Chapter 29

Development and Inheritance

Fertilization:

This occurs in the fallopian tube. Millions of sperms are discharged into vagina, less than 1% of the sperms enters the fallopian tube, the rest are destroyed by vaginal acid. Fertilization can occur 12 - 24 hours after ovulation. During fertilization, a sperm fuses with the egg to form a zygote.

Sperm:

A sperm has a flagellated tail, and a head that is covered with acrosome. The acrosome contains an enzyme called acrosin. The acrosin stimulates sperm motility.

Capacitation: Several sperms secrete enzymes like hyaluronidase, acrosin, and neuraminidase around the oocyte, this makes a sperm to be able to penetrate the ring of cells around the oocyte called corona radiata, and the zona pellucida inside the oocyte. Immediately, this happens, calcium ions are released by the oocyte that blocks any further penetration. The oocyte now develops into a mature ovum.

Cleavage: Rapid cell division of the zygote occurs; the process is called cleavage. Cleavage begins 24 hours after fertilization, and is completed within a few hours (6 hours). The zygote divides from 2, 4, 8, 16, 32, etc., to a solid mass of small cells called blastomeres, and this solid mass is called a morula.

Blastocyst: After about 4 days, there is a rearrangement of the cells producing a hollow region called blastocoele, and the tissue is now called a blastocyst. The outer covering of cells in the blastocyst is called trophoblast, and an inner cell mass called embryoblast.

Implantation: The blastocyst is now in the uterus after 6 days of fertilization. The endometrium is in its secretary phase. The trophoblast develops two layers of cells at the region of attachment (cytotrophoblast and syncytotrophoblast ). The trophoblast secretes a hormone called human chorionic gonadotropin (HCG). This hormone is functions like the LH by stimulating the corpus luteum to further secrete progesterone and estrogen. By the third week, the blood level of hCG is normally detected in a woman's blood. The hCG level rises, up to 2 months after conception, and starts to decline almost to zero level by the 4th month.

Placentation: This is the formation of the placenta from the trophoblastic and

endometrium tissues. Part of the trophoblastic tissues form the chorion. The chorion forms finger-like processes called chorionic villi. The blood vessels of the embryo are attached to the chorionic villi. A combination of the chorionic villi and another tissue called decidua basalis form the placenta. Through the placenta the fetus receives its nutrients. The placenta detaches itself and sloughs off after childbirth.

Embryonic Development:

Formation of the Primary Germ layers: After implantation and the formation of the blastocyst, the inner membrane differentiate to form three primary germ layers: ectoderm, mesoderm, and endoderm.

Gastrulation: This rearrangement of the inner cell mass to form these structured primary germ layers is called gastrulation.

Formation of the Embryonic Membranes:

Amnion /Amniotic Fluid: Within 8 days after fertilization, the cells of the cytotrophoblast differentiate to form amnion (a thin protective membrane) and a space called amniotic cavity that contains amniotic fluid. The fluid functions as a shock absorber for the fetus and separates the fetus from the surrounding tissues.

Yolk & Yolk Sac: By the 12th day, the endodermal layer differentiates to form the yolk sac. The yolk sac forms part of the digestive tube.

Allantois: From the yolk sac emerges a highly vascularized pouch, allantois, that later forms the umbilical cord. The chorion develops from the mesoderm that lines the trophoblast.

Chorion & Placenta: The chorion later forms the placenta that allows the exchange of materials between the fetus and the mother. The chorion also produces human chorionic gonadotropin (hCG ) which acts like luteinmg hormone (LH). This is accomplished by the 3rd month. Placenta is protective aganst certain microorganism and viruses, but some viruses like AIDS, German measles, chicken pox, measles, poliomyelitis and encephalitis can penetrate it t; and also IgG can penetrate it. The placenta allows nutrients to pass to the fetus and secrete estrogen and progesterone (which takes over from the hCG ) which are necessary to maintain pregnancy. It also secretes relaxin that helps to relax the public symphysis and dilate the uterus. The placenta also secretes human placental lactogen that enhances lactation. Many drugs can pass through the placenta. Nutrients and oxygen diffuse from the placenta into the umbilical vein, while wastes leave through the umbilical arteries. At birth, the umblical cord is cut as the placenta degenerates.

GENETICS:

Definition:

Alleles: Two alternative forms of a gene that code for the same trait, located on the same locus on the homologous chromosome.

Dominant Allele: An allele that masks the presence of another, and also its trait is a dominant allele, for example a trait (T).

Recessive Allele: An allele whose presence or trait is masked by the presence of a dominant allele for example, a trait (t). These genes can only be expressed if occurred in pairs or as homozygous, example is the genotype tt for short.

Homozygous: Same alleles on a homologous chromosome, example, TT (tall) or tt (short).

Heterozygous: Different alleles on a homologous chromosome, example, Tt

Phenotype: Expression of a genetic make-up.

Genotype: Gene make-up.

Incomplete Dominance: Neither member of the pair of alleles is dominant. The phenotype shows a trait which is a combination of the two individual traits, example of such a trait is color (hair & skin color).

Multiple Allele Inheritance: (also referred to as Codominance). Some traits have more than two allelic genes or alternate form, example is the ABO blood group. The genes A and B are equally dominant and can be inherited by an individual with both genes showing equal dominance, an example is the blood group AB.

Polygenic Inheritance: The phenotypes here are like those in incomplete dominance. The polygenic trait shows a continuous gradation between the two allelic genes, for example, skin or eye color. The expression of a polygenic inheritance is controlled not only the pair of genes for that trait, but also by other genes/traits and other factors.

Karyotype: There are 46 chromosomes (23 pairs) in a human cell; 22 pairs are called autosomes and a pair of sex chromosomes. In females, the pair of sex chromosomes is XX, but XY in the males. Karyotype is the arrangement of chromosomes showing the size and the shape.

Sex-Linked Inheritance: There are certain traits that are transmitted by the sex chromosomes only. Some traits (or genes) are located only on the Y- chromosome, or on the X- chromosome of the sex chromosomes and, therefore, can only be transmitted by these genes. An example is the trait of color blindness that occurs mainly in the males.