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Chapter 29

Nursing HAP201 Chapter Notes - Chapter 29: Zona Pellucida, Gestational Sac, Fallopian Tube


Department
Nursing
Course Code
Nursing HAP201
Professor
Judith Card
Chapter
29

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HAP201 Week Two/Chapter 29: Development
LO 29.1: Explain the major developmental events that occur during the embryonic and fetal periods
FIRST WEEK OF DEVELOPMENT
The embryonic period extends from fertilization through the eight week
The first week is characterized by several significant events including
o Fertilization
o Cleavage of the zygote
o Blastocyst formation
o Implantation
Fertilization
The genetic material from a haploid sperm cell (spermatozoon) and a haploid secondary oocyte merges into a single diploid
nucleus
Occurs in the uterine (fallopian) tube within 12-24 hrs after ovulation
Sperm swim from the vaginal into the cervical canal by whip-like movements of their flagella.
o The passage of sperm through the rest of the uterine and then into the uterine tube results from contraction of the walls of
these organs
o Prostaglandins in semen stimulate uterine motility during intercourse and aid in sperm movement through the uterus and
into the uterine tube
Sperm that reach the oocyte within mins after ejaculation cannot fertilize until seven hrs later. During this time in the uterine
tube of the female, the sperm undergo capacitation, a series of functional changes that cause the sperm’s tail to beat faster and
prepare a plasma membrane to fuse with the oocyte’s plasma membrane
o Sperm are acted on secretions in the female reproductive tract the cause the removal of cholesterol, glycoproteins, and
proteins from the plasma membrane around the head of the sperm cell
o Only capacitated sperm can attract and respond to chemical factors produced by surrounding cells of the ovulated oocyte
For fertilization to occur, a sperm cell must penetrate two layers
o Corona radiata: the granulosa cells that surround the secondary oocyte
o Zona pellucida: the clear glycoprotein layer between the corona radiata and the oocyte’s plasma membrane
Acrosome: helmet-like structure the covers the head of a sperm; contains several enzymes. These enzymes and strong sperm
tail movements help penetrate the cells of the corona radiata and come in contact with zona pellucida
ZP3: this is a glycoprotein in the zona pellucida that acts as a sperm receptor. Its binding to specific membrane proteins in the
sperm head triggers the acrosomal reaction the release of the contents of the acrosome. The enzymes digest a path through
the zona pellucida as the lashing sperm tail pushes the sperm cell onward. But, only the first sperm cell to penetrate the entire
zona pellucida and reach the oocyte’s plasma membrane fuses with the oocyte
Polyspermy: this is fertilization by more than one sperm cell and it is blocked by the fusion of a sperm cell with a secondary
oocyte. The cell membrane of the oocyte depolarizes, which acts as a fast block to polyspermy the inability of a depolarized
oocyte to fuse with another sperm.
Depolarization also triggers the intracellular release of calcium ions, which stimulate exocytosis of secretory vesicles from the
oocyte. Molecules released by exocytosis inactivate ZP3 harden the entire zona pellucida (slow block to polyspermy)
Once a sperm enters a secondary oocyte, the oocyte must first complete meiosis II. It divides into a larger ovum (mature egg)
and a smaller second polar body that fragments and disintegrates
The nucleus in the head of the sperm develops into the male pronucleus, and the nucleus of the fertilization ovum develops into
the female pronucleus. After the male and female pronuclei form, they fuse, producing a single diploid nucleus syngamy
Thus, the fusion of the haploid (n) pronuclei restores the diploid number (2n) of 46 chromosomes. The fertilized ovum now is
called a zygote
Dizygotic twins: fraternal twins; are produced from the independent release of two secondary oocytes and the subsequent
fertilization of each by different sperm. Genetically, they are dissimilar and they may or may not be the same sex
Monozygotic twins: identical twins; develop from a single fertilized ovum and contain exactly the same genetic material and
are always the same sex. They arise from the separation of the developing cells into two embryos, which occurs before 8 days
have passed. If separation is longer than 8 days, conjoined twins are produced, in which the twins are joined together and share
some body structures
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Cleavage of the Zygote
Cleavage: rapid mitotic cell divisions of the zygote that take place after fertilization.
The first division of the zygote begins 24 hrs after fertilization and is completed 6 hrs later.
The second day after fertilization completes the second cleavage and produces 4 cells
By the end of the third day, there are 16 cells.
Blastomeres: the progressively smaller cells produced by cleavage
Successive cleaves eventually produce a solid sphere of cells called the morula, which is still surrounded by the zona pellucida
and is about the same size as the original zygote
Blastocyst Formation
By the end of the fourth day, the number of cells in the morula increases as it continues to move through the uterine tube
towards the uterine cavity
When the morula enters the uterine cavity on day 4/5, a glycogen-rich secretion from the glands of the endometrium of the
uterus passes into the uterine cavity and enters the morula through the zona pellucida. This fluid is uterine milk, and along with
nutrients stored in the cytoplasm of the blastomeres of the morula, provide nourishment for the developing morula
At the 32-cell stage, the fluid enters the morula, collects between the blastomeres and reorganizes them around a large fluid-
filled cavity called the blastocyst cavity
Blastocyst: the developing mass formed from the blastocyst cavity. It is the same size as the original zygote, even though it has
hundreds of cells. During this formation, two distinct cell populations arise:
o Embryoblast: inner mass; located internally and eventually develops into the embryo
o Trophoblast: the outer superficial layer of cells that forms the sphere-like wall of the blastocyst. It will later develop
into the outer chorionic sac that surrounds the fetus and the fetal portion of the placenta
