There is an interactive of the stages of embryo development at the Endowment for Human Development site.
Three layers of cells are formed by the cells in the blastula.
During gastrulation, the blastula folds in on itself and cells migrate to form three layers of cells in a structure, the gastrula, with a hollow space that will become the digestive tract.
Each of the layers of cells is called a germ layer.
The cells in the blastula rearrange themselves to form the germ layers.
There are three germ layers.
The nervous system is one of the tissues given rise to by the ectoderm.
The muscles and tissue in the body are created by the mesoderm.
The gut has many internal organs.
The formation of the germ layers in the animal's body is a result of gastrulation.
The process of differentiation leads to the development of organs.
The ultimate cell type of the embryonic stem cells will be determined by specific sets of genes.
The genes specific to skin cells will be expressed by some cells in the ectoderm.
The shape and characteristics of epidermal cells will be taken on by these cells.
The process of differentiation is regulated by chemical signals from the cell's embryonic environment that set in play a cascade of events that regulates gene expression.
The adaptation for reproduction in humans is complex and involves specialized and different anatomies in the two sexes, a hormone regulation system, and specialized behaviors.
Testosterone causes the primitive gonads to differentiate into male sexual organs.
The primitive gonads are created when testosterone is absent.
There are tissues that produce a penis in males and clitoris in females.
The scrotum in a male will become the labia in a female.
There was a divergence in the development of what were once common embryonic structures.
The testes are external to the body so that a correct temperature can be maintained.
In land mammals, the environment of the sperm is about 2 degrees cooler than the body temperature to produce viable sperm.
If the testes don't descend during fetal development, the fertility of the individual will be reduced.
sperm and reproductive hormones are produced by the testes.
Each testis is approximately 2.5 by 3.8 cm and divided into wedge-shaped lobes by septa.
Seminiferous tubules are found in each wedge.
The vagina has an ejaculation after the organ is inserted.
During orgasm, the accessory organs and glands connected to the testes contract and empty the semen into the urethra and the fluid is expelled from the body by muscular contractions.
The penis becomes flaccid after blood is drained from it.
A flagellum for motility, a neck that contains the cell's energy producing mitochondria, and a head that contains the genetic material are found in sperm.
The protective coverings surrounding the egg allow the sperm to connect with the egg.
A ejaculate will contain between two and five liters of fluid and between 50 and 120 million sperm.
Human sperm has a flagellum, neck, and head, as seen in this scanning electron micrograph.
The sperm cells that make up the walls of the seminiferous tubule are the ones that are developing.
The epididymis lies along the top and bottom of the testes and is the site of sperm maturation.
A section of the vas deferens is removed during a vasectomy, preventing sperm from being passed out of the body during ejaculation and preventing fertilization.
The semen comes from the male reproductive system.
The sperm have important compounds from the accessory glands.
There are factors that affect sperm delivery.
The human male's reproductive structures are shown.
The vas deferens carries sperm from the testes to the seminal vesicles.
The urethra is joined by the ejaculatory duct.
Female reproductive structures are outside of the body.
The human female's reproductive structures are shown.
The breasts are made of fat.
The nursing child gets milk from the ducts that are empty at the nipple, as well as the milk that is rich in vitamins and minerals.
The uterus, vagina, and ovaries are internal female reproductive structures.
There is a system of ligaments that holds the ovaries in place.
The outer layer of the ovary is made up of follicles, each consisting of one or more cells that surround, nourish, and protect a single egg.
Follar cells develop and prepare their eggs for release during the menstrual period.
One egg is released at ovulation.
Estradiol and progesterone are hormones that help maintain the lining of the uterus during pregnancy.
Estradiol is one of the hormones produced by the ovaries.
The fimbrae is a fringe of fingerlike projections on the ends of the oviducts.
The fimbrae help the nonmotile egg enter the tube.
There is a ciliated epithelium over the smooth muscle in the walls of the oviducts.
The smooth muscle contracts as it moves the egg toward the uterus.
The embryo is usually moved to the uterus during fertilization.
It takes the embryo a week to travel through the oviduct.
A tubal ligation is similar to a vasectomy in that the oviducts are severed and sealed, preventing sperm from reaching the egg.
The uterus has a thick muscular wall and is lined with an endometrium rich in blood vessels and mucus.
The fertilized egg will implant itself in the endometrium if the endometrium is thick.
The embryo and fetus are supported by the uterus.
The smooth muscle in the uterus helps to force the baby through the vagina during labor.
The lining of the uterus is torn off if fertilization doesn't happen.
The endometrium builds up again.
The top of the vagina has a piece of the uterus in it.
The menstrual flow can leave the body.
The pathway for the delivery of offspring and the receptacle for the penis are what it is.
The process of meiosis is involved in the production of sperm and eggs.
