Chapter 40 - Human Reproduction & Development

Chapter 40 - Human Reproduction & Development


Chapter 40 - Human Reproduction & DevelopmentMale reproductive system organs

Scrotum

Supporting structure for the testes

The location of the scrotum & the contraction & relaxation of the cremaster muscle serve to regulate the temperature of the testes

Sperm production requires temperatures three degrees below core temperature

Cremaster muscle elevates the testes in response to sexual arousal or cold; depresses it for heat

Chapter 40 - Human Reproduction & DevelopmentSpermatozoan structure

1. Acrosome - modified, secretory vesicle/lysosome filled with enzymes that are needed to fertilize the egg

2. Midpiece - many mitochondria for energy production

3. Flagellum - movement

Chapter 40 - Human Reproduction & DevelopmentTestes

Contain coiled seminiferous tubules which are the site for sperm production

Seminiferous tubules are lined with spermatogenic cells = spermatogonia which are located next to the basement membrane of the tubule

Spermatogonia undergo spermatogenesis (meiosis) to produce sperm & mitosis to replenish themselves

Meiosis occurs from the basement membrane side to the lumen of the tubule

The wall of the tubule is actually composed of progressively more mature sperm: spermatogonia, primary spermatocytes, secondary spermatocytes, & spermatids

Spermatozoan are released into the lumen of the tubules



Chapter 40 - Human Reproduction & DevelopmentAmong the developing sperm cells are Sertoli cells that extend from the basement membrane to the lumen of the tubule

Sertoli cell functions:

1. Support, protect, & nourish developing spermatogenic cell

2. Mediate the effects of testosterone & follicle stimulating hormone (FSH) on the sperm

A. Testosterone - Development, growth, & maintenance of male sex organs; final maturation of sperm; stimulates descent of testes during embryogenesis

B. FSH - Act on Sertoli cells = stimulate spermatogenesis

3. Control movements of the spermatogenic cells & the release of spermatozoan into the lumen

4. Secrete fluid for sperm transport

Leydig cells are found in between the seminiferous tubules; Luteinizing hormone (LH) stimulates these cells to secrete testosterone


Chapter 40 - Human Reproduction & DevelopmentAccessory sex glands

1. Seminal vesicle secretion is/contains:

Alkaline to reduce the acidity in the female reproductive tract

Fructose for energy

Semenogelin (fibrinogen derivative) for the coagulation of the semen following ejaculation = helps keep the ejaculated sperm in the female reproductive tract following the withdrawal of the penis

Constitutes ~60% of the volume of the semen

Chapter 40 - Human Reproduction & DevelopmentDucts of the testes (continued)

1. Epididymis

Site of sperm maturation (10-14 days), sperm storage, & “dead” sperm reabsorption

2. Ductus (Vas) defrens

The vas defrens is the site of sperm storage, sperm transportation to the urethra, & sperm reabsorption

3. Ejaculatory ducts

Formed by the joining of the seminal vesicle duct & the vas defrens

These ducts eject spermatozoan into the urethra just before ejaculation & transport & eject seminal vesicle secretions

4. Urethra

Passageway for semen = mixture of sperm & accessory gland secretions

Chapter 40 - Human Reproduction & Development2. Prostate gland secretion is/contains:

Slightly acidic (pH 6.5), but is significantly more basic than the female reproductive tract (pH 3.5 - 4.0)

Citric acid for energy

Clotting enzymes which cause semenogelin to coagulate following ejaculation

Fibrinolysin which de-coagulates the semenogelin a short time later (5 –20 minutes) allowing the sperm to become motile following ejaculation

Constitutes ~25% of the volume of the semen

3. Bulbourethral (Cowper’s) gland

Secretion consists of alkaline mucus to lubricate the lumen of the urethra

Chapter 40 - Human Reproduction & DevelopmentSperm + accessory gland secretions = SEMEN

Ejaculate characteristics

2.5 - 5 ml; 150 million sperm per ml; <20 million = infertile

pH = 7.2 -7.7

It contains seminalplasmin, an antibiotic, which may protect the sperm from bacteria in both the male & female reproductive tracts

Penis

Body composed of three cylindrical tissue masses: Paired corpora cavernosa & corpus spongiosum

All three are well vascularized with blood sinuses

Under the appropriate stimulation, the penile arteries dilate & the blood sinuses become filled with blood & expand

This expansion compresses the penile veins, trapping the blood in the penis




Chapter 40 - Human Reproduction & DevelopmentFemale reproductive system organs

Ovaries

Ovarian follicles lie in the ovaries & consists of oocytes in different developmental stages surrounded by follicular cells

Graafian follicles are mature & will rupture, expelling a secondary oocyte via a process called ovulation

Corpus luteum contains the remnants of an ovulated mature follicle, & produces hormones such as progesterone & estrogen. It will eventually degenerate into the corpus albicans

Chapter 40 - Human Reproduction & DevelopmentOogenesis

Female meiosis begins in the developing female fetus.

