Anatomy of Human Development 3 Flashcards
What are the 4 main events of the first week of human development?
Fertilisation = The process by which the male sperm and female egg join together
Cleavage = The cell undergoes rapid cell division
Blastocyst formation = The process of cell differentiation
Implantation = Embryo attaches to the wall of the uterus
What are the four stages of fertilisation?
- Oocyte is released
During ovulation, an oocyte (egg) is released from the ovary into to fallopian tube. The oocyte must meet and fuse with the sperm within 24 hours after ovulation or it will degenerate. - Sperm and egg come into contact
At ovulation, the sperm speed up again and swim down the uterine tube where they meet the egg. - Fertilisation has occurred
The membrane surrounding the egg, called the zona pellucida, has two major functions in fertilisation. First, the zona pellucida contains sperm receptors that are specific for human sperm. Second, once penetrated by the sperm, the membrane becomes impermeable to penetration by other sperm. - Zygote is formed
Once it meets the egg, the nuclei from the egg and sperm merge and share their genetic material. The egg now contains all the genetic material it will need to become a person.
Where, within the uterine tube, does fertilisation take place?
Ampulla
What is cleavage and how does it progress?
Cleavage is a series of mitotic divisions of the zygote. Once the zygote is formed, the cell undergoes rapid cell division. The zygote cytoplasm is successively cleaved to form a blastula consisting of increasingly smaller blastomeres. The first blastomere stage consists of 2 cells (C), the next of 4 cells (D), the next of 8 cells (E) etc. At the 16 to 32 cells stage, the blastomeres form a morula (mulberry) consisting of an inner cells mass and an outer cell mass.
When does blastocyst formation occur and what does it form?
Blastocyst formation occurs when fluid secreted within the morula forms the blastocyst cavity. This fluid filled cavity is known as the blastoceole. Once this stage has been reached the outer layer of the blastocyst thins to single-cell thickness to become the trophoblast, enclosing the enlarging fluid-filled blastocyst cavity. The central group of cells move to one pole of the blastocyst (the embryonic pole) to form the inner cell mass now known as the embryoblast. At the same time, the zona pellucida begins to degenerate in preparation from implantation.
What do the trophoblast and embryoblast go on to form?
The trophoblast contributes to the foetal component of the placenta whereas the embryoblast will go on to form the embryo..
After how long does implantation occur? Where does the blastocyst implant?
By 4–5 days the fully formed blastocyst reaches the uterine lumen in preparation for implantation, which occurs a day later. The blastocyst implants within the posterior superior wall of the uterus.
During which phase of the menstrual cycle does implantation occur?
Secretory phase
This phase always occurs from day 14 to day 28 of the menstrual cycle. Progesterone stimulated by LH is the dominant hormone during this phase to prepare the corpus luteum and the endometrium for possible fertilized ovum implantation.
What is week 2 of human development known as? What happens during this week? What has happened by day 13? What symptoms may be seen at this point?
Week 2 of human development is known as a week of 2’s. The embryoblasts and trophoblasts begin to differentiate. Over the course of the week, the blastocyst gets more deeply embedded within the uterine wall. By day 13 the surface defect in the endometrium is healed as the embryo has now become fully embedded. There may be a little bleeding/spotting at this time, which can be confused with the menstrual cycle.
What is trophoblast differentiation during week 2 and what does it form? What do these cells contribute to?
During this period, the trophoblast layer proliferates and differentiates into two distinct layers: syncitiotrophoblast and cytotrophoblast.
The syncytiotrophoblast continues its growth into the endometrium to make contact with endometrial blood vessels and glands. Primary chorionic villi formed by the cytotrophoblast protrude into the syncytiotrophoblast.
It is these cells that will contribute to the formation and development of the placenta.
Which cells produce hCG?
Synctiotrophoblast
What does the formation of the major organ systems during the embryonic period cause?
All major organ systems begin to develop during the embryonic period, causing a craniocaudal and lateral body folding of the embryo.
What is gastrulation?
Gastrulation is a process that establishes the three primary germ layer (ectoderm, mesoderm and endoderm), thereby forming a trilaminar embryonic disk.
