Download Reproduction and Development

Reproduction and Development
Gametogenesis
Gametogenesis uses meiosis and produces haploid cells called gametes
Human Reproductive Systems
The primary reproductive organs are called gonads
Males » testes
Females » ovaries
These also secrete hormones important for the development of secondary
sexual traits
Males
Testes
These start to form in the abdominal cavity
They eventually descend into the scrotum
The sperm require the cooler temperatures in order to develop properly
Composed of many lobes
Each lobe contains two or three seminiferous tubules where the sperm
are produced
It’s all about the hormones
Spermatogenesis
Primary spermatocytes
are diploid cells,
undergo meiosis I, and
produce secondary spermatocytes
Secondary spermatocytes undergo meiosis II, each with the haploid number of
chromosomes
Semen
Sperm move from the testes into the epididymis where they mature
They travel through the vas deferens, into the ejaculatory duct, through the
urethra, and out the penis
The sperm are continually being mixed with secretions from the seminal
vesicles, the prostate gland, and the bulbourethral glands along the way to form
semen
These secretions provide nutrients, buffer, and lubricating fluids for the
sperm
Females
Ovaries
Located in the abdominal cavity
Produce both estrogen and progesterone
Oocytes are present in the ovaries
The oviduct connects the ovary to the uterus
The uterus is mainly muscle
The uterine lining is called the endometrium
The lower portion of the uterus is the cervix
The connection to the outside is the vagina
Oogenesis
Human females are born with all their eggs already present in their ovaries
These primary oocytes are arrested in meiosis I
Oogenesis occurs once per cycle
FSH triggers one primary oocyte to complete meiosis I to produce a secondary
oocyte
One secondary oocyte is produced per cycle
The other cells resulting from meiosis become polar bodies
The primary oocyte is located inside the follicle
As the follicle matures, it moves towards the wall of the ovary closest to the
oviduct (fallopian tube)
The follicle ruptures, releasing the secondary oocyte (ovulation)
The ruptured follicle develops into the corpus luteum, which secretes
hormones that prepare the reproductive tract for pregnancy and that maintain
it during the early phases of pregnancy
If fertilization occurs, then the corpus luteum continues to produce
hormones
If fertilization does not occur, then the corpus luteum degenerates
It’s really about the hormones
The female reproductive cycle is actually two cycles in one:
1. The ovarian cycle controls the growth and release of an egg
2. The menstrual cycle prepares the uterus for possible implantation of an
embryo
Hormonal signals coordinate the two cycles, keeping them in synch
If an embryo implants in the uterine wall, it will obtain nutrients from the
endometrium, and the thickened lining will not be discharged
Menstruation is uterine bleeding caused by the breakdown of the endometrium, the
blood-rich inner lining of the uterus and is a sign that pregnancy has not occurred
during the previous cycle
Fertilization
Sperm and egg have to meet
They do so in the oviduct
Development
Once fertilization has occurred, development takes over
The initial cell divisions are cleavage divisions - cell division without any major
increase in cell volume
The developing organism is called
A zygote at conception
An embryo from the zygote’s first division through the eighth week of
pregnancy
A fetus from the ninth week of pregnancy to the birth of the baby
About 6–7 days after fertilization, the embryo has reached the uterus as a fluid-filled
hollow ball of about 100 cells called a blastocyst
Protruding into the central cavity on one side of the blastocyst is a small clump
of cells called the inner cell mass, which will eventually form the fetus
The blastocyst will implant in the endometrium
About 9 days after conception, the cells of the early embryo begin an organized
migration that produces the gastrula, with three main layers
1. The ectoderm eventually develops into the nervous system and outer layer of
skin (epidermis)
2. The endoderm becomes the innermost lining of the digestive system and
organs such as the liver, pancreas, and thyroid
3. The mesoderm gives rise to most other organs and tissues, such as the heart,
kidneys, and muscles
During the pregnancy, the placenta develops to protect, serve, and nourish the
embryo
The three trimesters of human pregnancies are marked by significant events
First trimester
About 5 weeks after fertilization, the embryo has developed a brain and spinal cord,
four stumpy limb buds, a short tail, and primitive gill-like structures
Overall, a month-old human embryo looks pretty much like a month-old
embryo of any other vertebrate species
About 9 weeks after fertilization, the embryo is called a fetus
It has a clear sac, the amnion, around it
It has all of its organs and major body parts, and the embryo’s limb buds have
become tiny arms and legs with fingers and toes
The sex of the fetus can be determined by an ultrasound exam
Second trimester
The main developmental changes during the second and third trimesters involve an
increase in size and general refinement of the human features
A 14-week-old fetus is about 6 cm (2.4 inches) long
At 20 weeks, the fetus is about 19 cm (7.6 inches) long, weighs about half a
kilogram (1 lb), and has the face of an infant, complete with eyebrows and
eyelashes
The fetal heartbeat is detectable with a stethoscope, and the mother can usually
feel the fetus moving
Third trimester
The third trimester is a time of rapid growth as the fetus gains the strength
At birth, a typical baby is about 50 cm (20 inches) long and weighs 3–4 kg (6–8
lb)
Childbirth
Near the end of the third trimester, birth occurs
Occurs due to a series of rhythmic contractions of the uterus (labor)
Estrogen is a key hormone
It leads to an increase in the number of oxytocin receptors on the uterus
Oxytocin and prostaglandins stimulate the uterine contractions