BGDA Practical 12 - Second Trimester

From Embryology
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Practical 12: Embryo to Fetus | Second Trimester | Third Trimester | Birth | Neonatal | Abnormalities


Introduction

3D ultrasound static image of the 12 week fetus
  • Early fetal - week 12
  • placental changes and fetal membrane changes
  • length changes
  • clinical diagnosis (ultrasound)
  • fetal endocrine
  • sensory systems
  • Genital male and female external genital differences observable
  • Respiratory Month 3-6 - lungs appear glandular, end month 6 alveolar cells type 2 appear and begin to secrete surfactant

Week 12

Links: Ultrasound

Size

Fetal length change
Fetal head growth circumference
  • Biparietal diameter
  • Head circumference
  • Abdominal circumference
  • Femur length
  • Foot length (correlates with CRL)
  • Placental parameters (covered in next practical)

Head

Fetal head lateral.jpg Fetal head medial.jpg
Fetal head lateral Fetal head medial

Fetal head section 01.jpg

  • Tongue Week 12 - first differentiated epithelial cells

Week 16+ (4 months)

Respiratory Development Overview

Lung alveoli development cartoon

Pituitary Development

  • Week 4 - hypophysial pouch, Rathke’s pouch, diverticulum from roof
  • Week 5 - elongation, contacts infundibulum, diverticulum of diencephalon
  • Week 6 - connecting stalk between pouch and oral cavity degenerates
  • Week 10 - growth hormone and ACTH detectable
  • Week 16 - adenohypophysis fully differentiated
  • Week 20 to 24 - growth hormone levels peak, then decline

Thyroid Development

  • Functions from week10 - required for neural development, stimulates metabolism (protein, carbohydrate, lipid), reduced/absence = cretinism (see abnormalities)
  • Hormones - (amino acid derivatives) Thyroxine (T4), Triiodothyronine (T3)
Stage 13 and Stage 22 thyroid development
  • thyroid median endodermal thickening in the floor of pharynx, outpouch – thyroid diverticulum
  • 24 days - thyroid median endodermal thickening in the floor of pharynx, outpouch – thyroid diverticulum
  • Week 11 - colloid appearance in thyroid follicles, iodine and thyroid hormone (TH) synthesis
  • Fetal Thyroid Hormone
    • Initial secreted biologically inactivated by modification, late fetal secretion develops brown fat
    • Iodine deficiency- during this period, leads to neurological defects (cretinism)
    • Birth - TSH levels increase, thyroxine (T3) and T4 levels increase to 24 h, then 5-7 days postnatal decline to normal levels


Links: Endocrine - Pituitary Development | Embryo Images - Pituitary | Endocrinology

Skin Development

Fetal integumentary histology 01.jpg

  • 4 weeks
    • simple ectoderm epithelium over mesenchyme.
  • 1 - 3 months
    • ectoderm - germinative (basal) cell repeated division of generates stratified epithelium.
    • mesoderm - somite dermatome spreads out under the epithelium, differentiates into connective tissue and blood vessels.
  • 4 months
    • basal cell - proliferation generates folds in basement membrane.
    • neural crest cells - (melanocytes) migrate into epithelium. These are the pigment cell of the skin.
    • embryonic connective tissue - differentiates into dermis, a loose ct layer over a dense ct layer. Beneath the dense ct layer is another loose ct layer that will form the subcutaneous layer.
    • Ectoderm contributes to nails, hair follictles and glands.
    • Nails form as thickening of ectoderm epidermis at the tips of fingers and toes. These form germinative cells of nail field.
    • Cords of these cells extend into mesoderm forming epithelial columns. These form hair follicles, sebaceous and sweat glands.
  • 5 months
    • Hair growth initiated at base of cord, lateral outgrowths form associated sebaceous glands.
    • Other cords elongate and coil to form sweat glands.
    • Cords in mammary region branch as they elongate to form mammary glands. These glands will complete development in females at puberty. Functional maturity only occurs in late pregnancy.
    • Vernix cases begins to form and cover the skin (arms an intact visible coating in the third trimester)
Links: Integumentary System Development

Genital Development

Genital male and female external genital differences observable. In the ovary primary follicles begin to form and are characterized by the developing oocyte.

