Early Embryology

Question 1

What are the time periods, and the main events summarised in each of the 3 main periods of foetal development

Answer 1

1) Pre-embryonic = Conception to 2 weeks. Fertilised ovum undergoes mitosis, morula forms, blastocyst appearance emerges, impants, germ layers develop
2) Embryonic period = Week 2 to Week 8. Germ layers and the placenta develop. Main body systems form
3) Foetal period = Week 9 to birth. Where further growth and development of organs occur. Locomotor systemes become functional

Question 2

Define gametogenesis

Answer 2

When male and female germ cells (called oogenesis/spermatogenesis) undergo cytoplasmic and chromosomal changes to form the final oocyte and spermatozoon.

Question 3

Where does gametogenesis start? and when?

Answer 3

Remember starts during foetal development actually = Primordial germ cells arise from the wall of the yolc sac during the second week of development

Then in the 6th week they migrate into the body of the embryo and occupy the gonadal ridges. Here they undergo rapid mitotic division. Female germ cells differentiate into oogonia and rapidly multiply until the 5th month and reach 7 million in number. After this large numbers of them undergo degeneration (atresia).

IN male = Primordial germ cells instead differentiate into spermatogonia and will continue to proliferate from puberty throughout life.

Question 4

Outline the main steps of oogenesis

Answer 4

Oogenesis begins in the foetal period, but is not complete until puberty.

Foetal period = oogonia proliferate b mitosis and differentiate into primary oocytes.

At birth = Most enter the prophase of meitotic division (to get half of DNA). And are suspended here until puberty.

Arrest = Occurs due to oocyte maturation inhibitor (OMI). Small peptide produced by follicular cells sorrounding the primary oocyte.

During puberty = Primary oocyte grows. THe follicular cells become stratified to produce a layer of granulosa cells as well. Primary oocyte also becomes sorrounded by a layer of glycoprotein that is secreted by the granulosa cells. The ovarian connective tissue cells around follicle condense to form the theca folliculi. This theca layer then differentiates into an inner vascular and secretory layer, the theca interna, and an outer fibrous layer, the theca externa (2 layers)

Meiosis 1 = The diploid oocyte then becomes a haploid cell and generates 1 polar body. It then actually enters meiosis 2 here, but arrests at metaphase. This finally finishes once fertilisation occurs.

Question 5

How soon after ovulation does ffertilisation normally take place

Answer 5

12-24 hours after ovulation

Question 6

What are the 5 main steps of fertilisation

Answer 6

1) Sperm activation and penetration of the corona radiata
2) Attachment to the zona and penetration of the zona pellucida
3) Fusion of the oocyte and sperm cell membranes
4) Completion of meiosis in the oocyte and formation of pronuclei
5) Formation of the zygote

Question 7

Outline the steps of sperm activation and penetration of the corona radiata (first stage of fertilisation)

Answer 7

Before sperm reach distal fallopian tube (ampulla) needs to undergo capacitation, which makes it capable to penetrate ovum.

Capcitation = The glycoprotein and cholesterol on the acrosomal membrane of the sperm are removed by the secretions from the cervix and uterine tube.

Penetrate corona radiata = An acrosomal reaction occurs that releases hyaluronidase enzyme to penetrate through corona radiata. Active movement of sperm to push through are also important

Question 8

Outline the steps of attachment to and penetration of the zona pellucida (second stage of fertilisation)

Answer 8

Sperm binds to zona pellucida and rleases acrosomal enzymes = Enzymes for penetration are esterases, neuramindase and acrosin.

Once a spermatazoon binds to the zona pellucida it initiates a zona reaction. This changes to physical properties of the zona to prevent any additional sperm entering. It is thought that this is due to a cortical reaction where cortical granules release lysosomal enzymes into space between zona pellucida and oocyte cell membrane

Question 9

Outline the steps of fusion of the oocyte and sperm cell membranes (third stage of fertilisation)

Answer 9

Sperm makes its way through privitelline space (space between cell membrane of oocyte and zona pellucida) and comes into contact with cell membrane of oocyte causing fusion. Once fusion occurs the cell membranes of sperm and egg break down at the area of their contact.

Question 10

Outline the steps of completion of meiosis in the oocyte and formation of pronuclei (fouth stage of fertilisation)

Answer 10

The oocyte then resumes second meiotic division. To form a mature oocyte and a second polar body. The chromosomes of oocyte and sperm condense to a pronuclei. The haploid chromosomes become arranged on a spindle and split longitudinally to form chromatids.

Question 11

Outline the steps of formation of the zygote (fifth and final stage of fertilisation)

Answer 11

Male and female pronuclei meet. Membranes break down. Chromosomes mix to form a single cell called a zygote. Here fertilisation is complete, and zygote prepares for first mitotic division.

