Chapter 22 : Reproductive
Systems
Reproduction is the process by which: new
individuals of a species are produced and the genetic material is passed from
generation to generation.Human beings reproduce sexually because they produce
specialized reproductive cells called gametes : ova (ovum) in the female and
sperm (spermatozoa) in the male.
The fusion of the egg and sperm produce a cell called a
zygote, which eventually becomes a new individual. The advantage of sexual
reproduction is the mixing of the genetic material from two parents, which gives
greater variability and therefore adaptability, to the species.
Similarities in males and females:
Each reproductive system has
the same basic parts:
primary sex organs or gonads: Ova in females ; Sperm in males
accessory sex organs - internal and external
ducts (transport and store gametes)
accessory sex glands (produce materials that support gametes)
supporting structures (have various roles in reproduction).
The internal genital tracts of males and females develop from
separate structures. Every embryo produces two sets of ducts, Wolffian and
Mullerian ducts. In the male, the Wolffian ducts become the vas deferens and
associated structures. In the female, the Mullerian ducts become the oviducts,
uterus and vagina. In each sex, the set of ducts which is not used degenerates.
External genitals come from the same embryonic structures in
males and females.
The male:
Male gonads are the testes or
testicles, which are located in the
scrotum.
The scrotum is a sac that hangs from the root of the
penis and consists of loose skin and connective tissue. It supports the testes
and regulates the temperature of the testes by contraction of muscles (Cremaster
and dartos).The dartos muscle is found in the septum between the testes and
under the skin of the scrotum. Contraction causes the skin to wrinkle.
The cremaster muscle is an extension of the internal oblique
muscle and raises the testes during sexual arousal and when the temperature
becomes too cold, and brings them closer to the pelvic cavity. Relaxation causes
the testes to move farther from the pelvic cavity. The production and survival
of sperm depends on keeping them 3o C less than body temperature.
The testes descend into the
scrotum a month or two before birth. They
pass through the abdominal wall at the inguinal canal, guided
by a fibromuscular cord called the gubernaculum. It
is fastened to the testes and the skin of the scrotum. It
brings the vas deferens, blood vessels and nerves that become the spermatic
cord.
If fail to
descend:cryptorchidism = infertility. This
is corrected by injections of testosterone or surgery
Structure of the testes:
The testes are surrounded by a serous membrane called the tunica
vaginalis which is derived from the peritoneum, which ensures
that the testes can move freely in the scrotum. They are surrounded
by tough, white, fibrous capsule - tunica albuginea.
This layer penetrates into the testes and divides it into lobules. Each lobule
contains one to four seminiferous tubules, in which sperm cells
are made.
Sperm formation- spermatogenesis
(see also chapter 24)
Gametes differ from other
cells in the body because they only have half the usual number of chromosomes (haploid).
In humans, this would be 23 chromosomes. The body or somatic cells contain the diploid
number, or 46. To get the haploid number of chromosomes the cells have to
undergo a special type of division called meiosis (as opposed
to mitosis).
In humans, spermatogenesis takes about 65-70 days. It begins
with the spermatogonia, which contain the diploid (2n) number of chromosomes.
Spermatogonia are stem cells (undifferentiated cells) because when they
undergo division (mitosis), some of the daughter cells remain near the basement
membrane in an undifferentiated state to serve as a reservoir for future sperm
production. The rest of the cells lose contact with the basement membrane and
become primary spermatocytes. These are also 2n. Sperm
production stops at this stage until the onset of puberty, when, under the
influence of testosterone, it resumes.
In the first phase of meiosis (reduction division) the
primary spermatocyte enlarges before dividing. Then two nuclear divisions take
place. In the first, the DNA replicates and 46 chromosomes containing two
identical chromatids line up at the center of the cell. They line up as
homologous pairs, a process called synapsis, and at this point,
portions of a chromatid from the maternal homolog may be exchanged with the
corresponding part of the paternal homolog. This is called crossing
-over, and is form of recombination, a shuffling of
genetic material to increase variation. Next, the one member of each pair is
pulled to opposite ends of the cell. The cells thus formed are called secondary
spermatocytes.
In the second stage of meiosis or equatorial
division, there is no replication of DNA. The chromosomes, each composed of two
chromatids, line up on the equatorial plane, and the chromatids of each
chromosome separate from each other.
