ANA 205 Embyology 1st, 2nd & 3rd Week of Developement

Question 1

What occurs in the 2nd week of development

Answer 1

The following events take place during the 2nd week of development:
I. Completion of implantation of the blastocyst
II. Formation of the bilaminar embryonic disc (epiblast and hypoblast)
III. Formation of extraembryonic structures (amniotic cavity, amnion, umbilical vesicle [yolk sac], connecting stalk, and chorionic sac

Question 2

What occurs on day 8 of human development?

Answer 2

At the eighth day of development, the blastocyst is partially (slowly) embedded in the endometrium
the syncytiotrophoblast continues its invasion of the endometrium, thereby eroding endometrial blood vessels and endometrial glands
More cells in the cytotrophoblast divide and migrate into the syncytiotrophoblast, where they fuse and lose their individual cell membranes
Cells of the inner cell mass or embryoblast also differentiate into 2 layers:
the hypoblast layer, which is made up of small cuboidal cells, and it is adjacent(nearer) to the blastocyst cavity
the epiblast layer which is made up of high columnar cells, and it adjacent to the amniotic cavity
The hypoblast and epiblast layers together form a flat ovoid shaped disc called the bilaminar embryonic disc
At the same time, a small cavity appears within the epiblast which enlarges to form the amniotic cavity
Epiblast cells adjacent to the cytotrophoblast are called amnioblasts
Amnioblasts together with the rest of the epiblast, line the amniotic cavity
The endometrium adjacent to the implantation site is edematous and highly vascular

Question 3

What occurs on day 9 of human development?

Answer 3

The blastocyst is more deeply embedded in the endometrium, and the penetration defect in the surface epithelium is closed by a coagulum called fibrin
Vacuoles appear at the region of the trophoblast and they fuse to form lager lacunae
this phase of trophoblast development is known as the lacunar stage
the cells of the hypoblast adjacent to the cytotrophoblast form a thin membrane called the exocoelomic (Heuser’s) membrane
this membrane lines the inner surface of the cytotrophoblast
the exocoelomic (Heuser’s) membrane together with the hypoblast forms the lining of the exocoelomic cavity, or primitive yolk sac or primary umbilical vesicle

Question 4

What happens on days 11 and 12?

Answer 4

1. The blastocyst is completely embedded in the endometrium, and the surface epithelium almost entirely covers the original defect in the uterine wall
2. The blastocyst now produces a slight protrusion into the lumen of the uterus
   cells of the syncytiotrophoblast penetrate deeper into the stroma(tissue) and erode the endothelial lining of the endometrial capillaries
   These ruptured endometrial capillaries are called sinusoids
3. The lacunae then begin to communicate with the sinusoids, and maternal blood enters the lacunar system
   The communication of the eroded endometrial capillaries with the lacunae establishes the primordial uteroplacental circulation
   When maternal blood flows into the lacunae, oxygen and nutritive substances are available to the embryo
4. a new population of cells appears between the inner surface of the cytotrophoblast and the outer surface of the exocoelomic cavity
   These become confluent, they form a new space known as the extraembryonic cavity, or chorionic cavity or extraembryonic coelom
   This space surrounds the primitive yolk sac and amniotic cavity, except where the germ disc is connected to the trophoblast by the connecting stalk (which develops into the umbilical cord)
5. The extraembryonic mesoderm lining the cytotrophoblast and amnion is called the extraembryonic somatic mesoderm
   extraembryonic somatic mesoderm also forms the connecting stalk
   the lining covering the yolk sac is known as the extraembryonic splanchnic mesoderm
   these cells which are derived from yolk sac cells form a fine, loose connective tissue called the extraembryonic mesoderm
6. Soon, large cavities develop in the extraembryonic mesoderm.
   As the conceptus implants, the endometrial connective tissue cells undergo a transformation, called decidual reaction
   During this transformation, the cells of the endometrium swell because of the accumulation of glycogen and lipid in their cytoplasm, and they are known as decidual cells
   The primary function of the decidual reaction is to provide nutrition for the early embryo and an immunologically privileged site for the conceptus

Question 5

What happens on day 13/14 of embryonic development?

Answer 5

The surface defect in the endometrium has been completely covered by the surface epithelium
Occasionally bleeding occurs at the implantation site as a result of increased blood flow into the lacunar spaces
Cells of the cytotrophoblast proliferate locally and penetrate into the syncytiotrophoblast, forming cellular columns surrounded by syncytium
Cellular columns with the syncytial covering are known as primary villi
The primary yolk sac becomes reduced in size and is known as thesecondary yolk sac
This new cavity is known as the secondary yolk sac or definitive yolk sac or the secondary umbilical vesicle
In humans the yolk saccontains no yolkbut is important for the transfer of nutrients between the fetus and mother
This yolk sac is much smaller than the original exocoelomic cavity or primitive yolk sac
During its formation, large portions of the exocoelomic cavity are pinched off to form exocoelomic cysts
Exocoelomic cysts are often found in the extraembryonic cavity or chorionic cavity or extraembryonic coelom
Meanwhile, the extraembryonic coelom expands and forms a large cavity called the chorionic cavity
The extraembryonic mesoderm lining the inside of the cytotrophoblast is then known as the chorionic plate
The only place where extraembryonic mesoderm traverses the chorionic cavity is in the connecting stalk
With the development of blood vessels, the connecting stalk becomes the umbilical cord

Question 6

What is hCG and it’s function?

