Download Reproduction and Development

Reproduction
I.
A.
Topics
Development
B.
1.
2.
Gametogenesis
Spermatogenesis
Oogenesis
C.
Fertilization
D.Pregnancy and Birth
Male and Female Development
I.
Embryological Development
A.
Types of sex
1.
Chromosomal sex
a.
b.
c.
d.
Determined by sex chromosomes
i.
Male: XY
ii.
Female: XX
Mother always contributes an X
i.
Egg (gamete) always has an X chromosome
Chromosomal sex of the fertilizing sperm determines sex of the embryo
i.
Y results in the development of a male
ii.
X results in the development of a female
TDF-testis determining factor
i.
Present on Y chromosome
ii.
Promotes development of male
2.
3.
Gonadal sex
a.
Ovaries
b.
Testes
c.
Based on genetic makeup of the embryo
Phenotypic sex
a.
Primary
i.
Reproductive structures (internal)
b.
Secondary
i.
Genitalia
II.
Sexual Differentiation of the Reproductive System
A.
1.
Sexually indifferent stage
At 5 weeks
a.
Both male and female embryos share the same structures
Gonadal ridges
Müllerian ducts
a.
Become female ducts
i.
Oviducts and uterus
Wolfian ducts
a.
Become male ducts
i.
Epididymus, ductus deferens, seminal vesicle
2.
3.
4.
B. Formation of gonads
1.
At week 7 for males
2.
At week 8 for females
3.
Primordial germ cells
a.
Migrate from yolk sac
i.
Embryonic membrane
b.
Seed the developing gonads with stem cells that become gametes
4.
C.
1.
2.
3.
i.
Spermatogonia
ii.
Oogonia
Gonadal ridges differentiate into either testes or ovaries
a.
Depends on genetic makeup
b.
Consists of cortex and medulla
i.
Medulla becomes testis
ii.
Cortex becomes ovary
Testes determines which duct system develops
Testes produce testosterone
a.
Wolfian ducts develop
Testes produce AMH-anti Müllerian hormone
a.
Paramesonephric ducts degenerate
i.
No ovaries or oviducts develop
In the absence of testes, female form develops passively
D.
1.
2.
Differentiation of external genitalia
Male and female structures arise from the same structures
During indifferent stage, all embryos have:
a.
Genital tubercle
b.
Urethral groove
c.
Urethral folds
d.
Labioscrotal swellings
3.
During 8th week, external genitalia begin to develop
4. Male
a.
Genital tubercle enlarges and becomes the penis
b.
Urethral folds fuse along midline to form spongy urethra
i.
Tip of fold remains unfused
ii.
Urethral orifice
c.
Labioscrotal swellings fuse along midline to form scrotum
5. Female
a.
Genital tubercle becomes the clitoris
b.
Urethral groove persists
i.
Vestibule
c.
Urethral folds do not fuse
i.
Labia minora
d.
Labioscrotal folds do not fuse
i.
Labia majora
6.
Differentiation is dependent on the presence or absence of testosterone
a.
If testosterone is present, male external genitalia develop
b.
If testosterone is absent, female genitalia develop
i.
Female is the default state
III.
Puberty and Menarche
A.
1.
Pituitary events
Pituitary synthesizes and releases FSH and LH
a.
Levels are very low prior to puberty
b.
Begin to elevate between 10 and 14 years after birth
Onset of puberty is marked by an amplification of sleep related pattern of LH release
2.
B.
1.
2.
3.
4.
5.
6.
C.
1.
2.
IV.
A.
Ovarian events
Rising levels of gonadotropins stimulate the ovary to produce estradiol.
Estradiol is responsible for the development of secondary sexual characteristics
a.
Growth and development of breast
b.
Fat redistribution
c.
Bone maturation.
Follicle development occurs in waves but fails to reach ovulation stage
With successive follicular cycles, the uterine endometrium grows sufficiently so that withdrawal of
estrogen results in menstruation--menarche (first menstruation event)
No ovulation occurs
a.
Therefore, no corpus luteum
i.
Therefore no progesterone
Menarche occurs at an average age of 12
a.
Ovulation does not occur for another 6-9 months
Puberty in males
Correlated with alterations in testosterone levels
a.
During first trimester through gestation, testosterone is elevated
b.
Returns to near zero until 12
c.
Begins to rise
d.
Reaches peak at 17
e.
Stays there until about 50
f.
Decreases slightly
Anatomical changes are a result of testosterone secretion
a.
Increase in the size of internal and external genitalia
b.
Characteristic hair growth
c.
Androgen sensitive muscle growth (pect's; shoulders)
d.
Larynx enlarges and vocal cords thicken
e.
Enhanced rate of linear growth
i.
3"/yr for ~4 years
Male Reproductive System
Anatomy
1. Testes
a.
b.
c.
d.
e.
f.
g.
Lie within scrotum
Divided into 250-300 wedge-shaped compartments
i.
Lobules
Each lobule contains 1 - 4 seminiferous tubules
Lobules are connected to tubulus rectus
Tubulus rectus is connected to rete testis
Rete testis is connected to efferent ductule
Efferent ductules ae connected to epididymis
2.
Scrotum
a.
Sac of skin
b.
Hangs outside abdomen
i.
Sperm need to be at lower temperature than core body temperature
ii.
3ºC lower
c.