The cells that are formed are now called spermatids and
contain the haploid number of chromosomes.
Through all these divisions, the cells have been moving from
the basement membrane toward the lumen of the seminiferous tubule. During this
time they have also maintained contact with one another. In the final stage of
spermatogenesis, called spermiogenesis, the spermatids mature
into sperm. It develops a head with an acrosome and a flagellum or tail. See
fig. 22.6 in book
The head contains the DNA.
The acrosome contains enzymes which enable it to breakdown the layers
surrounding the oocyte and allow penetration and fertilization.
The midpiece contains many mitochondria to provide energy for swimming.
The tail propels the sperm.
About 300 million sperm mature each day from the onset of
puberty.
In addition to the sperm producing cells, there are other
types of cells in the testes. In addition to the cells that produce sperm,
in the semiferous tubules there are also supporting cells, called sustentacular
or Sertoli's cells. These are tall, columnar cells that extend the entire
thickness of the epithelium, from the base to the lumen of the seminiferous
tubule. They support, nourish and regulate the spermatogenic cells.
Just internal to the basement membrane are tight junctions
which form the blood-testis barrier . This barrier is important
because the spermatogenic cells have surface antigens which are recognized by
the immune system as foreign. This barrier prevents the blood cells from coming
in contact with the spermatogenic cells. These cells also phagocytize excess
spermatid cytoplasm during development, and produce fluid for sperm transport.
In between the seminiferous tubules are interstitial cells,
which produce testosterone.
The duct system of the testes includes the seminiferous
tubules, straight tubules, and rete testis, a network of channels. Sperm are
transported out of the testes through the efferent ducts into a single tube
called the ductus epididymis. The epididymis is located on the posterior surface
of the testes and has a head, a body and a tail. The ductus epididymis, which is
about 20 feet long, is the site of sperm maturation and storage. Over a 10-14
day period the motility of the sperm increases. The epididymus also helps propel
sperm through peristaltic contractions. Sperm may remain in storage for a month
or more. The ductus epididymis has psuedostratified columnar cells with long
branching microvilli (non-motile cilia) called stereocilia
which increase the surface area for the reabsorption of degenerated sperm. These
cells secrete glycogen and other substances that support the sperm and promote
maturation.
The ductus epididymis enlarges in the tail and becomes less
convoluted. At this point it is called the vas deferens. It passes into the body
and loops around the ureter and down the back of the urinary bladder. The
vas deferens stores sperm for up to several months and propels them
toward the urethra during ejaculation. Cutting of the vas deferens to achieve
sterility by blocking entry of sperm into semen is called vasectomy.
Along side the vas deferens as it ascends in the scrotum are the testicular
artery, autonomic nerves, veins, lymph vessels and the cremaster muscle; all
these structures together make up the spermatic cord. The
spermatic cord passes through the inguinal canal in the abdominal wall.
Each ejaculatory duct is formed by the union of the duct from
the seminal vesicle and vas deferens. It is the passageway for ejection of sperm
and secretions of the seminal vesicles into the first portion of the urethra,
the prostatic urethra. The male urethra is subdivided regionally into three
portions: prostatic, membranous, and spongy (penile).
Accessory Sex Glands or Male Internal Accessory Organs
The seminal vesicles secrete an alkaline,
viscous fluid that constitutes about 60 percent of the volume of semen and
contains fructose as an energy source for the sperm. Prostaglandins
contribute to sperm motility and viability, and cause muscular contractions of
the female reproductive organs - helping sperm to reach the ovum. The fluid
produced by the seminal versicles also contains semenogelin , a
coagulation protein unlike any in the blood, which causes the coagulation of
semen after ejaculation.
The prostate gland surrounds the urethra
just inferior to the urinary bladder. It secretes a milky, slightly alkaline
fluid that constitutes about 25 percent of the volume of semen and contributes
to sperm motility and viability. It also contains citrate as an
energy source for the sperm and several proteolytic enzymes which liquify
coagulated semen .
The two bulbourethral (Cowper's) glands lie
on either side of the membranous urethra and have ducts which empty into the
spongy urethra. They secrete mucus in response to sexual stimulation, and
lubricates the penis for intercourse. An alkaline substance neutralizes acid in
the urethra.
