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Sexual reproduction (XI. 4) - Coggle Diagram
Sexual reproduction (XI. 4)
Oogenesis and spermatogenesis
Spermatogenesis
Most mature sperm cells detach from the germinal epithelium and reach the inner part of the seminiferous tubule
In the walls of the tubule, Sertoli cells are present, that act as nurse cells
The outer layer of seminiferous tubules is called germinal epithelium, where sperm production begins
Germinal epithelium cells divide by mitosis to produce diploid cells. These cells grow and become primary spermatocytes
Testes are made of seminiferous tubules, with gaps between them called interstices (made of intersticial/Leydig cells)
The primary spermatocyte carries out the first division of meiosis and becomes a secondary spermatocyte
The production of sperm in the testes
The secondary spermatocyte carries out the second meiotic division and produced two spermatids
The spermatids becomes associated with the Sertoli cells that help it develop into a spermatozoon
The spermatozoon detaches from the Sertoli cell and is carried out of the testis in the liquid inside the seminiferous tubule
Oogenesis
At birth, there are approx. 40000 primary follicles (the cell that started meiosis with its follicle cells and no more are produced)
During the menstrual cycle a small number of primary follicles are stimulated by FSH to develop into a mature follicle containing a secondaty oocyte
When the fetus is 4-5 months old, these cells start dividing by meiosis, in the 7th month they stop in the first meiotic phase. They will start again during puberty
Germ cells in the fetal ovary divide by mitosis
The production of eggs in the ovary, that starts in the female fetus
The outconmes of oogenesis and spermatogenesis
The second meiotic division produces again one large cell and the second polar body, that is discarded
Egg formation occurs each menstrual cycle and usually only one egg per cycle is produced
The first meiotic division produces one large cell and the first polar body, that is discarded
Spermatozoa are produced continuosly, starting from puberty
After each meiotic division, four spermatids are obtained, reduing the cytoplasm. Eggs instead have to increase it
A spematozoon consists of a haploid nucleus, a system for movement and enzymes to enter the egg
Fertilisation
The acrosome reaction esposes the tip of the sperm, this contains proteins that can bind to the egg membrane
The sperm and egg membranes fuse together and the sperm nucleus enters the egg, fertilisation has occurred
The egg is coated with zona pellucida, made of glycoproteins, and it is digested by sperm enzymes contained in the acrosome (a sac in the sperm's head)
Only one sperm fertilises the egg, therefore polyspermy has to be avoided
Receptors on the sperm membrane detect chemicals released by the egg, allowing swimming towards the egg
The enzymes in the cortical granules digest binding proteins, preventing any more sperm cells to bind. They also harden the zona pellucida
Fertilisation is the union of an egg and a spermatozoon to produce a zygote
The sperm activates the egg, that releases the contents of cortical granules (vesicles near the membrane)
The beginning of pregnancy
Further, unequal divisions create a blastocyst (shaped like a hollow ball), this reaches the uterus after 7 days
The zona pellucida breaks down, and the egg has consumed all of its resources and needs external supplies
The fertilised egg cells divides by mitosis four times, yielding a four-celled embryo in 48 hours
Through a process called implantation, the blastocyst (thanks to newly developed finger-like projections) sinks into the endometrium of the uterus
The embryo starts absorbing oxygen and food from the mother's blood
By the 8th week it starts developing bone tissue and shows a human shape, it is now a phoetus
Role of hCG
This hormone stimulates the corpus luteum to continue producing estrogen and progesterone
Early in pregnancy the embryo produces human chorionic gonadotropin (hCG)
The maintainance of the endometrium depends on the continuous production of estrogen and progesterone
The placenta
Maternal blood flows in the inter-villous spaces around the villi
The functional unit of the placenta is a finger-like piece of fetal tissue called a placental villus
Fetal blood circulates in blood capillaries, close to the surface of each villus
The placenta is made of fetal tissues, the fetus also develops the amniotic sac that contains amniotic fluif
The cells that separate maternal and fetal blood form the placental barrier
The placenta is needed because the body surface area to volume ratio becomes smaller as the fetus grows larger
By the 9th week of pregnancy, the placenta starts to secrete estrogen and progesterone
The corpus luteum is no longer needed, but this switch of roles may lead to miscarriage
Parturition
Oxytocin stimulates contractions of the muscle fibres in the myometrium
These contractions are detected by stretch receptors, which signal to the pituitary gland to increase oxytocin secretion
At the end of the pregnancy, the phoetus releases hormones that stop the secretion of progesterone
Relaxation of muscle fibres in the cervix causes it to dilate
Progesterone inhibits secretion of oxytocin by
the pituitary gland and inhibits contractions of the myometrium (outer uterine wall)
Uterine contractions burst the amniotic sac and the amniotic fluid is released