Answer 6

The syncytiotrophoblast produces a hormone called the human chorionic gonadotrophin (hCG), which enters the maternal blood via lacunae keeps the corpus luteum secreting estrogens and progesterone

Question 7

How is hCG used in pregnancy tests?

Answer 7

hCG can be detected in maternal blood or urine as early asday 10of pregnancy and is the basis for pregnancy tests
Enough hCG is produced by the syncytiotrophoblast at the end of the second week to give a positive pregnancy test, even though the woman is probably unaware that she is pregnant

Question 8

What is extra-uterine implantation?

Answer 8

Blastocysts may implant outside the uterus
These implantations result in ectopic pregnancies
95% to 98% of ectopic implantations occur in the uterine tubes, most often in the ampulla and isthmus

Question 9

Exocoelomic cysts are often found where?

Answer 9

in the extraembryonic cavity or chorionic cavity or extraembryonic coelom

Question 10

Where is the only place where extraembryonic mesoderm traverses the chorionic cavity?

Answer 10

in the connecting stalk

Question 11

What is neurulation?

Answer 11

It is the process by which the neural tube is formed
During neurulation, the embryo may be referred to as a neurula

Question 12

What are the stages of neurulation?

Answer 12

Formation of:
Neural plate
Neural groove
Neural folds & their fusion
Neural crest cells
Neural tube

Question 13

What is Abortion?

Answer 13

This is the premature stoppage of development and expulsion of a conceptus from the uterus or expulsion of an embryo or fetus before it is viable-capable of living outside the uterus
The products of an abortion is called an abortus (i.e. the embryo/fetus and its membranes)

Question 14

List the types of Abortion and explain

Answer 14

i. A spontaneous abortion is one that occurs naturally and is most common during the third week after fertilization
Approximately 15% of recognized pregnancies end in spontaneous abortion, usually during the first 12 weeks

ii. A habitual abortion is the spontaneous expulsion of a dead or nonviable embryo or fetus in three or more consecutive pregnancies

iii. An induced abortion is a birth that is medically induced before 20 weeks (i.e., before the fetus is viable)
This type of abortion refers to the expulsion of an embryo or fetus induced intentionally by drugs or mechanical means

iv. A complete abortion is one in which all the products of conception are expelled from the uterus

v. incomplete abortion is one with retention of parts of the products of conception

vi. A missed abortion is the retention of a conceptus in the uterus after death of the embryo or fetus

vii. A miscarriage is the spontaneous abortion of a fetus and its membranes before the middle of the second trimester (approximately 135 days)

viii. Threatened abortion
is a condition that suggests amiscarriagemight take place before the 20th week of pregnancy
In this case, pregnant women have somevaginal bleeding with or without abdominal cramps during the first three months of pregnancy
When the symptoms indicate a miscarriage is possible, the condition is called a “threatened abortion.”
(This refers to a naturally occurring event, not medical abortions or surgical abortions

Question 15

What is the most critical stage of embryo development

Answer 15

The most critical stages of development occur during the first trimester (13 weeks) when embryonic and early fetal development is occurring

Question 16

What is Teratology?

Answer 16

This is the branch of science that studies the causes, mechanisms, and patterns of abnormal development

Question 17

What is a teratogen?

Answer 17

Any agent that can produce a congenital anomaly or increase the incidence of an anomaly in the population

Question 18

Causes of congenital anatomic anomalies or birth defects are often divided into:

Answer 18

1. Genetic factors such as chromosome abnormalities
2. Environmental factors such as drugs and viruses

Question 19

What events take place during week 1 of human development

Answer 19

Fertilization
Cleavage
Formation of blastomeres
Blastocyst Formation
Implantation

Question 20

What is fertilization?

Answer 20

Fertilization
Is a sequence of coordinated events that involves the union of the sperm and oocyte at the ampulla of the uterine tube
The fertilization process takes approximately 24 hours

Question 21

Stages of fertilization

Answer 21

I Passage of a sperm through the corona radiata
II. Penetration of the zona pellucida
III. Fusion of plasma membranes of the oocyte and sperm
IV. Completion of the second meiotic division of oocyte and formation of female pronucleus
V. Formation of the male pronucleus
VI. The 2 pronuclei fuse into a single diploid aggregation of chromosomes, and the ootid becomes a zygote

Question 22

Explain the Passage of a sperm through the corona radiata

Answer 22

For sperms to pass through the corona radiata, they must have been capacitated (removal of the glycoprotein coat and seminal plasma proteins from the plasma membrane that overlies the acrosomal region of the spermatozoa)