Semen is a mixture of sperm and accessory sex gland
secretions. The average volume of semen in an ejaculation is about 2 to 6 ml,
and contains about 50-150 million sperm per milliliter(ave. 120 million). When
the sperm count falls below 20 million per ml, the male in likely to be
infertile. Only a small fraction of sperm ever reach the oocyte, and it takes
the combined efforts of many sperm to penetrate the oocyte. Semen provides the
fluid in which sperm are transported, provides nutrients, and neutralizes the
acidity of the male urethra and female vagina. Once ejaculated, the semen
coagulates after about 5 minutes. After about 10-20 minutes it reliquifies. What
the function of the coagulation is, is unknown.
Sperm are nonmotile while in the testes and epididymis, but
become activated by the secretions if the accessory glands. Sperm cannot
fertilize an egg until they enter the female reproductive tract. Substances in
the female tract weaken the membranes of the acrosome, making the enzymes
available to break down the coverings of the ovum.
Sperm can be stored for many weeks inside the body, but
usually survive only a day or two after they leave the body.
The penis conveys urine and semen to the outside of
the body. It consists of a root, body, and glans penis. The body is composed of
three cylindrical masses of tissue each surrounded by a fibrous tissue called
the tunica albuginea. The paired dorsolateral masses are called the corpora
cavernosa and the smaller midventral mass is called the corpus
spongiosum . The corpus spongiosum contains the spongy urethra, and
functions in keeping the spongy urethra open during ejaculation.
The corpus spongiosum enlarges at its distal end to form the
glans penis. The glans contains the external urethral orifice, and many sensory
receptors. Covering the glans penis is the loosely fitting prepuce or foreskin,
which is removed in circumcision.
All three corpora consist of erectile tissue permeated by
blood sinuses. With sexual stimulation, the arteries supplying the penis dilate,
and large quantities of blood enter blood sinuses. When these spaces expand,
they compress the veins draining the penis, so the blood is trapped. These
vascular changes result in and erection, which is a
parasympathetic reflex caused by the release of nitric oxide. The penis
again becomes flaccid when the arteries constrict.
Emission, the movement of sperm cells from the test and
secretions from the prostate and seminal vesicles into the urethra is a sympathetic
reflex. As part of this reflex the sphincter at the base of the urinary bladder
closes, so that urine is not expelled during ejaculation, and semen does not
enter the urinary bladder. Ejaculation, the expulsion of the semen from the
body, is triggered by the presence of semen in the urethra through spinal nerves
in the sacral portion of the spinal cord. Immediately after ejaculation, the sympathetic
impulses constrict the arteries to the erectile tissue, reducing blood flow,
allowing the veins to remove the excess blood.
The Hypothalamus produces GnRH, stimulating:
LH (ICSH) stimulates the interstitial (Leydig) cells to
produce testosterone. This hormone may be converted in some
cells, such as in the prostate and seminal versicles, to a more potent androgen
called dihydrotestosterone (DHT).
FSH acts indirectly to stimulate spermatogenesis. FSH
and testosterone act together to stimulate the sustentacular or Sertoli
cells to produce androgen-binding protein (ABP)
which binds to the testosterone and keeps the levels of testosterone in
the seminiferous tubules high. Sustentacular cells are large cells in the
seminiferous tubules that extend from the basement membrane to the lumen. These
cells, in the presence of FSH and testosterone, stimulate the spermatogenic
cells to make sperm. The sustentacular cells also secrete the hormone inhibin,
which inhibits the anterior pituitary gland's production of FSH, preventing
oversecretion.
Testosterone stimulates the male pattern of development
before birth and stimulates the descent of the testes. At puberty, testosterone
and DHT bring about the development of the male sex organs and secondary sexual
characteristics, such as muscular and skeletal growth, and adult male hair
patterns, (depending on heredity,) and enlargement of the larynx which deepens
the voice. These androgens also contribute to male sexual behavior and libido.
Testosterone is an anabolic steroid hormone, which causes increased bone and
muscle mass in males.
FEMALE REPRODUCTIVE SYSTEM
The female organs of reproduction include the ovaries
(gonads which produce oocytes and hormones), uterine (Fallopian) tubes or
oviducts, uterus, vagina, and vulva. The mammary glands are also considered part
of the reproductive system.
