Embryology

Question 1

What happens during the first trimester?

Answer 1

Most critical Stage-rudiments of all major organs start to systems start to develop Most vulnerable period to effects of drugs,radiation and alcohol

Question 2

what happens during the second trimester

Answer 2

Nearly complete development of organ systems(human features)

Question 3

What happens during the third trimester

Answer 3

Rapid fetal growth(organs become functional)

Question 4

What is embryonic period

Answer 4

First 8 weeks of the starting from fertilisation

Question 5

When does Fertilisation occur?

Answer 5

It occurs within 12 -24 hours after ovulation

(sperm can remain viable for 48 hours whereas a secondary oocyte can only remain viable for 24 hours after ovulation

Question 6

what is capacitation?

Answer 6

It refers to a series of functional changes that causes the sperm’s tail to beat even faster when they are outside the secondary oocyte.This prepares the sperm to fuse with the oocyte plasma membrane

During capacitation the cholesterol,glycoproteins and proteins from the head of the sperm cell is removed by secretion of the uterine and only capacitated sperms are allowed to enter the plasma membrane of the egg

Question 7

Corona radiata

Answer 7

the granulosa cells that surround the secondary oocyte

Question 8

Zona Pellucida

Answer 8

the clear glycoprotein layer between the corona radiata and the secondary oocyte plasma membrane

Question 9

Acrosome

Answer 9

covers the head of a sperm ,contains several enzymes that help in penetrating the cells of the zona pellucida(bores a hole through the zona pellucida)

Question 10

Acrosomal Reaction

Answer 10

It is the reaction whereby the sperm binding to the zona pellucida ZP3 causes release of contents of the acrosome which allows a hole to drilled through the zona pellucida by the sperm with lashing tail

Question 11

How is polyspermy prevented(fusion of the more than one sperm to an egg)

Answer 11

It is prevented by depolarisation of the cell membrane of the secondary oocyte which triggers intracellular release of calcium ions which stimulate exocytosis of secretory vesicles from the oocyte .The molecules released by exocytosis inactivates ZP3 and hardern entire zona pellucida

Question 12

Cleavage of zygote

Answer 12

It happens whereby the zygote divides into 2(first division 24hrs after fertilisation)division continues till formation blastocyst (32 cells) on day 5

Question 13

How is the blastocyst formed?

Answer 13

Formed when uterine milk enters the morula and the cells get reorganised form the blastocyst cavity
During the formation 2 populations of cells are produced
Embryoblast(inside) and trophoblast(outside)

Question 14

what does the trophoblast eventually become?

Answer 14

It will ultimately become the outer chorionic sac that surrounds the fetus and the fetal portion of the placenta

Question 15

When does the blastocyst end up in the uterus?

Answer 15

Around day 5 the blastocyst “hatches” from the zona pellucida by digesting a hole in it with an enzyme and moves to the uterus

Question 16

Where and how does the blastocyst implant?

Answer 16

Implantation usually occurs in posterior portion of the fundus or the body of the uterus on around day 7
Enzymes released by the blastocyst allows it to burrow in to the endometrial wall

Question 17

Decidua Basalis

Answer 17

Following implantation the uterus the endometrium is know as the decidua ,the decidua separates from the uterus after the fetus is delivered
The decidua basalis is the endometrium b/n embryo and stratum basale

Question 18

Decidua Parietalis

Answer 18

The remaining part of the endometrium that remain uninvolved in the implantation process

Question 19

What is the function of the trophoblast?

Answer 19

the trophoblast splits into 2 layers the sychtiotrophoblast and the cytotrophoblast.The syntiotrophoblast secretes enzymes that enables the the blastoxyst to penetrate the endometrium by digesting the endometrial walls
cytotrophoblast exsists between the embryo and the syntiotrophoblast and these two layers eventually form the chorion.

Question 20

How is the bilaminar disc formed?

Answer 20

The bilaminar disc is formed when the when embryoblast differentiates into 2 layers to form the epiblast(aka primitive ectoderm) and hypoblast(primitive endoderm)(around Day 8)

Question 21

Describe the formation of the amnion and its function?

Answer 21

Amnion is formed as squamous cells roof over the epiblast cells forming a cavity and the cavity is filled with amniotic fluid
(The amnion will eventually envelop the entire embryo during folding)

The function of the amnion is to act as a shock absorber and regulate temperature of the embryo

Question 22

Why are sinusoids important?

Answer 22

Network of blood capillaries are established between mother and fetus through the maternal sinusoids
Maternal blood rich source of embryonic nutrition

Question 23

Describe formation of the yolk sac and its function

Answer 23

The yolk sac is formed when the mesodermal cells from the hypoblast migrate to inner surface of the blastocyst to form the exocoelomic membrane

the yolk sac is relatively small as the embryo receives nutrients from the endometrium however it is vital for weeks 2 and 3 and serves as a source of blood cells from week 3 to 6

The yolk sac eventually form the gut tube and primordial cells from here migrate to the developing gonads to form primitive germ cells and eventually gametes

Also functions as a shock absorber and helps prevent drying out of the embryo

Question 24

Describe the formation and the function of the chorion?

