starting at puberty Ovulation of one of thousands of oocytes occurs with each menstrual cycle, stops at menopause

External Fertilization- Female lays eggs - Male deposits sperm in area where

eggs were laid- Inefficient method due to many

eggs never becoming fertilized- Not associate with parental care- Many offspring die due to predation

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Internal Fertilization- Involves intercourse- Spermatozoa deposited into the female- Number of ova produced is less than with external fertilization- Associated with high parental care- Higher reproductive success

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Fertilization in Humans

- Millions of sperm are deposited into the female’s vagina

- Sperm absorb some of the fructose in the semen for energy

- Some find their way through the cervical opening- They swim up the endometrial lining and into the

fallopian tubes- Only a very small percentage will ever reach a

secondary oocyte

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Fertilization in Humans- Fertilization generally occurs in the fallopian tube- It takes many sperm to penetrate the follicle cell layer - Several sperm gain access to the zona pellucida- They release enzymes contained in the acrosome- One spermatozoon will reach the plasma membrane of the

secondary oocyte first and penetrate the egg- The plasma membranes of the gametes fuse - Spermatozoon donates its set of chromosomes to the

maternal set in the ovum- To prevent polyspermy, the cortical granules fuse with the

ovum’s internal plasma membrane and release their enzymes- This causes the zona pellucida to become impenetrable to

more sperm- The fertilized ovum is now a zygote

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Early Development of the Embryo

- Fertilization stimulates mitotic division- The embryo moves down the fallopian tube into the uterus- By the time it reaches the uterus, it’s about 100 cells in size (blastocyst)- Surrounded by a layer of cells (trophoblast) that will help form fetal portion of placenta- Inner cell mass becomes the body of the embryo- The embryo eventually stops moving along the endometrium and begins implantation

Endo-metrium

Blastocyst

Inner cell mass

Trophoblast

The Placenta- Forms from tissue from the embryo and the mother- The umbilical cord forms from the embryo side of the placenta- Has 3 blood vessels- Two carry blood to the placenta (deoxygenated, has waste products)- One carries blood back (oxygenated, nutrients added)

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Pregnancy Hormones- Early embryo secretes hCG

- hCG enters the blood stream; maintains the corpus luteum- Corpus luteum secretes progesterone to maintain the vascular endometrium

- Eventually, the placenta starts to secrete progesterone- Progesterone and estrogen prevent further oocyte development/ovulation

Birth Hormones- Birth is controlled by positive feedback

- Previous events lead to more forceful and frequent events until stimulus is removed

- No homeostatic factor controlled- Uterine contractions begin weak, infrequent;

become strong, frequent- The hormone involved is oxytocin

- Produced by the hypothalamus and secreted by the posterior pituitary

- Released into the blood stream when birth begins

- Receptors in the uterus respond with contraction

- First contraction signals for more oxytocin to be released

- Happens repeatedly, with increasing intensity until birth