The ovaries are female gonads located in the upper pelvic
cavity, on either side of the uterus and are held in place by a series of
ligaments. They have the same embryonic origin as the testes, and like the male
testes, descend from their original location near the kidneys to an area just
below the pelvic brim.
Each ovary has a hilus where blood vessels and nerves enter
the ovary. The ovary has several layers:
The germinal epithelium, which is a misnomer, is a layer of
simple squamous cells.
The tunica albuginea is a white capsule of dense connective
tissue.
The stroma, which is made of connective tissue, can be divided
into a cortex and medulla. The medulla is
mostly loose connective tissue that contains many blood and lymphatic vessels,
and nerve fibers.
The ovarian follicles are found in the
cortex and consist of oocytes at various stages of development and their
surrounding cells. When they are in a single layer, the surrounding cells are
called follicular cells, and later in development when they
form several layers they are called granulosa cells. A mature or
Graafian follicle is large, fluid filled follicle that will rupture and
release a secondary oocyte in the process called ovulation. A corpus
luteum is the remnant of a ruptured follicle, and produces progesterone,
estrogen and relaxin, until it degenerates into fibrous tissue and becomes a
corpus albicans.
Production of the egg, or secondary oocyte, is also a
complicated process, which is called oogenesis. During fetal
development, germ cells differentiate into oogonia, which are cells that divide
mitotically to produce millions of germ cells. Many of these cells degenerate,
but a few develop into primary oocytes that enter prophase I of meiosis, but do
not complete it until after puberty. At birth about a million oogonia and
primary oocytes remain in each ovary, and of these, about 400 will mature and
ovulate during a woman's reproductive lifetime. The rest undergo atresia or
degeneration.
Each primary oocyte is surrounded by a single layer of
follicular cells, and this structure is called a primordial follicle.
Periodically, a few primordial follicles begin to grow under the influence of
FSH. They first become primary follicles which have first one,
then several layers of surrounding cells. A layer of glycoprotein, called the zona
pellucida, separates the oocyte from the granulosa cells. At the same
time, the ovarian cells outside the follicle organize into two layers: an inner
vascular layer (theca interna) composed of hormone secreting
cells,connective tissue and blood vessels, and an outer fibrous layer (theca
externa) made of connective tissue.
The granulosa cells begin to secrete follicular fluid , which
builds up in a cavity called the antrum in the center of the
follicle. The follicle is now called a secondary follicle.
After puberty, each month one secondary follicle
resumes meiosis. The diploid primary follicle completes reduction division
(meiosis I) and two cells of unequal size are produced. The smaller cell is
called the first polar body, and is just a package of discarded
nuclear material. The large cell has received most of the cytoplasm, and is
called a secondary oocyte. The secondary oocyte begins meiosis
II, but stops at metaphase. At ovulation, the secondary oocyte and first polar
body are released. If fertilization does not occur, the secondary oocyte simply
degenerates. If a sperm does manage to penetrate the oocyte, meiosis resumes and
again there is an unequal division of cellular material. Another polar body is
formed, and in the larger cell, the ovum, the nucleus of the sperm and ovum
unite to form a zygote, which has the full (diploid or 2n) complement of
chromosomes. The first polar body may also undergo another division. The end
result is that one ovum and three polar bodies are produced from the primary
oocyte. The polar bodies all degenerate. Compare with spermiogenesis where four
sperm are produced from one spermatogonium. Why would the formation of only one
egg be beneficial? (Hint:cleavage)
The uterine (Fallopian) tubes extend laterally from
the uterus. The open end which faces the ovary is funnel shaped and is called
the infundibulum. It has many fingerlike projections called fimbriae,
one of which is attached to the ovary. Local currents produced by the movement
of the fimbriae draw the ovum into the Fallopian tube. The Fallopian tubes are
lined with ciliated columnar epithelium which create waves which move the ovum
down the tube to the uterus. There are also some secretory cells with microvilli
which may provide nutrition for the ovum. Peristaltic movements of the
muscularis layer also help to propel the ovum toward the uterus. The secondary
oocyte is usually fertilized in the outer one third of the Fallopian
tube, although fertilization may occur in the abdominopelvic cavity.