Answer 24

Also know as extra-embryonic coelom,formed as a result of large cavities that develop in the extraembryonic mesoderm
This cavities fuse to form the extraembryonic coelom(chorionic cavity)

The functions of chorion
(1)Secretes proteins that block the antibodies
(2)Secretes T Lymphocytes which blocks and suppresses normal immune responses in the uterus
(3)Chorion Produces HCG
Eventually chorion and amnion fuse

Question 25

Gastrulation

Answer 25

It is the process by which the bilaminar disc becomes trilaminar(Ectoderm,Mesoderm and Endoderm)
Gastrulation starts with primitive streak through which mesodermal cell from the epiblast enters the hypoblast

Question 26

Role of primitive streak

Answer 26

establishes caudal and cranial ends
establishes right and left sides
primitive nodes form on the head end

Question 27

Invagination

Answer 27

Process whereby epiblast cells migrate and detach from epiblast and enter the primitive streak

Question 28

Induction

Answer 28

Process whereby notochord induces adjacent unspecialised cells to become specialised

Question 29

What is the fate of Notochord?

Answer 29

Induces certain mesodermal cells to become vertebral bodies and it it becomes the nucleus pulposus of the intervertebral disc

Question 30

Neurulation

Answer 30

Process by which the neural plates fold and the and neural tube forms

Question 31

What happens to the head end of the neural tube

Answer 31

Primary Brain Vesicles ->Prosencephalon(forebrain)

                                  - >Mesencephalon(midbrain)
                                  - >Rhombencephalon(hindbrain)

Question 32

What does the prosencephalon become?

Answer 32

Telencephalon and Diencephalon(Form from the secondary vesicles)

Question 33

What does the mesencephalon become?

Answer 33

it does not form any secondary vesicles

Question 34

what does the rhombencephalon

Answer 34

Metencephalon (both formed from secondary vesicle)
Myelencephalon
Area adjacent to myelencephalon

Question 35

Development of Somites

Answer 35

Formed from the paraxial mesoderm

Mesoderm Layer has 3 segments (paraxial,intermediate and lateral mesoderm)

Question 36

what does the paraxial mesoderm develop into ?

Answer 36

It develops into somites
There are 3 types of somites
Dermatomes 
Myotome 
Sclerotome

Question 37

What does the myotome become?

Answer 37

The myotomes develop into the skeletal muscles of the neck, trunk, and limbs

Question 38

The dermatome becomes?

Answer 38

the dermatomes form connective tissue, including the dermis of the skin

Question 39

The sclerotome becomes?

Answer 39

The sclerotomes give rise to the vertebrae and ribs.

Question 40

What does the lateral plate mesoderm become?

Answer 40

It forms the splanchnic mesoderm and the somatic mesoderm(the somatic mesoderm is on top and splanchnic mesoderm is below)
Between them the intraembryonic coelom is formed

Question 41

what does the Splanchnic mesoderm become?

Answer 41

forms the heart and the visceral layer of the serous pericardium, blood vessels, the smooth muscle and connective tissues of the respiratory and digestive organs, and the visceral layer of the serous membrane of the pleurae and peritoneum.

Question 42

What does the somatic mesoderm become?

Answer 42

gives rise to the bones, ligaments, blood vessels, and connective tissue of the limbs and the parietal layer of the serous membrane of the pericardium, pleurae, and peritoneum

Question 43

Structures produced by Endoderm

Answer 43

Epithelial lining of gastrointestinal tract (except oral cavity and anal canal) and epithelium of its glands.
Epithelial lining of urinary bladder, gallbladder, and liver.
Epithelial lining of pharynx, auditory (eustachian) tubes, tonsils, tympanic (middle ear) cavity, larynx, trachea, bronchi, and lungs. Epithelium of thyroid gland, parathyroid glands, pancreas, and thymus.
Epithelial lining of prostate and bulbourethral (Cowper’s) glands, vagina, vestibule, urethra, and associated glands such as greater (Bartholin’s) vestibular glands and lesser vestibular glands.
Gametes (sperm and oocytes).

Question 44

Structures produced by Mesoderm?

Answer 44

All skeletal and cardiac muscle tissue and most smooth muscle tissue.
Cartilage, bone, and other connective tissues.
Blood, red bone marrow, and lymphatic tissue.
Blood vessels and lymphatic vessels.
Dermis of skin.
Fibrous tunic and vascular tunic of eye.
Mesothelium of thoracic, abdominal, and pelvic cavities.
Kidneys and ureters.
Adrenal cortex.
Gonads and genital ducts (except germ cells). Dura mater.

Question 45

what structures are produced by Ectoderm?

Answer 45

All nervous tissue. Epidermis of skin. Hair follicles, arrector pili muscles, nails, epithelium of skin glands (sebaceous and sudoriferous), and mammary glands. Lens, cornea, and internal eye muscles. Internal and external ear. Neuroepithelium of sense organs. Epithelium of oral cavity, nasal cavity, paranasal sinuses, salivary glands, and anal canal. Epithelium of pineal gland, pituitary gland, and adrenal medullae. Melanocytes (pigment cells). Almost all skeletal and connective tissue components of head. Arachnoid mater and pia mater.

