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Formation of Sex Cells
Spermatogenesis
Sperm are formed in semeniferous tubules of the testes by meiosis. Stem cells or spermatogonia in the semeniferous tubules divide forming primary spermatocytes. These cells have 46 chromosomes. This is the chromosome number found in the body cells and is called diploid or 2n. Sperm formation begins when the primary spermatocyte divides to form two secondary spermatocytes. During this division, the chromosome number is reduced by half. The resulting cells have 23 chromosomes, the haploid number (n). Next each secondary cell divides to form two spermatids. The spermatids cannot divide. Instead, they transform into sperm cells. The sperm leave the semeniferous tubules for the epididymus where they remain for a period of time to mature.
Oogenesis
At the time of her birth, all of the stem cells or oogonia in the ovaries of the female divide to form primary oocytes. As a result, the female is born with a limited number of gamete forming cells. During childhood up to puberty, these oocytes are dormant. At puberty, a group of primary oocytes begins to grow under the influence of the pituitary hormone follicle stimulating hormone (FSH). This is day one of the ovarian cycle. At day fourteen, the oocyte is released from the ovary. This process is called ovulation and is due to the action of the hormone luteinizing hormone (LH) When the follicle ruptures, the primary oocyte undergoes the first division of meiosis forming a secondary oocyte and a polar body. The first division of meiosis reduces the oocyte's chromosome number by half. Since the chromosome number in the primary oocyte is 46, the secondary oocyte has 23 chromosomes. The secondary oocyte is released from the ovary surrounded by follicle cells forming the corona radiata. Usually the oocyte is drawn into the opening of the fallopian tube by movements of the finger like fimbriae and the ciliary action of the lining of the fallopian tube.
First Two Weeks of Development
Fertilization - is a series of events which take place in a swollen portion of the oviduct called the ampulla. Fertilization includes all the events from sperm contact with the corona radiata (follicle cells) to fusion of the male and female nuclei.
1. Sperm activation includes the final maturation of sperm and the release of acrosomal enzymes.
2. Oocyte activation involves the unloading of vesicles from the secondary oocyte cytoplasm. This material forms a barrier around the oocyte to prevent polyspermy - more than one sperm fertilizing the egg cell.
3. The secondary oocyte finishes the second meiotic division producing an ovum (mature egg cell) and a polar body. The polar body formed during the first meiotic division divides also This results in one ovum and three polar bodies forming from one primary oocyte during meiosis.
4. The acrosome of the sperm head releases several enzymes which permits the sperm nucleus to enter the ovum.
5. The male nucleus and female nucleus fuse reestablishing the typical chromosome number of 46. The fertilized egg is referred to as the zygote.
Cleavage - occurs about 30 hours
after fertilization resulting in two identical cells. Cleavage divisions
continue for about 3 days producing a solid ball of 16 cells called a morula
(mulberry). During this cleavage period, the pre-embryo has moved from the
ampulla to the entrance of the uterus.
Blastocyst - By the time the morula had entered the uterus, on day six after fertilization , it had become a hollow ball of cells called the blastocytst. The outer cells of the blastocyst form the trophoblast. These cells will provide nourishment for the embryo. The inner space is called the blastocoel. Another type of cell forms the inner cell mass which is clustered within the blastocoel. These cells will form the embryo and are referred to as embryonic stem cells because any one cell has the capacity to form any one of the body's tissues.
Implantation - From
days 7 to 10 the blastocyst attaches to the lining of the uterus and begins to
burrow into the endometrium. By day 10, the blastocyst is completely
buried. By day 10, the cells of the inner cell mass have organized
themselves into two layers (Bilaminar stage), the epiblast and the hypoblast.
Cells of the epiblast divide and migrate to form a sac, the amnion. This
sac will eventually surround the fetus and fill with fetal urine
By day 12 a third layer (Trilaminar stage) the mesoderm forms
between the epiblast and hypoblast layers. By day 14, three distinct
embryonic tissues are formed, the ectoderm (formerly epiblast), the mesoderm and
the endoderm (formerly hypoblast)
Mesoderm grows around the inner surface of the blastocyst
next to the trophoblast. A new extra-embryonic membrane called the chorion
forms from the interaction of mesoderm and trophoblast. The chorion will
later become a layer rich with blood and blood vessels - the early placenta.
Embryonic Period - Weeks 3 to 8
1. All major organ systems are
present.
2. Cardiovascular system is functional.
3. Primary ossification begins in the long bones
Fetal Period - Weeks 9 to 40
1. Brain and spinal cord grow and
mature.
2. Skin is well-formed; nails are evident
3. Liver enlarges and bile is secreted.
4. Blood formation begins in bone marrow.
5. Sex of the fetus is evident.
Fetal Period - Weeks 13 to 16
1. Cerebellum develops
2. Sensory organs develop
3. Hard palate fuses
4. Kidneys are fully formed
5. Bones are well formed with joint cavities
6 Quickening occurs - spontaneous muscular movements
Fetal Period - Weeks 17 to 20
1. Fetal position assumed
2. Limbs become properly proportioned
Fetal Period - Weeks 21 to 30
1. Significant weight increase
2. If birth occurs now, survival is possible, however:
a. body temperature regulation is not mature
b. no surfactant is being produced by the lungs
3. Two thirds of the bronchiolar branching is complete
4. There is an increase in blood formation min the bone marrow
5. There is decreased blood formation in the liver.
Fetal Period - Weeks 31 to 40
1. Skin is well formed with
distinct toe and finger nails
2. Subcutaneous fat is laid down
3. Testes have migrated to the scrotum
Birth