Question 23

Explain the penetration of the zona pellucida

Answer 23

The zona is a glycoprotein shell surrounding the egg that facilitates and maintains sperm binding and induces the acrosome reaction
The intact acrosome of the sperm binds with a zona glycoprotein (ZP3/ zona protein 3) on the zona pellucida
Release of acrosomal enzymes (acrosin) allows sperm to penetrate the zona pellucida, thereby coming in contact with the plasma membrane of the oocyte
As soon as the head of a sperm comes in contact with the oocyte surface, the permeability of the zona pellucida changes
When a sperm comes in contact with the oocyte surface, lysosomal enzymes are released from cortical granules lining the plasma membrane of the oocyte
In turn, these enzymes alter the properties of the zona pellucida to :
prevent sperm penetration and inactivate binding sites for spermatozoa on the zona pellucida surface only one sperm seems to be able to penetrate the oocyte

Question 24

Explain the fusion of plasma membranes of the oocyte and sperm

Answer 24

The plasma or cell membranes of the oocyte and sperm fuse and break down at the area of fusion
The head and tail of the sperm enter the cytoplasm of the oocyte, but the sperm’s plasma membrane remains behind

Question 25

Explain the completion of the second meiotic division of oocyte and formation of female pronucleus

Answer 25

Penetration of the oocyte by a sperm activates the oocyte into completing the second meiotic division and forming a mature oocyte and a second polar body
The nucleus of the mature ovum/oocyte is now called the female pronucleus

Question 26

Explain the formation of the male pronucleus

Answer 26

Within the cytoplasm of the oocyte, the nucleus of the sperm enlarges to form the male pronucleus, and the tail of the sperm degenerates

NOTE: Since all sperm mitochondria degenerate, all mitochondria within the zygote are of maternal origin (i.e., all mitochondrial DNA is of maternal origin)
Morphologically, the male and female pronuclei are indistinguishable
The oocyte now contains 2 pronuclei, each having haploid number of chromosomes(23)
The oocyte containing two haploid pronuclei is called an ootid

Question 27

Explain how the 2 pronuclei fuse

Answer 27

the 2 pronuclei fuse into a single diploid aggregation of chromosomes, the ootid becomes a zygote
The chromosomes in the zygote become arranged on a cleavage spindle in preparation for cleavage of the zygote

Question 28

Discuss cleavage and blastomere formation

Answer 28

Cleavage is a series of repeated mitotic divisions of the zygote resulting in a rapid increase in the number of embryonic cells, known as blastomeres
Blastomeres become smaller with each successive cleavage division
Cleavage normally occurs as the zygote passes along the uterine tube toward the uterus

Division of the zygote into blastomeres begins approximately 30 hours after fertilization (day 2)
the 1st cleavage is the division of the zygote into a two-cell stage the 2nd cleavage is from the two-cell stage into a four-cell stage
the 3rd cleavage is from the four-cell stage into an eight-cell stage
After the eight-cell stage, the blastomeres change their shape and tightly align themselves against each other to form a compact ball of cells
This process is called compaction

Question 29

compaction is probably mediated by

Answer 29

cell surface adhesion glycoproteins

Question 30

Morula stage

Answer 30

Approximately 3 days after fertilization, cells of the compacted embryo divide again to form a 16-cell stage called the morula stage
When there are 16 to 32 blastomeres, the developing human is called a morula

Question 31

Discuss Blastocyst formation

Answer 31

Blastocyst formation
Shortly after the morula enters the uterus (approx. 4 days after fertilization), a fluid-filled space called the blastocystic cavity appears inside the morula
The fluid passes from the uterine cavity through the zona pellucida to form this space
As fluid increases in the blastocystic cavity, it separates the blastomeres into two parts
An inner cell mass of cells called the embryoblast, is surrounded by an outer cell mass of cells called the trophoblast

Question 32

Discuss implantation

Answer 32

Implantation
The wall of the uterus consists of 3 layers:
(a) endometrium or mucosa lining the inside wall
(b) myometrium, a thick layer of smooth muscle
(c) perimetrium, the peritoneal covering lining the outside wall
Approximately 6 days after fertilization (day 20 of a 28-day menstrual cycle), the blastocyst attaches to the endometrial epithelium
As soon as it attaches to the endometrial epithelium, the trophoblast starts to proliferate rapidly and gradually differentiates into 2 layers
An inner layer of cytotrophoblast
An outer layer of syncytiotrophoblast

Question 33

The 3rd week of embryonic development is characterized by

Answer 33

I. Appearance of the primitive streak
II. Development of notochord
III. Differentiation of three germ layers
IV. Further development of the chorionic villi

Question 34

Briefly comment on Sertoli cells and their contribution the the blood-testis barrier

Answer 34

The blood–testis barrier is a physical barrier between the blood vessels and the seminiferous tubules of the animal testes.
The barrier is formed by tight junctions, adherens junctions, and gap junctions between the Sertoli cells, which are sustentacular (supporting cells) of the seminiferous tubules, and divides the seminiferous tubule into a basal compartment (outer side of the tubule, in contact with blood and lymph) and an endoluminal compartment (inner side of the tubule, isolated from blood and lymph).

This barrier is critical for adult fertility, restricting contact between postmitotic germ cells and the immune system and preventing rejection of spermatogenic cells that form after immune tolerance.