Fertilization and Development Part III

Production of Sperm testicle fluids are removed and the sperm concentrated sperm mature here and develop the ability to swim Seminal vesicles: adds nutrients that include fructose sugar for respiration mucus to protect sperm in the cell Prostate: Prostate adds fluids that neutralise the vaginal acids and minerals ions mineral ions

Process of Fertilization

At the moment of ovulation, the secondary oocyte is surrounded by a zona pellucida and a corona radiata. Zona pellucida refers to the glycoportein membrane surrounding the secondary oocyte of mammals and is secreted by the ovarian follicle cells. Corona radiata is the name of the layer of follicle cells surrounding the zona pellucida The acrosome of the sperm cell contains proteolytic enzymes.

Cortical Reaction

As the sperm touches the cells of the corona radiata, the membrane around the acrosome fuses with the membrane of the cells, releasing the proteolytic enzymes and digesting the cell. This is called acrosomal reaction. The head of the sperm cell can now penetrate this layer.

-- The sperm cell then reaches the zona pellucida which has special receptors to bind the sperm cell and the sperm cell will pass through. The head of the sperm cell will fuse with the membrane of the secondary oocyte. When the two gametes fuse their plasma membranes, the cortical graneles fuse with the oocyte’s cell membrane and release their enzymes to the outside. The cortical granules will release an enzyme to thicken the zona pellucida so that it becomes a fertilization membrane. This prevents polyspermy.

--- The reaction of the cortical granules is also called cortical reaction. It takes place within a few seconds of the first sperm entering. The secondary oocyte now completes meiosis II and produces another polar body. The resulting fertilized ovum is now called as a zygote.

Pregnancy Zygote begins mitotic division within 24 hours after fertilization. As a result of cleavage a balstocyst is formed and is characterized by

Trophoblast will become the fetal portion of the placenta Inner cell mass will become the embryo A fluid filled cavity. The embryo enters the uterus and begins implantation about a week after fertilization. The embryo begins the secretion of its own hormone. The Human Chorionic Gonadotrophin (HCG)

HCG The trophoblastic cells secrete HCG It enters the bloodstream of the mother; its target tissue is the corpus luteum Maintains the sescretory function of corpus luteum. Corpus luteum secretes estrogen, progesterone and endometrium, in which the early embryo is embedded. Later in pregnancy the role of estrogen and progesterone is taken over by the placenta and corpus luteum is no longer necessary. HCG is excreted in the urine and can be detected with a pregnancy test

Placental Hormones and Pregnancy During early pregnancy the endometrium is maintained by a secretion of hCG from the embryo. Later the production of progesterone's is taken over by the placenta itself. Note that the concentrations rise throughout the gestation.

Early embryo development upto the implantation of the blastocyst

Cleavage After fertilization, the zygote undergoes the first mitotic division. This is a division, which is not followed by cell growth and is therefore called a cleavage division. Several of these cleavage divisions occur, which leads to the formation of a solid ball of cells called the morula. In humans morula reaches the uterus about four days after fertilization. This is followed by unequal divisions and results in a fluid filled space in the middle. The structure now is called a blastocyst and will implant in the endometrium (7 days after fertilization)

The cells on the outside of the blastocyst are called the trophoblast. The trophoblast will embed in the endometrium in the process of implantation. It will form trophoblastic villi into the endometrium to absorb nutrients from it. This is sufficient for two weeks and after which the placenta takes over.

Placenta The placenta forms From the trophoblast.

Placenta When the placenta is fully formed, two fetal blood vessels within the umbilical cord carry fetal blood to the placenta. The blood within these blood vessels is deoxygenated and carries waste products. This fetal blood exchanges materials with the maternal bloodstream and another fetal blood vessels within the umbilical cord and returns the blood to the fetus.

-- Materials passed from the fetus to the mother within the placenta: carbon dioxide, urea, water, and hormones (HCG) Materials passed from mother to fetus: oxygen, nutrients, (glucose, amino acids), water, hormones, vitamins, minerals, alcohol, drugs, nicotine, viruses such as HIV German measles. Blood of the fetus and the blood of the mother do not mix. Placenta serves as an endocrine organ especially during the second part of pregnancy.

The foetus develops and grows using materials obtained by exchange across the placental wall from mother to child. Excretory products are exchanged in the opposite direction from child to mother.

Materials are exchanged between the maternal and fetal blood in the placenta The fetus needs a good supply of nutrients and also needs to excrete waste products. Both of these are carried out by the placenta. The placenta is fetal tissue which invades maternal uterine tissue. The baby’s blood runs through blood vessels which go through spaces filled with maternal blood. Exchange of substances takes place by diffusion The fetal blood returns to the fetus enriched with nutrients and oxygen. The maternal blood has taken up the carbon dioxide and other metablolic waste products from the fetus, which it will exrete.

Placenta

Role of amniotic fluid Amniotic membrane is an extra embryonic membraneA fetus is surrounded by amniotic fluid which, in turn is surrounded by the amniotic sac. The amniotic sac keeps the fluid from leaking out and protects the fetus against infections. The amniotic fluid buffers shocks and protects the baby from mechanical harm. Provides an environment in which fetus has free movement and therefore well balanced exercise for all developing muscles and skeleton. Providing excellent thermal stability (amniotic fluid is mainly water and therefore has excellent temperature stability) Babies urinate in it It is constantly made and filtered by the mother.

Process of birth Normally, approximately 38 weeks after conception, the fetus is ready to be born. At the end of gestation it is believed that there is a drop in the high levels of progesterone. With the fall in progesterone the pituitary secretes this small polypeptide called oxytocin. Stretching of the lower uterus walls by the foetus and its production of prostaglandin's add to the stimulus for the pituitary to secrete oxytocin. The oxytocin causes the smooth muscle in the walls of the uterus to contract and labour has begun.

After nine months in the uterus the foetus is fully grown and takes up all the space available. These cramped conditions push the baby down stretching the lower walls of the uterus. This sends impulses to the mother brain. The foetus responds to the cramped conditions by producing hormones from the placenta (prostaglandin) which causes myometrial contraction Progesterone is the hormone of pregnancy and at this stage the high levels of this hormone become less active. All these changes stimulate the secretion of oxytocin from the pituitary and this causes the myometrial contractions of labour

Child Birth Positive feedback: In this system the stimuli to the brain increases the oxytocin production In turn the oxytocin stimulate myometrial contraction Myometrial contraction further stimulates the pituitary of the mother to release more oxytocin The strength and frequency of the myometrial contractions is further increased. In turn this further stimulates more oxytocin production The process builds with stronger and stronger contractions Final the child passes though the cervix and vagina to be born Contractions continue for a further period until the placenta is delivered (after birth).

Child Birth