Category:Second Trimester

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

hearing outer ear Week 13 - Meatal plug disc-like, innermost surface in contact with the primordial malleus, contributes to the formation of the tympanic membrane.  

ovary 100 days - primary follicles present

nail toenails appear

Head Development facial skeleton remodelling begins

Hearing - Inner Ear Development Week 14 GA 16 - neural-crest-derived melanocytes, now intermediate cells of the stria vascularis, tightly integrate with Na+ /K+ -ATPase-positive marginal cells, which started to express KCNQ1 in their apical membrane.^[1]

spleen Week 15 -alpha-smooth muscle actin (alpha-SMA)-positive reticulum cells scattered around the arterioles.^[2]

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.

Hearing - Inner Ear Development Week 16 GA 18 - cells in the outer sulcus express KCNJ10 and gap junction proteins GJB2/CX26 and GJB6/CX30, but these are not expressed in the spiral ligament.^[1] gap junction cartoon

neural - Cerebrum development of the periinsular sulci (week 16-17, GA 18-19 weeks)^[3]

integumentary 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

Cerebellum Magnetic Resonance Imaging (MRI) can study the developing cerebellum from 17 to 18 weeks (GA 19 to 20 weeks).

tooth Week 17 - First papilla of the permanent dentition appear (first molar) immediately behind the second milk molar, milk teeth are well advanced (Fetus 180 mm).

integumentary 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.^[2]

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

neural - Cerebrum central sulci and opercularization of the insula (week 18-20, GA 20-22 weeks)^[3]

integumentary lanugo, skin hair

integumentary 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.

touch pacinian corpuscle begin to develop^[5]

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. ^[2]

neural - Cerebrum covering of the posterior insula (week 22-24, GA 24-26 weeks)^[3]

spleen Week 24 - marginal zone appeared in the alpha-smooth muscle actin-positive reticular framework around the white pulp.^[2]

tooth Week 24 - Permanent incisors and canines appear.

Earliest potential survival expected if born

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

neural - Cerebrum closure of the laeteral sulcus (Sylvian fissure or lateral fissure) (week 25-26, GA 27-28 weeks)^[3]