Case 14- Embryology

Question 1

Ovulation

Answer 1

Production of eggs (-1 to -5 days)

Question 2

Ovarian follicular development

Answer 2

1. Germ cells form Primordial follicles.
2. Primordial follicles develop into Primary and secondary follicles through proliferation, within cortex of ovary.
3. At puberty FSH is released, follicles mature and develop an antrum (fluid filled space).
4. Mature (Tertiary, Graafian) follicle forms, large antrum. Last stage of development.
5. After antral stage you get Ovulation.
6. Luteinisation- empty follicle develops into the corpus luteum and then the Corpus albicans. Stimulated by LH

Question 3

Do all follicules ovulate

Answer 3

No, some undergo trisier

Question 4

Fertilisation

Answer 4

Day 0. Spermatozoon combines with the oocyte to form a one cell zygote.

Question 5

Process of Fertilisation

Answer 5

1. Sperm capacitation
2. Penetration of corona radiata
3. Attachment and penetration of zona pellucida with the acrosome of the sperm
4. Activation: Acrosome reaction (allows penetration of zona pellucida)
5. Sperm binds to the plasma membrane and sperm entry
6. Blocks polyspermy (prevents multiple sperms penetrating)

Question 6

Results of sperm entry

Answer 6

1. Oocyte stimulated to complete 2nd meiotic division
2. Diploid number of chromosomes restored
3. Genetic sex of embryo determined (XX or XY)
4. Genetic mingling produces unique individual
5. Metabolic activation of oocyte
6. Development proceeds

Question 7

Capacitation of sperm

Answer 7

1. Biochemical process- allows the sperm to fertilise with the egg
2. Post-ejaculation within female genital tract
3. Sperm interacts with female tract mucosa
4. Conditioning process – 7 hours
5. Removal of glycoproteins and sterol groups from the plasma membrane of the sperm
6. Reversible by exposure to prostatic secretions

Question 8

Activation of sperm

Answer 8

1. Irreversible morphological event
2. Whiplashing movements
3. In vicinity of/in contact with oocyte
4. Release of hydrolytic enzymes
5. Acrosome reaction: Fusion between plasma membrane of oocyte and acrosome of sperm
6. Allows sperm to successfully penetrate and fuse with the egg

Question 9

Polyspermy

Answer 9

Multiple sperm penetrating the oocyte, results in an inviable zygote due to multiple chromosome copies

Question 10

Fast block of Polyspermy

Answer 10

• 2-3 seconds after fusion
• Electrical depolarisation of plasma membrane of the ovum (Na+ channels)
• Prevents other sperm from adhering to membrane
• Lasts <5mins

Question 11

Slow block of polyspermy

Answer 11

• 10-60 seconds
• Oocyte secretes lyzozymes from cortical granules causing:
• Cortical reaction- Formation of impenetrable oocyte membrane
• Zona reaction- Prevention of sperm binding to zona pellucida through structural changes

Question 12

Pre-embryo

Answer 12

Pre-implantation embryo, cell division but no growth, fertilisation to days 11-12. Blastocyst.

Question 13

Embryo

Answer 13

Gastrulation, neurulation, folding and organogenesis. Embryonic period days 11-12 to week 8. ‘Critical period’ of development. Only after full implantation.

Question 14

Foetus

Answer 14

Growth, most organs are formed, fetal period week 9 to birth

Question 15

Result of sperm entry- polar bodies

Answer 15

• First polar body already formed from 1st meiotic division
• Further polar bodies formed from 2nd meiotic division
• Polar bodies are the genetic material which forms during meiosis
• Formation of male (from sperm) and female (from oocyte) pronucleus due to sperm entry
• Pronuclei dissolve and release genetic material
• First cleavage division, zygote splits into two separate cells.

Question 16

Polar bodies

Answer 16

• Polar bodies are formed by uneven cell division in meiosis, ensures the oocyte remains haploid until pronuclei combine
• Polar bodies are found within the zona and the Corona radiata but outside the plasma membrane. The genetic material is kept separate from the zygote
• One polar body is often reabsorbed by the zygote so that only two polar bodies are visible

Question 17

Movement of the pre-embryo

Answer 17

Fertilisation occurs in the ampulla of the uterine tube, cleavage occurs as the pre-embryo moves towards the uterus.In the Isthmus the Morulla forms, further cell divisions form a Blastocyst with a cavity. The Blastocyst implants in the uterine wall.

