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Human Anatomy: Embryology
The study of the developmental events that occur during the prenatal period
Begins with Fertilization:
A single fertilized cell divides by mitosis to produce all of the cells in the body.
The Prenatal Period
The first 38 weeks of human development
between fertilization and birth.
The pre-embryonic period:
first 2 weeks of development
zygote becomes a spherical, multicellular structure.
The embryonic period:
third through eighth weeks
all major organ systems appear.
The Prenatal Period
The Fetal Period:
Includes the remaining weeks of development prior to birth
The fetus continues to grow
Its organs increase in complexity
The Stages of Embryogenesis
zygote divides by mitosis
forms a multicellular structure called a blastocyst.
blastocyst cells form three primary germ layers
basic cellular structures from which all body tissues develop.
three primary germ layers arrange themselves in ways that give rise to all the organs within the body.
Following birth, an individual undergoes maturation.
the body grows and develops
the sex organs become mature
the sex organs then begin to produce gametes
Human somatic cells contain 23 pairs of chromosomes for a total of 46.
22 pairs of autosomes
one pair of sex chromosomes.
Autosomes contain genetic information for most human characteristics.
pair of similar autosomes
A cell is said to be diploid if it contains 23 pairs of chromosomes.
2N = 46
The Sex Chromosomes
The pair of sex chromosomes determines whether an individual is female (XX) or male (XY).
One member of each pair of chromosomes is inherited from each parent.
Begins with meiosis.
Produces secondary oocytes in the female.
Produces sperm in the male.
A type of cell division
Starts with a diploid parent cell
Produces haploid daughter cells (sperm or eggs/ova).
Meiosis results in the formation of gametes (sex cells).
In meiosis I:
homologous chromosomes are separated after synapsis
crossing over occurs.
In meiosis II:
sister chromatids are separated
sequence of phases resembles mitosis.
Homologous, double-stranded chromosomes in the parent cell form pairs (synapsis).
Pair of homologous chromosomes
occurs between the maternal and paternal chromosomes.
Homologous pairs of chromosomes line up above and along the equator of the cell.
Forms a double line of chromosomes.
Alignment is random with respect to maternal or paternal origin.
Pairs of homologous chromosomes separate and are pulled to the opposite ends of the cell.
Telophase I and Cytokinesis
Nuclear division finishes
The nuclear envelopes re-forms
The cytoplasm divides
Two new haploid cells are produced
Resembles the prophase stage of mitosis.
In each of the two new cells:
nuclear membrane breaks down
chromosomes collect together.
Crossing over does not occur in this phase.
The double-stranded chromosomes form a single line in the middle of the cell.
Spindle fibers extend from the centrioles at the poles to the centromere of each double-stranded chromosome.
The sister chromatids of each double-stranded chromosome are pulled apart at the centromere.
Each chromatid (single strand) is pulled to the opposite pole of the cell.
Telophase II and Cytokinesis
The single-stranded chromosomes arrive at opposite ends of the cell.
A cleavage furrow forms
Cytoplasm in both cells divides
Produces a total of four haploid daughter cells.
These daughter cells mature:
sperm in males
oocytes in females.
In females, the sex cell produced is called the secondary oocyte.
This cell will have 22 autosomes and one X chromosome.
parent cells that produce oocytes
reside in the ovaries
are diploid cells.
All the oogonia start the process of meiosis and form primary oocytes prior to birth.
They are arrested in Prophase I and remain this way until the female reaches puberty.
Each month usually only one becomes a secondary oocyte.
When the primary oocyte completes the first meiotic division, two cells are produced.
Division of the cytoplasm is unequal.
The secondary oocyte receives the bulk of the cytoplasm and is the cell that is arrested in Metaphase II.
The second cell, which receives only a tiny bit of the cytoplasm, is called a polar body.
The polar body is a nonfunctional cell and eventually degenerates.
Only the secondary oocyte has the potential to be fertilized.
The secondary oocyte is ovulated
The corona radiata and the zona pellucida form protective layers around the secondary oocyte.
If the secondary oocyte is not fertilized, it degenerates about 24 hours after ovulation, still arrested in metaphase II.
If the secondary oocyte is fertilized, it first finishes the process of meiosis. Two new cells are produced, and as before, the division of the cytoplasm is unequal.
The cell that receives very little cytoplasm becomes another polar body and eventually degenerates.
