Early Development and Disorders

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Testis Development:

Sry gene on Y chromosome.

• Testosterone (Leydig cells).

• MIF (Sertoli cells).

Meiosis I:

• Synapsis; crossing over; disjunction.

Meiosis II:

• No synapsis or crossing over.

• Disjunction with centromere splitting.


• Type A spermatogonia form Type B spermatogonia.

• Type B spermatogonia form primary spermatocytes after meiosis I.

• Primary spermatocytes form secondary spermatocytes.

• Secondary spermatocytes form 2 spermatids.

• Spermatids form sperm after spermiogenesis.


• Primordial germ cells form oogonia.

• Oogonia form primary oocytes after meiosis I.

• Primary oocytes arrested in prophase of meiosis I until puberty.

• Primary oocytes become secondary oocytes at puberty.

• Secondary oocytes arrested in metaphase of meiosis II and ovulated.

• Secondary oocyte forms mature oocyte and polar body after fertilization.

Ectopic Tubal Pregnancy:

• Blastocyst implantation in ampulla.

• Risk factors: endometriosis, PID, tubular pelvic surgery, diethylstilbestrol (DES) exposure.

• Uterine bleeding; abdominal pain; delayed menses; positive hCG test.

Urine Pregnancy Test:

• hCG can be detected by day 7 of conception.

• Low: ectopic pregnancy or spontaneous abortion.

• High: multiple pregnancy; hydatidiform mole; gestational trophoblastic neoplasia.

Sacrococcygeal Termaoma:

• Arises from remnants of primitive streak.


• Arises from remnants of notochord.

Caudal Dysplasia (Sirenomelia):

• Abnormal gastrulation; disturbed migration of mesoderm.

• Causes: VATER (vertebral defects, anal atresia, tracheoesophageal fistula, renal defects.)

• Also causes: VACTERL (vertebral defects, anal atresia, tracheoesophageal fistula, renal defects, upper limb defects.)

Additional Readings:

Basic Embryology

1. Early Development and Disorders
2. Differentiation and Anatomy of a Blastocyst
3. Differentiation of the Face
4. Effects of Drugs on Fetal Development

Related Topics

1. Genetic and Developmental Disorders
2. Notes on Fetal Circulation
3. Prenatal Infections

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