Question 18

Where does the pre-embryo remain during cleavage

Answer 18

Within the zona pelluda

Question 19

Pregnancy timing

Answer 19

Pregnancy is timed from the last period which is two weeks before fertilisation. Pregnancy is timed 2 weeks before development.

Question 20

Timing of pre-embryo development

Answer 20

Morula forms on day 3. Blastocyst forms on day 4, with cavity production. In the Blastocyst you get cell differentiation.
• 1-2 days= Cleavage stage (2-8 blastomeres)
• 5-7 days= Implantation begins
• 5+ days= hatching, removal of zona pellucida to allow implantation

Question 21

Blastocyst

Answer 21

Made of blastomere cells. Makes up the pre-implantation embryo.

Question 22

Types of blastomere cells

Answer 22

Start of specialisation. The inner cell mass (embryoblast) becomes the foetus, the trophoblast becomes the placenta. The blastocoele is the cavity.

Question 23

Specialisation and potential

Answer 23

Increased specialisation reduces potential.
• 2-4 cell stage= Cells are totipotent, potential to become any tissue
• Morula= Inner cells become pluripotent, potential to become any tissue except placenta. Inner cells have the potential to only become inner cell mass. Outer cells, have the potential to only become trophoblast cells
Blastocyst- pluripotent except placenta

Question 24

Attachment and invasion days 5-6 (implantation)

Answer 24

Trophoblast differentiates into Cytotrophoblast (cellular layer surrounding the conceptus) and Syncytiotrophoblast. The Syncytiotrophoblast is made of a syncytial epithelial layer, it invades maternal blood vessels by forming lacunae. The Syncytiotrophoblasts have shared nuclei, Cytotrophoblasts are made of individual cells, both form the placenta. Syncytiotrophoblast is source of hCG (signal of pregnancy to corpus luteum). Late Blastocyst, beginning of Bilaminar disk. hCG is detected in pregnancy tests.

Question 25

Incomplete implantation, embedding days 7-8

Answer 25

The inner cell mass forms, Pre-embryo ‘hatched’, the zona pellucida sheds before implantation. Attaches to endometrium in the posterior wall of the uterine cavity. The blastocyst begins to embed into the uterine wall. The lacunae attach to maternal blood. Inner cell mass forms epiblast cells which form the embryo. Hyperblast (extraembryonic cells), help in development of embryo but not part of the embryo itself.

Question 26

Interstitial implantation days 11-12

Answer 26

Uterine lining is repaired, interstitial implantation (embryo completely below lining). Maternal blood begins to flow through lacunae. Full implantation of blastocyst. Bilaminar disk is formed. Extraembryonic mesoderm lines up. Embryo completely surrounded by cytotrophoblasts, Syncytiotrophoblasts line up with maternal blood supply.

Question 27

Development of bilaminar embryo (days 6-12 during implantation)

Answer 27

The amnion surrounds the developing fetus. Epiblast forms the foetus. The Blastocyst is surrounded by Cytotrophoblasts, the Hypoblast gives rise to the yolk sac

Question 28

Development of bilaminar embryo (13-14 post implantation)

Answer 28

Primitive yolk sac shrivels up and secondary yolk sac is formed. Space within the Cytotrophoblast is the Exocoelomic cavity.

Question 29

Amnion

Answer 29

A single layer of extraembryonic ectoderm lined with extraembryonic mesoderm. It is formed from the amniotic ectoderm and protects the embryo during development. Amniotic fluid surrounds embryo/foetus.

Question 30

Chorion

Answer 30

A layer of trophoblast and extraembryonic mesoderm surrounding the embryo. Chorion has primary-secondary-tertiary chorionic villi – which protrude from the chorionic plate. Chorion eventually forms placenta (foetal component). The chorion is formed from the yolk sac.