The animations below shows the development of external female and male genitalia from the indifferent external structure, during the period of week 9 to 12. Note - this content will be covered in detail in BGDB Genital development.

Female external 001 icon.jpg
 ‎‎Female External‎‎
Page | Play
Male external 001 icon.jpg
 ‎‎Male External
Page | Play


All Genital cartoons


Neural

Brain ventricles and ganglia development 03.jpg Brain fissure development 02.jpg
Brain and Ventricular Development[1] Brain Fissure Development[1]
Sylvian Fissure Development 

Brain fissure development 02.jpg


At about 19 weeks (GA 21 weeks) neuronal migration ends and the radial glial cells, that aided this migration, now become transformed into astrocytes and astrocytic precursors.[2]

Links: Glial Development

Sensory Development

  • Hearing - middle ear bones (ossicles) - not free to move until birth.
  • Vision - months 3 to 4 retina development, eyelids closed (until 5-7 months), lens grows through third trimester, cannot focus even postnatally.


Ultrasound

About Ultrasound

Different body tissues reflect sound waves differently. In ultrasound, a beam of sound waves (frequency 3 to 10 MHz) are passed through the body, the reflected waves are analysed by a computer, and an image is then generated on a display screen. The sound source is usually a transducer placed on the surface of the abdomen.

Ultrasound Measurements

There can be number of different parameters, depending on gestational age, that are commonly recorded during an ultrasound procedure. These measurements (and ratios) include embryonic/fetal size and key lengths and sizes of specific structures, fetal membrane sizes/volumes, placental location/size. In addition, general body movements, including heartbeat, can be observed.

Ultrasound Fetal (GA 19 weeks) Ultrasound scan through the fetal trunk to measure abdominal circumference (AC).
Ultrasound Fetal (GA 19 weeks)

Ultrasound scan through the fetal trunk to measure abdominal circumference (AC).

placental cord knot

Embryonic/Fetal Size: crown-rump length (CRL), biparietal diameter (BPD), abdominal circumference (AC)

Embryonic/Fetal Structures: femur length (FL), head circumference, nuchal translucency, heart size/rate

Fetal Membrane: gestational sac diameter (GS), yolk sac diameter (YS), meconium peritonitis, umbilical cord stricture

Placenta: location, size, umbilical cord stricture

Ultrasound - Abnormalities

See Ultrasound movie for Cleft lip 1

Additional Information

Additional Information - Content shown under this heading is not part of the material covered in this class. It is provided for those students who would like to know about some concepts or current research in topics related to the current class page.
Normal Ultrasound
Ultrasound12wk 3D image2.jpg
 ‎‎Ultrasound 12wk
Page | Play
Week12 fetal heart rate-icon.jpg
‎‎12 Week Heart Rate
Page | Play
19weeklabel1.jpg
 ‎‎19 Week Fetus
Page | Play
  
Ultrasound day16 rabbit.jpg
 ‎‎Rabbit Embryo
Page | Play
Ultrasound: normal movies | abnormal movies | all ultrasound movies | Movies
Abnormal Ultrasound
Uterus Cleft Lip
Ectopic 01.jpg
 ‎‎Ectopic Pregnancy
Page | Play
Bicornuate uterus ectopic movie icon.jpg
 ‎‎Bicornuate Ectopic
Page | Play
Cleft lip 01.jpg
 ‎‎Cleft Lip 18 Week
Page | Play
Cleft lip 02.jpg
 ‎‎Cleft Lip 15 Week
Page | Play
Gastrointestinal Neural
Gastroschisis 01.jpg
 ‎‎Gastroschisis
Page | Play
Omphalocele 01 icon.jpg
 ‎‎Omphalocele
Page | Play
US Dandy-Walker 01.jpg
 ‎‎Dandy-Walker
Page | Play
US Spina bifida GA19week.jpg
 ‎‎Spina Bifida
Page | Play
Cardiac
US Transposition great arteries.jpg
 ‎‎Heart TGA
Page | Play
Hypoplastic left heart syndrome movie icon.jpg
 ‎‎Hypoplastic Left
Heart Syndrome
Page | Play
Postnatal persistant ductus venosus ultrasound 03.jpg
 ‎‎Patent
Ductus Venosus
Page | Play
Placenta
US Placenta Previa GA33week icon.jpg
 ‎‎Placenta Previa
Page | Play
US Vasa Previa GA32week.jpg
 ‎‎Vasa Previa
Page | Play
Velamentous cord movie icon.jpg
 ‎‎Velamentous Cord
Page | Play
Ultrasound: normal movies | abnormal movies | all ultrasound movies | Movies