Base of penis
d.
Skin is more heavily pigmented than other parts of the body
C.
1.
Duct system
Sperm travel from the testes to the outside via a series of ducts
a.
Accessory ducts
i.
Epididymus
ii.
Ductus deferens
iii.
Urethra
Epididymus
a.
Head
b.
Body
c.
Tail
d.
Uncoiled length
i.
6 m (20 feet)
e.
When sperm are release from seminiferous tubule, they are nonmotile
2.
f.
3.
4.
D.
1.
As they move through epididymous, some become motile
i.
20 days to move through epididymous
g.
During ejaculation, epididymous contracts vigorously
i.
Sperm are expelled from tail portion into ductus deferens
Ductus deferens
a.
Runs from epididymus through inguinal canal into pelvic cavity
b.
Joins with seminal vesicle
i.
Gland
c.
Union of duct of seminal vesicle and ductus deferens
i.
Ejaculatory duct
d.
Ejaculatory duct passes into prostate gland
e.
Prostate empties into urethra
f.
During ejaculation, muscle in the walls of the ductus deferens contracts
i.
Creates peristaltic waves that rapidly squeeze the sperm forward
Urethra
a.
Terminal portion of male duct system
b.
Conveys both urine and semen (at different times)
c.
Runs through penis and opens at the external urethral orifice
Accessory glands
Types
a.
Paired seminal vesicles
b.
Paired bulbourethral glands
c.
Single prostate
2. Seminal vesicles (size of small finger)
a.
Secretions account for 60% of semen
b.
Yellow, viscous, alkaline fluid
i.
Contains fructose, ascorbic acid (vitamin C), prostaglandins (breaks down cervical
mucus)
c.
Joins with ductus deferens to form ejaculatory duct
i.
Empties into prostate
3.
Prostate gland (chestnut size)
a.
Secretions account for 33% of semen
b.
Milky, white acid fluid
i.
Contains citrate (nutrient)
c.
Fluid enters urethra during ejaculation
4.
Bulbourethral glands (pea size)
a.
Produce thick, clear mucus
b.
Neutralizes acidic urine that may be present in urethra
E.
Penis
1.
Copulatory organ
a.
Delivers sperm into female reproductive tract
Penis consists of:
a.
Root
b.
Shaft (body)
i.
Ends in an enlarged tip, glans penis
Skin at end, prepuce (foreskin) is loose and slide forward around glans
a.
Surgically removed
i.
Circumcision
Internal anatomy of penis
a.
Spongy urethra
b.
Three long cylindrical bodies of erectile tissue
i.
Smooth muscle and connective with vascular space interspersed
c.
Erection
i.
Vascular spaces fill with blood
ii.
Penis enlarges and rigid
iii.
Permits penetration
2.
3.
4.
F.
1.
2.
3.
4.
5.
6.
Semen
Mixture of sperm and secretions from accessory glands
Transport medium
Source of nutrients
Enhance sperm motility
Helps neutralize acidic environment in vagina
2 - 5 ml/ejaculation
V.
Anatomy of Female Reproductive Tract
A.
Ovary
1.
2.
3.
Female gonad
Produces gametes (see Oogenesis Lecture)
Produces hormones
a.
Estrogens
b.
Progesterone
B.
Female duct system
1.
Uterine tubes
a.
Fallopian tubes (oviducts)
b.
Site of fertilization
c.
Transport fertilized egg to uterus
d.
Regions
i.
Fimbriae
ii.
Infundibulum
iii.
Ampula
iv.
Isthmus
e.
Beating cilia in fimbriae carries the oocyte into the uterine tube
f.
Smooth muscle contraction and beating cilia carry the egg along tube
g.
Not continuous with ovary
i.
Permits ectopic pregnancy
h.
PID-pelvic inflammatory disease
i.
Pathogens (sexually transmitted) infect peritoneal cavity
2. Uterus (see Fertilization Lecture)
a.
Sustains fertilized ovum
b.
Characteristics
i.
Anteverted
ii.
Muscular
c.
Parts
i.
Body
ii.
Fundus
iii.
Isthmus
iv.
Cervix
v.
Cervical canal
vi.
External os
vii.
Internal os
d.
e.
f.
3. Vagina
a.
b.
c.
d.
e.
Cervical mucus
i.
Secreted by gland in cervix
iii.
Characteristics change during menstrual cycle (see Menstrual Cycle Lecture)
Uterine wall
i.
Perimetrium
ii.
Endometrium
iii.
Myometrium
Characteristics
i.
Shaped like inverted pear
ii.
Muscular organ with thick walls
iii.
Glandular lining called endometrium
iv.
Narrow end is called cervix
v.
Projects into vagina
vi.
Canal of the cervix is flattened from front to back and is somewhat larger in its middle
part
vii.
Opening into the vagina is called the external os of the uterus
viii. Lined by a mucous membrane containing numerous glands that secrete a clear, alkaline
mucus
Female organ of copulation
Wall is distensible
Layers
i.
Adventia-elastic outer layer
ii.
Muscularis-smooth muscle layer
iii.
Mucosa-inner layer
Rugae
i.
Raised ridges of mucosal layer
Lubricated by mucus released by cervix
D.
Mammary glands
1.
2.
Present in both sexes but only function in females
Biologically important only during reproduction
a.
