The development of each new generation begins within a woman's body.
It is here that two living cells, the egg and sperm, will unite to form one cell, the zygote.
The fertilized egg carrying a unique complement of genes from the father's sperm and the mother's egg
then commences a program of development which will convert this one cell into a complex organism.
Within nine months the egg will divide repeatedly, forming a large number of cells
which will follow precise and sequential patterns of movement and differentiation
resulting in the formation of a human being.
This process of development begins within the pelvic region where the reproductive organs are located.
A closer look reveals the ovary where egg maturation occurs and the uterus with its outstretched fallopian tube or overduct.
The flat end of each fallopian tube, the ampulla, lies close to the ovary.
Each month during a woman's reproductive years egg maturation occurs in the ovary.
In this organ lie thousands of tiny follicle units each containing an egg cell known as a primary oocyte
surrounded by follicle cells.
These oocytes have ceased to divide by mitosis and have been arrested at the metaphase stage of the first meiotic division.
This transition from mitotic to meiotic division of egg cells occurs in a woman's ovary before her birth
so that these primary oocytes have been quiescent at least 12 or 13 years at the onset of puberty.
After puberty a few follicles mature each month until the onset of menopause some 40 to 45 years later.
While the development of several follicles is initiated each month, usually all die except one by the time of ovulation.
Assuming an average menstrual cycle of 28 days, counting from the first day of menstruation,
egg maturation takes place during the first 14 days of the cycle.
The various stages in the entire process will be shown on the ovary in the drawing.
In stage A and B, lasting 7 days, the primary oocyte enlarges and the surrounding follicle cells begin dividing.
Gradually, on day 8, a fluid-filled cavity, the antrum, forms within the follicle layer of cells as shown in C.
The enlarging follicle approaches the ovarian surface as seen in D, and during the final maturation process it bulges from the side of the organ.
At stage E, the ripe follicle is shown on day 14 just before it ruptures to expel the egg.
This oocyte has just completed the first meiotic division.
The follicle and the egg here are drawn very much enlarged to illustrate developmental events.
In reality, the diameter of the follicle is approximately one centimeter, and the egg is only one-tenth of a millimeter in diameter.
The follicle surrounding the oocyte contains a large fluid-filled antrum.
A combination of hydrostatic pressure from this fluid and enzymes from the follicle cells which weaken the ovarian wall result in rupture of the follicle and release of a secondary oocyte.
This release is called ovulation.
The secondary oocyte released from the ovary is drawn into the ampulla of the fallopian tube by currents created by the beating of thousands of hair-like cilia.
Following the expulsion of the egg from the ovary, the follicle again closes and seals.
The follicle then becomes known as the corpius luteum and secretes steroid hormones, estrogen, and progesterone.
These hormones signal the uterine lining to thicken in preparation for the entry of an embryo into the uterus.
The process of ovulation will now be repeated to emphasize important events.
The egg will be slowly propelled by cilia all along the tube towards the uterus, a journey which will require approximately five days.
However, the egg will survive an average of only 24 hours unless it is fertilized.
If fertilization is to be successful, sperm must reach the upper end of the fallopian tube.
Sperm are propelled through the reproductive tract by the thrust of their own motile tails and in the fallopian tubes by peristaltic contraction of the tube walls.
Of the millions of sperm released into the vagina, only a few hundred will reach the upper end of the fallopian tube.
When they encounter the egg, they stick to the outer surface.
Once the membrane of a sperm head contacts the egg cell membrane, the sperm is drawn into the egg and all further penetration by other sperm is prevented.
The first division occurs 18 to 24 hours after fertilization as the zygote slowly progresses along the interior of the fallopian tube.
As with each subsequent division, the number of cells doubles.
Because the embryo does not increase in size with each division, the resulting cells are small.
The solid ball of cells produced, known as a murula, begins to hollow out.
The cavity forms only in the center of the cell ball, but it is shown here as if the outer cells were transparent.
The hollow sphere is now called a blastocyst.
A mass of cells accumulates at one edge.
This pile of cells forms the inner cell mass and it is from these cells that the human body will form.
The remainder of the sphere, the trophoblast, will form the embryonic component of the placenta.
Four days have passed now since the egg was fertilized and the blastocyst is approaching the uterus.
This drawing, the size of the egg and the diameter of the fallopian tube, is enlarged for clarity.
On the fifth day after fertilization, the blastocyst enters the uterine cavity.
As a result of this growth and the erosion of the uterine tissue, the blastocyst penetrates the uterine lining.
This process is known as implantation.
By using a higher magnification to watch this process, the details of the enlarging blastocyst can be seen.
This view shows how the cells have arranged themselves.
Keep in mind the mass of tissues you will see develop consists of accumulations of many, many cells, although they will not be shown.