Question 12

Give us the time periods of the main landmarks that occur in the first week

Answer 12

First mitotic division = Occurs by the time the embryo reaches the middle of the tube, at about 30hrs post fertilisation

3 days = Morula of about 12-16 cells has reached the uterus

Day 4-5 = Fully formed blastocyst reaches uterine lumen in prep for implantation which occurs 1 day later

Question 13

Outline the main important stages that occur in the first week post fertilisation including mitotic division, and formation of a blastocyst

Answer 13

Within the first 3 days = Rapid mitotic division occurs resulting in a 16 cell stage embryo. Each of these cells is known as a blastomere.

Eventually a solid sphere of cells is known as the morula

The morula then starts to further differentiate = A cavity starts to appear called a blastocoele. This is then called a blastocyst. On one side of the blastocyst is a single layer of cells that sorrounds the blastocoele called the trophoblast. The rest of the cells move to one pole (the embryonic pole) to form an inner cell mass called the embryoblast.

Question 14

Which part of the blastocyst goes on to contribute to the foetal component of the placenta?

Answer 14

The trophoblasts that form a single layer around the blastocoele space.

Question 15

What is deciduation or the decidual reaction?

Answer 15

This is the process that occurs in the endometrium to get it ready for implantation. There are changes to the endometrial stroma as well as vascular changes including remodelling of maternal spiral arteries.

It is essentially getting reach for trophoblast invasion and placenta formation

Question 16

Outline the process of implantation including the differentiation of trophoblasts. This is in the second week after fertiliation.

Answer 16

Day 7/8 (start of the second week) =The trophoblasts differentiate into an inner and outer layer = Inner layer becomes only a single layer of cells, hence the name cytotrophoblasts. The outer layer is much more extensive and is the invasive layer called the syncytiotrophoblasts. At this stage it has invaded the endometrium but not yet the endometrial blood vessels.

Day 12 = Small clefts appear in the syncytiotrophoblasts called lacunae which communicate with maternal endometrial sinusoids which provies additional nutritional support. At the same time the cytotrophoblasts are also developing. A cleft/space emerges between the cytotrophoblasts and the exocoelomic membrane. These clefts then merge to form a full space called the extra-embryonic coelom.

Day 13 = Lacunae are much biffer. Cytotrophoblasts now form primary chorionic villi, finger like projections into the lacunae. By this stage the endometrium has completely engulfed the conceptus.

Day 14 = The syncytiotrophoblasts produce hCG which maintains the corpus luteum in the ovary which in turn sustains the thickness of the endometrium.

Question 17

Outline the process of blastocyst development in the second week including the differentiation, and formation of the amniotic cavity.

Answer 17

Day 7/8 = The inner cell mass of blastocyst also starts to differentiate into 2 layers. The epiblast and hypoblast. These 2 layers are in contact and form a bilaminar embryonic disc. Remember epiblasts are ‘on top’ ‘epi’ so they are the ones facing into the placenta. Within the epiblast layer the amniotic cavity starts to develop. Some epiblasts specialise into amniocystes and produce the amniotic fluid.

Day 9 = The primary yolc sac develops on the other side to the amniotic sac. The primary yoc sack is sorrounded by the ecocoelomic membrane = Which is derived from the hypoblasts. The fluid in the primary yolc sac is the nutrition for the embryo before the placenta is fully functional.

Day 13 = The two cavities continue to enlarge. The chorionic cavity (or extra-embryonic coelom) develops. This is just the fluid filled area that sorrounds the amniotic and yolc cavity/sac. But the presence of this choroinic sac now means the primary yolc sac changes name to the secondary yolc sac.
THe embryo is connected to the cytotrophoblast layer by a connecting stalk of extra-embryonic mesoderm. This stalk later becomes umbilical cord.

Question 18

Outline the processes of the third week after fertilisation including the formation of the trilaminar disc. Also list the general tings that the three layers lead to producing.

Answer 18

Day 15ish = Epiblast becomes the ectoderm, hypoblast becomes the endoderm. The ectoderm (prev epiblast) then gives off a third layer that lies between the original two germ layers = The meso derm.

As a general rule:
Ectoderm = Forms the the stuff that covers the body including skin, epidermis, the lens, internal ear, tooth enamel. It also forms neural tube and the neural crest that makes various things inc nervous system.

Mesoderm = Forms the skeletal, connective tissue, and muscle tissues of the body. Also the urogenital system and viscera

Endoderm = Forms the lining of GI and resp systems. And liver.

Question 19

Briefly describe the formation of the primitive streak and when this occurs.

Answer 19

By the end of the second week = A groove like depression forms in the midline at the caudal end of the bilaminar embryonic disk. This is the start of the primitive streak.

By beginning of week 3 = The streak deepens. At the further cephalic end of the streak the primitive node develops (the organiser for grastrulation in the embryo). Cells from the ectoderm migrate towards streak (ie the caudal end) and then spread out laterally. This forms a new germ layer = The intra-embryonic mesoderm which spreads out and lies between the ectoderm and endoderm.