During meiosis, two nuclear divisions separate the pairs of chromosomes in the nucleus and then separate the chromatids that were made during an earlier stage of the cell's life cycle.
haploid cells with half of their chromosomes normally found in diploid cells are produced by Meiosis and its associated cell divisions.
Spermatogenesis occurs in the wall of the seminiferous tubules, with the most primitive cells at the center of the tube and the most mature sperm at the end of the tube.
There are diploid, undifferentiated cells under the tubule.
Each stem cell is called a spermatogonium.
Two haploid cells called secondary spermatocytes are produced by the diploid primary spermatocyte.
Two cells called spermatids are produced by the secondary spermatocytes after meiosis II.
The spermatids eventually reach the tubule and grow a flagellum.
Four sperm result from each primary spermatocyte.
The sperm with their developing tails extended into the tubule are mapped onto the wall of the seminiferous tubule.
Oogenesis takes place in the outer layers of the ovaries.
oogenesis begins with a germ cell.
During the embryological development of the individual, this germ cell is called an oogonium.
One to two million oocytes can be produced by the time of birth.
Before birth, the primary oocytes begin meiosis.
In the first prophase stage, the meiotic division is arrested.
Future eggs are in prophase I at the time of birth.
The male reproductive system in which sperm are produced continuously throughout the life of the individual is different.
Each month, anterior pituitary hormones cause the development of a few follicles in an ovary.
The primary oocyte finished the first meiotic division.
The cell divides into two parts, with the main part going to one cell and a small part going to the other.
A polar body is a second cell.
The cell division was arrested at metaphase II.
The secondary oocyte is released at ovulation and travels to the uterus.
If the secondary oocyte is fertilized, the cell continues through meiosis II, producing a second polar body and haploid egg, which combines with the haploid sperm to form a fertilized egg (zygote) containing all 46 chromosomes.
The human male and female reproductive cycles are controlled by hormones from the hypothalamus and anterior pituitary with hormones from reproductive tissues and organs.
In both genders, the hypothalamus causes the release of hormones.
Although these hormones are named after their functions in female reproduction, they are produced in both sexes and play important roles in controlling reproduction.
The male and female reproductive systems use hormones.
The release of FSH and LH into the male system is caused by the hypothalamus at puberty.
Sertoli cells are located in the walls of the seminiferous tubules and are stimulated by FSH.
Leydig, located between the walls of the seminiferous tubules, is where testosterone is made and released into the testes and the blood.
Secondary sexual characteristics that develop in the male during adolescence are caused by this hormone.
The growth of facial, axillary, and pubic hair, an increase in muscle bulk, and the beginnings of the sex drive are some of the secondary sex characteristics in males.
A negative feedback system occurs in the male with rising levels of testosterone acting on the hypothalamus and pituitary to prevent the release of GnRH, FSH, and LH.
This will cause spermatogenesis to slow down.
The sperm count increases if the Sertoli cells stop releasing inhibin.
The female reproductive cycle is divided into two parts, the ovarian cycle and the menstrual cycle.
The average length of these cycles is 28 days.
The GnRH from the hypothalamus causes the release of hormones from the anterior pituitary.
Estradiol is responsible for the secondary sexual characteristics of females.
Breast development, flaring of the hips, and a shorter period for bone growth are included.
The ebb and flow of hormones causes the ovarian and menstrual cycles to advance.
Ovarian and menstrual cycles occur at the same time.
The first half of the ovarian cycle is called the follicular phase.
The growth of follicles on the surface of the ovary is caused by rising levels of FSH.
The egg is prepared for ovulation.
Estradiol is released as the follicles grow.
The first few days of this cycle coincide with menstruation.
After about five days, the menstrual cycle begins to proliferative.
The endometrium begins to grow again after the end of the last cycle.
The hormones produced by the hypothalamus, pituitary, and ovaries regulate the menstrual and ovarian cycles of women.
During the middle of the cycle, the high level of estrogen causes FSH and especially LH to rise and fall.
The egg is released when the most mature follicle breaks.
The eggs that did not break are lost.
The level of estrogen goes down when there are more follicles.
The ovarian and menstrual cycles run from about day 15 to 28.
The secretory and luteal phases are related.
A structure called a corpus luteum is created by the cells in the follicle.
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The regrowth of the uterus is aided by the regrowth of the progesterone.
Should a fertilized egg occur, the uterus is prepared to accept it.
The inhibition of FSH and LH prevents the development of any more eggs and follicles.
For the next few days, the level of estrogen produced by the corpus luteum increases.
The levels of estrogen and progesterone decrease if there is no fertilized egg in the uterus.
The next menstrual cycle begins as the endometrium begins to decline.
The hypothalamus can send GnRH to the pituitary if there is a decrease in progesterone.
A reproductive endocrinologist is a doctor who deals with a variety of hormonal disorders related to reproduction and infertility in both men and women.