What is embryoblast differentiation in week 2, and what does it form?
At the same time, the embryoblast differentiates into 2 distinct cell layers: the dorsal epiblast and the ventral hypoblast. Both layers together from a flat, ovoid-shaped disk known as the bilaminar embryonic disk.
Within the epiblast, clefts develop and eventually coalesce to from the amniotic cavity.
Hypoblast cells migrate and line the inner surface of the cytotrophoblast and eventually delimit the space called the definite yolk sac.
The epiblast and the hypoblast fuse to from the prochordal plate, which mark the future site of the mouth.
How does the primitive streak form during gastrulation in the embryonic period?
This process is first indicated by the formation of a primitive streak within the epiblast. This involves the coordinated movement and re-arrangement of cells in the epiblast. Even before the streak is visible, epiblast cells have started to move.
The primitive streak is a linear band of thickened epiblast that first appears at the caudal end of the embryo and grows cranially. At the cranial end its cells proliferate to form the primitive knot (primitive node). With the appearance of the primitive streak it is possible to distinguish cranial (primitive knot) and caudal (primitive streak) ends of the embryo.
How do germ layers form during gastrulation in the embryonic period?
The cells of the epiblast migrate, detach and slip underneath the epiblast in a process called invagination. These invaginating cells displace the hypoblast and create the endoderm. The epiblast cells that do not invaginate become the ectoderm and the cells that lie between the ectoderm and the endoderm become the mesoderm.
Which structures does the ectoderm give rise to?
Skin Posterior pituitary CNS Facial bones Retina
Which structures does the mesoderm give rise to?
Smooth muscle Heart Skeleton (not skull) Urogenital system Skeletal muscle Connective tissue
Which structures does the endoderm give rise to?
Pancreas
Lungs
Gastrointestinal tract
Liver
What is embryonic folding?
The flat trilaminar embryonic disk becomes a more cylindric embryo due to the longitudinal and transverse folding that occurs as a result of embryonic growth, especially the neural tube.
What is neurulation? When does it occur and what does it create?
Following gastrulation, the neurulation process develops the neural tube in the ectoderm, above the notochord of the mesoderm.
During the third week of gestation the notochord sends signals to the overlying ectoderm, inducing it to become neuroectoderm and from the neural plate (the notochord forms the nucleus pulposus of the intervertebral disk in adults).
The neural plate folds outwards to form the neural groove. Beginning in the future neck region, the neural folds of this groove close to create the neural tube. The neural tube is open at both ends at the anterior and posterior neuropores but will eventually close.
Sometimes the neuropores will fail to close.
What conditions may occur if the neuropores fail to close?
Spina bifida
The posterior neuropore usually closes around day 27. Failure of closure results in a developmental congenital disorder known as spina bifida.
Anencephaly
The anterior neuropore usually closes around day 25. Failure of closure of the anterior neuropore will lead to anencephaly, the failure of the brain and skull to develop.
What does the lumen of the neural tube give rise to?
The ventricular system of the brain and spinal cord
The rostal/cephalic part of the neural tube becomes the adult brain whereas the caudal part of the neural tube becomes the adult spinal cord.
The lumen of the neural tube gives rise to the ventricular system of the brain and central canal of the spinal cord.
What neural crest cells? What might they differentiate into?
Neural crest cells are multipotent cells. They arise at the border of the neural plate - the neural crest. The neural crest cells migrate throughout the embryo and later differentiate into multiple cell types.
Cranial neural crest cells differentiate into a variety of cells, such as bones of neurocranium, pia and arachnoid and several components of cranial nerves.
The trunk region neural crest cells differentiate into a variety of cells such as melanocytes, Schwann cells, dorsal root ganglia and parasympathetic nerves of the gut, abdomen and pelvis.
What appears at the cranial end of the neural tube in the fifth week of development? What do they give rise to?
In the fifth week of development, swellings appear at the cranial end of the neural tube. Three primitive vesicles appear first, and subsequently these develop into five secondary vesicles. These vesicles will give rise to all the structures of the brain and cerebellum, as well as the ventricular system.
What does the embryonic mesencephalon brain vesicle give rise to in the adult brain?
Midbrain