Within less than one week the embryo buries itself completely within the uterine wall.
The outer trophoblast develops into a syncytium where many nuclei are found within the cytoplasm of the large cells.
This is the syncytiotrophoblast.
The inner layer of the trophoblast, the cytotrophoblast, directly surrounds the inner cell mass and the trophoblaster cavity.
The implanted embryo is now completely surrounded by uterine lining.
The erosion caused by the invading syncytiotrophoblast has ruptured tiny blood vessels in the area of the embryo so that it is surrounded by pools of maternal blood.
A cavity appears within the inner cell mass and gradually enlarges.
This is the amniotic cavity.
A portion adjacent to that trophoblast is the amnion and the thicker interior region is known as the embryonic disc.
These cells of the embryonic disc eventually will form the body of the human embryo.
These cells separate into three distinct layers, the ectoderm, mesoderm and endoderm.
The separation of the three primary germ layers is called gastrulation.
By the tenth day after fertilization, the cells forming the endoderm begin to appear as a distinct layer at the periphery of the inner cell mass.
This migration of cells from within the embryonic disc and their aggregation to form a separate endoderm is known as delamination.
Delamination of the endoderm is the first part of gastrulation in humans.
The layer of endoderm continues to grow and gradually forms a fluid-filled sac, the yolk sac.
The name of this structure is misleading because the sac contains no yolk and does not function in the nutrition of the embryo.
After delamination of endoderm cells from the germinal disc, the remaining layer becomes known as the epiblast.
The epiblast contains cells which will become the ectoderm and mesoderm.
The separation of these two layers is the second part of gastrulation and occurs by primitive streak formation.
Cells of the epiblast will migrate toward the posterior midline to form a long pile called the primitive streak.
They will begin to drop below the epiblast and accumulate between the epiblast and endoderm.
The cells which pass through the streak will form the red mesoderm.
Those which will remain on the surface will be the ectoderm.
The growing mesoderm has split as it expanded.
One layer has migrated over the surface of the yolk sac while the other surrounds the amnion.
Mesoderm adjacent to the endoderm is splanctic mesoderm and is destined to become highly vascularized.
The combination of endoderm and splanctic mesoderm yields a tissue known as splanchnoplur.
Mesoderm which surrounds the amnion is called somatic mesoderm.
Ectoderm and adjacent somatic mesoderm together form somatoplur.
The somatic mesoderm now lines the inner surface of the cytotrophoblast.
At this stage the embryonic structure consists of four parts.
The trophoblast which will form the extraembryonic placenta.
The yolk sac and extraembryonic splanchnoplur where both the blood cells and the germ cells will originate.
The amnion and extraembryonic somatoplur surrounding a fluid filled sac that will form a hydrostatic cushion during later development.
And the triliminar sheet of ectoderm, mesoderm and endoderm which will form the body of the embryo itself.
After the movement of mesoderm through the primitive streaks ceases, the structure disappears and gastrulation is complete.
The three distinct germ layers are now formed and in the region of the embryo proper lies a triliminar sheet.
Ectoderm in blue, mesoderm in red and endoderm in yellow.
Along the midline the mesoderm segregates as a long rod, the notochord.
This will become an early embryonic supporting structure along the back.
Flanking the notochord along each side the mesoderm condenses to form a series of paired segments, the somites.
These will form the major muscles lying parallel to the spine and the bones of the spine itself.
Biochemical signals from the notochord and the somites induce the overlying ectoderm to form the central nervous system.
Along both sides of the dorsal midline, the region which will be the future back of the embryo, two ridges of ectoderm make their appearance.
These are the neural ridges or folds.
As they increase in size the folds extend towards each other and make contact all along the back from the future head to the future rump of the embryo.
Remember that you are viewing this process here in cross section at a point about half way down the back.
Once the folds have made contact they fuse, closing off a tube of ectoderm below the surface.
This is the neural tube and it will form the central nervous system.
The surface ectoderm above the neural tube will form the skin of the embryo's back.
Between the skin and the neural tube an important group of ectoderm cells originate.
They're too small and too many to show here correctly.
These are the neural crest cells which were located at the fusion point of the neural folds.
They remain separate below the skin but above the neural tube.
They remain here only briefly and then begin a rapid proliferation and extensive migration.
Eventually these cells will form such important and diverse tissues as the spinal gangula and the peripheral nervous system,
the adrenal medulla, pigment cells in the skin and connective tissue in the head.
To observe the embryo lengthwise it will first be rotated to reveal the profile of the hemisphere.
The uterine tissue and syncytiotrophoblast have been removed as we will concentrate on embryo development and identification.
A sagittal cut along the midline separates the hemispheres in two.
