Week 1 of Human Development Embryology Questions

Ectopic tubal pregnancy must always be an option in the diagnosis when a woman in her
reproductive years presents with such symptoms. Ninety percent of ectopic implantations occur
in the uterine tube. Ectopic tubal pregnancies result in rupture of the uterine tube and internal
hemorrhage, which presents a major threat to the woman’s life. The uterine tube and embryo
must be surgically removed. The symptoms may sometimes be confused with appendicitis

The blastocyst begins implantation by day 7 after fertilization

The mitochondrial DNA of all human adult cells is of maternal origin only. In human fertilization, the entire sperm enters the secondary oocyte cytoplasm. However, sperm mitochondria
degenerate along with the sperm’s tail. Therefore, only mitochondria present within the secondary oocyte (maternal) remain in the fertilized zygote.

At ovulation, a secondary oocyte begins meiosis II, but this division is arrested at metaphase.
The secondary oocyte will remain arrested in metaphase until a sperm penetrates it at fertilization. Therefore, the term “mature ovum’” is somewhat of a misnomer because it is a secondary
oocyte that is fertilized, and, once fertilized, the new diploid cell is known as a zygote. If fertilization does not occur, the secondary oocyte degenerates.At ovulation, a secondary oocyte begins meiosis II, but this division is arrested at metaphase.
The secondary oocyte will remain arrested in metaphase until a sperm penetrates it at fertilization. Therefore, the term “mature ovum’” is somewhat of a misnomer because it is a secondary
oocyte that is fertilized, and, once fertilized, the new diploid cell is known as a zygote. If fertilization does not occur, the secondary oocyte degenerates.

Cleavage is a series of mitotic divisions by which the large amount of zygote cytoplasm is
successively partitioned among the newly formed blastomeres. Although the number of blastomeres increases during cleavage, the size of individual blastomeres decreases until they resemble adult cells in size.

This scenario would result in a yellow- and black-coated offspring. Because EC cells and inner cell mass cells have very similar biochemical characteristics, they readily mix with each
other, and development proceeds unencumbered. Because the mixture contains cells with
yellow-coat genotype and black-coat genotype, offspring with coats of two colors (yellow and
black) will be produced. The offspring are known as mosaic mice.

This scenario would result in four genetically identical children. Blastomeres at the 4- to
8-cell stage are totipotent, that is, capable of forming an entire embryo. Since blastomeres arise
by mitosis of the same cell (zygote), they are genetically identical. This phenomenon is important in explaining monozygotic (identical) twins. About 30% of monozygotic twins arise by early
separation of blastomeres. The remaining 70% originate at the end of week 1 of development by
a splitting of the inner cell mass.

The zona pellucida must degenerate for implantation to occur. Early cleavage states of the
blastula are surrounded by a zona pellucida, which prevents implantation in the uterine tube

The blastocyst implants in the functional layer of the uterine endometrium. The uterus is
composed of the perimetrium, myometrium, and endometrium. Two layers are identified
within the endometrium: (1) the functional layer, which is sloughed off at menstruation, and
(2) the basal layer, which is retained at menstruation and serves as the source of regeneration of
the functional layer. During the progestational phase of the menstrual cycle, the functional layer
undergoes dramatic changes; uterine glands enlarge and vascularity increases in preparation
for blastocyst implantation.The blastocyst implants in the functional layer of the uterine endometrium. The uterus is
composed of the perimetrium, myometrium, and endometrium. Two layers are identified
within the endometrium: (1) the functional layer, which is sloughed off at menstruation, and
(2) the basal layer, which is retained at menstruation and serves as the source of regeneration of
the functional layer. During the progestational phase of the menstrual cycle, the functional layer
undergoes dramatic changes; uterine glands enlarge and vascularity increases in preparation
for blastocyst implantation.

The secondary oocyte is arrested in metaphase of meiosis II, and it will remain in this meiotic
stage until fertilization occurs. Following fertilization, the secondary oocyte completes meiosis II, forming a mature ovum and a polar body. The nucleus of the mature ovum is called the
female pronucleus, which fuses with the male pronucleus to form a zygote.