Fertilization can occur up to about 24 hours after ovulation. If the oocyte is
fertilized, it should arrive in the uterus in about 7 days.
The uterus is an organ the size and shape of
an inverted pear that functions in menstruation, implantation of a fertilized
ovum, development of a fetus during pregnancy, and labor. It also is part of the
pathway for sperm to reach the uterine tubes to fertilize a secondary oocyte.
The uterus is normally held in position by a series of ligaments. The uterus can
be divided into a fundus, body and cervix.
The secretory cells of the cervical mucosa produce about 20-60 ml of mucus per
day. Around the time of ovulation, this mucus becomes thinner and more alkaline.
The mucus provides for the energy needs of the sperm, protects sperm from the
hostile environment of the vagina, and protects them from phagocytes. At other
times of the cycle, the mucus becomes thicker and can form a cervical plug which
physically impedes the passage of sperm into the uterus.
Histologically, the layers of the uterus are: an outer
perimetrium, middle myometrium, and inner endometrium.
The perimetrium is a part of the visceral
peritoneum.
The bulk of the uterus is the myometrium. This
consists of an inner and outer layer of longitudinal muscle and a middle
circular layer. During labor, these muscles contract under the stimulation of
oxytocin.
The endometrium is highly vascular mucosa
and is divided into two layers: the stratum functionalis which
is closest to the uterine cavity is the layer which is shed during menstruation.
The deeper layer is the stratum basalis and is a permanent
layer, and gives rise to a new stratum functionalis after each menstrual cycle.
The vagina is a passageway for sperm and the
menstrual flow, the receptacle of the penis during sexual intercourse, and the
inferior portion of the birth canal. It is capable of considerable distension.
The mucosa of the vagina is continuous with that of the uterus, and consists of
nonkeratinized stratified squamous epithelium. There are dendritic cells in the
mucosa which are antigen-presenting cells and are thought to participate in the
transmission of HIV to a female after intercourse with an infected male. The
mucosa of the vagina contains large stores of glycogen, which decomposes to form
large amounts of organic acids. This lowers the pH of the vagina, making
it less susceptible to infection, but also making it less hospitable
to sperm.
The vulva is a collective term for the
external genitals of the female. It consists of the mons pubis
which is a pad of adipose tissue which cushions the pubic symphysis. The labia
majora are folds of skin analogous to the scrotum. They contain adipose
tissue, sebaceous glands and sweat glands. The labia minora are
folds of skin which are medial to the labia majora and which contain many blood
vessels and sebaceous or oil glands. The clitoris is homologous
to the penis in the male and is also composed of two corpora cavernosa and a
glans. The region within the labia minor is called the vestibule.
Here we find the external urethral orifice, on either side of which are the
openings of the ducts of the paraurethral glands. These glands
secrete mucus and are homologous to the prostate gland. On either side of the
vaginal orifice are the vestibular (Bartholin's) glands, which
produce mucus during sexual arousal and are homologous to the bulbourethral
glands.
Read about erection, lubrication and orgasm in your book.
FEMALE REPRODUCTIVE CYCLE
The female reproductive cycle is made up of two cycles :
the ovarian cycle which is the series of events leading to the
maturation and release of an oocyte, and the uterine or menstrual cycle,
which prepares the uterus to receive the fertilized ovum.
The uterine and ovarian cycles are controlled by GnRH from
the hypothalamus, which stimulates the release of FSH and LH by the anterior
pituitary gland. FSH stimulates the development of secondary follicles and
initiates secretion of estrogens by the follicles. LH stimulates further
development of the follicles, secretion of estrogens by follicular cells,
ovulation, and the secretion of progesterone and estrogens by the corpus
luteum.
Estrogens have four important functions:
1 )stimulate the growth, development, and maintenance of
female reproductive structures, secondary sex characteristics and the breasts
2) they help regulate fluid and electrolyte balance.
3) they stimulate protein synthesis.
4) they lower blood cholesterol level ( women under 50 have a
much lower risk of coronary artery disease than do men.
Moderate levels of estrogen in the blood inhibit the
release of GnRH and LH and FSH. This is the basis of birth control pills.