Second Trimester Intestinal Growth

Both the small and large intestine grow in a linear manner during the second trimester.[3][4]

Fetal small Intestine length growth graph.jpg Fetal large Intestine length growth graph.jpg

Second Trimester

(Clinical Week 14) Second Trimester


Week
Stage
Event
12
Clinical second trimester Fetal head lateral.jpg Week 12 - CRL 85 mm, femur length 15 mm, biparietal diameter 25 mm

Hearing Week 12-16 - Capsule adjacent to membranous labrynth undegoes vacuolization to form a cavity (perilymphatic space) around membranous labrynth and fills with perilymph

Genital male and female external genital differences observable

Respiratory Month 3-6 - lungs appear glandular, end month 6 alveolar cells type 2 appear and begin to secrete surfactant

Tongue Week 12 - first differentiated epithelial cells (Type II and III)

Genital female genital canal (80 days) formed with absorption of the median septum

13
  Tongue Week 12 to 13 - maximum synapses between cells and afferent nerve fibers

Hearing - Outer Ear Development Week 13 - Meatal plug disc-like, innermost surface in contact with the primordial malleus, contributes to the formation of the tympanic membrane.  

14
Tongue Week 14 to 15 - taste pores develop, mucous

Ovary Development 100 days - primary follicles present

Nail Development toenails appear

Head Development facial skeleton remodelling begins

15
  Pancreas glucagon detectable in fetal plasma.

Spleen Week 15 -alpha-smooth muscle actin (alpha-SMA)-positive reticulum cells scattered around the arterioles. [5]

16
14 cm Fetal size change.jpg Hearing Week 16-24 - Centres of ossification appear in remaining cartilage of otic capsule form petrous portion of temporal bone. Continues to ossify to form mastoid process of temporal bone.

Pituitary adenohypophysis fully differentiated

Respiratory Week 16 to 25 lung histology - canalicular

Hearing - Outer Ear Development Week 16.5 - External auditory meatus is fully patent throughout its length, lumen is still narrow and curved.

Skin 4 months - basal cell- proliferation generates folds in basement membrane; neural crest cells- (melanocytes) migrate into epithelium; embryonic connective tissue- differentiates into dermis, a loose ct layer over a dense ct layer. Beneath the dense ct layer is another loose ct layer that will form the subcutaneous layer. Ectoderm contributes to nails, hair follictles and glands. Nails form as thickening of ectoderm epidermis at the tips of fingers and toes. These form germinative cells of nail field. Cords of these cells extend into mesoderm forming epithelial columns. These form hair follocles, sebaceous and sweat glands.

primary follicles begin to form in the ovary and are characterized by an oocyte

glandular urethra forms and skin folds present

17
Brain week 17 histology.jpg Neural - Brain development histology week 17
18
Bailey095.jpgTongue Week 18 - substance P detected in dermal papillae, not in taste bud primordia

Skin vernix caseosa covers skin

Spleen Week 18 - alpha-SMA-positive reticulum cells increase in number and began to form a reticular framework. An accumulation of T and B lymphocytes occurred within the framework, and a primitive white pulp was observed around the arterioles. [5]

Hearing - Outer Ear Development week 18 - External auditory meatus is already fully expanded to its complete form.

19
   
20
  Pituitary week 20 to 24 growth hormone levels peak, then decline

Skin lanugo, skin hair

Skin 5 months - Hair growth initiated at base of cord, lateral outgrowths form associated sebaceous glands; Other cords elongate and coil to form sweat glands; Cords in mammary region branch as they elongate to form mammary glands.

