HUMAN ANATOMY
FOETAL DEVELOPMENT
INTRODUCTION:
In approximately nine months, a single cell—a fertilized egg—develops into a
fully formed infant consisting of trillions of cells with myriad specialized
functions. The dramatic changes of fertilization, embryonic development, and
fetal development are followed by remarkable adaptations of the newborn to life
outside the womb. An offspring’s normal development depends upon the
appropriate synthesis of structural and functional proteins. This, in turn, is
governed by the genetic material inherited from the parental egg and sperm, as
well as environmental factors.
Fertilization or conception
:
DEFINITION: It occurs when a sperm and an oocyte (egg) combine and their
nuclei fuse.It can also be defined as the union of the male sperm and the female
secondary oocyte to form a zygote.
Because each of these reproductive cells is a haploid cell containing half of the
genetic material needed to form a human being, their combination forms a diploid
cell. This new single cell, called a zygote, contains all of the genetic material
needed to form a human—half from the mother and half from the father.
1
Lecture Note by Mrs. Oge Nwankwo
,PROCESS OF FERTILIZATION
The process of fertilization involves a sperm fusing with an ovum. The most
common process begins with ejaculation during copulation, follows
with ovulation, and finishes with fertilization.
Various exceptions to this sequence are possible, including artificial
insemination, in vitro fertilization, external ejaculation without copulation, or
copulation shortly after ovulation. Upon encountering the secondary oocyte,
the acrosome of the sperm produces enzymes which allow it to burrow through
the outer shell called the zona pellucida of the egg. The sperm plasma then fuses
with the egg's plasma membrane and their nuclei fuse, triggering the sperm head
to disconnect from its flagellum as the egg travels down the fallopian tube to
reach the uterus.
In vitro fertilization (IVF) is a process by which egg cells are fertilized by sperm
outside the womb, in vitro.
There are six important steps which must occur for fertilization to be successful:
a) Sperm transport to the site of fertilization
b) Sperm capacitation
c) The acrosomal reaction
d) Polyspermy block
e) Completion of meiosis II
f) Zygote formation
2
Lecture Note by Mrs. Oge Nwankwo
, 1. Transit of Sperm
During ejaculation, hundreds of millions of sperm (spermatozoa) are released
into the vagina. Almost immediately, millions of these sperm are overcome by
the acidity of the vagina (approximately pH 3.8), and millions more may be
blocked from entering the uterus by thick cervical mucus. Of those that do enter,
thousands are destroyed by phagocytic uterine leukocytes.
Their journey—thought to be facilitated by uterine contractions—usually
takes from 30 minutes to 2 hours. If the sperm do not encounter an oocyte
immediately, they can survive in the uterine tubes for another 3–5 days. Thus,
fertilization can still occur if intercourse takes place a few days before ovulation.
In comparison, an oocyte can survive independently for only approximately 24
hours following ovulation. Intercourse more than a day after ovulation will
therefore usually not result in fertilization.
3
Lecture Note by Mrs. Oge Nwankwo
, 2. Sperm capacitation
During the journey, fluids in the female reproductive tract prepare the sperm for
fertilization through a process called capacitation, or priming which takes up to
7hours. The fluids (vaginal fluid) improve the motility of the spermatozoa. They
also deplete cholesterol molecules embedded in the membrane of the head of the
sperm, thinning the membrane in such a way that will help facilitate the release
of the lysosomal (digestive) enzymes needed for the sperm to penetrate the
oocyte’s exterior once contact is made. Sperm must undergo the process of
capacitation in order to have the “capacity” to fertilize an oocyte. If they reach
the oocyte before capacitation is complete, they will be unable to penetrate the
oocyte’s thick outer layer of cells.Contact Between Sperm and Oocyte
Upon ovulation, the oocyte released by the ovary is swept into—and along—the
uterine tube. Fertilization must occur in the distal uterine tube because an
unfertilized oocyte cannot survive the 72-hour journey to the uterus. The corona
radiata is an outer layer of follicular (granulosa) cells that form around a
developing oocyte in the ovary and remain with it upon ovulation. The underlying
zonapellucida (pellucid = “transparent”) is a transparent, but thick,
glycoprotein membrane that surrounds the cell’s plasma membrane that is the
oocyte. As it is swept along the distal uterine tube, the oocyte encounters the
surviving capacitated sperm, which stream toward it in response to chemical
attractants released by the cells of the corona radiate (the outermost layer of the
oocyte) allowing access to thezonapellucida
3. The acrosomal reaction
To reach the oocyte itself, the sperm must penetrate the two protective layers
(corona radiate and zonapellucida). The sperm first burrow through the cells of
the corona radiata. Then, upon contact with the zonapellucida, the sperm bind to
receptors in the zonapellucida. This initiates a process called the acrosomal
reaction in which the enzyme-filled “cap” of the sperm, called the acrosome,
releases its stored digestive enzymes. These enzymes clear a path through the
zonapellucida that allows sperm to reach the oocyte.