Produce milk
Each mammary gland is contained by a rounded skin-covered breast
a.
Anterior to pectoral muscle
Areola
a.
Ring of pigmented skin
Nipple
a.
Located in the center of the areola
b.
Autonomic NS controls smooth muscle of the areola and nipple
i.
Become erect when stimulated or cold
Sebaceous glands
3.
4.
5.
6.
7.
a.
Make surface of areola bumpy
b.
Produce sebum that reduces chapping and cracking of the skin of the nipple
Internal structures of mammary gland
a.
Lobes that radiate around and open at the nipple
b.
Lobes are padded and separated by fat and connective tissue
c.
Within lobes are lobules
d.
Lobules contain glandular alveoli
i.
Produce milk when a woman is lactating
e.
Alveolar glands pass milk into lacteriferous ducts
i.
Open to the outside at the nipple
Gametogenesis
I.
A.
1.
2.
General Concepts
Processes
Spermatogenesis
a.
Produces sperm
b.
Spermatogonium differentiate into spermatazoa
Oogenesis
a.
Produce ova
b.
Oogonium differentiate into ovum
B.
1.
2.
Takes place in gonads
Male: testes
Female: ovary
II.
Spermatogenesis
A.
1.
Overview:
Spermatogenesis is the process of producing sperm with half the number of chromosomes (hapliod) as
somatic cells.
a.
The germ cells progress first from the diploid to haploid state and then change shape to become
spermatozoa.
2.
The process of spermatogenesis then allows the recombination of male and female haploid gametes at
fertilization.
This provides genetic contributions from both parents without increasing the number of chromosomes
each generation.
3.
B.
1.
2.
Location
Spermatogenesis occurs in medullary sex cords known as seminiferous tubules.
Seminiferous tubules are part of the male gonad or testes.
C.
Cells involved in spermatogenesis
1.
Sertoli cells-"nurse cells"
a.
Nurse cells provide:
i.
Support for germ cells
ii.
Environment for germ cells to develop and mature
iii.
Substances initiating meiosis or the reduction from diploid to haploid cells
iv.
Hormonal signals effecting pituitary gland control of spermatogenesis
2. Leydig cells
a.
Produce testosterone
b.
Located adjacent to seminiferous tubules.
D.
1.
2.
3.
4.
5.
E.
1.
Spermatogenesis in the Sexually Mature Male
Function of the testes is to produce the male gametes or spermatozoa
a.
This process is termed, spermatogenesis
b.
The site of spermatozoa production is the seminiferous tubules
The spermatozoa originate from precursor cells that are called spermatogonia
a.
These cells line the basement membrane of the seminiferous tubule
Spermatogenesis can be divided into three parts:
a.
Spermatocytogenesis-proliferative phase
b.
Meiosis-production of the haploid gamete
c.
Spermiogenesis
i.
Spermatids mature into spermatozoa (sperm)
The adult male mammal that is a continuous breeder.
a.
Males continue to produce spermatozoa throughout life.
Spermiogenesis
a.
Morphological conversion of round spermatid into spermatozoa without a division
Overall result of spermatogenesis
Cell proliferation
a.
More cells are produced than originally present
b.
c.
d.
e.
f.
III.
A.
1.
Each spermatogonia may produce up to 256 spermatozoa per cycle (25 x 4)
Maintenance of a reserve germ cell population
i.
Production of new spermatogonia is faster than maturation of spermatozoa
Haploid gametes are produced
Genetic variability is introduced
i.
Independent assortment during meiosis
ii.
Crossing-over during Prophase I of meiosis
Spermatids mature into spermatozoa
Oogenesis: differentiation of female primordial germ cell into an unfertilized egg
Egg Cell
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
3.
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. Organization of the ovary
1.
2.
3.
D.
1.
2.
3.
D.
1.
2.
Ovaries complex organs composed of structural, accessory and endocrine cells in addition to the germ
line cells
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
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
Events during oogenesis
Primordial germ cells migrate to the ovary during fetal and embryonic development
Differentiation of primordial germ cell into an unfertilized egg requires:
a.
Proliferation
i.
Increase in number
Growth in size
Phases
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
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.
3.
E.
1.
2.
3.
4.
5.
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
Meiotic 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
Follicle development
Oocyte matures in close association with follicle cells
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
As the oocyte reaches maturity, the follicle becomes filled with fluid to form the Graafian follicle
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
Graafian follicle continues to differentiate to form the corpus luteum
IV.
A.
1.
2.
Hormonal regulation of oogenesis
Follicle stimulating hormone (FSH)
Immature oocytes are stimulated to grow and mature by FSH
FSH is an anterior pituitary hormone
B.
1.
Estrogen
Produced by follicle
C.
1.
2.
Luteining hormone (LH)
Stimulates corpus luteum development
LH is an anterior pituitary hormone
D.
1.
Progesterone
Produced by corpus luteum
V.
A.
Physiology of the Menstrual Cycle
Overview
1.
Major glands and organs
a.
Hypothalamus
b.
Anterior pituitary
c.
Ovary
d.
Uterus
e.
Two uterine tubes
i.
Fallopian tubes
ii.
Connect ovaries to the fundus of the uterus
B.
Hormones
1.
2.
3.
4.
5.
6.
7.
8.
C.
1.
Definition-substance produced in one part of the body that affects another part of the body
a.