The sagittal view is then completed by filling in the remaining portion of the section.
Much of the embryonic structure is spherical so it looks quite similar in a cross section or sagittal section.
The embryo itself however is now seen lengthwise with the anterior or head end to the left and the posterior or tail end to the right.
The entire triliminar sheet of ectoderm, mesoderm and endoderm will begin to lift and fold to form a distinct embryonic body.
Observe also that a posterior outpouching of splenknoplur will gradually penetrate an area known as the body stalk.
This outpouching is the allantois, another important extraembryonic membrane.
While these internal changes are taking place the trophoblast structure becomes more elaborate forming a chorion or embryonic placenta.
The extension of numerous villi provide a greater surface area for exchange of nutrients, gases and wastes between the blood of the mother and the blood vessels of the embryo.
As the embryo develops the exchange requirement increases and the surface area of the chorionic villi will be seen to increase to meet the need.
As the embryo folds the yolk sac is slowly pinched off while the amnion expands to surround the embryo body.
The connecting body stalk maintains the contact between the embryo and the surrounding trophoblast.
The endoderm which forms a blind tube in the head region is the foregut, its smaller counterpart in the posterior region is the hindgut.
When the folding process nears completion the connection between the yolk sac and the embryonic gut narrows to form the yolk stalk.
The yolk sac now lies outside the embryo's body and therefore is clearly an extraembryonic membrane.
The other extraembryonic membranes are the allantois, a splenknoplue extending into the body stalk.
The endodermal lining of the allantois remains insignificant in the body stalk but the important allantois mesoderm has extended through the stalk into the chorion providing blood vessels for circulatory exchange there.
The amnion, also an extraembryonic membrane, consists of a somatopleur providing a fluid filled sac around the embryo
and the trophoblast with its mesodermal lining which forms the embryonic placenta or chorion where nutrient, gas and waste exchange occurs with the mother.
Lifting off of the embryo has brought the fold of the amnion into a position to surround the yolk stalk and body stalk with the enclosed allantois.
This broad connecting structure between the embryo and the chorion is known as the umbilical cord.
In the mesoderm beneath the foregut formation begins with a hollowing out of two tubes, the heart primordia.
At this stage, four weeks after fertilization, the body pattern of the human embryo can be clearly distinguished.
The folding of the trilaminar sheet has resulted in a tube-within-a-tube body plant with an outer tube containing an inner tube of splenknoplue.
The inner tube which will form the digestive system is still continuous with the extraembryonic yolk sac and allantois.
The openings of the future mouth and anus have not yet broken through.
This view reminds you again of the position of the embryo within the uterine wall.
Continued growth of the embryo will soon obliterate the lumen of the uterus entirely.
The further development of the embryo will now be followed with the surface view of the body.
The yolk sac is shown here without the surrounding splenic mesoderm in order to distinguish it clearly from other mesodermal tissues.
Here at four and a half weeks, the limb buds of the arm and leg are visible.
The presence of rubella virus or drugs such as stalidomide at this stage often arrests further limb development and have also severe internal consequences.
Folds in the neck region are visceral arches, homologous to the gill bars of aquatic organisms.
The otic placode is visible adjacent to the hindbrain and the impression of the mesodermal somites is apparent under the skin along the back.
The eyes will appear on the side of the head above the first visceral arch.
The eyes will gradually be pulled forward to the front of the face by the movement of the visceral arches.
At this stage the embryo has a distinct tail structure which will shorten and disappear before birth.
As the limbs extend, flattened petals form at the distal ends.
By programmed cell death the petal is transformed into a hand or foot with five separate digits.
The face is recognizable as a human by fourteen weeks of development.
The actual size of the embryo is now five to ten centimeters long.
The position of the embryo in the uterus and the internal anatomy of the mother can now be seen at twenty-four weeks.
During the remaining weeks of pregnancy the growing fetus expands and stretches the uterus.
The mother's abdomen enlarges to accommodate the space required by the developing infant.
Cushioning of the baby's body is provided by the fluid encased in the anion.
As term approaches the infant usually assumes a position in the uterus with the head adjacent to the birth canal or vagina.
The film will now be reviewed again without narration.
Try to mentally narrate the events yourself as they occur.
Thank you for watching!
Thank you.
Thus we have followed the reproduction of the human species.
Complex physical growth and the movements of cells and tissues have been traced.
The underlying biochemical explanations for these complicated developments remains largely unknown.
The full understanding of the normal process of human development is the goal of considerable research worldwide.
The eventual benefits of these studies will include prevention of birth defects caused by organisms, chemicals or genetic influences,
and contribute to our knowledge of how the body controls and regulates its components.
When this regulation fails, as in the case of cancer and immune diseases,
this knowledge is essential to successful medical intervention.