4. เยื่อหุ้มตัวเด็กและรก (extraembryonic membrane placenta)

      4.1 amnion หรือถุงน้ำคร่ำ อยู่ล้อมรอบตัวอ่อน เจริญมาจาก trophoblast ภายในมีน้ำคร่ำ (amniotic fluid) ป้องกันมิให้ตัวอ่อนติด amnion ช่วยควบคุมอุณหภูมิ ป้องกันการกระเทือน ช่วยให้เด็กเคลื่อนไหวสะดวก กระตุ้นการเจริญของกระดูกและกล้ามเนื้อ amniochorionic sac ช่วยขยายปากช่องคลอดนำทางเด็ก หล่อลื่น ชะล้างช่องคลอด ประมาณน้ำคร่ำของทารกปกติใกล้คลอดมีประมาณ 1,000 มิลลิลิตร ทารกกลืนน้ำคร่ำและถูกดูดซึมไปทาง respiratory และ digestive tract




     4.2 chorion เป็นเยื่อหุ้มรอบ amnion เจริญมากจาก cytrophoblast ,  syncytiotrophoblast และ extraembryonic somatic mesoderm การเจริญอย่างรวดเร็วของ chorion เข้าไปใน endometrium ของแม่ ลักษณะคล้ายนิ้วมือเรียกว่า villi ซึ่งจะเปลี่ยนแปลงเป็นเส้นเลือดต่อไป

      4.3  yolk sac หรือถุงไข่แดง เจริญจาก endoderm และ extraembryonic mesoderm เป็นแหล่งอาหารและสร่างเม็ดเลือดแดง endodermal cell จากผนัง yolk sac จะเจริญเปลี่ยนแปลงเป็น primordial germ cell 

     4.4 Allantois เป็นถุงอยู่เหนือ yolk sac ช่วงแรกๆ ทำหน้าที่ excretion พอเกิดรกก็จะกลายเป็นท่อตันไม่ได้แลกเปลี่ยนก๊าซ หรือเก็บของเสียจำพวกยูริก เหมือนตัวอ่อนของไก่

     4.5 Umbilical cord หรือสายสะดือ ปกติยาวประมาณ 30 – 90 ซม. เส้นผ่านศูนย์กลาง  1 – 2 ซม. ประกอบด้วย whaton s jelly หรือ umbilical vessel มีเส้นเลือดแดง (artery) 2 เส้น และเส้นเลือดดำ (vein) 1 เส้น 

     4.6 Placenta หรือรก เจริญมาจาก villous chorion ของลูกและ decidua basalis ของมดลูกแม่ มีหน้าที่ลำเลียงอาหาร ก๊าซ ของเสีย สารคัดหลั่ง สร้างฮอร์โมน hCG , progesterone , estrogen , relaxin และเป็นแนวป้องกันไม่ให้จุลินทรีย์ผ่านไปยังทารก




          เยื่อหุ้มรกถูกนำมาใช้ประโยชน์ทางการแพทย์มานาน เช่น นำมาปลูกถ่ายรักษาผิวหนังที่ถูกไฟไหม้และโรคผิวดวงตาที่เกิดจากปัญหาขอเยื่อบุตา เพราะเยื่อหุ้มรกมีคุณสมบัติพิเศษสามารถต่อต้านจุลชีพทำให้ไม่เกิดการติดเชื้อ และไม่พบสารกระตุ้นภูมิคุ้มกันจึงไม่มีปัญหาเรื่องเนื้อเยื่อไม่เข้ากัน




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