Fetal Stages

----------Advance Biology NOTES (September 15, 2010)------------

Development-> concerns with the events and processes that occurs as a single cell becomes a complex organism.

Processes of Development

1. Growth- cell divides gets larger and divides again
2. Differentiation- when cells become specialized in structure & function
3. Morphogenesis- when body parts are shaped and patterned into a certain form

"Ontogeny Recapitulate Phylogeny"
- Latin saying which means the development of the embryo retraces the evolution of its species.
Example:
Gill Clefts are developed pharyngeal pouches in advanced vertebration.
1st pair- auditory cavity 3rd pair- thymus
2nd pair- tonsils 4th pair- parathyroids

Developmental Stages

A. Embryonic Development

1st Week- early developmental stages

1. Cleavage- cell division w/o growth (from fallopian tubes to uterus)

2. Morula- solid ball of cells (embryo reacts at 3rd day)

3. Blastula- (5th day) cavity is formed called blastocysts having 2 main parts: tropoblasts (outer and becomes the chorion) and inner mass (becomes the fetus)

4. Gastrula- (2nd week) implantation of the embryo at the uterine wall due to the enzyme secreted by the tropoblast that digest away some of the tissue and blood vessels of the uterine wall & secretion of HCG (Human Chorion Gonadotropin) as basis for pregnancy test that appears in the urine & maintains the corpus luteum in secreting estrogen and progesterone that lasts for 3 mos.
At 5 mos. the placenta begins to secrete estrogen and progesterone in greater amount as corpus luteum degenerates, formation of the 3 embryonic layers- ectoderm, mesoderm and endoderm

5. Neurula
- (3rd Week) appearance of the nerve cord and heart; development of the Nervous System
- (4th-5th weeks) formation of the 4th extraembryonic embrane (allantois) that later becomes the umbilical cord connecting the developing embryo to the placenta; human features appear (head, arms, legs)
- (6th-8th weeks) brain development; neck formation; NS development for reflex actions; all organ systems are established; placenta is mature and fully functioning; 1 1/2 inches long and weighs like an aspirin tablet

B. Fetal Development

2-4 mos.
- large head, flat nose, eyes apart and ears distinctively present, sex are determined; heart beat is felt; skeleton onsified; 6 1/2 inches long and 1/2 lb

5-7 mos.
- movement, eye lids open, pink-colored skin, have lanugs covered with greasy cheese like substance (vernix caseosa) for protection from amniotic fluid, 12 inches long and 3 lbs.

8-9 mos.
- fetus rotates so head is towards the cervix- 21 inches long and 7lbs

Gestation
- period of pregnancy, normally lasts for 38 weeks (266 days)

Parturition
- labor and expulsion of the fetus; contraction of the uterine wall every 15-20 minutes for every 20-30 seconds or 15 minutes for 40 seconds or more.

Stages of Labor:

1.) Dilation of the cervix
-> expulsion of the mucus plug from the cervical canal which prevents bacteria and sperm from entering the uterus during pregnancy

2.) Expulsion of the baby
-> epinotony is done or incision to enlarge the vaginal opening; stitched later then healed

3.) Expulsion of afterbirth
-> placenta is expelled after 15 minutes of delivery; (uterus) uterine muscles contract and uterus shrinks

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Gestation Period

Gestation Period

Definition:
For mammals the gestation period is the time in which a fetus develops, beginning with fertilization and ending at birth. The duration of this period varies between species.

For most species, the amount a fetus grows before birth determines the length of the gestation period. Smaller species normally have a shorter gestation period than larger animals.For example, a cat's gestation normally takes 58–65 days while an elephant's gestation takes 645 days. However, growth does not necessarily determine the length of gestation for all species, especially for those with a breeding season. Species that use a breeding season usually give birth during a specific time of year when food is available.

Various other factors can come into play in determining the duration of gestation. For humans, male fetuses normally gestate several days longer than females and multiple pregnancies gestate for a shorter period. Ethnicity may also lengthen or shorten gestation.In dogs there's a positive correlation between gestation time and a small litter size.