Question 46

How is the intraembryonic coelom formed?

Answer 46

In the third week of development, small spaces appear in the lateral plate mesoderm.
These spaces soon merge to form a larger cavity called the intraembryonic coelom (SE ¯-lom cavity).
This cavity splits the lateral plate mesoderm into two parts called the splanchnic mesoderm and somatic mesoderm

Question 47

Placentation

Answer 47

the process of forming the placenta (pla-SEN-ta flat cake), the site of exchange of nutrients and wastes between the mother and fetus. The placenta also produces hormones needed to sustain the pregnancy .
By the beginning of the twelfth week, the placenta has two distinct parts: (1) the fetal portion formed by the chorionic villi of the chorion and (2) the maternal portion formed by the decidua basalis of the endometrium

Question 48

Function of Placenta

Answer 48

The placenta allows oxygen and nutrients to diffuse from maternal blood into fetal blood while carbon dioxide and wastes diffuse from fetal blood into maternal blood
The placenta also is a protective barrier because most microorganisms cannot pass through it. However, certain viruses, such as those that cause AIDS, German measles, chickenpox can pass through it .(This includes alcohol and drugs that can cause birth defects,recall the FAS)

Question 49

What is present in the umbilical cord that connects placenta to the embryo?

Answer 49

The umbilical cord consists of two umbilical arteries that carry deoxygenated fetal blood to the placenta, one umbilical vein that carries oxygen and nutrients acquired from the mother’s intervillous spaces into the fetus, and supporting mucous connective tissue called Wharton’s jelly (WOR-tons) derived from the allantois.

Question 50

What happens in the fourth week embryonic development?

Answer 50

ORGANOGENESIS (refers to the formation of body organs and systems. By the end of the eighth week, all of the major body systems have begun to develop, although their functions for the most part are minimal)

Question 51

What is embryonic Folding?

Answer 51

The cylinder consists of endoderm in the center (gut), ectoderm on the outside (epidermis), and mesoderm in between.

The main force responsible for embryonic folding is the different rates of growth of various parts of the embryo, especially the rapid longitudinal growth of the nervous system (neural tube).
Folding in the median plane produces a head fold and a tail fold; folding in the horizontal plane results in the two lateral folds.

Question 52

How is the Gut tube formed?

Answer 52

The dorsal part of the yolk sac becomes the primitive gut during embryonic folding(towards the midline)

The primitive gut differentiates into an anterior foregut, an intermediate midgut, and a posterior hindgut

Question 53

What happens to the Oropharyngeal membrane?

Answer 53

• The OROPHARYNGEAL MEMBRANE is located in the head end of the embryo
• It separates the future pharyngeal (throat) region of the foregut from —-the stomodeum (stoˉ-moˉ-DE ¯-um; stomo- mouth),
• Because of head folding, the oropharyngeal membrane moves downward and the foregut and stomodeum move closer to their final positions.
• When the oropharyngeal membrane ruptures during the fourth week, the pharyngeal region of the pharynx is brought into contact with the stomodeum. (refer to image on totora page 1103)

Question 54

What happens to the cloacal membrane?

Answer 54

a developing embryo, the last part of the hindgut expands into a cavity called the cloaca

On the outside of the embryo is a small cavity in the tail region called the proctodeum (prok-toˉ-DE ¯-um; procto- anus)
Separating the cloaca from the proctodeum is the cloacal membrane (

During embryonic development, the cloaca divides into a ventral urogenital sinus and a dorsal anorectal canal.

As a result of tail folding, the cloacal membrane moves downward and the urogenital sinus, anorectal canal, and proctodeum move closer to their final positions.

When the CLOACAL MEMBRANE ruptures during the seventh week of development, the urogenital and anal openings are created.

Question 55

What are Pharyngeal arches?

Answer 55

They are swellings on the surface of the embryo.

Each pharyngeal arch consists of an outer covering of ectoderm and an inner covering of endoderm, with mesoderm in between.

Just as the somite gives rise to specified structures in the body wall, each pharyngeal arch, cleft, and pouch gives rise to specified structures in the head and neck.

Each pharyngeal arch is a developmental unit and includes a skeletal component, muscle, nerve, and blood vessels.(rmb there is a ectoderm,mesoderm) In the human embryo, there are four obvious pharyngeal arches and two less distinct arches

Question 56

What happens from the 5th week through to 8th week?

Answer 56

very rapid development of the brain, so growth of the head is considerable.

Following this the head grows even larger relative to the trunk, and the limbs show substantial development

In addition, the neck and trunk begin to straighten, and the heart is now four-chambered.

By the seventh week, the various regions of the limbs become distinct and the beginnings of digits appear

. At the start of the eighth week (the final week of the embryonic period),
the digits of the hands are short and webbed, the tail is shorter but still visible, the eyes are open, and the auricles of the ears are visible

By the end of the eighth week, all regions of limbs are apparent; the digits are distinct and no longer webbed due to removal of cells via apoptosis.

Also, the eyelids come together and may fuse, the tail disappears, and the external genitals begin to differentiate.

The embryo now has clearly human characteristics.