Progesterone is secreted mainly by the
corpus lutem and works with estrogens to prepare the endometrium for
implantation and the mammary glands for milk synthesis. High levels of
progesterone also inhibit GnRH and LH and FSH.
Relaxin is another hormone produced by the corpus
luteum. It relaxes the uterus by inhibiting uterine contractions which
presumably aids implantation. During pregnancy, relaxin is produced by the
placenta, and continues to relax the smooth muscle of the uterus. It also
relaxes the pubic symphysis and helps dilate the uterine cervix to aid in
delivery.
Phases of the Female Reproductive Cycle:
The typical female reproductive cycle runs about 24-35
days, with 28 days being average. It can be divided into three phases: The
menstrual phase, the preovulatory phase, and the postovulatory phase.
The menstrual phase takes place during
days 1-5. FSH release stimulates maturation of the
follicles. At this time 20 or so small follicles , some in each ovary,
begin to enlarge. In the uterus the stratum functionalis of the endometrium is
shed, discharging about 50 -150 ml of blood , tissue fluid, mucus, and
epithelial cells. The declining levels of estrogen and progesterone in the blood
cause the spiral arteries of the uterus to constrict, and the surrounding cells
become ischemic and die, and are sloughed off.
Ovaries: The preovulatory phase is the
time between menstruation and ovulation. This phase is the most variable in
length, and the differences account for the longer or shorter cycles. It is
usually from day 6 to 13 in a 28 day cycle. In the ovaries, under the influence
of FSH, our 20 follicles are continuing to grow.
Granulosa cells produce increasing amounts of estrogen and some
progesterone. The estrogen produced inhibits the release of LH, but causes it to
be stored in the anterior pituitary gland.
By day 6, one follicle has outgrown the others and is
the dominant follicle. Estrogen and inhibin secreted by the dominate follicle
inhibit the secretion of FSH by the pituitary and inhibits the development of
the other follicles, which then degenerate. The dominant follicle continues to
develop into a mature or Graafian follicle in preparation for ovulation. If two
(or more) follicles achieve codominance, both may ovulate and result in
fraternal twins. The rise in estrogen also causes the secretion of LH. Within
the follicle, the granulosa cells around the oocyte loosen the connection to the
inner wall, and fluid accumulates. With regard to the ovaries, the
menstrual phase and the preovulatory phase are called the follicular
phase because the follicles are developing.
Uterus: The estrogens produced by the
developing follicles cause the cells of the stratum basalis to undergo mitosis
and produce a new stratum functionalis. The glandular
epithelium of the uterine lining thickens, and the thickness of the
endometrium doubles. This is the proliferative phase in the
uterus.
About day 14 of a 28 day cycle , when
the estrogen levels are high enough, they exert a positive feedback on the
hypothalamus and anterior pituitary causing the the
anterior pituitary to releases the stored LH - resulting in a surge that lasts
about 36 hours. It is this surge in LH that tests which indicate
ovulation measure. This surge in LH weakens and
ruptures the wall of the follicle, and the oocyte is released =
Ovulation is the rupture of the mature follicle. The secondary oocyte
is released into the pelvic cavity; it takes with it two layers: a clear
glycoprotein layer called the zona pellucida, and the first layer of granulosa
cells.?
After ovulation, the mature follicle collapses, and the
follicular cells of the theca interna
enlarge and change to form the corpus luteum under the influence of LH. The
corpus luteum , under the influence of LH, secretes progesterone, estrogen, and
relaxin. As the corpus luteum becomes established,
blood levels of progesterone rise.
The slight rise in progesterone just before ovulation
increases the basal temperature by about 0.4 -0.6 o F. The next 24
hours are the most likely time for pregnancy to occur. Changes also occur in the
cervical mucus, and a woman may experience pain around the ovaries which is
called mittelschmerz ( MIT-el-shmarts).
Which is the ovulating hormone ? (LH) Could ovulation occur
if FSH remained low throughout the menstrual cycle? (no- because a follicle
would not develop.)
The postovulatory phase During the
postovulatory phase, is the most constant in duration, and lasts for 14 days,
from day 15 to 28 in the 28 day cycle. It is the time between ovulation and the
onset of the next menstrual cycle.