The cell that receives the majority of the cytoplasm becomes an ovum which can be fertilized.
Typically, only one secondary oocyte is expelled (ovulated) from one of the two ovaries each month.
The left and right ovaries alternate ovulation each month.
The parent or stem cells that produce sperm are called spermatogonia.
Spermatogonia are diploid cells that reside in the the testes.
Each one first divides by mitosis to make an exact copy of itself called a primary spermatocyte.
Primary spermatocytes then undergo meiosis and produce haploid cells called spermatids.
Spermatids contain 23 chromosomes, but they still must undergo further changes to form a sperm cell.
In spermiogenesis, spermatids lose much of their cytoplasm and grow a long tail called a flagellum.
The newly formed sperm cells are haploid cells that exhibit a distinctive head, a midpiece, and a tail.
From a single spermatocyte, four new sperm are formed.
All sperm have 22 autosomes and either an X chromosome, or a Y chromosome.
Two sex cells fuse to form a new cell containing genetic material derived from both parents.
Restores the diploid number of chromosomes.
Determines the sex of the organism.
Occurs in the widest part of the uterine tube (the ampulla).
Millions of sperm cells are deposited in the female reproductive tract during intercourse.
Only a few hundred have a chance at fertilization.
Only the first sperm to enter the secondary oocyte is able to fertilize it.
The remaining sperm are prevented from penetrating the oocyte.
Shortly after fertilization, the zygote begins to undergo a series of divisions.
Divisions increase the number of cells in the pre-embryo, but the pre-embryo remains the same size.
During each succeeding division, the cells are smaller and smaller.
Before the 8-cell stage, cells are not tightly bound together
after the third cleavage division, the cells become tightly compacted into a ball called a morula (16 cells).
Zona pellucida begin to disintegrate as morula enters the uterus.
Blastocyst cavity develops.
Pre-embryo now a blastocyst:
Inner cell mass or embryoblast
Implantation is the process by which the blastocyst burrows into and embeds within the endometrium.
Begun about day 7; done by day 9
Trophoblast cells invade
Formation of Bilaminar Germinal Disc
By day 8, embroblast begins to differentiate
Hypoblast layer: adjacent to blastocyst cavity
Epiblast layer: adjacent to amniotic cavity
Together called bilaminal germinal disc
Formation of Extraembryonic membranes
Eventually encloses the entire embryo in a fluid-filled sac called the amniotic cavity to prevent desiccation.
The amniotic membrane is specialized to secrete the amniotic fluid that bathes the embryo.
The outermost extraembryonic membrane, is formed from rapidly growing cells.
These cells blend with the functional layer of the endometrium and eventually form the placenta.
Functions in exchange of nutrients, waste products, and respiratory gases between the maternal and fetal bloodstreams.
Transmission of maternal antibodies to the developing embryo or fetus.
Production of hormones to maintain and build the uterine lining.
Begins with establishment of the three germ layer
By process of gastrulation
Ends at about week 8
Main organ systems laid in
Occurs during the third week of development immediately after implantation.
One of the most critical periods in the development of the embryo.
Cells of the epiblast migrate and form the three primary germ layers:
Trilaminar structure now called an embryo
Primitive streak formation
Dorsal surface of the bilaminar germinal disc
Depression on surface of epiblast
Cephalic end: primitive node
Inward movement of cells
Cells from epiblast detach, move from primitive streak to area between epiblast and hypoblast.
Other migrating cells replace the hypoblast: form endoderm
Remaining cells in epiblast become ectoderm
Folding of the Embryonic Disc
Begins late third and fouth weeks
Some areas grow faster than others.
Helps form head and buttocks
Transverse (or lateral) folding
Helps form trunk
Neurulation: differentiation of ectoderm
Notochord forms in area of primitive streak
This induces neurulation
Differentiation of Mesoderm
Somites: most bone, muscle, cartilage, dermis, CT
Urinary and reproductive systems
Lateral Plate Mesoderm
Cardiovascular, lining of body cavities, CT of limbs
CT and musculature of face
Differentiation of Endoderm
Linings of digestive, respiratory and urinary tracts.
Once the three primary germ layers have formed, and the embryo has undergone folding, organogenesis begins.
The upper and lower limbs attain their adult shapes, and the rudimentary forms of most organ systems have developed by week 8.
By the end of the embryonic period, the embryo is slightly longer than 2.5 centimeters (1 inch), and yet it already has the outward appearance of a human.