Endocrine-travels through blood
GnRH-gonadotropin releasing hormone
a.
Produced by hypothalamus
i.
Hypothalamus is part of the brain
b.
Travels via portal vein to anterior pituitary to control release of LH and FSH
FSH-follicle stimulating hormone
a.
Produced by anterior pituitary
b.
Controls gonadic function
LH-luteining hormone
a.
Produced by anterior pituitary
b.
Controls gonadic function
Estrogen
a.
Produced by gonads
i.
Follicle in females
ii.
Sertoli cells in males
Progesterone
a.
Produced by corpus luteum
b.
Prepares uterus for implantation and pregnancy
Testosterone
a.
Primarily a male hormone
i.
Also produced by adrenal gland
HCG-human chorionic gonadotropin
a.
Secreted by extramembryonic membranes during pregnancy
3.
Menstrual cycle
Prototypic: 28 days
a.
Range is 20-36 days
i.
Typically 26-32
b.
Only 10% of women have a 28 day cycle
Day 1: first day of menstruation
a.
Typical flow 2-7 days
i.
Duration, amount and pattern varies
Cycles varies in length at different times during female lifespan
II.
Phases of the menstrual cycle: based on ovarian events
A.
1.
Follicular phase
Hypothalamus secretes GnRH
a.
Controls activity of anterior pituitary
Anterior pituitary secretes FSH & LH
a.
Controls activity of ovary
FSH stimulates ovarian follicles to begin to develop
As follicle develops it begins to secrete estrogen
a.
Estrogen causes further follicular development
b.
Estrogen causes the endometrium to thicken
c.
Estrogen acts to signal hypothalamus to stop releasing GnRH
i.
Negative feedback-reduced production
ii.
Causes reduced FSH release
d.
Elevated estrogen levels causes LH release from anterior pituitary
i.
Positive feedback-increases production
Although there are many follicles in the ovary, one becomes dominant
2.
2.
3.
4.
5.
a.
b.
B.
1.
2.
3.
4.
5.
6.
7.
8.
9.
C.
1.
2.
3.
4.
5.
6.
Matures while others do not
Fraternal twins, two become dominant
Ovulation
Events:
a.
Egg is released from follicle
b.
Follicle differentiates into corpus luteum
Estrogen levels peak 1-1.5 days prior to ovulation
Elevated estrogen causes a surge in LH release from anterior pituitary
Ovulation is a response to LH surge
Follicle differentiates into corpus luteum
Corpus luteum produces progesterone
a.
Progesterone causes reduced LH levels
i.
Negative feedback-reduced production
b.
Progesterone causes reduced GnRH levels
i.
Negative feedback-reduced production
As follicle differentiates it produces testosterone
a.
As in males, testosterone increases libido
Rupture of the follicle
a.
Releases egg into fimbria
Egg moved by cilia and motility of fallopian tube
a.
Egg is viable for approximately 36 hours
b.
Sperm is viable for approximately 3 to 5 days
c.
Window for pregnancy can be as large as 7 days
i.
5 days prior to ovulation
ii.
1-1.5 days after ovulation
Luteal phase
Last 14 days of the menstrual cycle
a.
Corresponds to the life of the corpus luteum
Corpus luteum-"Yellow body"
a.
Derived from the follicle
Corpus luteum produces
a.
Progesterone
b.
Estrogen (estradiol)
c.
Testosterone
Progesterone
a.
Suppresses new follicle growth
i.
Prevents ovulation of other follicles
b.
Maturation of glandular and blood supply to endometrium of uterus
If no pregnancy:
a.
Corpus luteum degenerates into corpus albicans ("White body")
b.
Estrogen and progesterone levels fall
i.
Causes endometrial lining to degenerate-menstruation
If pregnant:
a.
Developing embryo produces HCG
i.
Maintains corpus luteum
ii.
Progesterone maintains uterus
III.
Phases of the menstrual cycle: based on uterine/cervical events
A.
1.
2.
Proliferative phase
Corresponds to follicular phase in ovary
Uterine events
a.
Endometrium thickens
b.
Glands and blood vessels grow in response to estrogen
Cervical events
a.
Estrogen causes changes to cervical mucus
i.
Thinner
ii.
Clearer
iii.
More elastic (spinnbarkeit)
iv.
Changes facilitate passage of sperm into uterus
v.
Cervix may feel softer and more open
3.
B.
Ovulation-egg released from follicle
C.
1.
2.
Secretory phase
Corresponds to luteal phase in ovary
Uterine events
a.
Endometrial glands and blood supply mature
i.
Prepared to secrete glycogen-nutrition for egg
b.
Progesterone maintains endometrium
i.
Increasing levels of progesterone cause a reduced quantity of thick and tacky cervical
mucus
Menstruation
Occurs if fertilization does not occur
Progesterone and estrogen levels fall
D.
1.
2.
3.
Uterine events
a.
Endometrial blood vessels constrict
i.
Loss of blood supply causes endometrium to be shed
b.
Menstrual flow is a combination of endometrial tissue and blood
c.
Cramps are associated with lack of progesterone and presence of prostaglandins secreted in
uterus
Fertilization
I.
A.
1.
2.
General Background
Gametes viability
Oocytes
a.
Only one oocyte is normally released during the menstrual cycle
b.