Question 31

Structure of embryo after 5 weeks

Answer 31

The embryo is formed from the epiblast and is surrounded by the amnion. The amniotic sac is the space between the amnion and the embryo. The extraembryonic mesoderm cells form the Chorionic plate which surrounds the amnion. The chronic villi plate forms from the Cytiotrophoblasts and Syncytiotrophoblasts. Surrounding the whole structure is a shell made of Cytotrophoblasts.

Question 32

What does the Hypoblast form

Answer 32

Extraembryonic tissue

Question 33

Where does implantation occur

Answer 33

In the posterior uterine wall

Question 34

What does the Extramembryonic epiblast give rise to

Answer 34

1) Embryonic ectoderm -> Neural crest 2) Primitive crest -> Extraembryonic mesoderm / Embryonic mesoderm / Notochordal process 3) Primitive crest -> Embryonic endoderm -> Allantoic endoderm

Question 35

Gastrulation

Answer 35

Cellular migration, germ layers established, germ layers form all the tissues in the human body and the Embryonic axes are established (polarity i.e. knowing which part of the body is the head). Occurs from day 12-17.

Question 36

Change that occurs in gastrulation

Answer 36

Bilaminar disk (Epiblast, Hypoblast) -> Trilaminar (Ectoderm, Mesoderm and Endoderm)

Question 37

Structure of trilaminar disk

Answer 37

The trilaminar disk still contains the amnion, which is associated with the ectoderm, and the yolk sac which is associated with the Endoderm. The whole structure is surrounded by the cytotrophoblast. The layers go ectoderm, mesoderm and endoderm. The ectoderm form at the dorsal end.

Question 38

Day 13 of gastrulation

Answer 38

The disk elongates and the primitive streak forms which has bilateral symmetry (can tell left and right from dorsal view). The primitive streak forms at the site of the future tail (caudal end). The Epiblast will form the mesoderm and endoderm as they move through the primitive streak. The primitive streak forms at the caudal end of the bilaminar disk and contains the primitive groove.

Question 39

Convergent extension cell movement

Answer 39

Causes elongation, the epiblast becomes longer and thinner. The primitive streak grows down the middle allowing you to separate it into left and right halves. The primitive groove is the opening the cells move through. Cells move through the primitive groove in order to form the trilaminar disk

Question 40

Primitive streak- gastrulation

Answer 40

Molecular signals are produced from the primitive streak which encourage cells to differentiate as they go through the primitive groove. First set of cells become the endoderm, then mesoderm and ectoderm. The endoderm cover the Hypoblast and contribute to the yolk sac. The Hypoblast differentiates into an Extra-embryonic endoderm.

Question 41

Wave of epiblast cells that move through the primitive streak and differentiate

Answer 41

* Second population of extraembryonic mesoderm (yolk sac, amnion) * Embryonic endoderm and Allantoic endoderm (extension of the embryonic endoderm) * Embryonic mesoderm (second population), notochord and node mesoderm which are important molecular signalling centres which organise the development of the embryo * Remaining epiblast differentiates into embryonic ectoderm

Question 42

Neural ectoderm

Answer 42

Induced to form the neural plate by the notochord. Forms nervous system and sensory organs

Question 43

Neural crest cells

Answer 43

Clinically important cell populations (cranial NC and cardiac NC). Genetic defects in neural crest cells can cause craniofacial and cardiac defects. The notochord forms the intervertebral disk

Question 44

What does the Embryonic mesoderm give rise to

Answer 44

The notochord, endothelium, bone, cartilage, skeletal muscle, smooth muscle, blood, lymph, heart, kidneys, spleen, reproductive system and limbs.

Question 45

What does the Embryonic endoderm give rise to

Answer 45

The gut tube, abdominal organs (liver, pancreas), trachea and lungs, auditory and urinary system.

Question 46

What does the Embryonic ectoderm give rise to

Answer 46

The neural crest cells and the epidermis of the skin

Question 47

Population of cells

Answer 47

* Extra-embryonic Ectoderm: Amniotic ectoderm * Extra-embryonic Endoderm: Two populations: Derived from hypoblast (contributes to yolk sac). Derived from primitive streak (contributes to allantois) * Extraembryonic Mesoderm: Two distinct populations: Blastocyst (contributes to amnion and chorion), Primitive streak (contributes to yolk sac)