There are a number of disorders, including menstrual problems, infertility, and pregnancy loss.
Doctors can use fertility drugs, surgery, or assisted reproductive techniques.
In order to become a reproductive endocrinologist, you have to go through a four-year residency in OB/Gyn and a three-year fellowship in reproductive endocrinology.
The physician needs to pass written and oral exams in order to be board certified.
An egg is fertilized and the baby is born.
Within 24 hours of fertilization, the egg nucleus has finished meiosis.
The cell is called a zygote with fusion.
The embryo goes through the oviduct to the uterus.
If the embryo doesn't implant into the uterus within seven days, it will die.
The outer layers of the developing embryo or blastocyst grow into the endometrium, and healing of the endometrium closes up the blastocyst into the tissue.
The endometrium of the uterus is maintained with adequate levels of progesterone.
The level of b-HCG in urine is determined by a pregnant woman.
The test is positive if the hormone is present.
The periods are divided into three equal periods.
Nutrition and waste are handled through the lining of the uterus during the first two to four weeks of the baby's life.
The embryo begins to form the endometrium and the placenta as the trimester progresses.
Chemicals from the fetus are removed from the mother's body.
Passive immunity is provided by the mother's immunoglobulins passing through the placenta.
Body structures begin to develop during the first trimester.
By five weeks, the limbs, eyes, heart, and liver have been formed.
Many of the organs, such as the lungs and liver, are not yet functioning because the individual is five centimeters in length.
All of the body's organs and structures are going through initial development during the first trimester, so exposure to any toxins is especially dangerous.
The survival of the fetus can be affected by interference with chemical signaling.
The fetus grows to about 30 cm during the second trimester.
The mother feels the first movements when it becomes active.
The structures and organs continue to grow.
The functions of nutrition and waste elimination have been taken over by the placenta.
The baby will be delivered with the help of the placenta.
The most rapid growth occurs during this time as all organ systems continue to grow.
You can see the stages of human fetal development on this website.
The labor is the process of expelling the fetus from the uterus.
Estradiol causes the uterus to develop and bind the hormone oxytocin at the end of the third trimester.
The baby reorients, facing forward and down with the back or crown of the head engaging the uterus.
Nerve impulses are sent to the hypothalamus, which signals the release of oxytocin, when the cervix is stretched.
The smooth muscle in the uterine wall contracts.
prostaglandins are released into the uterus by the placenta.
A positive feedback relay takes place between the uterus, hypothalamus, and the anterior pituitary.
The strength and force of the contractions increase as more smooth muscle cells are recruited.
There are three stages of labor.
This is needed for the baby to be born.
The cervix will start to dilate around 10 cm.
The baby is removed from the uterus during stage two.
The uterus contracts as the mother pushes to deliver.
After the baby has been born, the organ has completely disengaged from the wall of the uterus and the last stage is the passage of the placenta.
If labor should stop before stage two is reached, synthetic oxytocin, known as Pitocin, can be administered to restart and maintain labor.
When one individual produces genetically identical offspring, or when the genetic material from two individuals is combined to produce genetically diverse offspring, it may be asexual.
Sexual reproduction in animals occurs through a number of processes.
Sexual reproduction can take place inside or outside the body.
When the sexes are combined, they may be expressed at different times in the life cycle.
The sex of an individual can be determined by a number of factors.
A sperm and an egg are combined in a fertilization process.
Outside the bodies or inside the female, this can happen.
Some species release the egg and sperm into the environment, some species retain the egg and sperm in the female body, and still other species retain the developing embryo covered in a shell.
Only one sperm and one egg can be fertilized at a time.
The blastula is formed after fertilization.
The blastula is a hollow ball of cells that undergoes a process called gastrulation.
The nervous system, the skin cells, the muscles, and the internal organs are all part of the body.
Organogenesis involves the formation of organs from the germ layers.
The germ layer gives rise to different types of tissue.
The reproductive structures that evolved in land animals allow males and females to mate, fertilize internally, and support the growth and development of offspring.
Spermatogenesis and eggs are produced through the process of meiosis.
The male and female reproductive cycles are controlled by hormones.
The need for FSH and LH production is monitored by the hypothalamus.
The formation of sperm and the preparation of eggs for release are caused by FSH and LH.
Sertoli cells and Leydig cells in the testes are stimulated by FSH and LH.
THe Leydig cells produce testosterone, which is responsible for the secondary sexual characteristics of males.
Estradiol and progesterone are produced in females.
The female reproductive cycle is divided into the ovarian and menstrual cycles.
Human pregnancy begins with fertilization of an egg and progresses through the three trimesters.
The body's basic structures are laid out in the first trimester.
The development of the organs and systems continues during the second trimester.
The third trimester has the greatest growth of the fetus and ends in labor and delivery.
The labor process has three stages, each powered by hormones.
The sperm comes from the testes to the vas deferens.