Following ovulation, the oocyte starts undergoing an ageing process
c.
This regressive degeneration leads to egg infertility
i.
Egg is fertile for about 24 hours
Spermatozoa
a.
Each ejaculation releases 200 - 300 millions of spermatozoa
b.
A very low percentage of the spermatozoa will reach the site of fertilization
c.
The cervix and uterofallopian junction is a hostile environment
d.
Female immune system destroys most of the spermatozoa
i.
Neutrophil granulocytes and macrophages present in the uterine lumen
e.
These barriers also select the strongest spermatozoa
f.
Spermatozoa remain fertile for a maximum of two days in the female reproductive system
g.
For fertilization to take place, spermatozoa need to undergo two functional modifications:
i.
Spermatazoa undergo maturation in the epididymis
ii.
Spermatazoa undergo capacitation in the female reproductive system
h.
Changes enable the acrosomal reaction to occur
II.
General Overview of Events
A.
B.
C.
D.
E.
F.
Activation of sperm
Finding the egg
Sperm penetration
Prevention of polyspermy
Fusion of egg and sperm
Initiation of development
III.
Overview of preparation of the sperm for fertilization
A.
1.
2.
3.
Limiting factors
Because of their size, sperm have limited resources
Relative to their size, sperm have to travel a great distance to the site of fertilization
Motility is very energy consuming
B.
1.
Timing of motility is precise
Does not occur in testes or semen
C.
1.
Extra-cellular layers of the ovum form barrier to fertilization
Sperm must pass through these layers
IV.
Activation of sperm motility
A.
1.
In mammals little is known about the initial activation of sperm motility
Sperm maturation in the epididymis facilitates the acquisition of motility
a.
Motility is triggered by the ejaculation of semen into the female reproductive tract
Sperm gradually become more motile after they enter the female reproductive tract
Activation may be gradual because contractions of the female reproductive tract help conduct sperm to
the site of fertilization in the oviduct
2.
3.
4.
When sperm reach the egg there is an additional increase in motility
a.
This is caused by capacitation (see below)
V.
Attraction of sperm to the egg
A.
1.
2.
Activation of motility does not ensure that sperm will find an egg
Female reproductive tract is a big place
Chemotaxis
a.
Chemical signals released by ovum that attracts sperm to the egg
VI.
A.
1.
Sperm penetration of the egg
Capacitation
Sperm require a period of maturation in the female reproductive tract before they are capable of
fertilizing the egg
Capacitation takes 5 to 6 hours in the human
Capacitation takes place in the uterus and oviduct
Capacitation alters the sperm plasma membrane
Capacitated sperm show greater respiratory activity and motility than noncapacitated sperm
a.
Assists their penetration through the egg surface coats
2.
3.
4.
5.
B. Acrosome reaction
1.
Acrosome reaction of mammals is initiated as sperm proceed through the corona radiata and approach
the zona pellucida
2.
Acrosomal membrane fuses with the plasma membrane of the sperm
a.
Enzymes stored in the acrosome escape into the region surrounding the sperm
i.
Corona-penetrating enzyme
ii.
Enzymes loosen the connections between cumulus oophorus or corona radiata cells
iii.
Help the sperm to reach the zona pellucida
3.
Mechanism used by sperm to penetrate the zona pellucida is not fully understood
a.
Some investigators think that sperm penetrate this layer solely by the propulsive force of their
swimming
b.
Others believe that sperm digest their way through the zona pellucida using acrosin, a protease
(enzyme)
C.
Zona pellucida restricts inter-species fertilization
VII.
Fusion of sperm and egg
A.
1.
Zonal reaction
Eggs are surrounded by the zona pellucida
a.
After sperm penetration, zona pellucida undergoes a structural change known as the zonal
reaction
Sperm receptors present on the vitelline envelope
a.
Outer most layer of egg
b.
Vitelline envelope is called the fertilization envelope
Fertilization cone
a.
Extension of egg cytoplasm
b.
Engulfs the sperm
2.
3.
B.
1.
2.
Synkaryon
Sperm and egg nuclei combine to form the diploid zygotic nucleus
Late response to fertilization
a.
Completed 20 minutes after sperm attachment
C.
If fertilization does not occur, the ovum disintegrates and is destroyed by the fallopian tube
VIII.
Reaction of egg to fertilization
A.
1.
Activation program
Two responses
a.
Early reaction
i.
Prevents polyspermy
b.
Late responses
i.
Lead to formation of embryo
B.
1.
Prevention of polyspermy
Only one sperm can fertilize the egg
a.
Prevents polyploidy
i.
More than two sets of chromosomes
2. Fusion of a single sperm with the egg is a critical event in development
a.
Results in an equal genetic contribution from each parent
b.
Restores diploid chromosome number
3. Polyploidy will ultimately cause the death of the embryo
C.
1.
2.
Strategies for preventing polyspermy
Limited the number of sperm that reach the site of fertilization
a.
Sperm must migrate through female reproductive tract
Limit the number of sperm that can penetrate the extracellular layers of the egg
a.
Mediated by structural changes to the zona pellucida
D.
1.
Cortical reaction
It consists of the exocytosis of the contents of the cortical granules
a.
Contain proteolytic enzymes
2.
Begins at the point of sperm-egg fusion and propagated around the surface of the egg
a.
Takes about 1 minute
3.
