Lecture 3 Flashcards
Development involves:
o Differentiation of cells
o Reorganization of cells
Development can be characterized by different periods of time
Prenatal development – fertilization/conception to delivery
§ Involves Embryology – prenatal development focussing on the first two months
after fertilization
o Postnatal development – development from birth to maturity
Fertilization
Fertilization is the joining of two haploid cells to create a diploid cell
• Function of the haploid cells:
• Spermatozoon (see chapter 27)
o Delivers the paternal chromosomes to the ovum
o sperm has head, midsection and tail
o head has an acrosome and a nucleus containing 22 haploid chromosomes and one X or
Y chromosome)
o midsection has mitochondria to provide energy to move flagellum
o flagellum (tail) moves to allow sperm to swim
Ovum
o Outermost layer has loosely defined cells called corona radiata (protective layer)
o Deep to corona radiata is another protective layer the jelly-like zona pellucida
o Contains a polar body which is a non-functioning cell that eventually degrades
o Provides the maternal chromosomes (secondary oocyte that has 23 haploid
chromosomes, 22 autosomes and a second X chromosome, suspended in metaphase of
meiosis II)
o Ovum provides nourishment for embryonic development
Fertilization
An ovum is produced in one of the ovaries once a month and travels along the uterine tube to the
uterus (cilia in the tube move the egg along and it takes about 4-5 days for the egg to get to the
uterus)
• Fertilization usually occurs in the ampulla of the uterine tube
• 200 million sperm cells enter the vaginal canal
• Only about 10,000 make it to the uterine tubes
• Less than 100 actually contact the egg
• Only one will fertilize the egg
• Fertilization details
o Oocyte at Ovulation: Ovulation releases a secondary oocyte and the first polar body; both
are surrounded by the corona radiata, which protects the egg. The oocyte is suspended
in metaphase of meiosis II
o Fertilization and Oocyte Activation: Acrosomal enzymes (hyaluronidase) from multiple
sperm create gaps in the corona radiata. A single sperm then makes contact with the
oocyte membrane, and membrane fusion occurs, triggering oocyte activation and
completion of meiosis, cortical granules from inside the oocyte dissolve into the
cytoplasm changing the permeability of the egg and make it impermeable to other sperm
o Pronucleus Formation Begins: The sperm is absorbed into the cytoplasm, the female
pronucleus develops.
o Spindle Formation and Cleavage Preparation: The male pronucleus develops, and
spindle fibers appear in preparation for the first cleavage division.
o Amphimixis Occurs and Cleavage Begins: metaphase of first cleavage division
o Cytokinesis Begins: The first cleavage divison nears completion roughly 30 hours after
fertilization
o Once fertilized called a zygote
Prenatal Development
Prenatal development is gestation period (9 months)
• Prenatal development is divided into trimesters
o First trimester (rudiments of all organs appear)
o Second trimester (fetus looks like a human)
o Third trimester (organs become functional, rapid growth)
The First Trimester - 1 to 12 weeks
• Four events within the first trimester
o Cleavage (sequence of cell reproduction)
o Implantation (implantation into endometrial lining)
o Placentation (formation of the placenta)
o Embryogenesis (development of the embryo)
Cleavage and Blastocyst Formation
Cell division results in the formation of blastomeres (blast – precursor) – identical daughter cells,
get smaller with each subsequent division
• A solid ball of cells eventually develops – this is a morula (morus – mulberry, tightly packed)
• Some cells migrate to one “edge” of the morula creating a mass of cells and a hollow cavity
called the blastocoele
• The ball of cells is now called the blastocyst
• The outer layer of the blastocyst consists of cells called the trophoblast (tropho – food, blast –
precursor, these cells will provide food/nutrients to the developing embryo)
• The inner cell mass (embryoblast – will become the embryo) consists of stem cells that will
develop into all the cells and cell types in the body (note: stem cells that are used for cloning are from the blastocyst stage, these cells are pleuri (multi) potent (power) and can differentiate into
any cell or tissue type in the body) *these are the controversial “stem cells” you hear about
Implantation
Upon contact with the endometrial lining, the trophoblast cells divide rapidly
• The trophoblast cells “fuse” with the endometrial lining forming a syncytial trophoblast (a
syncitium occurs when the cell walls of a group of cells break down and they become one large
cell)
• This layer of cells releases hyaluronidase to erode away more of the endometrial lining so the
mass can implant
• Upon implantation, the inner cell mass separates from the trophoblast area
• When the inner cell mass separates from the
trophoblast, two cavities form:
o Amniotic cavity
o Blastocoele cavity
Formation of the Blastodisc
A layer of cells forms between the amnionic cavity and the blastocoele cavity called blastodisc
• The layers are called
o Epiblast (epi – above) – composed of columnal cells next to the amniotic cavity
o Hypoblast (hypo – below) – composed of cuboidal cells next to blastocoele
o Migration of the epiblast cells around the amniotic cavity is the first step in the formation
of the amnion (or amniotic membrane is specialized to secrete amniotic fluid that bathes
the embryo)
o Migration of hypoblast cells create a sac that hangs below the blastodisc. This is the first step in yolk sac formation – in humans it does not store yolk but is important for early
blood cell and vessel formation
Gastrulation
Eventually some cells of the epiblast move toward the center of the blastodisc creating a
primitive streak
o Formed by a ridge along the epiblast
o Defines body axis (midline)
o Cells in primitive streak (PS) divide very rapidly
• The formation of the three primary germ layers is called gastrulation
• As the cells move toward the primitive streak area, they begin to migrate between the epiblast
and hypoblast layers
• This creates three distinct layers of cells
Gastrulation and Germ Layer Formation
The three layers of cells are: o Ectoderm (ektos – outer, derm - skin) (derived from the epiblast layer)
o Mesoderm (meso – middle) (new layer between the epiblast and hypoblast, derived from epiblast cells)
o Endoderm (endo – inner) (derived from the hypoblast layer)
- The ectoderm, mesoderm, and endoderm are collectively known as the germ layers
- Each layer will form specific tissues and organs of the body (see Table 28.1 pg. 755)
• Germ layers will also form structures involved in embryonic survival called extraembryonic
membranes
• There are four major extraembryonic membranes:
o Yolk sac - pouch that extends from the hypoblast cells into the blastocoele and is
involved in early blood cell formation
o Amnion – contains amniotic fluid which surrounds and cushions the developing embryo or
fetus
o Allantois –gives rise to the urinary bladder
o Chorion – the mesoderm and trophoblast layers form together to form the chorion
§ The chorion will eventually develop extensions into the endometrium
§ Placentation
The placenta begins to form when the chorion produces villi (chorionic villi)
that extend into the endometrial lining
• The body stalk connects the embryo to the chorion
• As the fetus develops and moves farther into the uterus, it obtains its
nutrients via the umbilical cord
Embryogenesis
Shortly after gastrulation, embryogenesis begins
• By week 4, a head fold and tail fold develop
• The embryo is separated from the blastodisc area
