Human Anatomy: Embryology | PPT download

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Human Anatomy: Embryology | PPT download

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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.

Homologous chromosomes:

pair of similar autosomes

Diploid Cells

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 I

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.

Prophase I

Homologous, double-stranded chromosomes in the parent cell form pairs (synapsis).


Pair of homologous chromosomes

Crossing over

occurs between the maternal and paternal chromosomes.


Metaphase I

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.

Anaphase I

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

Prophase II

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.

Metaphase II

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.


Anaphase II

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.

Initiates cleavage.

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).


Blastocyst formation

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

Trophoblast subdivides




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.

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.


Embryonic Period

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

Primitive pit



Inward movement of cells

Cells from epiblast detach, move from primitive streak to area between epiblast and hypoblast.

Forms mesoderm

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.

Cephalocaudal folding:

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

Neural plate

Neural folds

Neural Groove

Neural tube

Differentiation of Mesoderm

Five categories:


Paraxial mesoderm

Somites: most bone, muscle, cartilage, dermis, CT

Intermediate mesoderm

Urinary and reproductive systems

Lateral Plate Mesoderm

Cardiovascular, lining of body cavities, CT of limbs

Head Mesenchyme

CT and musculature of face

Differentiation of Endoderm

Linings of digestive, respiratory and urinary tracts.

Thyroid,parathyroid, thymus, most of liver, pancreas and gallbladder.


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.




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