Vitelline envelope is elevated above the egg surface
a.
Vitelline envelope is called the fertilization envelope
b.
Elevation of vitelline envelope increases the distance between egg plasma membrane and
vitelline membrane
i.
Increases distance the supernumerary sperm have to travel
4.
Cortical reaction makes the vitelline envelope hard
a.
Resistant to digestion by sperm proteases
5. Sperm receptors present on the vitelline envelope are destroyed
IX.
Initiation of development
A.
B.
C.
1.
Prior to fertilization the egg is arrested in metaphase of the second meiotic division
Division process is reactivated by the penetration of the sperm
Second meiotic division is completed
Second polar body is emitted
a.
Egg is now called a zygote
b.
Egg contains contain two haploid sets of chromosomes
i.
Paternal from sperm
ii.
Maternal from egg
D.
1.
2.
3.
Development following fertilization
Fertilized ovum remains in the fallopian tube for about 72 hours
a.
Several cellular divisions
b.
Size of the fertilized ovum does not increase
Developing zygote migrates through fallopian tube
a.
Propelled by smooth muscle contraction and beating of cilia
84 - 96 hours after ovulation
a.
Cells of zygote divide mitotically to form morula
b.
Morula enters the uterus and rapidly transforms itself in a liquid filled structure called the
blastocyst
c.
After 2-3 days, the blastocyst will anchor to the uterine wall
i.
Implantation
Pregnancy and Childbirth
I.
A.
B.
1.
Terms
Pregnancy: events that occur from the time of fertilization until infant is born
Conceptus: developing offspring
Preembryo
a.
Two weeks following fertilization
2.
Embryo
a.
3rd through 8th weeks of development
3.
Fetus
a.
9th week through birth
C.
Gestation period: time during which development occurs
1.
Length
a.
280 days
2.
Onset
a.
End of the previous menstrual
i.
Pregnancy begins two weeks prior to fertilization
ii.
Includes the follicular phase of the menstrual cycle
3. Embryonic period
4. Fetal period
II.
Preembryonic Development
A.
B.
1.
2.
3.
C.
1.
Fertilization to implantation
Events
Cleavage
Blastocyst production
Implantation of blastocyst
Cleavage and blastocyst production
Cleavage
a.
Period of rapid mitotic divisions
b.
Little growth
i.
Daughter cells become smaller
Blastomeres
a.
Identical daughter cells produced during the first 72 hours
Morula
a.
16+ cell stage
Blastocyst
a.
Fluid-filled hollow sphere of a single layer of cells
b.
100 cells+
c.
Preembryo free from zona pellucida
d.
Trophoblast
i.
Placenta formation
e.
Inner cell mass
i.
Becomes embryonic disc
ii.
Forms the embryo
2.
3.
4.
D.
Implantation
1.
2.
3.
Blastocyst floats freely in uterus for 2 - 3 days
About six days after fertilization implantation begins
Endometrium must be ready
a.
Window of implantation
i.
Ovarian hormones must be at appropriate levels
ii.
Estrogen and progesterone
If endometrium is prepared, blastocyst implants high in the uterus
If endometrium is not ready, blastocyst continues to move lower in the uterus
a.
Implants where chemical environment is appropriate
Outer layer (trophoblast) adheres to endometrium
Trophoblast invades the endometrium
a.
Digests the uterine cells
b.
Blastocyst burrows into endometrium
i.
Surrounded by blood leaked from degraded endometrial blood vessels
c.
Blastocyst is covered over and sealed off from uterine cavity
Implantation takes about a week
a.
Completed by 14th day after ovulation
i.
Corresponds to timing of menstruation
Implanted preembyo secretes a hormone to maintain corpus luteum
a.
Human chorionic gonadotropin (hCG)
i.
Causes corpus luteum to continue secreting progesterone
b.
Conceptus takes over hormonal control of the uterus
i.
High for the first two months of pregnancy
c. Pregnancy tests are used to detect hCG
Placenta will ultimately produce progesterone and estrogen to support uterus
a.
Not until 3 -4 months
Nutrients for developing embryo initially comes from digested endometrial cells
4.
5.
6.
7.
8.
9.
10.
11.
a. By the second month, placenta provides nutrients
E. Placenta
1. Temporary organ
a.
Made from both maternal and fetal sources
2.
Embryonic tissue forms the inner part of the placenta
a.
Chorion
i.
Formed from trophoblastic tissue
ii.
Fetal blood connects to maternal sources via chorion
iii.
Do not intermix
iv.
Materials diffuse between the two sources
3.
Part of the endometrium becomes the deciduas basalis
4.
Function of the placenta
a.
Nutrition
b.
Respiration
c.
d.
Excretion
Endocrine organ
5. Placental hormones are necessary for pregnancy
a.
hCG maintains corpus luteum
i.
Only temporarily
ii.
2-3 months
b.
Placenta takes over much of the role of the ovaries after three months
i.
Provides peptide and steroid hormones for maintenance of pregnancy
c.
Hormones provided by placenta
i.
hCG (functionally similar to LH)
ii.
hCS (human chorionic somatomammotrophin
iii.
Progesterone
iv.
Estrogen
6. Function of hormones
a.
hCS
i.
Increases through pregnancy
ii.
Necessary for the development of the breasts
b.
Progesterone
i.