Finally, a single sperm makes contact with sperm-binding receptors on the
oocyte’s plasma membrane. The plasma membrane of that sperm then fuses with
the oocyte’s plasma membrane, and the head and mid-piece of the “winning”
sperm enter the oocyte interior. Then fertilization occurs
4
Lecture Note by Mrs. Oge Nwankwo
FOETAL DEVELOPMENT
INTRODUCTION:
In approximately nine months, a single cell—a fertilized egg—develops into a
fully formed infant consisting of trillions of cells with myriad specialized
functions. The dramatic changes of fertilization, embryonic development, and
fetal development are followed by remarkable adaptations of the newborn to life
outside the womb. An offspring’s normal development depends upon the
appropriate synthesis of structural and functional proteins. This, in turn, is
governed by the genetic material inherited from the parental egg and sperm, as
well as environmental factors.
Fertilization or conception
:
DEFINITION: It occurs when a sperm and an oocyte (egg) combine and their
nuclei fuse.It can also be defined as the union of the male sperm and the female
secondary oocyte to form a zygote.
Because each of these reproductive cells is a haploid cell containing half of the
genetic material needed to form a human being, their combination forms a diploid
cell. This new single cell, called a zygote, contains all of the genetic material
needed to form a human—half from the mother and half from the father.
1
Lecture Note by Mrs. Oge Nwankwo
,PROCESS OF FERTILIZATION
The process of fertilization involves a sperm fusing with an ovum. The most
common process begins with ejaculation during copulation, follows
with ovulation, and finishes with fertilization.
Various exceptions to this sequence are possible, including artificial
insemination, in vitro fertilization, external ejaculation without copulation, or
copulation shortly after ovulation. Upon encountering the secondary oocyte,
the acrosome of the sperm produces enzymes which allow it to burrow through
the outer shell called the zona pellucida of the egg. The sperm plasma then fuses
with the egg's plasma membrane and their nuclei fuse, triggering the sperm head
to disconnect from its flagellum as the egg travels down the fallopian tube to
reach the uterus.
In vitro fertilization (IVF) is a process by which egg cells are fertilized by sperm
outside the womb, in vitro.
There are six important steps which must occur for fertilization to be successful:
a) Sperm transport to the site of fertilization
b) Sperm capacitation
c) The acrosomal reaction
d) Polyspermy block
e) Completion of meiosis II
f) Zygote formation
2
Lecture Note by Mrs. Oge Nwankwo
, 1. Transit of Sperm
During ejaculation, hundreds of millions of sperm (spermatozoa) are released
into the vagina. Almost immediately, millions of these sperm are overcome by
the acidity of the vagina (approximately pH 3.8), and millions more may be
blocked from entering the uterus by thick cervical mucus. Of those that do enter,
thousands are destroyed by phagocytic uterine leukocytes.
Their journey—thought to be facilitated by uterine contractions—usually
takes from 30 minutes to 2 hours. If the sperm do not encounter an oocyte
immediately, they can survive in the uterine tubes for another 3–5 days. Thus,
fertilization can still occur if intercourse takes place a few days before ovulation.
In comparison, an oocyte can survive independently for only approximately 24
hours following ovulation. Intercourse more than a day after ovulation will
therefore usually not result in fertilization.
3
Lecture Note by Mrs. Oge Nwankwo
, 2. Sperm capacitation
During the journey, fluids in the female reproductive tract prepare the sperm for
fertilization through a process called capacitation, or priming which takes up to
7hours. The fluids (vaginal fluid) improve the motility of the spermatozoa. They
also deplete cholesterol molecules embedded in the membrane of the head of the
sperm, thinning the membrane in such a way that will help facilitate the release
of the lysosomal (digestive) enzymes needed for the sperm to penetrate the
oocyte’s exterior once contact is made. Sperm must undergo the process of
capacitation in order to have the “capacity” to fertilize an oocyte. If they reach
the oocyte before capacitation is complete, they will be unable to penetrate the
oocyte’s thick outer layer of cells.Contact Between Sperm and Oocyte
Upon ovulation, the oocyte released by the ovary is swept into—and along—the
uterine tube. Fertilization must occur in the distal uterine tube because an
unfertilized oocyte cannot survive the 72-hour journey to the uterus. The corona
radiata is an outer layer of follicular (granulosa) cells that form around a
developing oocyte in the ovary and remain with it upon ovulation. The underlying
zonapellucida (pellucid = “transparent”) is a transparent, but thick,
glycoprotein membrane that surrounds the cell’s plasma membrane that is the
oocyte. As it is swept along the distal uterine tube, the oocyte encounters the
surviving capacitated sperm, which stream toward it in response to chemical
attractants released by the cells of the corona radiate (the outermost layer of the
oocyte) allowing access to thezonapellucida
3. The acrosomal reaction
To reach the oocyte itself, the sperm must penetrate the two protective layers
(corona radiate and zonapellucida). The sperm first burrow through the cells of
the corona radiata. Then, upon contact with the zonapellucida, the sperm bind to
receptors in the zonapellucida. This initiates a process called the acrosomal
reaction in which the enzyme-filled “cap” of the sperm, called the acrosome,
releases its stored digestive enzymes. These enzymes clear a path through the
zonapellucida that allows sperm to reach the oocyte.
Finally, a single sperm makes contact with sperm-binding receptors on the
oocyte’s plasma membrane. The plasma membrane of that sperm then fuses with
the oocyte’s plasma membrane, and the head and mid-piece of the “winning”
sperm enter the oocyte interior. Then fertilization occurs
4
Lecture Note by Mrs. Oge Nwankwo