Around the 5th day after fertilization, the blastocyst “hatches” from the zona pellucida by digesting a hole in it with an enzyme,
and then squeezing through the hole
The shedding of the zona pellucida is needed for the next step
Implantation
The blastocyst remains free within the uterine cavity for 2 days before attaching to the uterine wall
The endometrium is in its secretory phase
6 days after fertilization, blastocyst loosely attaches to the endometrium through implantation. It will implant on either side of
the posterior portion of the fundus and will orient with the inner cell mass towards the endometrium
7 days after, blastocyst attaches to the endometrium more firmly, surrounding glands enlarge and the endometrium becomes
more vascularized (forms new blood vessels)
The blastocyst eventually secretes enzymes and burros into the endometrium, becoming surrounded by it
Following implantation, the endometrium is known as the decidua, and separates from the endometrium after the fetus is
delivered. Different regions of the decidua are named, based on their positions relative to the site of the implanted blastocyst
o Decidua basalis: the portion of the endometrium between the embryo and the stratum basale of the uterus; provides
large amounts of glycogen and lipids for the developing embryo and fetus. It later becomes the maternal part of the
placenta
o Decidua capsularis: the portion of the endometrium located between the embryo and the uterine cavity
o Decidua parietalis: the remaining modified endometrium that lines the non-involved areas of the rest of the uterus.
As the embryo and later the fetus enlarges, the decidua capsularis bulges into the uterine cavity and fuses with the decidua
parietalis, thereby obliterating the uterine cavity
By 27 weeks, the capsularis degenerates and disappears
SECOND WEEK OF DEVELOPMENT
Development of the Trophoblast
8 days after fertilization, the trophoblast develops into two layers in the region of contact between the blastocyst an
endometrium
o Syncytiotrophoblast: contains no distinct cell boundaries. During implantation, it secretes enzymes that enable the
blastocyst to penetrate the uterine lining by digesting and liquefying the endometrial cells
o Cytotrophoblast: this is between the embryoblast and syncytiotrophoblast that is composed of distinct cells
o These two layers become part of the chorion (one of the fetal membranes) as they undergo further growth
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The blastocyst becomes buried in the endometrium and inner one-third of the myometrium
Human chorionic gonadotropin (hCG) is another secretion of the trophoblast, which has actions similar to LH. It rescues the
corpus luteum from degeneration and sustains its secretions of progesterone and estrogen (maintain the uterine lining to prevent
menstruation). Peak secretion of hCG occurs during week 9, when the placenta is fully developed and produces the progesterone
and estrogens that continue to sustain the pregnancy. This hormone is also an indicator of pregnancy and is detected by home
pregnancy tests
Development of the Bilaminar Embryonic Disc
Cells of the embryoblast differentiate into two layers around 8 days after fertilization
o Hypoblast: primitive endoderm
o Epiblast: primitive ectoderm
Cells of the hypoblast and epiblast together form a flat disc referred to as the bilaminar embryonic disc
Soon, a small cavity appears within the epiblast and eventually enlarges to form the amniotic cavity
Development of the Amnion
As this cavity grows, a single layer of squamous cells forms a domelike roof above the epiblast cells called the amnion. It
eventually surrounds the entire embryo, creating the cavity that becomes filled with amniotic fluid
Thus, the amnion forms the roof of the amniotic cavity and the epiblast forms the floor
Most of the amniotic fluid is derived from maternal blood. Later, the fetus contributes to the fluid by excreting urine into the
amniotic cavity
The cavity serves as a shock absorber for the fetus, helps regulate fetal body temperature, helps prevent the fetus from drying
out, and prevents adhesions between the skin of the fetus and surrounding tissues
The amnion ruptures just before birth; it and its fluid constitute the “bag of water”
Embryonic cells are sloughed off into the amniotic fluid
Development of the Yolk Sac
Also on the 8th day, cells at the edge of the hypoblast migrate and cover the inner surface of the blastocyst wall
The migrating columnar become squamous (flat) and then form a thin membrane called exocoelomic membrane. With the
hypoblast, this membrane forms the wall of the yolk sac, the former blastocysts cavity during earlier development. Now, the
bilaminar embryonic disc is now positioned between the amniotic cavity and yolk sac
The yolk sac is relatively empty and small, decreasing in size as development progresses. It has several important functions in
humans
o Supplies nutrients to the embryo during the second and third weeks of development
o It is the source of blood cells from the third through the sixth weeks
o It contains the first cells (primordial germ cells) that will eventually migrate into the developing gonads, differentiate into
the primitive germ cells, and form gametes
o It forms part of the GI tract
o It functions as a shock absorber
o It prevents drying out of the embryo
Development of Sinusoids
By the 9th day after fertilization, the blastocyst becomes completely embedded in the endometrium. As the syncytiotrophoblast
expands, small spaces called lacunae develop within it
By the 12th of development, these spaces fuse to form larger, interconnecting spaces called lacunar networks. Endometrial
capillaries around the developing embryo become dilated and are referred to as maternal sinusoids.
As the syncytiotrophoblast erodes some of the maternal sinusoids and endometrial glands, maternal blood and secretions from
the glands enter the lacunar networks and flow through them. Maternal blood is a rich source of nutrition and a disposal site for
the embryo’s wastes
Development of the Extraembryonic Coelom
The 12th day after fertilization, the extraembryonic mesoderm develops. These cells are derived from the yolk sac and form a
connective tissue layer (mesenchyme) around the amnion and yolk sac.
Soon, the extraembryonic coelom (a number of large cavities develop in the extraembryonic mesoderm, which then fuse to
form a single, larger cavity)
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