Maintains the endometrium
ii.
Levels reach a peak after about 3-4 weeks after fertilization
iii.
Decline before increasing again up to the time of parturition
iv.
Second increase is important for the development of the breasts
c.
Estrogen
i.
Necessary for the uterus to develop to accommodate the growing fetus
ii.
Important for the latter stages of breast development
d.
hPL (human placental lactogen) stimulates the growth of fetus and causes a reduction in glucose
use by maternal cells
i.
Maternal cells use more fatty acids and amino acids
ii.
Glucose is spared for fetal use
e.
hCT-human chorionic thyrotropin increases rate of maternal metabolism
i.
Hypermetabolism
7.
ii.
Appetite changes can be attributed to hCT
Placenta and mother's body operate in concert
a.
Placenta is an interface between mother and baby
b.
It consists of both maternal and embryonic tissue
c.
From third month to parturition it is the intermediary between mother and developing baby
F.
1.
Umbilical cord
Three blood vessels
a.
2 umbilical arteries
i.
Bring fetal blood to the placenta
b.
1 umbilical vein
i.
Returns blood to baby's heart
III.
A.
1.
Embryonic Development
Blastocyst to gastrula
Fertilization
a.
One cell
Preembryo
a.
Morula
i.
Multiple cells
ii.
Inside zona pellucida
b.
Blastocyst
i.
Fluid-filled sphere of cells
ii.
Free from zona pellucida
2.
B. Embryonic membranes
1.
2.
Types
a.
Amnion
b.
Yolk sac
c.
Allantois
d.
Chorion
Functions
a.
Amnion
i.
Transparent membranous sac
ii.
Becomes fluid-filled
b.
c.
d.
C.
1.
2.
3.
4.
iii.
Extends around the embryo
iv.
Protects against physical trauma
v.
Prevents embryo parts from adhering to together
vi.
Allows musculoskeletal development
vii.
Initially derived from maternal blood
viii.
Fetal urine contributes to amnionic fluid
Yolk sac
i.
Sac hanging from embryo
ii.
In other animals, yolk is the main source of nutrition
iii.
In humans, placenta plays nutritive function
iv.
Forms part of digestive tract
v.
Produces earliest blood cells
vi.
Source of primordial germ cells for gonads
Allantois
i.
Becomes the umbilical cord that links embryo with placenta
ii.
Becomes part of urinary bladder
Chorion
i.
Helps form chorion
ii.
Encloses all other membranes and embryo
Gastrulation
Two layered embryo to a three layered embryo
Primary germ layers
a.
Ectoderm
b.
Mesoderm
c.
Endoderm
Function of gastrulation
a.
Lays down basic structural framework of the embryo
b.
Permits the development of tissues and organs
Fate of the primary germ layers
a.
Ectoderm
i.
Nervous system
ii.
Skin epidermis
b.
Endoderm ("inner skin")
i.
Epithelial linings of GI, respiratory and urogenital tracts
c.
Mesoderm
i.
Everything else
ii.
Muscle
iii. Blood
iv. Bone
IV.
Development of Fetal Circulation
A.
Cardiovascular system
1.
2.
Begins to form during 3rd week following conception
Heart beat by week 3.5
B.
Fetal modifications
1.
2.
V.
Umbilical arteries and veins
a.
Umbilical vein carries blood with oxygen and nutrients from placenta to embryo
i.
Veins normally have low oxygen content
b.
Umbilical arteries return waste-laden blood from fetus to placenta
c.
Mother delivers oxygen and nutrient-rich blood to placenta
Vascular shunts
a.
No need for blood to go through liver
i.
Ductus venosus
ii.
Mixes oxygen-rich blood with oxygen-poor blood
b.
No oxygen from fetal lungs
c.
Heart (overview blood flow)
i.
Get oxygen-rich blood coming from placenta into systemic circulation
ii.
Foramen ovale-shunt between right and left atria
iii.
Ductus arteriosus-shunt blood from pulmonary trunk to aorta (bypassing pulmonary
circuit)
Fetal Development
A.Weeks 9 through 40
1.
Growth and differentiation
B.
Dimensional changes
1.
Start of fetal development
a.
Crown-to-rump length
i.
30 mm (1 inch)
b.
Weight
i.
1 g (0.03 oz)
2.
At parturition
a.
Crown-to-rump length
i.
360 mm (14 inch)
b.
Weight
i.
2.7 - 4.1 kg (6 - 10 lb)
c.
Length
i.
550 mm (22 in)
C.
1.
2.
Developmental events of the fetal period
8 weeks-end of embryonic period
a.
Head equal to body length
b.
Large liver
c.
Limbs present
i.
Webbed digits
d.
Bones ossify
e.
Cardiovascular system is functional
f.
All body systems are present
9 - 12 weeks-3rd month
a.
Body elongates
b.
Head is still disproportionately large
i.
Brain continues to enlarge
c.
Skin epidermis is formed
i.
Crude facial features
d.
Sex can be determined
e.
Limbs well formed
f.
Crown-to-rump length
3.
4.
5.
6.
i.
90 mm (3.5 inch)
13 - 16 weeks-4th month
a.
Nervous system continues to develop
i.
Sensory organs are differentiated
ii.
Eyes blink
iii.
Sucking motions of lips
b.
Face looks human
i.
Body beginning to outgrow head
c.