Question 20

Outline the process of notochord formation, and describe what the notochord goes on to form

Answer 20

Cells from primitive node migrate cranially towards the buccopharyngeal membrane. This produces the notochordal plate which then turns inwards to form a solid cylinder which is the notochord.

The notochord actually sits just under the future neural tube (which is the brain + spinal cord). While the notochord actually forms the longitudinal axis for the embryo and later the centres forms the nuclei pulposi of the intervertebral disks.

Question 21

What is at the most cranial and caudal ends of the tri-laminar embryonic disck

Answer 21

Cranial = Origiinally this is the prochordal plate, but this is soon replaced by the Buccopharyngeal membrane, which forms a temporary seal for the future oral cavity. In week 4 this actually breaks down to join the amniotic cavity and the gut tube (foetus swallows amnio)

Caudal = Originally this is the cloacal plate, but replaced by the Cloacal membrane

Question 22

Outline the process of neurlation

Answer 22

Definition = The process that forms the brain and spinal ord

Day 19 = The ectroderm starts folding to form the neural plate. Initially the neural plate enlarges

Day 20 = The plate deepens to form the neural groove from which the neural tube will form.
Remember that the cranial and caudal ends of the neural tubes remain open and are the anterior (cephalic) and posterior (caudal) neuropores.

Question 23

What do the neuro crest cells go on to form? And outline how they are formed

Answer 23

Neural crests = These form at the edges of the neural tube, where the neuroectoderm is in contact with the ectoderm. Neural crest cells then detach themselves from the neural groove and form aggregations of neural crest cells.

Contribute to = Dorsal root, cranial/enteric/autonomic ganglia, connective tissues of face, bones of skull, adrenal medulla, glial cells, schwann cells, melanocytes, parts of the meninges, parts of teeth.

Question 24

Describe the further development of the mesoderm including the different mesodermal layers.

Answer 24

By day 17 = The most mesodermal cells are close to the midline, so this area is the thickest. The mesodermal areas further out are the intermediate, and the lateral plate mesoderms.

Paraxial mesoderm = The part of the mesoderm that flanks the neural tube

The lateral plate mesoderm = This is continuous with the extra-embryonic mesoderm that covers the amniotic and yolc sacs.

The mesoderm covering the amniotic sac = Parietal or somatic layer

Mesoderm covering the yolk sac = Visceral or splanchnic layer.

Intermediate mesoderm = Arises from the paraxial and lateral plate mesoderm to go on to form the urogenital system

Question 25

Describe how the paraxial mesoderm is further differntiated.

Answer 25

The paraxial mesoderm further differentiates into paired blocks of tissue in a cranio-caudal direction. These form on either side of the notochord and are called somites.

First pair = Forms at day 20, and continues at about 3 pairs per day until 42-44 are formed. This is how we calculate the age of embryo

Beginning of 4th week = The somites further differentiate into dermomyotomes (forming connective tissue and muscle) AND sclerotomes (forming bone and cartilage). Cells from sclerotome sorround notochord to form the spinal cord.

Question 26

Describe in further detail how the somites develop

Answer 26

Starting from 4th week = Somites start developing and differentiating more

Ventro-lateral somites = Become myotomes, and the remaining become dermatomes.

Myotomes = Further split into dorsal epimeres, and the ventral hypomeres.
Dorsal epineres = Give rise to epaxial muscles (muscles assoicated with skull, vertebrae and ribs), erector spinae muscle
Ventral hypomeres = Give rise to hypaxial muscles (Trunk muscles, abdo wall, diaphragm and limbs)

Question 27

Outline how the lateral plate mesoderm differentiates into the serous membranes

Answer 27

The mesodermal cells of the lateral plate arrange themselves in thin layers = These become the serous membranes of the body including pleura, pericardium, and peritoneum. They later become the serous and visceral layers of the coverings

Question 28

What does the endoderm go on to form?

Answer 28

The lining of the GI and Resp systems, the parenchymal cells of the liver, panceas, thyroid, and parathyroidism. As well as lining of the urinary bladder

Question 29

What time does folding of the embryo start

Answer 29

From the 4th week

Question 30

How many types of embryo folding is there in the 4th week. What are they?

Answer 30

Longitudinal or cephalocaudal

Lateral or transverse

Question 31

Describe the longitudinal and lateral folding that occurs int he 4th week of embryo development

Answer 31

Longitudinal folding or cephalocaudal = Occurs because cranial end of neural tube rapidly developments with the brain formation. This occurs between days 21-24. Ends up so that the head and tail are closer together. As

Lateral folding or transverse = Occurs because of the enlargement of the somites.

Question 32

Remember the yolk sac and the gut tube are connected. This narrows during folding to form what duct?

Answer 32

The vitello-intestinal (or vitelline) duct. This is later lost. If this remains patient you can get meconium coming out of umbilicus.