Prior to birth the fetal ovaries are filled with oogonia & some will enter prophase I but will arrest before completion

These “suspended” primary oocytes will remain dormant until puberty

As it grows , it forms a clear glycoprotein layer, zona pellucida, between the primaryoocyte & the granulosa cells

The innermost layer of granulosa cells will adhere strongly to the zona pellucida = corona radiata

This structure is now called the secondary follicle, & its maturation will continue once puberty is reached

The secondary follicle will develop into the mature (Graafian) follicle, during which the primary oocyte will divide into a secondary oocyte & a polar body, which is a packet of discarded nuclear material due to unequal division. This completes meiosis I

Chapter 40 - Human Reproduction & DevelopmentOogenesis (continued)

The secondary oocyte will enter meiosis II and arrest at metaphase II, until ovulation & subsequent fertilization

Once ovulated the secondary oocyte will either be fertilized or not

If it is not fertilized the secondary oocyte will degenerate

If it is fertilized, meiosis II will finish, resulting in the formation of the ovum (mature egg) & a second polar body (waste)

The nuclei of the ovum & the already penetrated sperm will combine & a zygote is formed


Chapter 40 - Human Reproduction & DevelopmentUterine (Fallopian) tubes

2 tubes that extend laterally from the uterus

Transport secondary oocytes & fertilized ova from the ovaries to the uterus

Divided into three sections:

A. Infundibulum which lies next to the ovaries. It ends in a fringe of fingerlike projections called fimbriae which attach to the lateral ends of the ovary

B. Ampulla - 2/3 of the tube

C. Isthmus connects the tube to the uterus

Chapter 40 - Human Reproduction & DevelopmentUterus

Site of menstruation, implantation of the zygote, development of the fetus, & labor

The superficial layer of the uterus is called the endometrium & it is the site for implantation of a fertilized egg

Vagina

Passageway for menstrual flow & childbirth

Receives semen

Chapter 40 - Human Reproduction & DevelopmentUterine & Ovarian Cycles

4 phases divided into:

1. Menstrual

2. Preovulatory

3. Ovulation

4. Postovulatory

Menstrual phase

Characterized by menstrual flow

Secondary follicles in the ovaries begin to enlarge

The endometrium is sloughed off due to a decrease in estrogen & progesterone

Chapter 40 - Human Reproduction & DevelopmentPreovulatory phase

FSH stimulates the growth & development of a Graafian follicle

Ovulation

A surge of LH induces ovulation

The ovum is ovulated & the remains of the follicle in the ovary will develop into the corpus luteum

Chapter 40 - Human Reproduction & DevelopmentPostovulatory phase

Two pathways:

If the ova is not fertilized the corpus luteum will decrease its secretion rate & degenerate into the corpus albicans over a period of two weeks

The decrease in corpus luteal hormones will lead to menstruation & the stimulation of the next round of follicular development

If the ova is fertilized, the embryo’s chorion will secrete hCG (human chorionic gonadotropin) which will maintain the corpus luteum beyond its two week lifespan

The chorion will develop into the placenta which will begin to secrete estrogen & progesterone to maintain the pregnancy



Chapter 40 - Human Reproduction & DevelopmentDevelopmental Anatomy of the Reproductive System

Early embryo has primitive gonads that have the potential to develop into either male or female organs

The embryo also possesses two sets of ducts: Wolffian (primitive male) & Mullerian (primitive female)

The male pathway depends upon the presence of the Y chromosome which contains the SRY gene (Sex determining Region of the Y chromosome)

During the 7th week, the SRY gene produces or regulates the production of the H-Yantigen

This antigen directs the differentiation of the primitive gonads into testes

Testes begin secreting testosterone & Mullerian-inhibiting factor

Mullerian inhibiting factor destroys the Mullerian ducts (female embryonic reproductive tubes), while testosterone transforms the Wolffian ducts into the internalmale reproductive tube organs

Testosterone is converted to dihydroxytestosterone which directs the development of the male external genitalia

Chapter 40 - Human Reproduction & DevelopmentThe female pathway depends upon the absence of the Y chromosome

No SRY gene

No development of the H-Y antigen

Gonads develop into ovaries, which don’t secrete testosterone or Mullerian-inhibiting factor

Mullerian ducts develop into the female internal reproductive organs

Lack of testosterone allows the development of the female external genitalia, & the degeneration of the Wolffian ducts