Crown-to-rump length
i.
140 mm (6 inch)
17 - 20 weeks-5th month
a.
Vernix caseosa covers body
i.
Fatty secretions of sebaceous glands
b.
Lanugo covers skin
i.
Silk-like hair
c.
Body is in the fetal position
i.
Body too bid for space restrictions of uterus
d.
Limbs reach final proportions
e.
Quickening occurs
i.
Mother feels spontaneous muscular activity of fetus
f.
Crown-to-rump length
i.
190 mm (8 inch)
21 - 30 weeks-6th and 7th months
a.
Substantial weight increase
b.
Survival is possible after week 27
i.
Requires temperature regulation
ii.
Requires ventilation
c.
Eyes are open
d.
Skin is red and wrinkled
e.
Toenails and fingernails are present
f.
Lean, well-proportioned body
g.
Limb bones begin to ossify
h.
Crown-to-rump length
i.
280 mm (12 inch)
30 - 40 weeks-8th and 9th months ("Term")
a.
Skin is whitish pink
b.
Fat in subcutaneous tissue
c.
Crown-to-rump length
i.
360 - 400 mm (14 - 16 inch)
VI.
Parturition-Birth
A.
1.
Timing
Within 15 days of calculated due date
a.
280 days following last menstrual cycle
B.
1.
Labor
Series of events that result in the expulsion of the fetus from the uterus
C.
Initiation of labor
1.
2.
Not clearly understood
Associated with high levels of estrogen (why estrogen increases is not known)
a.
Estrogen stimulates myometrial cells of the uterus to form oxytocin receptors
b.
Estrogen offsets the effects of progesterone
Resultant effect is for myometrium to become increasingly irritable with intermittent and irregular, weak
contractions
a.
Braxtin Hicks contractions
i.
False labor
Oxytocin is released by fetus
a.
Source not presently known (i.e., which fetal cells)
Oxytocin acts on placenta which then releases prostaglandins
Oxytocin and prostaglandins are uterine muscle stimulants
Physical and emotional stress activate hypothalamus
a.
Posterior pituitary releases oxytocin in response to hypothalamic activation
Oxytocin release is controlled by positive feedback
a.
Increasing muscle contraction in response to oxytocin causes the release of more oxytocin
3.
4.
5.
6.
7.
8.
VI.
A.
1.
2.
B.
1.
2.
3.
Stages of Labor
Stage 1: Dilation stage
Labor onset until full dilation of the cervix
a.
Longest phase
b.
6 - 12 hours
Events
a.
Upper part of uterus begins to contract
i.
15 - 30 minutes apart
ii.
Duration: 10 - 30 seconds
b.
Contractions become more vigorous and rapid
c.
Lower uterine segments become involved
d.
Baby's head is forced against cervix with each contraction
i.
Cervix softens
ii.
Cervix becomes thinner (effaces)
iii.
Cervix dilates (diameter of opening increases)
e.
Amnion ruptures-breaking the water
i.
Amnionic fluid is released
f.
Engagement
i.
Infant's head enters true pelvis
ii.
Head is aligned so its greatest dimension is along anterior-posterior line
Stage 2: Expulsion stage
From full dilation until the infant is delivered
Contractions
a.
Every 2 - 3 minutes
b.
1 minute apart
Without anesthesia the mother has an urge to bear down (push)
4.
Length of the phase
a.
First birth
i.
50 min
b.
Second birth
i.
20 min
5. Crowning
a.
Largest diameter of the baby's head is distending the vulva
b.
Episiotomy-incision made to widen vaginal orifice
i.
Incision between vaginal opening and anus
C.
1.
2.
3.
Stage 3: Placental stage
Delivery of the placenta
a.
Within 30 minutes of the birth
Uterus continues to contract
a.
Compress blood vessels
i.
Limits bleeding
b.
Causes placenta to be detached from uterine wall
Afterbirth
a.
Placenta and fetal membranes
VII.
Lactation
A.
1.
Production of milk by mammary glands
Hormonal changes at end of pregnancy stimulate the hypothalamus
a.
Placental estrogen, progesterone and lactogen increase
b.
Hypothalamus releases PRH-prolactin releasing hormone
c.
PRH acts on anterior pituitary
i.
Prolactin is secreted
During first few days following birth
a.
Colostrum is secreted from mammary glands
i.
Not true milk
ii.
Less lactose (sugar)
iii.
No fat
iv.
More protein, vitamin A and minerals
v.
Rich in antibodies
Prolactin release gradually decreases without mechanical stimulation of the nipples
a.
Nervous connection from nipple to hypothalamus
i.
Stimulate PRH release
b.
Prolactin is released
i.
Stimulates milk production for next feeding
Same nervous impulse also activates oxytocin release from posterior pituitary
a.
Let-down reflex
i.
Ejection of milk from the alveoli of the mammary gland (from both breasts)
Benefits of breast finding
a.
Oxytocin also acts on smooth muscle of the uterus
i.
Causes uterine contraction and return to pre-pregnancy proportions
b.
Nutrients of maternal milk are more readily absorbed and metabolized than cow's milk
c.
Beneficial chemicals uniquely present in maternal milk
i.
Antibodies
2.
3.
4.
5.
d.
ii.
Other immune products
Acts as a natural laxative
i.
Cleanses meconium from bowels