For the ovaries, this is called the luteal phase
because it is under the control of the secretions of the corpus luteum. If
fertilization does not occur, after two weeks the secretions of the corpus
luteum decline, and it degenerates into the corpus albicans. The decline in
estrogen and progesterone bring on menstruation and promote the release of GnRH,
FSH and LH, which begin the maturation of more follicles.
In the uterus, the progesterone produced by the corpus luteum
promote the growth of the endometrial glands which secrete glycogen,
vascularization of the stratum functionalis and an increase in the amount of
tissue fluids containing nutrients and electrolytes
to provide a favorable environment for development of the embryo..
These changes reach their peak about one week after ovulation, at about
the time the fertilized ovum would reach the uterus. For the uterus, this is
called the secretory phase because of the activity of the
glands of the uterus.
GnRH - FSH - follicles produce estrogen- estrogen causes the
release of LH- LH causes ovulation and formation of corpus luteum- corpus luteum
produces progesterone ( and estrogen)
PREGNANCY
A secondary oocyte can be fertilized for about 24 hours
after ovulation. Sperm can usually remain viable for about 48 hours (up
to 72). So there is a three day window during which fertilization can occur -
from two days before to one day after ovulation. Fertilization normally occurs
in the outer third of the Fallopian tube, although it may also occur in
the abdominopelvic cavity. Sperm swim up the up the female reproductive tract,
and are aided by muscular contractions the uterus stimulated by prostaglandins
in the semen. The oocyte may also secrete a chemical substance that attracts
sperm. The sperm undergo functional changes while in the female tract , and
these changes are called capacitation. These changes allow the
sperm to be able to fertilize the oocyte. During this process the membrane
around the acrosome becomes fragile, and the enzymes it contains are released.
Remember it requires the enzymatic action of many sperm to allow one sperm to
penetrate the ovum. Normally only one sperm penetrates the egg, and this is
called syngamy. When the first sperm enters the secondary
oocyte, the cell depolarizes causing the release of Calcium ions inside the
cell. The calcium stimulates the release of granules which promote changes in
the zona pellucida which blocks the entry of other sperm. It is at this point
the secondary oocyte completes its division, and the nuclei of the ovum and
sperm unite to form a cell with the diploid number of chromosomes, called a zygote.
Dizygotic (fraternal) vs. Monozygotic (identical) twins.
Fraternal twins occur when two separate eggs are ovulated and fertilized. These
individuals may be of different sexes, and are no more alike than any other
brother or sister. Identical twins occur when a single egg is fertilized. As the
cells grow and divide, they break apart and form two individuals with the same
genetic makeup (clones).
After fertilization the zygote undergoes rapid mitotic cell
division. Because these divisions do not increase the size of the zygote, they
are called cleavage. Over a period of days, these cleavages produce a solid
sphere of cells, which is still surrounded by the zona pellucida. At this point
it is called a morula.
At four and a half to five days, the cells have
developed into a hollow ball of cells called a blastocyst, and
enters the uterus. The blastocyst has an outer layer of cells called the
trophoblast, an inner cell mass and a fluid filled cavity called the
blastocele. The trophoblast and part of the inner cell mass will form the
membranes of the fetal portion of the placenta, and the rest of the inner cell
mass forms the embryo.
The blastocyst remains free in the uterus for a short time,
during which the zona pellucida disintegrates. The blastocyst receives
nourishment from the glycogen produced by the glands in the endometrium, and the
blastocyst enlarges. About 6 days after ovulation, the blastocyst attaches to
the wall of the uterus, a process called implantation. When the blastocyst
implants, it orients itself so that the inner cell mass is toward the
endometrium. The blastocyst secretes enzymes which allow it to penetrate into
(digest) the endometrial wall. The cells it digests serves to nourish the
blastocyst for about a week after implantation.
Implantation can also occur in the Fallopian tube, cervix, or
the abdominal cavity. Implantation anywhere outside the uterus is
called ectopic pregnancy. It is possible for a fetus to grow in
the abdominal cavity, but growth within the Fallopian tube causes the tube to
rupture, resulting in severe bleeding.
As early as 8 -12 days after fertilization, the blastocyst
begins to secrete human chorionic gonadotropin. The hCG keeps
the corpus luteum active, until the placenta can begin to produce estrogens and
progesterone. This is the basis for pregnancy tests.