Chapter 40 - Human Reproduction & DevelopmentPRACTICE QUESTIONS

1. What muscle regulates the temperature of the testes?

2. What is the acrosome?

3. What tubules are the site for sperm production?

4. What are the functions of Sertoli cells & Leydig cells?

5. Name the 3 male accesory sex glands & a key function of their secretions.

6. What structures fill with blood to cause the penis to be erect?

7. What is the function of the corpus luteum?

8. Name the 3 sections of the fallopian tubes?

9. What are the 4 phases of the female menstrual cycle? What happens during each phase?

10. Describe how the male reproductive organs develop under the influence of the SRY gene.

Chapter 40 - Human Reproduction & DevelopmentStages of Human Prenatal Development

Divided into 3 stages

1. Pre-embryonic stage – 1st 2 weeks, including fertilization

A. Fertilization

B. Cleavage (multiple cell division) of the fertilized ovum = zygote

C. Implantation into the uterus wall

D. Gastrulation – the initial folding of the cells into 3 distinctive germ layerswhich marks the 1st step in cell specialization

2. Embryonic stage – 3rd week to 8th week

A. The cells of the 3 germ layers grow, specialize, & interact to form all of the body’s organs = organogenesis

B. Support structures form – placenta, umbilical cord, extra-embryonic membranes

Chapter 40 - Human Reproduction & DevelopmentStages of Human Prenatal Development

3. Fetal period – 9th week to 38th week (pre-birth)

A. Organs begin to function & coordinate to form organ systems

B. Growth is very rapid

C. Pre-natal development ends with labor & the birth of the baby

Chapter 40 - Human Reproduction & DevelopmentPre-embryonic stage

1. Fertilization – penetration of the ovum by a sperm

The ovum is surrounded by:

A. Zona pellucida – protein / sugar coating

B. Corona radiata – multiple layers of cells

The sperm must get through both layers

What sperm structure is involved in digesting these 2 protective layers?

Acrosome



Once the sperm makes contact with the cell membrane of the ovum, 2 events prevent further sperm from entering the ovum (Polyspermy – fertilization by multiple sperm –why don’t we want this to happen?)

1. Fast block to polyspermy – when the sperm contacts the ovum’s cell membrane, an electric field is generated which repels the remaining sperm

2. Slow block to polyspermy – the electric field is a transient event, thus a more permanent barrier needs to be created. The zona pellucida will literally harden & form a concrete barrier impenetrable by other sperm

Once in the ovum the sperm pro-nuclei & the ovum pro-nuclei fuse = zygote

Fertilization occurs within the fallopian tubes


2. Cleavage

A series of cell divisions in which a single zygote will become a mass of cells

During cleavage a blastocyst will form, which marks the first signs of cell specialization

A. Inner cell mass will become the embryo

B. The trophoblast will become the placenta

What happens to the placenta at birth?

3. Implantation

The blastocyst attaches to the endometrium between days 5 & 7

The trophoblast will extend projections into the endometrium, which will from the chorion which develops into the placenta

The trophoblast will also secrete human chorionic gonadotropin (hCG), a hormone necessary for the maintenance of the endometrium & the blastocyst


4. Gastrulation

The inner cell mass develops into the 3 germ layers, which are cells that are destined to become specific tissues & organs

A. Ectoderm will develop into nervous tissue, the pituitary gland, skin

B. Mesoderm will develop into muscle, bone, kidneys, internal reproductive organs

C. Endoderm will develop into the liver, pancreas, lung / digestive tract linings



Chapter 40 - Human Reproduction & DevelopmentEmbryonic stage

1. Organogenesis – the transformation of the 3 layered embryo into an individual with distinct organ.

The embryo is very susceptible to environmental factors such as chemicals & viruses

Neuralation – development of the nervous system

Begins above a support structure called the notochord which developed from mesoderm

Development begins from midline ectoderm cells located above the notochord

These cells thicken to form the neural plate & extensions called neural folds extend upward eventually fusing together to form a tube aptly called the neural tube

The cells associated with this tube eventually become the brain & spinal cord

What happens to the notochord?

Its replaced by mesoderm cells which will differentiate (change) into the vertebral column & surrounding muscle



Chapter 40 - Human Reproduction & DevelopmentEmbryonic stage

2. Support structure formation

Placenta is formed from the chorion which is embryonic tissue & the blood vessels that come from the mother’s circulatory system

Even though the blood systems of the baby & mother are separate, they lie side by side, facilitating the transfer of materials between the mother & baby






Chapter 40 - Human Reproduction & DevelopmentPRACTICE QUESTIONS

1. What are the 3 stages of human pre-natal development?

2. Define cleavage & gastrulation.

3. What 2 protective layers surround the ovum?

4. Define polyspermy

5. What is the difference between the fast block & slow block to polyspermy?

6. What will develop from the inner cell mass & trophoblast?

7. What are the 3 primary germ layers? Name one organ that will develop from each layer.

8. What is neuralation?

9. What is the notochord?

Chapter 40 - Human Reproduction & Development

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