2009 Lecture 16

Genital Development

Introduction

This section of notes covers genital development. Differences in development are dependent on a protein product of the Y chromosome SRY gene. Mesonephric duct (Wolffian Duct) and paramesonephric (Mullerian Duct) contribute the majority of male and female internal genital tract respectively.

Stage 22 mesonephros

Objectives

• Understand the role of the Y chromosome in sex determination.
• Understand the differences in male/female duct develpoment (mesonephric/paramesonephric).
• Compare the development of the cloaca in the male and female.
• Understand the developmental abnormalities in male and female development.

Textbooks

• Human Embryology (2nd ed.) Larson Ch10 p261-306
• The Developing Human: Clinically Oriented Embryology (6th ed.) Moore and Persaud Chapter 13 p303-346

Three Stages

The mesonephric/paramesonephric duct changes are one of the first male/female differences that occur in development, while external genitaila remain indeterminate in appearance for quite a while.

1. Differentiation of gonad (Sex determination)
2. Differentiation of internal genital organs
3. Differentiation of external genital organs

The 2nd and 3rd stages dependent on endocrine gonad. Reproductive development has a long maturation timecourse, begining in the embryo and finishing in puberty. (More? Puberty)

Development Overview

Sex Determination

• Humans (week 5-6)
• Germ cells migrate into gonadal ridge
• Gonads (male/female) identical at this stage, indifferent

Gonad development

• dependent on sex chromosome
• Y testes
• No Y ovary

SRY

• SRY protein (Testes determining factor, TDF) binds DNA
• Transcription factor, Bends DNA 70-80 degrees

Internal Genital Organs

• All embryos form paired
• Mesonephric duct, see kidney development
• Paramesonephric duct, Humans 7th week Invagination of coelomic epithelium Cord grows and terminates on urogenital sinus
• Male Gonad (testes) secretes Mullerian duct inhibitory factor (MDIF) which causes regression of paramesonephric duct
• Male Gonad (testes) secretes Testosterone which retains mesonephric duct

External Genital Organs

• All embryos initially same (indifferent)
• Testosterone differentiates male

Sex Determination

Male (XY)

• Y chromosome - 200+ genes, 50 million base pairs
• Sry was discovered (1990) by studying a human XY female, resulting from a deletion in the Y chromosome that did not allow testis development. Subsequent mapping of this deletion allowed isolation and characterization of the SRY gene.

• SRY encodes: encodes a 204 amino acid protein (Mr 23884 Da) that is a zinc-finger transcription factor. Transcription factors bind to specific sites of DNA and regulates the transcription (expression) of other genes, we still do not know all the genes SRY regulates.

• SRY is expressed: when testes begin to form, in gonadal tissue and does not require the presence of germ cells.

Female (XX)

• 1400+ genes, 150 million base pairs
• In contrast to the Y chromosome, the X chromosome contains about 5% of the haploid genome and encodes house-keeping and specialized functions. The genetic content of the X chromosome has been strongly conserved between species.

• An early discovery (1961) was that in order to have correct levels of X chromosome gene/protein expression (gene dosage), females must "inactivate" a single copy of the X chromosome in each and every cell. The initiator of the X inactivation process was discovered (1991) to be regulated by a region on the inactivating X chromosome encoding an X inactive specific transcript (XIST), that acts as RNA and does not encode a protein.

• X inactivation occurs randomly throughout the embryo, generating a mosaic of maternal and paternally derived X chromosome activity in all tissues and organs.

Primordial Germ Cells

• thought to be the first population of cells to migrate through the primitive streak in early gastrulation.
• This population of cells then lie at the hindgut yolk sac junctional region and later migrate into the germinal ridge in early embryonic development.
• It is not the primordial germ cells which respond to SRY presence or absence, but the supporting cells within the developing gonad.

References

• Before We Are Born (5th ed.) Moore and Persaud Chapter 14 p289-326
• Essentials of Human Embryology, Larson Ch10 p173-205
• Human Embryology, Fitzgerald and Fitzgerald Ch21-22 p134-152
• Developmental Biology (6th ed.) Gilbert Ch14 Intermediate Mesoderm

Online Links

• UNSW Embryology Abnormalities | Y chromosome | X chromosome | Ovary | Stage 13/14 Embryo | Stage 22 Embryo | Stage 22 Highpower

• UNSW Embryology Movies: Urogenital Movies

• Embryo Images Unit: Embryo Images Online | Urongenital Development | Internal Genitalia | Definitive Kidney | External Genitalia

• Histology: Male Reproductive System | Female Reproductive System

Glossary Links

Glossary: A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | Numbers | Symbols | Term Link

Course Content 2009

Embryology Introduction | Cell Division/Fertilization | Cell Division/Fertilization | Week 1&2 Development | Week 3 Development | Lab 2 | Mesoderm Development | Ectoderm, Early Neural, Neural Crest | Lab 3 | Early Vascular Development | Placenta | Lab 4 | Endoderm, Early Gastrointestinal | Respiratory Development | Lab 5 | Head Development | Neural Crest Development | Lab 6 | Musculoskeletal Development | Limb Development | Lab 7 | Kidney | Genital | Lab 8 | Sensory - Ear | Integumentary | Lab 9 | Sensory - Eye | Endocrine | Lab 10 | Late Vascular Development | Fetal | Lab 11 | Birth, Postnatal | Revision | Lab 12 | Lecture Audio | Course Timetable

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Cite this page: Hill, M.A. (2024, May 2) Embryology 2009 Lecture 16. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/2009_Lecture_16

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