The inner cell mass forms two cavities: the yolk sac
and the amniotic cavity. The function of the yolk sac in humans
is not to provide food, but rather to produce blood cells and other functions.
The amniotic cavity becomes the fluid filled cavity in which the embryo floats.
The fluid is produced from fetal urine, and secretions from the skin,
respiratory tract, and amniotic membranes. In between the yolk sac and the
amniotic cavity is the embryonic disc, which gives rise to the three primary
germ layers: ectoderm, mesoderm and endoderm.
The gestation period, or the length of
pregnancy, is divided into three trimesters. During the first trimester the new
individual starts out as a zygote, then morula, then blastocyst at which stage
implantation occurs. The developing human then become known as an embryo .The
embryonic phase of development lasts from fertilization until the end of the 8th
week of gestation, when the embryo becomes a fetus. By day thirty-five the heart
is beating; the eyes and the limb buds are present. By month four, the rudiments
of all organ systems are formed and functioning, and from then on fetal
development is just a matter of growth. At the end of the third month, the
placenta is functioning.
The chorion, from the outer cells of the morula, develops
into the fetal part of the placenta. The chorionic villi
connect the fetal circulation to the placenta. The placenta is composed of both
fetal and maternal tissues, and serves five functions:
1. Transfer of gases , much as in the lungs.
2. transport of nutrients . Glucose , fatty acids, K, Na and Cl all diffuse
across the placenta, other substances are actively transported.
3. excretion of wastes such as urea, uric acid and creatinine diffuse into the
maternal blood.
4. Hormone production - the placenta is a temporary endocrine gland,
secreting estrogen and progesterone, until by the third month the corpus
luteum is no longer needed to maintain pregnancy, and prepares the mother's body
for delivery and lactation.
5. Formation of a barrier- There is normally no mixing of fetal and maternal
blood, and the membranes form a barrier to the passage of some drugs and toxins.
The barrier is incomplete and nonselective, so some substances, such as alcohol,
steroids, narcotics, anesthetics, some antibiotics and some organisms can cross
this barrier.
Quickening is the term used to describe
the first movements of the fetus felt by the mother, usually occurring during
months four or five of pregnancy. By month seven the fetus is quite active.
During the last month the fetus becomes less active, usually due to space
considerations.
At the end of pregnancy both the mother and the uterus become
"irritable", and the uterus undergoes Braxton Hicks contractions:
intermittent, painless contractions which can come 10 to 20 minutes apart. They
become more frequent as the pregnancy progresses, and can sometimes be mistaken
for the onset of labor. At the end of pregnancy the cervix begins to thin and
dilate. Labor has three stages:
Stage one- this is the period from the start of true
labor contractions until the dilation of the cervix is complete at ten
centimeters. When labor begins the uterine contractions cause the cervix to
dilate and the amniotic sac may rupture. Usually lasts from 6 to 24 hours,
depending on the number of previous deliveries.
Stage two- the period from maximal
cervical dilation to the birth of the baby, lasting a few minutes to an hour.
Contractions become more intense and frequent.
Stage three- the expulsion of the placenta, which
usually occur within 15 minutes after the birth of the baby, but can range from
5 to 60 minutes.
The mammary glands are modified
sweat glands whose function is to synthesize, secrete, and eject milk
(lactation).The breasts lie over the pectoralis major muscles, and are attached
to them by the connective tissue ligaments of Cooper. Breast size is determined
more by the fat around the glandular tissue, than by the glandular tissue. Each
breast has 15 to 20 lobes made up of several lobules. The lobules are made of
the milk-secreting cells arranged in alveoli.( Note the similarity with the
construction of the lungs). The alveoli are surrounded by myoepithelial
cells which contract to move the milk out of the alveoli. Milk passes
from the alveoli into secondary tubules, then into mammary
ducts. Close to the nipple the mammary ducts expand to form lactiferous
sinuses where some milk may be stored. Milk then passes into lactiferous
ducts which open onto the nipple. The colored area around the nipple is
the areola (a-REE- o-la).
Milk production is stimulated mostly by prolactin with some
help from estrogen and progesterone. Ejection or milk let down occurs under the
influence of oxytocin.