21
   
22
  Gray0038.jpg Neural brain cortical sulcation - sylvian fissure, interhemispheric fissure, callosal sulcus, parietooccipital fissure, and hippocampic fissures present[6]

Spleen - Week 22 - antigenic diversity of the reticular framework was observed, and T and B lymphocytes were segregated in the framework. T lymphocytes were sorted into the alpha-smooth muscle actin-positive reticular framework, and the periarteriolar lymphoid sheath (PALS) was formed around the arteriole. B lymphocytes aggregated in eccentric portions to the PALS and formed the lymph follicle (LF). The reticular framework of the LF was alpha-SMA-negative. [5]

23
   
24
  Respiratory Week 24 to 40 lung histology - terminal sac

Spleen Week 24 - marginal zone appeared in the alpha-smooth muscle actin-positive reticular framework around the white pulp.[5]

Earliest potential survival expected if born

ovarian follicles can consist of growing oocytes surrounded by several layers of granulosa cells

25
  Respiratory end month 6 alveolar cells type 2 appear and begin to secrete surfactant

Endocrine - Adrenal and Placenta

The feto-placental unit, and potential roles of dehydroepiandrosterone (DHEA) in prenatal and postnatal brain development[7]

"Synthesis of dehydroepiandrosterone (DHEA) by the fetal adrenal gland is important for placental oestrogen production, and may also be important for modulating the effects of glucocorticoids on the developing brain. ... Together, the studies outlined in this review indicate that the androgen DHEA is an important hormone of adrenal and Central Nervous System (CNS) origin in the fetal and postnatal spiny mouse. Disturbance of the development of these fetal tissues, and/or of the relationship between the fetal adrenal gland and placenta during pregnancy, may have significant consequences for fetal development, placental function, and maturation of the brain. It is proposed that such disturbances of normal adrenal function could account for some of the neuropathologies that arise in juvenile and adult offspring following illness and stress experienced by the mother during pregnancy."

References

  1. 1.0 1.1 Hao Huang, Rong Xue, Jiangyang Zhang, Tianbo Ren, Linda J Richards, Paul Yarowsky, Michael I Miller, Susumu Mori Anatomical characterization of human fetal brain development with diffusion tensor magnetic resonance imaging. J. Neurosci.: 2009, 29(13);4263-73 PubMed 19339620 | PMC2721010 | J Neurosci.
  2. H J Kadhim, J F Gadisseux, P Evrard Topographical and cytological evolution of the glial phase during prenatal development of the human brain: histochemical and electron microscopic study. J. Neuropathol. Exp. Neurol.: 1988, 47(2);166-88 PubMed 3339373
  3. J FitzSimmons, A Chinn, T H Shepard Normal length of the human fetal gastrointestinal tract. Pediatr Pathol: 1988, 8(6);633-41 PubMed 3244599
  4. John G Archie, Julianne S Collins, Robert Roger Lebel Quantitative standards for fetal and neonatal autopsy. Am. J. Clin. Pathol.: 2006, 126(2);256-65 PubMed 16891202
  5. 5.0 5.1 5.2 5.3 Takashi Satoh, Eiichi Sakurai, Hiroshi Tada, Tomoyuki Masuda Ontogeny of reticular framework of white pulp and marginal zone in human spleen: immunohistochemical studies of fetal spleens from the 17th to 40th week of gestation. Cell Tissue Res.: 2009, 336(2);287-97 PubMed 19255788
  6. C Garel, E Chantrel, H Brisse, M Elmaleh, D Luton, J F Oury, G Sebag, M Hassan Fetal cerebral cortex: normal gestational landmarks identified using prenatal MR imaging. AJNR Am J Neuroradiol: 2001, 22(1);184-9 PubMed 11158907
  7. Tracey A Quinn, Udani Ratnayake, Hayley Dickinson, Margie Castillo-Melendez, David W Walker The feto-placental unit, and potential roles of dehydroepiandrosterone (DHEA) in prenatal and postnatal brain development: A re-examination using the spiny mouse. J. Steroid Biochem. Mol. Biol.: 2015; PubMed 26485665


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Cite this page: Hill, M.A. 2017 Embryology BGDA Practical 12 - Second Trimester. Retrieved September 26, 2017, from https://embryology.med.unsw.edu.au/embryology/index.php/BGDA_Practical_12_-_Second_Trimester

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© Dr Mark Hill 2017, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G